Bidirectional Diffusion of Ammonium and Sodium Cations in Forward

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Bidirectional Diffusion of Ammonium and Sodium Cations in Forward Osmosis: Role of Membrane Active Layer Surface Chemistry and Charge Xinglin Lu,† Chanhee Boo,‡ 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: Systematic fundamental understanding of mass transport in osmosis-driven membrane processes is important for further development of this emerging technology. In this work, we investigate the role of membrane surface chemistry and charge on bidirectional solute diffusion in forward osmosis (FO). In particular, bidirectional diffusion of ammonium (NH4+) and sodium (Na+) is examined using FO membranes with different materials and surface charge characteristics. Using an ammonium bicarbonate (NH4HCO3) draw solution, we observe dramatically enhanced cation fluxes with sodium chloride feed solution compared to that with deionized water feed solution for thinfilm composite (TFC) FO membrane. However, the bidirectional diffusion of cations does not change, regardless of the type of feed solution, for cellulose triacetate (CTA) FO membrane. We relate this phenomenon to the membrane fixed surface charge by employing different feed solution pH to foster different protonation conditions for the carboxyl groups on the TFC membrane surface. Membrane surface modification is also carried out with the TFC membrane using ethylenediamine to alter carboxyl groups into amine groups. The modified TFC membrane, with less negatively charged groups, exhibits a significant decrease in the bidirectional diffusion of cations under the same conditions employed with the pristine TFC membrane. Based on our experimental observations, we propose Donnan dialysis as a mechanism responsible for enhanced bidirectional diffusion of cations in TFC membranes.

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process.22−24 TFC FO membranes showed higher water permeability and salt rejection as well as a wider pH operation range than the CTA FO membrane, thus further accelerating the growth and applications of ODMP.25−27 At the same time, significant efforts have been made in the development of draw solutes for various potential applications that can produce high osmotic pressure with low reverse draw permeation.28 Among those potential draw solutes, the thermolytic ammonia−carbon dioxide (NH3−CO2) draw solution has been investigated at the lab scale29,30 and more recently demonstrated in a pilot-scale desalination system.5 As a unique property of ODMP, bidirectional diffusion of solutesreverse diffusion of draw solutes and forward diffusion of feed solutesis detrimental to OMDP due to loss of draw solutes,31 acceleration of water flux decline by fouling,32 and decrease in product water quality.33,34 Previous studies35−39 investigated bidirectional ion diffusion in ODMP with the

INTRODUCTION Membrane technologies provide promising solutions to address the global challenge for adequate and safe water.1−3 As an emerging membrane technology, osmosis-driven membrane processes (ODMP), such as forward osmosis (FO) and pressure retarded osmosis (PRO), have been gradually developed in the past decade. Because of their low fouling propensity,4 ODMP have potential applications in treatment of a variety of high fouling potential source waters, including desalination of high salinity brines from shale gas produced water,5−9 municipal wastewater reclamation,10−16 and dilution of brine from reverse osmosis (RO) plants.17−19 Previous efforts on the development of ODMP have mainly focused on two technical obstacles: fabrication of high performance membranes and selection of appropriate draw solutes to meet the special needs of ODMP.20 Early studies on ODMP used a commercial cellulose triacetate (CTA) FO membrane fabricated on a polyester woven mesh support, which sparked resurgence in ODMP research and development in the past decade.21 More recently, thin-film composite (TFC) polyamide FO membranes have been successfully fabricated through a carefully tailored support layer fabrication © XXXX American Chemical Society

Received: August 24, 2014 Revised: November 19, 2014 Accepted: November 24, 2014

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before and after surface modification as “pristine TFC” and “modified TFC”, respectively. Prior to the experiments, all membranes were immersed in 25% isopropanol (Fisher Scientific) for 30 min to allow complete wetting.44 The pure water permeability coefficient, A, salt (NaCl) permeability coefficient, B, and structure parameter, S, were determined using an RO and FO characterization method as described in our previous study.22 The determined properties (A, B, and S) of the FO membranes are provided in Table S1 of the Supporting Information (SI). An electrokinetic analyzer (EKA, Brookhaven Instruments, Holtsville, NY) was used to determine the membrane surface zeta potential. Carboxyl group density was measured using toluidine blue O (TBO, technical grade, Sigma-Aldrich) as reported in our earlier publication.43 More details of these membrane surface characterization methods are given in the SI. Draw Solutions. Ammonia−carbon dioxide (NH3−CO2) draw solution was prepared by mixing 1.8 M ammonium bicarbonate (NH4HCO3, Sigma-Aldrich) and 0.2 M ammonium hydroxide (NH4OH, Sigma-Aldrich) in deionized (DI) water (Milli-Q ultrapure water purification system, Millipore, Billerica, MA). A total ammonium salt concentration of 2.0 M was achieved with an ammonia to carbon dioxide molar ratio of 1.1. The measured pH of the NH3−CO2 draw solution was 7.8. Ammonium chloride (NH4Cl) draw solution was also used to investigate the effect of solution pH on the bidirectional cation diffusion. ACS reagent grade ammonium chloride (NH4Cl, Sigma-Aldrich) was dissolved in DI water at a concentration of 2.0 M. The measured pH of the 2.0 M NH4Cl draw solution was 4.5. Measurement of Water Flux and Bidirectional Ion Flux. A laboratory scale crossflow FO unit with channel dimensions of 77 mm × 26 mm × 3 mm was used to measure the water flux and solute flux across the membrane. The experiments were performed using DI water or 0.2 M NaCl as feed solution and NH3−CO2 or NH4Cl as draw solution in FO mode (i.e., draw solution facing the membrane support layer and no applied hydraulic pressure on either side of the cell). All experiments were conducted for 1 h with a crossflow velocity of 21.4 cm/s. Temperatures of both draw and feed solutions were maintained at 25 ± 0.5 °C. Water flux, Jw, across the membrane was measured by recording the increase in draw solution weight at 1 min intervals. The data collected after the water flux had stabilized (always within 10 min) was averaged and used for the calculation of ion flux. Forward ion fluxes, JSforward, were determined by measuring the total concentrations of feed ions (i.e., Na+ and Cl−) in the draw solution after 1 h using

commercial CTA FO membrane. These studies found that ion valence and hydration radius influence the bidirectional diffusion rate of different ions36,37 and suggested electrostatic interactions can impact bidirectional diffusion when nitrate is present in the system.38,39 However, all observed phenomena and proposed mechanisms of bidirectional diffusion in those studies were based on the CTA FO membrane. Using commercial TFC and CTA FO membranes, Coday et al.38 indicated a higher diffusion of cations relative to anions for the TFC membranes, attributing this phenemenon to different surface charge of the support layers of TFC and CTA membranes. More recently, Arena et al.,40 using a TFC RO membrane in FO, have reported enhanced reverse diffusion of ammonium (NH4+) through the membrane when a strong electrolyte (i.e., NaCl) was used as a feed solution. They further observed substantially lower rejection of sodium than chloride in FO, attributing the result to cation exchange between the draw and feed solutions. Given the significance of TFC membranes in ODMP and the advantages of NH4+ based draw solutes, such as NH3−CO2 and concentrated fertilizers,41 a more fundamental understanding of the transport mechanisms in FO with TFC membranes and NH4+ based draw solutions is of significant importance for the future development of the ODMP. Such fundamental understanding can lead to the development of TFC membranes with reduced reverse diffusion of ammonium, which will significantly increase the efficiency of OMDP. The objective of this study is to investigate the bidirectional diffusion of ammonium (NH4+) and sodium (Na+) in FO and to relate this phenomenon to membrane material properties. Water and cation (NH4+ and Na+) fluxes are determined with different membrane materials at different solution pH to confirm the role of membrane surface chemistry and charge in bidirectional diffusion of NH4+ and Na+. Surface chemical modification of the TFC membrane active layer to alter its surface charge characteristics is also carried out to better understand the role of surface charge in bidirectional diffusion of NH4+ and Na+. The results of our experiments are used to elucidate the mechanism for the enhanced reverse transport of NH4+ and the bidirectional diffusion of cations in TFC FO membranes.

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MATERIALS AND METHODS

FO Membranes and Membrane Characterization. A commercial asymmetric cellulose triacetate (CTA) FO membrane was obtained from Hydration Technology Innovation (Albany, OR). This proprietary FO membrane, denoted as CTA in this paper, is thought to be prepared by phase inversion using the same polymer material for the entire membrane. CTA membranes have negligible or no surface charge because the polymer functional groups are converted to ester during the fabrication process.42,43 A high performance thin-film composite (TFC) polyamide FO membrane was acquired from Oasys Water (Oasys Water Inc., Boston, MA). The membrane consists of a dense selective polyamide active layer on top of a porous polysulfone support layer. The surface of the polyamide active layer is abundant with carboxyl groups,43 which renders the membrane surface negatively charged. To better understand the role of membrane surface charge in bidirectional ion diffusion, we modified the functional groups on the TFC membrane surface as described in the next subsection. We denote the TFC FO membrane

C D(VDinitial + Jw A m t ) = JSforward A m t

(1)

where CD is the feed ion concentration in the draw solution after 1 h, Vinitial is the initial volume of the draw solution, Jw is D the average water flux, Am is the membrane area, and t is the time. Similarly, reverse ion fluxes, Jreverse , were determined by S measuring the total concentrations of draw ions (i.e., NH4+ and HCO3−) in the feed solution using C F(VFinitial − Jw A m t ) = JSreverse A m t

(2)

where CF is the draw ion concentration in the feed solution after 1 h and Vinitial is the initial volume of the feed solution. F Concentrations of sodium and chloride ions in the draw solution (i.e., NH3−CO2 or NH4Cl solutions) were quantified B

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using inductively coupled plasma mass spectrometry (ICP-MS; Optima 8000, PerkinElmer, Fremont, CA) and ion chromatography (IC; ICS-3000, Dionex, Sunnyvale, CA), respectively. A 10 mL sample of draw solution was collected after a 1 h FO experiment and boiled until all water evaporated. During the boiling, the dissolved thermolytic ammonium salts decompose into NH3 and CO2 gases. Next, the remaining NaCl was dissolved in a known volume of DI water and the Na+ and Cl− concentrations were measured.29 Note that concentration of Cl− ion was analyzed only for experiments with NH3−CO2 draw solution. Concentrations of total ammonia and total carbon species in the feed solution (DI water or NaCl solution) were measured using the phenate method and total organic carbon (TOC) analyzer (TOC-V, Shimadzu Corp., Japan), respectively. We note that the detectable concentration ranges of Na+ by ICP-MS and Cl− by IC are several orders of magnitude lower than the measured values in this work. Also, the sensitivities of the phenate method for ammonia and the TOC analyzer for carbonate species are less than 1 ppm, indicating the accuracy and reliability of our analytical methods. FO Membrane Surface Modification. In order to alter the surface charge of the pristine TFC membrane, surface modification was carried out using a custom-made mold that isolated the polyamide active layer side of the membrane for the desired reaction. After membrane characterization through the experimental procedures described above, the same membrane coupon was placed on a glass plate with the polyamide active layer facing up. A soft rubber spacer and rectangular frame were then clamped on top of the membrane to isolate the polyamide active surface while in contact with the surface modification solution. Both the rubber spacer and rectangular frame have open windows of 77 mm × 26 mm to ensure that the modification takes place on the same membrane area which was previously characterized in the FO system. The modification reaction was carried out on an orbital shaker (Cole-Parmer, IL) at 60 rpm. For surface modification, the polyamide active layer was first contacted with an aqueous solution of 4 mM 1-ethyl-3-(3dimethlaminopropyl) carbodiimide (EDC, 98%, Thermo Scientific), 10 mM N-hydroxysuccinimide (NHS, 98%, SigmaAldrich), and 0.5 M NaCl (ACS grade, J.T. Baker) buffered at pH 5 using 10 mM MES monohydrate (BioXtra, 99%, SigmaAldrich) for 1 h. This step converted the native carboxyl groups on the polyamide surface into amine-reactive esters. After this step, the polyamide was washed with DI water for three times and subsequently contacted with an ethylenediamine (BioXtra, Sigma-Aldrich) aqueous solution containing 0.15 M NaCl and buffered at pH 7.5 with 10 mM HEPES (Sigma-Aldrich) for 30 min. In this step, ethylenediamine readily binds to the active layer by the formation of amide links with the amine-reactive esters. The membrane surface was then washed three times with DI water to remove unreacted ethylenediamine on the surface. Ethylenediamine concentration was varied from 10 to 100 mM to examine its influence on membrane transport properties and a concentration of 100 mM was finally chosen for further solute transport experiments.

Figure 1. Water flux, and forward and reverse solute fluxes, obtained from experiments with (A) CTA membrane and (B) pristine TFC membrane in FO mode. The experiments were conducted for 1 h using NH3−CO2 (1.8 M NH4HCO3 mixed with 0.2 M NH4OH) as a draw solution at an ambient pH ∼7.8. Two types of feed solutions, DI water (on the left side of the dashed line) and 0.2 M of NaCl (on the right side of the dashed line), at ambient pH ∼5.7 were used. Crossflow velocity was fixed at 21.4 cm/s and temperature was maintained at 25.0 ± 0.5 °C for both feed and draw solutions. Error bars represent one standard deviation.

Figure 1. When DI water was used as a feed solution (left side in both figures), the CTA and TFC membranes exhibited water fluxes of 12.79 ± 0.16 L m−2 h−1 and 40.97 ± 0.16 L m−2 h−1, respectively. The much higher water flux of the TFC membrane is attributed to the much lower structural parameter of the TFC membrane (271 μm) compared to the CTA membrane (573 μm) and to the higher intrinsic pure water permeability of the polyamide material (3.75 L m−2 h−1 bar−1) compared to the cellulose triacetate (0.99 L m−2 h−1 bar−1)45 (Table S1 of SI). After adding 0.2 M NaCl to the feed solution (right side in both figures), both membranes showed a reduced water flux21.3% for the CTA membrane and 35.9% for the TFC membrane due to a decrease in the net osmotic pressure driving force.29,46 Reverse solute fluxes of NH4+ and HCO3−, and forward solute fluxes of Na+ and Cl−, are depicted in Figure 1 as red, orange, blue, and green columns, respectively. For the CTA membrane (Figure 1A), the reverse solute fluxes of NH4+ and HCO3− were 0.86 ± 0.08 and 0.64 ± 0.10 mol m−2 h−1, respectively, when DI water was used as feed solution (left side of Figure 1A), indicating that slightly more NH4+ diffused from the draw solution to the feed solution than HCO3−. As we explained earlier, NH3−CO2 draw solution was prepared by

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RESULTS AND DISCUSSION Effect of Membrane Surface Chemistry on Bidirectional Diffusion of Cations. In this set of experiments, two types of commercial FO membranes (i.e., CTA and TFC) were investigated using NH3−CO2 draw solution. Measured water fluxes for both membranes are presented as black columns in C

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Figure 2. Effect of feed solution pH on bidirectional diffusion of cations with pristine TFC membrane. Water flux, and forward and reverse solute fluxes, were obtained from 1 h FO experiments using 2 M NH4Cl (at ambient solution pH 4.5) as a draw solution and DI water or 0.2 M NaCl at two different solution pH (pH 6.0 and 3.0) as feed solutions. All experiments were conducted with a crossflow velocity of 21.4 cm/s and a constant temperature of 25.0 ± 0.5 °C. Error bars represent one standard deviation.

an enhanced diffusion ratio (EDR) of ions (i.e., NH4+ and HCO3−):

mixing 1.8 M NH4HCO3 and 0.2 M NH4OH. The slightly higher reverse NH4+ diffusion is attributed to the higher total ammonia concentration (2.0 M) than bicarbonate species (1.8 M) in the draw solution. When 0.2 M NaCl was used as a feed solution, we observed slight increases in reverse diffusion of both NH4+ (1.04 ± 0.07 mol m−2 h−1) and HCO3− (0.74 ± 0.07 mol m−2 h−1) compared to those obtained from experiments with DI water feed solution. We note that our data are comparable to previous studies using the same type of membrane.47 Concomitantly, for the CTA membrane, the forward fluxes of Na+ and Cl− in the experiment with 0.2 M NaCl feed solution were 1.02 ± 0.02 and 0.24 ± 0.02 mol m−2 h−1, respectively. We attribute this observation to the higher reverse diffusion of NH4+, which is balanced by the forward diffusion of cations from the feed solution to the draw solution. In our case, two cations (i.e., Na+ and H+) are present in the feed solution. However, due to the much higher Na+ concentration than H+ in the feed solution, transport of Na+ plays a dominant role in balancing the solution charge, which is manifested as enhanced Na+ transport. Reverse and forward solute fluxes obtained from FO experiments with the TFC membrane are presented in Figure 1B. Although the TFC membrane exhibited a much higher water flux than the CTA membrane, the reverse solute fluxes of NH4+ and HCO3− were comparable for both the CTA and TFC membranes when DI water was used as a feed solution. However, a substantial increase in reverse NH4+ flux was observed with the TFC membrane when a 0.2 M NaCl feed solution was employed (right side of Figure 1B). We also observed a significantly higher forward Na+ flux compared to that obtained with the CTA membrane. Interestingly, there were only slight changes in the rates of anion diffusion, regardless of the types of feed solution and membranes used. The results of bidirectional ion diffusion with the TFC membrane indicate that with NaCl feed solution the cations and anions diffused through the membrane with an unequalmolar proportion. In order to evaluate the effect of feed solution on draw solute reverse diffusion in our study, we define

EDR ion =

RSFNaCl RSFDI

(3)

where RSFDI and RSFNaCl are the reverse solute fluxes of a specific ion when DI water and 0.2 M NaCl were used as feed solutions, respectively. The CTA membrane exhibited EDRNH4+ and EDRHCO3− of 1.22 ± 0.06 and 1.17 ± 0.09, respectively, indicating that NaCl feed solution has a slight influence on the transport of NH4+ and HCO3− for the CTA membrane. On the other hand, EDRNH4+ and EDRHCO3− were 7.49 ± 0.24 and 1.85 ± 0.24 for the TFC membrane, implying a dramatic increase in NH4+ diffusion, but a relatively mild enhancement in HCO3−. We note that previous publications38,40 also reported a similar phenomenon with commercial TFC membranes under conditions similar to those used in this study. Taken together, these results confirm that the presence of Na+ in the feed solution could significantly enhance the reverse transport of NH4+ for TFC polyamide membranes. The most important difference between the CTA and TFC membranes relevant to our study is the active layer surface chemistry. The CTA membrane is made of cellulose triacetate, which contains acetyl and hydroxyl functional groups. Since these functional groups do not dissociate under the pH range investigated, the membrane active layer does not possess fixed surface charge.48,49 On the other hand, the TFC membrane consists of a thin polyamide active layer, which is formed via interfacial polymerization between amine containing monomer and acyl chloride on top of a microporous support. After the fabrication, unreacted acyl chloride groups hydrolyze into carboxyl groups, which dominate the surface functional groups of the TFC membrane.48,50 Therefore, the enhanced bidirectional diffusion of NH4+ and Na+ with the TFC membrane might be attributed to the presence of carboxyl groups. Role of Membrane Surface Charge in Bidirectional Diffusion of Cations. To further unravel the mechanism for the enhanced bidirectional diffusion of cations with the TFC membrane, we performed a set of experiments using 2.0 M D

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those of the pristine membrane (P-value >0.05), suggesting that surface modification did not significantly affect the membrane transport properties. We selected ethylenediamine concentration of 100 mM to maximize the efficiency of the surface modification.

NH4Cl as a draw solution and 0.2 M NaCl at two different pH (pH 3.0 and 6.0) as feed solutions (Figure 2). The water fluxes (black column) were 47.06 ± 2.21 and 35.81 ± 0.66 L m−2 h−1 when using DI water (ambient pH of 5.8) and 0.2 M NaCl (ambient pH 6.0) as feed solutions, respectively. After adjusting the feed solution (0.2 M NaCl) to pH 3.0, the water flux was 34.66 ± 2.04 L m−2 h−1, indicating that the feed solution pH does not affect water transport for the TFC membrane. When using NH4Cl as the draw solution and DI water as the feed solution (left side of Figure 2), the reverse NH4+ flux was 0.68 ± 0.08 mol m−2 h−1, which is comparable to that obtained with NH4HCO3 draw solution (0.71 ± 0.04 mol m−2 h−1, Figure 1). In this case, the transport of NH4+ is determined by the concentration difference between the draw and feed solutions. When a feed solution of 0.2 M NaCl was used at pH 6.0 (middle of Figure 2), we observed a similar trend to that observed with the NH3−CO2 draw solution (Figure 1), namely, substantial increases in NH4+ (3.57 ± 0.04 mol m−2 h−1) and Na+ (2.10 ± 0.02 mol m−2 h−1) bidirectional diffusion. As evidenced by the EDRNH4+ of 5.32 ± 0.60 in this case, the enhanced transport of NH4+ also takes place when using a different NH4+ based draw solution with a different anion. After adjusting the feed solution (0.2 M NaCl) to pH 3.0, bidirectional diffusion of NH4+ and Na+ dramatically decreased compared to that at pH 6.0, as shown in the right side of Figure 2. The calculated EDRNH4+ in this case is about 1.90 ± 0.01, indicating there was only a mild increase in reverse NH4+ flux with 0.2 M NaCl feed solution at pH 3.0. Since all other experimental conditions were identical, the sharp decrease in bidirectional diffusion of NH4+ and Na+ could only be attributed to the change of feed solution pH. For the TFC polyamide membrane, the surface is abundant in carboxyl groups, which have a pKa ∼ 5.2.43,51 Therefore, under an ambient solution pH (pH ∼ 6.0), deprotonated carboxylate groups (COO−) dominate the surface charge (illustrated in the middle of Figure 2), resulting in a surface with negatively fixed charge. On the other hand, at pH 3.0, protonated carboxylic groups (COOH) dominate (illustrated in the right side of Figure 2), rendering the membrane surface less negative. Therefore, different bidirectional transport behaviors of cations under different solution pH are attributed to the fixed surface charge. Deprotonated carboxyl groups under higher feed solution pH makes the membrane surface more negatively charged, resulting in the increase of bidirectional diffusions of NH4+ and Na+. Reduced Bidirectional Diffusion of Cations by Surface Charge Modification. The TFC membrane negative fixed surface charge is attributed to the presence of inherent carboxyl groups. These carboxyl functional groups can be exploited as reactive sites for surface modification. To verify the role of carboxyl groups and fixed surface charge in bidirectional diffusion of cations, we modified the membrane surface with ethylenediamine using an EDC−NHS coupling reaction. The surface modification converts carboxyl groups into amine groups, thereby decreasing the negative fixed charge of the membrane surface. The pristine TFC membranes were first modified using ethylenediamine at different concentrations (10, 50, and 100 mM) to examine whether surface modification alters the membrane transport properties (SI Figure S1). After surface modification, the modified membranes showed comparable values of both water permeability, A, and salt permeability, B, as

Figure 3. Water and solute fluxes obtained from experiments with modified TFC membrane in FO mode. The experiments were conducted for 1 h using NH3−CO2 (1.8 M NH4HCO3 mixed with 0.2 M NH4OH) as a draw solution at ambient pH ∼7.8. Two types of feed solutions, DI water (on the left side of the dashed line) and 0.2 M NaCl (on the right side of the dashed line), at ambient pH ∼5.7 were used. All the experiments were conducted with a crossflow velocity of 21.4 cm/s and a constant temperature of 25.0 ± 0.5 °C. Error bars represent one standard deviation.

The water fluxes with the modified TFC membrane (Figure 3) were 37.26 ± 1.90 and 23.78 ± 0.61 L m−2 h−1 with DI water and 0.2 M NaCl feed solutions, respectively, which were comparable to the results obtained with the pristine TFC membrane (Figure 1B) with the same draw solution concentration (1.8 M NH 4 HCO 3 mixed with 0.2 M NH4OH). The reverse NH4+ flux with 0.2 M NaCl feed solution was still higher (3.62 ± 0.47 mol m−2 h−1) compared to that with DI water feed solution for the modified TFC membrane, but much less pronounced than with the pristine TFC membrane (5.32 ± 0.15 mol m−2 h−1 for the pristine TFC). The calculated EDRNH4+ for the modified TFC membrane (3.98 ± 1.11) was also 46.9% lower than that for the pristine TFC membrane (7.49 ± 0.24). With the reduced reverse NH4+ flux for the modified TFC, a concomitant decrease in forward Na+ flux was also observed, because less Na+ ions were needed to balance the solution charge on the draw side. On the other hand, the reverse transport of HCO3− was comparable for the pristine and modified TFC, implying that surface modification did not affect the transport of anions. In our surface modification, EDC and NHS convert carboxyl groups into intermediate amine-reactive esters,52 which form covalent bond with one end of the amine group on ethylenediamine, leaving the other end of the amine group dangling on the surface. Hence, the modified membrane surface is expected to possess less negative surface charge. Surface charge of the TFC membrane before and after modification was evaluated using two techniques, namely the toluidine blue O (TBO) method43 and zeta potential measurement.49,53 The TBO results (SI Table S2) showed a decrease in the density of E

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carboxyl groups after surface modification (21.0 ± 0.9 vs 18.5 ± 1.0 nm−2 for the pristine and modified TFC, respectively). The zeta potential (Figure 4) of the modified TFC membrane

conferring the membrane a cation exchange character.55,56 We propose Donnan dialysis as a main mechanism responsible for the enhanced bidirectional diffusion of cations in TFC membranes as schematically depicted in Figure 5.

Figure 4. Zeta potentials of pristine and modified thin-film composite (TFC) membranes as a function of solution pH. Measurements were performed at room temperature (23 °C) in the presence of 1 mM KCl as a background electrolyte solution. The solution pH was adjusted using aliquots of HCl and KOH. The estimated isoelectric points (IEP) of the membranes are indicated.

shifted upward compared to that of the pristine TFC, resulting in an increase of the isoelectric point (IEP) from 3.5 for the pristine to 4.2 for the modified TFC membrane. The above results indicate that both carboxyl group density and zeta potential of the TFC membrane did not change substantially after surface modification, which might be ascribed to limitations of both surface charge characterization methods. TBO method quantifies the surface charge density by utilizing electrostatic interaction between the cationic TBO molecule and the negatively charged functional groups on the membrane surface.43 However, the ionic TBO molecule is hydrated and has a relatively large molecular weight (∼270 g/mol), and thus cannot interact effectively with carboxyl groups embedded inside the small pores of the TFC membrane due to steric exclusion.43 Also, membrane surface zeta potential indicates the surface charge behavior, but not the exact fixed surface charge.49,54 In addition, our surface modification is likely limited to converting carboxyl groups mainly on the surface but less inside the pores of the TFC membrane because of the relatively large molecular size of EDC and NHS (191.70 and 115.09 g/mol, respectively). However, despite the limitations mentioned above, we still observed a qualitative decrease in the negative surface charge density after surface modification, which confirmed the significant role of carboxyl groups and surface charge of TFC membranes in the bidirectional diffusion of NH4+ and Na+. Mechanism for Enhanced Cation Bidirectional Diffusion in TFC Membranes. According to our results, the transport of cations (i.e., NH4+ and Na+) was dramatically enhanced with a more negatively charged membrane surface, while anions (i.e., HCO3− and Cl−) exhibited only a slight enhancement in transport. Specifically, the results suggest that negatively charged carboxyl groups on the TFC polyamide surface play a significant role in the bidirectional diffusion of cations. The TFC membrane surface is abundant in carboxyl groups, which could serve as fixed ionic groups, thereby

Figure 5. Schematic of the bidirectional diffusion of ions (Red: NH4+, Orange: HCO3−, Blue: Na+, Green: Cl−) for the TFC membrane in FO. The arrows represent diffusion of ions corresponding to the ions with same color. (A) Initial diffusion of NH4+. (B) Potential diffusions for solution charge balance. (C) Cation exchange phenomenon for the bidirectional diffusion of cations.

In the FO experiments, NH4+ readily diffuses through the TFC membrane from the draw solution to the feed solution due to its high electrostatic attraction to the negatively charged polyamide surface, the high NH4+concentration in the draw solution side, and the relatively small hydrated radius (0.33 nm57) of NH4+ (Figure 5A). This initial NH4+ diffusion creates negative and positive charge potentials in the draw and feed solutions, respectively. To maintain solution electroneutrality, two scenarios are possible (Figure 5B): (i) coupled anion (i.e., HCO3−) diffusion from draw to feed sides and (ii) forward Na+ diffusion from feed to draw sides. However, electrostatic repulsion generated by the negatively charged fixed surface sites hinders the coupled transport of anions. On the other hand, cations, namely Na+, readily adsorb onto the negatively charged surface and then diffuse through the polyamide layer, which explains the enhanced bidirectional diffusion of cations for the TFC membrane (Figure 5C). Implications for the Development of FO Membranes. In this work, we have demonstrated the significant role of membrane surface chemistry and charge in determining the bidirectional diffusion of cations, thereby enhancing our understanding of ion transport mechanisms in the FO process. In addition, we have shown that surface charge modification of the TFC membrane effectively improved the membrane selectivity for cations. Varying the surface modification conditions can further optimize the membrane surface functional groups and surface charge, thus improving TFC FO membrane performance. Robust membranes with miniF

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mized reverse and forward transport of ions are critical for the efficient application of the FO process in treating a variety of source waters. Our results further underscore the unique requirements for TFC membranes for the FO process compared to TFC membranes for pressure-driven membrane processes (i.e., RO and nanofiltration).

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ASSOCIATED CONTENT

S Supporting Information *

Details on the membrane surface characterization (zeta potential and TBO method) (S1); effect of ethylenediamine concentration on membrane transport properties (Figure S1); transport properties of pristine membranes (Table S1); carboxyl group density of TFC membranes (Table S2). This material is available free of charge via the Internet at http:// pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*(J.M.) Phone: +86 451 86283010; fax: +86 451 86283010; email: [email protected]. *(M.E.) Phone: +1 203 432 2789; fax: +1 203 432 4387; email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support received from the National Science Foundation under Award Number CBET 1232619 and the Funds for Creative Research Groups of China (Grant No. 51121062). We thank Oasys Water Inc. for providing the pristine TFC membranes and Dr. Helmut Ernstberger (Yale University) for assistance with use of the ion chromatography and inductively coupled plasma mass spectrometry. We also acknowledge a graduate fellowship (to X.L.) made possible by the China Scholarship Council (CSC), and the use of facilities supported by YINQE and NSF MRSEC DMR 1119826.

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REFERENCES

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