Antifouling Thin-Film Composite Membranes by Controlled

Article pubs.acs.org/est

Antifouling Thin-Film Composite Membranes by Controlled Architecture of Zwitterionic Polymer Brush Layer Caihong Liu,†,‡ Jongho Lee,‡ Jun Ma,*,† and Menachem Elimelech*,‡ †

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States

‡

S Supporting Information *

ABSTRACT: In this study, we demonstrate a highly antifouling thin-film composite (TFC) membrane by grafting a zwitterionic polymer brush via atom-transfer radicalpolymerization (ATRP), a controlled, environmentally benign chemical process. Initiator molecules for polymerization were immobilized on the membrane surface by bioinspired catechol chemistry, leading to the grafting of a dense zwitterionic polymer brush layer. Surface characterization revealed that the modified membrane exhibits reduced surface roughness, enhanced hydrophilicity, and lower surface charge. Chemical force microscopy demonstrated that the modified membrane displayed foulant-membrane interaction forces that were 1 order of magnitude smaller than those of the pristine TFC membrane. The excellent fouling resistance imparted by the zwitterionic brush layer was further demonstrated by significantly reduced adsorption of proteins and bacteria. In addition, forward osmosis fouling experiments with a feed solution containing a mixture of organic foulants (bovine-serum albumin, alginate, and natural organic matter) indicated that the modified membrane exhibited significantly lower water flux decline compared to the pristine TFC membrane. The controlled architecture of the zwitterionic polymer brush via ATRP has the potential for a facile antifouling modification of a wide range of water treatment membranes without compromising intrinsic transport properties.

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nonspecific adsorption of organic foulants.16 Despite its widespread application, PEG is susceptible to oxidation and therefore loses antifouling functionality in complex media.15 Other hydrophilic nanomaterials have also been grafted onto TFC membranes to increase surface hydrophilicity, including silica nanoparticles,9,21 graphene oxide,22,23 and carbon nanotubes,24,25 demonstrating varying degrees of antifouling performance. However, silica may be prone to adsorption of proteins,26−28 and certain surface functional groups, such as carboxylic groups that may be present in oxidized carbon materials (e.g., graphene oxide), can result in calcium-ion induced organic fouling.29 Zwitterion-based polymers have recently received considerable attention as promising antifouling materials due to their high hydrophilicity, long-term durability, and environmental stability.15,30−32 The presence of both cationic and anionic groups on the zwitterions with overall neutral charge results in significant hydration via electrostatic interactions.31,33,34 In addition, the arrangement of water molecules in the hydration shell around the zwitterions resembles that of free water, leading to a high affinity to water.32,35 The thick hydration layer on zwitterionic polymer brushes imposes a strong steric hindrance to adsorption of foulants.12 Accordingly, zwitterion-

INTRODUCTION Membrane-based desalination is widely considered as a crucial technology for augmenting fresh water supply due to its compactness, modularity, reliability, and high energy efficiency.1 Thin-film composite (TFC) membranes are the state-of-the-art technology for desalination, having a thin (∼100 nm) polyamide selective layer fabricated by interfacial polymerization.1 TFC membranes have become the industrial standard for nanofiltration (NF), reverse osmosis (RO), and forward osmosis (FO). 1,2 Despite the wide range of applications for seawater and brackish water desalination, as well as wastewater reuse, fouling still remains a major challenge for TFC membranes.3−6 The detrimental effects of membrane fouling, such as reduced water flux, increased energy consumption, and shorter membrane lifetime, have led to extensive efforts to develop efficient antifouling surface treatment methods to prevent adsorption of foulants.7−11 It is generally accepted that increased surface hydrophilicity reduces fouling propensity due to the formation of a dense hydration layer, which imposes an energetic barrier for foulant adsorption.12−14 Hence, numerous studies have reported antifouling modification of TFC membranes for enhancing surface hydrophilicity via grafting of hydrophilic materials.9,15−20 Of these, poly(ethylene glycol) (PEG) is the most widely used antifouling material. PEG possesses a neutral charge and binds water molecules via hydrogen bonding, resulting in increased surface hydrophilicity and reduced © 2017 American Chemical Society

Received: November 27, 2016 Accepted: January 17, 2017 Published: January 17, 2017 2161

DOI: 10.1021/acs.est.6b05992 Environ. Sci. Technol. 2017, 51, 2161−2169

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Environmental Science & Technology

Figure 1. Schematic illustration of TFC membrane modification by grafting zwitterionic polymer brush layer on the membrane surface. (1) Coupling of initiator and dopamine hydrochloride (i.e., BiBB-DA). (2) Immobilization of initiators on the TFC membrane via polymerization of BiBB-DA (i.e., TFC-PDA membrane). (3) Grafting of zwitterionic polymer brush via ATRP (i.e., TFC-PSBMA membrane). BiBB stands for αbromoisobutyryl bromide, TEA stands for triethylamine, SBMA stands for [2-(methacryloyloxy)-ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (also named sulfobetaine methacrylate), and PSBMA stands for poly(sulfobetaine methacrylate).

alginate, proteins, and natural organic matter), which simulate realistic fouling environments. Our work also provides insights into the role of the zwitterionic polymer brush layer and the antifouling mechanisms of the developed membrane.

based materials have recently been investigated for enhancing fouling resistance of TFC membranes via interfacial polymerization with zwitterionic monomers,36 membrane surface modification,37,38 and initiated chemical vapor deposition.39,40 While these approaches have shown fouling resistance against proteins and bacteria, additional efforts are needed to investigate the antifouling performance of zwitterionic polymer coated TFC membranes with complex mixtures of organic foulants. Furthermore, there is a need for improved surface modification methods that result in uniform and dense zwitterionic polymer brush layers on the TFC membrane surface. Previous studies have shown that the “graft-from” approach leads to denser surface functional groups than the “graft-to” approach,41,42 rendering it more suitable for antifouling surface modification. Surface-initiated polymerization methods, such as atom-transfer radical-polymerization (ATRP) and reversible addition−fragmentation chain transfer (RAFT), have been widely used to increase fouling resistance by grafting antifouling polymers from the surfaces.43−47 Specifically, the ability to control the size of polymers and to use a wide range of solvents, including aqueous media, render ATRP or RAFT a suitable technique for membrane modification. The narrow polydispersity and relatively slow polymerization enable the control of the thickness of the polymeric surface coating, thereby maintaining the membrane transport properties.2,45 Using benign and aqueous media is also more desirable for membrane surface modification than using toxic organic solvents.48 Therefore, a systematic study that fully explores these desired characteristics of surface-initiated polymerization, thoroughly investigates fouling resistance mechanisms, and carefully assesses the antifouling performance in realistic fouling environments could result in the development of robust antifouling TFC membranes. In this work, we developed an antifouling TFC polyamide membrane by surface grafting of a zwitterionic polymer brush layer with controlled architecture via ATRP. Fouling resistance of the modified membrane was quantified by chemical force microscopy and was demonstrated by static adsorption tests of proteins and bacteria. Fouling experiments with feed solutions containing high concentration of organic foulants further demonstrated the fouling resistance of the modified membrane. Our study is the first to systematically investigate the organic fouling behavior of zwitterionic polymer coated TFC membranes using a mixture of model organic foulants (i.e.,

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MATERIALS AND METHODS Materials and Chemicals. Polyamide thin-film composite (TFC) forward osmosis (FO) membranes (Porifera, Inc., CA) were used as pristine and base membranes for surface modification. Before use, the membranes were immersed in 25% isopropanol (IPA) solution for 30 min, followed by rinsing in deionized (DI) water for at least 3 h. The membranes were stored in DI water at 4 °C before modification. Dopamine hydrochloride, tris(hydroxymethyl)aminomethane (Tris) (>99.8%), α-bromoisobutyryl bromide (BiBB) (98%), triethylamine (TEA) (>99%), [2-(methacryloyloxy)-ethyl]dimethyl(3-sulfopropyl)ammonium hydroxide (SBMA), copper(II) chloride, tris(2-pyridylmethyl)amine (TPMA), L-ascorbic acid, and N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich. DI water was obtained from a Milli-Q ultrapure water purification system (Millipore, Billerica, MA). Bovine serum albumin (BSA, ≥ 98%, 66 kDa, Sigma-Aldrich), sodium alginate (12−80 kDa, Sigma-Aldrich), and Suwannee river natural organic matter (SRNOM, International Humic Substances Society, St. Paul, MN) were selected as model organic foulants for the dynamic fouling experiments. Stock solutions of each foulant (10 g/L) were prepared and stored at 4 °C. Membrane Modification. Dopamine hydrochloride (800 mg, 2.10 mmol) was dissolved in DMF (40 mL) in an amber bottle with a PTFE/silicone septum. After dry nitrogen was bubbled into the solution for 20 min, 2-bromoisobutyryl bromide (0.26 mL, 1.05 mmol) and triethylamine (0.3 mL, 1.05 mmol) were added. The solution was then stirred under nitrogen at room temperature for 3 h. Meanwhile, a nonmodified TFC membrane coupon was sandwiched between a clean glass plate and a rubber mat with a central hole cut out (10 × 6.5 cm). A polypropylene frame with the same dimensions as the central hole was combined with the rubber mat and secured by using steel clamps to create a sealed well. The above prepared BiBB-dopamine solution was added to a 200 mL aqueous tris(hydroxymethyl) aminomethane buffer (pH 8.5, 2.0 mmol), which was immediately added into the well to contact the membrane active layer; this step initiates formation of polydopamine (PDA) on the membrane surface. 2162


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step method to determine A, B, and S is described in our previous publication.49 Static Adhesion Tests. Membrane antifouling properties were evaluated by static adsorption tests of bovine serum albumin (BSA) and Escherichia coli (E. coli). A fluoresceinconjugated BSA (FITC-BSA) (Life Technologies, A23015) at a concentration of 0.05 mg/mL was prepared in a phosphatebuffer saline (PBS) solution at pH 7.4. A membrane coupon (2.1 cm in diameter) was cut and mounted in a custom-made membrane cell holder. FITC-BSA solution (3 mL) was added to the cell holder and left in contact with the membrane active layer (salt rejecting side) in the dark on a rocking platform at 60 rpm for 3 h. The solution was then removed and the membrane was gently rinsed twice with PBS buffer. The rinsed membrane coupon was placed on a glass slide with a droplet of PBS buffer on top, covered by a coverglass, with nail polish on the edges for sealing. Fluorescence images were taken by an inverted Axiovert 200 M epifluorescence microscopy (Carl Zeiss Inc., Thornwood, NY). Membrane adhesion property for bacteria was assessed using plate counting. E. coli (ATCC BW26437, Yale Coli Genetic Stock Center, New Haven, CT) cells were cultured in a Lysogeny broth (LB) overnight at 37 °C. After being diluted 25 times with a fresh LB media, the overnight cultured bacterial cells were cultivated to a log phase by growing for about 2 h at 37 °C with agitation, at which ∼109 colony-forming units (CFU)·mL−1 were found. The cultures were then washed three times with a sterile 0.9 wt % NaCl solution to remove excess macromolecules present in the growth media, and resuspended in a sterile 0.9 wt % NaCl solution at a final concentration of ∼109 CFU·mL−1. Finally, a bacterial suspension in a 0.9 wt % NaCl solution at 108 CFU·mL−1 was prepared for subsequent static adsorption experiments. Membrane coupons with an area of ∼3.5 cm2 were punched and mounted in custom-made membrane holders. An aliquot of the bacterial suspension (3 mL, 108 CFU·mL−1) was added into each holder and kept in contact with the membrane active layer for 3 h at room temperature. After exposure, the membranes were rinsed twice with 0.9 wt % NaCl to remove the unattached bacteria. The membrane coupons were then transferred into 50 mL Falcon tubes containing 10 mL of 0.9 wt % NaCl solution and sonicated for 10 min to remove the attached bacteria cells from the membrane surface. The supernatant was sequentially diluted, spread on LB agar plates, and incubated at 37 °C overnight. After incubation, the grown colonies were counted to quantify the attached bacteria cells on the membranes expressed as CFU·mL.−1 Dynamic Fouling Experiments in Forward Osmosis. The fouling propensities of the control and modified membranes were assessed by dynamic fouling experiments in a cross-flow FO test unit. The FO cell has dimensions of 77 mm × 26 mm × 3 mm with an effective membrane area of 20.02 cm2. The feed solution chemistry for the fouling experiments simulated secondary wastewater effluent from selected wastewater treatment plants in California9,50 and contained 0.5 mM NaHCO3, 0.93 mM NH4Cl, 0.5 mM CaCl2, 0.61 mM MgSO4, 9.2 mM NaCl, and 0.45 mM KH2PO4 with pH adjusted to 7.4. BSA, sodium alginate, and SRNOM were selected as model organic foulants to represent proteins, polysaccharides, and natural organic matter, respectively. High concentrations of the model foulants (100 mg/L each) were added to the feed solutions for the fouling experiments. The initial water flux of each fouling experiment was kept at 20 L·

After 10 min on a rocking platform at 60 rpm, the membrane was thoroughly rinsed with DI water. The washed membrane (named TFC-PDA hereafter) was stored in aqueous isopropanol (10% v/v) until further modification. An SBMA monomer (15.64 g, ∼56 mmol) was dissolved in a 1:1 volume ratio of isopropanol (IPA):DI water mixture (200 mL, v/v) in a 250 mL glass bottle covered with aluminum foil. After bubbling dry nitrogen through the solution for 10 min, 8 mL of copper chloride complex catalyst solution (Copper(II) chloride 0.010 g (14.8 μmol) and TPMA 0.140 g (0.095 mmol) in 20 mL of 1:1 IPA:DI water (v/v) mixture) were added into the bottle. The prepared TFC-PDA membrane was placed in the 250 mL glass bottle. After bubbling nitrogen for another 10 min, 12 mL of the ascorbic acid solution (with a concentration of 1 g in 10 mL of 1:1 IPA:DI mixture) were added to the glass bottle to initiate polymerization. After 1 h of polymerization, the bottle was exposed to open air to terminate the ATRP process and the membrane was thoroughly rinsed with DI water. The PSBMA modified membrane (named TFC-PSBMA hereafter) was then stored at 4 °C in aqueous isopropanol (10 v/v %). A schematic diagram for the membrane modification process is shown in Figure 1. Membrane Characterization. Membrane surface morphology was characterized by both scanning electron microscopy (SEM, Hitachi SU-70 FE-SEM, Hitachi High Technologies America, Inc.) and atomic force microscopy (AFM, Bruker Dimension Fastscan AFM, Bruker Corp., Santa Barbara, CA). Surface roughness was obtained by using AFM in Peak Force Tapping mode. A Scanasyst-air silicon cantilever with a backside coated by reflective aluminum was employed (Bruker Nano, Inc., Camarillo, CA). The cantilever has a spring constant of 0.4 N/m, a resonance frequency of 70 kHz, and a tip with a radius of curvature of 2 nm. The length and width of the cantilever are 115 and 25 μm, respectively. Membrane surface chemistry was assessed by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy (Thermo Scientific Nicolet 6700, Thermo Fisher Scientific Inc., MA). The hydrophilicity of the pristine and modified membranes was assessed by measuring the water contact angle using the sessile drop method (OneAttension contact angle meter, Biolin Scientific, Finland) at a minimum of 12 random locations for each sample. Zeta potentials of the membrane surface before and after modification were calculated from streaming potentials measured by a streaming potential analyzer (EKA, Brookhaven Instruments, Holtsville, NY) with an electrolyte consisting of 1 mM KCl and 0.1 mM KHCO3, with pH adjusted as needed by addition of KOH or HCl. Chemical force microscopy was used to quantify the membrane-foulant interactions, by measuring the adhesion force between the membrane and a model organic foulant. Specifically, a carboxylated latex particle with a diameter of 4 μm (CML, carboxyl content 19.5 μeq/g, Life Technologies, Eugene, OR) was attached on an AFM cantilever to serve as model organic foulant, as described in our previous studies.9 The adhesion force measurements were performed in an AFM liquid cell containing 50 mM NaCl and 0.5 mM CaCl2 aqueous solution. At least 40 measurements, each on five different, randomly selected spots, were taken for each sample at room temperature (23 °C). Transport parameters of the pristine and the modified membranes, namely water permeability coefficient (A), salt permeability coefficient (B), and structural parameter (S), were determined in a laboratory-scale cross-flow FO unit. The four2163


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Figure 2. SEM images of the polyamide active layer of (a) pristine TFC, (b) TFC-PDA, and (c) TFC-PSBMA membranes. AFM 3D images of (d) pristine TFC, (e) TFC-PDA, and (f) TFC-PSBMA membranes. (g) Surface roughness determined by AFM for pristine TFC, TFC-PDA, and TFCPSBMA membranes. The roughness parameters, that is, mean-square value (Rq) and average roughness (Ra), were calculated from AFM images from at least six different spots on each membrane sample. Asterisks above bars indicate that the TFC-PSBMA membrane roughness parameters were significantly different (p < 0.05) than the corresponding values of the pristine TFC membrane. (h) Thickness of PSBMA polymer brush layer at different durations of the ATRP reaction. PSBMA brush was grafted onto a polyamide film which was formed by a layer-by-layer deposition of mphenylenediamine and trimesoly chloride for 15 cycles on a silicon wafer, to mimic polyamide membrane surface (See Supporting Information (SI) Figure S1).

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m−2·h−1 by adjusting the NaCl draw solution concentration. A spacer was used to support the membrane (support layer side) in the draw solution channel. The cross-flow velocity of the draw solution was 4.25 cm/s while the feed solution cross-flow velocity was 9.56 cm/s. The solution temperature was maintained at 25 ± 0.5 °C by a temperature-controlling water bath (Neslab RTE 7, Newington, NH). The procedures for the fouling experiments were similar to those described elsewhere.9,50 Briefly, prior to each test, DI water was used as the draw and feed solutions to stabilize the system for ∼20 min until the water flux reached ∼0. Then, the aforementioned synthetic secondary wastewater effluent was added into the feed solution reservoir with pH adjusted to 7.4, followed by the addition of sodium chloride stock solution into the draw solution reservoir. Once the water flux across the membrane was stabilized and reached ∼20 L·m−2·h,−1 the fouling experiment was initiated by adding the three organic foulants (i.e., BSA, sodium alginate, and SRNOM) to the feed solution with pH readjusted to 7.4. The water flux and feed solution concentrations were recorded by a computer until 500 mL of permeate water volume was collected. To account for dilution of draw solution by the permeate, baseline water flux data were obtained prior to each fouling experiment using the same procedure for the fouling experiments, but with no added foulants.50,51 Water flux decline only due to fouling was then attained by subtracting the baseline (i.e., without foulants) from the measured water flux (i.e., with foulants).

RESULTS AND DISCUSSION Membrane Surface Characteristics. SEM and AFM images depicting surface morphologies of the pristine and modified membranes are shown in Figure 2. After the TFC membrane was modified by PDA (i.e., TFC-PDA), surface roughness slightly increased (Figure 2g), due to aggregates of PDA (Figure 2b, e). On the other hand, a smooth film was formed on the membrane surface, with a noticeably reduced surface roughness after 1 h of ATRP reaction for grafting the PSBMA brush layer (i.e., TFC-PSBMA, Figure 2c, f, g). Since the controlled nature of ATRP allows for formation of polymers with a narrow polydispersity,44−46 we measured the PSBMA film thickness for different durations of polymerization to estimate the brush layer thickness. The polyamide active layer of TFC membranes is typically fabricated via interfacial polymerization. This leads to a relatively high surface roughness (Figure 2a, d), making it difficult to accurately measure the thickness of the PSBMA layer formed on the membrane. Instead, we prepared a polyamide film on a silicon wafer via molecular layer-by-layer (mLbL) assembly.52 Two monomers, i.e., m-phenylenediamine and trimesoyl chloride, were spincoated one after another for 15 cycles, resulting in polyamide layers produced by the reaction between the two alternating monomers (SI Figure S1). A PSBMA film was then grafted on the polyamide-coated silicon wafer through identical procedures and chemical concentrations used for preparing the TFCPSBMA membrane, that is, the deposition of BiBB-DA followed by ATRP reaction. The thickness of the PSBMA 2164


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Figure 3. (a) ATR-FTIR spectra for pristine TFC, TFC-PDA, and TFC-PSBMA membranes. (b) Water contact angles for pristine TFC, TFC-PDA, and TFC-PSBMA membranes. Average values were obtained from contact angles measured on at least 12 random locations on each membrane sample. Error bars represent standard deviations. Asterisks indicate that the contact angle of the modified membrane showed a statistically significant difference compared to the pristine TFC membrane (p < 0.05). (c) Zeta potentials of pristine TFC and TFC-PSBMA membrane surfaces as a function of solution pH. Membrane zeta potentials were estimated by measuring streaming potentials in a background electrolyte solution of 1 mM KCl and 0.1 mM KHCO3. All measurements were performed at room temperature (23 °C).

film was measured by a profilometer (Figure 2h). As expected, the measured thickness increased with increasing polymerization time, providing a precise control on the thickness of PSBMA on the membranes. In the present study, 1 h of reaction was used, leading to an approximate thickness of 400 nm. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was performed to characterize the membrane surface chemistry and to further verify the successful grafting of PSBMA brushes. Figure 3a shows the spectra of the pristine and modified membranes for a wavenumber range of 1800−800 cm−1, which covers the main functional groups of the modified membranes. Absorbances at 1657 and 1536 cm−1 correspond to (amide I) CO stretching vibrations and (amide II) N−H band of amide group (−CONH−) in the active polyamide layer.53,54 In addition, strong peaks at 1243 and 1608 cm−1 refer to amide III and the hydrogen−bonded carbonyl of the amide in polyamide layer, respectively. The spectrum of polysulfone displays distinct peaks at 1487 and 1305 cm−1, representing C−C and SO groups, respectively.54 After the TFC membrane was modified with PDA (i.e., TFCPDA), no noticeable change in the spectra was detected, likely due to the minute thickness of the PDA and the incomplete coverage. For the PSBMA brush modified membrane (i.e., TFC-PSBMA), two additional peaks were observed at 1726 and 1039 cm−1, identifying the carbonyl group and sulfonate group, respectively, which are among the main functional groups of SBMA.54 In addition, sulfur was detected by energy dispersive spectroscopy (EDS) from the TFC-PSBMA membrane, proving the existence of the PSBMA brush layer (SI Figure S2). In order to investigate the membrane surface hydrophilicity, contact angle measurements were conducted. As shown in Figure 3b, the contact angle of TFC-PDA was significantly reduced after the deposition of PDA, showing its hydrophilic nature.55 Modification with the PSBMA brush layer further increased the surface hydrophilicity of the membrane due to the strong hydration capacity of zwitterionic polymers.32 The surface charge characteristics of the pristine and modified membranes were investigated by determining the zeta potentials calculated from streaming potential measurements using the Helmoltz−Smoluchowski equation.56 Since a typical TFC polyamide membrane is fabricated by interfacial polymerization of a monomeric polyamine with a polyfunctional acyl halide, the unreacted amine and carboxylic groups on the membrane surface dominate the membrane surface charge behavior.57 For the investigated pH range shown in

Figure 3c (i.e., pH 3−9), the surface zeta potential of the pristine membrane changes from positive to negative values with an isoelectric point at pH 3.3. As the pH increases, the membrane becomes more negatively charged due to the deprotonation of the carboxyl groups. Furthermore, the preferential adsorption of anions (Cl− and OH−) may also render the zeta potential more negative, since anions are less hydrated than cations in aqueous solutions, leading to a closer proximity to the membrane surface.56 After the PSBMA brush was grafted, the zeta potential became less negative due to the presence of the net-zero charged zwitterionic polymer shielding the functional groups on the membrane. Transport and Structural Properties of the Modified Membranes. The membrane transport parameters, namely water permeability coefficient, A, salt permeability coefficient, B, and structural parameter, S, were measured by the four-step FO characterization method.49 As expected, the structural parameter, S, an intrinsic parameter relevant only to the membrane support layer, did not change after the modification of the active layer (Figure 4). It was also observed that the water

Figure 4. Transport and structural properties of pristine TFC and TFC-PSBMA membranes. Water permeability coefficient, A, salt permeability coefficient, B, and structural parameter, S, were measured by the four-step FO characterization method as described in the Materials and Methods section.

permeability coefficient, A, of the TFC-PSBMA membrane decreased slightly while the salt permeability coefficient, B, increased. However, these changes were not statistically significant (based on a Student t test) compared to the pristine TFC membrane. The precise control on the growth of the PSBMA layer via ATRP allows for further optimization of the 2165


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Figure 5. Distributions of adhesion forces between a model foulant (carboxylatetd latex particle) and (a) control TFC, (b) TFC-PDA, and (c) TFCPSBMA membranes measured by AFM. The “NO” label (at the positive force range) stands for measurements where no adhesion force was observed. The test solution for the measurements comprised 50 mM NaCl and 0.5 mM CaCl2 at room temperature (23 °C). Five locations were randomly selected for each membrane sample, and at least 40 measurements were taken at each location. Average values of the adhesion forces are also presented.

Adsorption Propensity of Proteins and Bacteria. The antifouling character of the modified TFC membranes was further tested using BSA and E. coli, representing respectively proteins and bacteria as common foulants. Proteins are ubiquitously found in wastewater effluents and other source waters. Adsorption of proteins and other molecules on water treatment membranes not only reduces water permeability, but also leads to the formation of a conditioning layer, which may enhance microbial colonization and eventually biofouling.3,15,61 Therefore, demonstration of protein fouling resistance is crucial for successful antifouling modification of TFC membranes. We used a FITC-BSA as a model foulant to visualize membrane fouling by proteins. The fluorescence intensities of the microscopy images of the membranes after exposure to FITC-BSA qualitatively show the degrees of BSA adsorption on the surfaces (Figure 6). The bright fluorescence image of the

coating thickness such that the membrane transport properties are least affected while antifouling functionality remains highly effective. Relating Organic Fouling Propensity to Interfacial Adhesion Forces. The significantly increased hydrophilicity of the TFC-PSBMA membrane (Figure 3b) implies that the grafting of zwitterionic polymer brushes may enhance fouling resistance. We employed chemical force microscopy to assess the organic fouling propensity of the modified membranes, with a carboxylated latex particle attached to an AFM cantilever serving as a model organic foulant. Most organic foulants (e.g., natural organic matter, polysaccharides, and proteins) possess carboxylic groups, which aggravate fouling by complex formation with carboxylic groups on polyamide-based TFC membrane surfaces in the presence of calcium ions.29,58,59 Therefore, interaction forces between the membrane surface and the carboxylated particle can provide a quantitative measure of membrane fouling propensity. Figure 5 presents the frequency distributions of adhesion forces (normalized to the latex particle radius) for the pristine and modified membranes in aqueous solution containing calcium ions. For the pristine TFC membrane, the average interaction force was −0.92 ± 0.6 mN/m, exhibiting adhesive forces (i.e., negative values), with a very small fraction of nonadhesive events (indicated as “NO” in Figure 5a). The significant adhesive forces observed with the control TFC membrane are mainly attributed to intermolecular bridging mediated by calcium ions.60 After the membrane was modified by PDA (i.e., TFC-PDA membrane), the adhesion forces were significantly reduced to −0.43 ± 0.32 mN/m, with an increased frequency of nonadhesive events (indicated as “NO” in Figure 5b). Because PDA partially covers the carboxylic groups on the membrane surface, calcium-ion bridging is expected to be less prominent, which would result in a reduced adhesion force between the TFC-PDA membrane and foulants. The TFC membrane grafted with zwitterionic polymers (TFC-PSBMA) showed a much narrower distribution of adhesion forces centering at markedly lower value, −0.07 ± 0.15 mN/m, representing a 92% reduction in average adhesion force compared to the pristine membrane. The apparent PSBMA film, observed in SEM images (Figure 2c), shields the surface carboxylic groups and thus effectively prevents calciumion induced fouling. Swelling of the PSBMA brush layer can also contribute to a further reduction of the foulant-membrane adhesion force by increasing the distance between the solid− water interface and the membrane surface.32

Figure 6. Epifluorescence microscopy images of pristine and modified TFC membranes following protein adhesion tests using fluoresceinconjugated BSA (BSA-FITC) in PBS. (a) Pristine TFC, (b) TFCPDA, and (c) TFC-PSBMA membranes after 3 h exposure to BSAFITC in PBS solution at pH 7.4.

pristine TFC membrane indicates significant adsorption of BSA. The fluorescence intensity of the TFC-PDA membrane decreased noticeably, likely due to increased hydrophilicity by deposition of PDA. In contrast, the micrograph for the TFCPSBMA membrane shows virtually no fluorescence, implying negligible adsorption of BSA. Such a drastic difference in BSA adsorption clearly demonstrates the excellent protein fouling resistance of the PSBMA modified membrane. We attribute this observation to the exceptionally high affinity of zwitterions to water, more than other hydrophilic materials such as polyethylene glycol (PEG).26,33 Resistance to adhesion of microorganisms is also critical for membrane fouling mitigation because the attachment of microorganisms typically leads to subsequent colonization and formation of a biofilm.15,62 We investigated the fouling resistance against microorganisms using adsorption tests with E. coli as model bacteria. The adhesion of E. coli on each membrane sample was expressed as a percentage of CFU relative to that on the pristine TFC membrane (Figure 7). We 2166


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compared to the control TFC membrane. Specifically, our data show a 3-fold decrease in the rate of flux decline of the PSBMA modified membrane compared to the control TFC membrane (Section S1 and Figure S3 in SI). After 500 mL of permeate volume, the pristine TFC membrane showed a 25% decline in water flux, while the TFC-PSBMA membrane exhibited a remarkably lower flux decline of 15%. Again, the dynamic fouling experiments clearly demonstrate the excellent antifouling functionality of the zwitterionic polymer brushes, highlighting the promise of our surface modification technique for membrane fouling mitigation. Antifouling Mechanisms of Zwitterionic Polymer Modified Membranes. The static and dynamic fouling experiments confirmed the antifouling character of zwitterionic polymers possessing high water affinity and hydrophilicity. We also attribute the excellent fouling resistance to the complete and dense coverage of the polymer brushes on the membrane surface, resulting in reduced surface roughness. It has been reported in previous literature that fouling propensity is lower for membranes with lower surface roughness because of the reduced surface area for membrane-foulant interaction.63,64 Hence, the smooth film of PSBMA can mitigate fouling by decreasing the surface area. We also found that shielding carboxylic functional groups on the TFC membrane surface is critical to increasing organic fouling resistance. Carboxylic groups are present on typical organic foulants and they aggravate membrane fouling by forming complexes with carboxyl groups on TFC membranes via calcium-ion bridging. Therefore, complete coverage of the membrane surface by zwitterionic polymers can prevent calcium-ion induced fouling. The low adhesion forces probed by chemical force microscopy support our proposed mechanism of organic fouling mitigation via shielding of surface functional groups (Figure 5c). We emphasize that the above antifouling mechanisms are effective primarily at the early stage of fouling. The significantly lower rate of flux decline for the TFC-PSBMA membrane compared to the pristine TFC membrane at the initial period of the dynamic fouling experiments highlights the effectiveness of the antifouling modification (SI Section S1 and Figure S3). Due to the advection of foulants toward the membrane by the permeate flow and foulant rejection by the active layer, the membrane surface is continuously exposed to more highly concentrated foulants than those present in the bulk. Once foulants adsorb to the membrane surface, the aforementioned antifouling mechanisms cannot be effective and instead the dominant interaction occurs between the newly approaching foulants and the already deposited foulants. Despite the slow fouling rate of the zwitterionic polymer modified membrane at the initial period, the rates of flux decline for both pristine and modified membranes were eventually comparable because fouling progresses mainly by foulant−foulant interactions (SI Section S1 and Figure S3). Still, the zwitterionic polymer-coated TFC membrane exhibited significantly delayed fouling compared to the pristine TFC membrane, demonstrating great potential for water desalination and wastewater reuse. Careful optimization of the PSBMA layer thickness is anticipated to further improve the antifouling property, while maintaining the membrane transport properties. Moreover, our strategy in constructing an antifouling film with a precisely controlled thickness on arbitrary substrates using polydopamine could also have

Figure 7. Adhesion of E. coli on the surface of pristine and modified membranes after 3 h contact time. The adhesion of E. coli on each membrane sample was expressed as the percentage of colony-forming units (CFU) relative to that on the pristine TFC membrane (control). Error bars indicate standard deviations from three independent replicates.

again observed that the coating of PDA imparted a considerable fouling resistance, with the TFC-PDA membrane exhibiting an 80% reduction of CFU. After coating with the zwitterionic polymer brush layer, the membrane exhibited an even higher resistance against bacteria adhesion, showing nearly 90% CFU reduction compared to the pristine TFC membrane. Dynamic Fouling Behavior in Forward Osmosis. The results of the chemical force microscopy and static fouling experiments strongly suggest that zwitterionic PSBMA brushes impart TFC membranes with excellent organic fouling resistance. To further assess the antifouling property of the modified membranes in a more realistic setting and for longterm applications, FO fouling experiments with organic foulants were conducted for both pristine and zwitterion-modified membranes. Figure 8 displays water flux behavior (corrected for draw solution dilution effects) for the pristine TFC and TFCPSBMA membranes. The data represent the average of three independent fouling experiments. Our results demonstrate a significant fouling resistance for the TFC-PSBMA membrane

Figure 8. Normalized water flux due to fouling as a function of cumulative permeate volume of the pristine TFC and TFC-PSBMA membranes during FO dynamic fouling tests. Fouling conditions were as follows: composition of the feed solution simulated secondary wastewater effluent (0.5 mM NaHCO3, 0.93 mM NH4Cl, 0.5 mM CaCl2, 0.61 mM MgSO4, 9.2 mM NaCl, and 0.45 mM KH2PO4) with a mixture of three organic foulants: 100 mg/L of sodium alginate, 100 mg/L BSA, and 100 mg/L SRNOM. Solution pH was adjusted to 7.4 and initial water flux was 20 L m−2 h,−1 adjusted for each experiment by an NaCl draw solution (concentration ranged from 0.7 to 0.9 M). Cross-flow velocities of feed and draw solutions in the membrane cell were 4.25 and 9.56 cm/s, respectively. Temperature was maintained at 25.0 ± 0.5 °C. Fouling experiments were performed until 500 mL cumulative volume of permeate was collected, lasting ∼17.4 h for pristine TFC and ∼16 h for TFC-PSBMA membranes. 2167


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applications for organic fouling control in disparate industrial fields.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b05992. Illustration of molecular layer-by-layer assembly to produce polyamide film on silicon substrate (Figure S1); Energy dispersive spectroscopy spectrum of TFCPSBMA membrane (Figure S2); Rate of flux decline during dynamic fouling experiments (Section S1 and Figure S3) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.M.) E-mail: [email protected]. *(M.E.) E-mail: [email protected]. ORCID

Menachem Elimelech: 0000-0003-4186-1563 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Department of Defense through the Strategic Environmental Research and Development Program (SERDP, ER-2217). C.L is grateful for the support from the China Scholarship Council (CSC) Graduate fellowship. We thank Marissa Tousley for assisting polyamide film formation on a Si wafer by a molecular layer-bylayer technique. Facilities used were supported by Yale Institute for Nanoscience and Quantum Engineering (YINQE).

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REFERENCES

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