In Situ Surface Modification of Thin-Film Composite Polyamide

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In situ surface modification of thin-film composite polyamide membrane with zwitterions for the enhanced chlorine-resistance and transport properties Jing Wang, Si Zhang, Pengfei Wu, Wenxiong Shi, Zhi Wang, and Yunxia Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21572 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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In situ surface modification of thin-film composite polyamide membrane with zwitterions for the enhanced chlorine-resistance and transport properties

Jing Wang1,2, Si Zhang1, Pengfei Wu1, Wenxiong Shi1,3, Zhi Wang2*, Yunxia Hu1*

1. State Key Laboratory of Separation Membranes and Membrane Processes, National Center for International Research on Membrane Science and Technology, School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, P. R. China 2. Tianjin Key Laboratory of Membrane Science and Desalination Technology, State Key Laboratory of Chemical Engineering, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, P. R. China 3. Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798.

*Corresponding author: [email protected] ; Tel: +86-22-83955129 *Corresponding author: [email protected]; Tel: +86-22-27404533


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Abstract: High-performance chlorine-resistant thin film composite (TFC) membrane with zwitterions was fabricated by in-situ surface modification of polyamide with 2,6diaminopyridine and the subsequential quaternization with 3-bromopropionic. The successful modification of TFC polyamide surface with zwitterions was confirmed by various characterizations including surface chemistry, surface hydrophilicity and surface charge. The membrane transport properties were measured in both of the crossflow reverse osmosis (RO) and forward osmosis (FO) processes and the results found that the modified TFC membrane improved both of its water permeability and permselectivity with the increased A and A/B ratio upon modification with zwitterions. The chlorination challenging experiments were performed to demonstrate that the modified membrane enhanced its chlorine resistance without affecting its salt rejection upon 16000 ppm·h chlorination exposure. Chlorination mechanism study illustrated that the modified membrane with zwitterions could prevent the Orton rearrangement of benzene ring of the PA layer. Importantly and excitingly, the optimal chlorinated TFC membrane with zwitterions achieved a very high water flux of 72.15±2.55 LMH with 99.67±0.09% of salt rejection in the cross-flow RO process under 15 bar.

Keywords: Thin-film composite (TFC), polyamide, chlorine-resistance, perm-selectivity, zwitterions, reverse osmosis, forward osmosis




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1. INTRODUCTION Global water scarcity and water pollution is becoming increasingly serious, membranebased desalination technology offers a good option with low energy consumption to produce fresh and clean water from seawater and wastewater1-2. The thin-film composite (TFC) polyamide (PA) membrane, fabricated through interfacial polymerization of aromatic amine and aromatic acyl chloride on microporous support, is the state-of-the-art desalination membrane and widely used in the RO, NF and FO processes3-4. Aromatic polyamide selective layer determines the water permeability, solute rejection and also life-time of TFC desalination membranes5-6. However, the selectivity of polyamide drops rapidly from the attack of oxidizing agents, such as chlorine species, which is indispensably used as disinfectants in the water treatment but can break the amide bonds (-CO-NH-) of the polyamide resulting in the membrane performance deterioration7-8. Now, 10~30% cost of the current desalination process has to spend on the extra de-chlorination of water right before feeding into the TFC polyamide membrane and then the additional re-chlorination of water for disinfection9. Therefore, it is of great significance to develop chlorine-resistant TFC polyamide membranes for the economical and sustainable operation of desalination process. The polyamide of TFC membrane suffers from the chlorination of N-H bond into N-Cl first and then ring-chlorination via Orton rearrangement irreversibly under the desalination operation conditions with low concentration of chlorine in neutral pH10-11. Several strategies have been developed to improve the chlorine resistance of the TFC PA membrane, including the use of new monomers without primary amine groups or benzene ring for interfacial polymerization to prevent the formation of N-H groups or benzene ring in the TFC selective layer, and the coating of nanoparticles or protective layers to inhibit the ring-chlorination of polyamide via Orton rearrangement7. For example, monomers without benzene ring like 1,3-cylohexanebis(CHMA)12, Triethanolamine (TEOA)13 and Methyl-diethanolamine (MDEOA)14 and monomers with electron-with-drawing groups and high steric hindrance like m-phenylenediamine4-methyl (MMPD)15, N,N’-dimethyl-m-phenylenediamine (N,N-DMMPD)16 and



hexafluoro alcohol-m-phenylenediamine(HFA-MPD)17 were used to form the selective layers of TFC membranes and thus to effectively reduce chlorine attacks through the prevention of Orton rearrangement. However, such new monomers are difficult to synthesize and are expensive to use, and the selectivity of the fabricated membrane is generally much lower than that of the conventional TFC polyamide membranes. Recently, nanoparticles, such as graphene oxide (GO)18, carbon nanotubes (CNT)19-20 and layered double hydroxide21, have been added into the monomer solutions for the interfacial polymerization to improve the chlorine resistance of the TFC PA membranes, which may act as a steric barrier to protect the amidic sites from being attacked by chlorine. However, the stability and cost of the TFC PA membrane with nanoparticles still remains to be improved. The coating of a protective layer or a sacrificial layer onto the polyamide surface of TFC membrane has been developed as a facile and cost-effective approach for the enhancement of membrane chlorine resistance. The sacrificial or regenerative sacrificial layer with function groups would react with chlorine and finish it before attacking PA, and the protective layer would act as a barrier to stop chlorine contacting with PA9, 22-24. For example, the materials containing amine, amide or hydantoin groups like N-isopropylacrylamide (NIPAm)25, glycylglycine (Gly)24 and 3-allyl-5,5dimethylhydantoin (ADMH)22 can be coated on the PA and act as sacrificial or regenerative sacrificial layers to consume chorine. Moreover, the chlorine-resistant polymers like poly(vinyl alcohol)26, poly(4-vinylpyridine) (P4VP)27 and PDMAEMA28 have been used to shield the active sites of polyamide and thus to improve the chlorineresistance of TFC PA membrane. However, the sacrificial or regenerative sacrificial layer faces the chlorination saturation problem, and the protective layer does not coat all the PA active sites without sacrificing the membrane perm-selectivity. Furthermore, both of the sacrificial and protective layers have been introduced onto the PA membrane surface to form a durable chlorine resistant layer with their combined advantages of preventing the attack of chlorine and increasing the chlorine tolerance level29-30. However, the coating of these protective and sacrificial layers generally increases membrane transport resistance and sacrifices membrane perm-selectivity. It


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is still extremely challenging to improve the chlorine resistance property of TFC PA membrane without sacrificing its perm-selectivity. Fabrication of chlorine-resistant TFC polyamide membrane with superior transport properties is very important through the selection of cost-effective compounds and facile approaches for the polyamide modification. 2,6-diaminopyridine (DAP) is a cheap and common small molecule as a pyridine derivative containing amine groups and pyridine ring, which had been used for an antioxidant or antimicrobial agent, a nitrogen-rich oxygen reduction electrocatalyst, and soluble and thermally stable polyamide material31-33. DAP is very reactive with acyl chloride and can be grafted on nascent polyamide by our reported layer-by-layer interfacial polymerization (LBL-IP) method34. Importantly, the unreacted amine groups of DAP may act as sacrificial groups to react with chlorine, and the pyridine ring of DAP can be easily quaternized to form zwitterions having the steric and electronic-drawing effects of pyridine and quaternary ammonium groups on the polyamide layer for the prevention of Orton rearrangement. Therefore, DAP has a great potential to be grafted onto the PA layer for the improved chlorine-resistance without sacrificing the membrane perm-selectivity since it is a small reactive hydrophilic molecule. Herein, 2,6-diaminopyridine was used to in-situ modify the polyamide selective layer of TFC membrane for the improved chlorine-resistance via layer-by-layer interfacial polymerization (LBL-IP), followed by its quaternization with 3bromopropionic acid (3-BPA). In our previously work35, we had found that the carbon nanotube (CNT) interlayer would improve the permeability and selectivity of TFC membrane effectively, therefore, all the TFC membranes were fabricated on the porous support substrates containing CNT interlayer and porous PES membrane. As presented in Fig. 1, the polyamide selective layer was formed on the porous support substrate through the interfacial polymerization of m-phenylenediamine (MPD) aqueous solution and trimesoyl chloride (TMC) hexane solution, 2, 6-DAP was then introduced to react with acyl chloride groups from the nascent polyamide of TFC membrane, and then 3bromopropionic acid was used to quaternize the amine groups of modified polyamide for the formation of zwitterions. The impacts of such modification with 2, 6-DAP and



3-BPA on the polyamide surface properties including hydrophilicity, roughness and zeta potential was investigated. Water flux and reverse salt ﬂux or salt rejection of the modified membranes were measured in the reverse osmosis (RO) and forward osmosis (FO) process to determine the membrane perm-selectivity before and after modification. Chlorination challenging experiments were also performed to investigate the chlorine resistance and transport properties of modified TFC membranes.

Figure 1. Schematic illustration of the polyamide surface modification of TFC membrane by grafting of 2,6-DAP and 3-BPA.

2. MATERIALS AND METHODS 2.1 Materials Polyethersulfone (PES) microfiltration membrane (pore size: 0.22 μm) was purchased from YiBo Co. Ltd. (China). Single-walled carbon nanotubes (SWCNTs, diameter: < 2 nm, length: 5-30 μm, purity: > 95%), m-phenylenediamine (MPD, 99%) and 3bromopropionic acid (3-BPA, 98%) were obtained from Aladdin Chemical Co. Ltd. (China). 1,3,5-benzenetricarbonyl trichloride (TMC, 99%) and 2,6-diaminopyridine (2,6-DAP, 99%) were supplied by J&K Scientific Co. Ltd. (China). Dopamine hydrochloride (DOPA, 98%) and sodium hypochlorite solution (NaClO, 14.5%) was


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received from Energy Chemical Co. Ltd. (China). Tris was purchased from Labest Co. Ltd. (China). Other chemicals used in this work were all supplied by Kermel Chemical Reagent Co. Ltd. (China). All chemicals were used as received. 2.2 Fabrication of the QDAP modified TFC polyamide membranes Aromatic polyamide TFC membranes were fabricated via interfacial polymerization (IP) on the porous PES membrane support with a CNT interlayer, which was prepared following the references35-36. 6 mL of the PDA-coated CNT suspension (0.183 mg/mL) was deposited on the PES membrane surface through vacuum-filtration to prepare the PES-CNT membrane. After dried the PES-CNT membrane at 60℃ for 0.5 h, it was prewetted using DI water for 0.5 h. MPD aqueous solution (3.4 wt.%), TMC organic solution in n-hexane (0.15 wt.%), and 2, 6-DAP aqueous solution (0.2, 0.6 and 1.0 wt.%) were sequentially introduced to the surface of CNT interlayer of the PES-CNT membrane for 1 minute, followed by the removal of excess solution before switching to another solution. Then, the fabricated membrane was cured in an air oven at 80°C for 2 min to obtain the 2, 6-DAP modified TFC polyamide membrane. After thorough rinse with DI water, the 2, 6-DAP modified membrane was immersed in 50 mL 3-BPA solution (1.5 wt.%) for 4 hours at 30°C to quaternize the tertiary amino groups of 2, 6DAP, and the final quaternary pyridine modified membrane was obtained. The pH of all the solutions were in the range of 7.0±0.5 as naturally prepared without adjusting. The resulting membrane was named as the QDAP-0.2, QDAP-0.6 and QDAP-1.0 membranes, respectively, indicating the corresponding concentration of 2, 6-DAP solution. Pristine TFC polyamide membranes (labeled as PA membrane in the following Figures) were fabricated as controls through interfacial polymerization of MPD and TMC on the PES-CNT support membranes following the above procedure. As we had achieved the improved perm-selectivity of FO and NF TFC membranes by using the PES-CNT support membranes in our previous works35, 37, we found that the key role of PES-CNT support was to provide a favorable interface to form highly permeable and selective polyamide with thin thickness and no defects through interfacial



polymerization. and increase the perm-selectivity of the TFC membrane. Therefore, in this work, all the pristine and modified membranes were fabricated on the PES-CNT support membrane. 2.3 Membrane Characterizations Membrane surface chemical characterizations were characterized by ATR–FTIR spectrometer (Nicolet Is50, Thermo Fisher Technology, USA) and XPS (K-alpha, Thermo Fisher Technology, USA). The contact angle goniometer (DSA25S, Kruss, Germany) and analysis software (Advance) were used to test the static contact angles of membranes. The zeta potential of the membrane surface was measured by SurPASS solid surface Zeta potential analyzer (Anton Paar GmbH, Austria) at pH 7.0 ± 0.3 and 25 °C. The surface morphologies and roughness of the membranes were observed by scanning electron microscope (SEM) (Gemini SEM500, Zeiss, Japan) and atomic force microscopy (AFM) (Bruker Optics, Germany). All of the membrane samples were completely dried under vacuum at 40 °C for 4 hours before characterizations and coated with gold before imaging by the SEM. 2.4 Transport performance in the FO process The water flux and reverse salt flux of the TFC polyamide membranes were tested using a custom-designed cross-flow forward osmosis (FO) system, as described in our previous work35. The effective surface area of the membrane cell was 8.0 cm2 (2 cm х 4 cm). The feed solution and the draw solution were deionized water and 1.0 M NaCl solution, respectively. The membranes were evaluated under the AL-FS (active layer facing feed solution) operational mode, 25.0 ± 0.5 °C and 0.5 L/min cross-flow rates. The structural parameter (S) of TFC polyamide membrane was measured following the reported protocol38 and using 0.5, 1.0 and 1.5 M NaCl solutions as draw solutions in the FO system, respectively. 2.5 Transport performance in the RO process A lab-scale reverse osmosis (RO) system was applied to measure the transport


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performance of TFC polyamide membranes, i.e., water permeability coefficient (A), salt permeability coefficient (B) and salt rejections (R). A, B and transfer coefficient kf are calculated as the following Eqs. (1) ~ (3)39. 𝐽𝑅𝑂 𝑤

(1)

𝐴 = ∆𝑃

𝐵 = 𝐽𝑅𝑂 𝑤(𝑁𝑎𝐶𝑙)( 𝑘𝑓 =

𝐽𝑅𝑂 𝑤(𝑁𝑎𝐶𝑙)

1 ― 𝑅0

𝐽𝑅𝑂 𝑤(𝑁𝑎𝐶𝑙)

𝑅0

𝑘𝑓

[

𝑙𝑛

)exp ( ―

∆𝑃 𝜋𝑏 ― 𝜋𝑝

(

× 1―

(2)

)

𝐽𝑅𝑂 𝑤(𝑁𝑎𝐶𝑙) 𝐽𝑅𝑂 𝑤

)]

(3)

𝑅𝑂 Where 𝐽𝑅𝑂 𝑤 and 𝐽𝑤(𝑁𝑎𝐶𝑙) are the permeate flux of pure water and 2000 ppm NaCl

solution, respectively; △P is the applied hydraulic pressure; R0 is the NaCl rejection calculated by the NaCl concentrations in the feed and permeate solution; and πb and πp are the osmotic pressures of the feed and permeate solutions, respectively. The effective surface area of the membrane cell was 28.26 cm2. After the system stabilization with deionized water for 30 min, the test was conducted under 15 bar, 25 ± 0.5 °C and 0.5 L/min cross-flow rates. The water permeability coefficient of TFC polyamide membrane was measured using deionized water as a feed solution, while, its salt permeability coefficient and salt rejection were measured using a 2000 ppm NaCl aqueous solution as a feed solution. 2.6 Evaluation of membrane chlorine resistance Membrane chlorination challenging experiments were implemented by soaking the membranes in 2000 ppm chlorine solution (pH 7.0 ± 0.3) for 2, 4, 6 and 8 hours at room temperature, then thoroughly rinsed with deionized water and stored in deionized water for 48 hours before further characterization. The membrane surface functional groups of the chlorinated membranes were characterized by ATR-FTIR and AFM to explain the membrane chlorine resistance mechanism. The water flux and salt rejection of chlorinated membranes were also measured in the FO and RO processes following the above-mentioned protocols.

3. RESULTS AND DISCUSSION ACS Paragon Plus Environment


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3.1 Polyamide Surface Properties Before and After Modification The ATR-FTIR and XPS analysis were used to verify the successful surface modification of TFC polyamide membranes, and results shown in Figure 2(a) present that the modified PA has a similar IR spectrum as the pristine membrane except a new peak at 1729 cm-1 from the C=O stretching of carboxylic acid groups connecting with alkane40, which illustrates the successfully grafting of 3-BPA on the polyamide surface. As the adsorption bands of pyridine in 2,6-DAP are similar to those of benzene in PA membrane, the 2,6-DAP modified membrane (DAP-0.6) presents nearly the same IR spectrum with pristine PA membrane. The water contact angle of the modified PA surface decreased significantly compared with the pristine PA, indicating the enhanced surface hydrophilicity of modified PA upon the grafting of 2, 6-DAP and 3-BPA41. While, the higher concentration of 2, 6-DAP used for surface modification and the higher water contact angle of modified PA surface. Additionally, the surface zeta potential of modified PA became positive and increased with the concentration of 2, 6DAP due to the increase of positive charged amine groups from 2, 6-DAP. Obviously, the modified PA with 2, 6-DAP and 3-BPA was weak charged due to the formation of zwitterions. As a control, the pristine PA was negative charged due to free carboxylic acid groups from hydrolysis of acyl chloride groups after interfacial polymerization. The elemental compositions were determined by XPS and shown in Table 1. The elemental ratios of N/C and N/O were increased obviously after grafting 2,6-DAP and also increased with the concentration of 2,6-DAP. What’s more, as 3-BPA contains a higher content of O than DAP-modified membrane, all QDAP-modified membranes had a lower N/O ratio than DAP-modified membranes when using the same 2,6-DAP modification concentration. We also calculated the cross-linking degree of the PA membrane and the grafting ratios of 2,6-DAP and 3-BPA based on the following Eqs. (4) ~ (6)42. 𝑁 𝐶 𝑁 𝐶 𝑁 𝐶

=

3𝑛 + 2(1 ― 𝑛) 18𝑛 + 15(1 ― 𝑛)

(4)

3𝑛 + 2(1 ― 𝑛) + 3(1 ― 𝑛)𝑥

(5)

= 18𝑛 + 15(1 ― 𝑛) + 5(1 ― 𝑛)𝑥 3𝑛 + 2(1 ― 𝑛) + 3(1 ― 𝑛)𝑥

= 18𝑛 + 15(1 ― 𝑛) + 5(1 ― 𝑛)𝑥 + (1 ― 𝑛)𝑥 × 3𝑦


(6)

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Where n is the cross-linking degree of the PA membrane; (1-n) is the ratio of the residual acyl chloride group; x is the grafting ratio of 2,6-DAP, and y is the grafting ratio of 3-BPA. Results find that 58.57% of the residual acyl chloride groups had been reacted with 0.6 wt.% 2,6-DAP solution, and 53.83% of 2,6-DAP were quaternized by 3-BPA to obtain the QDAP-0.6 membrane. Therefore, all results present that 2,6-DAP and 3-BPA had been successfully grafted on the polyamide membrane surface. Table 1 The elemental compositions of pristine and modified TFC membranes measured by XPS. PA DAP-0.2 DAP-0.6 DAP-1.0 QDAP-0.2 QDAP-0.6 QDAP-1.0 2,6-DAP 3-BPA

C

N

O

N/C

N/O

73.18 72.61 72.08 71.92 72.30 71.63 71.44 62.50 60.00

11.03 12.45 13.72 14.17 12.15 13.23 13.48 37.50 0.00

15.79 14.94 14.20 13.91 15.55 15.14 15.08 0.00 40.00

0.15 0.17 0.19 0.20 0.17 0.18 0.19 0.60 0.00

0.70 0.83 0.96 1.02 0.78. 0.87 0.89 0.00 0.00

Surface morphology of the modified PA was observed using SEM to see characteristic ridge-and-valley and leaf-like structures with large leave size, similar as the pristine PA (Figure 3). The surface roughness (Rq) of the modified PA decreased notably in comparison with the pristine PA (Rq:142.3±4.7 nm), and decreased slightly with the concentration of 2, 6-DAP. Therefore, the QDAP modified PA surface presented the decreased roughness and the enhanced hydrophilicity with positive charge at neutral pH upon the grafting of 2, 6-DAP and 3-BPA.



Figure 2. Polyamide surface properties of the pristine and modified TFC membranes including (a) ATR-FTIR spectra (including pristine PA, DAP modified PA (DAP-0.6) and QDAP modified PA(QDAP-0.6)), (b) water contact angles, and (c) zeta potentials at neutral pH. The TFC polyamide membranes modified with different concentrations of 2, 6-DAP solutions (0.2, 0.6 and 1.0 wt.%) and then quaternized with 3-BPA (1.5 wt.%) were labeled as QDAP-0.2, QDAP-0.6 and QDAP1.0, respectively, indicating the corresponding concentration of 2, 6-DAP solution used for modification.


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Figure 3. SEM (a, b, c, d) and AFM (e, f, g, h) images of the PA surfaces from the pristine membrane (a, e), and modified membranes with QDAP-0.2 (b, f), QDAP-0.6 (c, g) and QDAP-1.0 (d, h).



3.2 Membrane Transport and Structural Properties Transport properties of the pristine and QDAP modified TFC PA membranes were tested in both of the FO process and RO process. Results shown in Figure 4 illustrate that the QDAP modified membranes exhibited the increased water flux with comparable low reverse salt flux less than 5 gm-2h-1 as the pristine PA membrane in the FO process. The QDAP modified membranes enhanced its water flux further with the concentration of 2,6-DAP solutions used for modification and reached to 25.09±1.81 LMH when 0.6 wt.% 2,6-DAP was used, when tested in the AL-FS mode of FO process using the 1M NaCl draw solution and the DI water feed solution. It can conclude that the QDAP modification process increased the membrane water flux but did not affect its salt rejection, which may be explained by the incorporated hydrophilicity of QDAP into the polyamide layer and the facilitated water transport across the hydrophilic membrane43. The grafting of QDAP had increased the hydrophilicity and roughness of the membrane, which would help to increase the water permeability of TFC membrane. What’s more, the LPL-IP process was limited by the amounts of the residual acyl chloride group on the nascent PA membrane, therefore the mass transfer resistance of the membrane would increase little with the thin grafting of QDAP-layer. Therefore, the water flux of QDAP-0.6 membrane was increased a little by the combination of the above two factors. However, the QDAP modified membranes did not further improve its water flux with the increase of 2,6-DAP concentration to 1.0 wt.%, which may due to the thick grafting layer of QDAP and thus the increased mass transfer resistance.

Figure 4. Transport properties of the pristine and QDAP-modified TFC PA membranes in the FO


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process including water flux and reverse salt flux (a) and in the RO process including water permeability coefficient A, salt permeability coefficient B and salt rejection R (b). FO operation conditions: 1M NaCl draw solution, DI water feed solution, the AL-FS mode, 25.0 ± 0.5 °C and 0.5 L/min flow rate. RO operation conditions: 15 bar operating pressure and the 2000 ppm NaCl feed solution kept at 25.0 ± 0.5 °C with the 2.0 L/min flow rate.

We thus chose the QDAP-0.6 membrane as the optimal modified membrane and measured its intrinsic transport properties to investigate the effect of QDAP grafting on the membrane performance. A cross-flow RO system was employed to evaluate the water flux and salt rejection of the pristine PA and QDAP modified membranes. When tested under 15 bar, the water flux of the QDAP modified TFC membranes was 31.50±1.20 LMH, and increased by 52% compared with the pristine PA without modification. The salt rejection of the QDAP modified TFC membrane (99.23±0.08%) was even slightly higher than that of the pristine TFC membrane (99.03±0.19%). The intrinsic water permeability coefficient (A) of the QDAP modified TFC membranes was increased significantly to 2.10±0.08 LMH/bar compared with 1.38±0.18 LMH/bar of the pristine membrane, and its salt permeability coefficient (B) was 0.17±0.02 LMH with comparable value as the pristine membrane. The pH of the feed solutions had little impact on the modified TFC membrane performance at the tested range of pH 5~9. It can conclude that the QDAP modification increased the membrane permeability without affecting its selectivity. The structure parameter (S) value of the pristine membrane (322±39 μm) and the QDAP modified membrane (332±53 μm) was measured in the FO process to find no obvious difference, which may due to that the S is mainly affected by the supporting layer rather than the selective layer of TFC membrane44. In conclusion, the QDAP modified TFC membrane obtained the higher value of A and also the higher ratio of A/B than the pristine membrane. This in-situ modification process with quaternized 2,6-DAP can increase the perm-selectivity of TFC PA membrane effectively.



3.3 Membrane Chlorine Resistant Properties A chlorination challenging experiment was conducted to investigate the chlorine resistance of the QDAP modified TFC polyamide membrane. As literature identifies that the degradation effect of TFC polyamide membrane depends on the pH and concentration of chlorine solution45. What’s more, the short-time exposure to high concentrations of chlorine would cause more serious problems to polyamide membranes than long-time exposure to low concentrations even under the similar total exposure intensity46. We thus run the chlorination challenging experiment in an extreme severe condition and investigated the membrane performance after 2~8 hours exposure to a high concentration of chlorine (2000 ppm NaClO solution, pH 7.0±0.3). Figure 5 compares the water flux and reverse salt flux of the pristine and QDAP modified membranes, and illustrates that all chlorinated membranes remarkably increased their water flux as a comparison with their control samples before chlorination. With the chlorination exposure intensity increasing, the TFC PA membrane without modification increase its reverse salt flux dramatically up and deteriorated its selectivity. In contrast, the QDAP modified membrane has not changed its reverse salt flux upon 12,000 ppm · h chlorine exposure, and slightly increased its reverse salt flux to 7.73±2.21 gm-2h-1 after 16,000 ppm·h chlorine exposure. These results demonstrate the improved chlorine resistance of the QDAP modified membrane and its chlorine tolerance level goes up to 16,000 ppm·h, which is higher than 2,000~12,000 ppm·h of the reported chlorine-resistant membranes from literatures19, 24, 27, 47. The improvement of membrane chlorine resistance may account for the sacrificial effect of amine groups in the grafted QDAP layer and the protective effect of pyridine groups and quaternary amine groups to prevent the Orton rearrangement of benzene ring of polyamide, which is further discussed in the following study.


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Figure 5. Chlorine resistance of the pristine and QDAP-modified TFC polyamide membranes including (a) water flux and (b) reverse salt flux in the FO process after exposure to 2,000 ppm NaClO for 2~8 hours. The optimal QDAP-modified TFC polyamide (QDAP-0.6) was selected for the chlorine challenging experiment. Operation conditions: 1 M NaCl draw solution, DI water feed solution, the AL-FS mode, 25.0 ± 0.5 °C and 0.5 L/min flow rate.

The above results also confirm that the chlorinated QDAP membrane can increase its water flux without affecting its reverse salt flux in the FO process, suggesting the chlorination process may be used as a post-treatment to further optimize the performance of the QDAP modified membrane. Therefore, we treated the QDAP modified membranes (QDAP-0.6) in 2,000 ppm chlorine solution for 6 hours and then measured their water flux and salt rejection in the RO system under 15 bar. As shown in Figure 6(a), the water fluxes of the chlorinated QDAP membranes were 72.15±2.55 LMH, and increased by 248% compared with the control membranes without the posttreatment of chlorination. The salt rejection of the chlorinated QDAP membranes (99.67±0.09%) was even slightly higher than that of the control membranes (99.03±0.19%). The reasons may be that the chlorination process can disrupt the intermolecular hydrogen bonds of polyamide and the free-volume of polyamide network is thus increased with the enhanced rotational freedom and flexibility of the polyamide, and the membrane water flux is thus improved as more water channels or voids are available7. Generally, most of the membranes improved their water flux but



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along with the decreased salt rejection upon chlorine treatment due to the degradation of PA. But for the QDAP modified membrane, the QDAP-grafted PA enhanced its water flux with the improved salt rejection because of the protection of PA from the grafted QDAP. The optimal chlorinated TFC membrane with zwitterions achieved a very high perm-selectivity (A, 4.81±0.17 LMH/bar, B, 0.22±0.06 LMH and 99.67±0.09% salt rejection), which is compared with various commercial and lab-made FO or membranes and shown in Table 3. The A/B ratio is a critical parameter to evaluate the permselectivity of TFC PA membranes, and the higher A/B ratio means higher permeable and selective membrane. The A/B ratio of the chlorinated QDAP membrane was measured in the RO process to be 21.86 bar-1 (A, 4.81±0.17 LMH/bar and B, 0.22±0.06 LMH). This value is significantly higher than the reported values (2.32-13.75 bar-1) in literature (Figure 6(b))48-49, proving that our optimal QDAP membranes exhibited superior perm-selectivity with a remarkably improved chlorine resistance.

Table 3. Performance of various commercial and lab-made FO or RO membranes. Membrane

Testing

Water flux or

Salt

Rejection, %

Ref.

names

Conditions

permeability,

permeabilit

LMH/bar

y, LHM

2.90

-

99.5

50

2.82~2.88

-

99.7

50

3.26

-

99.2

50

NaCl,

0.97±0.03

1.16±0.11

-

51

NaCl,

3.44±0.14

0.47±0.03

-

51

P(ADMH-co-

2000 ppm NaCl,

3.47±0.17

-

99.2±0.1

52

VAm)-PA

15.5 bar

TS-II

2000 ppm NaCl,

23.3±2.29

-

49.6±2.0

53

BW30a

2000 ppm NaCl, 15.5 bar

AGa


RE4021-TLa


HTI-CTAb

50

mM

27.6 bar Oasys

TFCb

50

mM

27.6 bar

4.8 bar


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sPA-1800

1000 ppm NaCl,

2.85±0.14

0.59±0.05

88.35±1.35

54

1.93±0.14

0.23±0.03

97.4±0.1

55

1.83

0.14

-

48

2.10±0.08

0.17±0.02

99.23±0.08

This

2 bar TFC-Psf-6

2000 ppm NaCl, 6 bar

MPD-DA/TMC

500 ppm NaCl, 2 bar

QDAP-0.6

2000 ppm NaCl, 15 bar

Chlorinated

2000 ppm NaCl,

QDAP-0.6

15 bar

a Commercial b

work 4.81±0.17

0.22±0.06

99.67±0.09

This work

RO membrane

Commercial FO membrane

Figure 6. Water permeability coefficient A, salt permeability coefficient B and salt rejection R of the chlorinated QDAP membranes (a), and the A/B ratio (A/B) vs water permeability coefficient A of TFC PA membranes fabricated in this work and also in literature (b). The chlorinated QDAP membrane were prepared by immersing the DAP-0.6 membrane in 2,000 ppm chlorine solution for 6 hours, then thoroughly rinsed with DI water and stored in DI water for 48 hours.

3.4 Membrane Chlorine Resistance Mechanism The polyamide of TFC membrane is attacked from chlorine through these two major ways including reversible N-chlorination and irreversible ring-chlorination. The Nchlorination process will not affect membrane perm-selectivity seriously, while, the ring-chlorination of polyamide will cause serious polymer degradation and membrane



selectivity deterioation.9 ATR-FTIR was used to examine the surface chemistry groups of PA before and after chlorination, and thus to understand the chlorine resistance mechanism of the QDAP modified PA. As shown in Figure 7 (a), the marked peaks at 1442, 1540 and 1660 cm-1 are from aromatic C-H bending vibration, N-H in-plane bending and C=O stretching 10, respectively. The N-H and aromatic C-H groups of PA without modification nearly disappeared upon chlorination, but the N-H and aromatic C-H groups still existed for the QDAP modified membrane after chlorination. The disappearance of 1442 cm-1 peak accounted for the chlorination of aromatic ring, while the disappearance of 1540 cm-1 peak was due to the conversion of the N-H groups to N-Cl groups10. Additionally, the peak of C=O stretching group was shifted from 1660 to 1668 cm-1 for the chlorinated PA membrane (PA-6h), indicating that the inter-chain hydrogen bonds were attacked and weakened seriously by chlorine10, 22. Therefore, the ATR-FTIR results confirmed that the aromatic ring of the QDAP modified PA did not suffer from Orton rearrangement and still had many N-H groups. Furthermore, the polyamide of TFC membrane may change their surface roughness and surface area upon chlorination because chlorine can break the intermolecular hydrogen bonds of the polyamide and thus increase the rotational freedom and flexibility of polyamide chains. AFM was then employed to observe the change of surface roughness and surface area of PA before and after chlorination. Figure 7 (b) and (c) show that the roughness of the chlorinated pristine PA membrane decreased seriously from 142.3±4.7 nm to 122.3±3.7 nm, while that of the chlorinated QDAP membrane (110.2±3.8 nm) were nearly same as the un-chlorinated membrane. Noticeably, the specific surface area ratio of the PA membrane was decreased from 2.07±0.03 to 1.97±0.05, and that of chlorinated and QDAP modified membrane was changed only from 1.83±0.05 to 1.85±0.04. The change of surface roughness and surface area of the PA membrane may account for the increased movement of the PA chains without intermolecular hydrogen bonds. Therefore, the results also confirmed that the pristine PA chains went through Orton rearrangement. In contrast, the QDAP modified PA did not change obviously before and after chlorination, suggesting the QDAP modified PA did not go through Orton rearrangement but achieved chlorine


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resistance.

Figure 7. ATR-FTIR spectra of the pristine and QDAP-modified TFC polyamide membranes before and after chlorination at 2,000 ppm chlorine solution for 6 hours (a), and AFM images of pristine PA (b) and QDAP-modified TFC polyamide membrane (QDAP-0.6, c) surfaces after chlorination at 2,000 ppm chlorine solution for 6 hours.

We also used the density function theory (DFT) calculation to identify the energy difference of three possible attacking sites in the QDAP modified PA (shown in Fig.8) and to analyze the chlorine resistance mechanism of QDAP modified PA membrane. The DFT calculation was performed with Becke three-parameter exchange and Lee−Yang−Parr correlation (B3LYP) functional using Gaussian 09 code.56-58 The 6311G++ basis set was applied for the single-point calculations.59-60 The energy



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difference ∆E between the production and the reactant was obtained by the eq. (7), ∆E = 𝐸PA ― QDAP ― CL + 𝐸H ― 𝐸PA ― QDAP ― 𝐸CL

(7)

Where EPA-QDAP-CL and EH denote the energy of the productions; EPA-QDAP and ECl denotes the energy of the reactants. As shown in Figure 8, the △E of three possible attacking sites a, b and c in the QDAP modified PA membrane is -27.07, -26.98 and 24.03 eV, respectively. Therefore, the site a and b with the lower △E are more reactive with chlorine than the site c, and thus, the QDAP would protect site c in the polyamide layer from chlorine attack.

Figure 8. Possible chlorine attacking sites in QDAP-modified membrane surface.

Based on the above results, we proposed the chlorine resistance mechanism of the QDAP modified membrane as the sacrificial and protective role of QDAP (Figure 9). The in-situ interfacial polymerization was performed on the polyamide surface to graft the QDAP layer, and the primary and secondary amine groups in the QDAP layer can react with chlorine as sacrificial groups to protect the polyamide layer from chlorine


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attack. Moreover, the pyridine groups and quaternary amine groups in the QDAP layer are both electron-withdrawing groups, and the quaternary pyridine has the steric hindrance effect. Both the electronic and steric factors of QDAP layer can prevent the Orton rearrangement of polyamide and thus increase the chlorine resistance of the polyamide17, 27.

Figure 9. Chlorination mechanism schematic of the pristine and QDAP-modified polyamide membranes. (The pink area is the chlorinated part and the grey area is the protected part of the PA layer with or without modification)

4. CONCLUSIONS 2,6-DAP was used with the additional quaternization by 3-BPA to in-situ modify the polyamide selective layer of TFC membrane with zwitterions for the improved chlorine resistance and the enhanced transport properties. Quaternized 2,6-DAP (QDAP) grafted on the polyamide surface dramatically improved the membrane surface hydrophilicity



and turned the membrane surface charge into positive at neutral pH, and slightly decreased its surface roughness. The transport properties of the QDAP modified TFC membrane were measured in both of the cross-flow RO and FO processes to exhibit the increased water permeability without affecting its salt permeability. The chlorination challenging experiment was performed to demonstrate that the QDAP modified membrane exhibited the improved chlorine resistance without affecting its salt rejection upon 16,000 ppm·h chlorination exposure intensity (pH 7.0±0.3). Our results found that the QDAP had formed a sacrificial and protective layer to protect the PA layer from chlorine attack and to prevent the Orton rearrangement of benzene ring of the PA layer, and thus improved the PA chlorine resistance. Importantly, chlorination was used as a post-treatment for the QDAP modified membrane to further improve its permselectivity significantly. The optimal chlorinated QDAP membrane achieved a superior perm-selectivity with a very high A/B ratio of 21.86 bar-1, much higher than the reported highest value of 13.75 bar-1 for the TFC polyamide membranes. Excitingly, the optimized TFC PA membrane increased its water flux to 72.15±2.55 LMH with 99.67±0.09% of salt rejection in the cross-flow RO process under 15 bar. Therefore, our work provides cost-effective materials of 2,6-DAP and 3-BPA, and a facile LBLIP surface modification method to fabricate high-performance TFC PA membranes with excellent chlorine resistance and superior perm-selectivity, which has a great potential to scale up for real applications.

AUTHOR INFORMATION All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS This work was supported by Postdoctoral Science Foundation of China (2017M621080), National Natural Science Foundation of China (No. 21476249, No. 51708408), Chang-jiang Scholars and Innovative Research Team in the University of Ministry of Education, China (No. IRT-17R80), Program for Innovative Research Team in University of Tianjin (No. TD13-5044) and the Science and Technology Plans


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of Tianjin (No. 17PTSYJC00060).

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