Chlorine-Resistant Polyamide Reverse Osmosis Membrane with

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Chlorine-resistant Polyamide Reverse Osmosis Membrane with Monitorable and Regenerative Sacrificial Layers Hai Huang, Saisai Lin, Lin Zhang, and Li'an Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16462 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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ACS Applied Materials & Interfaces

Chlorine-resistant Polyamide Reverse Osmosis Membrane with Monitorable and Regenerative Sacrificial Layers Hai Huang†‡, Saisai Lin†‡, Lin Zhang†* and Li’an Hou† † Key

Laboratory of Biomass Chemical Engineering, College of Chemical and Biological

Engineering, Zhejiang University, Hangzhou 310027, P. R. China ‡ These authors contributed equally. KEYWORDS: reverse osmosis, chlorination, grafting membrane, regenerative membrane, zetapotential.

ACS Paragon Plus Environment

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Scheme 1. EDC/NHS mediated Gly-grafting reaction on polyamide membrane surface.


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Figure 1. Effect of Gly concentration on membrane RO performance

Figure 2. Chlorination evaluation of Gly-PA and B-PA membranes


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Figure 3. SEM images of membrane surface: (a) & (b) intact B-PA and Gly-PA, (c) & (d) 1500 ppm∙h chlorinated B-PA and Gly-PA, (e) & (f) 3000 ppm∙h chlorinated B-PA and Gly-PA.


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(c) Figure 4. XPS spectra of Gly-PA (a), B-PA (b) and their quantitative analysis (c) of chlorine content after “chlorination and reduction”.


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Figure 5. FTIR spectra of B-PA (a) and Gly-PA (b) in the intact state (black) and chlorinated-reduced state (red)

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Figure 6. Normalized rejection (a) and flux (b) of B-PA and Gly-PA in cycling chloination. NA: intact membrane; C1, C2, C3: 1st, 2nd and 3rd chlorination; R1, R2, R3: 1st, 2nd and 3rd reduction.


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Figure 7. Relation between membrane rejection and zeta potential for B-PA (a) and Gly-PA (b)

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Figure 8. Mechanical schematic of Gly-induced chlorine-resistance


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Chlorine-resistant Polyamide Reverse Osmosis Membrane with Monitorable and Regenerative Sacrificial Layers Hai Huang†‡, Saisai Lin†‡, Lin Zhang†* and Li’an Hou† † Key

Laboratory of Biomass Chemical Engineering, College of Chemical and Biological

Engineering, Zhejiang University, Hangzhou 310027, P. R. China ‡ These authors contributed equally. KEYWORDS: reverse osmosis, chlorination, grafting membrane, regenerative membrane, zetapotential.

ABSTRACT: Improving chlorine stability is a high priority for aromatic polyamide (PA) reverse osmosis (RO) membranes especially in long-term desalination. In this paper, PA RO membranes of sustainable chlorine resistance was synthesized. Glycylglycine (Gly) was grafted onto the membrane surface as a regenerative chlorine sacrificial layer, and the zeta-potential was used to monitor the membrane performance and to conduct timely regeneration operations for chlorinated Gly. The Gly-grafted PA membrane exhibited ameliorative chlorine resistance in which the NH moiety of glycylglycine served as sacrificial pendants for chlorine attacks. Cyclic chlorination experiments, combined with FT-IR and XPS analysis, were carried out to characterize the membrane. Results indicated that the resulting N-halamines could be fast regenerated by a simple al-


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kaline reduction step (pH 10). A synchronous relationship between the zeta-potential and the chlorination extent of the sacrificial layer was observed. This indicated that the zeta-potential can be used as an on-site sensor to conduct a timely regeneration operation. The intrinsic mechanism of the surface sacrificial process was also studied.

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INTRODUCTION

As the state-of-the-art desalination technology for sustainable water-supply, reverse osmosis (RO) process has been undergoing a worldwide expansion, with around 80% share in worldwide desalination capacities installed over the past two decades.1-3 The large-scale applications are mainly due to the development of aromatic polyamide (PA) thin film composite (TFC) membranes with high ion-rejection rate and water-permeability. Although current advances in nanomaterial science has made much breakthrough in novel RO membranes development based on graphene, graphene oxide and MoS2, etc., the economic feasibility of scaling up those membranes remains questionable and aromatic polyamides is still the most commercially popular RO membrane material.4 Nevertheless, the stability and life expectancy of PA-TFC membranes continues to be issue in this rapidly growing industry.5-7 Biocides such as chlorine or hypochlorite, commonly used to prevent biofouling or as membrane cleaning agents,8-9 can react with the PA active layer even at ppm levels and then result in material degradation associated with performance deterioration.10-12 In current desalination operations, the following protocol is typically adopted to prolong the service life of PA-TFC membranes. Water to be purified is chlorine-pretreated to prevent biofouling of membranes, then dechlorinated before being fed into membrane units.13 After passing through the RO membranes, the product water is rechlorinated again before distribution.14 Considering these additional processing steps, it is of


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high urgency to develop chlorine-resistant polyamide reverse osmosis membranes especially in the long-term operation to lower the costs of water desalination. It has been reported that the deterioration of polyamide matrix due to free chlorine follows a twostep electrophilic substitution, including a reverse N-chlorination of the amide N-H group to NCl group, and subsequently an irreversible ring-chlorination via Orton rearrangement.12,15-21 Based on this chlorination mechanism, several tactics have been developed to improve the chlorine-resistant property of the aromatic PA-TFC membranes. One strategy is to use highly chlorine-tolerant polymers or monomers via substituting the N-position hydrogen atom by other moieties on the amide bond,22-23 or by introducing electron-withdrawing or steric hindrance groups into the benzene rings.24-26 However, the removal of the hydrogen atoms would destroy the hydrogen bonding among the polyamide inter-chains and inevitably reduce the ion selectivity and surface hydrophilicity. What’s more, the synthesis process for these alternative membrane materials are always difficult and tedious. Surface coating with chlorine-resistant or chlorine-consuming polymers on PA layers is another simple strategy to protect the membranes from chlorine attacks.27-29 However, these protective layers are prone to be washed away during long-term cross flow operations as a result of the weak adhesion forces (Van der Waals force). Furthermore, it would also suffer from water flux loss since the permeation resistance increases.27-30 Another alternative strategy of surface modification is to covalently graft some protective pendants to the polyfunctional active sites on the nascent membrane surface, such as to the residual unreacted acylchloride (-COCl) and amino (-NH2) groups, or the hydrolysis-induced carboxyl groups (COOH).31-32 Compared with the method of surface coating, covalently-grafted mono- or bi-layer generally imposes less mass transfer resistance and possess a higher stability of chlorine-resistance under cross-flow operations.


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However, the method of grafting modification also has its inherent limitations. For example, it is difficult to ensure the grafted barriers to have covered the whole membrane surface. That’s not only because these barriers are too thin but also they grow via the site-to-site grafting to the PA layer, both of which are likely to introduce defects in the barriers. Thus, the resultant chlorineresistance is generally much lower than that of physical coating as the latter works in a totally physical isolation . To improve the chlorine-resistance, a novel strategy of sacrificial grafting has been recently attempted by introducing some chlorine-reactive groups actually to consume the residual chlorine rather than simplly physical isolation.33-37 Wei et al. modified PA membranes by free radical grafting of 3-allyl-5,5-dimethylhydantoin (ADMH) and Zhang et al. brought 5chloromethylsalicylaldehyde (SA) to PA membrane surface via quaternarization.36-37 These grafting materials served as a sacrificial layer to consume the active chlorine by their N-H pendants or via chlorine oxidation. More importantly, the regenerative ability of the chlorine-resistant property of these sacrificial groups has also been taken into account, since the chlorination saturation resulted from the restricted number of the grafting groups is really a thorny issue. This concept has been firstly conducted in a series of research on surface grafting of hydantoin derivatives.33-35 The chlorinated product of N-halamines can be regenerated to its initial chlorine-active state by microorganisms. Considering N-halamines commonly show a higher biocidal efficiency against Staphylococcus aureus and Escherichia coli despite of its broad-spectrum sterilization,38 a chemical reduction may be preferable in the practical long-term RO operation. In addition, in that case PA layers still has the risk to be exposed to chlorine attacks once these sacrificial moieties have been chlorine-saturated while the regeneration operation is not implemented in time. Thus a measure that can monitor the chlorination extent and conduct a timely regeneration operation just


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before the chlorine saturation is quite essential for these sacrificial groups to sustain the chlorineresistant property. Therefore in this paper, a monitorable surface sacrificial process was designed to develop polyamide (PA) reverse osmosis (RO) membranes of sustainable chlorine-resistant property. Glycylglycine (Gly) groups, used as a regenerative sacrificial layer, were grafted onto the PA membrane surface. As shown in Scheme. 1, catalyzed by EDC/NHS system, the primary amine of Gly molecule was grafted onto the carboxyl group hydrolyzed from the residual acyl chloride after interfacial polymerization. Two N-H moieties on Gly groups could serve as the active chlorine-consumer and its chlorinated state of N-Cl can be rapidly regenerated into N-H again by a simple alkaline reduction (pH 10). Most interestingly, the protonation of amine (-NH) and carboxylic (-COOH) groups on Gly molecules would be depressed by the chlorine substitution of the hydrogen atom on N-H moieties, which increased the negative charges on the membrane surface. Such negative behavior obviously can be recovered along with the regeneration operation as the chlorine was removed from N-H moieties. Based on this synchronous relationship, the zeta-potential of the membrane was designed as an on-site sensor to monitor the chlorination extent of the Gly sacrificial groups and further conduct a timely regeneration operation just before chlorine saturation.


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Scheme 1. EDC/NHS mediated Gly-grafting reaction on polyamide membrane surface.

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RESULTS AND DISCUSSION

Grafting Condition Optimization. As shown in Scheme 1, the primary amine groups of Gly molecules reacted with carboxyl groups hydrolyzed from the residual acyl chloride after interfacial polymerization. By this way, Gly was grafted to the surface of polyamide membranes. However, excessive Gly-grafting would significantly increase the mass transfer resistance and in turn caused a permeation loss.31,39 Therefore, the separation performance of the grafted PA-TFC membranes was evaluated to optimize the Gly-grafting concentration. The results are shown in Figure 1. With the increase in the concentration of Gly solution, the water flux of the grafted membranes exhibited a continuous declination. When the concentration of Gly solution was increased to 0.45 wt%, the water flux decreased from 45.6 L∙m-2∙h-1 to 31.2 L∙m-2∙h-1. Such permeation loss was mainly resulted from the additional grafted-barriers, which was similar to the


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permeation resistance increase in physical coating.27-32 At the same time, a slight decrease was also observed in salt rejection. This negative behavior may probably be affected by Gly-loading, partially reducing the degree of cross-linking of the PA skin layer and neutralizing the membrane surface charges.30 When the concentration of Gly solution reached 0.2 wt %, the separation performance gradually kept balance as the signal of grafting saturation. Considering the overlapping and aggregation of the Gly molecules at higher concentrations, the concentration of Gly solution was fixed at 0.2 wt % as an optimum condition for the subsequent chlorination evaluation.

Figure 1. Effect of Gly concentration on membrane RO performance Chlorine-resistance Evaluation. A statically accelerating chlorination experiment was conducted to evaluate the chlorine-resistance property of the grafted membrane with 0.2 wt% Gly-grafting concentration. As shown in Figure 2, during the entire increase process of chlorination exposure intensity, an obvious and continuous deterioration behavior of separation performance was observed in the B-PA membrane, which indicated that a severe chlorine attack had occurred on membrane surface. In contrast, in the range of 0 to 1500 ppm∙h of chlorination exposure intensity, the Gly-PA membrane samples, under the protection of Gly sacrificial layer, kept with a steady separation performance in the same chlorination. Since each grafted-Gly molecule intro-


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duced two sacrificing amine groups on the PA surface, the active chlorine preferentially consumed these sacrificing moieties outside the bulk membranes. By this mean, PA selective layers in Gly-PA membranes were protected from the chlorine attack and exhibited a remarkably improved chlorine resistance. However, further chlorination was restricted to a specific number of Gly groups that would be eventually saturated by the active chlorine. Once up to saturation, the active chlorine would turn to attack the PA matrix. This phenomenon is found in Figure 2. When the chlorination exposure intensity exceeded 1500 ppm∙h, apparent performance degradation was observed in Gly-PA membranes. If these chlorine-saturated sacrificial groups could be restored back to their initially chlorine-active state by certain means, a more durable chlorine resistance of Gly-PA membranes might be expected.

Figure 2. Chlorination evaluation of Gly-PA and B-PA membranes The surface morphology of B-PA and Gly-PA membranes corresponding to the above chlorination process was characterized. As showed in Figure 3a and 3b, the Gly-PA and B-PA membranes presented a similar surface morphology with dense and “leaf-like” feature, indicating that the


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Gly-grafting reaction was a mild modification process without any significant damage on the membrane surface. After chlorination exposure to 1500 ppm∙h, the “leaf-like” feature of B-PA membranes was disappeared and gradually replaced by smoother morphology (see Figure 3c), implying a chlorine-reduced hydrolysis in the partial area of PA layers. As for Gly-PA membranes, Figure 3d manifested that it nearly remained the same with the aid of Gly protection, consistent with the above separation performance evaluation. As the chlorination intensity was increased up to 3000 ppm∙h, more severe and destructive decomposition occurred on those smooth defect areas of B-PA membranes. The PA layer even began to peel off from the polysulfone substrate as shown in Figure 3e. At this intensive chlorination, Gly-PA was also observed with some damaged regions similar to the ones present in Figure 3c, indicating that the polyamide matrix decomposition had also occurred while beyond chlorine saturation.

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Figure 3. SEM images of membrane surface: (a) & (b) intact B-PA and Gly-PA, (c) & (d) 1500 ppm∙h chlorinated B-PA and Gly-PA, (e) & (f) 3000 ppm∙h chlorinated B-PA and Gly-PA. Regeneration of Chlorine-resistance. In order to overcome the chlorine saturation issue and further enhance the durability of the chlorine resistance of Gly-PA membranes, an simple alkaline reduction (pH 10) was conducted. It was feasible because the Cl-substitution of amide N-H on Gly structure was actually a reversible process and its counteraction (eg. dechlorination reduction) was especially accelerated in the presence of OH-.15-17,40-42 A series of spectrum measurements were carried out to verify the feasibility of this alkaline reduction. Here all the test membrane samples were processed with one cycle of “chlorination-reduction” treatment. The XPS results of the original, chlorinated and reduced B-PA and Gly-PA membranes are shown in Figure 4, respectively. The absorption intensity of elemental characteristic peaks at 200eV, 280eV, 390eV and 530eV in Figure 4a and 4b symbolized the atomic content of Cl, C, N and O in the membrane samples. Obviously, a distinct Cl absorption peak was both found in the chlorinated and alkaline-reduced B-PA membranes. Conversely in the Gly-PA membrane, the chlorine characteristic peak also appeared after chlorination, but then almost disappeared after the alkaline reduction. A quantitative analysis of surface chlorine content, shown in Figure 4c, clearly displayed the significant differences in the chlorine removal capacity of the alkaline re-


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duction for B-PA and Gly-PA membranes. It indicated that in Gly-PA membranes, most of the active chlorine was consumed by N–H moieties on Gly groups via the reversible N-chlorination and could be effectively removed by this alkaline reduction. While in B-PA membranes without any protection, the active chlorine mostly entered into the irreversible ring-chlorination and the alkaline reduction no longer worked on these chlorine.17-21, 43 Thus, the XPS results suggested that compared to the ring-chlorinated B-PA membranes, alkaline reduction performed much better in the N-chlorinated Gly sacrificial layer for active chlorine removal.

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Figure 4. XPS spectra of Gly-PA (a), B-PA (b) and their quantitative analysis (c) of chlorine content after “chlorination and reduction". FT-IR spectrum was then taken to characterize the surface chemistry of B-PA (Figure 5a) and Gly-PA (Figure 5b) samples before and after "chlorination-reduction". In most cases, the typical spectrum of polyamide membranes presented two characteristic peaks around 1650 cm-1 and 1540 cm-1, identified as amide I (-N-C=O) and amide II (-C-N-H ) modes in secondary amides, respectively.43 Furthermore, a weak peak around 1600 cm-1 was always present as a typical indicator for chlorination extent as it corresponded to the C=O stretching but shifting to1650cm-1 with the effect of hydrogen bonds in PA layer.11,44 As seen in Figure 5, the spectra of the intact Gly-PA and B-PA were nearly identical because the grafted Gly segment mainly consists of simple amide bonds without any other special structure as shown in Scheme 1. After one cycle of chlorination-reduction, the peak at 1542 cm-1 corresponding to amide II (-C-N-H) was found a shrink but still could be clearly clarified both in B-PA and Gly-PA membranes. However, given that chlorine substitution (forming amide N-Cl) breaks up the hydrogen bonding among the N-H structure, a weaker peak at 1604 cm-1 was indeed found in Gly-PA membranes but almost disappeared in B-PA membranes. This implied that the alkaline reduction indeed reduced partial N-Cl moieties into its initial N-H moieties but exhibited a higher performance in the Gly sacrificial layer, as the absorption peaks at 1542 cm-1 and 1604 cm-1 in Gly-PA membranes were both stronger than that in B-PA membranes.


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Figure 5. FTIR spectra of B-PA (a) and Gly-PA (b) in the intact state (black) and chlorinated-reduced state (red) Since the XPS and FTIR spectrums confirmed the high performance of the alkaline reduction in N-H sacrificial moieties regeneration of Gly-PA membranes, a cyclic chlorination involving three “chlorination-reduction” cycles was carried out to further estimate the regenerative ability of the chlorine resistance property in these membranes. In each cycle, the chlorination exposure intensity was set with 500 ppm at pH 4.0, at which the grafted Gly sacrificial groups were far from chlorine saturation according to the chlorination results shown in Figure 2. As shown in Figure 6a, the salt rejection of B-PA and Gly-PA membranes both declined after chlorinated (C area) and always recovered to varying degrees after alkaline reduction (R area). Evidently the salt rejection of the chlorinated G-PA membranes always returned to its original level after alkaline reduction treatment, but that of the chlorinated B-PA membranes overall showed an downward trend. After three cycles, the decrease of the normalized salt rejections in B-PA membranes was accumulated to 8.0%, by contrast that in Gly-PA membranes was only 1.2%. Therefore, alkaline reduction did not work in terms of performance recovery for B-PA membranes since the PA layer had suffered an irreversibly severe and destructive decomposition in chlorination as present earlier in Figure


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3. For Gly-PA membranes, the de- and re-structure of hydrogen bonding framework due to the transformation from N-Cl to N-H in Gly moieties may have caused the variation in salt rejection, 5,6,11,12

but it was a reversible and recoverable process. Similarly, the increase in water flux in

Figure 5b was also significantly greater in B- PA than that in Gly-PA. The enhancement of the water flux in Gly-PA membranes could be reasonably attributed to the hydrolysis of partial amide bonds under alkaline condition.5,11,12,25 These performance results firmly supported the XPS and FTIR spectrums and verified the high generation ability of the chlorine resistance in Gly-grafting polyamide membranes. Moreover, it also implied that the Gly-PA membrane had the potential to sustain its good behavior for more cycles if the alkaline reduction could be implemented in time.

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Figure 6. Normalized rejection (a) and flux (b) of B-PA and Gly-PA in cycling chloination. NA: intact membrane; C1, C2, C3: 1st, 2nd and 3rd chlorination; R1, R2, R3: 1st, 2nd and 3rd reduction. Synchronous relationship between Zeta-potential and Chlorination Extent. Herein a synchronous relationship between the zeta-potential and chlorination extent was examined. Figure 7 showed the variation of the surfacial zeta-potential of membrane samples at each stage in the above cycling chlorination. In Figure 7a, same as shown in Figure. 6a, the salt rejection of B-PA membranes could always recover to a certain degree but overall showed an downward trend. Unfortunately, this recovery phenomenon was not observed in its zeta-potential curve, which exhib-


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ited a continuous declination in the entire cycling chlorination. In Figure 7b, the salt rejection of Gly-PA membranes was also taken into count and it almost all returned to its original level after the alkaline reduction in the three “chlorination-reduction” cycles. Fortunately, the variation in its corresponding zeta-potential perfectly coincided with that in the salt rejection of Gly-PA membranes. The zeta-potential value dropped in chlorination and then nearly recovered to its initial level after the reduction treatment. The drop in the zeta-potential was partially caused by the depression in the protonation of the amine functional groups both in B-PA and Gly-PA membranes, as the hydrogen bonding amine N-H group was converted to the non-hydrogen bonding N-Cl group.45,46 Such drop in Gly-PA membranes stemmed from the reversible N-chlorination could be recovered after the alkaline reduction. Meanwhile, the deprotonation of the carboxylic groups on the end of Gly molecules, due to the electron-induced effect caused by chlorine substitution, would also achieve a more negative zeta-potential and could be recovered by the active chlorine removal. However, the situation was quite different in B-PA membranes. The cleavage of polyamide chains due to chlorine attack led to an increase in free carboxylic groups and this process was especially pronounced under alkaline conditions,6,46 resulting in a more negative zeta-potential after alkaline treatment as shown in Figure 6a. That was the main reason that zetapotential would present a desired recovery phenomenon in Gly-PA membranes and further have a good synchronous relationship with the chlorination extent of these membranes but not in B-PA membranes. More importantly, during the three testing cycles, the zeta-potential value in C and D area of Figure 6b extremely stuck to a stable level, which indicated that the zeta-potential was a fully reliable quantitative parameter to serve as an on-site chlorination sensor.


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Figure 7. Relation between membrane rejection and zeta potential for B-PA (a) and Gly-PA (b) Chlorine-resistance Mechanisms. Here the intrinsic mechanisms of the surface sacrificial process was explored to understand the basic principles for more analogical strategies in this area. As shown in Figure 8, the B-PA membranes experienced a typical chlorination process: the active chlorine reversibly reacted with amide groups (S1) and formed N-chloro products with limited stability prone to dechlorination,10-11,17-18 then most of chlorine got into the irreversible ring chlorination via Orton-rearrangement (S2).6,11,17-18,35,43 The decrease of the hydrogen bonding as a result of N-chlorination led to freely available carbonyl groups and finally deteriorated the ion rejection of B-PA membranes.10-12 The irreversible ring chlorination could not be dechorinated by a reducing agent; a reduction treatment even could create a more open polyamide structure (S3) due to the alkaline promoted hydrolysis of C-N bonds.11,47-48 In Gly-PA membrane (S4), once exposed to chlorination, the N-H moieties of Gly groups would serve as the sacrificial pedant and preferentially consume the active chlorine (S5). By this way, most of the active chlorine was reversibly N-chlorinated to the N-H moieties on Gly groups and could be well dechlorinated by alkaline reduction as demonstrated in the above experiments. As described earlier, the chlorine substitution of the hydrogen atom on amine N-H groups in Gly molecules would inhibit


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the protonation of the amine and carboxylic groups on Gly segments, resulting in a more negative zeta-potential on Gly-PA membrane surface. With the removal of the Cl uptake by alkaline reduction, the zeta potential could precisely recover to its initial value. Such a good synchronous relationship between the zeta-potential and the chlorination extent of the sacrificial layer could be well designed to conduct a timely regeneration operation.

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Figure 8. Mechanical schematic of Gly-induced chlorine-resistance

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Conclusion.

In conclusion, an integrated surface sacrificial process involving of a glycylglycine (Gly) sacrificial layer and its chlorination extent monitoring via zeta-potential, was designed to obtain the


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polyamide (PA) reverse osmosis (RO) membrane of sustainable chlorine resistant property. With the aid of EDC/NHS catalytic system, the glycylglycine (Gly) group of regenerable chlorine resistance was grafted to the PA-TFC membrane surface. The –NH2 moieties on Gly could consume the active chlorine and the resulting N-halamines can be fast regenerated by a simple alkaline reduction (pH 10). A synchronous relationship between the zeta-potential and the chlorination extent of the sacrificial layer was observed. Cyclic chlorination experiments indicated that the zeta-potential can be used as an on-site sensor to conduct a timely regeneration operation. The intrinsic mechanism of the surface sacrificial process was also studied. In addition, the mechanism of this surfacial sacrificial process was also proposed to provide the basic principle for more analogical strategies in areas like membrane fouling or acidification.

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Experimental Section

Membrane surface grafting. The original PA-TFC membrane was fabricated via the interfacial polymerization as previously report.30,49 A pretreatment was conducted prior to the surface grafting of the PA-TFC membrane. The prepared membrane was immersed in a weak alkaline aqueous solution of 3.3 wt % Na2SO4 and 0.2 wt % Na2CO3 for 20 s to completely remove the unreacted monomers, as well as to induce more residual acylchloride (-COCl) to hydrolyze into carboxyl groups (-COOH).50-51 After a rinse with DI water, the pretreated PA-TFC membrane was activated in 2-(N-Morpholino) ethanesulfonic acid (MES, >99%, Aladdin) buffer solution (0.1 mol/L, pH 5.0) containing 0.5 wt % 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC, >98%, Aladdin) and 0.8 wt % N-hydroxysuccinimide (NHS, >98%, Aladdin) with an 80 r/min shaker under 37 °C for 1 hr. Afterwards, the membrane was washed with DI water to


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remove the residual catalyst. PBS buffer solutions (pH 7.2-7.4) with different Glycylglycine (Gly, >99%, Aladdin) concentrations (wt %) were applied to react with the activated membrane surface under the same shaking condition for 2 hr. Finally, the Gly-grafted PA membrane was rinsed with DI water to remove the excess Gly absorbed on the membrane surface and stored in DI water. The original PA membrane and Gly-grafted membrane were denoted as B-PA and GlyPA, respectively. Separation Performance Evaluation. A customized cross-flow RO cell was used to evaluate the membrane performance. The apparatus has been reported in our previous work.30,49 B-PA and Gly-PA membrane were tested with 2000 ppm NaCl aqueous solution at 25 °C. All samples were pre-compressed for 1 hr and evaluated under the pressure of 1.6 MPa, and then the volume of the permeate was collected to calculate the flux (J, L·m-2·h-1) as follows49: J = ΔV/(S×Δt) where ΔV (L) is the volume of the permeate collected over a time interval Δt (h), S (m2) is the effective membrane area. The ion rejection rate (R, %) was calculated by the following equation5, 49: R= (1 - Cp/Cf) /100% where Cp and Cf are the conductivity of the permeate and the feed, respectively, as measured by using a conductivity meter. The reported RO experimental results here were the average values achieved by at least three membrane samples fabricated at different time. The chlorine-resistance performance of RO membranes was evaluated by the normalized water flux and salt rejection as follows49:


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Jn = J/J0 Rn = R/R0 where J and R are the instantaneous flux and rejection rate measured at different times in the filtration process, respectively; while J0 and R0 are the initial flux and rejection rate, respectively. Membrane Surface Characterization: The membrane morphologies were imaged by scanning electron microscope (SEM, JSM-5610LV, Japan). Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Tensor 27, Bruker, Germany) and X-ray photoelectron spectroscopy (XPS, Shimadzu-KRATOS, Japan) was applied to characterize the surface groups. The grazing angle ATR-FTIR spectra were recorded with a resolution of 4 cm-1, an incidence angle of 80o, and 256 accumulating scans. A contact angle goniometer (Digidrop, GBX, Germany) equipped with video capture at room temperature was employed to quantify the surface hydrophilicity. Chlorine-resistance evaluation. A statically accelerating chlorination in our previous report was carried out at pH 4.0 to evaluate the chlorine resistance of the B-PA and Gly-PA membranes.49 Samples were soaked in an aqueous solution of NaClO (pH 4, active chlorine 500 ppm) at 25 °C for different time interval. Here, the ppm·h (chlorine concentration × soaking time) concept was employed for quantification of chlorination exposure intensity.40 After that, samples were thoroughly rinsed with DI water for 48 hrs before RO performance experiment. The whole accelerating chlorination was carried out in a concealed 1L brown Wheaton glass bottles with lined PTFE caps under a constant mix on a shaker at 25 °C. A cycling chlorination experiment was also designed for the evaluation of the regenerative ability of the chlorine resistant property. Firstly, membrane samples were equally treated with the above NaClO aqueous solution for 1 hr at 25


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°C, and then 48 hrs DI water rinse. After the RO test, the samples were taken off the RO cell and shaken in an alkaline sodium borate buffer (pH 10) for 2 hrs for chlorine-active regeneration. After the regeneration process, the samples were rinsed in DI water and RO tested again. This "chlorination-reduction" treatment was repeated for three cycles in the cycling evaluation. ACKNOWLEDGMENT The authors acknowledge financial support for this work from the National Natural Science Foundation of China (No. 51578485), and the National Basic Research Program of China (No. 2015CB655303); and the Research Fund for the Doctoral Program of Higher Education of China (No. 20130101110064), Zhejiang Provincial Collaborative Innovation Center Program 2011 (No. G1504126001900). chemical

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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

Tel. and Fax: +86-571-87952121.

Present Addresses † Hai Huang’s present address: The Institute of Seawater Desalination and Multipurpose Utilization, State Oceanic Administration, Tianjin 300192, P. R. China Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources 1. the National Natural Science Foundation of China (No. 51578485); 2. the National Basic Research Program of China (No. 2015CB655303); 3. the Research Fund for the Doctoral Program of Higher Education of China (No. 20130101110064), 4. Zhejiang Provincial Collaborative Innovation Center Program 2011 (No. G1504126001900).


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SYNOPSIS

A sustainable chlorine resistance of PA RO membranes with monitorable and regenerative sacrificial layer was developed. Glycylglycine is grafted on membrane surface as a regenerative chlorine sacrificial layer and then zeta-potential is introduced to conduct a timely regeneration operation for these chlorinated Gly. The mechanism of this process is also proposed.


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Chlorine-Resistant Polyamide Reverse Osmosis Membrane with

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