Superhydrophilic Thin-Film Composite Forward Osmosis Membranes

Sep 24, 2012 - This study investigates the fouling behavior and fouling resistance of superhydrophilic thin-film composite forward osmosis membranes f...
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Superhydrophilic Thin-Film Composite Forward Osmosis Membranes for Organic Fouling Control: Fouling Behavior and Antifouling Mechanisms Alberto Tiraferri,† Yan Kang,†,‡ Emmanuel P. Giannelis,†,‡ and Menachem Elimelech*,† †

Department of Chemical and Environmental Engineering, Yale University, P.O. Box 208286 New Haven, Connecticut 06520-8286, United States ‡ Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, United States S Supporting Information *

ABSTRACT: This study investigates the fouling behavior and fouling resistance of superhydrophilic thin-film composite forward osmosis membranes functionalized with surfacetailored nanoparticles. Fouling experiments in both forward osmosis and reverse osmosis modes are performed with three model organic foulants: alginate, bovine serum albumin, and Suwannee river natural organic matter. A solution comprising monovalent and divalent salts is employed to simulate the solution chemistry of typical wastewater effluents. Reduced fouling is consistently observed for the superhydrophilic membranes compared to control thin-film composite polyamide membranes, in both reverse and forward osmosis modes. The fouling resistance and cleaning efficiency of the functionalized membranes is particularly outstanding in forward osmosis mode where the driving force for water flux is an osmotic pressure difference. To understand the mechanism of fouling, the intermolecular interactions between the foulants and the membrane surface are analyzed by direct force measurement using atomic force microscopy. Lower adhesion forces are observed for the superhydrophilic membranes compared to the control thin-film composite polyamide membranes. The magnitude and distribution of adhesion forces for the different membrane surfaces suggest that the antifouling properties of the superhydrophilic membranes originate from the barrier provided by the tightly bound hydration layer at their surface, as well as from the neutralization of the native carboxyl groups of thin-film composite polyamide membranes.

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INTRODUCTION Fouling of water treatment membranes diminishes process performance, limits the range of feedwater sources that can be feasibly treated, and acts as the limiting factor for the advancement of membrane technologies. While all conventional membrane processes are hampered by membrane fouling, forward osmosis (FO) has been observed to inherently endure fouling compared to other systems that employ saltrejecting membranes, such as reverse osmosis (RO).1−3 Therefore, the implementation of FO-RO hybrid systems4,5 or of FO as a stand-alone process not preceded by feed pretreatment,6,7 is being explored. Successful developments in this area would allow the safe and economical advancement of desalination and wastewater reuse technologies.8 A number of previous FO studies using a variety of foulants have shown complete3 or considerable6,9−13 stable water flux when membranes were challenged with feed solutions containing foulants. More pronounced flux decline was reported when alginate was used as a model foulant in the presence of calcium ions, due the formation of a cake/gel layer.1,14 Calcium bridging was identified as the main cause of fouling layer development.1,14 However, flux was highly © 2012 American Chemical Society

recovered after a physical cleaning (rinsing) step, suggesting the reversible nature of this type of fouling.1,14 Although the factors controlling the fouling mechanism vary from foulant to foulant,14 the low flux decline in FO can be generally attributed to the different structure of the fouling layer compared to that which forms in pressure-driven membrane processes, such as RO.1,9 On the other hand, reverse salt diffusion of FO draw solutes was often found to increase the foulant−membrane interaction at the surface/water interface11 or to exacerbate cake-enhanced osmotic pressure (CEOP) within the already formed fouling layer.2,9 Recent FO studies have observed lower flux recovery due to silica scaling,15 and have underlined the important role of other divalent cations (Mg2+) in enhancing fouling by microalgae.11 Measurements of foulant−membrane interaction forces using atomic force microscopy were successfully employed to explain the FO fouling mechanism.1,14 These studies have highlighted Received: Revised: Accepted: Published: 11135

July 15, 2012 September 5, 2012 September 24, 2012 September 24, 2012 dx.doi.org/10.1021/es3028617 | Environ. Sci. Technol. 2012, 46, 11135−11144

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Membrane Fabrication. TFC FO membranes were fabricated via interfacial polymerization of polyamide on hand-cast polysulfone support layers. The PSf support layer was fabricated by nonsolvent induced phase separation, adapting the procedure outlined in our previous publications.28 PSf (9 wt %) was dissolved in DMF and then stored in a desiccator for at least 15 h prior to casting. To begin casting the membrane, the PET fabric was attached to a glass plate and wetted with NMP. The PSf solution was drawn down the PET fabric using a casting knife (Gardco, Pompano Beach, FL) with a gate height fixed at 350 μm (∼15 mils). The whole composite was immersed in a precipitation bath containing 3 wt % DMF in DI water at room temperature to initiate phase inversion. The support membrane remained in the precipitation bath for 10 min before being transferred to a DI water bath for storage until polyamide formation. Polyamide thin films were fabricated via interfacial polymerization of MPD (3.4 wt % in DI water) and TMC (0.15 wt % in Isopar-g), following the procedure described in our previous publication.28 The fabricated TFC membranes were rinsed thoroughly and stored in DI water at 4 °C. Nanoparticle Preparation and Membrane Functionalization. Superhydrophilic nanoparticles were fabricated by surface functionalization of silica nanoparticles with a radius of approximately 7 nm (Ludox HS-30, 30 wt %, Sigma-Aldrich).29 Briefly, 6 g of nanoparticles were suspended in 54 mL of deionized water and sonicated for 30 min. Then, 6.4 g of Ntrimethoxysilylpropyl-N,N,N-trimethylammonium chloride ( N(CH3)3+, 50 wt %, Gelest, Inc. SIT8415.0) were added to the dispersion under vigorous stirring. This step was followed by pH adjustment to pH ∼5 (by adding HCl) and a heating step to 60 °C for 18 h. Finally, the suspension was dialyzed in DI water using SnakeSkin tubing (7 kDa MWCO, Pierce Biotechnology) for 48 h. Free carboxyl moieties at the surface of polyamide membranes26 were exploited to irreversibly bind functionalized silica nanoparticles to the membranes via a simple dip coating protocol. The polyamide membranes were immersed in the nanoparticle suspension for 16 h at room temperature (23 °C), with only the active layer side in contact with the suspension. During this step, the positively charged ammonium groups at the surface of the nanoparticles bind to the negatively charged carboxylic groups at the surface of polyamide membranes via electrostatic attraction.29 The pH of the suspensions was adjusted to between 6.4 and 7.4 (by adding NaOH) before the dip coating protocol. Membrane Characterization. Control and functionalized membranes were tested using a cross-flow membrane filtration system.30 Two sets of experiments were conducted: one in FO mode (DI water as feed solution against the membrane active layer and 1 M NaCl as draw solution) and one in pressure retarded osmosis (PRO) mode (DI water feed solution against the membrane support layer and a 0.5 M NaCl draw solution). No mesh spacers were employed and both cocurrent cross-flow velocities were fixed at 21.4 cm/s. The setup was maintained at a constant temperature of 25 ± 0.5 °C. Water flux in both experiments was determined by monitoring the rate of change in weight of the draw solution for 30 min. During the FO experiment, NaCl concentration in the feed was also monitored at 3 min intervals with a calibrated conductivity meter (Oakton Instruments, Vernon Hills, IL) to quantify the reverse NaCl flux.31,32 These measurements allowed for the determination of the pure water permeability of the membrane active layer, A,

the importance of foulant−membrane physicochemical interactions in governing the fouling behavior and the role played by membrane surface properties to control fouling. For example, hydrophilicity is theorized to have a central function in the fouling behavior of membranes as well as surfaces utilized in many different applications.16−20 Water separation membranes should be designed to maximize their surface affinity with water with the goal being to increase their resistance to fouling.8 Antifouling properties arise due to the strong hydration layer of the hydrophilic surface, which opposes the adsorption of molecules and particles to the membrane surface.18,21 Despite increased efforts to understand fouling behavior in FO, investigations aimed at fabricating antifouling FO membranes are still scarce. Fabrication of hydrophilic membrane surfaces can be obtained by surface functionalization with polymers16,17,19,20,22 or nanomaterials.19,23,24 Combining nanoparticles and polymeric membranes is a particularly attractive route to achieve membrane functionalization. Such methods exploit the “processability” of polymers and the flexible tuning offered by nanoparticle fabrication. In particular, we have recently proposed a simple coating method to irreversibly bind superhydrophilic silica nanoparticles to a polyamide-based membrane.25 Polyamide membranes possess a relatively high density of native carboxyl functionalities at their surface,26 which can be exploited as binding sites for polymeric chains19 and nanomaterials.27 Positively charged silica nanoparticles were tethered to these carboxyl groups via electrostatic interaction to produce membranes with superhydrophilic surfaces. In this paper, we investigate the fouling behavior and antifouling mechanisms of thin-film composite forward osmosis membranes with superhydrophilic surface properties. The functionalization optimizes the polyamide surface chemistry and interfacial energy to reduce membrane fouling with model organic foulants, specifically alginate, bovine serum albumin (BSA), and Suwannee river natural organic matter (SRNOM). We also study the role of hydraulic pressure in membrane fouling by comparing membrane performance in FO (without hydraulic pressure) and RO (with hydraulic pressure) modes. Finally, interfacial force measurements are used to explain the fouling behavior and identify the antifouling mechanism of the superhydrophilic membranes.

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MATERIALS AND METHODS

Materials and Chemicals. Polysulfone (PSf) beads (Mn: 22 000 Da), 1-methyl-2-pyrrolidinone (NMP, anhydrous, 99.5%), N-N-dimethylformamide (DMF, anhydrous, 99.8%), 1,3-phenylenediamine (MPD, >99%), and 1,3,5-benzenetricarbonyl trichloride (TMC, 98%) were used as received (SigmaAldrich, St. Louis, MO). A polyester nonwoven fabric (PET, grade 3249, Ahlstrom, Helsinki, Finland) was used as a backing layer for the PSf membrane supports. For interfacial polymerization of polyamide, TMC was dispersed in Isopar-G, a proprietary nonpolar organic solvent (Univar, Redmond, WA). Chemicals used for post-treatment of polyamide membranes were sodium hypochlorite (NaOCl, available chlorine 10−15%, Sigma-Aldrich) and sodium bisulfite (NaHSO3, Sigma-Aldrich). Sodium chloride (NaCl, crystals, ACS reagent) from J.T. Baker (Phillipsburg, NJ) was used for the membrane performance tests. Unless specified, all chemicals were dissolved in deionized (DI) water obtained from a Milli-Q ultrapure water purification system (Millipore, Billerica, MA). 11136

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the NaCl permeability of the membrane active layer, B, and the structural parameter of the membrane support layer, S, by treating A, B, and S as adjustable parameters to simultaneously fit experimental data of water and reverse salt fluxes to the corresponding governing equations presented in our previous publications.31−33 In addition to the membrane performance characteristics, the membrane surface physicochemical and morphological properties were extensively characterized as described in detail elsewhere.25 The strength of the nanoparticle binding to the membrane was assessed by subjecting the surface of the functionalized membranes to physical (bath sonication, shear induced by high cross-flow velocity), chemical (solutions of low pH, high pH, or very high salinity), and combined physical and chemical stresses. Membranes were then rinsed thoroughly with DI water, followed by reevaluation of contact angles and imaging by SEM to appraise the presence of nanoparticles and the membrane wettability after stress. Model Foulants and Solution Chemistry. The model organic foulants chosen to represent proteins, polysaccharides, and natural organic matter were, respectively, bovine serum albumin (BSA, ≥98%, Sigma-Aldrich), sodium alginate (SigmaAldrich), and Suwannee river natural organic matter (SRNOM, International Humic Substances Society, St. Paul, MN). According to the manufacturer, the molecular weight of the BSA is about 66 kDa. BSA is reported to have an isoelectric point at pH 4.7.34 Sodium alginate has been widely used in membrane fouling research to represent polysaccharides that constitute a major fraction of soluble microbial products in wastewater effluent.1,35 According to the manufacturer, the alginate has a molecular weight in the range of 12−80 kDa. Other properties of the alginate used are given in our previous publication.36 SRNOM has been used extensively as a model organic foulant36−38 and its characteristics can be found elsewhere.39 The organic foulants were received in powder form. Stock solutions for BSA and alginate (10 g/L) and for SRNOM (2 g/L, adjusted to pH 10) were prepared by dissolving the foulant in DI water. The stock solutions were stored at 4 °C. The solution chemistry for the fouling and AFM experiments was based on secondary wastewater effluent from selected wastewater treatment plants in California, as described in our previous publication.35 Specifically, the solution comprised 0.45 mM KH2PO4, 9.20 mM NaCl, 0.61 mM MgSO4, 0.5 NaHCO3, 0.5 mM CaCl2, and 0.935 mM NH4Cl. The final pH of the solution was ∼7.4 and the calculated ionic strength was 14.7 mM (Visual MINTEQ 3.0). Assessing Fouling and Cleaning Behavior. The FO and RO fouling experiments were performed with cross-flow membrane systems.1,37 A constant solution temperature of 25 ± 0.3 °C was maintained by a water bath (Neslab, Newington, NH). We employed a cross-flow velocity of 21.4 cm/s during all fouling and cleaning experiments. The protocol for the FO fouling experiments comprised the following steps. First, a new membrane coupon was placed in the unit and characterized as described previously. Next, the system was thoroughly rinsed with DI water and cocurrent cross-flows of the DI water solutions were run for >1 h to stabilize the system. At this point, the feed solution was replaced with the testing solution described above, and an appropriate volume of a 5 M NaCl stock solution was added to the draw solution (∼1 M NaCl) to obtain a constant water flux of 19.5 ± 0.5 L m−2h−1(11.5 ± 0.3 gal ft−2day−1). After the flux became stable, 150 mg/L of the

foulant of interest were added to the feed solution and the fouling experiment was protracted for 8 h. The feed solution was continuously mixed using a magnetic stirrer. Water flux and solute concentration in the feed solution were recorded throughout the experiment by automatically logging data to a computer every 3 min. Baseline experiments were conducted to quantify the flux decline due to the decrease in the osmotic driving force during the fouling experiments as the draw solution was continuously diluted by the permeate water and by the reverse diffusion of NaCl into the feed solution. The baseline experiments followed the same protocol as that for the fouling experiments except that no foulant was added to the feed solution. Knowledge of A, B, and S for each coupon and of the solute concentrations, that is, osmotic pressures, of both the feed and draw solutions at any time during fouling, allowed us to correct for the small change in water flux associated with the loss in driving force. To confirm the reproducibility of the FO fouling and cleaning experiments, all runs were duplicated. Cleaning experiments were conducted immediately following the FO fouling runs. Conditions for cleaning experiments were as follows: 15 mM NaCl cleaning solution, cross-flow of 21.4 cm/s, and a 5 s long introduction of air bubbles every 3 min, for a total cleaning duration of 15 min. During the cleaning step, the draw solution was also replaced by 15 mM NaCl solution, so that there was no permeate flux through the membrane. Pure water and reverse salt fluxes of the cleaned membrane were determined after the cleaning experiment to determine the flux recovery. The protocol for RO fouling experiments comprised the following steps. The membrane was first compacted overnight with DI water under an applied pressure of 20.7 bar (300 psi). The membrane was then stabilized and equilibrated with the foulant-free testing solution described earlier in this paper for approximately 2 h. The applied pressure was adjusted in this step to obtain a permeate flux analogue to that used in the FO experiments, that is, 19.5 ± 0.5 L m−2h−1 (11.5 ± 0.3 gal ft−2day−1). Next, 150 mg/L of foulant were added to the feed solution and the fouling experiment was continued for 8 h at constant applied pressure while keeping the feed reservoir continuously mixed using a magnetic stirrer. At the end of the fouling run, the solution in the feed reservoir was disposed of and replaced with a 15 mM NaCl solution to clean the fouled membrane. Cleaning conditions were the same in RO and in FO. At the end of the cleaning stage, the cleaning solution in the reservoir was discarded, the reservoir was rinsed with DI water to flush out the residual chemical cleaning solution, and the cleaned RO membrane was subjected to the second baseline performance with the foulant-free synthetic wastewater solution to redetermine the pure water flux. AFM Contact Mode Force Measurements. Atomic force microscopy (AFM) was used to measure the foulant−foulant and foulant−membrane interfacial forces, adapting the procedures described by Li and Elimelech.38 The force measurements were performed with a colloid probe, modified from a commercial AFM probe (Veeco Metrology Group, Santa Barbara, CA). To prepare the colloid probe, a 4.0 μm carboxyl modified latex (CML) particle (Interfacial Dynamics Corp., Portland, OR) was attached to a tipless SiN cantilever using Norland Optical adhesive (Norland Products, Inc., Cranbury, NJ). The probe was cured under UV light for 20 min, and then coated with foulants by soaking it in organic foulant solution (2000 mg/L alginate, BSA, or SRNOM) for at 11137

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least 24 h at 4 °C to prevent organic degradation. During this step, the organic molecules adsorbed onto the surface of the CML latex particle. The adhesion force measurements were performed in a fluid cell. The foulant−membrane forces were measured after injecting into the fluid cell a testing solution with the same composition as that used in fouling experiments. To measure foulant−foulant intermolecular forces, 20 mg/L of organic foulant were introduced into the fluid cell and deposited to the membrane surface. In all cases, the membrane surface was equilibrated with the test solution for 45−60 min before force measurements were performed. The force measurements were conducted at five different locations, and at least 25 measurements were taken at each location to minimize inherent variability in the force data. Because the focus of this study was on the adhesion forces, only the raw data obtained from the retracting (pull-off) force versus cantilever extension curves were processed to obtain the force versus surface-to-surface separation curves. Force, rupture distance, and attraction energy distributions were obtained. The rupture distance represents the maximum extension distance where the probesurface interaction disappears in the process of probe retraction.1,40

Figure 1. Transport parameters of the fabricated membranes. The intrinsic water permeability of the active layer, A, the solute permeability coefficient of the active layer, B, and the structural parameter of the support layer, S, are presented as bars for the control polyamide membranes (black patterned) and the superhydrophilic membranes (solid red), functionalized with silica nanoparticles silanized with −N(CH3)3+-terminated chains. Values are an average of nine separately cast and functionalized samples for each membrane type. Error bars represent one standard deviation.

mono- and divalent ions and using single foulants (alginate, BSA, or SRNOM). Experiments were carried out for 8 h and were followed by physical cleaning in the absence of calcium and with addition of air bubbles to enhance the hydrodynamic shear in the feed channel. The results of duplicate runs for the control and superhydrophilic membranes are summarized in Figure 2 and Table 1. In Figure 2, the decreased flux after fouling (patterned bars) and the recovered flux after cleaning (solid bars) are presented as normalized fluxes. An unrealistically high foulant concentration (150 mg/L) was used to accelerate the fouling rate. In our subsequent discussion, the decline and recovery of water flux are used as heuristic parameters to describe membrane fouling and cleaning behavior, respectively. Alginate fouling was the most pronounced, followed by BSA and SRNOM, with the latter causing little change in water flux for both types of membranes. A faster decline in water flux caused by alginate fouling compared to proteins or natural organic matter was also observed in a previous study.14 This observation is attributed to complexation and bridging mechanisms that alginate molecules experience in the presence of calcium ions, resulting in the formation of a cross-linked alginate gel layer on the membrane surface.1,14,36 The thick alginate layer could also be observed by the naked eye at the end of the runs (data not shown). This thick layer provides resistance to water flux, as well as accelerated cake-enhanced osmotic pressure (CEOP), due to reverse salt diffusion, resulting in elevated osmotic pressure near the membrane surface on the feed side.2,9,41 A relatively low water flux decline due to fouling by humic substances in forward osmosis was also reported recently.10 In all cases, the superhydrophilic membranes experienced a lower overall flux decline compared to control membranes, which indicates a higher resistance to organic fouling by the functionalized membranes. Fouling resistance was significant for alginate, with water flux losses of about half the magnitude of those experienced for the control membranes. The antifouling mechanism of the superhydrophilic membrane surfaces was even more pronounced in the case of BSA fouling. These results confirm the antifouling properties of hydrophilic surfaces toward proteins, also discussed in numerous other studies.18,42−46 Furthermore, the decrease in water flux produced by BSA adsorption on the superhydrophilic

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RESULTS AND DISCUSSION Membrane Properties. Characterization of the membrane surface following functionalization showed that a layer of tightly bonded nanoparticles was present at the surface.25 The functionalization effectively rendered the surface superhydrophilic, attaining values of wettability and hydrophilicity that are the highest reported so far in the literature for materials similar to those employed in this study (Supporting Information (SI) Figure S1). We also observe that the cationic nanoparticles slightly decreased the average surface roughness and increased the overall zeta potential of the surface.25 Analysis of the membrane surface after subjecting the membrane to physical and chemical stresses (bath sonication, shear induced by high cross-flow velocity, and solutions of low pH, high pH, or very high salinity) suggested that the functionalization was virtually irreversible and shedding of nanoparticles from the surface unlikely during typical operational conditions (SI Figures S1 and S2). Figure 1 presents the characteristic transport parameters for both the control and superhydrophilic membranes. Average and standard deviation values of the intrinsic water permeabilities of the active layer, A, the intrinsic salt permeability of the active layer, B, and the structural parameter of the support layer, S, are shown as bars. As expected, the structural parameter of the membranes was not affected by the functionalization of the surface of the active layer. On the other hand, values for both A and B increased after surface functionalization. We attribute this increase to enhanced wetting of the more hydrophilic membrane surface that can result in a higher transport across the thin film, and possibly to some defects due to handling during membrane functionalization. The combination of transport parameters resulted in an average water flux of approximately 19.5 L m−2h−1 for 1 M NaCl draw solution and DI water feed, and would produce a water flux of 8.8 L m−2h−1 in the case of 1.5 M NaCl and seawater as draw and feed solutions, respectively, based on the governing equation for FO water flux.33 Organic Fouling in FO. The mechanism of fouling in forward osmosis was studied in the presence of a mixture of 11138

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Figure 2. Forward osmosis organic fouling of the control polyamide membranes (black) and functionalized superhydrophilic membranes (red): (A) alginate, (B) BSA, and (C) Suwannee River natural organic matter (SRNOM). The percentage of water flux, Jw, in FO at the end of the 8 h fouling step relative to the initial water flux, Jw,0, is shown as patterned bars. The percentage of water flux in FO recovered after the “physical” cleaning step is shown as solid bars. Duplicates are shown for each membrane type. Fouling conditions were as follows: feed solution as described in the Materials and Methods section with 150 mg/L of organic foulant (alginate, BSA, or SRNOM), initial water flux of 19.5 L m−2h−1, cross-flow velocity of 21.4 cm/s, for a total of 8 h of fouling. Cleaning conditions were as follows: foulant-free feed solution of 15 mM NaCl, no permeate water flux, cross-flow velocity of 21.4 cm/s, air bubbles introduced every 3 min, for a total cleaning time of 15 min. Temperature was maintained at 25 °C.

fouling can be easily broken apart and removed by simple physical cleaning with solutions that do not contain calcium ions.1,14,37 In contrast to alginate and SRNOM, no significant recovery of water flux was observed for membranes fouled by BSA under the examined conditions. However, because cleaning is attributed to physical removal of the fouling layer by hydrodynamic shear forces, it can be further improved by optimizing the shear force generated by the bubbles and the cross-flow.1 Role of Pressure in Fouling: Comparison of FO and RO Modes. To further understand the mechanism of fouling in FO and evaluate the role of the driving force (i.e., osmotic pressure versus hydraulic pressure difference) in membrane fouling, we tested the membranes in both FO and RO modes. The RO fouling and cleaning data are presented in Figure 3. An initial water flux identical to that used in FO was obtained in RO by adjusting the applied hydraulic pressure. Because different fabricated membranes had slightly different permeabilities, the calculated hydraulic resistances of the fouling layers

Table 1. Summary of the FO Fouling and Cleaning Data for the Different Foulants and Membranes Used in This Study foulant

polyamide membrane Jw/Jw,0 after fouling (%)

alginate BSA

79.7; 72.8 89.7; 91.3

SRNOM

97.2; 96.5

Jw/Jw,0 after recovery (%) 96.5; 98.6 no recovery observed 99.6; ∼100

−N(CH3)3+ functionalized membrane Jw/Jw,0 after fouling (%) 90.0; 86.6 99.2; 97.3 97.1; ∼100

Jw/Jw,0 after recovery (%) 98.7; 98.2 no recovery observed ∼100; ∼100

membranes occurred within the first 50 min of fouling, contrary to the behavior of alginate and SRNOM, which caused a more gradual flux decline (SI Figure S3). The water flux was completely recovered after physical cleaning in the case of SRNOM fouling. Remarkably, alginate fouling was also found to be almost completely reversible despite the significant flux decline observed during the fouling stage. The sparse and loose layer of alginate formed during

Figure 3. Comparison of organic fouling in RO and FO for the control polyamide membranes (black) and functionalized superhydrophilic membranes (red): (A) alginate, (B) BSA, and (C) Suwannee River natural organic matter (SRNOM). The percentage of water flux, Jw, at the end of the 8 h fouling step relative to the initial water flux, Jw,0, is shown as patterned (FO) and hollow (RO) bars. The percentage of water flux recovered after the “physical” cleaning step is shown as solid bars. Fouling conditions were as follows: feed solution as described in the Materials and Methods section with 150 mg/L organic foulant (alginate, BSA, or SRNOM), initial water flux of 19.5 L m−2h−1, cross-flow of 21.4 cm/s, for a total of 8 h. Cleaning conditions were as follows: foulant-free feed solution of 15 mM NaCl, no permeate water flux, cross-flow velocity of 21.4 cm/s, air bubbles introduced every 3 min, for a total cleaning time of 15 min. Temperature was maintained at 25 °C. 11139

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are also provided10,47 for meaningful comparison of the different RO tests. In the case of BSA and SRNOM, all membrane types were fouled more in RO mode than in FO mode. The lower susceptibility to fouling in FO processes was also observed in other studies.6,9,10 In RO mode, the compressible organic foulants form a compact and dense cake layer that increases hydraulic resistance, while foulants form a looser fouling layer on FO membranes where the sole driving force is an osmotic pressure gradient. Conversely, alginate fouling caused similar flux decline in both modes for the control polyamide membranes, consistent with our previous study.1 Although RO is also subjected to CEOP by the rejected salt, the effect is much more pronounced in FO because of reverse salt diffusion.9,41 Furthermore, CEOP is exacerbated in FO by the creation of a thicker alginate gel layer due to the lack of applied hydraulic pressure. For the superhydrophilic membranes, the more pronounced alginate-induced flux decline in RO is an indication that a significantly thinner or sparser alginate gel layer is formed in FO for these membranes. Except in the case of RO alginate fouling of the control membranes, the decrease in performance due to fouling followed the general rule: control membranes in RO > control membranes in FO ≥ superhydrophilic membranes in RO > superhydrophilic membranes in FO. These results confirm that the functionalized membranes can also mitigate fouling in RO mode. A similar sequence of performance was also found for membrane cleaning efficiency or fouling reversibility. No or lower flux recovery was observed for the control polyamide membranes in RO compared to the respective FO experiments, suggesting the difficulty of removing a more compact fouling layer from the membrane surface by simple physical cleaning.37,48 On the other hand, complete recovery or fouling reversibility was found for the superhydrophilic membranes fouled by SRNOM in RO. Some cleaning efficiency was measured also in the cases of BSA and alginate foulants, although it was not sufficient to recover the same water flux of the respective FO runs. The results suggest that operating in FO mode may have the important advantage of minimizing the need for chemical cleaning. Relating Fouling Behavior to Foulant-Membrane Interactions. To explain the surface properties responsible for the different fouling behavior of the two membrane types, we employed AFM force measurements to characterize the foulant−membrane and foulant−foulant interactions. AFM has been successfully employed to quantify the short-range intermolecular forces that govern the fouling behavior of surfaces.1,14,18,36,49−51 Figures 4 and 5 present the frequency distribution of foulant−membrane and foulant−foulant adhesion forces, respectively. Foulant-membrane force measurements provide information about the interaction of a clean membrane with foulants in solution and about the likelihood of initial adsorption of foulants to the membrane surface. In foulant−foulant force measurements, the fouled AFM particle probe contacts the deposited foulants and pulls them off the surface, thus measuring the strength of adhesion of already adsorbed molecules on the surface. We also report the corresponding average values of the adhesion force, rupture distance, and interaction energy (or work of adhesion); the latter is calculated as the negative area in the force versus distance curves. Although not all the parameters are distributed normally, these averages give a first order approximation of the magnitude and range of surface interactions.

Figure 4. Adhesion force measurements of foulant−membrane interaction by AFM contact mode. The different plots refer to interactions between membrane surfaces and a carboxyl-modified latex particle AFM probe fouled with (A) alginate, (B) BSA, and (C) Suwannee River NOM (SRNOM). Values related to the control polyamide membranes are presented as black patterned bars, whereas data measured for the functionalized superhydrophilic membranes are shown as red bars. The “NO” label at positive force values stands for measurements where no adhesion force was observed. The test solution chemistry for the measurements is as described in the Materials and Methods section. At least 25 retracting tip measurements on five random spots were taken for each sample at room temperature (23 °C). Note the graphs are plotted with a different scale for the x axis. Also presented are the corresponding average values of adhesion force, rupture distance, and interaction energy calculated as the negative area in the force vs distance curve.

Comparing the force measurements with the fouling data shows direct correlation between the magnitude of the adhesion forces and the extent of flux decline for the three organic foulants. Larger adhesion forces were measured for SRNOM, BSA, and alginate, in that order, for both foulant− 11140

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membrane and foulant−foulant interactions. Also, in all, but a few cases, the adhesion force distributions measured with the control thin-film composite polyamide membranes are shifted toward more negative values, indicating greater adhesion forces compared to the superhydrophilic membranes. The shift in adhesion force distribution does not occur in the cases of BSAmembrane interaction (Figure 4) and SRNOM−SRNOM fouled membrane interaction (Figure 5). However, in both of these cases, the difference in adhesion force distribution between the two membrane types was minimal and the overall measured adhesion forces were very low in magnitude. While the adhesion forces for the control membranes were distributed in a form resembling normal distribution, the forces measured with the superhydrophilic membranes can be better described by a quasi-Poisson law with low mean and variance (i.e., forces or events closer to zero are most likely to occur). Accordingly, the average adhesion forces for the control membranes were 2 to 3 times the values observed for the superhydrophilic membranes. The same trend existed for the interactions measured between membranes and carboxylmodified latex particles, often used as surrogates for carboxylcontaining organic foulants or for bacterial cells (SI Figure S4).1,52−54 The shape and the width of the distribution of the adhesion forces also inform us about the type of surface interaction for each foulant. (a). Alginate. The alginate-membrane adhesion forces are widely distributed for the control (polyamide) membranes (Figures 4A, 5A), consistent with a bridging mechanism. Bridging is caused by the divalent calcium ions in solution cross-linking the carboxyls of the membrane surface with the carboxyl groups of the alginate molecules, thus enhancing the attachment of these molecules to the membrane surface.37 This fouling mechanism resulted in longer rupture distances and multiple pull-off events of the AFM tip during its retraction from the membrane surface (SI Figures S5 and S6), corresponding to multiple bridging sites along the chain of an alginate molecule.1 However, this mechanism was thwarted for the superhydrophilic membranes, whose surface carboxyl groups were overlaid by the positively charged nanoparticles. The corresponding adhesion forces for the superhydrophilic membranes were distributed in a more compact fashion and centered at low adhesion force values (Figures 4A, 5A). Once a layer of alginate has formed at the surface, further bridging can occur between the alginate film at the surface and alginate molecules in solution, resulting in the continuous formation and growth of a cross-linked alginate gel layer on the membrane surface. Therefore, in the case of alginate, fouling is controlled by calcium bridging and by foulant−foulant interactions.1 This is likely the reason for the similar water flux losses in RO and FO for both membrane types, even when bridging could not occur between alginate and the superhydrophilic surfaces. However, in FO, the alginate gel layer was loosely packed and easily removed from the membrane surfaces when rinsing with calcium-free solution, as discussed above1 (Figure 2). (b). BSA. The mechanism of BSA fouling is different from alginate. Proteins adsorb to surfaces via van der Waals and electrostatic interaction, and through hydrophobic effects, that is, the favorable interaction between the nonpolar hydrophobic regions of the protein and of the membranes.43 In the case of hydrophilic materials, lower protein fouling has been observed due to unfavorable polar interactions43 and to the inability of protein molecules to displace the bound hydration layer and

Figure 5. Adhesion force measurements of foulant−foulant interaction by AFM contact mode. The different plots refer to interactions between membrane surfaces and a carboxyl-modified latex particle AFM probe both fouled with (A) alginate, (B) BSA, and (C) SRNOM. Values related to the fouled control polyamide membranes are presented as black bars, whereas data measured on the fouled functionalized superhydrophilic membranes are shown as red patterned bars. The “NO” label at positive force values stands for measurements where no adhesion force was observed. The test solution for the measurements is as described in the Materials and Methods section. At least 25 retracting tip measurements on five random spots were taken for each sample at room temperature (23 °C). Please note the graphs are plotted with a different scale for the x axis. Also presented are the corresponding average values of adhesion force, rupture distance, and interaction energy calculated as the negative area in the force vs distance curve. 11141

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adsorb on the surface.18,45,46 The resulting width of the AFM force distribution was found to be narrower than in the case of alginate (Figures 4B, 5B). Because AFM intermolecular force measurements are conducted in contact mode, the BSAmodified AFM colloidal probe can physically displace the water molecules at the membrane surface during its approaching movement in foulant−membrane interaction force measurement. Therefore, the adhesion forces shown in Figure 4B represent interactions between the surface of the silica nanoparticles and the proteins adsorbed on the probe. A more representative system to understand BSA-membrane interactions during real operation is that realized in foulant− foulant interaction force measurement, during which we pull a layer of BSA molecules that has already been deposited on the membrane surface (Figure 5B). In this case, the interactions measured for the control membranes were significantly larger than those on the functionalized membranes. During sample preparation in the liquid cell, the BSA molecules deposited on the superhydrophilic nanoparticles without the ability to displace the hydration layer, resulting in lower adhesion forces measured by AFM, which correlated well with lower fouling in both RO and FO experiments. For the superhydrophilic membranes, several interaction profile curves related to the approaching and retracting movement of the AFM probe overlapped, a phenomenon that is typically observed when adhesion forces are absent53 (labeled “NO” for no adhesion, in the graphs). (c). SRNOM. The type of interactions between SRNOM molecules and the membrane surface is a combination of the forces encountered with BSA and alginate. The SRNOM molecules contain several functionalities, including carboxyl groups. The presence of carboxyls is reflected in a distribution of SRNOM-membrane forces for the polyamide control surfaces that is relatively wider than the respective distribution observed with BSA (Figures 4C, 5C). However, the magnitude of these adhesion forces and the related SRNOM fouling were of low magnitude, even in the presence of dissolved calcium ions. Due to the weak interaction forces observed in all systems employing SRNOM and the low flux decline observed in fouling experiments, we suggest that hydrodynamic shear forces during fouling were dominant compared to molecular and physicochemical interactions with the membrane. Antifouling Mechanism in FO. The main fouling resistance mechanism of the superhydrophilic membranes is attributed to the strong affinity of the superhydrophilic surfaces to water (SI Figure S1).18,21,25 In the presence of hydrogen acceptor groups, the short-range acid−base forces promote the existence of an interfacial layer of tightly bonded water molecules, which provides a barrier against the adhesion of foulants.55,56 Water molecules at the membrane interface have low rotational and translational dynamics and their displacement occurs at the expense of a significant amount of enthalpy gain.57,58 Therefore, one of the strategies to fabricate foulingresistance surfaces should aim at maximizing the interfacial energy between the surface and water. In addition to the barrier provided by bound water molecules, the positively charged nanoparticles at the membrane surface neutralize or simply cover the surface carboxyls on the polyamide active layer, thereby preventing calcium bridging with carboxyl-rich fouling molecules. The positive charges at the surface of the nanoparticles may also give rise to electrostatic attraction with negatively charged foulants, although in our case, the overall fouling resistance

imparted by the bound water molecules was much more significant than the attractive electrostatic interactions. Further studies should focus on the fabrication and deployment of nanoparticles that are neutrally charged or that contain suitable functional groups that minimize attractive interactions with fouling molecules while maximizing interactions with the polyamide surface. For example, zwitterionic compounds have been shown to resist the adsorption of proteins,59 but facile methods for binding with polyamide membranes need to be developed. Previous studies have underlined the effect of higher crossflow to reduce fouling and to enhance cleaning efficiency.1,9,14,15,60 In a membrane system, surface energies play the most important role in preventing adsorption of foulants and the role of shear stress cannot be overly emphasized. The fouling resistance and cleaning efficiency of our functionalized membranes can be further improved by optimizing the flow conditions at the boundary layer on the feed side. At high shear rate, the role of hydrodynamic forces with respect to foulant− surface chemical interactions becomes more prominent. In this regime, we propose that the membrane surface would be rendered more “slippery” by thwarting surface dehydration, thus reducing opportunities for surface adhesion.18 This phenomenon would be even more conspicuous for superhydrophilic membranes. We also observed that CEOP plays an important role in flux decline in FO, when the reverse draw solute accumulates within the fouling layer. This phenomenon becomes less important when the membrane solute rejection is higher. Therefore, surface functionalizations should be conceived with the goal to maintain a low membrane permeability to salt, that is, a low solute permeability (B) coefficient. Finally, our results confirm the capability of AFM intermolecular forces to predict the membrane fouling behavior. In general, when the average work of adhesion is plotted against the loss in water flux due to fouling, a positive correlation exists between these two parameters for both foulant−membrane and foulant−foulant measurements using the three types of model organic foulants (SI Figure S7). In particular, the energies measured for foulant−foulant experiments scale well with the fouling rate.

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ASSOCIATED CONTENT

S Supporting Information *

Summary of the surface physicochemical properties of the functionalized membranes (Figure S1); analysis of the surface of the functionalized membranes after subjecting the membranes to physical and chemical stresses for the evaluation of the strength of the nanoparticle-surface interaction/binding (Figure S2); FO and RO fouling curves (Figure S3); adhesion force and rupture distance measurements for the interactions between a carboxyl-modified latex particle and membranes by AFM contact mode, showing similar trends as those observed using fouled AFM tips (Figure S4); rupture distance measurements of foulant−membrane and foulant−foulant interactions by AFM contact mode (Figure S5); representative force vs distance curve for alginate-membrane interaction using control polyamide membranes (Figure S6); plots of the water flux losses in RO and FO against the work of adhesion measured by AFM contact mode, showing correlation between fouling rate and the interaction energy among the foulants and the membranes used in this study (Figure S7).This material is available free of charge via the Internet at http://pubs.acs.org. 11142

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1 (203) 432-2789; fax: +1 (203) 432-4387; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication is based on work supported by Award No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST). We also acknowledge the NWRIAMTA Fellowship for Membrane Technology awarded to Alberto Tiraferri.

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