Article pubs.acs.org/est Degradation of Polyamide Nanofiltration and Reverse Osmosis Membranes by Hypochlorite Van Thanh Do,† Chuyang Y. Tang,†,‡,* Martin Reinhard,§ and James O. Leckie§ † School of Civil & Environmental Engineering and ‡Singapore Membrane Technology Centre Nanyang Technological University, Singapore, 639798 § Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information * ABSTRACT: The degradation of polyamide (PA) nanofiltration and reverse osmosis membranes by chlorine needs to be understood in order to develop chlorine-resistant membranes. Coated and uncoated fully aromatic (FA) and piperazine (PIP) semi-aromatic PA membranes were treated with hypochlorite solution and analyzed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). XPS results showed that in chlorine treated FA PA membranes the ratio of bound chlorine to surface nitrogen was 1:1 whereas it was only 1:6 in the case of PIP PA membranes. Surface oxygen of uncoated FA and PIP membranes increased with increasing hypochlorite concentration whereas it decreased for coated FA membranes. High resolution XPS data support that chlorination increased the number of carboxylic groups on the PA surface, which appear to form by hydrolysis of the amide bonds (C(O)−N). FTIR data indicated the disappearance of the amide II band (1541 cm−1) and aromatic amide peak (1609 cm−1) in both coated and uncoated chlorinated FA membranes, consistent with the N-chlorination suggested by the XPS results. Furthermore, the surface charge of chlorinated membranes at low pH (<6) became negative, consistent with amide-nitrogen chlorination. Chlorination appeared to both increase and decrease membrane hydrophobicity depending on chlorination exposure conditions, which implied that Nchlorination and hydrolysis may be competing processes. The effects of property changes on the membrane performance were also observed for NF90, BW30, and NF270 membranes. al. in 1994.4 The mechanisms that have been identified to explain the properties and performance changes of chlorinated polyamide membranes are summarized in Supporting Information S1. Chlorination of the amide nitrogen occurs when active chlorine species (i.e., hypochlorous acid) attack the lone electron pair of either the N or O atom of the amide group and rearrange to the N-chloroamide.4−7 A direct ring chlorination mechanism has also been proposed: the aromatic ring of mphenylene diamine is attacked by active (electrophilic) chlorine species and substitution occurs preferentially at the para position. 8,9 Indirect ring chlorination (termed Orton rearrangement) can be initiated by rapid N-chlorination followed by an intramolecular rearrangement in which chlorine migrates to the ring.10−12 Most of the previous studies have focused on the incorporation of chlorine into the membrane matrix, but systematic studies on oxygen composition changes and the role of chlorine in promoting amide hydrolysis as a significant competing degradation reaction are not reported.4 1. INTRODUCTION Polyamide-based thin film composite (TFC) membranes are the most widely used reverse osmosis and nanofiltration membranes today, mainly because of their high selectivity and water permeability.1 The basic types of membrane structures and chemistries are illustrated in Figure 1. The fully aromatic (FA) polyamide (PA) membrane is formed by interfacial polymerization of m-phenylene diamine and trimesoyl chloride (TMC) to obtain a three-dimensionally cross-linked rejection layer, where the degree of cross-linking, n ranges from 0 (fully linear) to 1 (fully cross-linked).2 The semi-aromatic PA membrane is similarly synthesized by piperazine (PIP) and TMC. Additional surface coating with polyvinyl alcohol (PVA) is used for some commercial membranes to improve membrane fouling resistance.2,3 A drawback of PA membranes is that they are degraded by chlorine, which is commonly added in form of sodium hypochlorite as a disinfectant to control biofouling or as a membrane cleaning agent.4 Addressing the need for chlorineresistant membranes requires a mechanistic understanding of the interactions between chlorine and the PA rejection layer of membrane and how these effects impact membrane performance. A critical review of prior studies was reported by Glater et © 2012 American Chemical Society Received: Revised: Accepted: Published: 852 September 3, 2011 December 1, 2011 December 14, 2011 January 5, 2012 dx.doi.org/10.1021/es203090y | Environ. Sci. Technol. 2012, 46, 852−859 Environmental Science & Technology Article Figure 1. Schematic diagram of basic membrane structures and chemistries. Diagram is not to scale. chemistry. In addition, the FA PA membranes SW30HR and BW30 are coated with a polyvinyl alcohol (PVA) surface coating. The physiochemical properties of these membranes are provided in the Supporting Information S2. Membranes were provided by Dow/FilmTec (Minneapolis, MN, U.S.), except HL, which was from GE Osmonics (Minnetonka, MN, U.S.). Sodium hypochlorite (∼10% NaOCl, reagent grade) was obtained from Sigma Aldrich (St. Louis, MO, U.S.). Exact concentrations for the soaking solutions were determined by a standard titration method using sodium thiosulfate.27 All other reagents and chemicals were of analytical grade with purity over 99%. Milli-Q water (Millipore, Billerica, MA, U.S.) was used in all preparations and experiments. 2.2. Membrane Chlorination Procedures. Membrane coupons, which were preserved as received at 4 °C, were cleaned and soaked in Milli-Q water for 24 h before chlorination experiments. Membranes were presoaked in NaOCl solutions at the same testing conditions for 1 min to remove excess water on the surface before being soaked in NaOCl solutions with concentrations of 10, 100, 1000, and 2000 ppm for either 1 or 24 h to accelerate laboratory degradation process.17,19,20 Since acidification is typically performed in NF/RO plants for scaling control,28 soaking solutions were adjusted to pH 5 by the addition of HCl or NaOH so that HOCl is the main species.20 Samples were contained in concealed 0.5 L Wheaton glass bottles with PTFElined caps, covered with aluminum foil (to protect the chlorine from sunlight degradation), and constantly mixed on a shaker at room temperature (∼21 °C). After exposure tests, the average chlorine depletion of the soaking solutions was less than 10%, and is within the magnitude reported in the literature.20 After soaking, the samples were thoroughly rinsed with MilliQ water. For XPS, FTIR, and contact angle measurements, the membranes were vacuum-dried while zeta potential measurements were performed on wet samples immediately after chlorination. 2.3. X-ray Photoelectron Spectroscopy (XPS). To obtain information about composition and bonding chemistry for the surface layer (top 1 to 5 nm sample thickness), XPS analysis was carried out on a Kratos AXIS Ultra spectrometer (Shimadzu, Columbia, MD, U.S.) with a monochromatic aluminum Kα X-ray source at 1486.7 eV. To compensate for membrane surface charging, the electron flood gun was Effects of chlorination on membrane performance have been reported in the literature but findings appear to be inconsistent. The decline in water flux after membrane chlorination13−20 has been attributed to an increase in membrane hydrophobicity caused by chlorine attachment,13 “membrane tightening”,14,15 or a more rigid polymer conformation.17,18,21,22 In contrast, a brief exposure to chlorine at high pH is used to treat PA RO membranes after fabrication to improve performance.23,24 The observed increase in flux is usually accompanied by a decrease in salt rejection13−20,25 and proposed to be the result of the polyamide chain cleavage,13−15 or physical separation of PA rejection layer from its polysulfone support.18 Meanwhile, better salt rejection of chlorine-treated membranes has been attributed to the result of membrane tightening,14,15 or enhanced PA chain cross-linking due to the formation of azocompounds or quinone-like species.18,26 Opposite trends are observed in membrane surface charges and wettability as well. Membrane zeta potential is observed to generally become more negative after chlorination.19,22 Such change in surface charge does not seem to be adequately explained by ring chlorination or the Orton rearrangement mechanism. Hydrophobicity of chlorinated membranes is reported to both increase and decrease depending on chlorine concentration, soaking pH and duration.19,22 To explain these observations by a single mechanism is difficult. Therefore, more investigations and possibly additional mechanisms are required to obtain insight into the membrane degradation process by chlorination. This paper presents evidence that chlorine treatment leads to the uptake of chlorine and oxygen by PA membranes. Changes in membrane surface and bulk chemistry as well as surface charge and wettability are interpreted that subsequent reactions with hypochlorite appear to hydrolyze the amide C−N bond of the PA layer. The results show that chlorine treatment can significantly modify the membrane properties. 2. MATERIALS AND METHODS 2.1. Chemicals and Materials. The commercial thin film composite cross-linked PA membranes investigated in the current study include three RO membranes (SW30HR, BW30, and XLE) and three nanofiltration membranes (NF90, NF270, and HL). According to our previous characterization work,2,3 NF270 and HL membranes have the semi-aromatic PIP PA chemistry, the other four membranes are of the FA PA 853 dx.doi.org/10.1021/es203090y | Environ. Sci. Technol. 2012, 46, 852−859 Environmental Science & Technology Article operated at 3.6 eV. Survey spectra were recorded 3 times per sample, over the range of 0−1000 at 1 eV resolution. High resolution scans had a resolution of 0.1 eV. Calibration for the elemental binding energy were done based on the reference for carbon 1s at 285 eV.29 Data were processed by standard software with Shirley background and relative sensitivity factor (RSF) of 0.78, 0.477, 0.278, and 0.891 for O 1s, N 1s, C 1s, and Cl 2p peaks, respectively. Replicate analyses were done at each chlorination condition for NF90, BW30, and NF270 membranes. 2.4. Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR). Bonding chemistry of the chlorinated membranes was obtained using an FTIR (IRPrestige-21, Shimadzu, Columbia, MD, USA) with 45° multi-reflection HATR ZnSe flat plate crystal (PIKE Technologies, Madison, WI, U.S.) as ATR element. Each spectrum was averaged from 50 scans over the range of 650−4000 cm−1 at a resolution of 2 cm−1. Baselines were corrected for atmospheric CO2 and water vapor. One replicate was done for each sample. 2.5. Zeta Potential and Contact Angle Measurement. Membrane surface charge was measured by a SurPASS electrokinetic analyzer (Anton Paar GmbH, Graz, Austria) with an adjustable gap cell and using 10 mM NaCl as electrolyte solution. The cell channel height was adjusted to ∼110 ± 5 μm. The solution pH was automatically titrated from 3 to 9 using 0.1 M HCl and NaOH. The zeta potential was calculated from the streaming potential using the Fairbrother− Mastin formula.30 Contact angle measurements were performed with an OCA Goniometer (Dataphysics Instruments, Filderstadt, Germany) by the sessile drop method. A syringe with a needle diameter of 0.525 mm was used to place a water droplet of 10 μL on the membrane. Tangent lines to both sides of the droplet static image were generated and averaged by the software SCA20. The reported contact angle of each membrane sample was the average of 40 measurements (2 independent membrane coupons and 20 different locations for each sample) with errors indicated by the standard deviations. 2.6. Membrane Performance Tests. The laboratory-scale NF/RO filtration system consisting of four identical cross-flow test cells (CF042, Sterlitech, Kent, WA, U.S.) is illustrated and fully described in Supporting Information S3. Each cell houses 42 cm2 (4.6 cm ×9.2 cm) of active membrane area. Spacers of 1.2 mm thickness from GE Osmonics (Minnetonka, MN, USA) were used for all filtration tests. Membrane performance tests were conducted for NF90, BW30, and NF270. Prior to each filtration test, virgin membranes (soaked in MiliQ water for 24 h) and freshly degraded membranes were cleaned thoroughly with MiliQ water. A 10 mM of NaCl feed solution at 21 °C and natural pH (pH ≈ 6) was recirculated through the system for 24 h at 1 L/ min cross-flow velocity, corresponding to a superficial velocity of 22.6 cm/s. The operating pressures for NF90, BW30, and NF270 were set at 100, 260, and 70 psi, respectively, to obtain similar virgin membrane fluxes (around 1.2 to 1.5 m/day). Permeate flux was determined by a gravimetrical method. Salt rejection was calculated from permeate and tank conductivity measured by an Ultrameter II conductivity meter (Myron L Company, Carlsbad, CA, U.S.). The equations to determine the water permeability coefficient, A (m/s.Pa) and the solute permeability coefficient, B (m/s) are provided in Supporting Information S3 . 3. RESULTS AND DISCUSSION 3.1. Chlorine and Oxygen on Membrane Surfaces Elemental Changes after Chlorination. Six membranes Figure 2. Atomic percent of chlorine and nitrogen of chlorinated uncoated fully aromatic (NF90 and XLE), PVA coated fully aromatic (SW30HR and BW30), and uncoated poly(piperazinamide) (HL and NF270) membranes at pH 5; 2000 ppm ×24 h, 1000 ppm ×24 h and 1000 ppm ×1 h. Figure 3. Ratio of (a) oxygen to carbon and (b) oxygen to nitrogen as a function of the atomic percent of bound chlorine for (●) NF270 and (▲) NF90 at different chlorination conditions: pH 5; 2000 ppm × 24 h, 1000 ppm × 24 h, 1000 ppm × 1 h, 100 ppm × 24 h, 100 ppm × 10 h and 10 ppm × 100 h. The dotted and dashed lines represent the (a) O/C and (b) O/N ratio for virgin NF270 and NF90, respectively. 854 dx.doi.org/10.1021/es203090y | Environ. Sci. Technol. 2012, 46, 852−859 Environmental Science & Technology Article Figure 4. High resolution XPS spectra and deconvoluted peak assignments of (a) O 1s, C 1s, N 1s for virgin and (b) O 1s, C 1s, N 1s, Cl 2p for chlorinated NF90 (2000 ppm × 24 h, pH 5). The Cl 2p peak of different bonding state is deconvoluted into Cl 2p3/2 and Cl 2p1/2. The peak intensity ratio of Cl 2p3/2 over Cl 2p1/2 is constant (=2) and the binding energy of Cl 2p3/2 peak was used to identify chemical bonding. In spite of the small chlorine uptake, the performance of chlorinated PIP membranes is reported to be impaired by chlorine exposure.19 This phenomenon requires a different explanation for the attack of chlorine on the PIP PA structure. The XPS results revealed that chlorination increased the O/ C ratio on the uncoated membrane surfaces (Figure 3a). This ratio increased from 0.2 (virgin) to 0.3 (2000 ppm ×24 h) for NF270 and from 0.15 (virgin) to 0.21 (2000 ppm × 24 h) for NF90. The O/N ratio, which indicates the degree of PA crosslinking, was plotted against the chlorine content (Figure 3b). Theoretically, the ratio of O/N is 1:1 for a fully cross-linked PA layer (n = 1, Figure 1) and is 2:1 for a linear PA layer (n = 0).2 The O/N ratio of NF90 increased from 1.15 for virgin membrane to 1.44 for the membrane treated with 2000 ppm for 24 h. NF270 experienced the same increase in O/N ratio from 1.24 to 1.69, which means that the chlorine treated membranes were less cross-linked and that more C−N bonds were broken. The increase of oxygen content and decrease in the number of C−N bonds are hypothesized to result from C− N bond hydrolysis, which leads to additional carboxylic acid groups. Mechanistically, facilitated hydrolysis of the chlorinated C−N bond may be explained by the polarization of the amide carbon due to chlorine substitution at the nitrogen, which makes the carbon more susceptible to nucleophilic attack by hydroxide. Additional evidence for chlorination promoted hydrolysis is presented in Sections 3.2 and 3.3. 3.2. Changes in Membrane Chemistry Due to Chlorination. The high resolution XPS spectra for O, C, N, and Cl of virgin and chlorinated (at 2000 ppm × 24 h) NF90 are illustrated in FIGURE 4. Changes in chemical bonding at with 3 different chemistries were exposed to 1000 ppm NaOCl solutions for 1 h at pH 5. In addition, NF90, BW30, and NF270 membranes were treated at 2000 and 1000 ppm for 24 h. The results from the XPS survey scan show that the amount of chlorine attached onto the PA surface decreased in the order: uncoated FA > coated FA > PIP membranes. The correlations between the chlorine (%) uptake of the surface layer and the nitrogen atomic percentage are presented in Figure 2. The chlorine to nitrogen atomic ratios (Cl/N ratio) of uncoated FA membranes (NF90 and XLE) cluster around the 1:1 ratio line, which suggests that chlorine attaches to the nitrogen of the secondary amide via the N-chlorination mechanism. The chlorine and nitrogen atomic percentages of the coated FA membranes were 1:1 at 1000 ppm NaOCl × 1 h; however, both percentages increased significantly at 1000 and 2000 ppm NaOCl × 24 h. The average surface nitrogen content was ∼6% at 1000 and 2000 ppm ×24 h, which is higher than nitrogen content of virgin BW30.2 This is further accompanied with a significant reduction in the oxygen content (Supporting Information S4 and S5). These observations suggest that the PVA coating was likely partially detached under severe chlorination conditions employed. In addition, elevation of the Cl/N ratio from 1 to 1.56 and 2.3 could suggest that chlorine also binds to the coating PVA layer. The observation that the PIP membranes NF270 and HL had Cl/N ratios much less than 1 is consistent with the fact that tertiary nitrogen is not chlorinated, as reported in the literature.6,12,26 The absence of amide protons accounts for the low chlorine incorporation into PIP membranes.4,12 Chlorination might still occur at the noncross-linked nitrogen atoms, which exist at low abundance. 855 dx.doi.org/10.1021/es203090y | Environ. Sci. Technol. 2012, 46, 852−859 Environmental Science & Technology Article Figure 6. Zeta potential for virgin and chlorinated (a) NF90 and (b) NF270 as a function of pH. Background electrolyte was 10 mM NaCl. The uncertain in zeta potential measurements is estimated to be ∼ ± 5 mV (Supporting Information S8 and Tang et.al41). Figure 5. ATR−FTIR spectra for (a) NF90, (b) BW30, and (c) NF270: virgin and chlorinated at pH 5; 2000 ppm × 24 h, 1000 ppm × 24 h and 1000 ppm × 1 h. Figure 7. Contact angle (deg) for virgin and chlorinated membranes. Error bar indicates the standard deviations of 40 measurements (2 independent membrane coupons and 20 different locations for each sample). the surface of the PA layer can be understood through analysis of the shifts in the binding energy (BE) of the deconvoluted peak spectra. The assignments for deconvoluted peaks are summarized in the table inserted in Figure 4.31−33 Only the chlorine peak representing covalent bonding (∼200.7 eV) is discussed. After chlorination, the second deconvoluted peak of O 1s (∼532.6 eV) corresponding to H···OC−N, OC−O bonds was shifted upward by about 0.6 eV; the second deconvoluted C 1s (∼ 286.2 eV) peak, which is assigned to C− O, C−N, C−Cl bonds, was shifted upward by about 0.13 eV. For NF270, an increase in peak intensity was observed for the second O 1s peak (∼532.6 eV, Supporting Information S6). Although these shifts are not sufficiently distinctive to deconvolute and assign new peaks, they may indicate an increase in the number of carboxylic groups due to the chlorine-promoted C−N bond hydrolysis. Bulk chemistry changes due to chlorination in both polysulfone and PA layers were obtained from the FTIR spectra for virgin and chlorinated NF90, BW30 and NF270 (Figure 5). Peak assignments for the FTIR spectra of these PA membranes were reported in detail elsewhere.3 The FA amide signature peaks at wave numbers of 1541, 1609, and 1663 cm−1 experienced prominent changes in case of the NF90 and BW30 membranes, consistent with literature data.7,17,20,34,35 The disappearance of the amide II band (1541 cm−1) for N−H in-plane bending and N−C stretching vibration and aromatic 856 dx.doi.org/10.1021/es203090y | Environ. Sci. Technol. 2012, 46, 852−859 Environmental Science & Technology Article ment and the ATR-FTIR measurement can reveal important information on the degree of membrane chlorination across the PA rejection layer.2 Although the XPS results showed less surface chlorine on BW30 than on NF90 at short exposure (1000 ppm × 1 h), FTIR spectra of chlorinated BW30 show a complete disappearance of the amide II band (1541 cm−1) and the aromatic amide peak (1609 cm−1). The latter suggests that the PVA coating did not protect the FA PA matrix from chlorine attack under the severe chlorination conditions applied in this study. Therefore, the lower XPS chlorine signal for the PVA coated membranes (Figure 2) was merely a reflection that less PA was exposed at the membrane surface due to the presence of the coating. Chlorine might have penetrated through the coating and caused the damage to the PA underneath. No changes in the polysulfone layer can be observed at a wave band 1145−1350 cm−1, which is consistent with the finding of Ettori et. al that polysulfone was not chemically modified by chlorination.20 FTIR spectra from 2700 to 3800 cm−1 for the three membranes are provided in the Supporting Information S7. For NF90 and BW30 FA membranes, the magnitude of broad peaks centered ∼3300 cm−1, which are assigned to N−H and/or O−H stretching3 depleted as chlorination conditions became more severe. On the other hand, for the NF270 PIP membrane, only a slight reduction in the C−H stretching peak at 2970 cm−1 was observed.3 3.3. Changes in Membrane Surface Charge and Hydrophilicity. The surface charge of NF90 and NF270 membranes before and after chlorination is presented in Figure 6 and that of BW30 is provided in Supporting Information S8. Both NF90 and NF270 membranes became more negative as the chlorine concentration increased, which agrees with previous observations.19,37 It is useful to note that the magnitude of this negative charge shift within a membrane is not constant; at low pH, the shift magnitude is relatively large. The iso-electrical point of virgin NF90 is at pH ≈ 5.5 and the positive charge at low pH is contributed by the protonated amide group.38 All chlorinated membranes were negatively charged at the lowest measured pH leading to the speculation that due to N−Cl bond formation, amide nitrogen can no longer form −NH2+ groups.19 Meanwhile, the negative charge at high pH is associated with deprotonation of carboxylic acid.38 The more negative zeta potential at high pH reconfirms that the number of carboxylic groups on the surface increased, consistent with the chlorine-promoted hydrolysis mechanism. The hydrophobicity of the membranes evaluated by contact angle is illustrated in Figure 7 and more data of other membranes is in Supporting Information S9. A higher value of the contact angle denotes greater difficulty in wetting the membrane. Despite the measurement uncertainty, some general trends can be observed. The membrane surfaces become more hydrophobic under severe chlorination conditions. However, NF270 and BW30 treated at 1000 ppm for 1 h became more hydrophilic. Both trends in wettability might be explained by the competing effects of N-chlorination and hydrolysis processes. The incorporation of chlorine on the membrane surface can cause an increase in hydrophobicity and inhibition of membrane wetting. However, an increase in carboxylic/ hydroxyl functional groups can increase membrane wetting. 3.4. Changes in Membrane Performance. To evaluate the changes in intrinsic membrane properties, water permeability coefficient, A (m/s.Pa) and solute permeability coefficient, B (m/s) for virgin and chlorinated membranes Figure 8. Virgin and chlorinated membrane performance: (a) water permeability coefficient, A (m/s·Pa) and (b) solute permeability coefficient, B (m/s). The operating pressures for NF90, BW30, and NF270 were set at 100, 260, and 70 psi, respectively. amide peak (1609 cm−1) for N−H deformation vibration and CC ring stretching vibration3 strongly suggests that chlorine replaced hydrogen of the amide nitrogen via electrophilic substitution in N-chlorination. This FTIR information is consistent with the Cl/N ratio from XPS results in Figure 2. It was observed that the amide I band (1663 cm−1), which represents CO stretching (major contributor) and C−N stretching and C−C−N deformation vibration, was shifted to a higher wavenumber. Since the CO stretching of benzoic acid is at 1680 cm−1,20,36 it can be hypothesized that the breakage of hydrogen bonds between CO and N−H groups17,20 and additional carboxyl groups by hydrolysis have contributed to this shift. In contrast to the obvious spectra changes for NF90 and BW30, there was no noticeable change in the FTIR peaks detected for the PIP membrane NF270 (Figure 5c). This result is consistent with the XPS survey spectra (Figure 2) results in which the nitrogen in the tertiary PA of NF270 was less prone to chlorine attack. Nevertheless, a small amount of chlorine was still incorporated into the PIP PA layer, assumed to be at nitrogen atoms of noncross-linked amine groups. It is also interesting to compare the XPS and FTIR results for the PVA coated membrane BW30. Since XPS measures only the top 1−5 nm thickness of a sample, its signal responds mainly to coating material and exposed surface PA. Meanwhile, ATR-FTIR has much deeper sample penetration depth and allows measurement of the PA properties across the active layer. Thus, a comparison between the surface XPS measure857 dx.doi.org/10.1021/es203090y | Environ. Sci. Technol. 2012, 46, 852−859 Environmental Science & Technology Article composite polyamide RO and NF membranes: I. FTIR and XPS characterization of polyamide and coating layer chemistry. Desalination 2009, 242 (1−3), 149−167. (4) Glater, J.; Hong, S.-k.; Elimelech, M. The search for a chlorineresistant reverse osmosis membrane. Desalination 1994, 95 (3), 325− 345. (5) Challis, B. G.; Challis, J. A., Reactions of the carboxamide group. In The Chemistry of Amides; Zabicky, J., Ed.; Wiley Interscience: New York, 1970; pp 731−857. (6) Jensen, J. S.; Lam, Y.-F.; Helz, G. R. Role of amide nitrogen in water chlorination: Proton NMR evidence. Environ. Sci. Technol. 1999, 33 (20), 3568−3573. (7) Kwon, Y.-N.; Tang, C. Y.; Leckie, J. O. Change of chemical composition and hydrogen bonding behavior due to chlorination of crosslinked polyamide membranes. J. Appl. Polym. Sci. 2008, 108 (4), 2061−2066. (8) Shafer, J. A., Directing and activating effects of the amido group. In The Chemistry of Amides; Zabicky, J., Ed.; Wiley Interscience: New York, 1970; pp 685−729. (9) Glater, J.; Zachariah, M. R. Mechanistic study of halogen interaction with polyamide reverse-osmosis membranes. ACS Symp. Ser. 1985, 345−358. (10) Orton, K. J. P.; Jones, W. J. CLXIII.Primary interaction of chlorine and acetanilides. J. Chem. Soc., Trans. 1909, 95, 1456−1464. (11) Orton, K. J. P.; Soper, F. G.; Williams, G. CXXXII.-The chlorination of anilides. Part III. N-chlorination and C-chlorination as simultaneous side reactions. J. Chem. Soc. (Resumed) 1928, 998−1005. (12) Kawaguchi, T.; Tamura, H. Chlorine-resistant membrane for reverse osmosis. I. Correlation between chemical structures and chlorine resistance of polyamides. J. Appl. Polym. Sci. 1984, 29 (11), 3359−3367. (13) Koo, J.-Y.; Petersen, R. J.; Cadotte, J. E. ESCA characterization of chlorine-damaged polyamide reverse osmosis membrane. ACS Polym. Prepr. 1986, 27 (2), 391−392. (14) Glater, J.; McCutchan, J. W.; McCray, S. B.; Zachariah, M., R., The effect of halogens on the performance and durability of reverseosmosis membranes. In Synthetic Membranes; Turbak, A. F., Ed. American Chemical Society: Washington DC, 1981; Vol. 153, pp 171−190. (15) Glater, J.; Zachariah, M. R.; McCray, S. B.; McCutchan, J. W. Reverse osmosis membrane sensitivity to ozone and halogen disinfectants. Desalination 1983, 48 (1), 1−16. (16) Kwon, Y. N.; Tang, C. Y.; Leckie, J. O. Change of membrane performance due to chlorination of crosslinked polyamide membranes. J. Appl. Polym. Sci. 2006, 102, 5895. (17) Kwon, Y.-N.; Leckie, J. O. Hypochlorite degradation of crosslinked polyamide membranes: II. Changes in hydrogen bonding behavior and performance. J. Membr. Sci. 2006, 282 (1−2), 456−464. (18) Soice, N. P.; Greenberg, A. R.; Krantz, W. B.; Norman, A. D. Studies of oxidative degradation in polyamide RO membrane barrier layers using pendant drop mechanical analysis. J. Membr. Sci. 2004, 243 (1−2), 345−355. (19) Simon, A.; Nghiem, L. D.; Le-Clech, P.; Khan, S. J.; Drewes, J. E. Effects of membrane degradation on the removal of pharmaceutically active compounds (PhACs) by NF/RO filtration processes. J. Membr. Sci. 2009, 340 (1−2), 16−25. (20) Ettori, A.; Gaudichet-Maurin, E.; Schrotter, J.-C.; Aimar, P.; Causserand, C. Permeability and chemical analysis of aromatic polyamide based membranes exposed to sodium hypochlorite. J. Membr. Sci. 2011, 375 (1−2), 220−230. (21) Avlonitis, S.; Hanbury, W.; Hodgkiess, T. Chlorine degradation of aromatic polyamides. Desalination 1992, 85, 321−334. (22) Kwon, Y.-N.; Leckie, J. O. Hypochlorite degradation of crosslinked polyamide membranes: I. Changes in chemical/morphological properties. J. Membr. Sci. 2006, 283 (1−2), 21−26. (23) Cadotte, J. E. Interfacially synthesized reverse osmosis membrane 1981. were presented in Figure 8. Higher A and B values indicate that the membrane is more permeable to water or solute.39 In the current study, chlorination decreased both A and B values. The decrease of A with increasing exposure to chlorine can be explained by the decrease of membrane wettability due to chlorine incorporation into the membranes13 in addition to any possible changes in the PA polymer conformation.17,18,21,22 The more negative membrane surface charge due to increased carboxylic functional groups can enhance electrostatic repulsion between the membrane and anionic solutes; therefore, salt passage through the chlorinated membranes was reduced19,40 despite of the reduced cross-linking degree (Figure 3b). In conclusion, the data presented in this study suggest that the chlorination of FA PA membranes promotes the hydrolysis of the amide C−N bond. The simultaneous occurrence of the N-chlorination and C−N hydrolysis leads to opposite effects. While the incorporation of chlorine into the PA backbone makes the membrane more hydrophobic and less permeable, the hydrolysis of the C−N bond has the potential to make the membrane more hydrophilic. This proposed mechanism could possibly explain the disparate and often contradictory observations reported in the literature. ■ ASSOCIATED CONTENT S Supporting Information * S1. Chlorination mechanisms of fully aromatic polyamide membranes. S2. Membrane physiochemical properties. S3. NF/ RO filtration system and performance evaluation. S4. Elemental compositions of virgin and chlorinated membranes by XPS. S5. Surface oxygen content for chlorinated BW30 membrane. S6. High resolution XPS spectra for virgin and chlorinated BW30 and NF270 membranes. S7. ATR−FTIR spectra at wavenumber from 2700 to 3800 cm−1. S8. Surface charge of virgin and chlorinated BW30 and replicates for virgin membranes. S9. Hydrophilicity of virgin and chlorinated membranes. This material is available free of charge via the Internet at http:// pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *Tel: (65) 67905267; fax: (65) 67910676; e-mail: cytang@ntu. edu.sg. ACKNOWLEDGMENTS This research was financially funded by the Tier 1 Research Grant No. RG6/07, Ministry of Education, Singapore. V.T.D. is supported by the Singapore Stanford Partnership Program. We thank Professor William B. Krantz for valuable discussions and help. The authors also thank the Singapore Membrane Technology Centre for technical support and GE Osmonics and Dow/FilmTec for providing the membrane samples. ■ ■ REFERENCES (1) Lee, K. P.; Arnot, T. C.; Mattia, D. A review of reverse osmosis membrane materials for desalination--Development to date and future potential. J. Membr. Sci. 2010, 370 (1−2), 1−22. (2) Tang, C. Y.; Kwon, Y.-N.; Leckie, J. O. Probing the nano- and micro-scales of reverse osmosis membranes: A comprehensive characterization of physiochemical properties of uncoated and coated membranes by XPS, TEM, ATR-FTIR, and streaming potential measurements. J. Membr. Sci. 2007, 287 (1), 146−156. (3) Tang, C. Y.; Kwon, Y.-N.; Leckie, J. O. 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