LBL Surface Modification of a Nanofiltration Membrane for Removing

Article pubs.acs.org/IECR

LBL Surface Modification of a Nanofiltration Membrane for Removing the Salts of Glutathione Solutions Tao Zhang, Haitao Gu, Peiyong Qin,* and Tianwei Tan Beijing Key Laboratory of Bioprocess, Beijing University of Chemical Technology, Beijing 100029, P. R. China ABSTRACT: Dilute glutathione solutions containing NaCl need to be purified and concentrated. Nanofiltration (NF) would be a good solution, provided that it could completely reject glutathione while permeating NaCl entirely. Commercially available NF membranes are incapable of meeting both of these targets. Therefore, in this study, we modified a commercially available NF membrane by the layer-by-layer (LBL) assembly of alternating polyelectrolyte on thin films, to increase both the glutathione rejection and the membrane selectivity for NaCl over glutathione. Poly(styrene sulfonate) (PSS) was selected as the anionic polyelectrolyte, and poly(diallyldimethylammonium chloride) (PDADMAC) was chosen as the cationic polyelectrolyte. The separation performances of the modified membranes were studied with 1 g/L synthetic glutathione solutions containing 2 g/L NaCl at a pressure of 800 kPa. The results demonstrated that the glutathione rejection increased from 74% (unmodified) to 97.27% ([PSS/PDADMAC]10/PSS) and the selectivity for NaCl over glutathione increased from 2.8 (unmodified) to 23 ([PSS/ PDADMAC]10/PSS).

1. INTRODUCTION Glutathione is a tripeptide-containing thiol, combining glutamic acid, cysteine, and glycine. It is a nonprotein thiol and is significantly in demand as a therapeutic drug because of its multiple biological functions in various tissues and its involvement in many diseases and malnutrition.1−3 A reduced glutathione ratio contributes significantly to some human diseases, including lung inflammation, chronic renal failure, diabetes, alcoholism, Parkinson’s disease, and cataract formation,4 because it has antioxidant and detoxification effects. Most glutathione is produced by fermentation. It is prepurified by ion exchange, resulting in a dilute glutathione solution containing considerable amounts of NaCl. Both the removal of NaCl and the further concentration of glutathione are further required. Separation techniques such as ion exchange, electrodialysis, and adsorption on activated alumina have been found to be nonselective,5 developing unwanted byproducts6 and suffering from low binding capacity depending on the composition of the solution.7 These drawbacks can possibly be overcome by membrane separation. Nanofiltration (NF), widely used in water-softening applications,8,9 is recognized as a very attractive alternative because of its selectivity and high flux. In its specific application to glutathione solutions, NF should completely reject glutathione while permeating NaCl entirely. Existing NF membranes fail to achieve these targets. NF membrane modification might achieve the desired results. There are many techniques for membrane surface modification such as dip coating,10 redox-initiated graft polymerization,11 UV-initiated graft polymerization,12 plasma-induced graft modification,13 ion beam implantation,14 electron-beam treatment,15 chemical treatment by immersion in inorganic acids,16 and layer-by-layer deposition.17−21 Layer-by-layer deposition is considered as most attractive, because it can control the thickness of thin films at the nanometer scale, the charge density, the composition of the active layer, and the ion rejection.22−27 The technique of layer-by-layer deposition was first introduced by Decher and Hong,28 who applied oppositely © 2013 American Chemical Society

charged polymers by consecutive alternating immersion of a substrate in baths containing positively and negatively charged polyelectrolyte aqueous solutions. Self-assembly of polyelectrolytes is a versatile method for building up controlled nanostructures on the surface of existing objects.29,30 The modification of the membrane surface can be monitored in a very simple way within the nanoscale range so that the surface properties can be precisely adjusted.31 It was reported that a thin film was formed by the layer-by-layer deposition of polyelectrolytes on the surface of a porous membrane. This film was able to achieve interesting performances in gas separation,32,33 pervaporation, 33 and ion separation.34 In this study, NF270 membranes were modified by the layerby-layer (LBL) electrostatic deposition of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(styrene sulfonate) (PSS) (Figure 1) with various numbers of alternating layers. The resulting membranes were applied to remove NaCl from glutathione solutions.

2. EXPERIMENTAL SECTION 2.1. Materials. Nanofiltration membranes (NF270) were obtained from FilmTec Corporation (Minneapolis, MN) and used as substrates for modification. Poly(diallyldimethylammonium chloride) (PDADMAC) and poly(styrene sulfonate) (PSS) were purchased from Aldrich Chemical Co. (Milwaukee, WI). NaCl was purchased from Beijing Chemical Works (Beijing, China). Deionized water (DI) was used in the preparation of aqueous solutions (Milli-Q water purification system, Millipore, Billerica, MA). Glutathione was purchased from Amresco Company (Solon, OH). Received: Revised: Accepted: Published: 6517

March 3, 2013 April 18, 2013 April 19, 2013 April 19, 2013 dx.doi.org/10.1021/ie400694q | Ind. Eng. Chem. Res. 2013, 52, 6517−6523

Industrial & Engineering Chemistry Research

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spectroscopy (XPS) (ESCALAB-250, Thermo Scientific, Waltham, MA) measurements were carried out using monochromatized Al Kα X-rays with a 45° angle on the [PSS/PDADMAC]2/PSS membrane. The surface morphology of the membranes was examined by scanning electron microscopy (SEM) (Hitachi S-4700, Tokyo, Japan). Quantitative surface roughness analysis of the composite membranes was performed using atomic force microscopy (AFM) imaging (Mutimode 8, Bruker, Billerica, MA). The surface average roughness (Ra) of the membranes was described in terms of the root-mean-square (RMS) roughness. 2.4. LBL Surface-Modified NF Membranes to Concentrate and Desalt Glutathione Solutions. To avoid interference from other impurities in the solutions, synthetic solutions of glutathione and NaCl were used in the experiments, but in line with the concentrations found in prepurified glutathione. These solutions contained 1 g/L glutathione and 2 g/L NaCl. The pH value of the feed solutions was 3.3, and the isoelectric point for glutathione is about 5.93, so the glutathione molecules were positively charged. Each membrane was housed in an NF setup (HP4750, Sterlitech, Kent, WA), at an effective membrane surface area of 14.6 cm2. Feed solution (50 mL) was added to the NF equipment and was concentrated 10 times. The pressure was 800 kPa (∼8 bar). The permeate solution flux (J, L/m2·h) was calculated using the equation

Figure 1. Structures of the polyelectrolytes used in this study.

2.2. Surface Modification of the NF Membrane. Prior to modification, the NF270 membranes were soaked in water for a minimum of 5 h. Each membrane was carefully cut into a suitable size to be used in the membrane cell, mounted on a custom-designed holder with the active surface facing upward, and held tightly to expose the effective membrane area. The NF membrane was rinsed with an aqueous solution of 10 g/L PSS in 0.5 M NaCl for 15 min and then with deionized water for 5 min (step 1). The membrane was then rinsed an aqueous solution of 10 g/L PDADMAC in 0.5 M NaCl for 15 min and then with deionized water for 5 min (step 2). This procedure was repeated n times until the desired [PSS/PDADMAC]n membrane coating was reached. If the [PSS/PDADMAC]n membrane was rinsed with an aqueous solution of 10 g/L PSS in 0.5 M NaCl again, a [PSS/PDADMAC]n/PSS membrane was obtained. The n of [PSS/PDADMAC]n and [PSS/ PDADMAC]n/PSS membranes is the number of layer pairs. The top surface of the [PSS/PDADMAC]n/PSS membranes was PSS, and PDADMAC was the top surface of the [PSS/ PDADMAC]n membranes. The pH values of PSS and PDADMAC are 4.4 ± 0.5 and 6.0 ± 0.5, respectively. 2.3. Characterization of Membranes. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was employed to analyze the functional groups of the membranes. ATR-IR spectra were recorded on a Varian 3100 FTIR spectrometer with a ZnSe crystal. X-ray photoelectron

J=

Q At

(1)

where Q is the amount of permeate collected, A is the effective membrane area, and t is the sampling time. The concentration of glutathione was determined by highperformance liquid chromatography (HPLC) (Waters 1525, Waters, Milford, MA). The HPLC column was C18 (4.6 × 150 mm), the flow rate was 0.8 mL/min, and the UV absorption wavelength was at 238 nm. The concentration of NaCl was determined by conductivity meter (DDS-307, Leici, Shanghai, China). The solute rejection of the NF membrane for glutathione and NaCl was calculated using the equation

Figure 2. ATR-IR spectra of unmodified and LBL surface-modified NF membranes. 6518

dx.doi.org/10.1021/ie400694q | Ind. Eng. Chem. Res. 2013, 52, 6517−6523

Industrial & Engineering Chemistry Research ⎛ Cp ⎞ R (%) = ⎜1 − ⎟ × 100 Cf ⎠ ⎝

Article

(2)

where Cp is the concentration of solute in the permeate and Cf is the corresponding concentration in the feed. The selectivity (S) for A over B was calculated as S=

CpACfB CfACpB

=

100 − RA 100 − RB

(3)

where CpA and CpB are the concentrations of solutes A and B, respectively, in the permeate and C fA and C fB the correspondingconcentrations in the feed. RA and RB are the rejections of A and B, respectively. After the membrane performance tests, the [PSS/PDADMAC]4 and [PSS/PDADMAC]4/PSS membranes were placed into 50 mL of deionized water and cleaned by ultrasonication. Then, the concentration of glutathione in the deionized water was determined by HPLC. The glutathione adsorption capacity of the membrane was thus measured. The membrane glutathione adsorption capacity (M, g/m2) was calculated as

M=

CV A

(4)

where C is the concentration of glutathione, V is the volume of aqueous solution, and A is the effective membrane area.

3. RESULTS AND DISCUSSION 3.1. Membrane Characterization. ATR-FTIR spectra are provided in Figure 2. The wavenumbers ranged from 800 to 1800 cm−1. The spectrum of the unmodified membrane (Figure 2a) did not show a peak at 1033 cm−1. The spectra of the modified [PSS/PDADMAC]2/PSS (Figure 2b) and [PSS/ PDADMAC]4/PSS (Figure 2c) membranes showed peaks at 1033 cm −1 . The height of that peak for the [PSS/ PDADMAC]4/PSS membrane was greater than that for the [PSS/PDADMAC]2/PSS membrane. The peak at 1033 cm−1 corresponds to the sulfonate stretch of PSS present on top of the base membrane.35 Its presence provides evidence of polyelectrolyte modification. The adsorption capacity of PSS on the membrane increased with the number of repeated depositions. To further elucidate the presence of PSS on the [PSS/ PDADMAC]2/PSS membrane, we performed XPS measurements. As shown in Figure 3, the XPS spectrum exhibited a peak at around 169 eV due to S 2p of the PSS moieties, and the O 1s spectrum showed a peak at around 531 eV, assigned to PSS moieties.36 Figure 4 shows the surfaces of the unmodified nanofiltration membrane and the LBL surface-modified membranes [PSS/ PDADMAC]2 and [PSS/PDADMAC]2/PSS. The surface morphology of the unmodified membrane had a smooth and regular structure. The surface morphology of the [PSS/ PDADMAC]2 membrane was rougher than that of the unmodified membrane, and there were defects on the surface. This might be because of the strong molecular rigidity of PDADMAC, making it difficult to form a dense morphology, which might affect the retention performance of the membrane. The surface morphology of the [PSS/PDADMAC]2/PSS membrane was denser than that of the [PSS/PDADMAC]2 membrane, as PSS is more flexible than PDADMAC. The retention performance of the membrane was correspondingly better. However, there were some bumps on the membrane,

Figure 3. XPS (S 2p, O 1s) spectra of [PSS/PDADMAC]2/PSS membrane.

which might be due to the agglomeration of PSS molecules. This might make the [PSS/PDADMAC]2/PSS membrane rougher than the [PSS/PDADMAC]2 membrane. AFM was used for the morphological characterization of the membrane surfaces (Figure 5). Table 1 lists the average roughness (Ra) values of the membrane surfaces (Figure 5). It can be seen that the Ra value of the membrane increased after the membrane was modified and that the Ra values of the membrane surfaces increased with the amount of absorbed layers. This was in good agreement with previous research.31 The Ra values of the [PSS/PDADMAC] 2 and [PSS/ PDADMAC]4 membranes were less than those of the [PSS/ PDADMAC]2/PSS and [PSS/PDADMAC]4/PSS membranes. This might be due to the occurrence of some bumps on the surface of the [PSS/PDADMAC]2/PSS and [PSS/PDADMAC]4/PSS membranes. These results are consistent with the SEM findings. 3.2. LBL Surface-Modified NF Membrane for Removing Salt from Glutathione Solutions. 3.2.1. Effect of the Number of Layer Pairs of [PSS/PDADMAC]n/PSS[PSS/ PDADMAC]n Membranes on the Removal of Salt from Glutathione Solutions. Figure 6 indicates that the glutathione rejection, sodium chloride rejection, and selectivity for sodium chloride over glutathione increased with increasing number of layer pairs in the [PSS/PDADMAC]n/PSS membranes. There was a large increase in glutathione rejection (from 75% to 90%) when the number of layer pairs increased from 0 to 1, whereas the flux decreased significantly (from 35 to 26 L/m2·h). For n = 0, the membrane was of the [PSS/PDADMAC]0/PSS type. 6519



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Figure 4. SEM images of unmodified and modified membrane surfaces: (1) unmodified membrane, (2) [PSS/PDADMAC]2 membrane, (3) [PSS/ PDADMAC]2/PSS membrane.

Figure 5. Three-dimensional AFM images (1 × 1 μm2) of unmodified and modified membrane surfaces: (a) unmodified membrane, (b) [PSS/ PDADMAC]2 membrane, (c) [PSS/PDADMAC]4 membrane, (d) [PSS/PDADMAC]2/PSS membrane. (e) [PSS/PDADMAC]4/PSS membrane.

This basic membrane first adsorbed PSS by weak hydrophobic interactions,31 and the membrane surface was relatively loose. Therefore, the separation performance of the membrane was not good. When the number of layer pairs was 1, the membrane was of the [PSS/PDADMAC]1/PSS type. PDADMAC and PSS adsorbed by strong electrostatic interactions,37 and the membrane surface was relatively dense; therefore, the separation performance of the membrane was better. With the further self-assembly of multilayers, the adsorption of PSS increased. The pore size of the membrane decreased with increasing numbers of bilayers.35 The membrane surface was denser, and the separation performance was better. The glutathione rejection was stable (about 97%) after the number of layer pairs reached 4, when PSS had reached adsorption saturation. The structure and properties of the membrane did not change significantly, and the performance of the membrane was fairly stable. The selectivity increased from 2.9 to 23.0, demonstrating the excellent performance in removing NaCl from glutathione solutions after the membrane was modified by layer-by-layer (LBL) assembly. The flux declined with the number of layer pairs of the membranes [PSS/PDADMAC]n/ PSS, as shown in Figure 7. This was because the pore size was reduced as the number of layer pairs increased.35 Therefore, the number of layer pairs influenced the membrane performance.38,39 Figure 8 indicates that glutathione rejection, sodium chloride rejection, and selectivity for sodium chloride over glutathione

Table 1. Average Roughness (Ra) Values of Unmodified and Modified Membrane Surfaces membrane

Ra (nm)

unmodified [PSS/PDADMAC]2 [PSS/PDADMAC]4 [PSS/PDADMAC]2/PSS [PSS/PDADMAC]4/PSS

2.63 3.54 3.75 3.89 5.50

Figure 6. Glutathione rejection, NaCl rejection, and selectivity of NaCl over glutathione as functions of the number of layer pairs in [PSS/PDADMAC]n/PSS membranes.

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MAC]n/PSS membranes was high, and the flux was low. Another reason for this observation is that the positively charged glutathione molecules absorbed on the negatively charged surface of the [PSS/PDADMAC]n/PSS membranes and formed a positive layer on the surface. The positively charged glutathione layer attached to the surface formed an extra layer that neutralized the membrane surface and could reject the glutathione due to Donnan exclusion.40 The positively charged [PSS/PDADMAC] n membranes also rejected the glutathione due to Donnan exclusion. Therefore, the glutathione adsorption capacities of the [PSS/PDADMAC]n/PSS membranes were higher than those of the [PSS/ PDADMAC]n membranes. This can be observed in Figure 9. It Figure 7. Fluxes of the [PSS/PDADMAC]n/PSS and [PSS/ PDADMAC]n membranes.

Figure 9. Glutathione adsorption capacities of the [PSS/PDADMAC]4 and [PSS/PDADMAC]4/PSS membranes.

can be seen that the glutathione adsorption capacity of the [PSS/PDADMAC]4/PSS membrane was 0.123 g/m2, which was higher than that of the [PSS/PDADMAC]4 membrane (0.041 g/m2). However, the surface of the [PSS/PDADMAC]n/PSS membranes was denser than that of the [PSS/ PDADMAC]n membranes, so the glutathione retentions of the [PSS/PDADMAC]n/PSS membranes were higher than those of the [PSS/PDADMAC]n membranes. Due to size exclusion and Donnan exclusion, the glutathione retentions of the [PSS/ PDADMAC]n/PSS membranes were higher than those of the [PSS/PDADMAC]n membranes. The selectivity for NaCl over glutathione of the [PSS/ PDADMAC]n/PSS membranes was much higher than that of the [PSS/PDADMAC]n membranes with the same value of n in Figures 6 and 8. The maximum selectivity values of the [PSS/ PDADMAC]n/PSS and [PSS/PDADMAC]n membranes were 23 and 6.1, respectively. The reason for the difference is that the glutathione rejection of the [PSS/PDADMAC]n/PSS membranes was much higher than that of the [PSS/ PDADMAC]n membranes with the same value of n, but the NaCl rejections of the [PSS/PDADMAC]n/PSS and [PSS/ PDADMAC]n membranes were almost the same. As a result, the selectivities of the [PSS/PDADMAC]n/PSS membranes were higher than those of the [PSS/PDADMAC]n membranes. It can be concluded that the [PSS/PDADMAC]n/PSS membranes are better than the [PSS/PDADMAC]n membranes for removing NaCl from glutathione solutions.

Figure 8. Glutathione rejection, NaCl rejection, and selectivity of NaCl over glutathione as functions of the number of layer pairs in the [PSS/PDADMAC]n membranes.

increased with the number of layer pairs in the [PSS/ PDADMAC]n membranes, and the flux declined with the number of layer pairs (Figure 7), a behavior similar to that of the [PSS/PDADMAC]n/PSS membranes. This occurred because the pore size of the [PSS/PDADMAC]n membranes was reduced with increasing bilayers.35 When the number of layer pairs was 0, the [PSS/PDADMAC]0 membrane was unmodified. It was the basic membrane, and the glutathione rejection was 74%. 3.2.2. Use of the [PSS/PDADMAC]n/PSS[PSS/PDADMAC]n Membranes for the Removal of Salt from Glutathione Solutions. Figures 6 and 8 indicate that the glutathione and NaCl rejections of the [PSS/PDADMAC]n/PSS membranes were higher than those of the [PSS/PDADMAC]n membranes for the same value of n. The maximum glutathione rejections of the [PSS/PDADMAC]n/PSS and [PSS/PDADMAC]n membranes were 97.27% and 89%, respectively. However, the flux achieved by the [PSS/PDADMAC]n/PSS membrane was lower than that achieved by the [PSS/PDADMAC]n membrane (Figure 7). One major reason for the difference is that the surface morphology of the [PSS/PDADMAC]n/PSS membranes was denser than that of the [PSS/PDADMAC]n membranes. This might be because the PSS molecular main chain is more flexible than the PDADMAC molecular main chain. A flexible molecular main chain could result in strong interaction forces, which would induce a dense surface morphology formation. This is consistent with the SEM results. Therefore, the retention performance of the [PSS/PDAD-

4. CONCLUSIONS The removal of NaCl from glutathione solutions by the layerby-layer (LBL) assembly of alternating polyelectrolyte on NF membranes was found to be excellent. The glutathione rejection increased from 74% (unmodified) to 97.27% ([PSS/ 6521



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(3) Meister, A.; Tate, S. S. Glutathione and related γ-glutamyl compounds: Biosynthesis and utilization. Annu. Rev. Biochem. 1976, 45, 559−604. (4) Wang, Q.; Guan, Y.; Yang, M. Application of superparamagnetic microspheres for affinity adsorption and purification of glutathione. J. Magn. Magn. Mater. 2012, 324, 3300−3305. (5) Lhassani, A.; Rumeau, M.; Benjelloun, D.; Pontie, M. Selective demineralization of water by nanofiltration: Application to the defluorination of brackish water. Water Res. 2001, 35, 3260−3264. (6) Rapp, H. −J.; Pfromm, P. H. Electrodialysis for chloride removal from the chemical recovery cycle of a Kraft pulp mill. J. Membr. Sci. 1998, 146, 249−261. (7) Veressinina, Y.; Trapido, M.; Ahelik, V.; Munter, R. Fluoride in drinking water: The problem and its possible solutions. Proc. Est. Acad. Sci. 2001, 50, 81−88. (8) Mohammad, A. W.; Hilal, N.; Al-Zoubi, H.; Darwish, N. A. Prediction of permeate fluxes and rejections of highly concentrated salts in nanofiltration membranes. J. Membr. Sci. 2007, 289, 40−50. (9) Mukherjee, P.; SenGupta, A. K. Some observations about electrolyte permeation mechanism through reverse osmosis and nanofiltration membranes. J. Membr. Sci. 2006, 278, 301−307. (10) Wu, D.; Liu, X.; Yu, S.; Liu, M.; Gao, C. Modification of aromatic polyamide thin-film composite reverse osmosis membranes by surface coating of thermo-responsive copolymers P(NIPAM-coAm). I: Preparation and characterization. J. Membr. Sci. 2010, 352, 76− 85. (11) Ben-David, A.; Bernstein, R.; Oren, Y.; Belfer, S.; Dosoretz, C.; Freger, V. Facile surface modification of nanofiltration membranes to target the removal of endocrine disrupting compounds. J. Membr. Sci. 2010, 357, 152−159. (12) Khayet, M.; Abu Seman, M. N.; Hilal, N. Response surface modeling and optimization of composite nanofiltration modified membranes. J. Membr. Sci. 2010, 349, 113−122. (13) Kim, M. M.; Lin, N. H.; Lewis, G. T.; Cohen, Y. Surface nanostructuring of reverse osmosis membranes via atmospheric pressure plasma-induced graft polymerization for reduction of mineral scaling propensity. J. Membr. Sci. 2010, 354, 142−149. (14) Mukherjee, P.; Jones, K. L.; Abitoye, J. O. Surface modification of nanofiltration membranes by ion implantation. J. Membr. Sci. 2005, 254, 303−310. (15) Linggawati, A.; Mohammad, A. W.; Ghazali, Z. Effect of electron beam irradiation on morphology and sieving characteristics of nylon66 membranes. Eur. Polym. J. 2009, 45, 2797−2804. (16) González Muñoz, M. P.; Navarro, R.; Saucedo, I.; Avila, M.; Prádanos, P.; Palacio, L.; Martínez, F.; Martín, A.; Hernández, A. Hydrofluoric acid treatment for improved performance of a nanofiltration membrane. Desalination 2006, 191, 273−278. (17) Aravind, U. K.; George, B.; Baburaj, M. S.; Thomas, S.; Thomas, A. P.; Aravindakumar, C. T. Treatment of industrial effluents using polyelectrolyte membranes. Desalination 2010, 252, 27−32. (18) El-Hashani, A.; Toutianoush, A.; Tieke, B. Use of layer-by-layer assembled ultrathin membranes of dicopper-[18]azacrown-N6 complex and polyvinylsulfate for water desalination under nanofiltration conditions. J. Membr. Sci. 2008, 318 (1−2), 65−70. (19) Guedidi, S.; Yurekli, Y.; Deratani, A.; Dejardin, P.; Innocent, C.; Altinkaya, S. A.; Roudesli, S.; Yemenicioglu, A. Effect of enzyme location on activity and stability of trypsin and urease immobilized on porous membranes by using layer-by-layer self-assembly of polyelectrolyte. J. Membr. Sci. 2010, 365, 59−67. (20) Liu, X.; Qi, S.; Li, Y.; Yang, L.; Cao, B.; Tang, C. Y. Synthesis and characterization of novel antibacterial silver nanocomposite nanofiltration and forward osmosis membranes based on layer-bylayer assembly. Water Res. 2013, 47, 3081−3092. (21) Shi, J. F.; Zhang, W. Y.; Su, Y. L.; Jiang, Z. Y. Composite polyelectrolyte multilayer membranes for oligosaccharides nanofiltration separation. Carbohydr. Polym. 2013, 94, 106−113. (22) Decher, G.; Schmitt, J. Fine-tuning of the film thickness of ultrathin multilayer films composed of consecutively alternating layers

PDADMAC]10/PSS). The selectivity for NaCl over glutathione increased from 2.8 (unmodified) to 23 ([PSS/PDADMAC]10/ PSS). The glutathione rejection increased with the number of layer pairs of the modified membranes, becoming stable (about 97%) after the number of layer pairs reached 4. The [PSS/ PDADMAC]n/PSS membranes were better than the [PSS/ PDADMAC]n membranes in removing NaCl from glutathione solutions.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-6444-8962 E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research 973 P r o g r a m o f Ch i n a ( G r a n t s 2 0 1 3 CB 7 3 3 6 0 0 a n d 2011CB200900), the National High Technology Research and Development 863 Program of China (Grant 2012AA021404), the National Natural Science Foundation of China (21276017), and the Chinese Universities Scientific Fund.

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NOMENCLATURE A = effective membrane area (m2) C = concentration of glutathione (g/L) Cf = concentration in the feed (g/L) CfA = concentration of A in the feed (g/L) CfB = concentration of B in the feed (g/L) Cp = concentration in the permeate (g/L) CpA = concentration of A in the permeate (g/L) CpB = concentration of B in the permeate (g/L) J = flux (L/m2·h) M = membrane glutathione adsorption capacity (g/m2) Q = amount of permeate collected (L) R = solute rejection (%) Ra = average roughness (nm) RA = rejection of A (%) RB = rejection of B (%) S = selectivity t = sampling time (h) V = volume of aqueous solution (L)

Abbreviations

AFM = atomic force microscopy ATR-FTIR = attenuated total reflectance Fourier transform infrared HPLC = high-performance liquid chromatography LBL = layer-by-layer NF = nanofiltration PDADMAC = poly(diallyldimethylammonium chloride) PSS = poly(styrene sulfonate) SEM = scanning electron microscopy XPS = X-ray photoelectron spectroscopy

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REFERENCES

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LBL Surface Modification of a Nanofiltration Membrane for Removing

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