Journal of Membrane Science 441 (2013) 112–119

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Journal of Membrane Science

0376-73

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journal homepage: www.elsevier.com/locate/memsci

Mineralized polyacrylonitrile-based ultrafiltration membranes withimproved water flux and rejection towards dye

Xiao-Na Chen, Ling-Shu Wan, Qing-Yun Wu, Suo-Hong Zhi, Zhi-Kang Xu n

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou

310027, China

a r t i c l e i n f o

Article history:

Received 27 July 2012

Received in revised form

13 February 2013

Accepted 16 February 2013Available online 6 March 2013

Keywords:

Polyacrylonitrile

Mineralization

Alternate soaking process

Calcium carbonate

Permeation

88/$ - see front matter & 2013 Elsevier B.V. A

x.doi.org/10.1016/j.memsci.2013.02.054

esponding author. Tel.: þ86 571 8795 2605;

ail address: xuzk@zju.edu.cn (Z.-K. Xu).

a b s t r a c t

In this work, we employed CaCO3 mineralization to greatly improve the water flux and dye rejection of

polyacrylonitrile-based (PAN) ultrafiltration membranes. PAN-based membranes were prepared with

1.0–5.0 wt% of poly(acrylic acid) (PAA) as a matrix for mineralization. These membranes were

subsequently mineralized with CaCO3 by an alternate soaking process (ASP). The optimized miner-

alization condition was 10 cycles of ASP with 100 mM CaCl2 and Na2CO3 solutions. The resulted

membranes were characterized with field emission scanning electron microscopy, X-ray diffraction,

and energy dispersion X-ray analysis combined with elemental distribution mapping. The mineralized

CaCO3 particles were found to be deposited throughout the membranes. Results also indicated that the

pure water flux of PAN-based membranes decreased with PAA if the content was larger than 1.0 wt%,

which was due to the stretching of PAA chains deriving from the electrostatic repulsion of –COO�

groups. In contrast, the mineralized membranes showed a dramatic increase of water flux owing to the

chain collapse of PAA caused by the formation of complexes with Ca2þ in the mineralization of CaCO3

as well as the enhanced hydrophilicity by CaCO3. The mineralized membranes even showed a high

rejection of Congo red, which makes them potential in nanofiltration for dye-polluted wastewater.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Polyacrylonitrile (PAN) is one of the most widely used poly-mers for making ultrafiltration membranes owing to its goodsolvent resistance [1–3], sufficient chemical stability [4,5], excel-lent mechanical properties [4,6], and superior thermal stability[7,8]. A series of investigations have been carried out to improvethe permeation performance or rejection ability of PAN-basedultrafiltration membranes. Pore-reduced PAN membranes wereprepared by chemical conversion of the nitrile (–CN) groups withNaOH or CH3ONa [6,9]. Surface modification, such as the UV- orplasma-induced graft polymerization of acrylic acid (AA) [10],styrene [11], and monomethyl polyoxyethylene methacrylates(MePEOMAs) [12], was also used to prepare PAN membraneswhich were able to reject small molecules. Other efforts wereimplemented to increase the permeation performance of this kindof membranes by addition of small or polymeric additives intocasting solutions and selection of coagulation bathes or post-treatments [1,13,14]. However, these strategies, either involving achemical reaction process or using a large amount of organic

ll rights reserved.

fax: þ86 571 8795 1773.

compounds, are environmentally unfriendly and relativelycomplicated.

Biomineralization, a widespread phenomenon among sustain-able system, has received more attention recently because theformed inorganic–organic hybrid composite has significantmechanical property and biocompatibility [15–17]. Calcium car-bonate (CaCO3) constitutes an attractive model mineral because itis one of the most abundant inorganic biominerals produced innature and possesses many important applications in industryproducts, such as papers, plastics, rubbers, and textiles [18]. Itwas found that controlling and directing the crystallization ofCaCO3 in nature generally involve acidic proteins carrying anionicgroups [19]. Although the mechanism for biomineralization hasnot been thoroughly understood up to now, it is believed thatsoluble acidic polymers and insoluble organic matrix are involvedin the mineralization process [20]. The anionic groups of poly-mers are able to enrich Ca2þ ions, leading to a local super-saturation and then inducing the nucleation of CaCO3 [21]. Manysoluble acidic molecules or polymers have been studied to controlthe mineralization process of CaCO3, including amino acids[22,23], polycarboxylic acids [24,25], synthetic peptides [26,27],dendrimers [28], and double hydrophilic block copolymers con-taining anionic groups [29,30]. Among these, poly(acrylic acid)(PAA) has been widely utilized in mimicking the mineralizationprocess of CaCO3 due to its intensive –COO� groups [24,31]. Up to

X.-N. Chen et al. / Journal of Membrane Science 441 (2013) 112–119 113

now, most studies concerning the mineralization process ofCaCO3 were carried out in aqueous solutions or on a modelsurface, such as self-assembled monolayer [32–34], Langmuirmonolayer [35,36], and layer-by-layer film [37]. To the best ofour knowledge, scarce reports were presented on the mineraliza-tion of CaCO3 in/on materials that have been practically used [16].

We suggest fabricating PAN/CaCO3 membranes via the miner-alization process and expect these membranes possessing highpermeation for water with desirable rejection to dyes. In thiswork, PAN membranes were prepared employing a typical phaseinversion method. A small amount of PAA was added as matrix forthe mineralization. These PAN membranes were mineralized withCaCO3 by the alternate soaking process (ASP), a simple method todeposit calcium salts (such as calcium carbonate and calciumphosphate) in polymeric materials [38,39]. We optimized themineralization factors and carefully studied the permeation andrejection as well as absorption properties of the mineralizedmembranes. The PAN/CaCO3 membranes have higher permeationand dye rejection than those of non-mineralized ones.

2. Experimental

2.1. Materials

PAN (Mw¼280,000) was purchased from Spectrum ChemicalCo. Ltd., China. PAA (Mw¼450,000) was commercially obtainedfrom Aladdin Chemistry Co. Ltd., China. Anhydrous sodiumcarbonate (Na2CO3, 99.8%) and N,N-dimethylformamide (DMF,99%) were purchased from Sinopharm Chemical Reagent Co. Ltd.Anhydrous calcium chloride (CaCl2, 96%) was acquired fromQuzhou Juhua Chemical Reagent Co. Ltd., China. Congo red inAR grade was obtained from Aladdin Chemistry Co. Ltd., China.PAN was dried before use and other chemicals were used withoutfurther purification.

2.2. Preparation of the PAN-based ultrafiltration membranes

PAN-based asymmetric membranes were prepared via thephase inversion process. A series of PAN/PAA mixtures weredissolved in DMF to form homogeneous casting solutions. All

Fig. 1. FESEM images for the cross-section morphologies of PAN-based membranes w

(d) 3.0 wt%, (e) 4.0 wt%, and (f) 5.0 wt%.

casting solutions had a total polymer concentration of 12.5 wt%,while PAA content in the total polymer varied from 0 to 1, 2, 3,4 or 5 wt%. Each casting solution was stirred at 60 1C for 12 h andkept for another 6 h without stirring to completely release airbubbles. Then, the solution and a glass plate were preheated to90 1C for 2 h before casting. Subsequently, the solution was caston the glass plate with a casting knife, and the plate wasimmediately immersed in a coagulation bath of water at 30 1C.The obtained membrane was left in deionized water overnight toallow complete release of residual solvent. It was then succes-sively immersed in 80:20, 60:40, 40:60, 20:80 and 0:100 water/ethanol mixtures for 15 min and afterwards in n-hexane over-night. Finally, the membrane was dried in vacuum at 60 1C for 6 hfor further characterization.

2.3. Mineralization of the PAN-based membranes

An ASP method was carried out to mineralize CaCO3 into thePAN-based membranes. A sample of membrane in the wet state(kept in ionized water) was first immersed in a CaCl2 solutionranging from 0.01 M to 0.5 M for 30 s, followed by rinsed withdeionized water for another 30 s. Subsequently, the sample wasimmersed in a Na2CO3 solution having the same concentrationwith the CaCl2 solution and rinsed with deionized water for 30 s,respectively. The above processes were referred to as 1 cycle(ASP1) and ASP10 means that we prepared a mineralized mem-brane with 10 cycles of ASP. A series of mineralized membraneswere prepared with 0, 1, 3, 5, 7, 10 and 15 cycles.

2.4. FESEM/EDX analysis of the membranes

The cross-section and surface structure of virgin membraneswere observed by field emission scanning electron microscopy(FESEM, S-4800, Hitachi, Japan). To observe the cross-section, themembrane was fractured in liquid nitrogen. All samples weresputtered with gold for 60 s prior to FESEM measurement and thethickness of gold for the sample is 10–20 nm. The elementcomposition and distribution of membranes before and aftermineralization were determined by energy dispersion X-ray(EDX) analysis employing the FESEM with a 20 keV energy beam.

ith PAA content in the total polymer blend of (a) 0 wt%, (b) 1.0 wt%, (c) 2.0 wt%,

Fig. 2. FESEM images for the surface morphologies of PAN-based membranes with PAA content in the total polymer blend of (a) 0 wt%, (b) 1.0 wt%, (c) 2.0 wt%, (d) 3.0 wt%,

(e) 4.0 wt%, and (f) 5.0 wt%.

Fig. 3. X-ray diffraction for PAN-based membranes before and after mineraliza-

tion. Both membranes contain 3.0 wt% PAA. CaCl2 and Na2CO3 concentrations of

100 mM were used in mineralization.

X.-N. Chen et al. / Journal of Membrane Science 441 (2013) 112–119114

2.5. XRD analysis

Dry membranes, having a thickness of 210 mm, were foldedlayer by layer and stuck with dual adhesive until the thicknessreached about 1 cm. Then the stacked membrane was cut laterallyto obtain a flat and dense cross-section sample for X-ray diffrac-tion (XRD) analysis. The X-ray diffraction study was carried out ona Rigaku D/Max-2550PC X-ray diffractometer (Panalytical,Netherlands).

2.6. FT-IR/ATR measurements

Attenuated total reflectance infrared spectroscopy (FT-IR/ATR)was carried out on a Nicolet 6700 spectrometer (Thermo FisherScientific, USA). The spectra were obtained in the region of 4000–500 cm�1 and collected by cumulating 32 scans at a resolution of4 cm�1. Each spectrum was baseline-corrected in the wholeregion using the automatic baseline correct function of the Omnicsoftware (Nicolet).

2.7. Water contact angle determination

Time-dependent static contact angle (SCA) was determined atroom temperature (25 1C) on a contact angle goniometer (CTS-200, Mighty Technology Pvt. Ltd., China). Samples were kept invacuum at 60 1C for 6 h before contact angle measurement.A total of 2 mL of ultra-pure water was dropped onto the drymembrane with a micro-syringe and the half-spherical shape ofthe droplet was captured and calculated. At least 10 contactangles were measured to get a reliable result.

2.8. Water permeation measurement

Water filtration was done by a dead-end stirred-cell filtrationsystem (Millpore 6700P05, USA) and effective area of each piece ofmembrane was 4.9 cm2. The membrane was first installed into thepermeation cell and conditioned by filtering ultra-pure water at0.12 MPa for 30 min. The pressure was then lowered to 0.10 MPato measure the water flux. All experiments were carried out atroom temperature (25 1C). The pure water flux (PWF) was deter-mined in terms of liter per square meter per hour (L/(m2 h)) and

calculated by the following equation:

PWF¼ V=ADt

where V, A, and Dt are the volume of permeated water (L), themembrane area (m2) and the permeation time (h), respectively. Foreach sample, three membranes were parallelly measured to get anaverage value.

2.9. Rejection and static adsorption of Congo red

Rejection of the membranes was measured with Congo red in thedead-end stirred-cell filtration system. Here, we choose 3.0 wt% PAN-based membranes both before and after mineralization as typicalmembranes. Before the test, all membranes were pressured at0.3 MPa for 30 min with ultra-pure water to reach a stable flux. Afterthat, ultra-pure water in the cell was changed to 0.05 mg/mL Congored solution for another 30 min before testing the permeation and

X.-N. Chen et al. / Journal of Membrane Science 441 (2013) 112–119 115

rejection of Congo red solution. The rejection (R) is defined as

R¼ ð1�Cp=Cf Þ � 100%

where Cp and Cf represent feed and permeated Congo red concentra-tion, respectively.

The static adsorption of Congo red was carried out by soakingeach membrane with a diameter of 2.5 cm in 10 mL 0.05 mg/mLCongo red solution for 6 h at room temperature (25 1C). Theconcentration of Congo red solution was analyzed by UV–visiblespectrophotometer (UV-2450, SHIMADZU, Japan) at 497 nm.To eliminate infiltration difference, the membrane before miner-alization was first rinsed with ethanol for 30 min and then

Fig. 4. EDX patterns of the mineralized membrane and element contents for

the membranes before (a) and after mineralization (b). Sample is a

PAN-based membrane with 3.0 wt% PAA. The mineralization was carried out

when CaCl2 and Na2CO3 of 100 mM were used in ASP10. (For interpretation of the

references to color in this figure, the reader is referred to the web version of this

article.)

Fig. 5. FESEM images (a1, b1) and EDX images of Ca (a2, b2) for PAN-based memb

interpretation of the references to color in this figure, the reader is referred to the we

transferred into ultra-pure water for another 30 min before theadsorption experiment.

3. Results and discussion

3.1. Effects of PAA content on the morphology of PAN-based

membrane

Figs. 1 and 2 show FESEM micrographs of the cross-section andsurface of PAN-based membranes with various PAA contents,respectively. All membranes have a typical asymmetrical structureconsisting of a dense top layer and a porous sub-layer as well asfinger-like macrovoids. As the amount of PAA increases, the macro-voids gradually enlarge while the top layer becomes thin andslightly dense. It is well known that PAA can affect the phaseseparation process due to its water solubility. PAA as an additive tocasting solution will decelerate the diffusional exchange ratebetween solvent and nonsolvent and the growth of the polymer-poor phase was hindered, resulting in an increase membraneporosity and suppression of macrovoids [40]. However, the FESEMimages did not show these results.

3.2. Mineralization of PAN-based membranes

ASP is a simple method to deposit calcium salts such ascalcium carbonate and calcium phosphate in polymeric materials[38,39]. In our cases, PAN-based membranes are alternatelysoaked in CaCl2 and Na2CO3 solutions which contain Ca2þ andCO3

2� to generate CaCO3 particles. XRD measurements can bedirectly used to confirm the deposition and the crystal form ofCaCO3 in the cross-section of the membrane (Fig. 3). Strong peaksat (104), (113), (202), (018) and (116) reveal the presence ofcalcite inside the membrane, while peaks at (110), (112), (114)and (300) suggest the existence of vaterite. The membrane cross-section was further examined by EDX spectrum and correspond-ing elemental distribution mapping. EDX data (Fig. 4) clearlydemonstrates that calcium element is present in the membrane

ranes with 3.0 wt% PAA before (a1, a2) and after (b1, b2) mineralization. (For

b version of this article.)

Fig. 6. FT-IR/ATR spectra for PAN-based membranes with 3.0 wt% PAA when CaCl2

and Na2CO3 of 100 mM were used in different ASP cycles.

Fig. 7. Water contact angle for PAN-based membranes with 3.0 wt% PAA when

CaCl2 and Na2CO3 of 100 mM were used in different ASP cycles.

Fig. 8. FT-IR/ATR spectra of PAN-based membranes with 3.0 wt% PAA when

different CaCl2 and Na2CO3 concentrations were used in ASP10.

Fig. 9. Water contact angle of PAN-based membranes with 3.0 wt% PAA when

different CaCl2 and Na2CO3 concentrations were used in ASP10.

X.-N. Chen et al. / Journal of Membrane Science 441 (2013) 112–119116

after mineralization, and its content reaches 22.68 wt%, which ismuch higher than that before mineralization (ASP0). This resultprovides an indirect evidence for the presence of CaCO3 in themembrane cross-section. Fig. 5 displays the elemental distribu-tion mapping of EDX for a typical PAN-based membrane beforeand after mineralization, in which the yellow points represent thesignal of the calcium element in the sample. Fig. 5(a2) showsscattered yellow points, while Fig. 5(b2) presents a much denserspread of yellow points, indicating that calcium is spread over thecross-section of the mineralized membrane. These results demon-strate that CaCO3 has been successfully deposited throughout themembrane.

3.2.1. Effects of repeated soaking on the mineralized PAN-based

membranes

Effects of ASP cycle number on the deposition of CaCO3 in PAN-based membranes were analyzed by FT-IR/ATR measurements(Fig. 6) and SCA (Fig. 7). Compared with the membrane beforemineralization, absorption peaks at 711 and 871 cm�1 for themineralized membranes indicate the presence of calcite. Vibrationat 849 cm�1 shows the precipitation of vaterite, and the relativelyweak absorption also suggests that the proportion of vaterite isquite small, which is in agreement with those results from XRD inFig. 3. In addition, the spectra also show strong absorption at 1387–1408 cm�1. According to literatures [41–43], absorption peaks at1384, 1420, 1430 and 1440 cm�1 are considered to be the char-acteristic bands of CO3

2�. On the other hand, along with theincreased cycles, FT-IR spectra of CaCO3 exhibit increased absorptionsignals at the characteristic bands, indicating an increase of CaCO3 inthe membranes. As shown in Fig. 7, the initial water contact angle ofthe membrane containing 3.0 wt% PAA before mineralization is 631,while it decreases with an increase of ASP cycles for mineralization.This is due to the enhanced hydrophilicity of the membrane aftermineralization by the deposition of CaCO3 particles. After 10 cycles,the initial water contact angle drops to 281, indicating a highlyhydrophilic surface. However, as ASP cycles further increased to 15cycles, it was time consuming and the decrease of water contactangle was not so obvious. Given the above results, we choose 10cycles as the optimized cycle numbers.

3.2.2. Effects of CaCl2/Na2CO3 concentration on the mineralized

PAN-based membranes

We changed the concentrations of CaCl2 (Ca2þ) and Na2CO3

(CO32�) solutions from 10 mM to 500 mM for ASP mineralization.

X.-N. Chen et al. / Journal of Membrane Science 441 (2013) 112–119 117

As shown in Fig. 8, with the increase of concentration, FT-IR spectraof CaCO3 show increased absorption intensity at the characteristicbands (711, 871 cm�1, 749 cm�1, and 1392 cm�1), which suggestsan increased amount of CaCO3 in the membrane. Fig. 9 shows adecrease in initial water contact angle along with increasing CaCl2and Na2CO3 concentrations. The value drops from 631 to below 201within the experimental range of concentration. Increasing the

Fig. 10. (a) Water flux of PAN-based membranes with different PAA contents

before and after ASP mineralization; (b) ratio of water flux for the membranes

after and before mineralization.

Fig. 11. Schematic illustrations for (a) stretching of PAA chains leading to block

concentration to 200 mM or 500 mM, brittle mineralized mem-branes are also obtained due to formation of large CaCO3 particlesin the membrane. Thus, 100 mM CaCl2 and Na2CO3 are consideredto be an appropriate concentration for ASP mineralization.

3.3. Properties of the mineralized PAN-based membranes

Pure water flux (PWF) is a key performance of ultrafiltrationmembranes. It is directly related to the thickness of the skin layer,which is considered to be the main mass transfer resistance ofasymmetric membranes. Generally speaking, the thinner the skinlayer is, the higher the water flux is. Fig. 10(a) shows that the PWFof PAN-based membrane with 1.0 wt% PAA (176 L/m2 h) isslightly higher than that of pure PAN membrane (160 L/m2 h).However, the PWF dramatically decreases to 72 L/m2 h for thePAN-based membrane containing 2.0 wt% PAA and eventuallydecreases to 51 L/m2 h in the case with 5.0 wt% PAA. In contrast,the permeation properties of PAN-based membranes can beeffectively improved by mineralization. For example, the mem-brane containing 3.0 wt% PAA rises from 71 L/m2 h to 501 L/m2 h.Fig. 10(b) illustrates water flux ratios of mineralized membranesto corresponding blank ones. The ratio gradually increases withthe increase of PAA content.

This result may be owing to the thin skin layer (Fig. 1) andimproved hydrophilicity of the PAN membrane by adding smallamount of PAA. As we known, PAA is a weak polyelectrolyte(pKa�4.9) [44–46], and thus the carboxylic groups of PAA couldpartially ionize into –COO� under pure water condition (pH¼7). Theaddition of PAA brings two impacts, one is the increase of hydro-philicity and a thinner top-layer, the other is the blocking of themembrane pores by chain stretching (Fig. 11a). When the amount ofPAA is small, the former effect is dominant and results in anincreasing water flux. Otherwise, the latter effect becomes significant,and a decrease in water flux is obtained.

On one hand, the deposited CaCO3 enhances the hydrophilicityof the mineralized membranes. On the other hand, Ca2þ can formcomplex with –COO� of PAA chains, leading to chain collapse[47], making it easy for the passage of water through themembrane pores (Fig. 11(b)).

Table 1 compares the permeation, rejection and static adsorptionof 0.05 mg/mL Congo red solution for a typical PAN-based membranebefore and after mineralization. Congo red is a negatively chargedacid dye with a molecular weight of 696 Da. The mineralizedmembrane displays a higher rejection (98%) of Congo red solutionthan that of membrane before mineralization (24%). It indicates thatthe mineralized membrane has potential in nanofiltration. Accord-ingly, the membrane shows a lower flux of Congo red after miner-alization than before. Most –COOH in the membrane beforemineralization can be ionized into –COO� under 0.05 mg/mL Congo

age of membrane pore and (b) collapse of PAA chains after mineralization.

Table 1Rejection, adsorption and permeation properties for a PAN/PAA membrane before

and after mineralization to Congo red solution. The membrane contains 3.0 wt%

PAA.

Sample Rejection (%) Adsorption (mg/m2) Flux (L/m2 h)

Before mineralization 24 81.6 83.3

After mineralization 98 427.0 25.5

X.-N. Chen et al. / Journal of Membrane Science 441 (2013) 112–119118

red solution (pH¼8.45), and thus the rejection of Congo red ofmembrane before mineralization mainly results from the electrostaticrepulsion between –COO� and the negatively charged Congo red. Incontrast, the mineralized membrane is more likely to electrostaticallyadsorb Congo red, which will narrow the pore size and hinder thepassage of Congo red [48]. On the other hand, only 82.7 mg/m2 Congored has been adsorbed on the membrane before mineralization,whereas the adsorption increases by 5 times after mineralization.The difference in Congo red adsorption is supposed to be an effect ofthe membrane surface charges [49].

4. Conclusion

Polyacrylonitrile-based membranes can be facilely mineralized byCaCO3 deposition with an alternate soaking process. The mineralizedmembranes show obvious improvement in surface hydrophilicity,water flux, and rejection towards Congo red. It seems that miner-alization collapses the stretching of poly(acrylic acid) chains via theformation of complexes of –COO� with Ca2þ . We suggest thesemineralized membranes have great potentials in nanofiltration forwastewater contaminated by dyes.

Acknowledgments

The research is financially supported by the National NaturalScience Foundation of China (Grant no. 21174124) and the State KeyLaboratory of Chemical Engineering (No. SKL-ChE-10D02).

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