Synthesis of AgCl Mineralized Thin Film Composite Polyamide

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Synthesis of AgCl mineralized thin film composite polyamide membranes to enhance performance and antifouling properties in forward osmosis Haiyang Jin, Frederick Rivers, Huidan Yin, Tianmiao Lai, Pinar Cay Durgun, Afsaneh Khosravi, Mary Laura Lind, and Ping Yu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04287 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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Industrial & Engineering Chemistry Research

Synthesis of AgCl mineralized thin film composite polyamide membranes to enhance performance and antifouling properties in forward osmosis Haiyang Jina,b, Frederick Riversb, Huidan Yinb, Tianmiao Laib, Pinar Cay Durgunb,c, Afsaneh Khosravib , Mary Laura Lindb,c,* and Ping Yua,* a

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei, 430072 People's Republic of China

b

School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ, 85287 USA

c

Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment, Arizona State University, Tempe, Arizona, United States *corresponding authors: [email protected]; [email protected].

Key words: forward osmosis; thin film composite membrane; surface modification; silver chloride Abstract This is the first report of using an alternate soaking process (ASP) to mineralize the surfaces of thin film composite (TFC) polyamide membranes with silver chloride (AgCl) for forward osmosis (FO). Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) analysis confirmed even distribution of AgCl particles on the top of the membrane surfaces. Surface roughness, contact angle, and zeta potential measurements show that the AgCl mineralized membranes have smoother, more hydrophilic, and more negatively charged surfaces than un-modified membranes. Under FO operation (with a deionized water feed and 1M NaCl draw) we found that the mineralized membranes exhibit higher salt rejection and water flux than the original membranes. Fouling experiments with bovine serum albumin (BSA) show that the mineralized membranes have lower water flux decline ratios in BSA aqueous solution and higher water flux recovery ratios after simple hydraulic washing than un-modified TFC membranes. 1. Introduction Recently the emerging membrane technology forward osmosis (FO) has attracted increasing attention 1, 2. FO membranes are semipermeable and separate a concentrated draw solution from a more dilute feed solution. The osmotic pressure difference between the concentrated draw solution and the dilute feed solution results in water permeation across the membrane. Unlike conventional pressure driven processes (e.g., ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO)), FO requires no applied hydraulic pressure and has lower energy consumption 3, lower membrane fouling 4, higher water recovery 5 and easier membrane cleaning 6. Currently, FO is applied in many fields including wastewater treatment 4, 7, seawater and brackish-water

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desalination 1, 2, food and pharmaceuticals processing 8, controlling drug release 9 and electrical power generation 10. However, some of the critical challenges facing FO are membrane fouling 11, concentration polarization 12, reverse solute diffusion 13, improved membrane development 14 and improved draw solute design 15. FO membranes with improved properties are the key to solve the above challenges. High rejection of both feed and draw solutes, high water flux, robust mechanical strength, and good resistance to chemicals and fouling are the characteristics of ideal FO membranes 1, 2. Increasing number of studies focus on the development of FO membranes and different types of membranes and various preparation methods have emerged. Chung’s group 16 used dry-jet wet phase inversion of polybenzimidazole (PBI) to make nanofiltration hollow fiber membranes and they modified the membrane to improve the salt rejection through cross-linking with p-xylylene dichloride 17. By supplementing the casting solution with polyvinylpyrrolidone (PVP) and polyethersulfone (PES) Chung’s group further improved the FO performance of the membrane 18. Emadzadeh et al. used a direct blending method to incorporate commercial TiO2 nanoparticles into the microporous substrate of composite FO membranes and obtained a much higher water flux than TFC membranes with identical test conditions 19. Setiawan et al. fabricated a nanofiltration-like active layer with positive charge and coated it onto a woven fabric support layer by polyelectrolyte posttreatment to reduce inner concentration polarization (ICP) 20. Surface properties are critical factors for FO membranes in performance and fouling behavior. Researchers have carried out various methods of surface modification to obtain FO membranes with improved performance. Saraf et al. enhanced performance of RO membranes in FO by coating poly (vinyl) alcohol onto the support layer of reverse osmosis membranes to increase the hydrophilicity of the support layer and reduce ICP 21. Elimelech et al. used PEG to modify TFC polyamide membranes and illustrated that the membrane performance could be completely recovered through flushing after being fouling by organics 22. Tiraferri et al. optimized the surface properties of FO polyamide membranes by functionalization with fine-tuned nanoparticles 23. By attaching silver nanoparticle-decorated graphene oxide nanosheets to the feed surface of TFC polyamide membranes Adel et al. improved the antibacterial properties and hydrophilicity of the membranes 24. Zhou et al. enhanced the water flux and fouling resistance of commercial thin-film composite polyamide RO membrane through surface mineralization by barium sulfate 25. Zhi et al. fabricated organic–inorganic composite membranes through surface mineralization of calcium carbonate (CaCO3) to improve water flux and Congo red rejection 26. In this work, we prepared thin film composite (TFC) polyamide FO membranes via interfacial polymerization and then we conducted surface mineralization modification by depositing silver chloride (AgCl) particles on the TFC membrane surfaces. This is the first report of AgCl surface mineralization for FO membranes. We varied the extent of mineralization on the membranes. We investigated the effects of AgCl deposition on membrane morphology and surface properties through a series of characterizations. We compared the FO performance and antifouling


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properties in bovine serum albumin (BSA) of the mineralized membranes and the un-modified TFC polyamide membrane. 2. Experimental 2.1 Materials We purchased polysulfone support membranes as substrate from NanoH2O, USA. We used anhydrous ethanol solution (Fisher, Scientific CO LLC) to pretreat and wash the substrates. We used 1,3-Phenylenediamine (MPD, purity 99%, Sigma Aldrich), trimesoyl chloride (TMC, purity 98%, Sigma Aldrich) and Isopar-G Fluid (purity 100%, Fisher, Scientific CO LLC) for interfacial polymerization. For post-treatment of the TFC membranes, we used sodium hypochlorite solution (NaClO, reagent grade, available chlorine 10%~15%, Sigma Aldrich) and sodium sulfite (NaHSO3, purity≥98%, Sigma Aldrich). We used aqueous solutions of silver nitrate (AgNO3, purity≥99%, Sigma Aldrich) and sodium chloride (NaCl, purity≥99.5%, Sigma Aldrich) for surface mineralization. We performed FO tests with sodium chloride as draw solute. The model foulant we used was bovine serum albumin (BSA, purity≥98%, Sigma Aldrich). We used all chemicals as received. Deionized (DI) water was the solvent for all aqueous casting solutions and we also used DI water for the soaking and rinsing steps during surface modification. We prepared feed and draw solutions for FO membrane testing with DI water and we used DI water for the physical cleaning step of flushing the fouled membranes. 2.2 Preparation of thin film composite polyamide membranes We synthesized thin film composite (TFC) polyamide membranes through interfacial polymerization based on the method previously published by Lind et al. 27. At first, we cut suitable size of polysulfone support, then we sprayed the surface of the support uniformly with ~20 mL of ethanol from a spray-bottle, finally we rinsed the support with 500 mL of deionized water.. Then we stored the support membrane in DI water before casting. After preparing a 3.5 wt. % MPD in deionized water solution and 1.5 wt. % TMC in Isopar-G solution, we taped the support membrane onto a glass plate and then placed the glass into MPD solution with active side down for two minutes. We removed excess droplets of the aqueous MPD solution from the membrane surface by using a fingerprint ink roller. Subsequently, we dipped the glass plate with the membrane into a vertical membrane holder with TMC solution for one minute to form a polyamide film. Then we removed the glass plate with the membrane from the vertical holder, placed the glass plate in a vertical position for two minutes, and put it into hot water (95 °C) with active-side up for two minutes. At last, we removed the composite membrane from the glass plate and placed it into 1.5% (v/v) sodium hypochlorite solution for two minutes, 1 % (v/v) sodium bisulfite solution for 30 seconds and hot water (95 °C) with active-side down for two minutes in sequence. Before surface mineralization modification, we rinsed the synthetized TFC membranes fully with DI water. Then we stored the membranes in DI water at 25℃ until performance and characterization testing.



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2.3 Surface mineralization modification We conducted surface mineralization modification of TFC polyamide membranes using an alternate soaking process (ASP), a method for depositing mineral compounds on a membrane surface 28, 29. Before modification, we prepared separate aqueous solutions of 0.1 M AgNO3 and 0.1 M NaCl in deionized water. The specific operation steps of ASP have been described previously 30. First, we soaked the TFC membrane sample in the 0.1 M AgNO3 aqueous solution for 60 seconds, and then we flushed it with deionized water for 60 seconds to remove any residual unreacted AgNO3. Subsequently, the membrane sample was soaked in 0.1 M NaCl aqueous solution for 60 seconds and then flushed with deionized water for 60 seconds to remove residual ions of NO3- and Na+. We consider the two step soaking and rinsing process as a single ASP cycle. Mineralization degree (MD) was controlled through changing the total number of ASP cycles. Mn (n=0, 1, 2, 4, 5 and 6) denotes as mineralized membranes with different numbers of ASP cycles. MD was determined through the weight change before and after the mineralization of each membrane sample as described by Zhi et al 26. We analyzed the weight change of the membrane samples by first thoroughly flushing them with deionized water, then we dried the membranes at 40 ℃ under vacuum until we observed no change in weight. The equation to calculate the extent of MD is:

MD (g/cm2 ) =

w1 − w 0 A

(1)

where w1 [g] is the weight of mineralized dry membrane sample, w0 [g] is the weight of the original dry membrane sample, and the surface area of the membrane sample is denoted as A [m2]. 2.4 Surface characterizations We thoroughly rinsed the membrane samples with deionized water and completely dried them under vacuum at 40 °C for 24 hours before surface characterizations. We used a scanning electron microscope (SEM) (FEI/Philips, XL30 ESEM-FEG Hillsboro, Oregon, USA) with an operation voltage of 20 kV to obtain the surface images of the original and mineralized TFC membranes. We also used the energy dispersive X-ray spectroscopy (EDX) (Apollo detector with Genesis software both from EDAX, USA) attached to the SEM at an operating voltage of a 20 k eV to acquire elemental composition information of our membranes. We characterized surface roughness of the synthetized original and mineralized TFC membranes in air with atomic force microscopy (AFM) in tapping mode (Digital Instruments, Santa Barbara, CA, USA) using a silicon nitride probe. Using a scanning rate of 0.5 Hz we obtained 5 µm ×5 µm images and we reported the root mean square surface roughness (Rq). We investigated the surface hydrophilicity of our original and mineralized membranes by sessile drop water contact angle measurement using a goniometer (Kruss Easy Drop DSA-20,


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PaloAlto, CA, USA). To eliminate experimental errors, we averaged 15 contact angles on three coupons of each membrane type to eliminate errors. We measured surface streaming potential of our original and mineralized membranes with a Zeta potential analyzer (SurPASS solid surface, Anton Paar GmbH, Austria). We performed experiments at 25 °C from pH 4 to 10 in a background electrolyte solution containing1.0 mM KCl. We used the Helmholtz-Smoluchowski equation to calculate the zeta potential of the membrane 31. 2.5 Forward osmosis performance evaluation We tested forward osmosis performance, water flux, and reverse salt flux of our original and mineralized membranes in a custom built cross-flow forward osmosis testing cell (Fig.1) based on the design of McCutcheon et al. 32. The cell contains two identical channels in each half of the cell with 18.86 cm2 effective area. During the baseline testing, we loaded membranes in pressure retarded osmosis (PRO) orientation in which the polyamide selective layer faced the concentrated draw solution while the polysulfone substrate faced the feed solution 33. During later fouling tests we placed the membranes in forward osmosis (FO) mode with the polyamide selective layer facing the feed solution. The draw solution was 1 M sodium chloride (NaCl) and feed solution was DI water. The volumes of draw solution and feed solutions were both 500 mL. The feed and draw solutions were kept in separate 1 L tanks. We set the flow rate of both solutions at 5 gallons per hour (GPH) and conducted triplicate 1 hour long measurements for each membrane sample at 25 °C. The water flux (Jw) through the membrane was defined as the mass of water permeating from the feed side to the draw side per unit time per unit membrane surface area (L·m-2·h-1). During testing we used a weighing balance (Denver Instrument, S-2002, USA) to record the weight change of draw solution and we also put feed solution on a platform at the same height as the draw solution to eliminate any gravitational effects. We calculated water flux by using the equation as follows:

Jw =

∆w ρ × A m × ∆t

(l / m

2

⋅ h or LMH )

(2)

where ∆w is the weight change of the draw solution in kilograms during the test, ρ is the density of water in kilograms per liter, Am is the effective membrane area in square meters and ∆t is the test time n hours. The reverse salt flux (RSF) is a phenomenon in which draw solute permeates from the draw side through the membrane to the feed side, opposite to the direction of water transport. We calculated RSF by measuring the mass of the NaCl that diffuses from the draw side to feed side per unit time per unit membrane surface area. We obtained the amount of NaCl present in the



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feed solutions by measuring the conductivity of each feed solution during each flux measurement. We used equation (3) to calculate reverse salt flux as follows: J s-NaCl =

w (g/m 2 ⋅ h) A m × ∆t

(3)

where w is the mass of NaCl in grams, Am is the effective membrane area in square meters and ∆t is the testing time in hours.

Fig. 1. Schematic of custom fabricated cross-flow forward osmosis testing cell. The draw and feed solutions are re-circulated with peristaltic pumps, the flow rate is maintained by rotameters. The feed and draw solutions are in a counter-flow configuration across the membrane.

2.6 Antifouling property evaluation We placed the membranes in forward osmosis (FO) mode for the antifouling tests, in which the active layer faced the feed solution. We selected BSA as model material for fouling experiments.


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We used concentrated NaCl solutions (0.5~1.5 M) as draw solution and 10 mM NaCl solution containing 200 mg/L BSA as feed solution. We set the initial flux across the membrane at 25 LMH by adjusting the NaCl draw solution concentration (from 0.5 – 1.5 M NaCl) and carried out a 6 hour long test with a cross-flow rate of 5 GPH in both draw and feed sides at 25℃. At the conclusion of each fouling experiment, after 6 hours of FO testing, we used DI water to perform physical membrane cleaning with 30 min of back flushing at a cross-flow rate of 5 GPH. After completion of cleaning the membrane by the DI water backflush, we measured pure water flux with a 10 mM NaCl BSA-free draw solution at the same cross-flow conditions and temperature as the fouling experiments in order to measure water flux recovery. During the testing, the polyamide selective layer faced the feed solution which contained the foulant BSA. We used the following equation to calculated flux recovery ratios (FRR): FRR=

Jc Ji

(4)

Where Jc (LMH) is the water flux after physical back flushing cleaning and Ji (LMH) is the initial water flux.

3. Results and discussion 3.1 TFC polyamide membrane surface mineralization We conducted surface mineralization of lab-synthesized TFC polyamide forward osmosis membranes via an alternate soaking process (ASP). We alternately soaked TFC polyamide membranes in aqueous solutions of AgNO3 and NaCl which deposits AgCl particles onto the membrane surface. Fig. 2 schematically depicts this deposition process.



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Fig.2. Concept of deposition of AgCl particles on the surface TFC polyamide membrane during surface mineralization. In the first step of ASP, Ag+ ions will interact with the pendent carboxyl groups (-COO−) in the active layer of TFC polyamide membrane through electrostatic attraction. Subsequently Ag+ ions will attract Cl- ions to generate AgCl deposits which will quickly and evenly distribute on the membrane surface because of the local supersaturated environment. The weakly bound ions (Na+ and NO3-) and unreacted ions (Ag+ and Cl-) were removed from the membrane samples after each soaking step with a DI water flush. Fig. 3 reports the calculation results of mineralization degree (MD); this figure shows that there is a nearly linear correlation between the weight of the TFC polyamide membrane and the number of ASP cycles. We further confirm the existence of the AgCl mineral coating through SEM and EDX analyses. Fig. 4 (a) presents surface SEM images of the original TFC polyamide membrane and Fig. 4 (b)-(f) presents the membranes M1M6 which were mineralized with ASP cycles 1-6. The SEM images show that AgCl particles deposited evenly over the membrane surface and the amount of AgCl visible on the surfaces increases the ASP cycles increase. It is noteworthy that previous research by Zhou et al. has shown that surface mineralization does not alter the inner ridge-and-valley structure of TFC polyamide membrane 25. Table 1 presents EDX measurements of the elemental atomic percentages of C, O, S, N, Ag and Cl in the original and mineralized TFC membranes. Table 1


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clearly shows that elements of silver and chloride are present on the membrane surface after surface mineralization. Finally, it is important to note that the quantity of Ag and Cl present on the membrane surface as measured by EDX increases as the ASP cycles increase.

Fig. 3. Measured mineralization degree as a function of the number of alternate soaking process (ASP) cycle.



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Fig. 4. SEM micrographs of the surfaces of: (a) M0; (b) M1; (c) M2; (d) M4; (e) M5; (f) M6.


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Table 1: EDX measurements of the elemental atomic percentages of the original and mineralized TFC polyamide membranes. membranes TFC M1 M2 M4 M5 M6

C 62.13 61.29 60.83 60.01 59.76 59.03

Atomic Percentage (%) N O S 11.59 15.55 10.74 11.43 15.34 10.59 11.35 15.22 10.51 11.19 15.02 10.36 11.14 14.95 10.33 11.01 14.77 10.20

Ag 0 0.71 1.11 1.78 1.97 2.68

Cl 0 0.64 0.98 1.64 1.85 2.31

3.2 Surface properties of original TFC and mineralized membranes Fig. 5 shows the measured DI water contact angles of the original and AgCl mineralized polyamide membranes and Table 2 reports the measured DI water contact angles. We can see that the contact angles of the TFC polyamide membranes decline from 65.6±1.8 to 39.8±1.3 as the number of ASP cycles increases from 0 to 6. The decrease in contact angle indicates that mineralized membranes have more hydrophilic surfaces than unmodified TFC membranes. It is worthwhile to note that the contact angles are nearly the same after 4 ASP cycles which means excessive mineralization will not further improve the membrane surface hydrophilicity. With enhanced surface hydrophilicity, the mineralized membranes could have both higher water flux and lower fouling tendency 34, 35. Fig. 6 presents AFM images of samples of the M0 and M4 membranes and the values of root mean square (Rq) of original and mineralized TFC polyamide membranes. Table 2 also presents the AFM data. We can see that mineralized membranes have smoother surfaces than the original membrane; the native TFC polyamide membrane has a RMS of 104.3±2.13 nm while the M4 membrane has a RMS of 73±2.19 nm. However, when more than four ASP cycles are performed the surface roughness increases, this may be the result of the agglomeration of AgCl particles on the surface. Lower surface roughness of mineralized membranes is desirable because a smoother membrane surface has lower fouling tendency 36, 37. Fig. 7 shows the zeta-potential of original and mineralized TFC polyamide membranes, from which we can investigate the change of surface charge as a result of mineralization. We find that the zeta potential of all the membranes changes with pH value and that membrane surfaces are more negatively charged as the pH increases from 4 to 10. In addition, the surfaces of the mineralized membranes are more negatively charged than the surfaces of the unmodified TFC membranes. As the number of ASP cycles increases the negative surface charge of the AgCl mineralized membranes also increases. This results from the adsorption of negative ions which



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are attracted to the AgCl particles that are deposited on membrane surface. Our results are similar to the results found in previous research 28. The mineralized membranes have more negatively charged surfaces than un-mineralized membranes. Therefore, through electrostatic repulsion, the mineralized membranes could show enhanced resistance to negatively charged salt ions and foulants than un-mineralized membranes.

Fig. 5. Contact angles of the original TFC and mineralized TFC polyamide membranes.

Table 2 Contact angles and surface roughness of the original and mineralized TFC membranes. membranes TFC M1 M2 M4 M5

Contact angle(o) 65.6±1.8 44.7±1.3 42.9±1.5 40.1±1.2 39.9±1.2

Root Mean Square (nm) 104.3±2.13 96.8±1.55 83.7±1.23 73±2.19 85.5±1.10


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M6

39.8±1.3

88.6±0.92

Fig. 6. 5×5 (µm) AFM images of: (a) M0 and (b) M4.



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Fig. 7. Zeta potentials of the surfaces of the mineralized and original TFC membranes.

3.3 FO performance of original and mineralized polyamide membranes Fig. 8 and Table 3 present the FO performance results including reverse salt fluxes and water fluxes using a draw solution of 1 M sodium chloride (NaCl) and a feed solution of DI water. We can clearly see that mineralized membranes exhibit lower reverse salt flux and higher water flux than the un-mineralized TFC polyamide membranes. The maximum flux performance of the mineralized membranes was for the M4 mineralized membrane which had a water flux of 24.0±2.1 LMH compared to 14.3±1.2 LMH for the unmodified TFC membrane. The reverse salt flux for the M4 mineralized membrane was lower (2.8±0.5 g/m2 h) compared to that of the TFC membrane (7.9±0.8 g/m2 h).Beyond 4 ASP cycles, the salt flux increases and the water flux decreases for the M5 and M6. There are two competing factors affecting water flux in the mineralized membranes. First there is enhancement of the water flux of the AgCl mineralized


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membranes as a result of the improved surface. Second there is competition impeding water flux from the increased resistance added by the mineralized AgCl coating. However, beyond 4 ASP cycles the surface hydrophilicity of the mineralized membranes does not increase significantly (as indicated by the contact angle results presented in Fig. 5 and Table 2). Above 4 ASP cycles, for membranes M5 and M6, the permeation resistance from the larger amount of deposited AgCl mineral coating is intensified, resulting in the reduction of water flux of the mineralized membranes. The mineralized membranes also displayed lower reverse salt flux. This is attributed to the improved surface hydrophilicity which promotes preferential adsorption of water to the membrane instead of salt ions. In addition, it is widely accepted that charged membranes will repel co-ions and attract counter-ions because of electrostatic interaction 38, 39. The surface of mineralized membranes become more negatively charged (as shown in Fig. 7) which increased the electrostatic repulsive force to Cl- ions. Hence, mineralized membranes could reject more Clions than un-modified membranes. The reverse salt flux of the mineralized membranes increased slightly for M5 and M6 as compared to M4, which is expected as reverse salt flux tends to have the opposite trend of water flux (increasing water fluxes yield decreasing reverse salt fluxes, while decreasing water fluxes yield increasing reverse salt fluxes). Reverse salt flux is directly proportional to the measured salt rejection coefficient R. A lower value of reverse salt flux corresponds to a higher value of salt rejection 1. Therefore, the mineralized TFC polyamide membrane with moderate AgCl deposition can exhibit both high water flux and high salt rejection, which meet the standards of ideal FO membranes.

Table 3 FO performance of original and mineralized TFC polyamide membranes. membranes TFC M1 M2 M4 M5 M6

Water flux Jw (LMH) 14.3±1.2 17.8±1.4 19.3±1.6 24.0±2.1 22.2±1.3 21.4±1

Reverse salt flux Js-NaCl (g NaCl/m2 h) 7.9±0.8 6.5±0.4 4.9±0.3 2.8±0.5 3.4±0.4 3.8±0.4



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Fig.8. (a) measured water flux and (b) measured reverse salt flux of the TFC and mineralized TFC membranes 3.4 Antifouling properties of TFC and mineralized TFC polyamide FO membranes We selected BSA as foulant to evaluate the antifouling property and water flux recovery ratio of our original and mineralized TFC membranes. We adjusted the draw solution concentration to ensure that original and mineralized TFC membranes had an equal initial flux of 25 LMH since membrane fouling in FO is heavily affected by initial water flux 40, 41. Before the long term fouling test, we carried out baseline experiments (i.e., feed without BSA foulants). We used the baseline experiments to correct the flux decline resulting from the dilution of the draw solution and the continuous concentration of the feed solution. The fouling behavior of original and mineralized TFC polyamide membranes are signified by FO water flux over a 6 hour test as shown in Fig. 9. We can see that AgCl mineralized membranes have lower water decline ratios than original TFC polyamide membrane after the 6 hour fouling test. In addition, the mineralized membranes display higher flux recovery ratios (FRR) than the TFC polyamide membranes after the physical back flushing with DI water as presented in Fig. 10. The lower flux decline ratio illustrates that the mineralized membranes have better antifouling property to BSA and the higher water flux recovery ratio indicates that we could


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more easily clean the adsorbed BSA off the mineralized membrane through hydraulic flushing. It seems that no further benefit is accrued from increasing the number of ASP cycles above 4. This is because the antifouling property and flux recovery ratio of the mineralized membrane increase as ASP cycles increase from 0 to 4, and then decrease with further increase in ASP. The improved antifouling property and water flux recovery is mainly the result of the enhancement of either surface negative charge or surface hydrophilicity. The adsorption of the protein foulant BSA to membrane surfaces relies on hydrophobic and electrostatic interactions 42, 43 . The mineralized membranes have more hydrophilic and negatively charged surfaces. As a result, the electrostatic repulsion is higher and interaction is lower between the BSA and mineralized membrane surface than between the BSA and the original TFC polyamide membrane. In addition, the surfaces of mineralized membranes are smoother than the original membrane. Hence, fewer BSA molecules will deposit on the surfaces of mineralized membranes which could lower membrane fouling and decrease flux decline ratio. As seen from the deionized water contact angle results in Table 2, after 4 ASP cycles, there is essentially a lower limit to the measured contact angle. Using a low contact angle as an approximation of membrane surface hydrophilicity implies that at there is no further increase in surface hydrophilicity of the mineralized membranes above 4 ASP cycles. However, as also seen in Table 2, membranes M5 and M6 have increased surface roughness compared to membranes M4. As seen in Figure 8, the water flux of M5 and M6 decreases compared to the flux of M4 which implies that the permeation resistance of the AgCl coating is intensified. Simultaneously, the membrane surface roughness is also increasing. We hypothesize that this increased permeation resistance and increased surface roughness is directly correlated with the observed decline in the antifouling properties of membranes M5 and M6 as seen in Figures 9 and 10. Previous works showed that membranes with higher surface roughness have higher fouling tendency 36, 37. Fig. 11 presents the EDX spectra of mineralized membrane with 4 ASP cycles (M4) and shows the elemental composition (At%) of Ag and Cl before and after the long term fouling testing. We can clearly see that the loading of Ag and Cl in the mineralized membrane stays almost the same after the 8 hour fouling test, which demonstrates AgCl particles are stable in the membrane system and will not release into the feed solution during long term testing.



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Fig. 9. Fouling behavior of original and mineralized TFC polyamide membranes at 25 ℃. Draw solution: 0.5 to 1.5 M NaCl solutions; feed solution: 10 mM NaCl containing 200 mg/L BSA; cross-flow rate: 5 GPH; membrane orientation: active layer facing the feed solution.

Fig. 10. Water flux recovery ratio of original and mineralized TFC polyamide membranes.


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Fig. 11 EDX spectra of mineralized membrane with 4 ASP cycles (M4) before and after the long term fouling testing

4. Conclusions In this work, we prepared TFC polyamide FO membranes via interfacial polymerization and then conducted surface mineralization modification by depositing AgCl particles on the TFC membrane surfaces though an alternate soaking process. We used SEM and EDX analysis to confirm the even distribution of AgCl particles on the surfaces of the mineralized membranes. We find that surfaces of mineralized membranes are more hydrophilic, smoother and more negatively charged than the original TFC membrane surfaces. We evaluated membrane performance in FO and found that the mineralized membranes exhibit both higher water flux and salt rejection than the un-mineralized, original TFC membranes. Fouling tests using the model foulant BSA showed that the mineralized membranes display lower water flux decline ratios in BSA aqueous solution and higher water flux recovery ratios after simple hydraulic washing. However, we note that excessive deposition of AgCl could add an additional layer of resistance to permeation through the membrane and thus decrease the water flux, selectivity and antifouling property of the membranes. We conclude that, in FO operation, TFC polyamide membranes with an AgCl surface mineralization have reduced tendency to foul compared to unmodified TFC membranes and strong future potential for FO applications.



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Acknowledgments The authors gratefully acknowledge the National Science and Technology Support Program (2012BAC02B03), the Fundamental Research Funds for the Central Universities, China (Awards No. 2015203020213) and the China Scholarship Council. We gratefully acknowledge the use of facilities with the LeRoy Eyring Center for Solid State Science and the use of facility in Biodesign institute at Arizona State University. We also gratefully acknowledge Rui Zheng from The Institute of seawater Desalination and Multipurpose Utilization, SOA in China for the zetapotential testing. We also acknowledge support from the National Science Foundation (NSF) NSF Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (ERC-1449500) and NSF CAREER CBET-254215. Finally, we acknowledge this work was supported by NASA Oﬃce of the Chief Technologist’s Space Technology Research Opportunity Early Career Faculty Grant No. NNX12AQ45G.

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Graphical abstract


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Synthesis of AgCl Mineralized Thin Film Composite Polyamide

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