Controlled TiO2 Growth on Reverse Osmosis and Nanofiltration

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Controlled TiO2 Growth on Reverse Osmosis and Nanofiltration Membranes by Atomic Layer Deposition: Mechanisms and Potential Applications Xuechen Zhou,†,‡,∥ Yang-Ying Zhao,†,§,∥ Sang-Ryoung Kim,† Menachem Elimelech,†,‡ Shu Hu,*,† and Jae-Hong Kim*,†,‡ †

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States Nanosystems Engineering Research Center for Nanotechnology Enabled Water Treatment (NEWT), Yale University, New Haven, Connecticut 06520, United States § State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China

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‡

S Supporting Information *

ABSTRACT: Enhancing the chemical and physical properties of the polyamide active layer of thin-film composite (TFC) membranes by surface coating is a goal long-pursued. Atomic layer deposition (ALD) has been proposed as an innovative approach to deposit chemically robust metal oxides onto membrane surfaces due to its unique capability to control coating conformality and thickness with atomic scale precision. This study examined the potential to coat the surface of TFC reverse osmosis (RO) and nanofiltration (NF) membranes via ALD of TiO2. Our results suggest that the optimal ALD conditions, the film growth kinetics, and the depth of deposition are different for RO and NF membranes due to the different diffusive transport of ALD precursors through the membrane pores. The TiO2 coating mainly located at the surface of the RO membrane; in contrast, the TiO2 coating extended to the depth of the NF membrane. The TiO2 coating degraded membrane water permeability and salt rejection beyond 10 cycles of ALD, the condition commonly employed in previous ALD-based membrane modification studies. Instead, this study showed that with fewer than 10 cycles, the TiO2 coating of RO membrane increased the membrane surface charge without negatively impacting water permeability and salt rejection. For the NF membranes, the coating of TiO2 inside their pores led to the tuning of pore sizes and increased the rejection of selected solutes.

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INTRODUCTION Atomic layer deposition (ALD) is an emerging chemical deposition technique that enables ultrathin coating of a substrate surface with various materials (e.g., metal oxides). An atomic-scale coating is achieved in one ALD cycle by chemisorbing the first precursors to the target substrate and subsequently chemisorbing the second precursors onto the first surface-bound precursors; this cycle is repeated until the desired film thickness is achieved.1 The capability of depositing an atomic layer each ALD cycle allows the precise control of the film conformality across the substrate surface and the coating growth rate (i.e., the growth per cycle). ALD has been particularly preferred to conventional coating techniques such as chemical vapor deposition when uniform coating over a nonflat surface is intended.1 Accordingly, an opportunity to improve the chemical resistance and the hydrophilicity of polymeric membranes (i.e., substrates with porous surface) by coating them with inorganics such as metal oxides by ALD has been recognized.2 Past ALD studies mostly focused on modifying the surface property of microfiltration (MF) and ultrafiltration (UF) © XXXX American Chemical Society

membranes made of hydrophobic polymers. Uniformly coating not only the top surface but also the inner-pore surface with aluminum oxide or titanium oxide, and consequently shielding the exposed polymer surface, were found to improve the water permeability, rejection capability, and fouling resistance of these membranes.3−8 A similar rationale was used in past attempts9,10 that applied ALD coating onto polyamide (PA) thin-film composite (TFC) membranes (i.e., the benchmark architecture for commercial reverse osmosis (RO) and nanofiltration (NF) membranes11). However, unlike MF and UF membranes whose pore sizes are much larger than the ALD coating dimension, the small pore sizes of RO and NF (18.2 MΩ·cm) produced from a Milli-Q ultrapure water purification system (Integral 10, Millipore, Billerica, MA) was used to prepare all the experimental solutions. ALD Condition and Procedure. As-received membranes were first immersed in 25% (v/v) isopropyl alcohol solution for 30 min to remove any extractable components and preservatives that are soluble in solvents,16 after which the membranes were washed thoroughly with DI water and dried in vacuum (∼200 Torr) for 8 h at 60 °C. The membrane samples were then loaded into the ALD reaction chamber (Fiji G2, Veeco-CNT; Figure S2), preheated at 100 °C for 30 min under vacuum (∼0.1 Torr). During the ALD, both membrane samples and reaction chamber were kept at 100 °C.17 Each ALD cycle (see Text S1 for details) consisted of (1) pulsing the TDMAT vapor preheated at 75 °C into ultrahigh purity Ar (99.999997%) gas for varying durations to control the amount of precursor delivered to the reaction chamber, (2) allowing TDMAT to react with the membrane surface in the closed chamber for a predetermined exposure time, (3) purging the reaction chamber with Ar gas for 60 s to remove remaining precursors, and (4) repeating the same sequence (steps (1) to (3)) using H2O instead of TDMAT. This cycle was repeated for different numbers of times (i.e., the number of cycles). TiO2 Growth Characterization. Energy dispersive X-ray spectroscopy (EDS; XFlash 5060FQ, Bruker, Billerica, MA) attached to a field emission scanning electron microscope (FESEM; SU8230, Hitachi, Japan) was employed to measure the total amount of Ti atoms, which is proportional to the Ti EDS intensity (Figure S3).18 The TiO2 growth on the top surface of both membranes was monitored by X-ray photo-

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MATERIALS AND METHODS Materials. Commercial RO (SW30 XLE; 99.5% NaCl rejection according to manufacturer’s specification) and NF (NF270; 99.2% MgSO4 rejection) membranes were purchased from Dow Chemical. These membranes consist of three polymer layers: a thin PA active layer (SW30 with fully aromatic PA synthesized using m-phenylenediamine and B

DOI: 10.1021/acs.est.8b03967 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 2. Normalized Ti Kα peak area in SEM-EDS spectra for SW30 and NF270 membranes as a function of (A) TDMAT pulse time (TDMAT exposure time fixed at 60 s) and (B) TDMAT exposure time (TDMAT pulse time fixed at 1.5 s). For both cases, the number of deposition cycles was fixed at 50. Acceleration voltage was 10 kV and working distance was 14.2 ± 0.4 mm. Each spectrum were acquired over a spot on the membrane surface of approximately 300 × 200 μm2 for 60 s. Error bars represent the triplicate measurements. Data without normalization can be found in Figure S8.

Figure 1) involves the chemisorption of TDMAT onto the PA active layer of the membrane. Carboxyl terminal functional groups of PA, formed during TFC membrane fabrication through the hydrolysis of unreacted acyl chloride groups after interfacial polymerization,22 are considered to serve as the sites to bond with Ti in TDMAT precursors (with dimethylamine as a leaving group).23 The abundance of carboxyl groups on the PA material, estimated to be more than 1 × 1014 per cm2 of PA surface,24 and the lack of hydroxyl groups (i.e., default sites in most TDMAT-based ALDs) support this premise. The TDMAT reacts with only these target sites; the unreacted TDMAT is purged by Ar such that a single molecular Ti layer forms in a self-terminating manner. The subsequently dosed H2O reacts with residual dimethylamino groups on the surface to form hydroxyl groups, which serve as the sites to bind with TDMAT in subsequent cycles. The reactions for the rest of cycles thus can be described as follows:25

electron spectroscopy (XPS; VersaProbe II, Physical Electronics, Chanhassen, MN) using Al Kα radiation (hν = 1486.6 eV). XPS Ar-beam sputtering depth profile was employed to examine the TiO2 growth within the active layer subsurface (see Figure S4 for details). The deposited TiO2 layer was then visualized by transmission electron microscopy (TEM; Tecnai Osiris, FEI, Hillsboro, OR) at 200 kV. Samples for TEM analysis were prepared by first embedding membranes in epoxy resin for 12 h at 60 °C and then cutting them into thin sections (∼100 nm), using a diamond knife mounted on the ultramicrotome (EM UC7, Leica Microsystem, Germany). Membrane Characterization. The fraction of linear portion of PA was estimated using O/N ratio (determined by XPS; Text S2).19 The zeta potential of the membrane surface before and after modification was determined using a streaming potential analyzer (EKA, Brookhaven Instruments, Holtsville, NY). The measurements were conducted with an electrolyte containing 1 mM KCl and 0.1 mM KHCO3 at pH 7.0. The filtration performance was evaluated using a laboratory-scale crossflow filtration system (Figure S5). Before testing, membranes were first soaked in pure ethanol for flux recovery (Figure S6) and then washed thoroughly with DI water.20 SW30 membranes (filtration area of 20 cm2) were first compacted for over 8 h under the pressure of 31.8 bar (460 psi), and the filtration was performed at the reduced pressure of 27.6 bar (400 psi) to determine water permeability and NaCl rejection (50 mM). NF270 membranes were compacted under 8.3 bar (120 psi) for 3 h and then water permeability and solute rejection were measured under the pressure of 6.9 bar (100 psi). Both symmetric (NaCl) and asymmetric (CaCl2 and Na2SO4) salts at different concentrations (2 and 20 mM) were employed as target solutes. The salt concentrations in feed and permeate were monitored by conductivity measurement. The average pore size of NF membranes was estimated by the rejection of neutrally charged organic molecules (erythritol, xylose, and dextrose; Text S3).21

TiOH* + Ti(N(CH3)2 )4 → TiOTi(N(CH3)2 )3* + HN(CH3)2

(1)

TiN(CH3)2* + H 2O → TiOH* + HN(CH3)2

(2)

where the asterisks denote the surface species. Since each cycle involves the reactive transport of the precursors to the target active sites inside the membrane pores, the amount of precursors and exposure time affect the growth rate and mass loading of ALD TiO2 over membranes. The TDMAT pulse time and exposure time were first optimized by fixing the number of ALD cycles at 50. While these two variables are not completely independent (e.g., a longer pulse time should translate into a shorter necessary exposure time), they are two critical variables that can be independently controlled during the ALD process. Sufficient H2O pulse and exposure durations were provided, 0.60 and 60 s, respectively, to ensure that H2O is not a limiting factor (see Figure S7 and Text S5 for analysis). The total amount of Ti deposited, determined using EDS (i.e., 1−3 μm penetration depth by 10 keV electron beam26), initially increased and reached a plateau as the TDMAT pulse time was increased from 0 to 2.5 s (Figure 2A). Note that the TDMAT dose was controlled by pulse duration not the partial pressure (Figure S9). A similar trend was observed when the TDMAT exposure

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RESULTS AND DISCUSSION Optimization of Deposition Condition. The first ALD cycle (i.e., on the pristine carboxyl-terminated membrane; C

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Figure 3. Surface Ti percentage obtained from XPS measurement of SW30 and NF270 membranes deposited with TiO2 under varying (A) TDMAT pulse times and (B) TDMAT exposure times. Error bars represent the duplicate measurements. XPS depth profile of (C) SW30 and (D) NF270 membranes coated on different conditions. The normalized intensity for each point was estimated from the integration of the peak areas. Al Kα radiation (hν = 1486.6 eV) was used as an X-ray source. The spectra were shifted using adventitious C 1s (C−C peak) at 284.8 eV as the internal reference to correct for charging. For depth profiling, an Ar ion beam was used in the Zalar mode to raster over an area over 2 × 2 mm2 on the membrane surface. Detailed experimental condition and original spectra of depth profile can be found in Figure S4. The analysis of nitrogen and sulfur spectra can be found in Text S6.

thickness.27,28 Overdosing by extended pulse time or exposure time can be also avoided, considering the precursor cost and overall process time.29 ALD Growth Kinetics on Different Substrates. The amount of TiO2 deposited only at the surface measured using XPS (i.e., average analysis depth at approximately 5 nm26), in contrast to the aforementioned total TiO2 loading measured by EDS, appeared similar between SW30 and NF270. As more TDMAT was dosed into the reaction chamber (i.e., longer pulse time), the amount of TiO2 deposited on the top surface of both SW30 and NF270 first increased and then reached a plateau (i.e., saturation) at the TDMAT pulse time of 1.0 s (Figure 3A). The effect of exposure time was also similar between SW30 and NF270, both reaching the saturation within a very short exposure time of 5 s (Figure 3B). It is noteworthy that the TiO2 growth curves as a function of both TDMAT pulse time and exposure time were the same for SW30 whether the Ti was measured across the depth (Figure 2) or on the surface (Figure 3). In marked contrast, a significant difference was observed with NF270: the saturation of TiO2 surface coverage was reached with much shorter pulse time (1.0 s) and exposure time (5 s; Figure 3) compared to the saturation of all the active sites across the depth (1.5 and 20 s, respectively; Figure 2). This indicates that the kinetics of TiO2

time was increased from 5 to 60 s (Figure 2B). The initial phase with the increase in Ti indicates that not all the active sites were consumed when less than a sufficient amount of TDMAT or less than the required reaction (exposure) time were provided. The plateau phase without further Ti deposition reflects the self-limiting nature of the ALD process; when all the active sites are fully saturated at each cycle, TiO2 loading will not be further increased by increasing either the Ti precursor dose or the reaction time for the fixed number of ALD cycles. Note that a long purge time (60 s) ensured the removal of all the unreacted precursors. The optimal ALD condition in terms of TDMAT pulse time and exposure time differed between SW30 and NF270. The SW30 membrane was saturated by a lower TDMAT pulse time of 1.0 s, compared to 1.5 s for NF270 (Figure 2A). Likewise, SW30 was saturated with the exposure time of 10 s, much shorter than the 20 s needed for NF270 (Figure 2B). On the basis of these observations, the optimal TDMAT pulse time was determined to be 1.5 s for both membranes, and the TDMAT exposure time to be 20 and 40 s for SW30 and NF270, respectively. Operating ALD under this saturation condition would ensure not only the full consumption of all the active sites at each cycle (i.e., coating uniformity) but also the reproducibility of the ALD and precise control of film D

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Figure 4. HAADF TEM cross-section images of (A) SW30 and (D) NF270 membranes. Corresponding EDS elemental mappings of (B) SW30 and (E) NF270 membranes. Scale bars are 50 nm. TEM cross-section images of (C) SW30 and (F) NF270. Scale bars are 20 nm. All membranes were coated with 50 cycles of TiO2. Sectioned samples were visualized with an accelerating voltage of 200 kV. Crack observed in the middle of TiO2 layer on NF270 is ascribed to defect formation during microtome processing.46

layer of RO membrane consists of approximately 53% of the network free volume (i.e., small spaces between polymer segments constituting the polymer aggregate) with the diameter of 0.28−0.46 nm, and the aggregate free volume (i.e., large spaces surrounding the polymer aggregates) with the diameter of 0.70−0.82 nm.19 The former provides the route for water/solute transport, but is inaccessible to TDMAT, suggesting that the TiO2 growth would be mainly confined to the top surface of SW30. On the contrary, the pore size of NF270 is broadly distributed12,32 with the average at around 0.82 nm (measured in this study; Table S1). The presence of a fraction of pores with much larger diameter (continuous free volume in the case of NF270) than that of TDMAT suggests that TiO2 growth can extend inside the active layer. Visualization of Titanium Dioxide Growth. The successful growth of uniform TiO2 layer on the surface of both SW30 and NF270 under the optimized deposition condition was confirmed by the cross-sectional TEM images. A ridge-and-valley morphology (Figure 4A) and a flat surface (Figure 4D), typical of RO and NF TFC membranes, respectively, were observed.33 Bright and uniform layers were noted on both ALD-treated samples; an EDS elemental mapping (Figure 4B and E), where Ti on the PA active layer was color-contrasted against sulfur in PSF support layer, confirmed these layers as TiO2 coating. TEM cross-sectional images (Figure 4C and F) provided a closer look at the difference in coating layers between SW30 and NF270. Although the average thickness of the PA layer containing TiO2 was ∼200 nm in SW30 (i.e., including the ridge-and-valley structure) which is 20 times thicker than the TiO2 layer in NF270 (∼10 nm) according to TEM images, the actual thickness of the TiO2 layer was thinner in SW30 than NF270, possibly due to the aforementioned differences in pore

growth below the top membrane surface are slower for NF270 compared to SW30.30 A difference was also noticed when the distribution of TiO2 loading was examined as a function of depth from the surface using an XPS depth profiling. Consecutive XPS analyses with increasing sputtering time were performed after removing the deposited TiO2 on membrane top surface by an argon beam after each XPS acquisition; but it should be noted that the increased sputtering time would roughly correlate with the physical depth from the top surface and should be interpreted only as a relative, not a quantitative, measure. Results obtained for SW30 coated with TiO2 under three different conditions (pulse time of 1.0 or 1.5 s and exposure time of 5 or 60 s), shown in Figure 3C, suggest that Ti deposition was not confined to the top surface but spanned the subsurface with a sharp decline near the surface. The Ti signal at high sputtering time likely resulted from the TiO2 deposited on the surface of PA that forms a ridge-and-valley structure. These distributions were the same regardless of ALD conditions, suggesting that the same degree of subsurface TiO2 deposition would be achieved as long as the top surface was saturated. In contrast, the Ti intensity depth profile in NF270 (Figure 3D) was dependent on pulse and exposure times. With either shorter pulse time (1.0 s) or shorter exposure time (5 s), the profile exhibited a shaper decline from the surface, indicating that the active nucleation sites were not saturated. In other words, a higher precursor dosage and/or a longer reaction time would be required for saturation of the subsurface growth for NF270 compared to SW30. The above differences between SW30 and NF270 are likely to result from the difference in the pore diameter (free volume) of the PA active layer in comparison to the size of TDMAT molecule (kinetic diameter of 0.70 nm31). The active E

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Figure 5. (A) Ti Kα peak area in SEM-EDS spectra as a function of the number of ALD cycles for SW30 and NF270. The intensity was normalized so that the slope in the linear stage (after 40 cycles) equals 1 for both membranes to exclude the issue of different top surface area. (B) The amplification of the first 40 cycles in (A). One ALD cycle on SW30 was composed of 1.5, 20, 60, 0.24, 20, and 60 s with the sequence of TDMAT pulse, exposure, purge, H2O pulse, exposure, and purge. The exposure time was 40 s for both precursors for NF270. Error bars represent the triplicate measurements.

Figure 6. Schematic illustration of TiO2 growth mechanisms with increasing deposition cycles. Differential subsurface ALD growth phenomenon occurs depending on the relative size of TDMAT precursor and the polymer free volume during Stage 1. This is followed by Stage 2 where isolated TiO2 islands start merging into continuous film on membrane top surface. After a continuous film has formed, TiO2 will be deposited on it layer by layer (Stage 3). This microscopic view does not include the ridge-and-valley structure of SW30 since its dimension is orders of magnitude larger.

stages were observed during the ALD growth on NF270. These stages were less discernible with SW30 (Figure 5). For the first several cycles, the growth rate (i.e., slope of the curve) gradually decreased after the initial fast increase (Figure S10). This was followed by the second stage in which the growth rate started to increase again and the final third stage with a linear growth (characteristic of the nonporous surface). We postulate that this unique growth mode results from unique morphologies of SW30 and NF270. The first stage (Stage 1 in Figure 6) results from the presence of a large number of active sites that are consumed during the first few ALD cycles. For SW30, these extra sites are likely given by the aggregate free volume. While these sites are relatively quickly consumed in SW30, extra sites in NF270 are likely more abundant, as they reside within the continuous polymer free volume whose diameters are larger than that of TDMAT. A large surface area, i.e., the combination of the top surface and subsurface area, contributes to the high growth rate detected at the beginning of this stage in the case of NF270. As the deposited TiO2 narrows this free volume entrance, hindering the further penetration of TDMAT, the number of accessible active sites decreases and so does the growth rate.35 The second stage (Stage 2 in Figure 6), characterized by a gradual increase of the growth rate, is believed to result from

sizes. The average thickness (out of six measurements) of the TiO2 layer deposited on SW30 after 50 cycles of ALD was measured to be 4.4 ± 0.7 nm. The growth rate calculated (0.088 nm/cycle) is in good agreement with the previously reported TDMAT-based TiO2 growth rate on nonporous substrates at 100 °C (∼0.090 nm/cycle),25 indicating that TiO2 mainly grows on the top surface of SW30 PA layer. On the contrary, the average thickness of the deposited TiO2 layer on NF270 was 7.4 ± 0.9 nm. The calculated growth rate (0.15 nm/cycle) was 66.7% higher than that on nonporous substrates. The TiO2 growth on the top surface should be independent of its subsurface growth (i.e., the TiO2 will continue to grow through each ALD cycle regardless of additional active sites elsewherein this case, the subsurface). Therefore, in case of NF270, the only possible explanation is that the growth of TiO2 occurs under the subsurface, in addition to the top surface. Titanium Dioxide ALD Growth Mode. The dependence of TiO2 growth on the number of ALD cycles (a.k.a. growth mode34) provides another critical piece of information to differentiate the properties of different substrates. If the substrate were a flat surface with active sites ideally spaced at the length scale of precursor, then the growth mode would be linear during all stages of growth. However, three discernible F

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Figure 7. Water permeability and solute rejection of pristine and ALD-modified membranes. (A) Normalized water permeability of modified SW30 and NF270 membranes. (B) The amplification of the first 10 cycles in (A). (C) Salt rejection of SW30 membranes coated with different cycles of TiO2. The concentration of NaCl in feed solution was 50 mM. (D) Salt rejection of pristine and ALD-coated NF270 membranes measured with NaCl, CaCl2, and Na2SO4 at concentrations of 2 and 20 mM. Solution pH was adjusted to 8.0 by NaOH. All experiments were conducted using a bench-scale crossflow system under a pressure of 27.6 and 6.9 bar (400 and 100 psi) for SW30 and NF270, respectively. Temperature was maintained at 22.0 ± 0.5 °C, crossflow velocity was set at 21.4 cm/s. Membranes with zero cycle refer to the ethanol-rewetted pristine membranes (i.e., IPA washing, drying, and then ethanol rewetting). Error bars represent the triplicate measurements.

Since TiO2 growth occurs only on the existing TiO2 layer in this linear growth stage, the slope of the growth curve becomes proportional only to the substrate top surface area. This would translate to approximately 2.8 times higher surface area for SW30 (ridge-and-valley structure) than that of NF270. This is consistent with values previously measured based on AFM.38 A slightly larger value obtained in this study is likely due to the access of TDMAT to the empty space under the ridges that AFM cannot detect.39 Water Permeability. As more ALD cycles were performed and more TiO2 was deposited, the water permeability of both SW30 and NF270 membranes decreased significantly (Figure 7A). After 50 cycles, the water flux of both membranes decreased to below 10% of the initial flux. The observed decrease at the large number of cycles confirms the formation of a continuous and dense TiO2 layer on both membranes that blocks the membrane surface, presumably similar to inorganic scaling.40 In previous membrane ALD coating studies, more than 100 ALD cycles were conducted to enhance UF/MF fouling resistance.4,7 However, our results suggest that even a relatively small number of cycles would have a detrimental effect and that a large number of ALD cycles should be avoided when the ALD technique is employed for TFC membrane modification. A difference in the permeability change pattern between two membranes was also noteworthy during the initial stage of

the transition of TiO2 coating in dispersed island configuration (due to available carboxyl terminations) to continuous coverage across the top surface. As more growth sites hydroxyl groups on the deposited TiO2are created as the surface coverage of TiO2 increases, the growth rate increases.36 A similar observation has been made during the ALD of a flat polymer surface that has limited active sites available for ALD growth.37 The duration of this phase would be dependent on the distance between nucleation sites; i.e., the higher nucleation site areal density on substrate surface, the shorter the second stage. The fractions of linear portion of PA on the membrane top surface, indicative of carboxyl group areal density and obtained based on O/N ratio measured by the XPS technique (Text S2), are estimated to be 0.73 and 0.60 for SW30 and NF270, respectively (Table S2), consistent with the observation that Stage 2 was less distinct in SW30 than NF270. Finally, once a continuous TiO2 film forms on the membrane top surface, the active site density becomes constant. Therefore, a fixed amount of TiO2 is deposited at each cycle, and the Ti growth becomes linear with respect to the number of cycles (Stage 3 in Figure 6). Since both Stages 1 and 2 proceeded with SW30 with a smaller number of cycles than with NF270 as discussed above, Stage 3 started earlier with SW30 at around 25 cycles than NF270 at around 40 cycles. It is noteworthy that the slope of the growth curve was higher with SW30 (0.104) than NF270 (0.0378) (Figure S10). G

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pristine value of −23 mV with even a few cycles, therefore lessening charge interaction (Figure S11). It is noteworthy that the surface charge of NF270 reached −15 mV with a larger number of ALD cycles. As a continuous TiO2 layer formed on both membranes, membrane surface charge converged to a single value regardless of the membrane type. Implications. Results of this study collectively indicate that caution must be taken when the ALD technique is considered to enhance the properties and performance of TFC RO and NF membranes through inorganic surface coating. Due to the nonporous nature of the densely packed coating layer, applying a large number of ALD cycles will inevitably lead to the detrimental loss of water flux, which is not desirable in most practices. Instead, only a few cycles of ALD performed under the optimized condition might be potentially advantageous for both RO and NF surface modification. In the case of RO, a small number of ALD cycles caused a drastic increase in surface charge through replacing carboxyl groups with hydroxyl groups, which is potentially important for both silica and gypsum scaling reduction,40,45 without affecting water permeability and salt rejection. The NF membrane appears as a more appealing substrate for ALD modification; a small number of ALD cycles can be potentially regarded as a unique approach to tune the pore properties in terms of both pore size and pore chemistry.

ALD growth (i.e., lower number of cycles) (Figure 7B). After five cycles of coating, the change in water permeability of the TiO2-coated SW30 membrane was within 3% of that of the pristine SW30, while the water permeability of NF270 decreased by 25%. The small change in water permeability with SW30 is likely related to the aforementioned postulation that, during the first few ALD cycles, TDMAT nucleates only within the aggregate free volume near the membrane top surface; this action does not significantly affect water transport. In the case of NF270, pores governing the water transport are accessible to TDMAT. Therefore, pore narrowing during the initial TiO2 layer growth likely resulted in the decrease in the average pore size (Table S1) and the drastic flux decline. A similarly drastic flux decline of ceramic NF membranes during the initial ALD growth has been observed before.41 Solute Rejection Property. Fifty cycles of ALD coating were found to decrease the salt rejection of SW30 from 99.7% to 91.8% (Figure 7C). This undesired outcome appears contrary to the observation that TiO2 layers blocked the pores and caused permeate flux decline. One possible explanation is the formation of TiO2 layers around the ridge-and-valley structure, narrowing the valley structure (openings) and aggravating concentration polarization (i.e., similar to cakeenhanced concentration polarization).10,42 However, when the number of ALD cycles was below five (i.e., less than 3% change in permeability), the salt rejection remains nearly constant (99.5%); this is contrary to the previous report in which salt rejection significantly deteriorated after 10 cycles of Al2O3 ALD.10 As TiO2 was deposited, the surface zeta potential of SW30 became more negative, attaining −15 mV compared to its pristine value of −10 mV (Figure S11). Such a surface charge increase did not affect the salt rejection. The effect of TiO2 coating on solute rejection by NF270 was assessed using both symmetric (NaCl) and asymmetric (CaCl2 and Na2SO4) salts (Figure 7D). The NaCl rejection of pristine NF270 is dominated by Donnan (charge) exclusion, namely the electrostatic repulsion between the negatively charged membrane carboxyl functional groups and Cl− ions. Therefore, at a higher feed concentration (20 mM), the screening of the membrane surface charge decreased the NaCl rejection. The low rejection of CaCl2 (34.7% at the feed concentration of 2 mM) is attributed to the electrostatic attraction between membrane surface and divalent Ca2+ ions. Thus, the enhanced rejection at higher feed concentration is likely due to the neutralization of membrane surface charge. The strong electrostatic repulsion between the PA layer and the divalent SO42− ions resulted into the high rejection (>90%) of pristine NF270 membrane for Na2SO4. No significant change was observed at higher Na2SO4 concentrations.43 The rejection behavior of the pristine NF270 membrane for NaCl, CaCl2, and Na2SO4 is characteristic of “loose” NF membranes where charge exclusion plays a more important role than size exclusion.44 As Figure 7D shows, the same ion rejection trend was observed for ALD coated NF270 membrane at different feed salt concentrations, indicating a rejection mechanism of ALD coated membrane similar to that of NF270 membrane. Different from the impact of TiO2 coating on SW30 ion rejection, the rejection of NF270 to both NaCl and CaCl2 increased with just five cycles of ALD. The increase in NaCl rejection is likely due to the enhanced size exclusion effect. For CaCl2, the change in the surface charge might have played an additional role. With TiO2 coating, the surface charge of NF270 was found to quickly increase from its

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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.8b03967. ALD exposure mode (Text S1); fraction of polyamide linear portion calculation (Text S2); membrane average pore radius calculation (Text S3); TDMAT based TiO2 ALD growth (Text S4); H2O pulse time and exposure time optimization (Text S5); nitrogen and sulfur spectra in XPS depth profile (Text S6); pharmaceutical rejection enhancement (Text S7); estimated pore radii obtained from organic tracer experiments (Table S1); estimation of the degree of cross-linking (Table S2); physicochemical properties of the pharmaceuticals (Table S3); polyamide chemical structure (Figure S1); schematic diagram of ALD machine (Figure S2); representative Ti EDS spectra detected during the TiO2 growth on SW30 (Figure S3); XPS argon sputtering condition and original XPS sputtering spectra (Figure S4); laboratory-scale NF/RO test unit (Figure S5); membrane permeability and NaCl rejection change during pretreatment (Figure S6); H2O pulse time and exposure time optimization (Figure S7); TDMAT pulse time and exposure time optimization without normalization (Figure S8); dependence of ALD chamber pressure on TDMAT pulse time (Figure S9); period determination for the ALD growth (Figure S10); membrane zeta potential change during the TiO2 growth (Figure S11); and pharmaceutical rejection of ALD coated NF270 membranes (Figure S2) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +1 (203) 432-4386; fax: +1 (203) 432-0199; e-mail: [email protected]. H

DOI: 10.1021/acs.est.8b03967 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology *Tel: +1 (203) 437-6521; fax: +1 (203) 432-4387; e-mail: shu. [email protected].

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ORCID

Menachem Elimelech: 0000-0003-4186-1563 Shu Hu: 0000-0002-5041-0169 Jae-Hong Kim: 0000-0003-2224-3516 Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the US National Science Foundation (NSF) through the Nanosystems Engineering Research Center for Nanotechnology Enabled Water Treatment (Grant EEC-1449500). Y.Y.Z. was supported by the Chinese Scholarship Council (CSC). We thank Y. Choo and M. Rooks for assistance with TEM imaging; M. Li for assistance with XPS and SEM; and Yale University YINQE (Yale Institute for Nanoscience and Quantum Engineering; TEM), and West Campus MCC (Materials Characterization Core; XPS and SEM) for shared facilities.

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DOI: 10.1021/acs.est.8b03967 Environ. Sci. Technol. XXXX, XXX, XXX−XXX