spiral.imperial.ac.uk...i abstract one of the drawbacks of current solvent stable nanofiltration...
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Imperial College London
Department of Chemical Engineering
Adsorptive Cellulose Membranes for Fluid
Separation
By
Nilay Keser Demir
Supervisor: Prof. Kang Li
A thesis submitted for the degree of Doctor of Philosophy of Imperial College
London and the Diploma of Imperial College London
2017
I certify that the work in this thesis is my own and that the work of others is
appropriately acknowledged
“The copyright of this thesis rests with the author and is made available under a
Creative Commons Attribution Non-Commercial No Derivatives licence.
Researches are free to copy, distribute or transmit the thesis on the condition that
they attribute it, that they do not use it for commercial purposes and that they do
not alter, transform or build upon it. For any reuse or redistribution, researches
must make clear to others the licence terms of this work”
To my dear husband Ishak Demir and
My lovely daughter, Inci Azra Demir
I
Abstract
One of the drawbacks of current solvent stable nanofiltration membranes is environmentally
harsh preparation methods which are being used to improve the stability of the membrane materials
because of the lack of viable membrane materials stable in organic solvents. This thesis describes
research into the utilization of cheap, natural, biodegradable polymer, cellulose, as a material for the
development of solvent stable membrane for different liquid separation applications. A simple high
temperature dissolution process in an environmentally friendly solvent N-methylmorpholine-N-
oxide (NMMO) was used to improve the greenness of the process. Membranes showed significantly
high permeances for polar protic and polar aprotic solvents, including acetone, acetonitrile,
tetrahydrofuran (THF), ethyl acetate, and alcohols. Dependency of the fluxes on the viscosities of
the solvents was explained by the homogenous symmetric membrane structure formed by phase
inversion process. The thickness of the membrane was decreased five times and fluxes were
improved dramatically without compromising the mechanical strength of the membranes at high
pressure and the resistance of them in harsh conditions. SEM images, Hagen-Pouiseille type
transport behavior, and drastic increase in the permeances by decreasing thickness confirmed the
homogenous symmetric membrane structure. Rejection experiments conducted for water and
organic solvents confirmed that the separation mechanism through the membranes is governed by
the adsorption taking place on the membrane surface. The adsorption capability depends on the
solvent and the charge of the dyes used as markers in rejection experiments.
When the membrane is saturated during adsorption, dyes were permeated through it and
rejection failed. Some chemical modifications were proposed to modify the membrane surface to
improve their efficiency in organic solvent nanofiltration applications. Cellulose membranes showed
an exceptional stability in modification conditions while the commercial backing paper was failed.
II
Solvent stable nanocellulose paper (NCP) backing material with very similar chemical stability as
the membranes was prepared in order to produce a completely stable and green end product. Since
one of the main objectives of this thesis is the development of green membrane fabrication methods,
chemical modifications were not being focused in detail, however, they should definitely be
investigated in future to open a new perspective and a more sustainable association for OSN
applications.
The main challenge in this study is to make use of the natural ability of ‘cellulose’ without
compromising its green image. Therefore, we reported the usage of cellulose membranes for metal
removal (i.e. silver and arsenic) from aqueous solutions by using their high potential on adsorption
processes. Very promising results were reported for silver adsorption. Addition of metal organic
framework, UIO-66 with high surface area in cellulose matrix improved the adsorption capacity of
membranes. If the regeneration of these membranes could be achieved, then large-scale industrial
membrane modules could be built especially for silver removal application.
III
Acknowledgements
I would like to thank my supervisor Professor Kang Li for his encouragement and support throughout
the course of my PhD. It has been a great honour for me working and learning from him. I would
like thanks to Republic of Turkey Ministry of National Education for the sponsorship during my
PhD studies.
I am also grateful to all my colleagues in the Li’s group who were always there to advise me and
support me. Special thanks to Dr. Solomon for working closely with me during some stages of my
PhD, and to Xinlei, Ana and Joa for their important input in my work, and their support during my
PhD. I would like to thank Gildas and Mulahim for their help in my experimental works. The last
but not the least I would like to thank to Farah for her support, kind friendship and precious
encouragement. To the above-mentioned and many other unnamed colleagues and friends, who have
also contributed to the completion of this work and gave me their support, I extend my sincere
thanks.
I would like to thank Zoheb Karim and Associate Prof Aji P Mathew from Lulea University of
Technology for collaborating with me.
Foremost, I owe thanks to my family for their unconditional love and unlimited patience from the
very beginning. Words cannot express how grateful I am to my beloved husband, Ishak Demir, for
his endless love and support throughout the course of 4 years. I would not be able to achieve this
without him.
IV
List of Figures
Figure 1.1 Cellulose chemical structure [3] ............................................................................... 3
Figure 2.1 Classification of membrane processes according to operating pressure, retained
solute/pore size [nm], molecular weight cut-off [g mol−1], transport mechanism, and examples
of applications [1] .................................................................................................................... 12
Figure 2.2 Schematic approach of A) Osmotic equilibrium, B) Forward Osmosis, C)Reverse
Osmosis, D) Pressure Retarded Osmosis [24] ......................................................................... 14
Figure 2.3 Schematic representations of (a) dead-end and (b) cross-flow geometries [27] .... 15
Figure 2.4 Structure of cellulose (n is the degree of polymerisation) [49] .............................. 25
Figure 2.5 Phase diagram cellulose- NMMO-water [65] ........................................................ 29
Figure 2.6 (a) Six-centre octahedral zirconium oxide cluster. (b) FCU unit cell of UiO-66; blue
atom – Zr, red atom – O, white atom – C, H atoms are omitted for clarity [10] ..................... 37
Figure 3.1 Schematic representation of dead-end filtration set-up ......................................... 55
Figure 3.2 Schematic representation of cross-flow filtration set-up in which membrane cells
connected in series ................................................................................................................... 56
Figure 3.3 Schematic representation of NCP production ........................................................ 61
Figure 3.4 Cross-flow filtration system ................................................................................... 69
Figure 4.1 Photograph of the 25 µm-thick membranes a) without backing, b) with backing. 71
Figure 4.2 X-ray diffractograms of cellulose powder (black) and 25 µm-thick membrane (red).
.................................................................................................................................................. 73
Figure 4.3 Cross-sectional views of pure cellulose membranes without backing with different
thickness; A) 500-µm-cast on polyester backing, B) 250- µm-cast on polyester backing, C)
100-µm-cast on polyester backing, D) 50-µm-cast on polyester backing ............................... 76
Figure 4.4 Cross-sectional view of pure cellulose membranes (500-µm-cast) without backing
.................................................................................................................................................. 77
Figure 4.5 Zeta potential of cellulose membrane at different pH values ................................. 78
V
Figure 4.6 Thermal decomposition profiles of (A) cellulose powder and (B) cellulose
membrane. The corresponding first order derivatives of TGA curves for cellulose powder and
membrane sample are included for comparison with dashed line. .......................................... 79
Figure 4.7 Tensile strength and the maximum load with respect to thickness of the membranes.
Membranes were tested for tensile strength and the maximum load without backing paper under
them.......................................................................................................................................... 81
Figure 4.8 Pure solvent fluxes through 25-µm-thick membrane for various solvents.
Nanofiltration experiments have been performed in dead-end system at 10bar and 25 ºC. .... 82
Figure 4.9 A) Pure water flux for 24h through 25 µm-thick membrane prepared by phase
inversion B) Pure acetone flux for 24h through 25 µm-thick membrane prepared by phase
inversion. Nanofiltration experiments have been performed in cross-flow filtration system at
5bar and 25 ºC. ......................................................................................................................... 83
Figure 4.10 Inversely proportional relationship between viscosities of organic solvents and
their fluxes through (A) 25-µm-thick cellulose membrane at 10 bar, (B) 10-µm-thick cellulose
membrane at 10 bar, (C) 5-µm-thick cellulose membrane at 2 bar; (D) 2.5-µm-thick cellulose
membrane at 2 bar; (E) Relationship between applied pressure and water flux through a 10-
µm-thick cellulose membrane. Nanofiltration experiments have been performed in dead-end
system at 25 ºC. ........................................................................................................................ 85
Figure 4.11 Permeances of various solvents versus A) thickness and B) 1/thickness for
cellulose membranes. Nanofiltration experiments have been performed in dead-end system at
10bar and 25 ºC. ....................................................................................................................... 87
Figure 4.12 Permeability of various solvents versus thickness of cellulose membranes.
Nanofiltration experiments have been performed in dead-end system at 10bar and 25 ºC. .... 89
Figure 4.13 Solvent permeance performance of a 25 µm-thick cellulose membrane disc for
eleven successive filtration experiments; orange for water, black for acetonitrile, grey for
acetone, red for ethyl acetate, blue for THF, green for 1-butanol. Filtration experiments have
been performed in dead-end system at 10bar and 25 ºC. ......................................................... 90
Figure 4.14 MWCO curve of cellulose membrane. Nanofiltration of feed solutions comprising
different dyes dissolved in water have been performed separately at 10 bar and 22°C. ......... 93
Figure 4.15 (A) Ultra-violet visible absorption spectra of CSG; blue for permeate, red for
retentate, black for feed. (Inset) Photograph of membrane after rinsing with MeOH after
rejection test. (B) Ultra-violet visible absorption spectra of MO; blue for permeate, red for
retentate, black for feed. (Inset) Photograph of membrane after rinsing with MeOH after
VI
rejection test. (25μm-thick membrane). All experiments were conducted at pH 5.5 conditions.
.................................................................................................................................................. 95
Figure 4.16 Photographs of permeate (left) and retentate (right) of CSG dye at different pH
values (25μm-thick membrane) ............................................................................................... 97
Figure 4.17 (A) Ultra-violet visible absorption spectra of MO; red for before experiment, black
for after experiment (Up) Photographs of membranes before and after adsorption experiments.
(B) Ultra-violet visible absorption spectra of CSG; red for before experiment, black for after
experiment (Up) Photographs of membranes before and after adsorption experiments. (25μm-
thick membrane) ...................................................................................................................... 98
Figure 4.18 Normalised concentration over time for pure cellulose membrane tested in water,
acetonitrile and acetone.......................................................................................................... 105
Figure 4.19 Experimental results of cross-flow filtration of CR dissolved in water by 25 µm-
thick cellulose membrane. Filtration experiments were run at 5 bar operation pressure and 55
L h-1 flow rate. Results for 2 identical membrane pieces are shown in the figures for
repeatability. A) Flux performance of the membrane for CR-water solution with respect to time,
B) Percentage rejection of CR in water (inset) Photograph of the membrane after 1-week cross-
flow experiment. .................................................................................................................... 109
Figure 4.20 MWCO curve of cellulose membrane in alcohols. Nanofiltration of feed solutions
comprising different dyes dissolved in methanol, ethanol, and 1-butanol have been performed
separately at 10 bar and 22°C. ............................................................................................... 110
Figure 4.21 Characterization results for nanocellulose paper A) SEM image of the surface view
of the nanocellulose paper with a grammage of 40 g m-2, B) Permeance of pure water with
respect to time through the nanocellulose paper with a grammage of 40 g m-2, C) Relationship
between grammage and paper thickness and pure water permeance ..................................... 117
Figure 4.22 SEM images of A) NCP-2 surface view; B) PBP surface view. NCP-2. ........... 119
Figure 4.23 Pictures of NCP-2 pieces before and after 12 months’ stability experiments .... 120
Figure 4.24 SEM images of NCP-2 samples after 12 months’ stability experiments in A)
ethanol, B) THF, C) acetone, and D) ethyl acetate……..…………………………………..121
Figure 4.25 Pictures of pure cellulose membranes; A) cast on NCP-2, b) cast on PBP........123
Figure 4.26 Cross- sectional SEM images for 25-µm-thick cellulose membranes cast on A)
NCP-2, B) PBP backing papers ............................................................................................. 124
Figure 4.27 Biodegradability study of fabricated cellulose membranes on NCP-2 and PBP in
water (a) and in soil (b)……………………………………………………………..………125
VII
Figure 4.28 A) XRD patterns, B) FTIR patterns of pure cellulose (black) and cellulose/UIO-66
membrane (red), and pure UIO-66 powder (blue). ................................................................ 128
Figure 4.29 SEM images of UIO-66 powder synthesized by solvothermal technique at 120 °C
for 48 hours. ........................................................................................................................... 129
Figure 4.30 Effect of pH on silver adsorption capacity onto the UiO-66 powder during batch
adsorption experiments conducted for 24 hours . .................................................................. 130
Figure 4.31 Adsorption kinetics of silver onto the UiO-66 powder at pH 2 conditions. ....... 131
Figure 4.32 Adsorption isotherms of silver onto the UIO-66 powder for 24 h of contact time
(A) Comparison of the experimental and the Langmuir isotherms, (B) The maximum
adsorption capacity results and constant parameters, (C) Experimental results . .................. 133
Figure 4.33 XRD pattern of UIO-66 powder after silver adsorption .................................... 134
Figure 4.34 Characterization results of UIO-66 powders; (A) EDX analysis result, (B)
percentage amounts of elements, (C) SEM image after
adsorption………………………………………………………………………………...…135
Figure 4.35 SEM images of cellulose/ UIO-66 membranes at different magnifications. These
membranes were prepared by phase inversion precipitation technique containing 9 g of
NMMO, 1 g of cellulose, 0.2 g of UIO-66.. .......................................................................... 136
Figure 4.36 A) SEM image of the membrane after Ag (I) adsorption, (B) corresponding EDX
data of membranes after adsorption of As (V), (C) corresponding EDX data of membranes after
adsorption of Ag (I). .............................................................................................................. 143
Figure A.1 Pure solvent fluxes through 12-µm-thick membrane for acetone, acetonitrile, ethyl
acetate, THF, water, and 1-butanol. Nanofiltration experiments have been performed in dead-
end system at 10bar and 25 ºC. .............................................................................................. 170
Figure A.2 Pure solvent fluxes through 5-µm-thick membrane for water, acetone, acetonitrile,
ethyl acetate, THF, and 1-butanol. Nanofiltration experiments have been performed in dead-
end system at 2bar and 25 ºC. ................................................................................................ 171
Figure A.3 Pure solvent fluxes through 2.5-µm-thick membrane for water, acetone, acetonitrile,
ethyl acetate, THF, and 1-butanol. Nanofiltration experiments have been performed in dead-
end system at 2bar and 25 ºC. ................................................................................................ 171
Figure B.1 UV calibration curves for CR in water and RB in acetone .................................. 172
Figure B.2 Visual representation of dye rejections in acetone (R: retentate, P: permeate) ... 172
VIII
List of Tables
Table 2.1 Basic properties and structure of N-methyl morpholine N-oxide [73] .................... 28
Table 2.2 The adsorption capacity of different metal ions by cellulose membranes in both cross-
flow mode and static mode. The adsorption capacity in static mode is written with in
parenthesis to compare results [3] ............................................................................................ 40
Table 3.1 Properties and structure of the dyes used for rejection tests (in H2O and organic
solvents) ................................................................................................................................... 45
Table 4.1 Obtained dry membrane thickness when cast on polyester backing using different
adjusted casting knife thicknesses ........................................................................................... 72
Table 4.2 Physical properties of the organic solvents used for nanofiltration and permeances
.................................................................................................................................................. 86
Table 4.3 Separation performance of the membrane for different charged dyes in water (25μm-
thick membrane) ...................................................................................................................... 92
Table 4.4 Separation performance of the membrane in water at different pH values (25μm-
thick membrane) ...................................................................................................................... 96
Table 4.5 Rejection performance of the membrane in ethyl acetate, and THF (25μm-thick
membrane) ............................................................................................................................... 99
Table 4.6 Affinities between membrane-solute and membrane-solvent ............................... 101
Table 4.7 Adsorption of MO and CSG on the membrane surface ......................................... 103
Table 4.8 The amount of adsorbed RB on the membrane surfaces in different solvents (V is
assumed constant at 300 mL (no effect of sampling) and S is 14 cm2) ................................. 106
Table 4.9 Comparison of performances of prepared cellulose membranes (25μm-thick
membrane) and Duramem300................................................................................................ 113
Table 4.10 Stability results for surface modification reaction conditions ............................. 115
Table 4.11 Stability results for cross-linking reaction conditions ......................................... 122
Table 4.12 Comparison of cellulose membranes’ flux-rejection performances cast on NCP-2
and PBP .................................................................................................................................. 124
IX
Table 4.13 Silver and arsenic adsorption capacity of different types of membranes at 25 ppm
initial metal concentration under static conditions ................................................................ 139
Table 4.14 Adsorption performance of cellulose and cellulose/MOF membranes in cross-flow
filtration (Recovery time is provided in parenthesis for comparison) ................................... 142
Table C.1 Hansen solubility parameters of the dyes calculated by group contribution method
[143, 144] ............................................................................................................................... 173
Table C.2 Physical properties of the solvents [202] .............................................................. 174
Table D.1 Comparison of the maximum adsorption capacities of silver on different adsorbents
in literature ............................................................................................................................. 175
Table E.1 IR absorption bands of membranes ....................................................................... 176
X
List of Abbreviations
AMIMCl 1-allyl-3-methylimidazolium chloride
AA Acetic anhydride
AC Acetic acid
[BMIM]Cl 1-butyl-2-mrthylimidazolum chloride
BDC 1,4-benzenedicarboxylate
BET Brunauer–Emmett–Teller
BSA Protein solution
CR Congo red
CV Crystal violet
CSG Chrysoidine G
CWT Cellulose weight total
CI Crystallinity index
DMAc Dimethylacetamide
DMF N,N-dimethylformamide
DMSO Dimethylsulfoxide
DP Degree of polymerization
DI Deionized
EA Ethyl acetate
EDTA Ethylenediaminetetraacetic acid
FT-IR Fourier transfer IR spectroscopy
FCU Face-centred-cubic
FO Forward osmosis
HP Hagen-Poiseuille
HTMC 6-Hydroxy-2,5,7,8-
tetramethylchroman-2-carboxylic acid
HNSA 6-Hydroxy-2-naphtalenesulfonic
acid sodium salt
HPLC High-pressure liquid chromatography
IP Interfacial polymerization
IEP Isoelectric point
ISA Integrally skinned asymmetric
ICP-OES Inductively coupled plasma emission spectrometer
ICP Inherently conducting polymer
MeOH Methanol
MO Methyl orange
MCC Microcrystalline cellulose
MF Microfiltration
MWCO Molecular weight cut-off
MOF Metal organic framework
XI
NMMO N-methylmorpholine-N-oxide
NF Nanofiltration
NAS National Academy of Sciences
NB Naphthelene brown
NMP N-methyl pyrrolidone
NCP Nanocellulose paper
NDSA 1,5-naphthalenedisulfonic acid
OSN Organic Solvent Nanofiltration
PBP Polyester backing paper
PPy Polypyrrole
PAN Polyacrylonitrile
PBI Polybenzimidazole
PDMS Polydimethylsiloxane
PEEK Poly(ether ether ketone)
PEG Polyethylene glycol
PE Polyethylene
PES Polyethersulfone
PI Polyimide
PP Polypropylene
PIM Polymer inclusion membrane
PVSA Poly(vinyl)sulfonic acid
PMIA Poly m-phenylene isophthalamide
PVA Poly(vinyl alcohol)
PVDF Poly(vinylidene fluoride)
RO Reverse Osmosis
RB Rose Bengal
SEM Scanning electron microscopy
SEM-EDX Scanning electron microscopy coupled with energy-
dispersive X-ray spectroscopy
SDS Sodium dodecyl sulfate
TFC Thin film composite
TGA Thermal gravimetric analysis
THF Tetrahydrofuran
TFNC Thin film nanofibrous composites
DBX Tetrabutyloxide
UV Ultra-violet
UF Ultrafiltration
WHO World Health Organization
XRD X-Ray Diffraction
XII
Table of Contents
Chapter 1.Introducton ................................................................................................................ 1
1.1 Background ...................................................................................................................... 1
1.2 Research Objectives ......................................................................................................... 5
1.2.1 Fabrication of cellulose membranes for organic solvent applications ...................... 6
1.2.2 Fabrication of a green backing paper from nanocellulose ......................................... 6
1.2.3 Fabrication of cellulose and cellulose/MOF membranes for metal removal
applications ......................................................................................................................... 6
1.3 Thesis structure ................................................................................................................ 7
Chapter 2.Literature Review .................................................................................................... 10
2.1 Background .................................................................................................................... 10
2.1.1 Membrane classification .......................................................................................... 11
2.1.2 Membrane filtration processes ................................................................................. 11
2.1.3 Flow unit operations ................................................................................................ 14
2.1.4 Transport models ..................................................................................................... 16
2.2 Organic solvent nanofiltration (OSN) ............................................................................ 18
2.2.1 Most commonly used materials for OSN ................................................................ 19
2.3 Polymer membrane types for OSN ................................................................................ 21
2.3.1 Integrally skinned asymmetric membranes (ISA) ................................................... 21
2.3.2 Symmetric Membranes ............................................................................................ 22
2.3.3 Thin film composite (TFC) ...................................................................................... 22
2.4 Formation of polymeric membranes .............................................................................. 23
2.4.1 Phase inversion ........................................................................................................ 23
2.5 Cellulose membranes ..................................................................................................... 25
2.5.1 Cellulose .................................................................................................................. 25
2.5.2 Methods to regenerate the cellulose ........................................................................ 26
2.5.3 Thin film nanofibrous composites (TFNC) ............................................................. 30
2.5.4 Cellulose membranes from NMMO technique ....................................................... 31
2.6 Challenges in OSN application ...................................................................................... 32
XIII
2.6.1 Chemical resistance ................................................................................................. 33
2.6.2 Membrane fouling ................................................................................................... 34
2.6.3 Compaction .............................................................................................................. 34
2.6.4 Greener OSN membranes ........................................................................................ 34
2.7 Cellulose composite membranes .................................................................................... 35
2.7.1 Metal organic frameworks (MOFs) ......................................................................... 36
2.8 Potential cellulose applications ...................................................................................... 37
2.9 Prospects and challenges ................................................................................................ 43
Chapter 3. Experimental .......................................................................................................... 44
3.1 Fabrication and the structural characterization of cellulose membranes ....................... 44
3.1.1 Materials .................................................................................................................. 44
3.1.2 Membrane preparation ............................................................................................. 46
3.1.3 Cellulose membranes characterization .................................................................... 46
3.1.4 Pure solvent flux measurements .............................................................................. 54
3.1.5 Rejection Tests ........................................................................................................ 56
3.1.6 Batch adsorption experiments ................................................................................. 58
3.1.7 Calculation of Hansen Solubility Parameters .......................................................... 58
3.2 Preparation and the structural and performance characterization of nanocellulose paper
.............................................................................................................................................. 59
3.2.1 Materials .................................................................................................................. 59
3.2.2 Preparation of nanocellulose paper.......................................................................... 59
3.2.3 Characterization of PBP and NCP backing papers .................................................. 61
3.2.4 Composite stability/biodegradability study ............................................................. 62
3.3 Metal adsorption through cellulose and cellulose/ UIO-66 membranes ........................ 63
3.3.1 Materials .................................................................................................................. 63
3.3.2 Synthesis of UIO-66 ................................................................................................ 63
3.3.3 Characterization of UIO-66 ..................................................................................... 64
3.3.4 Preparation of cellulose/UIO-66 composite membranes ......................................... 66
3.3.5 Characterization of cellulose/UIO-66 composite membranes ................................. 66
Chapter 4. Results and discussion ............................................................................................ 70
4.1 Structural and performance characterization of cellulose membranes ........................... 70
4.1.1 Cellulose membranes appearance ............................................................................ 70
4.1.2 Cellulose membranes characterization .................................................................... 71
4.1.3 Pure solvent flux measurements .............................................................................. 81
XIV
4.1.4 Rejection performances ........................................................................................... 91
4.1.5 Cleaning of membranes- reusability ...................................................................... 110
4.1.6 Comparison with industrial membranes ................................................................ 112
4.1.7 Surface modification .............................................................................................. 113
4.2 Structural and performance characterization of nanocellulose paper .......................... 116
4.2.1 Morphology and performance of the nanocellulose paper .................................... 116
4.2.2 Comparison of NCP-2 and PBP ............................................................................ 118
4.2.3 Composite stability/biodegradability study results................................................ 124
4.3 Metal adsorption through pure cellulose and cellulose/ UIO-66 membranes .............. 127
4.3.1 Characterization of UIO-66 powders .................................................................... 127
4.3.2 Adsorption Studies on pure UIO-66 ...................................................................... 129
4.3.3 Characterization of UIO-66 after adsorption ......................................................... 134
4.3.4 Characterization of cellulose/UIO-66 membranes ................................................ 135
4.3.5 Adsorption studies on cellulose/UIO-66 membranes ............................................ 137
4.3.6. Characterization of UIO-66 after adsorption ........................................................ 143
4.4 General achievements .................................................................................................. 145
Chapter 5. Conclusion ............................................................................................................ 148
5.1 Final conclusions .......................................................................................................... 148
5.1.1 Structural and performance characterization of cellulose membranes .................. 148
5.1.2 Structural and performance characterization of nanocellulose paper .................... 149
5.1.3 Metal adsorption through pure cellulose and cellulose/ UIO-66 membranes ....... 150
5.2 Future directions ........................................................................................................... 151
List of publications ................................................................................................................ 154
Bibliography .......................................................................................................................... 156
Appendices ............................................................................................................................. 170
Appendix A ............................................................................................................................ 170
Appendix B ............................................................................................................................ 172
Appendix C ............................................................................................................................ 173
Appendix D ............................................................................................................................ 175
Appendix E ............................................................................................................................ 176
Appendix F………………………………………………………………………………….177
1
Chapter 1
Introduction
1.1 Background
Separation processes account for up to 70 % of the overall costs in the oil and gas, chemical,
and pharmaceutical industries. Nanofiltration is a membrane filtration method used to separate total
dissolved solids from surface water and fresh ground water. Organic solvent nanofiltration (OSN) is
an emerging technology for molecular separation and purification processes carried out in organic
solvents. Its only difference from nanofiltration is the usage areas. Due to its favourable benefits over
classic methods, such as lower energy consumption, easy processibility [1, 2], it has been successfully
applied in a variety of chemical processes such as product purification and concentration, solvent
exchange and recycling, homogenous catalyst recovery, chiral separations or ionic liquid separation.
There are, however, three main technical challenges remaining today for the successful industrial
application of OSN: i) to solve the trade-off problem between tight membranes and the poor fluxes, ii)
to increase the number of viable membrane materials that are stable in a broad range of organic solvents
including polar aprotic solvents, and iii) to improve currently environmentally harsh preparation
methods used to improve the membrane stability.
2
Global energy and environmental problems highlight the urgent need for green membrane
materials and preparation processes for OSN applications. Since OSN technology started to be mature
nowadays, there are various strategies in literature to improve the greenness of the processes. It is not
possible to have completely green process since the usage area of these membranes is not green itself.
However, reducing the any negative impact on the environment and human beings will improve the
greenness of the processes. These strategies might be listed as using renewable or raw materials, and
greener and non-toxic solvents during fabrication, reducing the number of steps in fabrication
procedure, and dissolving polymer at room temperature [2]. In literature, several different polymers,
ceramics and organic-inorganic hybrid materials have been explored as OSN membrane materials.
There are variety of polymeric materials have been used to prepare OSN membranes such as
polyacrylonitrile, polyimide, polyaniline, polysulfone/ sulfonated poly(ether ether ketone) blends,
poly-benzimidazole, poly (ether ether ketone), and polypropylene [1]. The polymer membranes require
a mechanical support and chemical post-treatments (i.e crosslinking) to have high durability in harsh
organic solvents, and the preparation methods require large quantities of solvents, chemicals, and
energy [1, 2].
As mentioned before, it is important to find a green material and preparation process for OSN
applications. Cellulose, of which chemical structure is shown in Figure 1.1, is one of the most abundant
organic materials; it is also biodegradable, inexpensive, and a sustainable polymer as it conserves
natural resources. Cellulose does not melt in ordinary solvents due to very strong hydrogen bonds
between cellulose chains. This characteristic making cellulose a very good candidate for organic
solvent related applications without needing any conditioning or post treatments. On the other hand,
the semi-crystalline structure and the strong hydrogen bonds make the cellulose a very tough candidate
to work with. Even the commercially available techniques might not be as easy as the dissolution of
ordinary polymers. Type of cellulose used is another important factor for the ease of dissolution and
the properties of the product. Cellulose could be extracted from different sources such as wood, bast,
3
leaf, seed, grass stem, animals, microbes and bacteria. Commercially available ones are obtained by
purifying the raw cellulose from any of these sources and sold with high crystalline contents.
Commercial microcrystalline cellulose powder was used in this study.
Figure 1.1 Cellulose chemical structure [3]
Cellulose derivatives and regenerated cellulose are widely used cellulose types for membrane
fabrication for decades. They have lots of usages; however, the cellulose is degraded during the
preparation processes, and therefore it loses its demanding properties such as high crystallinity, and
high mechanical stability and high resistance to organic solvents [4]. Moreover, lots of dangerous
chemicals employed and formed during the degradation processes, which have negative impacts on the
environment. Due to these drawbacks of regeneration methods, efficient dissolution methods should
be developed to fabricate cellulose membranes by using the full of cellulose resources [5].
One of the potential dissolution method is achieved by the use of ionic liquids, which is a
efficient utilization method for cellulose resources [5]. For instance, cellulose membranes with a
performance in the nanofiltration range using an environmentally friendly method using the ionic liquid
1-allyl-3-methylimidazolium chloride (AMIMCl) as the solvent by Li et al. [5]. They reported cellulose
membranes with high water flux and a molecular weight cut off (MWCO1) [6] of 700 Da, by dissolving
cellulose completely at 90°C. This was the first reported nanofiltration membranes fabricated from a
1Membranes discriminate between dissolved molecules of different sizes and are usually characterized by their
molecular weight cut-off (MWCO), which is used to classify membranes in terms of selectivity. It is defined
as the molecular weight of the molecule which is 90% rejected by the membrane. The value is interpolated
from a curve of MW vs. rejection
4
cellulose/ionic liquid dope solution. One of the drawbacks of ionic liquids is their high cost. Since they
have low vapour pressure, they could be recycled by distillation, and by this way the cost and the
chemical waste generation could be minimized. Chen et al. [7] have suggested to use ionic liquid 1-
butyl-2-methylimidazolum chloride [BMIM]Cl to dissolve wheat straw cellulose and form the casting
solution. After they produced the cellulose membranes, they applied the vacuum distillation to recover
the residual [BMIM]Cl in the coagulation bath, and dried in a vacuum drying oven for 1 day. They
reported the recovery ratio as 95.2%, and the recovered ionic liquid was successfully used to prepare
other cellulose membranes.
N-methylmorpholine-N-oxide (NMMO) process is another environmentally friendly cellulose
dissolution method without any chemical reaction and by-products. NMMO can dissolve the cellulose
via one step high temperature dissolution without the formation of cellulose derivatives or complex
structure [8]. Since NMMO can dissolve the cellulose directly, its structure is not degraded or changed,
and the end products preserves the initial characteristics of cellulose raw material. Moreover, the
prepared cellulose membranes are still biodegradable. Cellulose membranes prepared by the
environmentally friendly NMMO dissolution method were reported for water applications [4, 8, 9].
The first one is done by Zhang et al. [8], in which flat sheet cellulose membranes were prepared by
simple one-step high temperature dissolution technique. The effect of different parameters on the
formation and characterization of membranes were studied in detail such as cellulose type, cellulose
concentration, precipitation bath temperature, and precipitation bath content. In another study, a
hydrophilic cellulose hollow fibre membranes have been developed by Li et al. [4] for oil-water
separation. Cellulose material obtained from wood pulp was dissolved in NMMO solvent, by using the
polyethylene glycol 400 as an additive. They reported highly efficient ultrafiltration membranes for the
oily water treatments, which are stable in a wide range of pH conditions. Mao et al. [9] developed
similar cellulose membranes using NMMO as the solvents for isopropanol dehydration application.
These membranes have shown much higher separation factors than the most of the other polymer
5
membranes with very acceptable flux ranges under the working conditions of 20 wt.% water-containing
IPA feed and at 65 ºC. Moreover, they reported significantly higher degree of crystallization and better
mechanical strength compared to cellulose acetate membranes. This promising ultrafiltration and
nanofiltration performances reported for the cellulose membranes prepared using NMMO and ionic
liquids, gave an insight about their potential for organic solvent nanofiltration applications.
1.2 Research Objectives
The main objective of this thesis is studying, elucidating and developing a new generation of
‘green’ solvent stable membranes for a wide range of organic solvents applications. Cellulose has been
selected as the membrane material since it is one of the greenest and cheapest feedstock in the world
with very benign structural properties due to very strong hydrogen bonds in its structure. NMMO was
selected as the solvent since it is the one of the greenest solvents which could dissolve cellulose without
destroying its crystalline structure. Membranes were fabricated by phase inversion via immersion
precipitation technique. Although this preparation method exists in literature, no one has utilized the
stable structure of cellulose for organic solvent related applications. Subsequently, a simple paper-
making method is introduced for the fabrication of a backing paper from nanocellulose to obtain a
completely green and stable end-product (cellulose membrane on nanocellulose backing) which could
tolerate harsh cross-linking and chemical modification conditions. Since this thesis mainly focuses on
the green ways of the membrane fabrication, cross-linking or other chemical modifications are
suggested as a future work, and the natural adsorption ability of cellulose is utilized for adsorptive
metal removal applications in the next objective. Silver and arsenic in water supplies are targeted, and
composite membranes consisting one type of metal organic framework (MOF), UIO-66 are proposed
for better removal efficiency. The specific thesis objectives are summarised as the following:
6
1.2.1 Fabrication of cellulose membranes for organic solvent applications
• to prepare flat sheet cellulose membranes via phase inversion method,
• to investigate the membrane structure by using various characterisation techniques including
X-Ray Diffraction (XRD), scanning electron microscopy (SEM), contact angle, streaming
potential, thermal gravimetric analysis (TGA), and mechanical test,
• to investigate the stability of membranes in a wide range of different organic solvents,
• to investigate the short and long-term flux and rejection performance of the membranes in
water, and organic solvents to understand the separation mechanisms taking place through the
membrane.
1.2.2 Fabrication of a green backing paper from nanocellulose
• to fabricate a green and solvent-stable backing paper using nanocellulose as raw material,
• to investigate the prepared backing paper in terms of flux, stability and biodegradability
performance.
1.2.3 Fabrication of cellulose and cellulose/MOF membranes for metal removal
applications
• to synthesize and characterize pure MOF powder, and to investigate its silver adsorption
capacity,
• to fabricate and characterize cellulose/MOF composite membranes via phase inversion method,
• to investigate the silver and arsenic adsorption capacity of cellulose and cellulose/MOF
composite membranes under static and kinetic conditions using dead-end and cross-flow
filtration configurations.
7
1.3 Thesis structure
This thesis is comprised of five main chapters. Chapter 1 provides an overview of the thesis
and its objectives as well as briefly explains the motivations of the project. Chapter 2 is a literature
review that includes a brief definition, as well as the fundamentals of membranes and a review of the
properties of the cellulose material and its membranes. It also includes a review of cellulose membrane
production methods, such as viscose technology, cupraamonium process, and direct dissolution of
cellulose in some solvents. Nanofiltration membrane types and membrane filtration processes are
summarized and different applications for cellulose membranes are represented.
In chapter 3, experimental procedures are firstly summarized for membrane preparation and
structural characterization. Then, flux and rejection performance experiments are explained in detail as
well as the static and kinetic adsorption procedures. Finally, preparation techniques for nanocellulose
backing paper is explained.
Chapter 4 is the results and discussion part of this thesis which includes 3 different sub-sections.
In section 4.1, symmetric cellulose membranes were developed via immersion precipitation method on
polyester backing material which provide mechanical support to the membranes. In this work, four
different membranes with overall dried thickness of 2, 5, 12, and 25 µm have been fabricated. A
detailed study on the morphology, porosity, and surface properties of the prepared membranes was
undertaken to understand the structure of the membranes in detail. The flux performance and the
stability of the membrane were investigated in different solvents such as water, acetone, acetonitrile,
ethyl acetate, tetrahydrafuran (THF), methanol, ethanol, 2-isopropanol, 1 -butanol. After proving that
membranes are stable in all the tested organic solvents as well as exhibiting very promising fluxes
compared to the literature values, their rejection performance were tested in water and some organic
solvents. Eight different dyes were used as markers in the solvents to analyse the MWCO of the
prepared membranes in water and different organic solvents. Electrostatic interactions were found to
be dominant for the separation mechanism in water. Since the surface of cellulose membranes are
8
strongly negative at neutral conditions, positively charged dyes are rejected more by adsorption. On
the other hand, rejection behaviour of the membrane in organic solvents is difficult to explain due to
the very different structures and properties of the organic solvents, but adsorption was still active for
the removal of dyes from the solutions.
When the membrane is saturated during adsorption, dyes permeated through it and rejection in
organic solvents failed. Some chemical surface modification techniques (i.e. cross-linking or
acetylation) could be used to improve its separation performance in organic solvents. Some preliminary
experiments were tried first to check the stability of the membrane and backing materials since very
harsh conditions are applied during these modifications. However, both polyester and polypropylene
backing materials failed in this conditions while no visible changes were observed in the membranes.
More than the stability of the membrane itself, improvements need to be done in regards to selecting
an adequate non-woven backing first. Therefore, the fabrication of nanocellulose paper (NCP) backing
material are illustrated in section 4.2, which allows us to produce a completely green end product which
is biodegradable and also stable in harsh media. A simple paper-production method was used for NCP
preparation in which only water was used as the dissolution media, which qualifies this process as an
environmentally friendly one. In order to compare the performance of the prepared backing paper, pure
cellulose membranes were cast on NCP and polyester backing paper (PBP), and the membranes were
compared in terms of flux, stability and biodegradability performance. Since this thesis mainly focuses
on the green ways of the membrane fabrication, cross-linking or other chemical modifications are not
desired. However, they should definitely be investigated in future to open a new perspective and a
more sustainable association for OSN applications.
The main challenge in this study is to make use of the natural ability of ‘cellulose’ without
compromising its green image. Therefore, in the last section (section 4.3), we reported the usage of
cellulose and cellulose/UIO-66 membranes for silver and arsenic metal ions removal from aqueous
solutions due to their adverse effect on the environment and the human health by using their high
9
potential on adsorption processes. Pure cellulose membranes exhibited very promising silver uptake
capability due to strong –OH bonding on the membrane surface, while no arsenic was adsorbed.
Superior arsenic adsorption capacity was reported for pure UIO-66 crystals before [10], and their silver
adsorption capacity was tested in this study. The exceptional fast silver adsorption performance and
high stability of UIO-66 in water provides promising insights to the water treatment applications [10,
11]. Incorporation of MOF particles in cellulose resulted in highly stable green membranes across a
broad pH range from very acidic (1) to neutral (7) conditions with promising adsorption performances
for silver and arsenic. Moreover, cross-flow filtration geometry improved their efficiency further due
to the penetration of pollutants through the membrane by applied positive pressure across the
membrane. If the regeneration of these membranes could be achieved, large-scale industrial membrane
modules could be built especially for silver removal application.
Finally, Chapter 5 is a summary of the main conclusions made from this work covering various
important findings from Chapter 4, followed by some suggestions for future work on this subject. In
the next part the list of papers published and oral and poster presentations were listed made in
conferences by the author. Appendices, given after Bibliography represent the additional data (which
are being referred to in the main sections) required for results and discussion parts.
10
Chapter 2
Literature review
This review seeks to provide insight into the state-of-the-art research in cellulose membranes
for both aqueous and organic applications, as well as into different applications which suits the intrinsic
properties of the membranes.
2.1 Background
Separation is an inevitable issue for various chemical industries (such as pharmaceuticals, the
oil industry, cosmetics etc.) and also one of the most expensive processes to run. Separation processes
has been estimated that to account for 40-70% of both capital and operating costs in industries [1, 2].
Indeed, current separation technologies, despite being ‘mature technologies’, are highly energy-
intensive. For example, distillation, which has a preeminent position in the field of separation, requires
a huge amount of energy for heating [2]. In order to improve both profitability and sustainability, novel
separation methods need to be investigated. Membrane technologies are considered a high priority
target to impact process economics [1].
Membranes are semi-permeable barriers between two phases. The passage of permeates is
selective, which means that some molecules pass through while others are rejected, and is induced by
a driving force. Membranes have been studied for a long period since the 1860s when Graham reported
11
his first dialysis experiments with a synthetic membrane [12]. There have been rapid developments
during the past half century, and currently, they are widely used at industrial scales for filtrations, water
treatment, desalination, pervaporation. They lead to lower investment, ease of processing and low
weight and space requirements [13].
2.1.1 Membrane classification
Membranes can be classified by their morphology/structure in terms of symmetric or
asymmetric [13, 14]. Symmetric membranes have a uniform structure which may be either porous or
non-porous throughout the thickness which can range between 10 and 200 µm, and the mass transfer
is controlled by the total membrane thickness. Therefore, permeation rates could be increased by
decreasing the total membrane thickness [13]. Asymmetric membranes consist of a very dense skin
layer with a thickness between 0.1 and 0.5 µm supported by a porous sublayer with a thickness of 50
to 150 µm. The resistance to mass transfer is determined by the thin skin layer. Both polymeric and
inorganic materials can be used to prepare membranes. Development of asymmetric membranes was a
breakthrough for industrial applications since they combine the high selectivity of a dense membrane
and high permeation rate of a thin membrane [13].
2.1.2 Membrane filtration processes
Membrane filtration processes are induced by a driving force, which could be a pressure
difference ΔP, an electrical potential ΔE, a concentration difference Δc, or a combination of those,
sometimes with a temperature difference ΔT [13]. This study focuses on filtration induced by a pressure
difference. Membrane processes are classified in four categories based on their pore sizes and operating
pressure and they are microfiltration, ultrafiltration, nanofiltration and reverse osmosis as shown in
Figure 2.1, and they are described in more detail in this section.
12
Figure 2.1 Classification of membrane processes according to operating pressure, retained solute/pore
size [nm], molecular weight cut-off [g mol−1], transport mechanism, and examples of applications.
Adapted from refence [1] which is an open access paper.
2.1.2.1 Microfiltration
Microfiltration corresponds to the separation of particles from 0.1 to 10 µm from a solution by
a membrane and the working pressure goes from 0.1 to 2 bar [13, 15]. These membranes enable the
filtration of particles bigger than bacteria, whose sizes are around
1 µm, such as yeast and colloids.
2.1.2.2 Ultrafiltration
This method can filter molecules with sizes between 0.01 and 0.1 µm. Hence, the selectivity of
the membrane is greater than microfiltration and is widely used in biotechnology to reject viruses and
clean the biopharmaceutical products [16]. The pressure range varies for this kind of membrane
13
between 1 and 5 bar [13, 15]. Ultrafiltration has been proved to be the ideal type of porous support for
membrane casting and needs to be as smooth as possible [17].
2.1.2.3 Nanofiltration
In nanofiltration, the principle is the same as in micro- and ultrafiltration, but the selectivity of
the membrane is much higher and the solutes can be separated very accurately according to their
molecular weights. Microsolutes and proteins can be removed with this method as it can retain particles
as small as 2 nm [18]. The pressure range is said to vary between 5.0 and 20 bar [1]. However, some
nanofiltration membranes are now used at pressures higher than 20 bar, therefore studies have proposed
to extend this limit to 40 bar [1]. Nanofiltration have found many industrial applications, such as in
desalinization of sea water [19, 20], and also the filtration for organic solvents, which will be discussed
in greater details in Section 2.2.
2.1.2.4 Reverse osmosis
Osmosis is a spontaneous natural phenomenon based on solvent molecules diffusing across a
selectively permeable membrane separating two solutions of different concentrations named as FO
(forward osmosis) in Figure 2.2(B) [21, 22]. Actually forward osmosis is used for the same meaning
with the osmosis in literature. The diffusion of solvent occurs from the less concentrated solution
(hypotonic) to the highly concentrated solution (hypertonic), until both solute concentrations are
equalled (isotonic). When the solute concentration in both side equalled, the system reached the
equilibrium as seen in Figure 2.2 (A). The driving force of this is the high entropy in the hypertonic
solution created by the solute dissolution which corresponds to a chemical potential increase.
Therefore, the osmotic pressure Δπ is introduced and corresponds to the mechanical pressure needed
to be applied on the highly concentrated medium to cancel this phenomenon. When the pressure applied
Δp exceeds Δπ, the opposite diffusion takes place, named as reverse osmosis as seen in Figure 2.2(C)
enabling the separation of solvent from its solution. This technique is widely used for water treatment
14
and the desalination of seawater to provide a source of drinkable water [23]. Pressure retarded osmosis
is mostly used for electricity generation applications. Figure 2.2 gives a schematic approach of the
situation.
Figure 2.2 Schematic approach of A) Osmotic equilibrium, B) Forward Osmosis, C)Reverse
Osmosis, D) Pressure Retarded Osmosis. Adapted from reference [24] which is an open access
paper.
2.1.3 Flow unit operations
Two different filtration unit operations are described in literature: the dead-end and the cross-flow
filtration, which are presented in Figure 2.3.
2.1.3.1 Dead-end filtration
Dead-end filtration is a batch-type process where the flow of the feed solution is orthogonal to
the membrane. It is an easy-to-implement method, especially for lab-scale experiments. However, it is
only applicable either for solutions with really low particle concentrations or solutions with very low
solid content, which is the case in OSN since the aim is to separate dissolved solutes [13]. Even so, the
flux decreases over time due to an increase of concentration polarization [25]. If this technique is
applied to larger-size particles, a cake (agglomeration of particles) can even be seen [26]. Therefore,
15
the membrane needs to be cleaned regularly to maintain good efficiency. Cross-flow filtration is usually
preferred to lower this phenomenon.
Figure 2.3 Schematic representations of (a) dead-end and (b) cross-flow geometries. Adapted
from refence [27] with the permission from John Wiley and Sons.
2.1.3.2 Cross-flow filtration
Cross-flow filtration is a continuous process where the flow of the feed solution is parallel to
the membrane. The feed flows tangentially across the surface of the membranes at positive pressure
instead of into the membrane as in the dead-end filtration. Hence, deposits on the membrane are
hindered by a non-stopped sweeping induced by the flow, resulting in a better hydrodynamics. Cross-
flow filtration is a suitable method for feed solutions containing high amount of solid with small
particle size dissolved inside, because these high amount could block the membrane pores easily in
dead-end filtration. The
The cross-flow method is harder to implement at small scales, but is widely used for pilot and
industrial scales. Indeed, the volume treated per surface area and per time is greater than that in dead-
end filtration thanks to the continuity of the technique, hence it is more profitable. However, cross-
flow is more expensive than dead-end filtration to implement and it is a labour-intensive process.
16
Cross-flow is reported to be the efficient mode for industrial level applications due to the high
penetration power of pollutants through the membranes [28].
2.1.4 Transport models
Understanding the transport mechanism through membranes is important in order to approach
new situations with confidence and to predict new phenomena [1]. Transport models are practical tools
to predict membrane transport. Solution-diffusion and pore-flow models are the two main models used
to explain the transport mechanisms through the membranes. Moreover, some modified transport
models are used to explain the separation phenomena occurs through nanofiltration membranes. Since
most of the nanofiltration membranes have charged surfaces, electrostatic and affinity interactions are
also important. Donnan exclusion mechanism considers the electrostatic interactions.
In the solution diffusion model, the permeates firstly dissolve in the membrane material, and
then diffuse through the membrane. Separation occurs due to difference in the solubilities and
diffusivities of the permeates, and the chemical potential difference across the membrane is expressed
as a concentration difference, while the pressure difference across the membrane is uniform [29]. It is
usually used to describe the transport through dense membranes, in which the pores in the membrane
(the free-volume elements) appear and disappear on approximately the same timescale due to statistical
fluctuations of the polymer molecules. This model can be applied to different membrane process of
reverse osmosis, dialysis, pervaporation, and gas separation [30]. The solution-diffusion model is
based on the Fick’s law of diffusion:
𝐽𝑣,𝑗 = −𝐷𝑗𝑑𝑐𝑗
𝑑𝑥 (2.1)
17
where is the flux of compound j, Dj is the diffusion coefficient is the measure of the mobility of the
individual molecules, and dcj/dx is the compound j concentration difference. The minus sign indicated
that the direction of diffusion is down the concentration difference.
In the pore-flow model, the permeates are transported by pressure driven convective flow
through the pores. The permeates are separated from the retentates due to their size differences: one of
the permeates is excluded from some pores. The chemical potential difference is expressed as a
pressure difference, while the solute and solvent concentrations within the membrane are assumed to
be uniform. Pressure- driven convective flow can be expressed by the Hagen-Poiseuille model:
𝐽 = −𝜋𝑟𝑝
2
8𝜇𝛿𝜏∆𝑝 (2.2)
where J is the permeation flux, ∆𝑝 is applied pressure difference across the membranes, 𝑟𝑝 is pore
radius, µ is viscosity, 𝛿 is membrane thickness, 𝜏 is tortuosity.
Transport through membrane is sometimes not so simple to explain by using only one transport
mechanism. Surface properties of the membranes and the permeates, and the interaction between them
start to become dominant when determining the transport mechanism [1]. More complex transport
models were obtained by modifying the two models above, which considers the specific characteristics
of the membranes and the permeates in order to predict the membranes’ performances more precisely
[1]. The transport mechanisms in solution-diffusion and pore flow methods are explained by diffusion
and convection, respectively, while the complex models also include the electrostatic and affinity
interactions. For instance, Donnan steric pore-flow model considers diffusion, convection and
electrostatic interactions mechanisms, while surface-force pore flow model is explained by diffusion,
convection and affinity interactions [1]. In the Donnan exclusion mechanism, membranes repel the co-
ions (i.e. the ions which have the same charge with the membrane surface), and an equivalent number
18
of counter-ions are also retained to satisfy the electroneutrality [31]. This means negatively charged
membrane surface rejects the negatively charged ions.
Adsorption-based membrane separation is another method encountered during membrane
separation applications and mostly takes place due to electrostatic and/or affinity interactions between
the membranes’ surface and the permeates [28, 32]. Separation is governed by this mechanism when
the charged molecules (dyes, metals, etc.) are separated by membranes with charged surfaces when a
driving force (pressure difference) is applied or not. Two different types of adsorption experiments
were conducted in this study, in the first type, membranes were subjected to solid containing feed
solutions without any driving force and adsorption was recorded. In the second type, membranes were
subjected to the same solutions under pressurized filtration conditions.
2.2 Organic solvent nanofiltration (OSN)
Molecular separation by organic solvent nanofiltration is a relatively new technology that has
been developed to find a more sustainable way of separating particles in an organic solvent after its
synthesis. Its sustainability induces a wide range of applications [33] and the key to the OSN process
is the membrane. The targeted dissolved solute is retained as it is larger than the pore size and cannot
pass into the downstream compartment, whereas the solvent and the smaller molecules do. Most of the
time, it is not really easy to explain the separation performance of the membranes with only the pore
size of the membranes, since there are lots of different parameters affecting the transport. Different
solvents have different properties and they all affect the surface characteristics of membrane and the
dissolved solutes. Therefore, the separation mechanism is not really straightforward in organic solvents
and many different separation mechanisms might be used to explain it efficiently. Each nanofiltration
membrane has a characteristic molecular weight cut-off (MWCO), which is defined as the molecular
weight at which 90% the rejection of a solute occurs [6]. However, that value might be different for
different organic solvents and dissolved solutes.
19
The driving force of the OSN process is the pressure difference, applied on the upstream
compartment. As stated in section 2.1, the pressure difference for NF can be up to 40 bar. The
membrane is fragile, and so the applied pressure needs to be clearly controlled. Excessive pressure
could lead to rupturing of the membrane. The flux is proportional to the applied pressure, and having
the highest flux possible is important for treating the largest volume per unit time. The equation for the
filtrated flux is as follows:
𝐽 = 𝑃 ∙ ∆𝑝 (2.3)
with the flux J, the permeability P and the applied pressure Δp. The permeability is a common value
for OSN membranes and often calculated in L m-2 h-1 bar-1.
To measure the ability of a membrane to reject a dissolved solute, the rejection factor can be
defined as the following:
𝑅𝑖(%) = (1 −𝐶𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒,𝑖
𝐶𝑟𝑒𝑡𝑒𝑛𝑡𝑎𝑡𝑒,𝑖) ∙ 100 (2.4)
where i corresponds to the dissolved solute, Cpermeate, i to the concentration of i in the permeate and
Cretentate, i to the concentration of i in the retentate.
2.2.1 Most commonly used materials for OSN
Both inorganic (ceramic) and organic (polymer) OSN membranes are studied in the scientific
literature and they each have their own advantages and drawbacks. Indeed, ceramics are known to have
better thermal, mechanical and chemical properties, but they are more complicated to scale-up.
Polymeric ones are easier to manufacture, but their thermal, mechanical and chemical stabilities are
worse than ceramic ones [1].
20
2.2.1.1 Ceramic OSN membranes
Ceramic NF membranes have been proved to be applicable to OSN filtration, and silica-zirconia
membrane have been synthesised via a sol-gel process and were successfully tested in alcoholic
solvents [34]. This study proved that it is possible to control the pore size of the silica-zirconia
membrane by the appropriate choice of colloidal particles according to their sizes and the sizes
conducted were MWCO of 300, 600, 1000 and > 1000 in methanol.
Moreover, TiO2 membranes have also been studied [35]. The work on TiO2 membranes was
performed with n-hexane as the organic solvent and the effect of adding water to the n-hexane was
studied. It was proved that the higher the ppm of water the larger the decrease in permeation flux.
Water concentration of up to 70 ppm at 30°C and 280 ppm at 60°C where studied. Those values
correspond to the saturated water concentration in hexane. It was established that the drop in
permeation flux was due to water blocking the membrane nanopores. Indeed, the hydrophilic ceramic
membranes are likely to interact with water molecules, therefore methylated SiO2 hydrophobic
membranes have been synthetized to limit this phenomenon and the addition of water to the same
solvent from 0 to 80 ppm induced almost constant fluxes [36].
The natural hydrophilicity of ceramic membranes induces good fluxes with polar solvents [35].
On the contrary, non-polar solvents present naturally low fluxes [37]. The authors proved that a
chemical treatment of ceramic membranes was possible to enhance those fluxes for non-polar solvents.
They grafted linear alkyl (C1, C5, C8 and C12) groups on the surface of commercial asymmetric
tubular TiO2 membrane with 1 nm pore size with Grignard reactions to give hydrophobicity properties,
with retention results were comparable to those of the commercial DuramemTM 300. The result was
that the higher the length of the carbon chain, hence the hydrophobicity, the higher the fluxes [37].
The high chemical, thermal and mechanical stability of ceramic OSN membranes combined
with good separation characteristics and a long lifetime makes them a good alternative to organic OSN
membranes, but they are also harder to scale up and also more expensive to produce. A spin-off
21
company in Germany named as Inopor commercially produces mono- and multi-channelled
hydrophilic and hydrophobic tubes, which have different MWCO performances in different organic
solvents. For instance, hydrophobic ones have 99% rejection of Victoria Blue (506 gmol-1) in methanol
and Erythrosine B (880 gmol-1) in acetone [1].
2.2.1.2 Polymer OSN membranes
Polymer membranes have huge advantages compared to ceramic OSN membranes such as the
amphiphilic characteristics that they present, which give them good permeability both in polar and in
non-polar solvents [37]. On the other hand, their processing is complicated, and, although they both
need to be cast, meaning the polymer needs to be soluble in the casting solvent, they also preserve a
great chemical resistance when the membrane is used.
There are many different polymers used in the literature to make polymeric membranes.
Marchetti et al. [1] give an overview of classical polymers for OSN: polyacrilonitrile, polyimide,
polyaniline, polybenzimidazole, polysulfone & sulfonated poly (ether ether ketone), poly(ether ether
ketone), polypropylene.
2.3 Polymer membrane types for OSN
2.3.1 Integrally skinned asymmetric membranes (ISA)
Integrally skinned asymmetric membranes are made up of a top skin layer above a porous
sublayer composed of the same material, and a non-woven material as a support made of a different
material [17]. They are created by casting a polymeric dope solution on a non-woven support, before
undergoing the phase inversion technique developed by Loeb and Sourirajan [38]. This method consists
of bathing the cast membrane and support into a solvent in which the used polymer is not soluble in.
Hence, the polymer precipitates with an adjustable speed that determines the membrane skin layer,
which accounts for the membrane selectivity and permeance properties. Furthermore, the support also
22
plays a crucial role. It has been shown that the choice of the UF support accounts for the quality of the
upper layer, and therefore should be chosen to be as smooth as possible [1].
2.3.2 Symmetric Membranes
Phase inversion technique which was developed by Loeb and Sourirajan [38] could also result in
symmetric membrane structure. There are different precipitation techniques (will be described in
section 2.4.1) applied during phase inversion techniques, and they are all resulted in different
membrane structures. Precipitation by solvent evaporation results in homogenous dense membranes
while precipitation induced by vapour phase results in homogenous porous membrane structure [39].
Moreover, in the case of phase in inversion by immersion precipitation technique, symmetric porous
membranes might be obtained by controlling the rate of precipitation. For instance, high precipitation
rate results in asymmetric membranes with finger-type structure, while low precipitation rate results in
asymmetric membranes with denser skin layer and sponge-like structure. When the rate of precipitation
is very low, symmetric membranes with no defined skin layer is obtained [40].
2.3.3 Thin film composite (TFC)
Thin film composite membranes differ from ISA membranes by their top layer. Indeed, TFCs
top layers are added onto a membrane support, which itself is cast on a non-woven support. Different
techniques have been developed to fabricate TFC: casting, interfacial polymerization, dip-coating a
solution of polymer or depositing a barrier film [1]. However, the TFCs are harder to make compared
to ISA. Their formation process is very sensitive since the added layer is very thin, which is in the
nanometre range. Therefore, the control over TFC permeation is difficult. Yet, Jimenez Solomon et al.
successfully synthetized a DMF-resistant TFC, via an interfacial polymerization (IP) with solvent
activation, proving that a novel way of forming TFC could lead to higher quality TFC [41]. The key of
this technique lies in the solvent activation that occurs as a pre-treatment.
23
2.4 Formation of polymeric membranes
2.4.1 Phase inversion
Phase inversion is a very versatile technique, which allows all kind of morphologies (i.e.
symmetric, and asymmetric) to be obtained. Thus, most of the commercial polymeric membranes are
produced by phase inversion. In this technique, a homogenous polymer solution is transferred from a
liquid to solid state by a controlled solidification process. The initial stage of the solidification process
in which the polymer solution is transferred to a two-phase system (a solid polymer-rich phase: forming
the membrane, and a liquid polymer-poor phase : forming the pores) is dominant for controlling the
membrane morphology, i.e. porous, or nonporous [13, 42]. In the end of the process, the polymer-rich
phase is precipitated to form the membrane by different techniques such as precipitation by solvent
evaporation and controlled evaporation, precipitation induced by vapour, and thermally induced phase
separation, and immersion precipitation [39].
Immersion precipitation is the most commonly used method for commercial membrane
production, which usually results in an asymmetric structure [39, 43]. Phase inversion by the immersion
precipitation technique usually results in an integrally skinned asymmetric structure due to the phase
separation taking place differently on the two surfaces of the membrane [39, 43, 44]. Strathmann [39]
et al. reported in 1977 that microporous membranes with sponge/or finger type structure with a dense
layer on the top were obtained by the immersion precipitation technique, while a symmetric membrane
with sponge like structure was produced by introducing the precipitant from the vapour phase. It may
produce asymmetric, porous membranes in cases of low polymer concentration, high mutual affinity
between solvent and non-solvent, or addition of non-solvent to the polymer solution [13]. Wijmans et
al. [43] reported in 1983 that it is also possible to prepare symmetric membranes by the immersion
precipitation technique by adding solvent to the coagulation bath, since addition of solvent into
coagulation bath slows down the rate of precipitation. They illustrated that the rate of nonsolvent inflow
and solvent outflow during coagulation process has a significant impact on the membrane structure.
24
Zhang et al. [8] produced cellulose membranes by immersion precipitation method using NMMO •
H2O as solvent and H2O as the non-solvent. They reported symmetric membranes with a homogenous
cross-section when the membranes were coagulated in a pure water bath. However, the structure
changed to asymmetric porous one when the solvent NMMO • H2O was added to coagulation bath.
2.4.1.1 Casting of flat sheet membranes
Membranes can also be presented in various geometries, both in their structures (tubular or flat)
and in their module arrangement (plate-and-frame or spiral-wound). The main method of preparing flat
sheet membranes is the casting technique: (i) the polymer is first dissolved in a convenient solvent (ii)
the polymer solution is cast on a non-woven support with a casting knife. The support and membrane
are then bathed in a non-coagulant order to induce the (iii) phase inversion during which the solvent is
removed and replaced by water, and hence, the polymer precipitates [13]. Flat sheet membranes are
commonly used in spiral-wound and plate-and-frame configurations. They are practical lab-scale
membranes because they can easily be cast and tested for experiments.
2.4.1.2 Spinning of hollow fiber membranes
The spinning technique is used to prepare hollow fibre membranes; the process consists of
pumping a highly viscous solution or slurry through a tube-in-orifice spinneret, while a bore solution
is injected in the centre of the spinneret. The membrane enters a bath were coagulation occurs and is
finally washed and dried [45]. Three tubular membrane types can be specified according to their
diameters: hollow fibre membranes (up to 0.5 mm), capillary membranes (0.5-5 mm), and tubular
membranes (more than 5 mm) [13]. Hollow fibres are widely used in the industry due to their large
surface area per unit volume and they are also self-supporting. Moreover, back-flushing can be
performed regularly to limit fouling with hollow fibres [27].
25
2.5 Cellulose membranes
2.5.1 Cellulose
Cellulose (C6H10O5)n is a straight-chain insoluble polysaccharide presenting glucose molecules
which are linked by β-1, 4 glycosidic bonds [46, 47]. As a crucial structural material for plant cell
walls, it is the most abundant organic polymer on earth [48]. The structure is depicted in Figure 2.4.
Figure 2.4 Structure of cellulose (n is the degree of polymerisation). Adapted from reference
[49] with the permission Royal Society of Chemistry.
Cellulose can exist in at least 5 allomorphic forms [49] and possesses strong hydrogen bonds
due to the presence of the three hydroxyl groups on the cycle. This key feature gives it high resistance
towards ordinary organic solvents [2]. Therefore, cellulose is to be considered potentially a great
material for OSN membranes. The interesting property of cellulose is that this polymer is not soluble
in most common organic solvents (methanol, ethanol, butanol, acetone, tetrahydrofuran (THF),
acetonitrile) and in water. On the one hand, this makes it harder to cast a solution containing cellulose,
but at the same time it gives cellulose membranes strong stability. This key property in addition to its
biodegradability enables cellulose to meet high expectations when applied to OSN filtration.
Cellulose is used under various forms for processing which presents different characteristics.
Miao and Hamad presented an overview of cellulose fibres, nanofibers, all-cellulose composites and
microcrystalline cellulose (MCC) which is the form that will be used during this project. MCC is
26
produced from acid hydrolysis of a cellulosic material before undergoing a mechanical treatment,
which leads to microcrystals and, gather to form MCC when dried. During this last step, MCC acquires
its key properties: a porous crystalline structure [50].
2.5.2 Methods to regenerate the cellulose
Regenerated cellulose is an important membrane material to use in different areas such as
dialysis, ultrafiltration, and release of pharmacon [7]. As mentioned above, cellulose cannot be
dissolved in water and common solvents due to its partially crystalline structure. There are conventional
methods in the literature used for production of cellulose regenerated materials, in which complex
chemical procedures are applied with various shortcomings such as low cellulose solubilities, low
degree of polymerization, hazardous by-products, and environmental issues [51]. Recently, a number
of new solvent systems have been reported which can be used to dissolve cellulose such as N-
dimethylformamide (DMF), paraformaldehyde (PF)/dimethyl sulfoxide (DMSO) [52], N2O4/N [53],
LiCl/N,Ndimethylacetamide (DMAc)[54], urea/NaOH [55], urea/lithium hydroxide [56], and ionic
liquids [57, 58]. Each solvent has its drawbacks, such as toxicity, high cost, and corrosivity [57].
Moreover, these solvent systems may cause a loss in the excellent properties (e.g. chemical resistance,
crystallinity) of the cellulose material. Researchers have many attempts to avoid the complicating
processing routes and protect the excellent intrinsic properties of cellulose, and they reported that cyclic
amine oxides are able to dissolve it without destroying the structure. N-methylmorpholine-N-oxide
(NMMO) is reported to be the best choice for cellulose due to its environmentally benign properties.
2.5.2.1 Viscose technology
In order to dissolve the cellulose, the strong hydrogen bonds need to be weakened. The cellulose
is hence converted in a soluble derivative and with the viscose technology, it is converted to xanthate
[59, 60]. This technology was used historically for the creation of cellophane. In this method, firstly
27
the pulp is steeped in an aqueous NaOH (sodium hydroxide) solution (17-19%) whereby the fibers start
to swell and cellulose converts to sodium cellulosate (alkali cellulose) [61-64]. Under controlled
temperature conditions, the alkali cellulose is aged by depolymerisation of the cellulose, which leads a
higher degree of polymerization (DP). Then it is reacted with carbon disulphide to form sodium
cellulose xanthate which is a yellow to orange crumb. After dissolving the xanthate in a dilute sodium
hydroxide solution, a yield of viscous orange solution, named viscose, if formed. The solution acquires
the desired properties for spinning after filtration and deaeration processes. The solution is then
extruded through a spinneret into a bath containing sulphuric acid, sodium sulphate, zinc sulphate,
water and a low level of surfactant. After the cellulose xanthate was neutralised and acidified in the
spin bath, it was stretched and decomposed into cellulose. Finally, the filaments are washed and
chemically desulphurised [14, 65]. The viscose technology has many environmental drawbacks, such
as having to recover the hazardous byproducts of this method such as H2S, CS2, and heavy metals [66].
2.5.2.2 Regenerated cellulose with cuprammonium
The cuprammonium process is another classical way used for the production of regenerated
cellulose (cupro silk, cuprophane) [66, 67]. In this method, cellulose is first dissolved in an aqueous
cuprammonium solution, and extruded through a capillary to get the fibers. The solution is washed to
remove the attached fatty and resinous materials, and then filtered through sand to remove any
undissolved matter. The spin bath in which the solution is extruded into contains a dilute acid (e.g.
hydrochloric acid, formic acid, citric acid, tartaric acid, or succinic acid), alcohol, and a concentrated
cresol solution. The hard solid filaments, which precipitate immediately in the spin bath, are stretched
in dilute hydrochloric acid using winder, spool and drum, and then washed and dried. Recovery of
copper and ammonia is economically and environmentally significant for the industrial value of this
method. However, it is also handicapped by the huge areas of space and amounts of water required [68,
69].
28
2.5.2.3 N-methylmorpholine-N-oxide (NMMO) technology
There are many attempts in literature to find new ways to avoid the complicated processing
routes and dissolve cellulose in a solvent directly [66, 70-72]. Li et al. [4] reported that cyclic amine
oxides are capable of dissolving the cellulose directly, and N-methylmorpholine N-oxide (NMMO) of
which its basic properties and structure is shown in Table 2.1, is the best solvent among these. NMMO
is a heterocyclic amine oxide organic which can dissolve cellulose in a physical way. Effectively,
NMMO possesses a highly electronegative atom of oxygen which is able to break through the hydrogen
bonds of the cellulose [14].
Table 2.1 Basic properties and structure of N-methyl morpholine N-oxide [73]
After overcoming the initial problems encountered during the process development, a
commercial production process for cellulosics with the generic name of Lyocell was introduced [66,
74, 75]. The NMMO technology is a relatively simple process when compared to other processes
mentioned above, because it does not involve any chemical reactions. It can dissolve cellulose without
any derivatization, complexation or special activation with its strong N-O dipoles [8]. It is
environmentally benign because NMMO is a non-toxic solvent, it can be almost totally recycled, and
no chemical byproducts are formed [8, 70]. The dope solution is prepared by the addition of cellulose
The molecular weight 115.2 g
The melting point 170°C
The initial decomposition
temperature 100-110°C
Water composition of
NMMO • H2O 13.5% wt.
The melting point of
NMMO • H2O 71-75°C
29
into the solvent NMMO • H2O, and the mixture is then heated up to 100°C while being stirred in a
vessel. Temperatures higher than 150°C are reported to be dangerous since the solvent is decomposed
undesirably which may result in explosions.
Figure 2.5 shows the ternary phase diagram for the cellulose-NMMO-H2O system in which the
dissolution region is indicated with grey colour. This phase diagram shows the percentage of NMMO,
H2O and cellulose required for the successful dissolution of cellulose. This relatively small region
implies that cellulose is completely dissolved in some NMMO/H2O mixtures with high NMMO
concentrations between 60% and 85% [66]. According to the same diagram, homogenous cellulose
solutions can be produced with only minor amounts of water. The reason is explained by the
competition between the hydroxyl groups in water compounds and the cellulose (containing also
hydroxyl groups) for NMMO molecules [66].
Figure 2.5 Phase diagram cellulose- NMMO-water. Adapted from reference [66] with the
permission of Elsevier.
Due to the strong hydrogen bonding in the highly crystalline structure of cellulose, the
dissolution of it is very difficult at ambient conditions. Dogan et al. [76] proposed a new
30
environmentally friendly microwave heating method for dissolving the cellulose in NMMO in shorter
times and with lower energy consumption. They prepared flat sheet membranes with different cellulose
contents at different heating conditions (microwave power) and characterized them in terms of
crystallinity and degree of polymerization and reported that microwave heating with a power of 210 W
is an efficient way for cellulose dissolution in an NMMO • H2O medium [76].
There is no chemical reaction in this method, so cellulose is not broken down and preserves its
main characteristics and also no by-products are formed [14]. Therefore, the production of cellulose
membranes from NMMO solution seems more sustainable than the previously discussed techniques.
2.5.3 Thin film nanofibrous composites (TFNC)
Composites are materials made from at least two different materials which have significant
physically and/or chemically different properties and nanocomposites exhibit reinforcements usually
smaller than 100nm. Nanocomposite membranes have reinforcements which can be continuous fibres,
short fibres, particles, or woven material [50, 77, 78]. TFNC membranes are prepared by using three
different fiber layers of which top layer consists of nanosized cellulose fibers as a barrier. The smaller
sized fibers on the top are filling the pores between the bigger fibers at the bottom and tight membranes
were obtained in this way. Since the cellulose fibers are not dissolved in any solvent, all the mechanical
and structural properties are preserved. TFNC is a relatively new technology applied for OSN, and
until now, most of polymeric membranes for OSN were prepared either by ISA or TFC technique,
which are now common and well-studied [79].
TFNC have become more and more investigated at the current moment because when compared
to usual polymeric membranes, they offer the huge advantage of enhanced mechanical properties due
to the inorganic components and the great processability of organic components [80]. Also, they are
energy efficient and offer high permeate fluxes. Indeed, Ma et al. [81] showed that the flux of a
cellulose nanocomposite TFNC membrane applied for oil/water emulsion separation was 10 times
31
larger than conventional UF polyacrylonitrile, PAN10 and PAN400 membranes, with a rejection factor
above 99.5%. Cellulose composites were made by the oxidation of bleached wood pulp with the
TEMPO/NaBr/NaClO technique.
2.5.4 Cellulose membranes from NMMO technique
The NMMO technique will be used during this project, and therefore it is crucial to understand
it in depth. The dope preparation process is as follows: first the NMMO is heated so that it starts
melting, and before being completely liquid, the cellulose is added in a homogenous way and stirred
at high temperature. The conditions in the literature are flexible, as Ichwan et al. [14] have successfully
prepared membrane with a temperature of 110°C for 1 hour with a cellulose weight total (CWT) of
between 8 and 11%, whereas Abe and Mochizuki [68] performed it at 90°C until transparency of the
solution is reached. After that, the solution is cast on a polymer support and subsequently undergoes a
phase inversion by being bathed in demineralised water.
Various parameters such as cellulose concentration, bath temperature, and NMMO
concentration in the coagulation bath are significant for the morphology of cellulose membranes
produced with NMMO method. For instance, higher cellulose concentration in dope solution generally
results in lower flux and higher rejection performances, because the pore size of the membranes gets
smaller due to higher polymer density. Moreover, pore size changes with temperature of coagulation
bath temperature, i.e. when the temperature is increased, the pore size is also increased. Another
important parameter, the NMMO concentration in water bath, is changing the structure of the
membrane completely by affecting the immersion precipitation rate. Since the higher NMMO
concentration in the coagulation water bath reduces the precipitation rate, and tighter membrane
structure could be obtained [13].
Zhang et al. [8] have studied the formation of cellulose UF membranes produced from NMMO
and subsequently characterised and applied them to water filtration. They showed that higher degrees
32
of polymerization of the cellulose induced higher viscosity of the dope. The kind of pulp they used also
had an impact on the permeation performances. The casting solution concentrations proved that
increasing cellulose concentration increases the rejection but decreases the flux, and higher polymer
density in the solution results in smaller pores after the formation of the membrane. Another important
result is that the higher the NMMO concentration in the concentration bath, the lower the rejection.
Moreover, the pore size is said to be designable by controlling the temperature bath. Indeed, between
25°C and 65°C, the pore size increases from 16.36 nm to 41.53 nm. Mao et al. [9] investigated the
addition of cellulose to the NMMO for preparing the dope solution to cast a pervaporation membrane
and observed that the fluxes increased from 5 to 20 wt% by the addition of water to the concentration
feed.
2.6 Challenges in OSN application
OSN process performances are evaluated according to flow, separation properties and stability.
The breakthrough between flux and rejection performance of the membranes is always being a
challenge in the separation technology, tighter membranes have poor flux performance. The best
solution method is suggested as the decreasing the membrane thickness. The chemical stability needs
to be considered, where the major challenge for OSN membranes is at. Van der Bruggen et al. [82]
listed other challenges such as: (i) avoiding fouling, (ii) improving separation, (iii) treatment of
concentrates, (iv) improving diffusion through the membranes. Compaction of the membrane is another
challenge. The last but not the least challenge is (v) producing new strategies for greener OSN
membranes. The details of all the challenges will be discussed in detail with the suggested solution
methods below.
33
2.6.1 Chemical resistance
The biggest challenge of OSN membranes is the stability in a wide range of organic solvents
other than the one they are dissolved in for producing the casting solution. Swelling and even
dissolution can be observed on polymeric membranes [13, 83, 84]. Two strategies have been studied
in the literature to mitigate the problem, by using high chemical resistant polymers, or by treating the
membrane after fabrication.
2.6.1.1 Use of high chemical resistant polymers
The first and most instinctive way to enhance the chemical stability is to use a more chemically
stable polymer. The structure of the membrane fabricated using chemically stable polymer is indeed
tougher towards organic solvents. But the major issue regarding this method is that only soluble
polymers can be cast with a controlled top layer during phase inversion [85]. Therefore, a specific
membrane cannot be global in terms of solvent stability.
However, when targeted to a specific application, using a high chemical resistant polymer offers
great results. Peeva et al. [86] have shown that the excellent thermal and chemical stability of poly
(ether ether ketone) could be applied to a hard-conditions Heck reaction. The reaction was studied in
DMF at 80°C with a base concentration higher than 0.9 mol.L-1.
2.6.1.2 Post-casting treatment process
Since the previous strategy has its limitations, another idea would be to treat the membrane
after casting. Cross-linking consists of a radical-initiated reaction of one polymer on another. Realizing
this treatment after casting the membrane permits it to have the desired membrane structure and
properties.
The use of diamines for the cross-linking of polyimide membranes has been proven to give
chemical resistance in DMF, THF and NMP after a chemical initiated reaction [87]. DuramemTM
34
commercial membranes are cross-linked polyimide membranes and are already operating in industrial
processes. Cross-linked polyaniline membranes [88] and cross-linked polybenzimidazole (PBI) [89]
are also other alternatives.
2.6.2 Membrane fouling
Membrane fouling is a significant issue and consists of the accumulation of particle close to the
membrane, which reduces the flux [18, 90]. In nanofiltration, the process of accumulation is hard to
understand due to the nanoscale interactions [82]. This study reckons pre-treatment methods, cleaning
the membranes, or modification of the membranes as classical solutions to this problem. The most
sustainable is said to be potentially the latter. Li et al. [4] were able to make anti-fouling UF hollow
fibre membranes with the addition of polyethylene glycol (PEG) in the cellulose matrix.
2.6.3 Compaction
Compaction is a phenomenon which occurs over time for OSN membranes because they face
high mechanical stresses when being pressurised up to 60 bars. If the structure is not strong enough, a
compaction occurs. Soroko et al. have shown that mixed matrix TiO2/PI membranes were less likely
to undergo compaction compared to PI membranes, due to a stronger porous structure [91].
2.6.4 Greener OSN membranes
OSN technology is becoming mature nowadays, and different strategies should be produced in
order to improve the environmental sustainability of this technology. The preparation and modification
procedures usually include several steps of chemical reactions and many hazardous waste are produced
at the end. In order to minimize the waste generated during fabrication, the energy consumption and
35
costs without comprising the performance of the membrane, the principles of green chemistry shown
below (described in detail in a recent OSN review) should be followed [2, 92, 93].
1. Substituting conventional solvents being used for membrane fabrication with green solvents,
2. Using low toxicity chemicals,
3. Reducing the number of steps in manufacturing,
4. Using renewable or raw materials,
5. Dissolving polymers and crosslinking at room temperature,
6. Designing degradable membranes.
Polymers are the most widely used membrane materials for OSN applications and most
polymeric flat sheet membranes are cast on a non-woven backing material to provide mechanical
stability [1]. Therefore, using a “degradable backing material” can be added in the principles of green
chemistry listed above in order to develop a completely green membrane.
2.7 Cellulose composite membranes
Transport mechanism through mixed matrix membranes could be governed by solution
diffusion or pore flow mechanism depending on the structure of the membranes. As discussed in the
transport models section (section 2.1.4), transport through membranes may also be governed by
adsorption mechanisms due to the electrostatic interaction between membrane surface and the
permeates. Adsorption capacity of the membrane material is the most significant issue in such cases,
and using composite membranes (including inorganic porous fillers like zeolite, carbon nanotube, or
metal organic framework (MOF)) could improve the adsorption performance. This is because,
composite membranes combine the advantages of both inorganic fillers such as high surface area and
adsorption capacity, and organic membranes such as low pressure drop, high mass transfer and easy
scale-up [2, 94, 95]. Since the adsorptive properties of composite membranes come from both the filler
material and polymer base, characteristics of the materials and their adsorption properties should be
36
evaluated separately [95]. For instance, Wang et al. [96] proposed a UIO-66/α-alumina ceramic
composite hollow fibre membranes to take advantage of the fast kinetics of UiO-66 adsorbents and to
make it industrially applicable. They reported a novel geometry for adsorption experiments which
could solve the challenging spent particles-separation issues and severe safety concerns caused by
possible leaking problems of dispersed particles.
2.7.1 Metal organic frameworks (MOFs)
Metal organic frameworks (MOFs) are a new type of porous materials that are constructed by
the inorganic and organic building units linked via coordination bonds. MOFs are one of the most
attractive porous materials due to their superior properties such as high surface area, high porosity, and
high degree of crystallinity [97], adjustable structure, and chemical functionalities [10]. They have a
wide range of usage areas such as membrane separation [98], gas [99] and water adsorption [100],
sensing [101], catalysis [102], energy storage [103], toxic gas removal [104].
One of the biggest issue in literature regarding MOFs is their instability in aqueous medium,
and researches show that hydrothermal stability of MOFs still remains an obstacle for the water-
containing applications [105]. There is an ongoing effort in the research world to improve the
hydrothermal stability of MOFs, which will open new perspective for water adsorption applications.
Recently, some water stable MOFs have been reported, such as ZIF-8, MIL-53, Fe-BTC, Zr-MOF [11,
97].
Nowadays, UiO-66 (stands for University of Oslo) is reported as one of the strongest MOFs in
aqueous media under acidic conditions and it is suggested for adsorption applications such as the
adsorption of Rhodamine B (RhB) [106], uptake of arsenic [10] and removal of
methylchlorophenoxypropionic acid from water [107]. It is constructed with Zr6O4(OH)4 clusters and
terephthalate (1,4-benzenedicarboxylate, BDC) linkers. Figure 2.6 shows the crystal structure of the
UiO-66 framework. It has an octahedral cluster, which includes six-centred Zr cations and eight μ3-O
37
bridges. Furthermore, each cluster unit is connected to 12 neighbouring clusters by BDC linkers to
establish an expanded face-centred-cubic (FCU) arrangement [10, 108].
Figure 2.6 (a) Six-centre octahedral zirconium oxide cluster. (b) FCU unit cell of UiO-66; blue
atom – Zr, red atom – O, white atom – C, H atoms are omitted for clarity. Adapted from
reference [10] which is an open access paper.
2.8 Potential cellulose applications
One-fifth of the world population, which is almost 1.2 billion people, are affected by the water
scarcity all around the world [109]. Apart from the scarcity of natural water, the contaminated industrial
wastewater also causes a life challenge for human and animals. Thus, industrial wastewater treatment
has been important in the last 2-3 decades [110, 111]. Wastewater from industrial production units
usually contains several different components such as surfactants, dyes, organic and inorganic
chemicals, heavy metals, precious metals etc. All these components must be removed before reusing
or disposing to nature, because industrial wastewater is usually mixed with domestic wastewater.
Heavy metals are non-biodegradable elements with high atomic weights (63.5-200.6) and
specific gravities (>5.0) [112, 113]. Heavy metals, many of which are toxic or carcinogenic, tend to
accumulate in living organisms and pose a danger to human health. Due to the rapid development in
metal plating facilities, mining operations, fertilizer industries, tanneries, batteries, paper industries,
(a) (b)
38
and pesticides industries, heavy metal pollution has increased, and has become one of the serious
environmental problems of the world [107]. Because of their drawbacks, heavy metals are priority
pollutants to the environment and need to be removed from water sources. Different methods have
been reported in the literature for the removal of heavy metal ions from water sources, namely ion-
exchange, chemical precipitation, adsorption, flotation, electrochemical methods, capacitive
deionization technique[114] and membrane filtration. Membrane filtration is a promising technique for
removal of heavy metal ions from the water sources due to its high efficiency, easy operation and space
efficiency. Ultrafiltration, nanofiltration, reverse osmosis, and electrodialysis are the membrane
filtration techniques used for this purpose. Moreover, there are several studies in the literature
suggesting the modification of membrane surfaces using different functional groups in order to enhance
the metal ion sorption. For instance, carboxylic acids are more selective towards multivalent cations
than monovalent ones, and amine-based ligands selectively adsorb the metal ions such as Pt4+, Cu2+,
Pd2+, Zn2+, Hg+. Meanwhile, phenol-based ligands remove Cd2+, phosphorus-based ligands remove
Pt4+ [115].
Silver and arsenic are two important contaminants in industrial wastewater and groundwater.
Silver (Ag) is valued as a precious metal and not very abundant in nature. It has been widely used in
various areas such as chemicals [116], batteries [117], aerospace [118], filming and imaging, and
photographic industries [119], as well as electronics and electrical applications [120] due to its unique
properties such as high electrical and thermal conductance, reflectivity, and attractive luster colours
[2]. Furthermore, silver is very useful in antibiotics production and some medical applications thanks
to its antimicrobial and anti-inflammatory features [118]. There is a huge and dangerous contamination
of groundwater due to considerable amount of silver consumption in industrial processes [121]. It is
considered a toxic compound as great as that of mercury when absorbed in living organisms [122], and
may cause various unfavourable health impacts (e.g. algiria that related to skin pigmentation, liver and
kidney degeneration and respiratory impairment) [117]. Silver resource is depleting rapidly, because
39
the amount of silver being used in industry cannot be reduced [123]. Accordingly, the effective
recovery of silver from wastewater has turned into a significant concern due to its value, and
environmental and human health concerns.
Even though metallic silver is not regarded as toxic, its ions are toxic to many organisms. These
salts might gather with biological molecules and cause some serious health problems. For instance,
ingestion of 10 gram of silver nitrate is usually fatal [124]. Furthermore, silver is usually found at very
low concentrations in natural waters. The average concentration of silver is 0.2 µg L-1 and 0.24 µg L-1
in natural freshwater and seawater, respectively. There have been no limitations on silver in drinking
water until 1962. World Health Organization (WHO) and National Academy of Sciences (NAS) last
reviewed the value for silver in the drinking water and the current standard for silver in drinking water
is 50 µg L-1 [125].
Adsorption is one of the promising methods among all the convenient technologies because it
is economically feasible, its operation is technically easy, and its yield is high [107]. Many researches
have been carried out to improve the efficiency of adsorption processes by decreasing the cost and
using different sorbents. Zhu et al. [119] have studied the adsorption of Ag (I) from aqueous media by
cellulose and its derivatives. Jintakosol and Nitayaphat [126] used the composite chitosan/bamboo
charcoal beads to uptake silver and examined several factors such as pH value, contact time, and
adsorbent dosage. They reported the maximum adsorption capacity as 53 mg g-1 at pH 6 with a
consistent behaviour with the Langmuir model. They also reported that these adsorbents are reusable
according to the desorption experiments. Cantuaria et al. [117] studied the batch adsorption of silver
by using pre-treated bentonite clay, and they reported the maximum adsorption capacity to be 61.48
mg g-1 at 283 K. They explained that the silver adsorption is an exothermic, spontaneous and physical
process.
All studies mentioned above reported that the silver adsorption results on porous adsorbents
under static conditions. Adsorption-based membrane separation is a relatively greener technology
40
compared to conventional separation techniques in terms of energy consumption, up-scalability and
flexibility. Since it could be used in large scale cross-flow filtration conditions with different
geometries, it has improved the chance of industrial scale applications. Recently, Karim et al. [28]
studied the removal of silver ions from industrial effluents by using cellulose nanocomposite membrane
in cross-flow operation mode. To the best of their knowledge, it was the first study using cellulose
membranes for metal ion recovery from wastewater. They reported 100% recovery of silver from the
mirror industry effluent, while the adsorption capacity (0.33 mg g-1) is very low due to the very low
initial concentration of mirror industry effluent (Table 2.2). They showed that cross-flow operation is
improving the removal efficiency towards metal ions in comparison to static mode (Table 2.2, in
parenthesis). They anticipated that the membrane might be reused after the recovery of the silver ions
by acid washes.
Types of
membranes
pH C0
(mg L-1)
Ci
(mg L-1)
Sorption Capacity Removal
Rate (%) Membrane
(mg m-1)
CNCs
(mg m-1)
Cu2+
2.3
330.2
S-G/CNCSL 285 9.6 (8) 28 (11) 13
S-G/CNCBE 211 24 (22) 67 (64) 36
S-G/PCNCSL 43 79 (66) 358 (233) 86
Fe3+/ Fe2+
2.3
550.5
S-G/CNCSL 472 16.7 (14) 48 (20) 14
S-G/CNCBE 369 37 (34) 102 (100) 33
S-G/PCNCSL 140 113 (109) 512 (391) 74
Ag+
9.1
1.48
S-G/CNCSL 0 0.33 (0.29) 0.82 (0.42) 100
S-G/CNCBE 0 0.33 (0.29) 0.87 (0.87) 100
S-G/PCNCSL 0 0.33 (0.29) 0.81 (1.00) 100
Arsenic is the second metal investigated in this study for adsorption studies. According to
United Stated Environmental Protection Agency and World Health Organization (WHO), arsenic is
one of the most dangerous contaminants in the industrial wastewater and the groundwater, due to its
Table 2.2 The adsorption capacity of different metal ions by cellulose membranes in both cross-
flow mode and static mode. The adsorption capacity in static mode is written with in parenthesis
to compare results. The table is adapted from reference [28] which is an open access paper.
41
toxicity [10]. While the concentration of arsenic in contaminated groundwater is 0.5 to 2.5 ppm,
industrial wastewater has higher than 100 ppm. The concentration of arsenic in drinking water may
contain up to 10 ppm based on the regulation of WHO [127] but since it can be easily accumulated in
the human body, it might result in serious health problems in the liver, kidneys, lungs, and skin.
Therefore, many studies in recent years have given attention to the effective recovery of arsenic from
wastewater, and adsorption is the most promising method for wastewater purification because of the
ease of operation, low cost, high performance and availability of broad range of adsorbents [10, 127].
Zeolite [128], activated carbon [129], iron oxide [129], zirconium [127] are the traditional adsorbents
used for arsenic recovery, but scientists and engineers are still seeking for new attractive materials. γ-
Fe2O3 nanoparticles embedded silica, yttrium–manganese binary composite, and metal organic
framework are the promising alternatives because of their high efficiency, low particle size,
hierarchically ordered structures, and high surface area. Ma et al. [127] proposed using the zirconium
nanoparticles (sizes ranging from 60 to 90 nm) as sorbents to remove the arsenate from aqueous
solutions. They tested the adsorption capacity of the nanoparticles under several parameters such as
pH, contact time, and coexisting anions, and they reached the maximum adsorption capacity of 243 mg
g-1under optimal pH from 2.5 to 3.5. Wang et al. [10] have proposed the water stable zirconium metal-
organic framework (UiO-66) to be used for the first time as adsorbent to remove aquatic arsenic
contamination. They claimed that they got the maximum adsorption capacity (303 mg g-1) ever reported
in the literature. Their superior adsorbent is useful at a wide range of pH values from 1 to 10.
Membrane technologies can be used for metal ion removal from wastewater/groundwater using
different processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse
osmosis (RO), and forward osmosis (FO). NF, RO, and FO processes requires high working pressure
and high cost membranes while MF and UF membranes need lower operation pressure. However, they
are not tight enough to remove the dissolved metal ions with high efficiency [130]. Recently, Zhao et
al. [131] examined the As removal by utilizing self-made PMIA (poly m-phenylene isophthalamide)
42
nanofiltration membrane and they have reached 90% As rejection in their work. Furthermore, Jin et al.
[132] used forward osmosis membranes made from cellulose triacetate to remove As, and they have
reached 90% rejection.
Composite membranes, which include a porous adsorptive material, are promising alternatives
for the removal of dissolved inorganic pollutants from wastewater sources. Zhenga et al. [133] studied
PVDF/zirconia blend flat sheet membranes for the adsorptive removal of As(V). Their membranes
showed a good performance for uptaking arsenate in batch adsorption experiments in a wide range of
pHs from 3 to 8. They reached the equilibrium in 25 h and the maximum adsorption capacity was
reported as 21.5 mg g-1, which is comparable the most of the current sorbents reported in the literature.
Two different membrane flow geometries could be applied for the adsorptive removal of metal
ions from aqueous systems, which are dead-end and cross-flow operations. The pollutants in the liquid
media are coagulated on the surface rapidly and form a cake, because the flow is perpendicular to the
membrane surface in the dead-end filtration. However, the turbulent flow being created in cross-flow
filtration systems reduce the cake formation and the lifetime of the membrane surface gets longer.
Generally, two points should be discussed to understand the impact of cross-flow on membrane
adsorption performance; i) the performance might be better in cross-flow due to the polarization
control, ii) because the pressure drop is longitudinal, the hydrodynamic in flow channel may affect the
breakthrough behaviour of the membranes during loading [134]. There are some studies in literature
examining the adsorption performance of the membranes in both dead-end and cross-flow filtration
conditions. For instance, Crespo et al. [40] tested the filtration of protein solution (BSA) with ion-
exchange membrane. They reported a better adsorption capacity for the membranes in cross-flow
conditions, due to improved control of pore blockage. This is clearly demonstrated as the cross-flow
mode operation had higher yields in comparison to dead-end mode operation [40]. Bayhan et al. [135]
have investigated the removal of heavy metal ions (Ni2+, Cu2+ and Pb2+) by yeast in cross-flow method.
They reported that the cross-flow microfiltration is an effective, low-cost method to uptake heavy metal
43
ions from water via yeast cells. Additionally, Mavrova et al. [136] have studied the combined
adsorption, membrane separation and flotation technique for heavy metal removal from wastewater.
They have successfully utilised cross-flow microfiltration for low-contaminated wastewater.
The materials used to produce composite membranes are important in terms of performance,
cost, and sustainability. Cellulose and a new type of Zr-based metal organic framework were chosen
in this study. Cellulose is one of the most abundant organic materials; it is also inexpensive,
biodegradable and a sustainable semi-crystalline polymer. UIO-66 was proven to be hydrothermally
stable in acidic conditions [11, 108], which is the mostly desired characteristic for water applications.
Moreover, its high surface area, and exceptional As uptake capacity [10] made it a potentially precious
candidate for future studies.
2.9 Prospects and challenges
The great potential of organic solvent nanofiltration technology for many different industrial sectors
such as oil, food, fine chemical, and pharmaceutical has been proved and OSN technology became
mature nowadays [1, 2]. The most important requirement for OSN applications is the resistance of
membranes in wide range of organic solvents, and this problem has been solved by using different
chemical modification methods, i.e. crosslinking. Crosslinking has been applied successfully in
literature to produce more stable polymeric membranes for different applications, although it
generated extra steps during manufacturing and produced more chemical wastes [1, 2]. Green aspects
of the manufacturing procedures (such as using renewable membrane materials, and greener solvents
for synthesis and reducing the waste production during synthesis) should be considered to ensure the
environmental sustainability of OSN technology.
44
Chapter 3
Experimental
3.1 Fabrication and the structural characterization of cellulose
membranes
3.1.1 Materials
N-methylmorpholine N-oxide monohydrate (NMMO • H2O) with a 13.3 wt. % water
composition and a melting point of 72°C was purchased from Sigma-Aldrich and used as received.
Microcrystalline cellulose (MCC) powder with 20 μm particle size was purchased from Sigma-Aldrich
and dried at 80°C under vacuum for 5 h prior to use. Holytex 3329 polyester non-woven backing was
purchased from Freudenberg. Acetonitrile, acetone, ethyl acetate, tetrahydrafuran (THF), methanol,
ethanol, 2-propanol, 1-butanol were purchased from VWR International. All solvents were reagent
grade, and were used as received. All the dyes of which properties and structures are given in Table
3.1 were purchased from Sigma-Aldrich and used as received.
45
Table 3.1 Properties and structure of the dyes used for rejection tests (in H2O and organic solvents)
Name Structure* Charge
[137]
Molecular
weight
(g.mol-1)
Volume
(Å3) [137]
UV-Vis
absorption
peak (nm)
Rose Bengal (RB)
- 1018 NA 548.5
Congo Red (CR)
0 696 NA 498.0
Crystal Violet (CV)
+ 408 1219.1 586.5
Naphtelene Brown (NB)
- 400 955.2 371.5
Methyl Orange (MO)
- 327 858.9 420.0
6-Hydroxy-2,5,7,8-
tetramethylchroman-2-
carboxylic acid (HTMC)
0 250 723.8 202.5
Chrysoidine G (CSG)
+ 249 737.9 439.0
6-Hydroxy-2-
naphtalenesulfonic
acid sodium salt (HNSA)
- 246 593.5 233.0
*All the molecular structure figures were obtained from the webpage of Sigma-Aldrich.
46
3.1.2 Membrane preparation
10 wt. % of MCC was dissolved in NMMO sol. at 90 °C for 4 hours under stirring. The solution
was kept heated without stirring for 1 hour to remove the air bubbles. The obtained dark yellow solution
with 6000 cP viscosity was cast hot on a polyester non-woven fabric taped to a stainless steel plate
heated at 80 °C using a bench casting machine (Elcometer 4340) at a speed of 3 cm s-1. The casting
knife was set at a thickness of 50, 100, 250 and 500 µm and heated at 80 °C prior to casting.
Temperature and relative humidity of the casting room were held constant at 21±1 ºC and 33-34 %,
respectively; to get repeatable and uniform membranes in performance. Immediately after casting, the
membrane was placed in a water bath at 21°C where phase inversion took place. The membrane was
washed with 3 L of deionized water three times and kept in water for further use. To assess
repeatability, at least two discs of each membrane were tested for performance studies and each
thickness was repeated at least 3 times.
3.1.3 Cellulose membranes characterization
X-ray diffractometer (XRD)
X-ray diffractometer is a rapid analytical technique used to identify the phase of a crystalline
material. The XRD instrument has three basic components an X-ray tube, a sample holder, and a X-
ray detector. X-rays are generated in a cathode-ray tube by heat. These produced X-rays are filtered
through foils or crystal monochrometers to produce monochromatic X-rays, and then directed onto the
samples. The detector is recording the X-ray signals to convert them to a count rate. For typical powder
patterns, the data are collected between 5 and 70º 2𝜃 angles.
An incident beam of X-rays diffract into many specific directions when leaving the crystal due
to the atomic planes in its structure, and diffracted beams are produced. All possible diffraction
directions should be attained by scanning the sample through a wide range of 2θ angles, because
powder samples have random orientation. The angles and the intensities of these diffracted beams give
47
someone very valuable information about crystalline structure of the material [138]. For instances, the
distance between the atomic planes that constitute the sample could be measured by applying Bragg’s
Law which is given below:
𝑛𝜆 = 2𝑑 𝑠𝑖𝑛𝜃 (3.1)
where the integer n is the order of diffracted beam, 𝜆 is the wavelength of the incident X-ray beam, d
is the distance between adjacent planes of atoms, and 𝜃 is the angle of incidence of X-ray beams.
Moreover, crystallinity index of cellulose materials could be calculated using simple empirical Segal
method in which the intensity of the highest crystalline peak and lowest amorphous peak were
considered as shown in equation 4.1. This method will be applied in section 4.1.2 to calculate the
crystallinity index of pure cellulose powder and cellulose membranes produced in this study, and all
details will be discussed.
Phase identification of the cellulose powder and prepared membranes was made by PANalytical
X'Pert PRO X-ray diffractometer using nickel-filtered Cu-Kα radiation operation at 40kV voltage and
40mA current. Cellulose powder and membranes were dried well before the experiment, the powder
sample was finely ground and homogenized and sufficient amount of samples were used during
experiment.
Density- Porosity
Gas pycnometer is recognized as one of the most reliable methods for measuring the skeletal,
true, absolute volume and density because of fully automatic, high-speed and high-precision volume
measurement in the equipment. Inert gases such as helium, nitrogen could be used as displacement
medium. In this study, helium pycnometer is used to analyze the true density of porous materials by
48
measuring the pressure change and true density is defined as the ratio of mass of substance to its volume
excluding open and closed pores.
In this technique, the sample with known weight is sealed into the instrument compartment
with a defined volume, then the inert gas is admitted and pressure is measured. After that, the same gas
is discharged to a new empty chamber with known volume and the pressure change resulting from
displacement of gas by a solid object was calculated based on Archimedes’ principle [139]. When the
sample weight is divided by the sample volume calculated from Archimedes’ principle, true density of
the sample was obtained.
Density of the cellulose powder and dried 25 µm-thick cellulose membranes was measured
experimentally using the helium pycnometer equipment AccuPyc 1330 from micrometrics. The density
of the dry membrane was also calculated by measuring the size and the weight of the several cut
samples.
Porosity is defined as the available free volume in the membrane structure. It could be measured
by using the density information of membrane and the polymer material. The porosity of the dry
membrane was calculated using the density of the cellulose powder (𝑑𝑀𝐶 𝑝𝑜𝑤𝑑𝑒𝑟) and dry membrane
(𝑑𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒) using the following equation:
𝑃 (%) = (1 −𝑑𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒
𝑑𝑀𝐶 𝑝𝑜𝑤𝑑𝑒𝑟) 𝑥 100 (3.2)
N2 Adsorption Analysis
Another technique used to measure the porosity of the membrane is gas adsorption/desorption
analysis. Moreover, surface area and pore size of porous materials could be measured by this technique.
Generally, nitrogen is used as condensable gas at its boiling point and the volume of adsorbed gas is
recorded at various vapour pressures. The data is drawn on an adsorption isotherm (amount of gas
49
adsorbed versus relative pressure) and analysed by assuming capillary condensation [140]. Brunauer–
Emmett–Teller (BET) theory is the mostly used one to explain the physical adsorption of gas molecules
on the surface of a solid material. According to this theory, the monolayer molecular adsorption
(Langmuir theory) is extended to multilayer adsorption by assuming that gas molecules adsorb
infinitely and the layers have no interaction between each other, and more importantly, Langmuir
theory is applicable to each separate layers.
This technique is not really reasonable for asymmetric membrane structure, since the porosity
of the whole membrane could be determined, not only the top layer. Luckily, the membranes prepared
in this study are homogenous, and do not have any separation layer. Moreover, samples should be dried
very well before BET analysis to avoid any pore blocking due to remaining solvents or moisture in the
membrane structure. In case of cellulose membranes, drying and degassing was not really simple since
it is a hydrophilic material (can adsorb the water molecules from air easily) and swelling in water
significantly (washed and stayed in water for long time before drying). When the drying/degassing
process before the BET analysis was not successful, no data was recorded for long time taking
experiments. After several attempts, drying and degassing at 100°C for overnight was selected as the
optimum treatment before BET analysis.
Cryogenic nitrogen adsorption experiments were used to determine the BET (Brunauer–
Emmett–Teller) surface area of cellulose powder and cellulose membranes using TriStar surface area
analyser (Micrometrics). Samples were dried at 100°C under vacuum for overnight before the analysis.
Membranes were found to be stable in liquid nitrogen.
Scanning electron microscopy (SEM)
Scanning electron microscopy applications focus on the characterization of membrane
structure. Imaging the nanostructure of a membrane is a very valuable information for understand the
50
structure-performance relationship. Since the polymeric membranes are not conductive, they need to
be coated before the imaging, but still low electron contrast prevents to take high magnification images.
SEM is using the radiation with an electron beam to obtain an image of a sample. The sample
surface produces low energy secondary electrons due to excitations in the sample itself produced by
electron beam, and SEM measures these energies. The applied voltage is the decisive for the resolution.
Since the high voltages damage the polymeric materials, the resolution is generally not larger than
5nm, so it gives information about the macrostructure of the membranes [140].
The surface and the cross-sectional morphologies of the synthesized membranes were
investigated by scanning electron microscopy (FEGSEM LEO1525). The membranes were dried
carefully under vacuum conditions and then were fractured in liquid N2 to obtain a tidy cross-section.
After that the membrane films were mounted horizontally and vertically on a circular aluminium
sample holder with carbon tape. After that, the samples were coated with 20 mA gold for 2 mins under
an argon atmosphere (Emitech) to achieve the necessary electrical conductivity and various
magnifications were used between 6000 x -20000 x during the analysis. At least three images of each
membrane were scanned and membranes prepared from at different times were analysed in repeats of
two to ensure there was no variation between samples of the same membrane type. Cellulose
membranes were charging significantly during the SEM experiments, which make the perfect imaging
impossible. Although many different coating conditions (gold and chromium coatings with different
thickness) were applied to find the optimum conditions, no improvement was achieved and the pictures
reported in this thesis are the best ones ever.
Contact Angle
This technique is used to measure the hydrophobicity of solid materials. When the water contact
angle is measured lower than 90º, the material is named as hydrophilic, and when the water contact
angle is measured higher than 90º, the material is named as hydrophobic. The angle where the
51
vapor/liquid phase and solid surface meets named as contact angle, and it is defined by the mechanical
equilibrium of three different interfacial tensions. It is defined by the Young’s equation shown below:
𝛾𝑙𝑣 cos 𝜃 = 𝛾𝑠𝑣 − 𝛾𝑠𝑙 (3.3)
where θ is contact angle and 𝛾𝑙𝑣, 𝛾𝑠𝑣 and 𝛾𝑠𝑙 represent the interfacial tensions at liquid-vapor, solid-
vapor and solid-liquid surfaces, respectively. Two methods are used for contact angle measurements:
i) the captive bubble point method; and ii) the sessile drop method. The measurement is conducted in
a wet phase in the captive bubble point method, while dry material is used for the sessile drop method
[140]. In the membrane terminology, the contact angle corresponds to the wettability of the membrane.
An ideal hydrophilic surface requires a whole water droplet of which contact angle is zero degree [1].
This characterization technique gives lots of useful information about the material surface in the
applications of painting, coating, cleaning, printing, bonding, dispersing.
Contact angle measurements were performed with an EasyDrop Instrument (manufactured by
Kruess) at room temperature using the sessile drop method, in which a 15 µl drop of liquid was
deposited on the surface of a piece of membrane using a micropipette. All membranes were dried prior
to contact angle measurements. The contact angle was measured automatically by a video camera in
the instrument using the drop shape analysis software. Contact angle measurements were performed
five times per sample and the average value was reported. In order to evaluate the repeatability,
measurements were performed on several different membranes.
Streaming Potential
Streaming (zeta) potential is defined as the electrokinetic potential in colloidal dispersions,
which indicates the stability. The higher the magnitude of zeta potential, the higher the electrostatic
interaction between similarly charged particles in a dispersion. In surface chemistry, it indicates the
52
surface properties of a material in terms of electrostatic loading, and this is what we used in this study
for estimating the surface properties of the cellulose membranes prepared. In membrane science,
especially in the field of nanofiltration, electrostatic properties of a membrane give very significant
information about its separation performance. Because in this range of separation limits, lots of
different transport mechanisms such as the electrostatic interactions or adsorption become dominant
for determining the separation mechanism instead of simple molecular sieving effect. Moreover,
determining the zeta potential is critical to analyse the membrane fouling phenomena especially in the
case of nanofiltration and reverse osmosis membranes. Membrane fouling is a significant issue for lots
of industrial applications, and some surface modification techniques are suggested in literature to avoid
the membrane fouling caused by electrostatic interactions.
This part of experiment was done by our collaborators in Austria. The zeta potential of the
membrane surfaces was measured based on the streaming potential method using the SurPASS
electrokinetic analyzer from Anton Paar (Graz, Austria). Membranes were placed on either side of an
open channel (100 μm apart) using an adjustable gap cell. 1 mM KCl electrolyte solution was pumped
through the cell and the pressure was steadily increased from zero to 300 mbar. The streaming current
was measured as a function of pH at 25 °C using two electrodes placed at both ends of the sample. ζ =
f (pH) was measured in the range of pH 2.3 to pH 9.5 with a standard deviation of ±0.2 by titrating
0.05 M KOH and 0.05 M HCl into the electrolyte solution. 25μm-thick dried membrane was used for
zeta potential experiment and different samples from different membranes batches were tested for
reproducibility. Same results were obtained.
Thermal gravimetric analysis (TGA)
Thermal gravimetric analysis is a thermal analysis method used to analyze the physical and
chemical changes occurred in properties of materials as a function of increasing temperature or time.
It can provide information about physical phenomena (i.e. adsorption, absorption, desorption,
53
vaporization, sublimation) as well chemical phenomena (i.e. chemisorption, desolvation,
decomposition) [141]. TGA is a high precision technique working on three different measurements;
mass change, temperature, and temperature change, and a precision balance and a programmable
furnace is required for the measurements. The sample regardless form its form was loaded in a pan and
weighed by a precision balance in the equipment and taken inside the equipment automatically. The
weight of the sample is weighed continuously as it is being heated to high temperatures, and some
weight decrease are recorded due to decomposition of some components inside the sample. The mass
loss data is plotted with respect to the temperature change and a curve is obtained. When TGA analysis
is used to evaluate the thermal stability of a material as in our case, the sample could be heated up to
2000 ºC to find the upper use temperature of the material. Ceramics, for instance, melts before
degradation due to very high thermal stability, therefore TGA is not feasible technique for them.
However, most of the polymers melt or degrade before 200 ºC, while some stable ones could stand up
to 300 ºC in air, and to 500 ºC in inert gases.
TGA was performed for both cellulose powder and membrane by TGA Q500 (TA Instruments).
Samples were heated up to 600 °C at a rate of 10 ºC min-1 in N2 atmosphere with a nitrogen flow rate
of 60 ml min-1. These measurements were conducted for pure cellulose powder and cellulose
membranes for at least two times for reproducibility and exactly same results were obtained. The first
reason of using this technique was to test the thermal stability of the prepared membranes, and the
second one was to compare the thermal behavior of cellulose powder and the membrane in order to
understand the effect of NMMO dissolution method on the characteristics of the cellulose material.
Mechanical Test
Tensile testing, in which a sample is subjected to a controlled tension until failure, is a
fundamental technique used in materials science [142]. The most important stability measurements for
organic solvent nanofiltration membranes could be sorted as the stability under high operation pressure
54
and the stability in different organic solvent conditions, and tensile testing may not have primary
significance for performance evaluation. However, for overall performance evaluation of a membrane
material, different aspects should be considered and tensile testing should be evaluated for the
conditions at which backing paper is not used, or different potential applications.
The tensile strength and maximum load that the membranes can stand at breakage point were
determined using a Lloyd EZ 50 tensile test machine. The measurements were carried out at a constant
elongation velocity of 1 mm min-1 and at room temperature. Membranes were tested without non-
woven backing material, testing was quite tricky. Since the surface of membranes are smooth, fixing
was not really easy during the measurement. Gripping surface must have a sufficient friction to hold
the membrane samples stable, but also should be gentle enough to not to tear the membranes. Soft
sticky tapes were used in this study to fix the samples to the specimen. At least two different samples
were tested to see the repeatability of the experiments, and to ensure the reproducibility of the
membranes, and very similar results were recorded.
3.1.4 Pure solvent flux measurements
A dead-end filtration cell (HP 4750 Stirred cell) shown in Figure 3.1 was used to measure the
flux (J) for water and different organic solvents. Flux was calculated as J= V / A x t, where V is the
volume of permeate, A is the effective membrane area, and t is permeation time. For the flux
measurements, a circle with 49 mm diameter was cut from the cellulose membrane and placed on a
stainless steel porous membrane support disk and fixed using a teflon o-ring. Then, the amount of
solvent that passes through the membrane in a defined time interval was weighed and the solvent flux
calculated. After the solvent flux reached steady state, data was collected for at least one hour. Most of
the dead-end nanofiltration experiments were carried out at a pressure difference of 10 bar and 25 ºC;
and only for membranes with a thickness less than 5 µm a pressure difference of 2 bar was used.
55
In this study, cross-flow operation mode was used as well as static mode because it is reported
to be the efficient mode for industrial level applications due to high penetration power of pollutants
through the membranes [28]. Long-term flux performance of the membranes was tested using a cross-
flow filtration system in which two membrane modules were connected in series, and a solvent-stable
high-pressure liquid chromatography (HPLC) pump was used. The effective membrane area was 14
cm2, which is the same area as the dead-end filtration set-up, and 24 h experiments were performed
with a working pressure between 4 and 5 bars, and a feed flow of 55 L h-1. Permeate samples for flux
measurements were collected at intervals of 1 h. The schematic diagram of the experimental apparatus
for the cross-flow filtration test is presented in Figure 3.2.
Figure 3.1 Schematic representation of dead-end filtration set-up (from
http://media.sterlitech.com/wysiwyg/HP4750_Manual_V1.2.pdf)
56
Figure 3.2 Schematic representation of cross-flow filtration set-up in which membrane cells
connected in series
3.1.5 Rejection Tests
Several dyes with different charges and different molecular weights ranging between 245 and
1020 g mol-1 (their properties and their structures are shown in Table 3.1) were selected as markers to
determine the MWCO of the membranes. Generally, rejection measurements were conducted at 10 bar
pressure, but for the membranes with a thickness of less than 5µm, a pressure difference of 2 bar was
used because of the very high fluxes. Before the rejection experiments, 100 ml of the pure solvent of
interest was pressurized to 10 bar to condition the membrane for possible compaction effects. Then, 20
ml solutions of 20 mg L-1 of the chosen dye in organic solvent were used as feed for the rejection tests,
and were pressurized by a nitrogen cylinder until 5 ml of the solution has passed through the
membranes. The quantitative analysis of feed and permeate solutions was determined by UV-visible
spectrophotometer (UV-1800, Shimadzu). The concentrations of feed and permeate were calculated
using the absorption values at the characteristic wavelength of dyes (seen in Table 3.1) and then the
rejection values were calculated by the equation 2.4 given in section 2.2.
Membrane cells
57
UV calibration curves for congo red (CR) in water and rose bengal (RB) in acetone shown in Figure
A.1 represent almost a linear relation between absorbance and dye concentrations. The concentration
in the retentate solution was also measured to confirm the mass balance and any significant solute loss
or adsorption within the membrane. As explained above, we evaluated the rejection values for several
dyes in water, acetone, acetonitrile, ethyl acetate, butanol, and THF through cellulose membranes with
different thicknesses.
3.1.5.1 Rejection tests in cross-flow system
Long-term rejection experiments were conducted in a cross-flow filtration system of which the
set-up is shown in Section 3.1.4. The membranes were put in two modules connected in series with
two sheets of non-woven backing PE support to ensure that no leakage could lead to the wrong
permeate sampling. 300 mL of feed solution at 20 mg L-1 of dye concentration was prepared, and the
working pressure was set between 4 and 5 bars by a valve and measured with a manometer. Before the
rejection experiments, the system was first filled with pure solvent and was run for approximately one
hour to let compaction occur, then the system was drained as much as possible before adding the feed.
To limit the initial dilution due to remaining solvent in the circuit, the retentate valve was remained
open until the liquid coming out was coloured. The first five droplets coming out of the permeate tubes
were disposed of, then the sampling at initial time was taken before putting the permeate tubes back
into the feed bottle. The system was then operated for 24 hours, during which samples were taken
regularly.
After rejection experiments, the system was cleaned in two steps: upstream part and
downstream cleaning. In the first step, the liquid was not allowed to permeate through the membrane,
and was washed with solvent in an open circuit. When the washing liquid came out almost clean, the
system was closed again so that clean liquid permeated through the membrane in an open circuit.
58
3.1.6 Batch adsorption experiments
Adsorption of dyes onto cellulose membranes was determined by batch experiments [143] in
which the membrane was inserted in a dead-end membrane cell and filled with the dye solutions with
a known concentration. Than the module was sealed well to prevent any water evaporation and the
concentration of an organic compound in an aqueous solution was determined after a 1 h contact with
the membrane material. No pressure was applied. The initial concentration was kept at 20 mg L-1 for
all the dyes tested. This concentration was chosen in order to compare it with the filtration conditions.
The volume of the solution was 20 ml and the surface area of the membrane tested for adsorption was
the same as that of the membranes tested for filtration. The dye concentration difference before and
after 1 h was determined by UV as described in Section 3.1.5. The amount of adsorbed dye was
calculated by subtracting the final concentration from the initial value.
3.1.7 Calculation of Hansen Solubility Parameters
The solubility parameter expresses the interactions between molecules due to dispersion forces,
polar forces, and hydrogen bonding in a polymer, solvent, or a solute. The total solubility can be
expressed in terms of these components and could be measured experimentally as the square root of
the cohesive energy density. For the larger molecules, the contributions of each functional group of the
structure to the cohesive energy and the molar volume have been accounted by using the group
contribution method which is expressed in equation (3.4)
𝑆 = (∑ 𝐸𝑐𝑜ℎ𝑖
∑ 𝑉𝑚𝑖
)1
2⁄ (3.4)
where 𝑆 is the solubility parameter, 𝐸𝑐𝑜ℎ𝑖 is the cohesive energy for the i functional group on the
molecule, 𝑉𝑚𝑖 is its molar volume [144]. Cohesive energy and molar volume data taken from Fedors
[145].
59
Solute-membrane and solvent-membrane affinities were calculated by subtracting the solubility
parameters of individuals as |𝑆𝑠𝑜𝑙𝑣𝑒𝑛𝑡 − 𝑆𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒| or |𝑆𝑠𝑜𝑙𝑢𝑡𝑒 − 𝑆𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒|. If the solubility
parameters of two species are similar, they tend to have high interaction. When the solute-membrane
affinity is higher than the solvent-membrane affinity, permeate is enriched with solute (low rejection).
When both solvent and solute have high affinity, the species with the solubility parameter closest to
the solubility parameter of the membrane polymer govern the rejection. Moreover, the higher the
solute-solvent affinity, the lower the rejection [144].
3.2 Preparation and the structural and performance characterization of
nanocellulose paper
3.2.1 Materials
Nanofibrillated cellulose (NFC), was produced by grinding never-dried bleached birch kraft
pulp (Betula pendula), which was conducted using a Masuko Mass Colloider (Masuko Sangyo Co.,
Kawaguchi, Japan). The pulp was passed through the grinder seven times and the final composition of
the aqueous gel-like NFC was approximately 1.8 wt. %. NFC fibrils have fibrous structure with the
dimensions of approximately 50 nm in diameter and several micrometres in lengths [146]. Hollytex
3329 polyester non-woven backing paper (PBP) was purchased from Freudenberg.
3.2.2 Preparation of nanocellulose paper
The nanocellulose paper preparation process [147] is very similar to the production of paper
(shown in Figure 3.1). Moreover, only water is being used during the process with no addition of
chemicals or solvents, which qualifies this process as an environmentally friendly one.
To produce NFC-based papers, the nanocellulose-in-water suspension was adjusted from a
starting consistency of 1.8 wt% prior to mechanical blending, and then blended (Breville VBL065-01,
60
Oldham, UK) for 3 min in deionized water to get a homogeneous suspension. The prepared suspension
was then vacuum-filtered onto a filter paper (VWR 413, 125 mm diameter, 5–13 µm pore size). The
wet filter cake was wet-pressed between blotting papers (3MM Chr, VWR) under a weight of 10 kg
for 5 mins to reduce the water content. These partially dried nanocellulose cakes were then sandwiched
between fresh blotting papers and metal plates, and consolidated and dried in a hot-press (25-12-2H,
Carver Inc., Wabash, USA) under a compression weight of 1000 kg for 1 h at 120 °C. Using the hot-
press prevents the shrinking of the nanocellulose papers during the drying process, since shrinkage of
nanocellulose papers will reduce flexibility in the fiber network. This would lead a decrease of the load
bearing capability of the resulting papers. After the drying procedure, the membranes were ready for
use. The thickness of the papers were controlled by changing the grammage of the nancellulose fibers
used (g m-2). Four different papers were prepared with the grammages of 20, 40, 60, 80 g m-2 to
investigate the relationship between thickness and grammage, and also the relationship between the
thickness and pure water flux.
Another nanocellulose paper, which will be termed as NCP-2 in the rest of the chapter, was
provided by Lulea University of Technology. For the NCP-2 fabrication, a larger scale of the NCP
production process summarized in Figure 3.3 was used. 2 wt. % of sludge suspension (200 ml) was
filtered through 146 cm2 area using suction pump. After 12 h drying at room temperature, the prepared
porous support was pressed between aluminium plates in a compression-molding machine (Fontune
Presses, Elastocon, Sweden) with load of 6000-7000 kg with heating to obtain compacted 3-D
structure.
61
3.2.3 Characterization of PBP and NCP backing papers
The grammage of the prepared nanocellulose papers was calculated by dividing the weight of
the used nanocellulose by the cross-sectional area of 49 mm diameter filter paper. The thickness of the
dried nanocellulose papers was measured by using a digital micrometre (Mitutoyo Digimatic
Micrometer, 0-25 mm).
The morphology of the backing papers (PBP, NCP, NCP-2) was examined using low (JSM
6010 LA) and high (FEGSEM LEO1525) magnification scanning electron microscopy (SEM) at 5kV
voltage. The backing papers were viewed only for the surface structure at low magnification SEM.
After that the samples were coated with gold to have an electrically conductive layer and various
magnifications were used between 6000x-20000x during the analysis.
Pore size distribution of the samples was determined by the mercury intrusion technique on an
AutoPore III mercury porosimeter (Micromeritics Instrument Co.). The 50μm thick composite
membranes was cut into a rectangular shape (1 × 2 cm) and weighed. The sample was placed in the
cup of the penetrometers (£s/n-14, 3Bulb, 0.412 Stem, Powder) and closed tightly. The penetrometer
and sample was put into the pressure chamber to measure pore size distribution. This characterization
experiment was conducted by our collaborators in Lulea University of Technology.
http://www.tesco.com
/direct/breville-vbl065
http://www.carverpress.
com/3856.html
http://glossary.peri
odni.com
Blending
Vacuum
filtration
NCP
Hot Press
Figure 3.3 Schematic representation of NCP production
62
Pure water flux measurements were performed using dead-end filtration system (described in
Section 3.1.4) to investigate the flux performance of the prepared nanocellulose papers and the backing
papers (PBP, NCP-2).
Moreover, the stability experiments have been conducted in different organic solvents and
surface modification solution to compare the resistance of the NCP-2 and the PBP. Small pieces of
nanocellulose paper with known dimensions and weights were soaked in acetone, ethyl acetate,
ethanol, and THF for 12 months. Subsequently, the membranes were immersed in methanol to remove
the high boiling point residual solvents, and the membranes were dried completely before weighing.
The visual appearance and the dried weights of the samples before and after 12-month experiment were
compared. The organic solvent resistance of nanocellulose paper was evaluated by measuring the
weight difference before (wbefore) and after (wafter) the test, as reported in another study [148].
𝑅𝑠𝑜𝑙𝑣𝑒𝑛𝑡 (%) = (1 − (𝑤𝑏𝑒𝑓𝑜𝑟𝑒−𝑤𝑎𝑓𝑡𝑒𝑟
𝑤𝑏𝑒𝑓𝑜𝑟𝑒)) 𝑥 100 (3.5)
3.2.4 Composite stability/biodegradability study
This part of the experiments was conducted by our collaborator in Lulea University of
Technology, Sweden. Two experiments were designed to understand the stability and biodegradability
of fabricated nanocellulose paper and commercial backing paper with the pure cellulose membranes
cast on them in real water and soil. For stability test, membrane samples on NCP-2 and PBP backings
were dipped in real wastewater having different pH values (2.1, 7.2 and 10.2) at 40 oC in stirring
condition. For biodegradability experiments, pure cellulose membranes on NCP-2 and PBP were
embedded in soil at 40oC. In order to determine the stability/degradability, they were tested for 45 days,
with 15 days intervals. The membranes were recovered from the soil, cleaned, and the weight was
determined after 24 h of drying at room temperature. The degradation rate was calculated based on the
63
weight before and after biodegradation in soil. This part of work was also conducted by our
collaborators in Lulea University of Technology.
3.3 Metal adsorption through cellulose and cellulose/ UIO-66
membranes
3.3.1 Materials
Zirconium (IV) chloride (ZrCl4, >99.5%), 1,4-benzenedicarboxylic acid (BDC, 98%), Silver
Nitrate (AgNO3, >99.0%), Acetic Acid (AC >99.7%) and sodium arsenate dibasic heptahydrate
(Na2HAsO4•7H2O, 98%) were purchased from Sigma Aldrich and used without any purification and
modification. N,N-Dimethylformamide (DMF, 99.8%) was purchased from VWR, and deionized (DI)
water that was used in all experiments was supplied by the Analytic lab in Chemical Engineering
Department of Imperial College.
3.3.2 Synthesis of UIO-66
ZrCl4, BDC, and H2O were dissolved in 180 mL DMF under stirring in order to give a molar
composition: Zr4+/BDC/H2O/DMF=1:1:1:500 [108]. The solution was homogenized in the ultrasonic
bath for 1h, and transferred into a Teflon-lined stainless steel autoclave. The autoclave was put into a
convective oven (UF30, Memmert), at 120±2 °C for 48 hours and then naturally cooled to room
temperature. UIO-66 crystals were collected by centrifugation (Thermo Scientific Legend X1R) at
15000 rpm for 10 minutes, and washed by ethanol several times. UIO-66 crystals were activated at
120±2 °C overnight under vacuum (Fistreem Vacuum Oven) before using.
64
3.3.3 Characterization of UIO-66
The crystal structure of MOF adsorbents was examined with X-ray diffractometers (XRD)
using Ni-filtered Cu Kα radiation 40 kV and 40 mA. XRD measurements were also conducted before
and after adsorption experiments in order to analyse the stability of the material in adsorption
conditions. Fourier transform infrared (FTIR) spectroscopy using an (Bruker Vertex 70) instrument
was performed in order to confirm the presence of functional groups in/on the adsorbents. For each
spectrum, 40 scans were carried out at a spectral resolution of 4 cm-1 over wavenumber range 600-
4000 cm-1. The morphology of the UIO-66 powders was characterized by using a scanning electron
microscope (SEM- LEO Gemini 1525) at voltage of 5kV. MOF crystals were mapped before and after
adsorption experiments by the scanning electron microscopy coupled with energy-dispersive X-ray
spectroscopy (SEM-EDX) at voltage of 20kV.
3.3.3.1 Adsorption experiments
Water stable UIO-66 crystals [10] were tested as adsorbents for the removal of silver (I) from
aqueous solution. Since As (V) adsorption capacity of UIO-66 powders was reported previously by
Wang et al. [10], here only membranes were examined for As adsorption. Three different types of batch
adsorption experiments were conducted to investigate the impact of several parameters on the
performances. All the adsorption experiments were run at the room temperature (25 ± 1°C) and
conducted twice to check the reproducibility. 1000 mg L-1 (ppm) silver stock solution was acquired by
dissolving silver nitrate, AgNO3, in 1 L DI water and diluted to require initial concentrations for the
experiments.
To study the effect of pH, a series of 100 mL silver solutions with an initial concentration of
100 ppm were prepared in glass bottles with different pH values ranging from 0 (± 0.2) to 7 (± 0.2).
The pH of the solutions was adjusted with HNO3 and NaOH and controlled by a JENWAY 4330
Conductivity/ pH Meter. High pH conditions were not investigated since Ag (I) precipitates as silver
65
hydroxide at high pHs [126]. For a typical experiment, 50 mg UIO-66 powder (adsorbents with a
dosage of 0.5 g L-1) was dispersed in 100 mL of 100 ppm AgNO3 solution. The mixture was shaken at
room temperature at a rate of 220 rpm for 72 h (IKA platform orbital shaker, KS 260 Control). Then,
samples were filtrated through a 0.45 µm filter and analysed for residual Ag (I) concentration by the
inductively coupled plasma emission spectrometer (ICP-OES, Optima 2000 DV, PerkinElmer). All of
the experiments were run for at least 3 times to analyse the reproducibility of experiments, and average
results were reported.
To study the kinetics of the adsorbents, UIO-66 with a dosage of 0.5 g L-1 (shown in equation
3.7) were added (250 mg powder) into 500 mL silver solutions with an initial concentration of 100
ppm at pH 2 and pH 7. The solution was shaken at room temperature for 72 h. During this operation,
10 samples (2 mL each sample) were collected using disposable plastic pipette in different time
intervals between 1 minute to 72 h. Then, samples were filtrated through the filter and their silver
concentrations were measured using the ICP-OES. The equilibrium time for the batch adsorption
experiments was found to be 24 h by the kinetics experiments.
To obtain the adsorption isotherm, a series of 100 mL silver solutions with 8 different
concentrations (from 5 ppm to 200 ppm) were prepared by diluting the stock solution. The pH of the
solutions was adjusted at pH 2 and pH 7. UIO-66 adsorbent with a dosage of 0.5 g L-1 was added (50
mg powder) to each solution and they were shaken at room temperature. Since the equilibrium time for
batch adsorption experiments was found to be 24 h, samples (2 ml each sample) were collected after
24 h of contact time and filtered. The filtrate was analysed by ICP-OES to obtain the silver
concentration. These adsorption isotherm experiments (static) were carried out to investigate the
relation between the adsorbent and adsorbate. The equilibrium adsorption capacity (qe, mg g-1) and
dosage of adsorbent (dosage, g L-1) was calculated using following equations (3.6) and (3.7):
𝑞𝑒 = (𝐶0−𝐶𝑒) 𝑥 𝑉
𝑚 (3.6)
66
𝑑𝑜𝑠𝑎𝑔𝑒 = 𝑚
𝑉 (3.7)
where, C0 and Ce are initial and final concentration of metal ions (mg L-1), respectively. V (L) is initial
volume of the solution, and m (g) is mass of adsorbent used in the experiment. Furthermore, Langmuir
isotherm model has been considered to study the adsorption equilibrium.
3.3.4 Preparation of cellulose/UIO-66 composite membranes
Cellulose/UIO-66 composite membranes with 10 wt.% cellulose were prepared by the phase
inversion method with the following steps.
• 0.1 g of UIO-66 MOFs was first dissolved in 2.5 g of NMMO solution while 1 g of MCC was
dissolved in 6.5 g NMMO solution at 90°C under stirring for 1 hour. These solutions were
mixed.
• The obtained dark yellow mixture after 4h stirring was blade cast on a stainless steel plate at 80
±1 °C temperature with the same casting knife (described in Section 3.1.2) of 500 μm slit and
a speed of 3 cm s-1.
• After the casting is finished the plate was bathed in DI water at 21°C where phase inversion
took place, and the membrane was washed with 3 L of DI water, and kept in DI water for further
use.
• UIO-66 concentration in composite membranes was set at 20 wt.% of the cellulose amount.
3.3.5 Characterization of cellulose/UIO-66 composite membranes
Surface and cross-sectional morphology of the cellulose/UIO-66 composite membranes was
determined by SEM as described in Section 3.1.3. Membrane surfaces were also mapped before and
67
after adsorption experiments by SEM-EDX at voltage of 20kV to detect the residual silver and arsenic
in/on the membrane. Cellulose/ UIO-66 membranes were also examined by ATR- FTIR spectroscopy
to confirm the presence of cellulose and UIO-66 powders. All of the characterization experiments were
conducted for two different membrane samples to analyse the reproducibility of the membrane
preparation method and very similar results were obtained.
3.3.5.1 Batch adsorption experiments for the membrane
Pure cellulose and cellulose/UIO-66 composite membranes were tested for arsenic (As (V))
adsorption in addition to silver (Ag (I)). As (V) stock solution with 1000 ppm concentration was
prepared by dissolving sodium arsenate dibasic heptahydrate, Na2HAsO4•7H2O, in 1 L DI water and
then diluted to acquire initial concentrations for the experiments. The preparation of Ag (I) stock
solution has already been described in Section 3.3.3.1. The batch adsorption experiments for the
membranes were very similar to the ones for UIO-66 powders. All the adsorption experiments were
run at room temperature (25±1°C).
In the batch experiment, a series of 100 mL silver and arsenic solutions with initial
concentration of 5, 10, 25, 50, 100 ppm were prepared in glass bottles using the stock solution. 50 mg
membrane (~7 cm x 5.5 cm x 0.015 mm) was dispersed in 100 mL of metal solution (adsorbents with
a dosage of 0.5 g L-1). The mixture was shaken at room temperature with a rate of 220 rpm for 72 h.
The samples were taken at the selected time intervals using disposable plastic pipette. Other
procedures were the same with those in the Section 3.3.3.1. The quantity of the metal adsorbed by the
surface of the membrane was calculated by considering the initial and final concentrations of the feed
solution.
68
3.3.5.2 Cross-flow adsorption experiments for the membrane
In this study, cross-flow operation mode was used as well because it is reported to be the
efficient mode for industrial level applications due to high penetration power of pollutants through the
membranes [28]. Homemade cross-flow filtration cell was used to investigate kinetic adsorption
capacity of the cellulose/UIO-66 membranes for silver and arsenic. Long-term adsorption experiments
were run at 2 bar operating pressure with a water flow rate of 600 cm3 min-1 (36 L h-1). The total
surface area of membrane used was approximately 18.9 cm2 (the weight was approximately half of
the membrane used in batch adsorption experiments) which was sufficient for the 100 mL volume of
feed water. 2 mL samples were collected at different time intervals and tested for their metal
concentration by ICP-OES. The schematic diagram of the experimental apparatus for the cross-flow
filtration test is presented in Figure 3.4.
Percentage adsorption (A (%)) was used to express the adsorption performance of the
membranes under continuous conditions. The percentage adsorption of metals was calculated by the
formula given below:
𝐴 (%) = (1 −𝐶𝑡
𝐶0) 𝑥 100 (3.8)
where, 𝐶0 is the concentration of feed solution; 𝐶𝑡 is the concentration of feed solution at time 𝑡.
Moreover, the equilibrium adsorption capacity of the cellulose membranes was calculated
using the equation (3.5) given in powder section (Section 3.3.3.1) where m (g) is the weight of dried
membrane used in experiment. All of the batch and cross-flow experiments were run using at least 2
different membrane samples from different batches to test the reproducibility and the average values
were reported.
69
Figure 3.4 Cross-flow filtration system
No detailed experiments were conducted for the recovery of metal ions from the membrane
surface, but some preliminary ones were done. Membrane samples that were used in adsorption
experiments were put in DI water for 24 hours and then the water was tested by ICP to understand
that if any metal ions desorbed spontaneously. Moreover, the same experiments were repeated for
methanol.
70
Chapter 4
Results and discussion
4.1 Structural and performance characterization of cellulose membranes
4.1.1 Cellulose membranes appearance
Figure 4.1 shows a photograph of two different 25µm-thick cellulose membranes with and
without backing material. It is clearly seen that cellulose membranes prepared by the phase inversion
procedure were highly transparent. Since the membranes swell significantly in water due to water
adsorption, the non-woven backing material was used to allow easy handling of the membranes. In order
to further investigate the swelling behavior of the membranes, small pieces of dried membranes with
known dimensions and weights were immersed in water, acetone, or THF and left overnight. The
membranes were weighed as soon as they were taken out from the solvents and wiped by a piece of
paper. Increase in weight and dimensions showed that the membranes are swelling 65%, 6%, and 5%
in water, acetone and THF, respectively. One reason for the low degree of swelling calculated for
solvents might be because the solvents evaporated off during weighing.
Most of the experiments were carried out using 25 µm-thick membranes, but the effect of
thickness on the membrane performance was also investigated by changing the blade thickness from
71
500 µm to 50 µm. The dry thicknesses of the prepared membranes were measured using micrometrics
equipment (manufactured by Mitutoyo) and SEM analysis.
Figure 4.1 Photograph of the 25 µm-thick membranes a) without backing, b) with backing.
Table 4.1 shows the obtained dry membrane thicknesses, which corresponds to the adjusted
casting knife thickness. When the membrane was cast directly on a glass plate without a non-woven
support using 500 µm blade thickness, the dried thickness was measured to be 50 µm as expected, since
the concentration of the polymer in the dope solution is 10 % by weight. However, the thickness of the
dried membranes cast on polyester non-woven backing is almost half of the membrane cast directly on
a glass plate, probably because of penetration of the casting solution into the backing support.
4.1.2 Cellulose membranes characterization
Morphological and mechanical properties of the prepared membranes were measured using
different characterization techniques including XRD, SEM, BET, contact angle, streaming potential,
TGA, and tensile test.
a) b)
72
Table 4.1 Obtained dry membrane thickness when cast on polyester backing using different adjusted
casting knife thicknesses
Knife Thickness
(µm)
Dry membrane thickness
(µm)
500 27±2
250 13±2
100 5±1
50 2±1
Cellulose powder and membrane crystallinity
An X-ray diffractometer was used to analyse the crystalline phase of the prepared membranes
and to see the effect of NMMO dissolution on the semi-crystalline structure of cellulose. The XRD
measurements were run twice for each sample and the experiments were repeated for three different
samples. The X-ray diffractograms of the cellulose powder and the cellulose membranes are shown in
Figure 4.2. Cellulose I and II are two crystalline phases of cellulose, and regenerated celluloses are
enriched in cellulose II, which is derived by the treatment of natural cellulose. This is an irreversible
conversion since cellulose II is thermodynamically more stable than cellulose I due to shorter H-bond
lengths in its structure [9]. Upon dissolution of cellulose powder in NMMO solvent and coagulation,
the crystalline structure of cellulose was transformed from cellulose I into cellulose II due to the
interaction between cellulose and NMMO [9, 149, 150]. This result is in agreement with previous
reports that use cellulose NMMO and other solvent systems [5, 9, 151]. From Figure 4.2(a), it can be
observed that the cellulose powder has three diffraction peaks around 2θ= 15.2º and 16.4º for (101)
and (101-), respectively, and at 22.8º for (002), which are very close the characteristic peaks of cellulose
I structure [149]. The first two peaks cannot be distinguished very easily due to the very close positions
and similar intensities of the peaks. On the other hand, regenerated cellulose generally shows a
diffraction pattern for cellulose II at 2θ= 12º, 20º, 21.7º for (101) and (101-), and (002), respectively.
73
The most significant diffraction peak for the cellulose membrane prepared in this work (see Figure 4.2
(b)) appeared at around 2θ=12.1º, but the two peaks at 20º-22º are again not distinguished clearly, due
to very close 2θ degrees.
Figure 4.2 X-ray diffractograms of cellulose powder (black) and 25 µm-thick membrane (red).
The percent crystalline material in total cellulose was expressed as crystallinity index (CI), and
CI of MCC powder and cellulose membrane was calculated using Segal equation (4.1) as shown below.
𝐶𝐼 =𝐼𝑐𝑟𝑦−𝐼𝑎𝑚
𝐼𝑐𝑟𝑦 𝑥 100 (4.1)
where Icry and Iam represents the intensity of crystalline and amorphous phase [152, 153]. The peak
with the highest intensity was selected for crystalline phase indicator, which are at 22.8° for cellulose
I and at 12.1° for cellulose II structure. The height of minimum position between 101- and 002 peaks
and the height of minimum position between 101 and 101- peaks were chosen for cellulose I and
cellulose II, respectively; as an amorphous phase indicator as described in literature [152]. CI of
5 10 15 20 25 30 35 40
Inte
nsi
ty (
Counts
)
2θ degree
15.2°
22.8°
21.7°
Type equation here.
12°
74
microcrystalline cellulose powder was generally reported between 83.0 and 65.0 % in literature
depending on the X-ray method and experimental conditions [154]. It was calculated as 80.0 % with a
standard deviation of 3.0 in this study.
The crystallinity index decreased from 80.0±3.0 to 63.0±8.0 % after processing the cellulose
powder into a membrane; in other words, the degree of crystallinity of the powder is higher than the
obtained cellulose membranes. This could be due to the inter- and intra-molecular hydrogen bonds
being destroyed in the cellulose powder by NMMO. Nevertheless, membranes prepared by dissolution
in NMMO still show a high crystallization degree compared to other membranes such as cellophane
membranes (nearly amorphous) [149]. Preserving the intrinsic properties of cellulose is important for
improving the stability of the membrane in the organic solvents.
Density, Porosity, and N2 Adsorption isotherm
In order to understand the structural characteristics of the prepared membranes’ densities,
porosity measurements have been carried out, and BET analysis was used. N2 adsorption at subcritical
temperatures is a routine method for specific surface area determination. The surface area of
microcrystalline powder was determined as 1.5±0.3 m2 g-1, which is in good agreement with values
reported in literature [155, 156]. The measurements were then repeated on pure cellulose membranes,
and a BET surface area of 12.0 ±2.0 m2 g-1 was obtained. Although BET analysis is rarely used for the
characterization of dense polymer membranes, it has applications in the characterization of membranes
made of glassy, semi-crystalline polymers [157].
The theoretical density of cellulose is reported to be between 1.54 and 1.63 g cm-3 in the
literature [158, 159], and it was also tested using the helium pycnometer equipment in our laboratory
giving a density value of 1.6 g cm-3. The weight of the 25-µm-thick dry membrane was measured for
ten different pieces of membranes with known areas and the density is calculated to be 1.1 ± 0.2 g cm-
75
3 while the experimental result obtained from the equipment is 1.2± 0.03 g cm-3. Using the average
values for the density of the powder and the membranes, the porosity of the membrane was calculated
as 25 %.
In addition to the intrinsic porosity of the microcrystalline cellulose (MCC) powder, it is
thought that the crystalline structure and the membrane preparation procedure might result in the
formation of nanopores through the membrane structure. Phase inversion by immersion precipitation
technique usually results in an integrally skinned asymmetric structure due to the phase separation
taking place differently on two surfaces of the membrane [44]. It may also produce asymmetric, porous
membranes in cases of low polymer concentration, high mutual affinity between solvent and non-
solvent, addition of non-solvent to the polymer solution [13].
Scanning electron microscopy
SEM was used to investigate the cross-sectional morphology of the prepared membranes. Pure
cellulose membranes were prepared on polyester support with four different casting knife thicknesses.
Since the membranes were peeled off from the support when dried, all the SEM images were only
taken from the membrane samples without the non-woven backing. SEM photographs of the cross-
sections of cellulose membranes with different thicknesses are shown in Figure 4.3, where the adjusted
knife and real thicknesses are written on each image. As highlighted in section 4.1.1, the thickness of
the dried membranes is almost half of when they were cast on a polyester backing, probably due to the
penetration of the polymer dope solution into the backing support, and this behavior is completely
consistent for all casting thicknesses since the dope concentration was kept constant for all membrane
thicknesses.
All membranes with different thicknesses show very similar cross-sectional morphologies with
homogenous and dense structures. Even in the images at very high magnification no visible separation
layer could be detected by the SEM technique as shown in Figure 4.4. In contrast, Zhang et al. [8] have
76
reported integrally skinned asymmetric structure with very visible sponge-like and finger-like sections
for the membranes prepared with the same method. However, they also observed that membranes with
sponge-like structures without any separation layers is prepared only when using different cellulose
pulp, and it was concluded that the effect of using different cellulose pulps has a great effect on the
membrane morphology [8]. Moreover, low cellulose concentration (10 wt.%), or high affinity between
NMMO • H2O and H2O could form symmetric-porous structures.
Figure 4.3 Cross-sectional views of pure cellulose membranes without backing with different
thickness; A) 500-µm-cast on polyester backing, B) 250- µm-cast on polyester backing, C) 100-µm-
cast on polyester backing, D) 50-µm-cast on polyester backing
C Knife thickness: 100 µm
5 µm 2 µm
D Knife thickness: 50 µm
A Knife thickness: 500 µm B Knife thickness: 250 µm
25 µm 12 µm
77
Figure 4.4 Cross-sectional view of pure cellulose membranes (500-µm-cast) without backing
Contact Angle
Since the prepared membranes were used for liquid applications, their hydrophilicity properties
are important. Cellulose is a naturally hydrophilic polymer with a contact angle of 20-30 [160] due to
the presence of a large number of hydroxyl groups in its structure. The cellulose membranes prepared
via the phase inversion method from NMMO solvent in this work have a contact angle of 40±4º, in the
range of hydrophilic materials [144]. Pure cellulose membrane sample with different thickness were
tested for contact angle measurements and similar results were obtained, because they have similar
morphologies and surface properties. This result implies that water molecules attracts the membrane
surface strongly, therefore water drop tries to spread out on the membrane surface. Moreover, since the
rough surfaces may increase the possibility of hydrophobic surface properties, this result is another
indication for the smooth membrane surface in this study.
Zeta Potential
Streaming potential measurement was performed in order to analyze the surface properties of
the membrane in more detail. The results shown in Figure 4.5 indicate that the membrane surface has
a slightly positive zeta potential (~+10mV) below its isoelectric point (IEP) at pH 3.43 and is negatively
charged above this pH, reaching -50mV zeta potential at the highest pH (~9.5). According to these
78
results it can be concluded that the surface charge of the cellulose membranes prepared in this work is
highly negative at neutral pH conditions.
Significantly negatively charged membrane surface is expected to affect the separation
properties of the membranes due to possible electrostatic interactions. In water, the neutral conditions
result in a negatively charged membrane surface, and the adsorption of positively charged molecules
on the membrane surface is expected. Moreover, the surface charge of the membranes is highly pH
sensitive, and different adsorption behavior could be observed at different pH conditions. In the case
of OSN, the surface charge information is not as meaningful as in the NF, because the zeta potential
experiments were conducted in aqueous medium. Since every organic solvent has its own properties,
they affect the charge of membrane surface and the solid particles in a different way. The adsorption
behavior taking place on the membrane surface is not predictable in organic solvents nanofiltration.
Figure 4.5 Zeta potential of cellulose membrane at different pH values
Thermal gravimetric analysis
TGA was performed to determine the thermal decomposition profiles of cellulose powder and
membrane. Cellulose powder has a different thermal decomposition profile than the cellulose
membrane with a decomposition onset at 270ºC, the maximum decomposition rate temperature is
2 3 4 5 6 7 8 9 10
-60
-50
-40
-30
-20
-10
0
10
Zet
a P
ote
nti
al
pH
79
between 290-320ºC, and complete decomposition occurred at 400ºC. It is shown in Figure 4.6 that
cellulose membrane has a two-step decomposition profile with a final decomposition at 500 ºC. The
two-step decomposition might be an indication of the presence of amorphous cellulose components in
the membranes, whereas MCC powder is showing a complete crystalline structure, as shown before in
the XRD results. For both samples, very slight weight loss was observed up to 100ºC.
Figure 4.6 Thermal decomposition profiles of (A) cellulose powder and (B) cellulose membrane. The
corresponding first order derivatives of TGA curves for cellulose powder and membrane sample are
included for comparison with dashed line.
0
2
4
6
8
10
0
20
40
60
80
100
0 100 200 300 400 500 600
[---
] -d
m/d
T (
% p
erc)
[ __ ]
Wei
ght
(%)
Temperature (ºC)
0
0.2
0.4
0.6
0.8
1
0
20
40
60
80
100
0 100 200 300 400 500 600
[---
] -d
m/d
T (
% p
erc)
[ __ ]
Wei
ght
(%)
Temperature (oC)
A
B
80
The first order derivatives of TGA curves, which are represented with dash lines, are
significantly informative for determining temperatures ate which the maximum mass loss occurs. The
temperature where the maximum weight loss occurs is 314°C for cellulose powder, while it is 306°C
for cellulose membranes. However, significant amount of mass loss rate was also measured at around
470°C for cellulose membrane decomposition.
Mechanical Test
Mechanical properties are not considered as a prior characteristic in membrane processes since
the membrane is usually held by a backing material [1]. However, the membrane still has to be strong
enough to withstand the applied pressure difference. Moreover, not all of the backing materials are
stable in organic solvents and sometimes stable membranes need to be self-supporting for harsh
conditions. Tensile test is not giving the direct information about the stability of the membranes under
high operation pressures but still gave an insight about it. Especially for the industrial scale usage, the
maximum load that membranes can stand might be significant due to harsh working conditions.
Most of the backings are made from non-biodegradable polymers, and using one of these
materials makes it impossible to have a completely green membrane fabrication process from the
making to the disposal of the membrane [2]. The tensile strength and maximum load that membranes
can stand at breakage point for the prepared cellulose membranes with different thicknesses are shown
in Figure 4.7. These mechanical properties were not measured for the thinnest membrane because it is
not really easy to peel off the membranes from the backing as a one-piece sample when they dried,
because of the polymer solution penetration through the backing paper. It is clearly shown that both
the tensile strength and the maximum load that membrane can stand increases when the thickness of
the membrane was increased, as expected. Even for the thinnest membrane measured (5µm), the
maximum load is around 6 N and the tensile strength is 45 MPa, which are significantly high compared
to the reported nanofiltration membranes in literature, because of the semi-crystalline structure of the
81
cellulose. The tensile strength for the cellulose films prepared using trifluoroacetic acid as a co-solvent
was also reported to be quite high at around 63 MPa by Wu and co-workers [161]. On the other hand,
Soroko et al. reported the tensile strength for one of the mostly used OSN membrane material,
crosslinked polyimide, around 10MPa, and they showed that it could be improved to 13MPa by the
addition of 5 % wt. TiO2 in the polymer matrix [91]. The high mechanical strength results obtained in
this section is another proof for that NMMO is not destroying the crystalline structure of the cellulose,
and it preserves all of its characteristics, therefore strong membranes were obtained.
Figure 4.7 Tensile strength and the maximum load with respect to thickness of the membranes.
Membranes were tested for tensile strength and the maximum load without backing paper under them.
4.1.3 Pure solvent flux measurements
All prepared membranes with different thicknesses were tested for pure water and organic
solvent flux measurements in dead-end filtration set-up. Solvent permeance through polymeric
membranes generally decreases significantly over time due to membrane compaction with applied
0
20
40
60
80
100
0 20 40 60
Load
(N)
or
Str
ength
(M
pa)
Membrane thickness (µm)
maximum load (N)
tensile strength (Mpa)
82
pressure [147]. Therefore, most of the OSN membranes should be pre-conditioned with a pure solvent
until a steady flux is reached in order to have reproducible membrane behaviour [162]. The cellulose
membranes prepared in this work respond to pressure quickly, reaching steady state almost
immediately for all thicknesses as clearly seen in Figure 4.8, which might be a consequence of the
semi-crystalline structure of the polymer. Moreover, Figure 4.9 shows membrane performance in a
cross-flow system over a longer period of time. The cellulose membrane flux remains steady without
any compaction for both water and acetone filtrations over 24 hour at 5 bars transmembrane pressure.
Figure 4.8 Pure solvent fluxes through 25-µm-thick membrane for various solvents. Nanofiltration
experiments have been performed in dead-end system at 10bar and 25 ºC.
0 50 100 150 200
20
40
100
120
140
160
180
Flu
x (L
m-2
h-1)
Time (min)
acetonitrile
acetone
ethyl acetate
THF
methanol
water
ethanol
2-propanol
1-butanol
83
Figure 4.9 A) Pure water flux for 24h through 25 µm-thick membrane prepared by phase inversion B)
Pure acetone flux for 24h through 25 µm-thick membrane prepared by phase inversion. Nanofiltration
experiments have been performed in cross-flow filtration system at 5bar and 25 ºC.
Acetone, acetonitrile, and ethyl acetate permeances through the cellulose membrane with 25µm
thickness were 16.4, 13.9, 13.7 L m-2 h-1 bar-1, respectively. In contrast, the most viscous solvent 1-
butanol (2.95 cP) gave the lowest flux of 2.2 L m-2 h-1 bar-1 (Table 4.2). These values are significantly
high in terms of nanofiltration membranes for organic solvents [1, 146, 147, 163]. It is believed that
the homogenous symmetric structure of the highly porous cellulose membranes with nano-sized pores
allow very fast passage of organic solvents that depend on their viscosities as tabulated in Table 4.2.
Moreover, dielectric constant and molar volume values are also tabulated in Table C.2. The molar
volume of the organic solvents was calculated by dividing the molar mass by its density. No direct
relationship was observed between flux values and the molar volumes of the organic solvents.
The fluxes of the solvents were plotted along with their viscosities from Figure 4.10(A) to (D),
and as shown, the solvent flux is inversely proportional to the solvent viscosities. Moreover, we
observed that the flux through the membrane increases linearly with applied pressure in the range 2 bar
to 30 bar (Figure 4.10(E)). Solvent transport through nanofiltration membranes usually occur by
diffusive transport and generally gives very low fluxes [137]. However, viscosity-flux and pressure-
0 5 10 15 20 25
0
10
20
30
40F
lux
(L
m-2
h-1
)
Time (h)0 5 10 15 20 25
0
20
40
60
80
100
120
140
Time (h)
A B
84
flux relationships strongly indicate that Hagen-Poiseuille (HP) equation is applicable to the cellulose
membrane prepared in this work. HP equation is used to explain the viscous flow through the
membranes with nano-sized pores [137] which corroborates the symmetric membrane fabricated in
this study.
Water is slightly deviating from the HP equation, which might be explained by the strong
hydroxyl interactions that occurred between the cellulose membrane and water, creating friction on the
pore walls and hindering the water flow. In similar cases water transports through activated diffusion
with hydrophilic groups in the membranes with hydrogen bonding ability [163]. If a preferential
adsorption of H2O takes place on the pores of cellulose membrane, which are rich in hydroxyl groups,
then the adsorption-desorption process might reduce the water flux while other solvents have no
interaction, and thus leads to higher fluxes. Moreover, the pore size of the cellulose membranes might
get smaller due to the hydration effect of adsorbed H2O molecules, which again can cause a reduction
in fluxes [164]. These low water fluxes may be the result of a combination of adsorption, viscosity,
and hindrance effects.
85
Figure 4.10 Inversely proportional relationship between viscosities of organic solvents and their fluxes
through (A) 25-µm-thick cellulose membrane at 10 bar, (B) 10-µm-thick cellulose membrane at 10
bar, (C) 5-µm-thick cellulose membrane at 2 bar; (D) 2.5-µm-thick cellulose membrane at 2 bar; (E)
Relationship between applied pressure and water flux through a 10-µm-thick cellulose membrane.
Nanofiltration experiments have been performed in dead-end system at 25 ºC.
0 1 2 3 4
0
50
100
150
200F
lux
(Lm
-2h-1
)
Viscosity (cP)0 1 2 3 4
0
50
100
150
200
250
300
350
400
Flu
x (L
m-2
h-1)
Viscosity (cP)
0 1 2 3 4
0
50
100
150
200
250
Flu
x (L
m-2
h-1)
Viscosity (cP)0 1 2 3 4
0
50
100
150
200
250
300
350
400F
lux (
Lm
-2h
-1)
Viscosity (cP)
0 5 10 15 20 25 30 35
0
20
40
60
80
100
120 Flux
Permeance
Flu
x (
Lm
-2h
-1)
Pressure (bar)
E
1. acetonitrile
2. acetone
3. ethyl acetate
4. THF
5. methanol
6. water
7. ethanol
8. 2-propanol
9. 1-butanol
1
3 2
4
5
6
7 8
9
9 9
9
1
1 1
6
6
6
2
2 2
3
3 3
4
4 4
A B
C D
86
Table 4.2 Physical properties of the organic solvents used for nanofiltration and permeances
Solvent MW
(g mol-1)
Surface
Tension
(mNm-1)*
Viscosity
(cP)
Permeance (Lm-2h-1 bar-1)
25µm 12µm 5µm 2.5 µm
acetonitrile 41.1 29.3 0.35 13.9±1.0 31.4±1.7 101.6±4.0 142.0±8.1
acetone 58.1 25.2 0.36 16.6±3.5 34.2±4.6 112.2±7.3 175.6±11.0
ethyl acetate 88.1 23.9 0.42 13.7±1.9 31.9±3.5 99.1±22.3 165.9±32.8
THF 72.1 26.4 0.46 12.9±0.5 23.3±2.7 83.5±11.9 155.0±15.7
Water 18.0 72.8 0.89 3.5±0.3 6.5±1.0 20.4±4.5 41.0±4.6
1-butanol 74.1 25.4 2.95 2.2±0.4 4.9±0.9 18.4±2.0 20.4±1.3
* Surface tension values (20 oC) were obtained from webpage [165] and the reference [137]
**Viscosity values were obtained from CRC Handbook of Chemistry and Physics [166]
4.1.3.1 Effect of thickness on cellulose membrane performance
Permeance is a key parameter, and high flux is desirable for industrial applications for
evaluating any process from an economic point of view. This can be achieved by increasing the
operating pressure, increasing the membranes area, or reducing the membrane thickness [167]. We
have prepared cellulose membranes with four different thicknesses by adjusting the casting knife
thickness. Figure 4.11 shows solvent permeance values for various solvents versus thickness and
1/thickness of cellulose membranes.
87
Figure 4.11 Permeances of various solvents versus A) thickness and B) 1/thickness for cellulose
membranes. Nanofiltration experiments have been performed in dead-end system at 10bar and 25 ºC.
0 5 10 15 20 25
0
50
100
150
200
Per
mea
nce
(L
m-2
h-1 b
ar-1
)
Thickness (m)
acetonitrile
acetone
ethyl acetate
THF
water
butanol
0
40
80
120
160
200
0 0.1 0.2 0.3 0.4 0.5
Per
mea
nce
(L
m-2
h-1
bar
-1)
1/thickness (µm-1)
acetonitrile acetone ethyl acetate THF water butanol
A
B
88
When the entire thickness of the membrane is decreased to half, the fluxes are almost doubled
for all tested solvents which is expected for membranes with a symmetric structure. As the membranes
are dense and symmetric, the membrane thickness is directly proportional to the casting knife thickness
as it is observed in this study. This is another significant approval for our porous symmetric membrane
structure speculation. On the other hand, for ISA membranes, the flux performance does not depend
on the entire thickness of the membrane, instead it is inversely proportional to the skin layer thickness
[44, 168]. Moreover, the thickness of the separation layer in the integrally-skinned asymmetric
membranes cannot be controlled just by changing the casting knife thickness. It is related to fabrication
parameters such as polymer concentration, solvent ratio, forced-convective evaporation time and
casting shear rate [168, 169].
Figure 4.12 presents the thickness- normalized permeance which is named as permeability of
various solvents versus thickness of the membranes. Permeability term is not widely used for liquid
separation applications when using ISA membranes, since the real thickness of the ISA membranes
cannot be determined accurately. Moreover, permeability is a material property while flux and
permeance show the economic benefits of a membrane. It is mostly used for the dense polymeric gas
separation membranes to eliminate the effect of the membrane thickness when comparing the
performance of different membranes [170]. Here it is observed that the permeability of all solvents
through the cellulose membranes is nearly constant for the 2.5 – 25 µm thickness range, cellulose as a
membrane material has a constant permeability characteristic for each different solvent regardless of
the thickness. Only slight deviation is observed for 5µm-thick membrane.
89
Figure 4.12 Permeability of various solvents versus thickness of cellulose membranes. Nanofiltration
experiments have been performed in dead-end system at 10bar and 25 ºC.
4.1.3.2 Stability of cellulose membranes in organic solvents
Structural stability is one of the most important characteristics of an OSN membrane for an
efficient and economic process. Instability can result in negligibly low solvent fluxes due to shrinkage
of the membrane matrix or extremely high solvent fluxes due to swelling or cracking of the membrane
[171]. Yang et al. [171] suggested that visual observation of membranes soaked in solvents could be
used to provide an insight into the stability/durability of a membrane.
Cellulose membranes prepared in this study exhibited outstanding stable filtration performance in
water and in various polar protic and aprotic organic solvents for long durations. Although the
membranes were hydrated in water, the performance was almost in line with the other organic solvents
according to the HP equation without any extra pre-conditioning step needed or a compaction period,
due to their semi-crystalline structure. In order to investigate the stability of the membranes in organic
solvents, membrane discs were soaked in organic solvents for a week, and no visual change was
0 5 10 15 20 25
0
1
2
3
4
5
6
THF
water
butanol
Per
mea
bil
ity (
Lm
-2h
-1bar
-1m
)*10
4
Thickness (m)
acetonitrile
acetone
ethyl acetate
90
observed for any of the solvents. Moreover, to test the stability under experimental conditions, one
membrane disc was soaked in one organic solvent overnight, then 3 hours of pure solvent filtration
experiment was run with the same solvent under a 10 bar operating pressure. The same membrane disc
was used to test all the organic solvents in a random sequence and then the first solvent was tested
again to prove that there is no performance change after the membrane was subjected to several
solvents. Figure 4.13 shows the organic solvents permeance performance of 25 µm-thick cellulose
membrane for eleven successive filtration experiments in a random sequence. The high stability
observed in the performance of the membranes was not affected by decreasing the cellulose membrane
thickness. No stability change has been recorded for the thinner membranes for long term experiments
either.
Figure 4.13 Solvent permeance performance of a 25 µm-thick cellulose membrane disc for eleven
successive filtration experiments; orange for water, black for acetonitrile, grey for acetone, red for ethyl
acetate, blue for THF, green for 1-butanol. Filtration experiments have been performed in dead-end
system at 10bar and 25 ºC.
It is also reported by Livazovic et al. [148] that cellulose membranes prepared by ionic liquid
dissolution are resistant to THF, hexane, DMF, NMP and DMAc. They tested the stability by
0
2
4
6
8
10
12
14
16
18
0 1 2 3 4 5 6 7 8 9 10 11 12
Solv
ent
per
mea
nce
(L
m-2
h-1
bar
-1)
Order of experiments
91
measuring the weight of the membranes before and after immersing in these solvents for 1h, 12h, 24h,
and one week, and they did not observe any visible change and weight loss probably due to strong
hydrogen bonds and crystallinity [148].
4.1.4 Rejection performances
25-µm-thick cellulose membranes were tested for rejection capabilities in water and organic
solvents that were tested in section 4.1.3. Eight dyes with different charges (+, neutral, -) and different
molecular weights changing between 245 and 1020 g mol-1 were being used for the dead-end and cross-
flow filtration methods. Pure solvent fluxes through the membrane were reported in section 4.1.3.
4.1.4.1 Water
In this section, filtration of some dyes in water through the cellulose membranes is presented
in the dead-end configuration. All dead-end rejection experiments were conducted at 10 bar
transmembrane pressure. All rejection experiments were repeated for at least 4 times for different discs
from separately cast membrane sheets to see the reproducibility of membrane synthesis and
experimental procedure, and average of them are reported. Maximum deviation for the rejection values
was calculated as 7% for water experiments. The structural characteristics of the dyes used was not
considered in detail to avoid the complexity, only their molecular weight and the charges were
considered.
Pure water flux was measured as 35 L m-2 h-1 through the 25µm-thick membrane, and no
difference was recorded for each dye solution in long time rejection experiments. Flux decline is
generally caused by concentration polarization or adsorption of molecules onto the membrane surface
[25, 143, 163, 172-174], and this fouling causes practical problems such as lower permeate yield,
higher energy consumption due to higher operation pressures, more chemical agents required for
membrane cleaning, and shorter membrane life time [143]. Van der Bruggen et al. [143] reported
92
significant water flux decline due to adsorption of organic compounds onto the surface of the two
commercial membranes namely UTC-20 (made of polypiperazineamide) and NF-70 (made of
polyamide). In our case, similar fluxes of dye solutions and neat solutions show that there is no
significant concentration polarization effect at this dye concentration [163]. Gomes et al. [173] also
suggested that flux decline by concentration polarization is insignificant for the dye concentration in
the range of wool textile industry (30- 50 mg L-1).
Table 4.3 Separation performance of the membrane for different charged dyes in water (25μm-thick
membrane)
Dyes Charge MW (gmol-1) Rejection (%)
RB - 1018 99.9
CR neutral 696 99.3
CV + 408 99.1
NB - 400 99.3
MO - 327 71.5
CSG + 249 94.4
HNSA - 246 55.0
HTMC neutral 250 67.7
Table 4.3 tabulates the rejection values for several dyes with different charges in dead-end
module at neutral pH conditions and MWCO curve of the membrane is represented in Figure 4.14.
Results in the table could be evaluated in two parts: large molecules and small molecules in terms of
molecular weight. These two different rejection behaviours could also be seen clearly from Figure 4.14.
In the case of large molecules, rejection was calculated around 99% regardless from the solutes’ charge.
93
It might be concluded that up to 400 gmol-1, molecular sieving effect is more dominant on the
separation mechanism occurring through the membrane in water.
Molecular charge can be a decisive factor for determining the retention of the molecule only
when the size of the membrane pores is much larger than the size of the molecule (charge effect),
otherwise the rejection of the molecules is governed by size exclusion namely sieve effect [164, 171].
Van der Bruggen et al. [175] investigated the effect of molecular size, polarity, and charge of the dyes
on the retention performance of nanofiltration membranes in aqueous solutions, and they concluded
that charge effect is important when the pores’ size is much bigger than the solutes’ size.
Figure 4.14 MWCO curve of cellulose membrane. Nanofiltration of feed solutions comprising
different dyes dissolved in water have been performed separately at 10 bar and 22°C.
The transport mechanism for smaller molecules seems to be more complex than that of larger
molecules, of which results are framed with red lines in Table 4.3. Three dyes with molecular weights
around 250 g mol-1(they are assumed to have similar molecular sizes), and with different charges (+,
neutral, -) were chosen to investigate the behaviour of the membrane for smaller sizes. They do not
seem in line according to the MWCO curve, especially the rejection value with the blue star on it
0
10
20
30
40
50
60
70
80
90
100
200 400 600 800 1000 1200
Rej
ecti
on (
%)
Molecular weight (g mol-1)
94
deviated a lot. Positively charged dye CSG (which is the blue star in curve) is 96.5 % rejected while
negatively charged MO is only 71.5 % rejected even though it has a higher molecular weight than CSG.
In addition, negatively charged HNSA and neutral HTMC are rejected around 65% by the membrane,
which implies that the negatively-charged cellulose membranes (reported in section 4.1.2.) are
preferentially rejecting positively charged dyes with a MW below 400 g mol-1 as opposed to the
working mechanism of Donnan exclusion [163, 164, 171, 175, 176]. In the Donnan exclusion
mechanism, membranes repel the co-ions (i.e. the ions which have the same charge with the membrane
surface), and an equivalent number of counter-ions are also retained to satisfy the electroneutrality
[31]. This means negatively charged membrane surface rejects the negatively charged ions. Van der
Bruggen et al. [175] reported Zirfon membranes with wide pores and negatively charged surface,
rejected the negatively charged dyes more than positively charged and neutral dyes with similar
molecular weights. Zhao et al. [164] reported significantly higher rejections for positively and
negatively charged dyes than for the neutral dyes.
In our case, the electrostatic interaction between the positively charged dye and the membrane
surface was visible after rejection experiments, the membrane was dyed yellow after CSG, while it was
almost clean after MO rejection test as shown in Figure 4.15. According to a mass balance, the sum of
the absorbance of permeate and retentate should be equal to the absorbance of feed. However, it is not
valid for CSG case due to the adsorbed amount of dye on the membrane surface. Both ultra-violet
visible absorption spectra of CSG and MO, and photograph of membranes after rinsing with MeOH
after rejection tests prove that CSG (+) is adsorbed on the membrane surface more than MO (-), when
the surface charge of the membrane is negative.
Since most of the nanofiltration membranes are charged, adsorption is an expected phenomena
[177]. All the dye molecules have functional substituents such as sulphonic, amino, and hydroxyl
groups bound to the aromatic rings. These groups could interact with the membrane since the
membrane also has functional hydroxyl groups. Multiple interactions could be responsible for the
95
adsorption mechanism, such as van der Waals, electrostatic, hydrophobic, and hydrogen bonds [173].
The adsorption experiments were conducted at different pH values where the membrane surface has
negative (pH: 6.0), neutral (pH: 3.4), and positive (pH: 2.4) zeta potentials according to the result of
streaming potential experiment presented in section 4.1.2, in order to provide more evidence for
electrostatic interactions during adsorption. The performance of the membrane was tested again at
neutral pH conditions after different pH experiments so as to investigate the stability of the membrane
at harsh pH conditions. The pH of deionized water was adjusted using HCl solution, and the pH values
were measured with JENWAY 4330 conductivity & pH meter.
Figure 4.15 (A) Ultra-violet visible absorption spectra of CSG; blue for permeate, red for retentate,
black for feed. (Inset) Photograph of membrane after rinsing with MeOH after rejection test. (B) Ultra-
violet visible absorption spectra of MO; blue for permeate, red for retentate, black for feed. (Inset)
Photograph of membrane after rinsing with MeOH after rejection test. (25μm-thick membrane). All
experiments were conducted at pH 5.5 conditions.
At pH 2.4, the membrane surface has a positive zeta potential, and the rejection of HNSA (-) is
higher than the rejection of CSG (+), because of the adsorption between the positively charged
membrane surface and negatively charged solutes. HNSA rejection (77.8%) by the positively charged
membrane surface is not as high as the rejection of CSG (95%) by the negatively charged membrane
surface, which might be explained by the lower level of positive zeta potential (+8.2 mV) than negative
0
0.5
1
1.5
2
2.5
300 350 400 450 500 550 600 650
Abso
rban
ce
Wavelength (nm)
0
0.4
0.8
1.2
1.6
2
300 350 400 450 500 550 600 650Wavelength (nm)
A B
96
conditions (-30mV). Moreover, the experimental confidence could be questioned due to insignificant
pH difference between 3.43 and 2.43, and membrane might be close to neutral conditions.
On the other hand, when the surface charge of the membrane is neutral around isoelectric point
(IEP), very similar rejection values around 72 % were obtained for all dyes regardless of their charge.
At the isoelectric point, the membranes are likely to present non-ionised acid and basic groups, so the
uptake of dyes is lower [173]. The similar rejection levels could be explained only by the molecular
sieving mechanism without any adsorption or by the adsorption mechanism governed by other types
of weaker forces. For instance, van der Waals forces contributes to dye aggregation, which enhance
the adsorption efficiency [173]. Gomes et al. [173] suggested that adsorption is the main phenomena
for the separation of acid orange 7 from a wool textile dye solution. They also reported that pure water
flux through membranes decreased because the pore size of the membrane was being reduced due to
the adsorption of dye molecules on the membrane surface and inside the pores. In our study, no flux
decline was being observed during dye solution filtration in dead-end system.
Table 4.4 Separation performance of the membrane in water at different pH values (25μm-thick
membrane)
Moreover, as seen in Table 4.4, the performance of the membrane did not change after testing
at different pH conditions, and almost same rejection values were obtained in the first and the last
experiments conducted at pH 6 conditions proving that our membranes are stable at harsh pH
pH Zeta potential
of membrane
Pure water flux
(Lm-2h-1)
Rejection (%)
CSG (+) HTMC (0) HNSA (-)
6.0 -30 mV 36.0±6.3 95.0±2.8 55.0±9.7 67.7±7.3
3.4 0 41.0±8.5 72.8±4.2 72.6±15.2 70.7±0.9
2.4 +8.2 mV 40.0±8.5 69.1±5.2 90.8±4.0 77.8±4.5
6.0 -30 mV 40.0±8.5 93.7±0.9 55.4±4.3 62.9±4.2
97
conditions. Figure 4.16 visualizes the rejection results for CSG at different pH values with the
photographs of permeate and retentate collected from the experiments. Analysis of variance (ANOVA)
test was used to evaluate the statistically significance of the variations in the rejections at different pH
conditions tabulated in Table 4.4. Test results suggest that the variations are statistically significant
with 99 % confidence for CSG (+) and HTMC (0), while the variation in HNSA (-) rejections is 90 %
significant. Therefore, it can be concluded that the effect of pH is important on the rejection capacity
of the membrane.
Figure 4.16 Photographs of permeate (left) and retentate (right) of CSG dye at different pH values
(25μm-thick membrane)
Batch adsorption experiments were also conducted to understand the mechanism occurring
between the membrane surface and solutes, and it was found that the adsorption process had already
started before any pressure was applied. In the batch adsorption experiments, membrane samples were
cut and inserted in the same membrane cell which is used during filtration experiments. Then the dye
solutions were poured into the membrane cell, and it is sealed very well to prevent any water
evaporation, and no pressure is applied for 1-hour experiment. As soon as the solution was poured into
the membrane cell, solute molecules adsorbed onto the surface. Figure 4.17 shows the batch adsorption
results conducted for 1h for MO and CSG. While only 1% of the total MO (0.004mg, calculated by
pH: ~6.00
R: 96.6 % pH: 3.43
R: 75.7% pH: ~6.00
R: 94.3 % pH: 2.43
R:72.8 %
98
mass balance) was adsorbed by the membrane surface, 28% of CSG (corresponds to 0.113mg) was
adsorbed in just 1 hour of batch experiment. Since the extent of adsorption could be determined by the
type of solute, the solute concentration, and the pH [177], more detailed adsorption results will be
reported in next section. Finally, membranes after rejection or batch adsorption experiments were
washed with fresh water for 2-3 hours in order to examine if the membrane could be cleaned by
filtrating the dyes through it. If the adsorbed dyes were not attached strong enough or the size of the
dyes smaller than the pore size of the membrane, they would go through the membranes when they are
washed with water. However, no dye came out from the permeate side of the membrane because the
adsorption is strong.
Figure 4.17 (A) Ultra-violet visible absorption spectra of MO; red for before experiment, black for
after experiment (Up) Photographs of membranes before and after adsorption experiments. (B) Ultra-
violet visible absorption spectra of CSG; red for before experiment, black for after experiment (Up)
Photographs of membranes before and after adsorption experiments. (25μm-thick membrane)
0
0.4
0.8
1.2
1.6
2
300 400 500 600
Abso
rban
ce
wavelength (nm)
A
0
0.2
0.4
0.6
0.8
1
300 400 500 600
Abso
rban
ce
wavelength (nm)
B
Before adsorption After adsorption
MO (-) CSG (+)
Before adsorption After adsorption
99
4.1.5.2 Organic Solvents
Rejection performance of the 25µm-thick cellulose membranes were tested using several dyes
in acetone, acetonitrile, ethyl acetate, THF, ethanol, methanol, and 1-butanol in a dead-end filtration
system at 10 bar operating pressure. No variations were recorded again between pure solvent and dye-
solvent fluxes for all systems, implying insignificant concentration polarization effect [163].
Rejections results are summarised in tables 4.5 and 4.6 for all organic solvents and dyes tested.
All rejection experiments were repeated at least 4 times using different discs from separately cast
membrane sheets to see the reproducibility of the membrane synthesis and experimental procedure.
The average values were taken, and the maximum deviation for the rejection values was calculated to
be 12% for the organic solvent experiments. Considering the rejection values in Table 4.5 and 4.6
together, it could be easily said that there is no straightforward theory that can explain the behaviour
of the membrane. While MWCO concept does work for water case, it does not work at all for organic
solvents. Percent rejection of a dye through the same membrane depends on the solvent tested, while
the percent rejection of dyes in the same solvent depends on the molecular weight of the dyes tested.
The steric and electrostatic separation mechanisms cannot be extended to non-aqueous systems easily
due to very different structures and properties of the organic solvents [163]. Therefore, several
variables that are related to the solvent, solute, membrane, and process properties should be considered
[144], and each solvent should be discussed separately.
Table 4.5 Rejection performance of the membrane in ethyl acetate, and THF (25μm-thick membrane)
Solvents Dye rejection (%)
RB
1018 g mol-1
CV
696 g mol-1
MO
327 g mol-1
Ethyl Acetate 99 96 98
THF 99 99 insoluble
100
As discussed in previous section, CSG (+) was the most rejected dye in water by adsorption
mechanism (due to charge effect), even though it has the smallest molecular weight. However, it is not
rejected as much as one of the bigger dyes MO (-) by the membrane in organic solvents. This could be
explained by using four different theories from literature [163].
The effective size of a dye might be smaller in organic solvents than in water, because of the
complexation of water molecules with the solute. Yang et al. [171] discovered higher dye rejection
values in water than in methanol and used this assumption to explain their results. In this study, this
explanation might be used for CSG rejection, however when it comes to MO, for instance, the higher
rejections were observed in acetone, acetonitrile, ethyl acetate, and butanol rather than in water.
In another explanation, researchers claimed that the reason of lower rejections in organic
solvents is the improved mobility of polymer chains due to their contact with organic solvents [174].
If this is the case, MO rejection should not be so high in organic solvents (94-99%), while it is only
rejected 70% in water.
In the third theory, hydration/solvation mechanism was used to explain the lower rejections in
organic solvents than in water for hydrophilic membranes. In this mechanism, rejection profile strongly
depends on the hydrophilic/hydrophobic nature of the membrane. When the membrane has a
hydrophilic nature, hydration of the membrane pore walls decreases the effective pore size and the
rejection in water is improved, vice versa, when the membrane is hydrophobic, it is solvated when in
contacted to organic solvents and the rejection is improved [163, 164]. Geens et al. [178] explained
higher raffinose rejections in methanol compared to water through hydrophobic membranes is caused
by the solvation effect, while the hydration effect is the reason for higher raffinose rejections in water
through a hydrophilic membrane. Membranes prepared in this work are hydrophilic [144] with a
contact angle of 40°. Pure water flux through them should be lower than the other solvents (except
butanol due to very high viscosity), and rejections in water should be higher for all dyes due to the
hydration effect, but MO is again an exception for this explanation.
101
Finally, the charge effect may be deactivated in non-aqueous systems because the zeta potential
of membranes’ surfaces change in different organic solvents [163]. Zhukov et al. [179] investigated
the electro-surface properties of non-aqueous system for a wide range of organic solvents, and they
reported that surface charge might be present in solvents, but the charge formation mechanisms depend
on solvent classes. In this study, it could be said that the charge effect is obviously not dominant in any
of the solvents for CSG rejection while it was a strong parameter for the water case. Both Zhao et al.
[164] and Yang et al. [171] suggested that charge effect is negligible in organic solvents.
None of the mechanisms above is adequate to explain the rejection behaviour of the membranes
alone. In such systems the molecular affinity between the solvent, solute, and membrane becomes
critical [163], and the combined effects of the mechanisms should be discussed. Physical properties
and Hansen solubility parameters of the solvents are given in Appendix C, Table C.1. Moreover, Table
C.2 represents the Hansen solubility parameters for dyes and cellulose calculated by the group
contribution method.
Table 4.6 Affinities between membrane-solute and membrane-solvent
Dyes
(g mol-1) Solvents
Acetone
Butanol Acetonitrile Ethanol Methanol
RB (1018)
Rejection 99 99 99 88 74
Membrane-solute 15.3 15.3 15.3 15.3 15.3
Membrane-solvent 13.8 10.5 9.3 7.2 4.3
Solute-solvent 29.1 25.8 24.6 22.5 19.6
CV (696)
Rejection 94 Nt* 98 Nt* Nt*
Membrane-solute 12.7 - 12.7 - -
Membrane-solvent 13.8 - 9.3 - -
Solute-solvent 1.1 2.2 3.4 5.5 8.4
MO (327)
Rejection 97 94 97 67 57
Membrane-solute 10.8 10.8 10.8 10.8 10.8
Membrane-solvent 13.8 10.5 9.3 7.2 4.3
Solute-solvent 3.1 0.2 1.4 3.5 6.4
CSG (249)
Rejection 88 76 70 65 50
Membrane-solute 8.2 8.2 8.2 8.2 8.2
Membrane-solvent 13.8 10.5 9.3 7.2 4.3
Solute-solvent 5.6 2.3 1.1 1.0 3.9
*Nt: not tested
102
The membrane-solute, membrane-solvent, and solute-solvent affinities calculated by
subtracting the solubility parameters of individual membrane, solvent, and solutes are given in Table
4.6 for five different solvents and four different dyes. THF and ethyl acetate were not included in the
list due to solubility constraints of some dyes in them. From top to bottom, for the same solvents, since
the membrane-solvent affinities are the same for all dyes, the effect of different solutes on the rejection
performance was investigated by looking at the membrane-solute affinity values. From the dye RB to
CSG, the membrane-solute affinity increases from 15.3 to 8.2, and this results in a slight decrease in
rejection performance as expected [144].
Considering each solvent separately, it could be said that the rejection of CSG is lower than the
rejection of RB in acetone, because |𝑆𝑠𝑜𝑙𝑢𝑡𝑒 − 𝑆𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒| is lower than |𝑆𝑠𝑜𝑙𝑣𝑒𝑛𝑡 − 𝑆𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒| for
CSG, but higher for RB. If the experiments were run for a longer period of time, the concentration of
MO and CSG in the permeate might be higher than the concentration in the retentate, which results in
negative rejections. Matsuura et al. [180] and Burghoff et al. [181] reported significant negative
rejections for phenol separation, while Koops et al. [182] reported for docosanoic acid in hexane
through cellulose acetate membranes. Moreover, negative rejection of solvent dyes was also reported
in hexane by polyimide membranes. For ethanol and methanol cases, |𝑆𝑠𝑜𝑙𝑢𝑡𝑒 − 𝑆𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒| is always
higher than |𝑆𝑠𝑜𝑙𝑣𝑒𝑛𝑡 − 𝑆𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒|, which is expected due to high rejections. However, the relative
difference is decreasing from top to bottom. Moreover, solute-solvent affinity is increasing from RB
to CSG in ethanol and methanol solvents, which results in lower rejections [144]. In addition to the
affinity properties, the molar volume and the molecular weight of the dyes are shown in Table 3.1 and
are decreasing in the same way, which might give the same response.
From the left to right, increasing solute-solvent affinities could explain the decrease in the
rejection performances for all dyes, with only three exceptions that are shown inside the red boxes in
Table 4.6. Ethanol and methanol are behaving slightly different to the other solvents in terms of
103
rejection performances even though their fluxes are consistent with others, the details will be discussed
in section 4.1.5.
Electrostatic interactions are obviously not dominant for the separation of dyes in organic
solvents, but they can still have a slight effect on the mechanism. Therefore, batch adsorption
experiments conducted in water were repeated for acetone and acetonitrile and the results are given in
Table 4.7. Data for water is presented again for comparison. The highest percentage of MO (-)
adsorption by the membrane surface was measured in acetonitrile, while the lowest adsorption took
place in water. 28% of CSG was adsorbed in water by the negatively charged surface in 1-hour contact
time while only 3 and 9% were adsorbed in acetone and acetonitrile, respectively.
Table 4.7 Adsorption of MO and CSG on the membrane surface
Solvents
Dyes
MO CSG
Adsorbed amount
on surface (mg)
Percent adsorption
of the total dye (%)
Adsorbed amount
on surface(mg)
Percent adsorption
of the total dye (%)
Acetone 0.020 5 0.013 3
Acetonitrile 0.030 7 0.040 9
Water 0.004 1 0.113 28
Since the surface charge of the membrane is negative in aqueous media, the electrostatic
interactions, thus the adsorption of CSG is maximum while the adsorption of MO on the membrane
surface is minimum, as expected. To the best of our knowledge, there is no information about the
surface charge of the cellulose membranes in organic solvents. Zhukov et al. [179] investigated the
electro-surface properties of non-aqueous systems for a wide range of organic solvents, and they
104
reported that surface charge might be present in solvents, but the charge formation mechanisms depend
on solvent classes. Considering the low adsorption percentages of dyes, it might be speculated that the
surface charge of the membrane is not strongly negative or positive in acetone and acetonitrile media.
It might be slightly positive in acetone (MO was adsorbed more), and slightly negative in acetonitrile
(CSG was adsorbed more). Moreover, high dissolution of the dyes in water might result in stronger
electrostatic interactions, and thus more dominant adsorption.
4.1.4.3 Cross-flow filtration experiments
It is to be noted that due to limited time (since the cross-flow filtration set-up does not belong
to our group), only a few cross-flow experiments have been conducted. Rejection of the biggest dye
RB (-) was tested in water, acetone, and acetonitrile, while CR (0) was tested only in water. The test
duration for CR was 100 h, and 24h for RB, which are long enough to observe the transitional state
and the establishment of the steady state, and therefore is a good compromise between time and
accuracy of the final value.
Firstly, RB rejection experiments were conducted in water, acetone, and acetonitrile using a
new membrane sample for each solvent, in order to restrain a high volume of MeOH waste for cleaning
the membrane and the system. After sampling regularly, it was observed that the concentration in
permeate was greatly increasing, while the feed concentration was decreasing, which implies that the
membrane is not rejecting properly in continuous system. It seems to reject until the third measurement
(15 minutes for water), and after that the feed and permeate have the same concentration, implying no
further rejection. The situation is similar for acetonitrile and acetone.
However, according to the mass balance made using the fluxes of the feed, two permeates and
the dye concentrations in them, a significant amount of dye was removed from the solution, which
should have adsorbed onto the membrane surfaces. To assess that, the evolution of the normalised
105
concentration Ct/C0 over time for water, acetonitrile and acetone were plotted in Figure 4.18. C0, the
dye concentration in the feed solution, was 20 mg.L-1.
Figure 4.18 Normalised concentration over time for pure cellulose membrane tested in water,
acetonitrile and acetone
The Ct/C0 ratio is useful to observe and estimate the amount of adsorption taking place in the
system. Indeed, it gives us the mass percentage of dye still present in the solution. The first observation
that can be made is that adsorption has a very low transitional state for acetonitrile and acetone when
compared to water, and indeed, after 1 hour of experiment, the ratio Ct/C0 has already reached its
constant value of 0.45 and 0.60, respectively. Water’s adsorption transitional regime is much longer
and can be estimated to be finished after 23 hours, where the ratio is 0.30, and adsorption is much
higher than in the solvents.
Those values for steady state gave us the amount of adsorption on the membranes. Two
membranes were tested in series with recirculation, and the upstream feeds were assumed to be
comparable due to mixing. Therefore, the adsorbed amount by a single membrane was calculated using
the following equation by dividing by 2:
(Ct/
C0)
106
𝑚𝑔 𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑 𝑝𝑒𝑟 𝑐𝑚2 =1
2𝑥 (1 −
𝐶
𝐶𝑜 ) 𝑥
𝑉.𝐶𝑜
𝑆 (4.2)
where V is the volume of the feed and S is the surface area of each membranes.
Percentage dye adsorption is calculated by the following equation [28]:
𝐶(%) =1
2𝑥 (1 −
𝐶𝑡
𝐶𝐹) 𝑥100 (4.3)
where 𝐶𝐹 is the absorbance/concentration of feed solution, 𝐶𝑡 is the absorbance/concentration of feed
solution at time 𝑡. The equation was divided by 2 again to obtain the percentage adsorption by a single
membrane. These experiments have shown that adsorption is a huge phenomenon taking place in
continuous processes, with membranes saturating and then letting the dye permeating through.
Therefore, it can be assumed that the RB rejections in batch processes were mainly due to temporary
adsorption.
Table 4.8 The amount of adsorbed RB on the membrane surfaces in different solvents (V is assumed
constant at 300 mL (no effect of sampling) and S is 14 cm2)
Solvent Adsorbed dye per area (mg cm-2) Percent dye adsorption (%)
Water 0.15 35
Acetonitrile 0.12 23
Acetone 0.09 20
Comparing the amount of RB dyes adsorbed by the membranes in water and in other solvents,
it is obvious that water is a more suitable medium for adsorption to be take place, even though RB has
107
a negative charge. Actually, it is not really fair to compare the amount of RB adsorption in water and
organic solvents directly, since they have completely different characteristics such as polarity, Hansen
solubility parameters and dissolution capacities for ionic compounds. While water is the best medium
for ion dissociation of molecules, there is not enough data for the organic solvents. Moreover, it is easy
to detect the zeta potential of the membranes and predict the behaviour of the dye molecules in water
medium, while it is not possible in organic solvents. Cellulose membranes have highly negative
surfaces with functional hydroxyl groups, and since RB is dissociated as positive (Na+) and negative
ions in water completely, Donnan exclusion mechanism [31] could be responsible for co-ions
adsorption by the membrane surface. Na+ ions interacted with the membrane surface and to satisfy the
electroneutrality condition, an equivalent number of negative ions were adsorbed on the surface. 1-
hour batch adsorption results for + and – charged dyes gave an insight that adsorption of positively
charged dyes would be significantly higher than this value in 24h cross-flow conditions, however, we
had no chance to measure it in this study. Due to the very strong interaction of the solutes with the
membrane, affinity properties seem less important for the separation mechanism.
On the other hand, the adsorbed amounts in acetone and acetonitrile cannot be underrated. Since
there is no information about the charge of the membrane surface and the dyes, it is not easy to discuss
the adsorption mechanism. It might be attributed to charge effects but in a different way [179], or the
adsorption mechanism might be governed by other forces such as van der Waals, hydrophobic, or
hydrogen bonds [173].
The rejection experiments for neutral dye CR in water gave very promising results compared
to RB. Figure 4.19 shows the flux and rejection performances of two identical membrane discs for the
100 h cross-flow experiments. The two membranes showed very similar performances in flux and
rejection.
108
In Figure 4.19A, it is shown that the membranes have very stable flux performance over 100 h
as in pure flux measurements, and the addition of CR did not affect their stability. Flux was slightly
decreasing with the effect of compaction for the first 3 hours, and then addition of CR resulted in a
further decrease (4% for one membrane, 8% for the other) within 1 h, which might be still caused by
compaction. High reductions of up to 65% in flux values were reported for NF membranes in literature
[143, 171, 177]. Flux reductions could also be explained by concentration polarization, fouling,
blocking of pores, adsorption, and hindered diffusion within the pores [171]. Since the flux decline was
observed in the first hour of the dye addition, it is not easy to evaluate it as a concentration polarization,
fouling or adsorption outcome. Because they are all expected have taken place slowly through the 100
h period. The membranes’ pores, which are in the same size with CR, were more likely blocked when
in contact with the dye solution.
Rejection of the CR neutral dye by the membrane was measured to be around 95% for the 100
h experiment. The permeate side was always clear while the concentration of feed decreased with time,
which is an indicator of adsorption. The photograph of the membrane after the experiment is also
proving that the adsorption phenomena can be seen visually, in Figure 4.19B. Percentage adsorption
of CR by the membrane surface was calculated as 35%, which is the same amount of RB adsorbed.
However, it should be noted that the CR experiment was run for 100 h, 4 times longer than the RB
experiment. Charge effect on the adsorption capacity was obvious for short-term dead-end and batch
adsorption experiments, but it started to be insignificant when the experiments got longer. The
adsorption of all dyes with positive, negative and neutral charges on the membrane surface might be
explained by their interactions with the hydroxyl groups on the surface of the membranes [179].
Because of the very limited data for long-time cross-flow experiments, it is not reasonable to make
global deductions. Additional experiments are required to understand the phenomena in detail, to
extend the boundary of the discussion.
109
Figure 4.19 Experimental results of cross-flow filtration of CR dissolved in water by 25 µm-thick
cellulose membrane. Filtration experiments were run at 5 bar operation pressure and 55 L h-1 flow rate.
Results for 2 identical membrane pieces are shown in the figures for repeatability. A) Flux performance
of the membrane for CR-water solution with respect to time, B) Percentage rejection of CR in water
(inset) Photograph of the membrane after 1-week cross-flow experiment.
20
22
24
26
28
30
0 20 40 60 80 100
Flu
x (
Lm
-2h
-1)
60
80
100
0 20 40 60 80 100
CR
rej
ecti
on (
%)
Time (h)
CR
addition
B
A
110
4.1.5 Cleaning of membranes- reusability
Flux values of alcohols through the membrane seem in line with all other solvents according to
HP type behaviour. However, the rejection performance of the membranes in alcohols is lower than
those in water and other solvents, and the rejections are decreasing in the order of butanol> ethanol>
methanol. Darvishmanesh et al. [144] reported higher methanol fluxes through STARMEM
membranes, which might be caused by i) higher affinity of alcohols to the membrane, or ii) increase in
the pore size of the membrane due to swelling. Since they recorded high rejection performances in
methanol, they accepted the first reason for their case. As shown in Figure 4.20, MWCO of the
membrane is 300 gmol-1 for butanol, while it does not even reach 1000 g mol-1 for methanol. MWCO
is not a sufficient descriptor for organic solvents [176], but Figure 4.20 gives an insight about the
dependency of rejection capability on the solvents.
Figure 4.20 MWCO curve of cellulose membrane in alcohols. Nanofiltration of feed solutions
comprising different dyes dissolved in methanol, ethanol, and 1-butanol have been performed
separately at 10 bar and 22°C.
In our case, since the rejections are lower in alcohols than in other solvents, the pore size of the
membranes should be enlarged reversibly due to swelling in alcohols. The degree of swelling increases
with increasing membrane-solvent affinity for all dyes regardless of the membrane-solute affinity. Not
any specific swelling experiment was conducted, these deductions are made from the flux and rejection
0
20
40
60
80
100
200 300 400 500 600 700 800 900 1000 1100
Rej
ecti
on
(%
)
Molecular weight (g mol-1)
1-butanol
ethanol
methanol
111
performance of the membranes in methanol, ethanol, and 1-butanol. Since the swelling is reversible,
and all the properties of the membranes returns to the original after testing with alcohols, MeOH was
proposed as a cleaning agent between dye rejection experiments. In literature, very harmful chemicals
are being usually suggested as cleaning agents, such as; sodium hydroxide (NaOH), hydrochloric acid
(HCl), trisodium phosphate, sodium tripolyphosphate, ethylenediaminetetraacetic acid (EDTA),
sodium dodecyl sulfate (SDS) [183-186].
For all of the solvents tested, the membranes were washed with MeOH in between the dye
rejection experiments, and before the second dye the solvent of interest was filtrated through the
membrane to make sure that all the MeOH was removed. The length of MEOH washing and solvent
filtration times were optimized after several experiments as 15-30 min (depending on the dye), and 10
min, respectively. Both pure flux and dye rejection measurements were conducted for the
characterization of membranes after MeOH cleaning. Controlled rejection experiments which have
been conducted for the same dye in the same solvent through the MeOH washed and the neat membrane
showed that neither the membrane structure is destroyed nor undergone swelling due to the collapsed
MeOH molecules during cleaning. In other words, the present study suggests that this cleaning method
does not have any major effects on the flux and rejection performance of the membranes. The
adsorption is thought to be physisorption due to electrostatic interactions between membrane surface
and dyes, given the zeta potential of the membrane and the reversibility of the process. Indeed, a
chemisorption of the products would have changed the cellulose surface structure. Ahmed Al-Amodui
[183] reported that chemical cleaning had a major effects on the performance and the surface properties
of several commercial NF membranes. In another work of his, they reported that the cleaned NF
membranes have higher permeability and lower rejection than the virgin NF membranes due to lower
adsorption phenomena, probably because of modified surface charge properties [184].
Each membrane disc was used for rejection tests in several solvents in a non-specified order,
because it was shown in section 4.1.3.2 that membranes are stable in all organic solvents regardless
112
from testing order. Moreover, cleaning cycles were repeated at least 40 times for one membrane disc
(each time membranes were cracked or destroyed for another reason, thus changed to other discs), and
the performances were maintained perfectly. In other words, membranes prepared in this study were
reusable for many times, and significantly improves the efficiency of the process from the economic,
scientific, and environmental point of view. Many studies in literature reported that the performances
or surface properties of membranes change after repeated cleaning cycles because of the harsh cleaning
agents used and unstable membrane materials [183-186]. Once more these results proved the stability
of cellulose in organic solvents.
4.1.6 Comparison with industrial membranes
We tested a commercially available organic solvent nanofiltration membrane, DURAMEM 300
provided by Evonik, (crosslinked polyimide, molecular weight cut off, MWCO, 300 g mol-1) under
similar operating conditions (i.e., room temperature and 10bar operating pressure) to compare with the
performance of our cellulose membranes. We examined the performance of the industrial membrane
for pure water, butanol, and acetone fluxes, and MO rejections were also tested in each solvent. As
seen in Table 4.9, these membranes have very similar MO rejections in acetone while the cellulose
membranes (the thickest membrane prepared in this work) give fluxes 25 times higher than that of
DURAMEM 300. Furthermore, we obtained a butanol flux using our cellulose membranes 13 times
higher than the commercial OSN membranes while maintaining higher MO rejection. The only
exception was observed for water fluxes through the membranes, where water flux through the
DURAMEM 300 is double the flux through our cellulose membranes. Moreover, the rejection
behaviour of the membranes in water is different from each other due to the electrostatic interactions
that occurred in water. On the other hand, while pure methanol filtration is performed for 10 mins to
wash the cellulose membranes after dye rejection test, it took around 24h to clean the DURAMEM 300
membrane in the same way. This part of experiments implied that the cellulose membranes prepared
113
in this study have quite high solvents fluxes compared to the commercially available membranes, while
they do not have any defined MWCO behaviour due to very dominant effect of electrostatic interactions
on the separation mechanism. However, reusability of cellulose membranes after a quick cleaning
procedure, their environmentally-friendly preparation procedure, and cheap, highly stable and
biodegradable nature improve their advantages over the other membranes.
Table 4.9 Comparison of performances of prepared cellulose membranes (25μm-thick membrane)
and Duramem300
Membrane
Permeance (Lm-2h-1bar-1) Rejections (%)
Water Acetone Butanol
CSG
in H2O
MO
in H2O
MO
in acetone
MO
in butanol
Cellulose 3.5±0.3 16.6±3.5 2.2±0.4 97.4±2.3 75.2±5.9 97.4±3.3 99.2
DURAMEM 300 6.3 0.65 0.17 91.9 99.9 99.6 81.9
4.1.7 Surface modification
A solution to limit the adsorption phenomenon could be to chemically modify the membrane
surfaces. Indeed, it is assumed that the huge adsorption observed – at least in water – can be explained
by the negative zeta potential of pure cellulose membranes and the hydroxyl groups present in the
polymeric cellulose structure. By trying to graft different polymers on those hydroxyl groups, it can be
expected to reduce the adsorption phenomenon due to the removal of labile hydrogens. The strategy is
as follows. Here, the main aim is to acetylate the surface, as acetylation is widely known for other
cellulose compounds [49]. To achieve this aim, two different pathways are available: either the
membrane undergoes a straightforward acetylation in organic solvent, or a cross-linking reaction is
done before this final step. As cellulose acetate exhibits lower stability when compared with cellulose
114
in organic solvents due to less hydrogen bonds reinforcing the structure, the cross-linking path was
suggested to strengthen the structure of the membrane before acetylation of the surface. A recent study
[187] showed some possibilities of cellulose cross-linking modification for membranes cast by
dissolving cellulose in ionic liquids. They reported very good separation of sucrose from NaCl with a
rejection of more than 80% of sucrose while no salt was rejected. However, the cross-linking reaction
has harsh acid-basic conditions. Indeed, a strong enough base is needed to reinforce the cellulose
hydrophobicity before the reaction with tetrabutyloxide (DBX).
The experiments consisted of testing parts of pure cellulose membranes in reaction conditions:
- A section of pure cellulose membrane on PE non-woven backing was added into 10 mL of acetonitrile.
1 mL methylimidazole was added to the mixture, then after a minute, 1 mL of acetic anhydride (AA)
was added. The membrane was still observable on the support. (Acetylation)
- A section of the pure cellulose membrane on PE non-woven backing was added into 10 mL of THF
with approximately 100 mg of tertbutyloxide. A whitish solid dispersion was observed after 1 hour of
stirring, while the membrane was still observable. (Cross-linking)
- A section of the pure cellulose membrane on polypropylene (PP) non-woven backing was added into
10 mL THF with approximately 100 mg of tertbutyloxide. A beige solid dispersion was observed after
1 hour of stirring, while the membrane was still observable. (Cross-linking)
- A section of the pure cellulose membrane without support was added into 10 mL THF with
approximately 100 mg of tertbutyloxide. After 12 hours stirring, the membrane was still observable
and seemed to have kept its flat shape. (Cross-linking) The results can be summarized in the following
table:
115
Table 4.10 Stability results for surface modification reaction conditions
Pure cellulose Pure cell. on PP Pure cell. on PE
Acetylation Membrane
Support No support
Cross-linking Membrane
Support No support
Acetylation procedure seemed fine for both the cellulose membrane and PE backing, but as
stated above, as cellulose acetate is not as stable as cellulose in organic solvents, direct acetylation
method is not ideal for this work. For the cross-linking case improvements need to be done for the
selection of an adequate non-woven backing first, because cellulose membrane already exhibited great
stability. Due to time restrictions it was not possible to optimize all the conditions, but one possible
solution will be discussed in the next section by replacing the commercial backing materials with a
home-made nanocellulose backing paper which expected to have similar chemical stability as the
membranes, and a more sustainable association for OSN.
116
4.2 Structural and performance characterization of nanocellulose paper
4.2.1 Morphology and performance of the nanocellulose paper
Morphology of the prepared nanocellulose paper was characterized using SEM, and one of the
surface images was given in Figure 4.21 A. It is clearly seen that the prepared papers have very
homogenous fibre distribution on the surface and the fibres have uniform dimensions. There is no
visible big holes or defects on the paper surface and this is a good property for a strong and defect-free
backing paper. Pure water flux experiments have been conducted using dead-end cell filtration set-up
at an applied driving pressure of 10 bar in order to characterize the prepared nanocellulose papers.
Figure 4.21.B represents the compaction of the nanocellulose paper with a grammage of 40 g m-2 over
3 hours of filtration. The pure water permeance decreased almost 300 times in the first 30 mins, which
could be attributed to the compaction effect of the applied pressure, as Mautner et al. [146] reported
previously. The time required to reach the equilibrium in this work is shorter than the time they
reported, probably because of the higher operating pressure applied in this work.
The permeance decreased significantly because the thickness of the paper is reduced with
applied pressure, which led to an increase in the density and a decrease in the pore volume. The reduced
pore volume resulted in lower solvent transportation through the paper.
The relationship between the grammage and the thickness on the permeance of the paper is
depicted in Figure 4.21.C. The grammage of nanocellulose used for paper preparation and the thickness
of the dried paper exhibit almost a linear relationship. Slight deviations occurred only due to the water
content of the starting material, which could change slightly due to experimental errors and the water
content remaining in the produced paper after the drying process. The thickness of the papers could be
easily controlled by choosing the appropriate amount of base material. The permeance is dependent on
the grammage (and thus the thickness) of the nanocellulose paper, as clearly seen in the same figure
(Figure 4.21.C).
117
Figure 4.21 Characterization results for nanocellulose paper A) SEM image of the surface view of the
nanocellulose paper with a grammage of 40 g m-2, B) Permeance of pure water with respect to time
through the nanocellulose paper with a grammage of 40 g m-2, C) Relationship between grammage and
paper thickness and pure water permeance
1µm
A
C
0 20 40 60 80 100
0
10
20
30
40
50
60
70
80
Th
ick
nes
s (µ
m)
Grammage (g m-2)
0.0
0.4
0.8
1.2
1.6
2.0
permeance (right axes)
Perm
eance (L
m-2h
-1 b
ar-1)
0 20 40 60 80 100 120 140 160 180
0
50
100
150
200
250
Per
mea
nce
(L
m-2h
-1bar
-1)
Time (min)
B
118
The pure water permeance through the paper with a grammage of 80 g m-2 was measured to be
0.5 L m-2 h-1 bar-1, while the thinnest paper with a grammage of 10 g m-2 was 1.6 L m-2 h-1 bar-1. As a
result, the flux performance of the prepared papers could be tailored very well by changing the
thickness, which depends on the grammage used for preparation. To sum up, the flux performance of
the prepared nanocellulose papers strongly depends on their thickness, but still, the thinnest paper is
giving a performance in the range of tight ultra- filtration or nanofiltration membranes, which is not
desired for a backing paper.
Moreover, the rejection performance of the nanocellulose paper with 65 g m-2 thickness was
reported in a range close to NF with a MWCO of 25 kDa corresponding to a hydrodynamic radius of
5 nm [146]. Moreover, they reported that both the flux performance and the MWCO of the
nanocellulose papers are determined by the nanofibrils’ dimensions. They are suggesting that the
overall performance of the nanocellulose papers could be tailored by selecting different nanofibrils
with different dimensions, which can open doors to different applications [146]. Therefore, we
collaborated with one of the authors of this reported work who is an expert on nanocelullose paper
production for producing cellulose backing papers with higher flux performances in Lulea University
of Technology, Sweden, and they provided us an open nanocellulose paper (NCP-2) to be used as a
backing material.
4.2.2 Comparison of NCP-2 and PBP
4.2.2.1 Morphological structure
The thickness of the support layers was measured to be 270 and 100 μm, for NCP-2 and PBP,
respectively. The morphology of the supports at the micro/nanometer length scale were observed using
SEM and are shown in Figure 4.22. In Figure 4.22.A, the surface of the NCP-2 is given, where
microsized fibers are clearly visible and the fibers are loosely bound together in a 3D network, with
119
micro-scaled voids in between. The morphology seems very homogenous through the membrane
sample with long microfibers. The integrated fibres structure provides very high mechanical strength
to the nanopaper while the micro-scaled voids improve the flux through it, which are two significant
desired characteristics for a backing paper. No drastic difference was recorded in PBP, however, longer
and thicker fibers are clearly visible on surface of PBP support.
Figure 4.22 SEM images of A) NCP-2 surface view; B) PBP surface view. NCP-2 was prepared
and sent to us by our collaborators in Lulea Technology, while PBP is a commercial backing
paper.
Pore size of the prepared backing paper is measured around 5-6 µm, which resulted in a very
high water flux performance around 7000 L m-2 h-1 bar-1. The micro-scaled porosity of the support layer
was expected to provide high flux during water purification while providing sufficient mechanical
strength. These finding are in agreement with one of the previous research of this group, where a
vacuum-filtration was applied for the fabrication of bi-layer composite membranes having a support
layer with microporous cellulose residues and nanocellulose functional layer [28].
4.2.2.2 Stability in organic solvents
The resistance of the NCP-2 to the organic solvents were tested in acetone, ethyl acetate,
ethanol, and THF. Small square pieces of papers with 2 cm x 2 cm dimensions were soaked in these
A B
120
solvents for 12 months. Figure 4.23 shows the pictures of the samples before and after the experiment
with the dry weights written on each picture. Both the similar visual appearance and the dried sample
weights imply that NCP-2 is stable in all tested solvents, and no structural deformation and/or
degradation was observed. These results are consistent with literature.
ethanol THF acetone ethyl ace.
Figure 4.23 Pictures of NCP-2 pieces before and after 12 months’ stability experiments
The NCP-2 samples kept in the solvents for 12 months’ stability experiments were also tested
by SEM technique in order to have more reliable indicator about the stability. The samples first were
taken out of the solvents and washed with DI water, and then they were kept in DI water overnight to
ensure the complete solvent removal. After that, they were dried under fume hood and tested for surface
structure. It is clearly shown in Figure 4.24 that, the structure of the nanopaper is perfectly preserved
in all of the organic solvents tested for 12 months, and no change was observed in terms of structural
properties.
Mautner et al. suggested to use cellulose nanopapers prepared in a similar way in organic
solvents nanofiltration applications and showed that the nanopapers are stable in two organic solvents;
THF and n-hexane [147]. Similarly, multilayer cellulose membranes prepared using ionic liquids have
been reported to be stable in five different organic solvents (THF, hexane, DMF, NMP and DMAc) for
up to 1 week [148].
0.0504g 0.0523g 0.0532g 0.0522g
0.0505g 0.0522g 0.0523g 0.0531g
121
Figure 4.24 SEM images of NCP-2 samples after 12 months’ stability experiments in A) ethanol, B)
THF, C) acetone, and D) ethyl acetate. Samples were washed with DI water and dried very well before
SEM.
4.2.2.3 Stability in surface modification solution
As discussed in section 4.1.8, chemical surface modifications could be utilized as a method to
reduce the adsorption phenomenon taking place on the membrane surface to improve the separation
performance of the cellulose membranes in organic solvent nanofiltration applications [1, 188, 189].
However, the preliminary experiments conducted for the surface modification suggested that the
commercial backing papers used (PP, PE) were not stable enough to withstand the harsh acid-basic
conditions of the cross-linking reaction, while the cellulose membrane was perfectly fine. A very
similar experiment was done again to test the stability of NCP-2 in cross-linking reaction conditions as
in section 4.1.8 and the results are summarized in Table 4.11:
A
C
B
D
122
- A part of the pure NCP-2 backing was added into 10 mL of THF with approximately 100 mg
of tertbutyloxide. After 12 hours of stirring, the NCP-2 was still observable and seemed to have kept
its shape.
- A part of pure cellulose membrane on NCP-2 backing was added into 10 mL of THF with
approximately 100 mg of tertbutyloxide. After 12 hours of stirring, both the membrane and the NCP-
2 were still observable and seemed to have kept their shapes.
Table 4.11 Stability results for cross-linking reaction conditions
Pure cellulose Pure NCP-2 Pure cell. on NCP-2
Cross-linking
(THF, addition of
Tertbutyloxide)
Membrane
Stability
-
NCP-2
Stability
-
As expected, the nanocellulose paper, NCP-2 has a very similar chemical stability to the
membranes, which enables possible surface modifications to improve the separation performance of
the membranes in OSN applications.
4.2.2.4 Solvent flux and dye rejection performance
Pure cellulose membranes were cast on NCP-2 and PBP papers to investigate the effects of
different backing papers on the performance of the membranes. Figure 4.24 shows the pictures of the
membranes on both backing papers. As seen clearly, the membranes are not highly visible, just some
shine on the papers can be observed since the pure cellulose membranes are completely transparent.
The only difference between two membranes might be their dried thicknesses due to different amounts
of penetration of the dope solution through the backing supports. However, no significant difference
123
between the dry thicknesses was observed, probably due to the very similar morphological structures
shown in Figure 4.25. This is further supported by the similar pure water flux values through the
backing papers.
Figure 4.25 Pictures of pure cellulose membranes; A) cast on NCP-2, b) cast on PBP
Moreover, the membranes cast on NCP-2 and PBP papers were tested for pure solvent flux and
dye rejection performances in water and ethyl acetate, and the results are tabulated in Table 4.12. As
expected, no significant difference was observed in terms of flux and rejection values, because the
performance of the membrane does not depend on the backing papers if there are no stability-related
issues in the organic solvents, mechanical strength issues to the transmembrane pressure, and the
compatibility issues between membrane and backing. SEM characterization was also conducted in
order to compare the morphological differences between the membranes casted on NCP-2 and PBP.
Since the cellulose membrane was peeled off from the backing paper when it is dried, only pure
cellulose membranes could be tested under SEM. As shown in Figure 4.26, there is no morphological
difference between these two membranes, and exactly same structures were obtained.
A B
124
Figure 4.26 Cross- sectional SEM images for 25-µm-thick cellulose membranes cast on A)
NCP-2, B) PBP backing papers
The results suggest that using NCP-2 as the backing paper instead of PBP does not have a
remarkable effect on the performance of the membranes. As described and discussed in detail in
previous sections, higher MO rejection performance was recorded in ethyl acetate than in water.
Therefore, PBP can be replaced by NCP-2 without any consideration in terms of performance.
Table 4.12 Comparison of cellulose membranes’ flux-rejection performances cast on NCP-2 and PBP
Membrane
Permeance
(L m-2 h-1 bar-1)
R (%) in Water R (%) in Ethyl
Acetate Water Ethyl
Acetate
CR MO CSG MO
Cellulose on NCP-2 3.9±0.3 12.8±2.1 99.9 69.7 99.4 99.9
Cellulose on PBP 3.5±0.3 13.7±1.9 99.9 75.7 96.6 96.1
4.2.3 Composite stability/biodegradability study results
The visual illustration of composite membranes is shown in Figure 4.25, which confirms the
zero degradation rate in polluted water on both NCP-2 and PBP backing papers. An interesting point
is that the NCP-2 showed the same stability as PBP over 45 days. Thus, the used composites were
stable in real polluted water for up to 45 days. This result confirms the suitability of the fabricated
A B
125
nanocellulose papers for use in real water purification systems. It is a well-known fact that the
degradation rate depends on weight/size of used samples, composition of samples, and the effect of
living or dead organisms. The real wastewater that was used came from the mining industry and was
not a suitable habitat for microorganisms to grow in, thus, this effluent did not contain any living
microorganisms. Therefore, the predominant factor, which might be responsible for the increased rate
of degradation, is the pH, but no sign of weight loss was reported for up to 45 days.
Figure 4.27 Biodegradability study of fabricated cellulose membranes on NCP-2 and PBP in water (a)
and in soil (b).
Figure 4.27 further illustrates the biodegradation study of the composite in soil. Zero
degradation of PBP composite was reported for up to 45 days of incubation but on the other hand 88%
weight loss of NCP was determined within 15 days of incubation, and a further increase in the
incubation time increased the degradation rate (Figure 4.25). Many factors are responsible for the
degradation of composite in soil, some of these factors are soil structure and composition (mineral and
organic), temperature, water activity, pH, and the oxygen and carbon dioxide content. These factors
126
directly have a bearing on the physical properties of the polymer composite other than
influencing/determining the microbial population of the soil. Hence, the extent of biodegradation of
the composite can be expected to vary with region and from season to season. However, degradation
by microbial attack is the major mode of degradation of the natural composites in soil. In this study,
all the above-mentioned factors were maintained uniformly for all the samples and were carried out
under laboratory conditions. The used temperature was also suitable for the growth and colonization
of microbes present in soil. Their previous study confirms the increase in the degradation rate of
cellulose-based composite membranes with time and applied temperature; and the maximum
degradation was achieved at 40°C [28]. Thus, used membranes showed a high rate of biodegradability.
Our results are also in agreement with previous published data where cellulose film was incubated with
soil which contained fungus microbes. A porous structure with fungal mycelia on the surface of the
decayed film was observed, indicating microbial degradation of cellulose film [190].
Completely green and stable membranes were obtained by replacing the commercial backing
paper with a home-made nanocellulose backing paper. They have very high potential to be used in
organic solvent nanofiltration applications due to their exceptional stability in a wide range of solvents
if suitable surface modification methods are applied. They have a significant potential, because using
a cheap, sustainable, and biodegradable raw material and a non-toxic solvent and producing a
completely stable and biodegradable membrane using a one-step preparation technique is not possible
in the organic solvent nanofiltration literature so far. If this potential of cellulose membranes could be
utilized and applied in organic solvent nanofiltration area, hundreds of dangerous chemicals and
complex reaction preparation steps could be replaced by the environmentally friendly ones, and
greenness of this technology could be improved. Since this thesis mainly focus on the green ways of
membrane fabrication, cross-linking or other chemical modifications are not desired in the scope of
this work. The challenge is to make use of the natural ability of ‘cellulose’ without compromising its
green image. Therefore, in the next section, we report the usage of cellulose membranes for metal
127
removal from aqueous solutions by using their high potential on adsorption processes. On the other
hand, chemical modifications should be investigated in the future to open a new perspective for OSN
applications.
4.3 Metal adsorption through pure cellulose and cellulose/ UIO-66
membranes
4.3.1 Characterization of UIO-66 powders
Crystalline structure of the synthesized UIO-66 powder was identified by X-ray diffractometer.
Figure 4.28(A) shows the XRD patterns of pure UIO-66, cellulose membrane and cellulose/UIO-66
membrane together for comparison. The pattern of pure UIO-66 agrees well with the literature studies
[191] showing the main characteristic peak at 2θ= 7.5º. ATR- FTIR spectroscopy analysis was also
done in order to better understand the chemical structure of the UiO-66 powders. As seen in Figure
4.28(B), the absorption peaks observed in 1580, 1510 and 1392 cm−1 correspond to the carboxylate
groups and peaks at 691 cm−1 and 728 cm−1 associated to Zr-(μ3)O [10]. The results of XRD and FTIR
indicated that UIO-66 powders have been successfully synthesized without any other crystalline phase.
Scanning electron microscopy was used to analyse the morphology and the crystal size of the
synthesized UIO-66 crystals and the images at different magnifications are shown in Figure 4.29.
Figure 4.29 shows that UIO-66 crystals are in micron particle size and the crystals are well intergrown
with sharp edges [10]. SEM images of UIO-66 powder show that the surface topologies are similar to
previously reported synthesised framework [10, 191].
128
Figure 4.28 A) XRD patterns, B) FTIR patterns of pure cellulose (black) and cellulose/UIO-66
membrane (red), and pure UIO-66 powder (blue).
5 10 15 20 25 30 35 40 45 50
Inte
rnsi
ty (
Counts
)
2θ degree
7.5°
25.9°
11.5°21.1°
20
30
40
50
60
70
80
90
100
400 1000 1600 2200 2800 3400 4000
Tra
nsm
itta
nce
(%
)
Wavenumber ( cm-1)
-OH stretch
C-H symetrical
stretch
C=C
stretch
-C-H bending
C-C, C-OH- C-H ring
Zr-(μ3)O
B
A
129
Figure 4.29 SEM images of UIO-66 powder synthesized by solvothermal technique at 120 °C
for 48 hours. The powder was washed by ethanol several times and dried at 120±2 °C overnight
under vacuum before characterization.
4.3.2 Adsorption Studies on pure UIO-66
4.3.2.1 pH effect study
Since pH value has significant impacts on speciation of metal ions and the surface charge of
the adsorbent, it plays a key role on metal ion adsorption in the wastewater treatment applications [192-
194]. The effect of pH on the silver adsorption performance of the UIO-66 was investigated by
conducting several batch adsorption experiments at different pH values ranging from 0 to 7. Adsorption
experiments were not performed at high pH values, because Ag (I) had started to precipitate as silver
hydroxide at alkaline solutions [192]. The amount of silver ions adsorbed by UIO-66 was calculated
using Eq. (3.6) given in section 3.3.3.1. The results of pH effect experiments are shown in Figure 4.30.
According to the results shown in Figure 4.30, the adsorption of Ag (I) depends on the pH levels. The
uptake of silver is better at acidic conditions with the highest adsorption capacity around 73 mg g-1 at
pH 2, while it is reduced to 50 mg g-1 at high pH values. This result might be caused by adverse effect
of basic conditions on the structure, because UIO-66 was reported to be not stable at very basic
conditions [10].
130
The surface charge of UIO-66 was reported as positive at acidic pH conditions [10], so the
adsorption at pH<3.9 should be unfavourable for metal cations due to the repulsion with the positively
charged UIO-66 [194]. Therefore, it could be concluded that electrostatic interactions seem like
insignificant for the adsorption mechanism. Massoudinejad et al. [193] explained the adsorption
mechanism by electrostatic interaction because the lower fluoride adsorption on UIO-66 decreases at
pH > 7 where the adsorbent has negative surface charge. Wang et al. [10] proposed to explain the
adsorption mechanism of As onto UIO-66 by charge effect using the zeta potential characterization
results. However, they found that the maximum adsorption capacity was obtained at pH 2, where the
surface charge of UIO-66 positive and the dominant arsenate species (H3AsO4) have zero valance.
Therefore they explained the adsorption mechanism by two coordination processes similar to the acid-
base interaction.
Nevertheless, similar adsorption capacities obtained different pH values give the opportunity
to uptake the silver from the industrial wastewater at wide range of pH conditions without significant
reduction in the performance.
Figure 4.30 Effect of pH on silver adsorption capacity onto the UiO-66 powder during batch adsorption
experiments conducted for 24 hours (Feedconc= 100 ppm; Fvolume=100 mL; Masspowder = 50 mg; contact
time= 24 h)
0
10
20
30
40
50
60
70
80
0 1 2 3 4 5 6 7
Ag a
dso
rpti
on
cap
acit
y [
mg L
-1]
pH
131
4.3.2.2 Kinetic study
Several experiments were performed in order to see the adsorption kinetics after the best pH
condition was investigated (as described in previous section). As illustrated in Figure 4.31, the
adsorption capacity increased nonlinearly with increased contact time after a very quick 10 minutes’
adsorption. Maximum adsorption capacity (64 mg g-1) is reached after 1h contact time, and then the
adsorption occurred via a relatively slower process. Finally, it reaches the plateau and the adsorption
equilibrium is established in 4 h. Additionally the adsorption process could be evaluated in two parts:
an initial rapid step and following slow step, which is the indication of the Langmuir adsorption model
(will be discussed in section 4.3.2.3). The uptake of Ag (I) is very fast in the first 60 minutes and then
the adsorption is occurred very slow and almost constant up to 72 h. It can be concluded that there is
no crucial change in adsorption capacity for long contact time. The kinetic experiments demonstrated
that the adsorption rate of Ag is so fast due to the interaction between structure of UIO-66 and Ag+. It
is a desirable feature for industrial wastewater treatment to have fast adsorption process. Thus, UIO-
66 adsorbents could be promising candidates for rapid recovery of silver. The adsorption of Ag+ might
be concerned by the physical adsorption due to the specific morphology of UIO-66 with large active
sides (accessible part) for the Ag+ adsorption.
Time (h)
0 10 20 60 70
Adsorp
tion c
apacity
(m
g/L
)
45
50
55
60
65
70
Figure 4.31 Adsorption kinetics of silver onto the UiO-66 powder at pH 2 conditions (Feedconc= 100
ppm; Fvolume=500 mL; Masspowder = 250 mg; contact time= 72 h)
132
4.3.2.3 Adsorption isotherm study
Adsorption isotherms describe the mass-transfer equilibrium between the adsorbents and the
adsorbates [193]. The silver sorption studies onto UIO-66 were investigated at pH 7 (neutral condition),
and pH 2 (since the maximum performance was obtained at this value) by changing the initial adsorbent
concentrations ranging from 5 to 80 mg L-1. Langmuir isotherm model was used to determine the
adsorption capacity of adsorbent and to investigate the mechanisms of adsorption. The basic
assumption of this model is that the maximum adsorption takes place when a saturated monolayer of
solid molecules appears on the adsorbent surface. The Langmuir adsorption isotherm is calculated by
the following Equation 4.4.
𝑞𝑒 =𝑞𝑚𝑎𝑥 𝑏𝐶𝑒
1+𝑏𝐶𝑒 (4.4)
where 𝑞max (𝑚𝑔 𝑔−1) and 𝑏 (𝐿 𝑚𝑔−1) are maximum adsorption capacity and Langmuir constant,
respectively. The linearized equation is expressed as follows:
𝐶𝑒
𝑞𝑒=
1
𝑞𝑚𝑎𝑥 𝐶𝑒 +
1
𝑏𝑞𝑚𝑎𝑥 (4.5)
Experimental results are shown in Figure 4.32. Both experimental results and Langmuir
isotherm are shown in Figure 4.32 (A) for pH 2 and pH 7 conditions. The maximum adsorption capacity
results and constant parameters are summarized in table inserted in Figure 4.32. According to Figure
4.32(B), the correlation coefficient (r2) of the Langmuir equation is around 0.9 and these results
demonstrate a uniform, monolayer adsorption formation of the silver within the adsorbent. According
to the Langmuir model, the maximum adsorption capacity of UIO-66 for silver is 76.9 mg g-1 and 65.2
mg g-1 at the optimal pH condition (pH 2) and neutral condition (pH 7), respectively. These values are
133
considerably better than the most of the current sorbents reported in Table D.1 in appendix D. The
better capacity could be based upon the unique properties of MOFs such as high porosity, high surface
area and crystal structure [10].
0 10 20 30 40 50 60 70 80
0
10
20
30
40
50
60
70
80
90
100
qe
(m
g/g
)
Ce (mg/L)
experimental data pH 7
Langmuir pH7
experimental data pH 2
Langmuir pH2
Figure 4.32 Adsorption isotherms of silver onto the UIO-66 powder for 24 h of contact time
(A) Comparison of the experimental and the Langmuir isotherms, (B) The maximum adsorption
capacity results and constant parameters, (C) Experimental results
0
0.3
0.6
0.9
1.2
0 10 20 30 40 50 60 70 80
Ce/
qe
(g L
-1)
Ce (mg L-1)
pH 7
pH 2
C
pH
Langmuir isotherm
qmax (mg g-1) b (L mg-1) r2
2.0 76.899 0.82 0.9
7.0 65.2 1.177 0.87
A B
134
4.3.3 Characterization of UIO-66 after adsorption
Crystalline structure of UIO-66 powder was characterized after adsorption (without any
washing to investigate the adsorbed silver) by XRD, SEM and SEM-EDX methods in order to
investigate their stability. XRD pattern after adsorption given in Figure 4.33 indicated that there was
no destruction in the crystal structure of UIO-66 after silver adsorption experiments. It still showed a
relatively high crystallization degree after adsorption, although it is covered with silver particles, which
verifying the good stability of UIO-66 framework. Some additional peaks were observed between 30
and 50 degrees corresponding to the characteristic XRD peaks of silver particles reported in the
literature [195], because the powders were not washed after adsorption. XRD pattern proves the
presence of silver species within the UIO-66 framework.
Figure 4.33 XRD pattern of UIO-66 powder after silver adsorption
Figure 4.34(C) shows the SEM image of UIO-66 crystals after silver adsorption experiment
with a very similar structure to UIO-66 crystals before adsorption in Figure 4.29. It can be clearly
observed that the framework morphology was reserved after the adsorption process. Although silver
particles cannot be detected from SEM images, the elemental mapping of used adsorbents done by
SEM-EDX proves the presence of silver species within the UIO-66 framework (Figure 4.34(B)) with
a 0.63 % weight amount. The adsorption of silver on MOF crystals occurs by the complexing
interaction between Ag and benzene ring of UIO-66 structure while the adsorption of arsenic on UIO-
32.45°
46.39°
5 10 15 20 25 30 35 40 45 50
Inte
nsi
ty (
Counts
)
2θ degree
38.41°
belongs to Ag +
135
66 was explained by the complexation via Zr-O-As and Zr-OH coordination bonds by one of my
colleague in our research group [10]. In that study, they reported that UIO-66 has a 303 mg g-1
adsorption capacity for arsenic at pH 2 conditions. Moreover, UIO-66 showed a great stability
throughout the test and no damage of the crystal structure was observed.
Figure 4.34 Characterization results of UIO-66 powders; (A) EDX analysis result, (B) percentage
amounts of elements, (C) SEM image after adsorption.
4.3.4 Characterization of cellulose/UIO-66 membranes
Remarkably opaque membranes were obtained by incorporating UIO-66 powder in cellulose
matrix, while pure cellulose membranes were completely transparent. The crystal structure of the
prepared pure cellulose membrane and cellulose/UIO-66 membrane was examined and compared by
XRD. From Figure 4.27(A), it can be seen that the pure cellulose membrane has a diffraction peak
A
B
C
136
around 2θ= 11.5º which is verified with literature studies [5]. The cellulose/UIO-66 membrane has
three peaks around 2θ= 7.6º, 2θ= 11.5º, and 2θ=20-21º. The first peak is representation of UIO-66, and
the second one comes from the cellulose powder. Another peak (2θ= 20º) cannot be distinguished very
easily due to the very close positions and similar intensities of the peaks. Infrared spectroscopy analysis
was also performed for comparison of the membranes as displayed in Figure 4.28(B). The characteristic
IR absorption peaks of membranes are presented in Table E.1. The FTIR spectrum confirms the
presence of cellulose and UIO-66 powders in the cellulose/UIO-66 membranes and these results are
matched with the literature studies [10, 196].
Figure 4.35 SEM images of cellulose/ UIO-66 membranes at different magnifications. These
membranes were prepared by phase inversion precipitation technique containing 9 g of NMMO,
1 g of cellulose, 0.2 g of UIO-66.
A B
C
137
Figure 4.35 presents SEM images of cellulose/UIO-66 membranes at different magnifications.
It is clearly observed in the high magnification image (Figure 4.35 (C)) that MOF crystals were formed
agglomerations, probably due to high loadings of filler (20 wt.%). This agglomeration problem should
be solved by lower MOF concentrations as explained in the similar examples in literature [197].
However, the agglomerated filler particles were homogenously distributed in the cellulose matrix.
Moreover, cellulose/UIO-66 membranes have rough surfaces unlike pure cellulose membranes, and
MOF crystals are covered with cellulose very well in most places.
4.3.5 Adsorption studies on cellulose/UIO-66 membranes
This part of the work includes just preliminary experimental results, and a comprehensive study
was planned, and will be done as a future work personally.
Pure cellulose and cellulose/UIO-66 membranes were tested for silver and arsenic adsorption
capacity in batch (static) and cross-flow mode (kinetic). Cross-flow operation mode is reported to be
the efficient mode for industrial level applications due to high penetration power of pollutants through
the membranes [28] due to applied pressure during process and longer contact times of membranes and
solutions. Therefore, different results were expected.
4.3.5.1 Static adsorption
The removal of silver and arsenic ions from aqueous solutions by pure cellulose and
cellulose/UIO-66 membranes were analysed in static mode at room temperature and at neutral pH
conditions. Different initial ion concentration of metal solutions between 5ppm and 100ppm was tested,
and contact time was chosen as 72 h. Table 4.13 is summarising all the static adsorption results
performed for 2 different membranes for both silver and arsenic at 25 ppm initial metal concentration,
because not too much difference was observed for different concentrations. The driving force for the
138
static adsorption is considered as the concentration difference between the feed solution and the
membrane surface.
As (V) exhibits no adsorption tendency towards pure cellulose membranes in static conditions,
which could be explained by a lack of electrostatic interactions between them. Cellulose naturally has
a sorption capacity for metal ions due to the presence of reactive hydroxyl groups on its structure,
however, at neutral pH values pure membranes exhibit strongly negative surface charge as reported in
section 4.1.2, and the predominant species of arsenate in water bodies exist as H2AsO4− and
HAsO42−[10]. In order to prove the significance of electrostatic interactions, static As (V) adsorption
experiments should be conducted at different pH values where the surface and the adsorbate have
the converse charges. However, it is not easy, because arsenate speciation becomes only neutral or
negative while the membrane never has a strongly positive surface charge at any pH values.
On the other hand, pure cellulose membranes exhibit a silver adsorption capacity around 3.5
mg g-1 (regardless from the initial metal concentration) which is probably due to electrostatic
interactions between Ag+ ions and negatively charged membrane surface.
Significant improvements in both silver and arsenic adsorption capacities were observed with
the addition of 20% wt. UIO-66 into cellulose matrix at all initial concentrations. The As (V) adsorption
capacity was increased from 0 to 12.5 mg g-1, by combining the superior As (V) adsorption capacity of
UIO-66 with the high stability and sustainability of cellulose. In the case of Ag (I), the adsorption
capacity was tripled (from 3.5 to 13.0 mg g-1). Addition of organic-inorganic fillers offers several
advantages such as high surface area, high porosity, tuneable hydrophilicity /hydrophobicity and
surface charge [198] while the continuous polymer matrix provide low pressure drop, easy
processability [2, 94]. Gohari et al. [199] prepared polyethersulfone(PES)/ hydrous manganese dioxide
(HMO) composite membranes for adsorptive removal of Pb(II) from aqueous solution. They combined
the high adsorptive capacity of HMO with the easy processability of PES to make the process
applicable to industrial adoption.
139
Table 4.13 Silver and arsenic adsorption capacity of different types of membranes at 25 ppm
initial metal concentration under static conditions. The pH of the solution was measured as 5.5.
Metal ions
Type of membrane
Adsorption capacity (mg g-1)
Ag (I) As (V)
Pure cellulose 3.5 0.0
Cellulose/UIO-66 (20%) 13.0 12.5
4.3.5.2 Cross-flow adsorption
The removal of As (V) and Ag (I) metals from aqueous solution with different initial
concentrations was also performed in cross-flow mode in order to see the impact of penetration through
the membrane due to applied pressure. The cross-flow experiments have been run for a period of over
48 hours. All experiments were run at room temperature and pH 5.5 conditions. The adsorption
capacity in cross-flow mode operation is expected to be higher in comparison with static mode
operation to remove metals ions from wastewater, both because of the penetration of metals [28]
through the membranes and also because of the full usage of MOFs inside the membrane structure as
well as the ones on the surface by the increased contact time.
Three different expressions were used to evaluate the performance of the membranes under
cross-flow operation conditions. First one is the adsorption capacity (equation 3.6) which was defined
previously in Chapter 3.
𝑞𝑒 = (𝐶0−𝐶𝑒) 𝑥 𝑉
𝑚 (3.6)
where, C0 and Ce are initial and final concentration of metal ions (mg L-1), respectively.
Second one is the percent removal, which was calculated by the Equation 4.6:
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Percent removal(%) = (C0−Cf
C0) 𝑥 100 (4.6)
Where, 𝐶0 (𝑚𝑔/𝐿) is the concentration of feed solution; 𝐶𝑓 is the final concentration of feed solution.
The last one is the amount of adsorbate calculated by Eq. 4.7 as below. Indeed, this value gives us the
adsorbates mass per unit area.
Amount 𝑜𝑓 adsorbate (mg 𝑚−2) = (1 −𝐶𝑡
𝐶0) 𝑥
𝑉𝐶0
𝑆 (4.7)
Where, 𝐶0 (mg L-1) the concentration of feed solution; 𝐶𝑡 is the concentration of feed solution at time
𝑡, 𝑆 (m2) is surface are of the membrane and 𝑉 (𝐿) is volume of feed solution n.
All the results obtained from cross-flow filtration experiments are tabulated in Table 4.14. For
all types of membranes synthesized, silver and arsenic adsorption experiments were conducted at
different initial metal concentrations. Low initial concentrations were consistent with the industrial
levels. Karim et al. [28] reported 100% recovery of silver from the mirror industry effluent with a 1.48
ppm initial concentration.
Both pure cellulose and cellulose/UIO-66 membranes had almost 99% silver removal in just 1
h contact time, which is extremely fast compared to the reported literature studies. Nasser et al. [200]
used polymer inclusion membrane (PIM) for the removal and recovery of silver cyanide complex from
aqueous solutions, and they reached 73% removal rate after 50 h of the process. Table 4.14 shows that
UIO-66 addition into cellulose matrix does not have any effect on the adsorption performance of pure
cellulose membranes. This should be explained by extremely higher Ag (I) adsorption capacity of pure
cellulose compared to UIO-66 powder due to electrostatic interactions. Moreover, adsorption
experiments were performed for 10 days, and any silver ion desorption was not observed. Maximum
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Ag (I) adsorption capacity of the composite membrane was calculated as 832.5 mg g-1 when the initial
Ag (I) concentration was 500 ppm. Since 99% of the dyes were adsorbed by the membrane surface,
real adsorption capacity was not determined. But still this adsorption capacity is the highest
performance ever reported in literature. One of the best results in literature was reported by Dimeska
et al. [201]. They have investigated the electroless recovery of silver by inherently conducting polymer
(ICP) membrane. While silver capacity (mg Ag on g of polymer) of the PPy/NDSA (polypyrrole 1,5-
naphthalenedisulfonic acid) membrane was 260 mg g-1, the PPy/PVS (polypyrrole poly(vinyl)sulfonic
acid) membrane's capacity was reported as 510 mg g-1 at 100ppm initial concentration. They have
demonstrated that the materials show a strong capability to recover silver. I am planning to continue
the experiments in future when I go back to my home country to understand the real Ag (I) adsorption
capacity of cellulose.
On the other hand, the effect of UIO-66 addition in kinetic As (V) adsorption experiments is
very obvious. While pure cellulose is not adsorbing at all, cellulose/UIO-66 membranes showed 96.7
% As (V) removal from the solution with 15 ppm initial metal concentration. Zhenga et al. [133] studied
PVDF/zirconia blend flat sheet membranes for the adsorptive removal of As(V). Their membranes
showed a good performance for uptaking arsenate in batch adsorption experiments in a wide range of
pH from 3 to 8. They reached the equilibrium in 25h and the maximum adsorption capacity was
reported as 21.5 mg g-1, which is comparable the most of the current sorbents reported in the literature.
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Table 4.14 Adsorption performance of cellulose and cellulose/MOF membranes in cross-flow
filtration (Recovery time is provided in parenthesis for comparison)
Membrane Metal Ions Feed solution
C0(mg L-1)
Percent
Removal (%)
Amount of ads.
(mg m-2)
Ads. Capacity
(mg g-1)
Pure Cellulose Ag (I) 10 99.9 (1 h) 625 16.7
250 99.2 (1 h) 14292 413.4
As (V) 15 0 (96 h) 0 0
Cellulose/UIO-66
Ag (I)
10 99.9 (1 h) 625 16.7
15 99.9 (1 h) 937 27.1
250 99.6 (1 h) 14348 415.0
500 99.1 (1 h) 30970 832.5
As (V) 15 96.7 (24 h) 841 24.3
Furthermore, As (V) adsorption capacity of our composite membranes at static conditions is
increased from 12.5 (given in Table 4.13) to 24.3 mg g-1 with the effect of cross-flow geometry. It is
an expected result according to literature. For instance, Sen et al. [202] have investigated the arsenic
uptake from contaminated groundwater at different pH and operation pressure conditions by using
nanofiltration membranes. They reported that arsenic rejection rises slightly when applied pressure for
cross-flow operation is increased. The main reason may be related to the solution-diffusion mechanism
that applies to nanofiltration.
Considering the Table 4.13 and 4.14, it is very obvious that the cross-flow operation mode is
improving the adsorption capacity of the membranes significantly except As (V) adsorption capacity
of pure cellulose membranes. In literature, cross-flow method is reported as more efficient than dead-
end method for industrial level applications due to high penetration power of pollutants through the
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membranes [32]. For instance, Crespo et al. [40] tested the filtration of protein solution (BSA) with
ion-exchange membrane. They reported a better adsorption capacity for the membranes in cross-flow
conditions, due to improved control of pore blockage. This is clearly demonstrated that the cross-flow
mode operation had higher yields in comparison to dead-end mode operation [40]. Bayhan et al. [135]
have investigated the removal of heavy metal ions (Ni2+, Cu2+ and Pb2+) by yeast in cross-flow method.
They reported that the cross-flow microfiltration is an effective, low-cost method to uptake heavy metal
ions from water via yeast cells.
4.3.6. Characterization of UIO-66 after adsorption
After conducting silver and arsenic adsorption experiments in cross-flow system, EDX was
operated to confirm the existence of metal ions on the surface of membranes as illustrated in Figure
4.36 (B) and (C). It can be clearly observed from Figure 4.36(A) that the membrane morphology was
reserved after adsorption experiments, and no metal ions is visible on the surface. The elemental
mapping of used membranes verifies the presence of arsenic and silver species within the membranes.
Recently it is reported that, the adsorption and desorption mechanisms of positively charged ions on
cellulose surface are largely unknown [119]. The possible mechanisms might be electrostatic
interactions, ions exchange, microprecipitation or interaction followed by nucleation effect [28]. Since
the zeta potential of the cellulose membranes are shown to be strongly negative in section 4.1.2,
electrostatic static interactions with positively charged ions become more reasonable.
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Figure 4.36 A) SEM image of the membrane after Ag (I) adsorption, (B) corresponding EDX data of
membranes after adsorption of As (V), (C) corresponding EDX data of membranes after adsorption of
Ag (I).
No detailed experiments were conducted for the recovery of metal ions from the membrane
surface, but some preliminary ones were done. Membrane samples that were used in adsorption
experiments were put in DI water for 24 hours and then the water was tested by ICP in order to
understand that if any metal ions desorbed spontaneously. Almost 25 % of the adsorbed metal ions
are desorbed and recovered in the water without any special chemical cleaning method. Recovery
experiments made in methanol could not be conducted successfully, because the alcohol solution
could not test by ICP. Some detailed experiments will be planned in future by me in order to complete
this study.
Furthermore, amount of adsorbate on unit area is an important information for industrial
implications for these membranes. For instance, 1 m2 of cellulose/UIO-66 membrane could adsorb 4 g
of silver in 1 h cross flow operation, and a typical 0.2 m diameter spiral wound flat sheet membrane
module can achieve 800 g silver adsorptions in 1 h operation.
B
C
A
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4.4 General achievements
This chapter demonstrated the formation of high flux cellulose membranes via phase inversion,
showing high stability in polar protic and polar aprotic solvents, including acetone, acetonitrile, THF,
ethyl acetate, and alcohols. To date, it is the most extensive research investigating the stability and flux
behaviour of cellulose membranes in organic solvents compared to published work. Cellulose is a good
candidate for organic solvent related applications due to it very stable structure in different solvents,
but this highly stable structure makes it also very difficult to prepare the cellulose membranes. NMMO
is one of the best solvents for cellulose reported in literature due to its non-toxic and environmentally
benign properties. Moreover, the semi-crystalline structure of the cellulose material was resulted in a
rigid membrane structure which is not compacted when high operating pressures were applied up to 30
bar, therefore no pre-conditioning step was needed for a reliable flux performance through these
membranes. Long-term cross-flow filtration experiments were further proving the stability of the
membranes in different organic solvents up to 1-week continuous operation time.
Short-term and long-term rejection experiments conducted in dead-end and cross-flow filtration
set-up showed that electrostatic interactions were dominant for the separation mechanism in water.
Since the zeta potential of the membrane surface could be straightforwardly determined in aqueous
systems, the adsorption phenomena occurs on the membrane surface was explained easily with the
electrostatic interactions caused by the charge effects. The positively charged CSG dye was rejected
97% by the negatively charged membrane surface by the adsorption effect although it has a very small
molecular weight compared to the membranes’ pore size. Moreover, the CSG rejection by the
membrane could be altered by modifying the pH levels of the dye solutions since the zeta potential of
the membrane surface changed. On the other hand, rejection behaviour of the membranes in organic
solvents was difficult to explain due to very different structures and properties of the organic solvents.
Since it is not feasible to measure the zeta potential of the membrane surface and the dyes in different
organic solvents, the electrostatic interactions could not be discussed easily. Adsorption was found to
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be still active for the removal of dyes from the solutions based on the batch adsorption experiments,
but different adsorption mechanism might be responsible for it such as van der Walls, hydrogen and
hydrophobic bonds. Although cellulose membranes exhibited 99% rejection of all dyes in organic
solvents when tested in dead-end filtration because of very fast testing periods, long-term cross-flow
filtration has failed in terms of rejection of which reason was mostly explained by using the Hansen
solubility parameters. The synthesized cellulose membranes are very promising product for OSN
applications but some surface modifications might be necessary for better separation performances,
and these surface modifications techniques are not environmentally friendly techniques. Since the
objective of this thesis is mainly based on the green membranes and the production methods, these
surface modification techniques were not considered in detail, but some preliminary stability
experiments were conducted to give some insights for future projects. It was found that the cellulose
membrane prepared in this work is completely fine in harsh cross-linking and acetylation conditions,
but the backing papers used during membrane fabrications are not. Therefore, a green and solvent
stable backing paper was synthesised from nanocellulose using a simple paper production method in
this study, and at the end a completely stable and biodegradable product was obtained for the potential
OSN applications. Replacing the commercial backing paper by this home-made nanocellulose paper
will give the opportunity to modify the surface of cellulose membranes to improve their performance
in OSN applications. No significant difference was recorded between the nanocellulose paper and the
commercial backing paper in terms of stability in the organic solvents and the structural properties.
Moreover, a completely safe product will be discharged to the environment when life time of the
membrane comes to the end according to the results of biodegradability experiments.
Finally, cellulose membranes were suggested for adsorptive metal removal applications from
aqueous solutions to make use of the natural ability of ‘cellulose’ without compromising its green
image. Since the membranes were found to have very high adsorption capacity for the positively
charged ions, metal ions were decided to be good alternatives for the removal applications. UIO-66
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crystals were prepared and characterized for silver adsorption performance, and very high capacity was
detected. Arsenic adsorption capacity of UIO-66 was also reported by one of my colleagues in the
group previously. Pure cellulose and cellulose/ UIO-66 composite membranes were investigated for
silver and arsenic adsorption performances in static and kinetic conditions. While no arsenic adsorption
was recorded on the surface of pure cellulose membranes due to lack of electrostatic interactions
between them, the silver ions were adsorbed by the membrane surface significantly. Cellulose/UIO-66
composite membranes adsorbed a good amount of arsenic due to interaction of arsenic and UIO-66
crystals, but no improvement was recorded for silver adsorption with the UIO-66 addition. Some
preliminary metal ions recovery experiments were conducted for pure cellulose and composite
membranes and promising results were recorded, more detailed further studies will be done by Nilay
Keser Demir in future. In conclusion, incorporation of UIO-66 particles in cellulose resulted in highly
stable green membranes across a broad pH range from very acidic (1) to neutral (7) conditions with
promising adsorption performances for silver and arsenic. Moreover, cross-flow filtration geometry
improved their efficiency further due to penetration of pollutants through the membrane by applied
positive pressure across the membrane.
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Chapter 5
Conclusion
5.1 Final conclusions
This thesis demonstrated that cellulose, as an abundant and renewable polymer, has a very high
potential for OSN applications. Its exceptional stability in various organic solvents is resulted from its
semi-crystalline structure with strong hydrogen bonds in it. Using an environmentally friendly solvent
for fabrication improved the greenness of the membrane, and opened a new perspective for OSN
membranes.
5.1.1 Structural and performance characterization of cellulose membranes
Cellulose membranes were fabricated by phase inversion method using NMMO as a solvent,
and they exhibited exceptionally high solvent permeances depending on the viscosities of the solvents,
which confirm the Hagen-Pouiseille type viscous flow through the membrane. The reason for the high
flux values was speculated to be the homogenous symmetric membrane structure with nano-sized pores
formed by freeze-drying occurred during phase inversion process. SEM images, Hagen-Pouiseille type
transport behavior, and drastic increase in the permeances by decreasing thickness confirmed this
membrane structure speculation.
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Short-term dead-end experiments were conducted in order to obtain preliminary information
about the performances, and very high rejection values ranging between 60-99% were recorded
depending on the properties of the solvents. Electrostatic interactions were dominant for the separation
mechanism in water, since the surface of cellulose membranes are strongly negative at neutral
conditions, positively charged dyes are rejected more by adsorption effect. The adsorption behaviour
of the membrane could be adjusted easily by changing pH of the solution which changes the surface
charge of the membrane. Moreover, 1 week of cross-flow filtration experiments were conducted for
two dyes in water, and 95% rejection was obtained for congo red, a neutral dye with 690 gmol-1 MW,
while negatively charged dye rose bengal (MW: 1018 gmol-1) passed through the membrane
completely. On the other hand, rejection behaviour of the membranes in organic solvents was difficult
to explain due to very different structures and properties of the organic solvents. Although cellulose
membranes exhibited 99% rejection of all tested dyes in organic solvents when tested in dead-end
filtration, long-term cross-flow filtration has failed in terms of rejection of which reason was mostly
explained by using the Hansen solubility parameters. Adsorption was still active for the removal of
dyes from the solutions.
5.1.2 Structural and performance characterization of nanocellulose paper
When the membrane became saturated during adsorption, dyes permeated through it and
rejection failed. Some chemical modifications were proposed to modify the membrane surface such as
acetylation and cross-linking. Acetylation procedure seems fine for both cellulose membrane and
commercial backing, but since cellulose acetate (acetylated cellulose) is not as stable as cellulose in
organic solvents, direct acetylation method is not ideal for this work. For the cross-linking case, other
than the stability of the membrane itself, improvements need to be done within the selection of an
adequate non-woven backing first. Due to time restrictions it was not possible to optimize all the
conditions, but one possible solution was discussed in the Section 4.2. By replacing the commercial
150
backing materials with a home-made nanocellulose backing paper with very similar chemical stability
as the membranes, a completely green product was obtained at the end. Stability experiments run in
real wastewater media (in harsh pH conditions at 40°C for 45 days) and chemical modification
conditions (cross-linking conditions) showed that nanocellulose backing paper was perfectly stable.
Moreover, it was almost completely degraded within 15 days of incubation in soil while no degradation
was reported for commercial backing (PBP) up to 45days of incubation. Since this thesis mainly focus
on the green ways of the membrane fabrication, cross-linking or other chemical modifications are not
desired. However, they should be investigated in future to open a new perspective and a more
sustainable association for OSN applications.
5.1.3 Metal adsorption through pure cellulose and cellulose/ UIO-66 membranes
The main challenge in this study was to make use of the natural ability of ‘cellulose’ without
compromising its green image. Therefore, in the last section (section 4.3), we reported the usage of
cellulose and cellulose/UIO-66 membranes for metal removal (i.e. silver and arsenic) from aqueous
solutions by using their high potential on adsorption processes. Pure cellulose membranes exhibited
very promising silver uptake capability due to strong –OH bonding on the membrane surface, while no
arsenic was adsorbed. Superior arsenic adsorption capacity was reported for pure UIO-66 crystals
before [10], and silver adsorption capacity of UIO-66 was tested in this study. UIO-66 crystals have
reached the equilibrium after 1-hour contact time with a remarkable silver uptake capacity of 77 mg g-
1. This exceptional fast silver adsorption performance and high stability of UIO-66 in water provides
promising insights to the water treatment applications. Incorporation of MOF particles in cellulose
resulted in highly stable green membranes across a broad pH range from very acidic (1) to neutral (7)
conditions with promising adsorption performances for silver and arsenic. Moreover, cross-flow
filtration geometry improved their efficiency further due to penetration of pollutants through the
membrane by applied positive pressure across the membrane. Finally, 4 g m-2 silver adsorption rate
151
was achieved with the cellulose/UIO-66 membrane in a 1-hour experiment. If the regeneration of these
membranes could be achieved, then large-scale industrial membrane modules could be built especially
for silver removal application.
5.2 Future directions
In this dissertation, cellulose membranes were prepared and tested for stability and flux
performances in water and several organic solvents. There was some attempt to understand the
transport mechanism better. Flux behaviour and the stability of the membranes could be tested for some
non-polar solvents under dead-end and cross-flow conditions to extend the scope of the work.
In Section 4.1, the prepared membranes were also tested for rejection capabilities using
different markers in water and tested organic solvents. Since the transport mechanism in organic
solvents is very complicated, more systematic experiments could be conducted to understand it better.
For instance, due to limited time and equipment, not enough cross-flow experiments were run. Long-
term experiments should be run for +, -, and neutral dyes with similar MW in water, and adsorption
capacity of the membrane should be evaluated. By these experiments, the charge effect on the
adsorption phenomena can be explained more easily, because short-term experiments are giving just
an insight. Also, cross-flow experiments in solvents should be conducted for differently charged dyes.
Cross-flow experiments have shown that adsorption is a huge phenomenon taking place in continuous
process, with membranes saturating and then letting the dye permeating through. A better
understanding of the factors influencing the membrane adsorption is needed to lead to the development
of cheap and renewable OSN cellulose membranes. Several parallel experiments should be conducted
to analyse the effect of different parameters (i.e. temperature, pH, feed concentration, etc.) on the
membrane adsorption.
Purifying effluents of the dye industry is indeed a huge concern, and further studies could be
made to apply cellulose membranes to the purification of those effluents. An interesting factor to look
152
at would be the adsorption of a double dye solution, and double dye competitive adsorption/filtration.
For example, an interesting idea would be to study the possibility of adsorbing the surface with a dye
while filtering the other one. Moreover, undesirable solutes present in small amounts in water or
solvents could be removed by physisorption.
Adsorption was found as the most important phenomenon taking place for cellulose
membranes, and the dye permeates through only when the membranes became saturated. The
adsorption phenomena could be controlled by grafting different polymers on the hydroxyl groups on
membrane surface using some chemical surface modification techniques (i.e. cross-linking or
acetylation). Being abundant and renewable would make cellulose a very good raw material for any
industry. However, very little cellulose modification can be found in the literature for OSN. Preliminary
experiments showed that both polyester and polypropylene backing materials failed in cross-linking
conditions while cellulose membrane was perfectly stable. Therefore, nanocellulose paper (NCP)
backing material were prepared in section 4.2, which allows us to produce completely green and stable
end product. Due to time restrictions and because it is out of scope of this thesis, surface modification
conditions were not optimized. However it should definitely be investigated in future to open a new
perspective for OSN applications.
In order to utilize the advantage of the adsorptive nature of cellulose membranes, heavy metal
removal studies were conducted in section 4.3. Pure cellulose and cellulose/UIO-66 membranes were
tested for silver and arsenic removal performance from aqueous solutions. Very promising results were
reported. Further studies should be conducted to understand the adsorption mechanisms taking place
between metal ions and membrane surfaces and for optimizing the conditions for best metal adsorption.
Moreover, regeneration of membranes should be investigated in detail to improve the efficiency of
process. If the regeneration is possible, then large scale membrane modules could be built for high
surface adsorptive systems. Since the metal adsorption is very rapid, the system efficiency will be very
153
high. Moreover, different organic-inorganic fillers could be tried to improve the efficiency of the
membranes.
154
List of publications
JOURNAL ARTICLES COVERED BY SCIENCE CITATION INDEX:
1. Xinlei Liu, Nilay Keser Demir, Zhentao Wu, Kang Li, ‘Highly Water-Stable Zirconium
Metal–Organic Framework UiO-66 Membranes Supported on Alumina Hollow Fibers for
Desalination’, Journal of the American Chemical Society, 2015 137 (22) : p. 6999-7002.
DOI: 10.1021/jacs.5b02276
2. Wang Chenghong, Xinlei Liu, Nilay Keser Demir, Paul Chen, Kang Li, ‘Applications of
water stable metal-organic frameworks’, Chemical Society Reviews, 2016, 45, p. 5107-5134.
DOI: 10.1039/C6CS00362A
INTERNATIONAL CONFERENCES ATTENDED:
1. Nilay Keser Demir, Andreas Mautner, Alexander Bismarck, Kang Li, ‘Development of bio-
based nano-cellulose membranes for wastewater treatment’, Poster presentation, Chemical
Engineering Day UK, Imperial College London, London, UK, March, 2013.
2. Nilay Keser Demir, Maria Jimenez Solomon, A. Livingston and K. Li, ‘Novel cellulose
membranes for organic solvent nanofiltration’, Oral presentation, International Conference on
Membranes, Suzhou, China, July, 2014.
155
3. Xinlei Li, Nilay Keser Demir, Kang Li, ‘Water stable MOF membranes on hollow fibres’,
Poster presentation, International Conference on Membranes, Suzhou, China, July, 2014.
4. Nilay Keser Demir, Maria Jimenez Solomon, A. Livingston and K. Li, ‘Green High Flux
Organic Solvent Nanofiltration membranes’ Oral presentation, Postgraduate Symposium on
Nanotechnology, University of Birmingham, UK, December, 2014.
5. Nilay Keser Demir, Maria Jimenez Solomon, A. Livingston and K. Li, ‘Ultra-high flux OSN
membranes made from a renewable polymer’, Poster presentation, Euromembrane, Aachen,
Germany, July, 2015.
6. Xinlei Li, Nilay Keser Demir, Kang Li, ‘Water stable MOF membranes on hollow fibres for
desalination’, Poster presentation, Euromembrane, Aachen, Germany, July, 2015.
SUMMER SCHOOL ATTENDED:
1. Attendee to ‘Membranes and Membrane Processes Design’, University of Duisburg, 30th
European Membrane Society Summer School, Essen, Germany, July 2013.
156
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Appendices
Appendix A
Pure solvent fluxes through cellulose membranes
Figure A.1 Pure solvent fluxes through 12-µm-thick membrane for acetone, acetonitrile, ethyl acetate,
THF, water, and 1-butanol. Nanofiltration experiments have been performed in dead-end system at
10bar and 25 ºC.
0 50 100 150 200
0
50
100
250
300
350
400
450
Flu
x (
Lm
-2h
-1)
Time (min)
acetonitrile
acetone
ethyl acetate
THF
water
1-butanol
171
Figure A.2 Pure solvent fluxes through 5-µm-thick membrane for water, acetone, acetonitrile, ethyl
acetate, THF, and 1-butanol. Nanofiltration experiments have been performed in dead-end system at
2bar and 25 ºC.
Figure A.3 Pure solvent fluxes through 2.5-µm-thick membrane for water, acetone, acetonitrile, ethyl
acetate, THF, and 1-butanol. Nanofiltration experiments have been performed in dead-end system at
2bar and 25 ºC.
0 50 100 150 200
0
50
150
200
250
300
Flu
x (L
m-2
h-1)
Time (min)
acetonitrile
acetone
ethyl acetate
THF
water
1-butanol
0 50 100 150 200
0
50
100
250
300
350
400
Flu
x (
Lm
-2h
-1)
Time (min)
acetonitrile
acetone
ethyl acetate
THF
water
1-butanol
172
Appendix B
Rejection results
Figure B.1 UV calibration curves for CR in water and RB in acetone
Figure B.2 Visual representation of dye rejections in acetone (R: retentate, P: permeate)
R² = 0.9999
R² = 0.9981
0
3
6
9
12
15
0 20 40 60 80 100
UV
Abso
rban
ce
Dye concentration (mgL-1)
CR in water
RB in acetone
Increasing MW
MO CV RB
173
Appendix C
Hansen solubility parameters and physical properties
Table C.1 Hansen solubility parameters of the dyes calculated by group contribution method [144,
145]
Name Groups Hansen Solubility Parameter
(MPa1/2 )
RB
1x phenyl (hexasubstituted)
2x phenyl (pentasubstituted)
2x ring closure 2 or more
3x conjugation in the ring
2x -OH
1x –O-
1x CO2
4x halogen attached to C
with double bond
4x Cl attached to C with
double bond
48.99
CV
3x phenylene (o, m, p)
1x C
3x N
6x CH3
21.02
MO 2x phenylene (o, m, p)
1x -N=N-
1x N
1x SO3
2x CH3
22.97
CSG 1x phenyl
1x phenyl (trisubstituted)
2x NH2
1x -N=N-
25.50
Cellulose 1x Ring closure 5 or more
atoms
2x OH (disubstituted or on
adjacent C atoms)
2x O
1x OH
4x CH2
33.72
174
Table C.2 Physical properties of the solvents [203]
Solvents Hansen Parameters Hansen Solubility
Parameter
Molar
Volume (L) Dielectric Constant
(Polarity) dD dP dH
Po
lar
Ap
roti
c
Ethyl acetate 15.8 5.3 7.2 18.1 98.5 6.0
THF 16.8 5.7 8.0 19.4 81.7 7.5
Acetone 15.5 10.4 7.0 19.9 74.0 21.0
Acetonitrile 15.3 18.0 6.1 24.4 52.5 37.5
Po
lar
Pro
tic
Water 15.5 16.0 42.3 47.8 18.0 80.0
Methanol 14.7 12.3 22.3 29.4 40.7 33.0
Ethanol 15.8 8.8 19.4 26.5 58.5 24.6
n-Butanol 16.0 5.7 15.8 23.2 91.5 18.0
MPa1/2 (equivalent to joules/cubic centimeter; 2.0455 x (cal/cc)1/2) @ 25oC (298.15 K): Hansen Solubility Parameters: A
User's Handbook, 2nd Edition, Charles M. Hansen, CRC Press, Boca Raton, FL, 2007, except as noted. The total solubility
parameter is the geometric mean of the three components dD (from non-polar, or dispersion interactions), dP (from polar
attraction), and dH (from hydrogen bonding).
175
Appendix D
Comparison of silver adsorption capacities
Table D.1 Comparison of the maximum adsorption capacities of silver on different adsorbents in
literature
Sorption material
pH
T/K
Max. adsorption
capacity [mg g-1]
Ref.
Rice husk - - 1.6 42
Expanded perlite - - 8.5 43
Chitosan - - 26.9 44
Natural clinoptilolites - - 31.4 45
Clinoptilolite - - 43.0 46
Mesoporous silica - - 46.0 47
Ureaformaldehyde chelating resins - - 47.4 48
Calcium alginate beads 4 295 52 49
Chitosan/bamboo charcoal composite - - 52.9 44
6-mercaptopurinylazo resin 6 - 56.1 50
Thiourea-formaldehyde chelating resins - - 58.1 48
MFT chelating resin - - 60.1 51
Verdeloda Clay - 283 61.5 7
Manganese oxide-modified vermiculite - - 69.3 52
Valonia Tannin resin (VTR) 5 295 97.1 53
Carbon adsorbents 6 - 114.3 54
PS-TMT chelating resins - - 187.1 55
Graphitic carbon nitride - 293 400.0 24
Poly(o-phenylenediamine) micro particles 5 303 533.0 56
UiO-66 MOFs 2 298 73.0 This study
176
Appendix E
IR absorption bands
Table E.1 IR absorption bands of membranes
Wave number [cm−1] Absorbing group and type of vibration
3300-3400 -O-H stretching
2910-2925 -C-H symmetrical stretching
1390-1410 -C-H bending
1000-1030 C-C, C-OH- C-H ring and side groups
vibrations
1580, 1510 and 1392 Carboxylate groups
691 and 728 Zr-(μ3)O
177
Appendix F
Permission for third part copyright works
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work
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publication
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Publisher
Requested Licence
Number
15 Figure 2.3 Wiley oBooks John Wiley and
Sons
4071241431582
25 Figure 2.4 Chemical Society
Reviews
Royal Society of
Chemistry 4071241136490
29 Figure 2.5 Progress in
Polymer Science
Elsevier 4071240792313
Third part copyright from open access papers
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12 Figure 2.1 Chemical
Reviews
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Chemical
Society
Open Access
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14 Figure 2.2 Polymers Elsevier Open Access
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37 Figure 2.6 Scientific Report Nature Open Access
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40 Table 2.2 RSC Advances Royal Society of
Chemistry
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