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 ( 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
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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···OC−N, OC−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
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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 CC 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 CO stretching (major contributor) and C−N stretching and C−C−N deformation vibration, was shifted to a higher wavenumber. Since the CO stretching of benzoic acid is at 1680 cm−1,20,36 it can be hypothesized that the breakage of hydrogen bonds between CO 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
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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.
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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.
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AUTHOR INFORMATION
Corresponding Author
*Tel: (65) 67905267; fax: (65) 67910676; e-mail: cytang@ntu. edu.sg.
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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.
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REFERENCES
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