Deterioration Mechanism of a Tertiary Polyamide Reverse Osmosis

Jul 5, 2019 - (3−5) Therefore, a system to measure and control the residual free ... biofouling control;(24) however, its durability to deterioratio...
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Cite This: Environ. Sci. Technol. 2019, 53, 9109−9117

Deterioration Mechanism of a Tertiary Polyamide Reverse Osmosis Membrane by Hypochlorite Koki Hashiba,† Satoshi Nakai,*,† Masaki Ohno,‡ Wataru Nishijima,§ Takehiko Gotoh,† and Takashi Iizawa† †

Department of Chemical Engineering, Graduate school of Engineering, Hiroshima University, Hiroshima 739-8527, Japan Department of Applied Life Science, Niigata University of Pharmacy and Applied Life Science, Niigata 956-8603, Japan § Environmental Research and Management Center, Hiroshima University, Hiroshima 739-8513, Japan Downloaded via NOTTINGHAM TRENT UNIV on August 10, 2019 at 07:42:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: A tertiary polyamide membrane was synthesized using N,N′-dimethyl-m-phenylenediamine. The durability of this membrane to chlorination by hypochlorite treatment followed by sodium hydroxide treatment was examined, and then deterioration mechanisms were proposed. The tertiary polyamide membrane demonstrated better durability to free chlorine than a conventional secondary polyamide one; however, the former was deteriorated by hypochlorite for 24 h at 2000 ppm of free chlorine below pH 7.5. The salt rejection and permeation performance of the membrane were almost unchanged, and the least chlorination of the active layer occurred during hypochlorite treatment at pH 10. These results indicated that hypochlorous acid rather than hypochlorite ion was the free chlorine species that induced membrane deterioration. The deterioration became severe as chlorination progressed, resulting in collapse of the active layer below pH 7.5. Chlorination and hydrolysis of the model tertiary amide N-methylbenzanilide and Fourier transfer infrared spectroscopy of a deteriorated membrane showed that chlorination of the tertiary polyamide occurred via direct chlorination of the benzene bound to the amidic nitrogen. Silver ion probing of the deteriorated membrane revealed that amide bond scission occurred in the active layer, which might be related to the electron deficiency of the amidic nitrogen caused by chlorination of its benzene ring. of such membranes.3,4,7−11 After N-chlorination of the amidic nitrogen, the Orton rearrangement of chlorine occurs, where the chlorine atom transfers from the amidic nitrogen to the neighboring benzene ring. Finally, the amide C−N bond is dissociated by hydroxide ions (OH−) via hydrolysis.3,4,7−11 On the basis of these findings, many researchers have attempted to develop chlorine-resistant membranes through surface modification or monomer alteration. Surface modification approaches include surface coating and grafting. Graphene oxide,12,13 polyethylene glycol diacrylate,14 and poly(vinyl alcohol)15 have been used as coatings for conventional polyamide membranes employed in membrane filtration. In addition, graft polymerization with the amidic N− H in the active layer has also been studied using copolymers and polymers, such as poly(3-sulphopropyl methacrylate-comethylene-bis-acrylamide) in the presence of Ce(IV)/poly(vinyl alcohol)16 and poly(vinyl sulfonic acid)17 and sorbitol

1. INTRODUCTION Currently, 86.5 million m3/day of drinkable water is produced by desalination of several water sources such as seawater,1 and 65% of this water is produced by reverse-osmosis membranes. In a reverse-osmosis process for water purification, multiple pretreatments, such as coagulative precipitation and microfiltration, are normally required to maintain the reverseosmosis performance, and membrane fouling by bacteria and organic matter is an inevitable problem.2 Sodium hypochlorite (NaOCl) is useful for preventing fouling; however, conventional secondary polyamide membranes are not durable to free chlorine species because the nitrogen of the amide group is sensitive to chlorine.3−5 Therefore, a system to measure and control the residual free chlorine concentration has been installed after chlorination for fouling control.6 To realize a simple and robust reverse-osmosis process, it is important to develop chlorine-resistant membranes. To develop chlorine-resistant membranes, it is necessary to understand the deterioration mechanisms of conventional secondary polyamide membranes by hypochlorite (ClO−). Previous studies reported that chlorination of chlorinesensitive amidic N−H bonds is a trigger of the deterioration © 2019 American Chemical Society

Received: Revised: Accepted: Published: 9109

January 31, 2019 June 27, 2019 July 5, 2019 July 5, 2019 DOI: 10.1021/acs.est.9b00663 Environ. Sci. Technol. 2019, 53, 9109−9117

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Environmental Science & Technology polyglycidyl ether,18 to prevent the chlorination of the amidic nitrogen. However, there is a trade-off between the improvement of chlorine resistance and loss of filtration performance.12−18 In addition, improvement of the chlorine resistance of secondary polyamide membrane products by coating might have a disadvantage in terms of structural durability because of the possible dissociation of the functional surface.19 Therefore, monomer alteration has attracted attention as an alternative approach to obtain chlorine-resistant membranes. In previous studies, the monomers used to synthesize conventional secondary amide membranes, such as trimesoyl chloride and m-phenylenediamine, were replaced by other monomers.20,21 Kawaguchi and colleagues reported that tertiary amides were inert to free chlorine species according to their results using model compounds such as Nmethylbenzanilide and N-phenylbenzanilide.22 Because of its chlorine resistance, N,N′-dimethyl-m-phenylenediamine has been used in interfacial polymerization as an acidic chloride monomer;23 the resulting tertiary polyamide membrane showed excellent tolerance to free chlorine species compared to that of the conventional secondary polyamide23,24 because of the physical protection of the chlorine-sensitive nitrogen. This tertiary polyamide membrane is ready to be produced at the commercial scale, and a recent paper reported the performance of tertiary polyamide membrane in the longterm pilot plant study using direct chlorination for biofouling control;24 however, its durability to deterioration by free chlorine and the related deterioration mechanisms should be studied to facilitate its practical application and further improvement. In this study, we evaluate the chlorine resistance of the tertiary polyamide membrane synthesized using N,N′-dimethyl-m-phenylenediamine and examine its deterioration mechanisms. To investigate the tolerance of the tertiary polyamide membrane to free chlorine, the membrane was subjected to chlorination treatment using NaOCl, followed by hydrolysis treatment using sodium hydroxide (NaOH). After confirming deterioration, the deteriorated membranes were investigated by X-ray photoelectron spectroscopy (XPS), attenuated total reflection Fourier transfer infrared (ATR-FTIR) spectroscopy, and field-emission scanning electron microscopy (FE-SEM) to determine the deterioration mechanisms. Chlorination and hydrolysis of a model tertiary amide N-methylbenzanilide were performed to validate the proposed membrane deterioration mechanisms.

ration was replaced with a fresh one after soaking for 2, 12, 24, and 36 h to maintain greater than 80% of the set Cfc value. Because the membrane performances were almost unchanged, the tertiary polyamide membrane was treated at Cfc = 2000 ppm for 24 h (CT value = 48000 ppm·h). To confirm the reproducibility for deterioration of the tertiary polyamide membrane, chlorination treatment was carried out at Cfc = 1000 ppm for 48 h. During soaking, the solution was rotated at 45 rpm, and pH was controlled using 1 M NaOH and 1 and 10 M hydrochloric acid (HCl). After chlorination treatment, hydrolysis treatment was performed by soaking the hypochlorite-treated membrane in alkaline solution adjusted to pH 12 by 1 M NaOH at 25 °C for 2 h. The chlorinated and hydrolyzed membranes were characterized and subjected to filtration testing. 2.3. Filtration Testing. Virgin and treated membranes were washed with Milli-Q water and then set in a cross-flow cell (Figure S1, Tritec Co., Ltd., Tokyo, Japan), in which the effective membrane surface area was 37.4 cm2. Model salt water25 (1 L) containing 0.015 M NaCl, 0.01 M NaH2PO4· 2H2O, and 0.01 M NaOH (pH 8) was fed through the cell at a flow rate of 50 mL/min (a crossflow rate of 1.3 cm/min) at 25 °C under 1.5 MPa. During the first 4 min of filtration, the permeate and concentrate were discarded; after this time, both were cycled back to the feed tank. During the circulation, the electrical conductivity of the feed and permeate was analyzed to determine the salt rejection R (%) using eq 1. The permeate flux was calculated by dividing the measured permeate flow rate by the effective membrane surface area. Flow rate was determined from the amount of permeate produced during the measurement time. ij permeate conductivity (S/m) yzz R (%) = jjj1 − z × 100 j feed conductivity (S/m) zz{ k

(1)

2.4. Membrane Characterization. The virgin, chlorinated, and hydrolyzed tertiary polyamide membranes were washed with Milli-Q water and then dried in a vacuum oven at 25 °C for 6 h. Carbon was deposited on the hydrolyzed membrane samples using a carbon coater (CADE; Meiwafosis Co., Ltd., Tokyo, Japan), and then the membrane surface was observed by a field-emission scanning electron microscope (S5200; Hitachi High-Tech Manufacturing & Service Corp., Ibaraki, Japan). Chlorine and carboxylate (R-COO−) and other functional groups in the active layer of the membrane samples were analyzed. To investigate whether or not chlorination resulted in an increase of hydrophobicity, the chlorinated and hydrolyzed membrane samples were analyzed using the water contact angle measuring instrument (DropMaster 300, Kyowa Interface Science Co., Ltd., Saitama, Japan) in triplicate. Briefly, the contact angle was measured at 10 different locations for each sample by the sessile drop method using ultrapure water with a droplet size of 0.5 μL. For detection of chlorine at the active layer surface, the chlorinated membrane samples were analyzed by XPS (ESCA3400HSE; Shimadzu Corp., Kyoto, Japan) using Mg Kα radiation as the X-ray source (1253 eV). To quantify R-COO− groups produced by amide bond scission, the chlorinated and hydrolyzed membrane samples were subjected to the silver-ion (Ag+) probing method. Briefly, the membrane samples were sequentially treated in 10−3 M silver nitrate (AgNO3) for 10

2. MATERIALS AND METHODS 2.1. Reverse-Osmosis Membranes and Model Tertiary Amide. The tertiary polyamide membrane was synthesized on a polysulfone support using N,N′-dimethyl-m-phenylenediamine as reported previously.23 The conventional secondary polyamide membrane (NTR-759HR, Nitto Denko Co., Osaka, Japan) was obtained commercially. The model tertiary amide N-methylbenzanilide was purchased from Sigma-Aldrich Co. LLC (St. Louis, MO). 2.2. Membrane Chlorination and Hydrolysis. The tertiary and conventional secondary polyamide membranes were soaked in pH-adjusted NaOCl solution (300 mL) with an initial free chlorine concentration (Cfc) of 200 ppm for 48 h at 25 °C in the dark. Cfc was measured by N,N′-diethyl-pphenylenediamine absorption photometry using a residual chlorine meter (HI96711; Hanna Instrument Japan, Inc., Chiba, Japan). The soaking solution for membrane deterio9110

DOI: 10.1021/acs.est.9b00663 Environ. Sci. Technol. 2019, 53, 9109−9117

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Environmental Science & Technology

Figure 1. Normalized salt rejection (left) and permeate flux (right) of the tertiary polyamide membrane after chlorination at a free chlorine concentration of 2000 ppm for 24 h and hydrolysis. Black and white plots show the normalized values after chlorination and hydrolysis, respectively. The dashed line shows the ratio of HOCl molecules (pKa = 7.5).30

min at pH 5.5, 10−5 M AgNO3 for 20 min at pH 9.85, and 2 × 10−6 M AgNO3 for 30 min at pH 10.5,26−28 and then the Ag+probed samples were analyzed by XPS. Survey scans at binding energies from 0 to 1150 eV were collected with an instrument resolution of 1.0 eV. High-resolution scans were also performed with a resolution of 0.1 eV for C (C 1s), N (N 1s), O (O 1s), Cl (Cl 2p), and Ag (Ag 3d). To confirm the change of the functional groups on the active layer surface caused by membrane deterioration, the membrane samples were analyzed by ATR-FTIR spectroscopy using a spectrometer (ALPHA-G; Bruker Corp., MA, USA) equipped with a diamond crystal. 2.5. Chlorination and Hydrolysis of the Model Amide Compound. Aqueous N-methylbenzanilide solution (26.7 ppm, 0.135 mM) was treated with NaOCl at pH 4, 7.5, and 10 and 25 °C in the dark. During the chlorination treatment, the pH was kept constant using 1 M NaOH and 1 and 10 M HCl. Note that the concentration of N-methylbenzanilide was one hundredth of the Cfc of NaOCl (1000 ppm, 13.5 mM) based on molarity. After 48 h, the solution was adjusted to pH 12 using 1 M NaOH. Hydrolysis was conducted for 2 h in a manner similar to that of the tertiary polyamide membrane. To analyze the remaining N-methylbenzanilide and its transformed products, the solution was extracted with n-hexane (50 mL) after chlorination and then again after hydrolysis. The extract was concentrated under a gentle stream of nitrogen and then analyzed by gas chromatography−mass spectrometry (GC/MS; HP6890-MSD5973, Agilent, Hanover, Germany) in a scanning mode (Table S1). The aqueous layer was adjusted to pH 2 or 12 with 1 M HCl or 1 M NaOH and then passed through a conditioned solid-phase extraction cartridge (Oasis HLB, Waters, MA). The cartridge was eluted with methyl acetate (1 mL), and then the eluate was dehydrated with anhydrous Na2SO4. After evaporation of the solvent under a gentle stream of nitrogen, the residue was derivatized with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA; 200 μL). After the derivatization, excess derivatization reagent was evaporated under a stream of nitrogen, and then the derivatized samples were analyzed by GC/MS in scanning mode (Table S1).

200 ppm at pH 12, and the subsequent hydrolysis treatment revealed its deterioration in the pH range from 4 to 12 (Figure S2). Because HOCl is rare at pH 12, this result indicates that OCl− deteriorated the secondary polyamide membrane. The chlorinated amide bond is known to be easily hydrolyzed by hydroxide ions.27 Under alkaline conditions, hydroxide ions are abundant, which means that N-chlorination and hydrolysis may occur simultaneously at pH 12. Conversely, under acidic conditions, HOCl is abundant, which means that chlorination is accelerated compared with that under alkaline conditions.29,30 Under acidic conditions, Orton rearrangement may occur because of the low concentration of hydroxide ions and dominant ring chlorination.29,31,32 The salt rejection and permeate flux of the virgin tertiary polyamide membrane were 84.5% and 0.59 m3·m2·d−1 on average, respectively, and lower than that for the secondary polyamide (99.2% and 0.99 m3·m2·d−1, respectively) (Table 2S); however, the former membrane performances were almost unchanged by chlorination for 48 h at Cfc = 200 ppm and subsequent hydrolysis. In fact, the normalized salt rejection before and after hydrolysis treatment was 0.92 and 0.89, thereby confirming the excellent durability of the tertiary polyamide membrane to free chlorine. Considering this concentration and contact time, the concentration−time of contact (CT) value for this tertiary polyamide membrane was calculated to be 9,600 ppm·h. In our previous study, the intermittent addition of NaOCl at Cfc = 10 ppm for 10 min every 24 h prevented biofouling during the reverse osmosis membrane filtration of secondary treated sewage from HigashiHiroshima Wastewater Treatment Plant, Japan.24 Comparing the CT value and intermittent chlorination conditions, the tertiary polyamide membrane was expected to be useful for an operation period of 7.9 years. At Cfc = 2,000 ppm, the salt rejection and permeate flux deteriorated over time, as shown in Figure 1. These results provide the first evidence that the tertiary polyamide membrane can be deteriorated by hypochlorite, even though the tertiary amides N-methylbenzanilide and N-phenylbenzanilide are reported to be inert to free chlorine.22 In addition, the observed deterioration behavior was surprisingly different from that of the conventional secondary membrane (Figure S2). Deterioration of the tertiary polyamide was the most extensive at pH 7.5. Considering the production of HOCl, it was reasonable that the tertiary polyamide membrane showed the least deterioration at pH 10 because of the absence of HOCl at this pH. On the basis of the

3. RESULTS AND DISCUSSION 3.1. Deterioration of the Tertiary and Secondary Polyamide Membranes. The salt rejection of the conventional membrane decreased after chlorination for 48 h at Cfc = 9111

DOI: 10.1021/acs.est.9b00663 Environ. Sci. Technol. 2019, 53, 9109−9117

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Figure 2. SEM images of the virgin and hydrolyzed tertiary polyamide membrane surfaces at 10000× magnification: (a) virgin membrane, (b) membrane after chlorination at pH 10, (c) membrane after chlorination at pH 7.5, and (d) membrane after chlorination at pH 4.

higher productivity of HOCl at lower pH, severer deterioration was expected; however, under acidic conditions, deterioration was weaker than that at pH 7.5 in terms of both salt rejection and permeate flux. At the same CT value (48000 ppm·h), similar trends of deterioration were confirmed in the salt rejection and permeation performances of the tertiary polyamide membrane (Figure S3). This finding allowed us to expect that unknown phenomena affected the membrane performance under acidic conditions, though chlorination might be severest under acidic conditions. SEM images of the membrane surfaces after chlorination at pH 4, 7.5, and 10 and subsequent hydrolysis are shown in Figure 2. The surface morphology of the membrane did not change by hypochlorite treatment at pH 10; conversely, the surface gathering structure disappeared, and melted-looking protuberances were observed on the membrane samples chlorinated at pH 7.5 and 4. Although it is possible that the vacuum-drying might affect the surface structure, the change of the surface morphology was the greatest at pH 4. This indicates that the severest damage of the tertiary polyamide membrane occurred at pH 4, although this observation was inconsistent with the observed changes of salt rejection and permeate flux (Figure 1). Note that the permeate flux decreased after hypochlorite treatment under acidic conditions, suggesting that hypochlorite treatment caused the active layer to become more hydrophobic. Actually, the water contact angle of the active layer increased after hypochlorite treatment at pH 4 (Figure 3). This is consistent with the fact that the powerful free chlorine agent HOCl is abundant under acidic conditions (Figure 1). In addition, after hydrolysis, the normalized permeate flux exceeded 1, suggesting the loosening of the polymer network. The degree of increase of the permeate flux induced by the hydrolysis treatment was bigger under acidic conditions than under neutral or basic ones (Figure S4). Considering these

Figure 3. Change of the water contact angle of the tertiary polyamide membrane after chlorination and hydrolysis. Bars indicate standard deviation (n = 3).

results, the most plausible explanation for the observed deterioration of the membrane performances is that the hypochlorite treatment at pH 4 resulted in the severest chlorination and damage of the active layer, as was apparent in the SEM images (see Figure 2). However, the salt rejection and permeate flux showed the biggest changes at pH 7.5. To verify the proposed membrane degradation mechanisms, the changes of the chemical characteristics of the active layer during chlorination under different pH conditions were analyzed. 3.2. Changes in the Chemical Characteristics of the Active Layer. Figure 4 shows XPS survey spectra of the active layer of the tertiary polyamide membrane after hypochlorite treatment at pH 4, 7.5, and 10. Because no peaks associated with sulfur derived from the support polysulfone were detected, the signals were attributed to the active layer surface. The peak intensity from Cl 2p increased for the membranes treated below pH 7.5 compared with that of the virgin membrane (Figure 5), thereby confirming that lower pH 9112

DOI: 10.1021/acs.est.9b00663 Environ. Sci. Technol. 2019, 53, 9109−9117

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Figure 4. XPS survey scans of tertiary polyamide membranes before and after chlorination at different pH (left) and the Cl 2p high resolution spectra (right).

Although the Cl 2p spectra of the tertiary polyamide membranes revealed the chlorination of the benzene ring by hypochlorite at pH 4 and 7.5 (Figure 4), chlorine binding to the amidic nitrogen was unable to be detected. This is because N−Cl bonds are unstable under X-ray irradiation, so Cl quickly transfers to the benzene ring.32 Therefore, ATR-FTIR analysis was conducted to investigate how hypochlorite changed the polyamide chains in the membrane. The ATR-FTIR spectra of the tertiary polyamide membrane before and after chlorination are shown in Figure 6. The Amide I band (about 1640 cm−1) is mainly attributed to carbonyl CO stretching, and the Amide II band (about 1540 cm−1) is caused by N−H bending and C− N stretching vibrations.35,36 Since the Amide II band was not detected on this tertiary polyamide membrane, the result indicates that N−H bonds might not be produced by chlorination. On the other hand, a peak shift of the Amide I band was observed. Although further evidence is necessary, the possible explanation for this peak shift might be changes of intermolecular/intramolecular interactions such as electrostatic repulsion by incorporation of chlorine atoms into the polymer chains. The fact that the peak shift became more obvious at lower pH is consistent with the presence of the strong free chlorine species HOCl. If N-chlorination occurs, the subsequent Orton rearrangement may cause desorption of

Figure 5. Cl atomic concentration of the tertiary polyamide membrane after chlorination at different pH.

resulted in severer chlorination of the active layer. Tertiary amides are reported to be inert to free chlorine;22 however, our results confirmed that chlorination of the tertiary amide occurred during hypochlorite treatment. The pH dependence of chlorination was same as that observed for conventional secondary polyamide membranes in several papers;5,10,29,31,33−35 however, the least chlorination was observed at pH 10 for the tertiary polyamide membrane.

Figure 6. ATR-FTIR spectra of the tertiary polyamide membranes before and after chlorination at different pH. 9113

DOI: 10.1021/acs.est.9b00663 Environ. Sci. Technol. 2019, 53, 9109−9117

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3.3. GC/MS Analysis of the Model Compounds. NMethylbenzanilide is reported to be inert to hypochlorite;22,37 however, we confirmed chlorination of N-methylbenzanilide at pH 4 and 7.5. As shown in Figure 8, four peaks appeared in the mass chromatogram of the hexane solution after chlorination. The peaks at 12.7, 13.0, and 13.7 min were consistent with two monochlorinated N-methylbenzanilides and dichlorinated Nmethylbenzanilide, respectively (Figures S5−S7). The amidic methyl groups remained in the three chlorinated products, and chlorine atoms appeared on the benzene ring to which the amidic nitrogen was bound. These results indicate that the tertiary amide was chlorinated via direct ring chlorination, which differs from the chlorination mechanism of the secondary amide membrane. In addition, greater chlorination at lower pH was confirmed by estimating the peak areas of chlorinated N-methylbenzanilide (Figure S8). This result is consistent with the severer chlorination of the tertiary polyamide membrane by hypochlorite treatment at lower pH, resulting in formation of a more hydrophobic active layer than was the case for the membrane chlorinated at higher pH. Note that trimethylsilyl benzoic acid was detected in the solid extract of the hydrolyzed sample after hypochlorite treatment at pH 4 (Figure S9). This acid might be produced by hydrolysis of chlorinated N-methylbenzanilide, which is consistent with chlorination of the benzene ring to which the amidic nitrogen atom was bound (Figures 8 and S5−S7). Chlorination of the benzene ring can decrease the electron density at the carbonyl carbon, which may facilitate the attack of the amide bond by hydroxide ions. This may be the mechanism of the deterioration of the chlorinated tertiary polyamide membrane during the subsequent hydrolysis. 3.4. Proposed Deterioration Mechanisms. Figure 9 summarizes the proposed deterioration mechanisms of the novel tertiary polyamide membrane. Based on the ATR-FTIR spectra, the dominant chlorination mechanism of its active layer was the direct chlorination of the benzene ring to which the amidic nitrogen atom was bound (Figure 4). XPS detected a higher abundance of chlorine atoms for the membrane chlorinated at lower pH (Figure 5), which increased the hydrophobicity of the active layer and led to lower permeability. Furthermore, the Ag-probed tertiary polyamide

amidic methyl groups, which results in the formation of N−H bonds.3 However, no Amide II band was detected (Figure 6). This suggests that N-chlorination may not occur in the tertiary polyamide membrane. The direct chlorination of the benzene ring might be dominant over N-chlorination; this chlorination process is different from that of the secondary polyamide membrane. Figure 7 shows the Ag atomic concentration in the active layer of the Ag-probed tertiary polyamide membrane samples

Figure 7. Ag atomic concentration of the tertiary polyamide membrane after chlorination at different pH and subsequent hydrolysis. Black and white plots show the values after chlorination and hydrolysis, respectively. Bars indicates standard deviation (n = 3) with two exceptions at pH 4 and 7.5 after hydrolysis treatment (n = 5). The dashed line indicates the Ag atomic concentration of the virgin membrane.

as detected by XPS. Although statistical significance was not confirmed, it was reasonable that the Ag concentration in the hydrolyzed membrane samples was higher than that in the chlorinated one. The possible reason for the lower Ag concentration at pH4 than that at pH7.5 may be less amount of OH− causable for hydrolysis. As for the hydrolyzed membrane samples, the Ag concentration became higher on average as the membranes were treated by hypochlorite at lower pH. Because the Ag atomic concentration is proportional to the production of R-COO− groups, these results showed that the chlorination in the active layer by hypochlorite was associated with amide bond scission.

Figure 8. Mass chromatograms of N-methylbenzanilide and the chlorination products detected in the hydrolyzed samples. The pH values indicate the hypochlorite treatment conditions. 9114

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Figure 9. Deterioration reactions of the tertiary polyamide reverse osmosis membrane by sodium hypochlorite (NaOCl) and sodium hydroxide (NaOH).

yl-m-phenylenediamine. Our results showed that the tertiary polyamide membrane was deteriorated by hypochlorite below pH 7.5. This indicated that HOCl rather than OCl− was the free chlorine species that caused membrane deterioration. The deterioration became severer as chlorination progressed, leading to obvious collapse of the active layer after chlorination below pH 7.5. Chlorination of the tertiary polyamide occurred via direct chlorination of the benzene ring to which the amidic nitrogen was bound, which was different from that of the secondary polyamide for which Orton rearrangement occurred following the chlorination of the amidic nitrogen. Amide bond scission occurred in the active layer of the tertiary polyamide membrane, which might be related to the electron deficiency of the amidic nitrogen caused by chlorination of the amidic nitrogen-bound benzene ring. Most recent studies have aimed to develop chlorine-resistant secondary polyamide by protecting N−H bonds because of their chlorine sensitivity. We confirmed that the tertiary polyamide membrane showed excellent tolerance to free chlorine; however, this was deteriorated by HOCl via direct ring chlorination. These results provide some ideas for development of novel reverse osmosis membranes with improved durability to free chlorine. For example, protecting another part of the tertiary polyamide, such as protecting the benzene ring bound to the amidic nitrogen by substitution with hydrophilic groups, may aid development of membranes that display permeability and chlorine tolerance simultaneously.

membrane samples showed that amide bond scission was associated with chlorination (Figure 7). As chlorination progresses at the benzene ring on which the amidic nitrogen atom is bound, the electron deficiency of amide bonds might increase, allowing hydroxide ions to easily attack the amide bond and cause hydrolysis. On the basis of these results, the most severe deterioration of the tertiary polyamide membrane was expected at pH 4, which was consistent with the SEM images, in which the gathering structure obviously disappeared from the membrane surface after chlorination at pH 4 (see Figure 2). Despite of this evidence, the greatest deterioration of the tertiary polyamide membrane in terms of both salt rejection and permeate flux occurred at pH 7.5. A possible explanation for this result is the collapse of the active layer induced by the weakening of intermolecular/intramolecular interactions in the polyamide structure following chlorination at pH 4.9,38 It has been documented that an active layer of a secondary polyamide membrane was separated from its support layer because of free chlorine penetration.39 In fact, the active layer of this tertiary membrane sample easily peeled away from the polysulfone support after chlorination. The collapsed active layer might contribute to maintaining the salt rejection and permeation performances of the tertiary polyamide membrane via densification by the operation pressure of 1.5 MPa; however, this might be temporary because of the damaged membrane structure. In contrast, the tertiary polyamide membrane was hardly deteriorated at pH10 by hypochlorite treatment. Because HOCl was absent at pH 10, this result indicates that the tertiary polyamide membrane was inert to OCl− but sensitive to HOCl. In this study, we investigated the chlorine resistance of a tertiary polyamide membrane synthesized using N,N′-dimeth-

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b00663. 9115

DOI: 10.1021/acs.est.9b00663 Environ. Sci. Technol. 2019, 53, 9109−9117

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Environmental Science & Technology

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GC/MS operational conditions (Table S1), performance of the secondary and tertiary polyamide membranes used in this study (Table S2), schematic diagram of the cross-flow test cell (Figure S1), normalized salt rejection of the conventional secondary polyamide membrane after chlorination and hydrolysis (Figure S2), normalized salt rejection and permeate flux of the tertiary polyamide membrane after chlorination (Figure S3), increase of permeate flux of the tertiary polyamide membrane after the hydrolysis treatment (Figure S4), mass spectra of the monochlorinated N-methylbenzanilide (Figure S5 and S6), mass spectrum of the dichlorinated N-methylbenzanilide (Figure S7), production of the dichlorinated and two monochlorinated Nmethylbenzanilide by chlorination of N-methylbenzanilide (Figure S8), and mass spectrum of the trimethylsilyl benzoic acid (Figure S9) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +81-82-424-7621. Fax: +81-82-424-7621. E-mail address: [email protected]. Researcher ID: D-72252011 (Web of Knowledge). ORCID

Satoshi Nakai: 0000-0002-6139-6649 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Hiroshima University. The authors are immensely grateful to Ms. Yuki Nishiumi for her technical support and Dr. Makoto Maeda for technical assistance with SEM measurement. We thank Natasha Lundin, Ph.D., from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

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DOI: 10.1021/acs.est.9b00663 Environ. Sci. Technol. 2019, 53, 9109−9117