Journal of Scientific & Industri al Research Vol. 60, July 2001 , pp 560-563

Nanofiltration Membranes as a Suitable Alternative to Reverse Osmosis/

Ultrafiltration Membranes in Separation Processes

U Razdan and V J Shah*

R 0 Division, Central Salt and Marine Chemicals Research Insti tute,

Gijubhai Badheka Marg, Bhavnagar 364 002

The paper reviews on thin-film composite membranes covering membranes, nanofiltration membranes, their advant ages over

other separation techniques and utility. Focuses on a limited area comprising nanofiltration (NF) membranes which appear as a

su itable alternative to reverse osmosis (RO) and ult ratiltration (UF) membrane processes. Nanofiltration (NF) has emerged as a

promising area which has extended membrane applications. F membranes are generally polyam ide memb~anes and are charged.

They have higher flux and lower salt rejecti on facilitati ng a diverse cut-ofl based on molecul ar weight or a specie with more than 92

per cent rejection.N F membranes are considered to be best alternative or purification.

Introduction Semjpermeable membranes useful for desalination and

separation processes are prepared by forming a polymeric ultrathin film having semipermeable properties on a microporous support. These are thin-film composite mem­branes having bilayer film which comprises an ultrathin film (0.3 to 3 urn) deposited on a porous support 1• The two layers have different composition and diverse func­tions. The top barrier layer is selective for solutes, while maintaining a good flux, whereas the porous layer gives support and resistst compression thus providing unhin­dered flow. Membrane process is a low energy process and can operate at mild temperatures and is a compact process in terms of space2• It can achieve si milar perfor­mance at lower power and at a lower capital cost. It does not involve any energy intensive phase change as required in other purification processes. It is a clean process and economical compared to other separation techniques . Membranes which can be "tailor-made" to meet spec ific separation properties may be non-porous, microporous, or macroporous.

*Author for correspondence

Nanofiltration Membranes N F is a relatively new membrane process which falls

be'tween R 0 and U F in its separation. characteristics (Figure I ). N F membrane is termed as "Loose" reverse osmosis membrane. It operates at low pressures, whereas R 0 membrane requires pressure of about >600 psi. Many industrial processes require separation of large molecules from small molecules . U F membranes are not able to discriminate efficiently between low molecular weight molecules, whereas reverse-osmosis membranes reject both low molecular-weight organics and salt. N F is a membrane process which retains organ ics >300 g/ mol based on cut-off of the solute wit!h rejection above 92 per cent\ and certai n multivalent salts (in case of negatively charged N F membranes) but passes s bstantial amounts of monovalent salts . The retention characteristics are based on free volume.

The utility of these membranes is that the membranes can be used for dewatering large molecules in the mo­lecular weight range 300 to I 000 g/mol, separating salts and organic acids from other organic compounds such as sugars and proteins, concentration, liquid phase separa­tion, desalting of higher organics and passing solutions with high osmotic pressure at low pressures4

·0

. Rejection

RAZDAN & SHAH: NANOFILTRATION MEMBRANES 561

Suspended ~:...:.L.L.tJ particles

MF : MICROFIL TRA TION UF : UL TRAFIL TRA TION NF NANOFIL TRA TION RO : REVERSE OSMOSIS

Proteins Antibiotics Macromolecules

Cl w 1-u UJ --, w 0::

UJ I­=> .....J 0 Vl

salts

Monovalent salts Small molecules

Water

MF UF NF MEMBRANES

RO

Fi gure I- Separat ion characteri sti cs of different types of membranes

of ions is based on size and valency, whereas the rejecti on

of organics is based on size and in vo lves siev ing effect 1.

On the other hand, R 0 membranes do not ex hibit ion

se lectivity. Because of its abi lity to reject hi gh percent­

age of many dissolved components N F membranes offer

a single treatment alternative7.

R 0 membranes are considered to be homogeneous,

whereas it has been establi shed using Atomic Force Mi­croscopy (AFM) that N F membranes have discrete pores of nm dimensionsx. They have an average pore-size eli am of -2 nm . N F membranes are more porous compared to

R 0 membranes. Be ing more porous might result in con­centration polarization and fouling . It can be overcome by better des ign and selecting materia ls possessing des ir­

ab le properties. N F membrane's low sa lt rejecti on and high water flux at low pressures, its abi I ity to separate

low molecular weight organics from high molecular weight organ ics, and also abi lity to pass solutions with high os­motic pressure at low pressure make it idea l for water softening, desalting, food process ing, waste-water treat-

ment and for various separation processes in indu stry. N F membranes can a lso be used for c leaning contam inated

ground water and for producing ultrapure water required

in e lectronics & semiconductor industry. Another re lated advantage of N .F. membranes is the reduction in di ssolved

organics based on cut-off of the membrane. When used as a pre-treatment instead of conventi onal chl orination ,

the consumption of dis infectant can be reduced signi fi­cantl y. Al so, N .F. membranes giving hi gh sa lt rejecti on can be used for desalination at a re latively low pressure or a pre-treatment for desalination. Thus N F membranes which operate at low pressure have tremendous app lica­

tions. N F membranes are charged which is utili zed in sepa­

rating sa lts based on charge and va lency. They are mostly

negati ve ly charged and se lect ively reject di va lent an ion . Bipolar membranes can be used for separating both di va­lent cations and ani ons from monovalent sa lts~. In bipo­lar membranes, pos iti vely charged layers are formed on negati vely charged N F membranes, such as NTR-74 1 0

562 J SCI IND RES VOL 60 JULY 200 1

and 7450, by us ing adso rpti on meth od us ing polyethyleneimine or quarternary ammonium polyelec­trolyte. Negative charge on membrane cause strong Donnan repulsion to divalent anions and not monovalent and divalent cations. Whereas, bipolar membranes showed good ion selectivity for both divalent cati ons and ani ons.

Although the mechanism oftranspo11 through N F mem­branes is yet not well understood, its app lications are in­creasing rapidly. Various models have been developed to describe performance of N F membranesx. The salt trans­port through a membrane is governed by different mecha­nisms such as diffusive fl ow, convective fl ow coupled to solvent flow, and convective flow through membrane de­fects. Performance of membrane systems is governed by fluid dynamics at the interface and chemical potential gra­dient. These factors can be varied by chemical modifica­tion of the membranes.

Th e composite membranes NTR-729HF and NTR-739HF, having significant chlorine resistance, have been commerci a li zed by Nitt o- Denk o. It is a poly(vinylalcohol) membrane also having a polyamide composition. NF-40 and NF-70 membranes have a bar­rier layer which is fully aromatic po lya mid and crosslinkecl 1. 10

. They are anionic in nature which is re­flected by their high Mg SO rejection compared to aCI rejection11

• They are lo~ pr~ssure membranes which are used for textile effluents, dye salts and pigments, water softening or purification, and pulp and paper industry. It also has great potential in food and pharmaceutical indus­tries, removal of sulphate from sea water and colour re­moval from caustic bleach effluents. NF-40 is the ti ght­est membrane and is free from defects. It can be used for the concentration of organics in the molecular weight range of 300 - I 000 g/mol with simultaneous removal of so­dium chloride and has high flux due to the presence of relatively charged hydrophillic groups attached to a hy­drophobic group. Due to surface active groups, they have improved fouling resistance against hydrophobic organ­ICS.

Other NF membranes rep011ed are hydroxyalkyl cellu­lose crosslinked with a dialdehyde and coated on an asy m­metric polyetheramide support with operating at > 7~"C , useful for separating compounds with molecular wetght 300-2000 12• Interpenetrating polymeric NF membranes deve loped by polycondensation reac ti on between trimcsoy l chloride and different amines in a dense layer of poly(ethyleneoxide-b-amide) 13

• A mines containing eth­ylene glycol blocks resulted in best performance hydro­phobic membranes with a cut-off as low as 600 g/mol and

a reasonable water flu x. Reinforced dry NF membranes were synthesized from acrylonitrile grafted on cellulose acetate 14

•

Reverse Osmosis (R O)membranes may be transformed into N F membranes useful for separati on of di ssolved substances from solvents by incorporati ng inorganic am­moni um cation salts before the membrane is cured. Am­monium sa lt of gluconic ac id (2.0per cent) was used in an aqueous so lution of polyamine to impart desirable rejec­tion characteri stics 15

• Flux of a composi te reverse osmo­sis or nanofiltration membranes having a polyamide dis­criminating layer is enhanced by using alkyl amines such as butylamine, cyc lohexy lamine,a ncl I ,6-hexanediam ine, N,N-dimethylethanalamine or benzylamine.. By incor­porating additi ves solute rejections can also change lead­ing to conversion of RO to NF membranes 16

•

Most of the nanofiltration membranes are polyamide ba eel containing -NH groups which are susceptible to chlorine attack. The major requirement for a membrane is that its separation characteristics should not change due to oxidative degradation . Membranes should be tolerant to attack by oxid ising species and chemical attack. This can be achieved with polyester membranes whi ch are known to be more resistant to oxidative degradation. Also, fou ling problem can be reduced by increasing hycl rophi­licity. Polysulphone backed polyesteramide based resor­cinol membranes were synthesized by in-situ interfacial condensation reaction 17

• Such membranes can be used for separation of solutes having molecular weight >100 g/mol. The resultant membranes can be used for dewater- . ing large molecules_, separating salts and organic acids from other organic compounds such as sugars and proteins and for desalting. Negatively charged phthalein based N F membranes based on polyes terlpo lyersteramide with cut-off of higher molecular weight species were also syn­thesized (Razdan U & Kulkarni S S, applied for Indian Patent, 1999). They can be used for separating higher organics and multivalent anions from monovalent salts. New membranes are being developed which have im­proved performance, separation characteristics, and higher flux .

NF membranes have introduced a new perspecti ve fo r applications in the processing of foods, pharmaceuticals , demineral ising water, for downstream processes, treatment of industrial effluents, and separation processes.

References Peterson R J, .I Memb Sci, 83 ( 1993) ~ 1- 150.

RAZDA & SHAH : NANOFILTRATION MEMBRANES 563

2 Simpson A E, Kerr C A & Buckl ey C A, Desalinarion, 64 ( 1987)

305.

3 Bruggen B V, SchaepJ & Wilms D V,J Me111b Sci ,156( 1) ( 1999)

29-41 .

4 Rautenbach R & Grosch! A, Desalinarion. 77 ( 1990) 73-84.

5 Bindoff A, Davier C J, Kerr C A & Buckley C A. De.wlinarion.

67 ( 1987) 455-465.

6 Cadotte ] , Forester R, Kim M, Petersen R & StockerT, Desalilw­

rion, 70 ( 1988), 77-88.

7 Watson B M & Hornburg C D. Desalinarion. 72 ( 1989) II .

8 Bowen W R, M ohammed A W & Hila! , J Me111h Sci, 126 ( 1997)

91- 105.

9 Urari M , Tsuru T, akao S & Kimura S, J Me111 b Sci, 70 ( 1992)

153- 162.

I 0 Freeman S D & StockerT F, Desalinarion, 62 ( 1987) 183- 19 1.

II Raman L P, Cheryan M & Raj agopalan N , Che111 Eng Pmg. 90

(3)( 1994) 68-74.

12 Schmidt M & Peinemann K-V. Ger DE 19,545.70 I (to GKSS­

Forschungszentrum Geesthacht Gmbh) 28 M ay 1997; Chen1 A w r

127 ( 1997) 83194.

13 Sforca M L, Nunes S P & Peinemann K-V, J Me111b Sci. 135(2)

( 1997) 179- 186.

14 Liu Y. Chen Y & Wang L , Water Treat. 9( I ) ( 1994) Il -l X.

15 Cadotte J E, Batze l D A & Stocker T F, US Pat. 5.65X.460 ( to

Dow Chemicals Co.) 19 August 1997; Che111 Ahsrr, 127 ( 1997)

206689.

16 Mickol s WE, PCT lnt Appl WO 97 27,935 (to Dow Chemica ls

Co.)O 7 August 1997; Chern A bstrl27 ( 1997) 19 1644.

17 Razdan U, Kulkarni S S & Joshi S G, Souvenir In DA An1111 Se111in.

Desai in water 111anage (NCL. Pune, India) 27 February 1999.