Antiorganic Fouling and Low-Protein Adhesion on Reverse-Osmosis

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Antiorganic Fouling and Low-Protein Adhesion on Reverse-Osmosis Membranes Made of Carbon Nanotubes and Polyamide Nanocomposite Yoshihiro Takizawa,†,⊥ Shigeki Inukai,†,⊥ Takumi Araki,†,§ Rodolfo Cruz-Silva,†,‡ Noriko Uemura,§ Aaron Morelos-Gomez,† Josue Ortiz-Medina,† Syogo Tejima,†,§ Kenji Takeuchi,†,‡ Takeyuki Kawaguchi,†,‡ Toru Noguchi,†,‡ Takuya Hayashi,†,‡ Mauricio Terrones,‡,∥ and Morinobu Endo*,†,‡ †

Global Aqua Innovation Center, and ‡Institute of Carbon Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan § Research Organization for Information Science & Technology, 2-32-3, Kitashinagawa, Shinagawa-ku, Tokyo 140-0001, Japan ∥ Department of Physics, Department of Materials Science and Engineering, Department of Chemistry, Center for 2-Dimensional and Layered Materials and Center for Atomically Thin Multifunctional Coatings (ATOMIC), The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: We demonstrate efficient antifouling and low protein adhesion of multiwalled carbon nanotubes-polyamide nanocomposite (MWCNT-PA) reverse-osmosis (RO) membranes by combining experimental and theoretical studies using molecular dynamics (MD) simulations. Fluorescein isothiocyanate (FITC)-labeled bovine serum albumin (FITC-BSA) was used for the fouling studies. The fouling was observed in real time by using a crossflow system coupled to a fluorescence microscope. Notably, it was observed that BSA anchoring on the smooth MWCNT-PA membrane was considerably weaker than that of other commercial/laboratory-made plain PA membranes. The permeate flux reduction of the MWCNT-PA nanocomposite membranes by the addition of FITC-BSA was 15% of its original value, whereas those of laboratory-made plain PA and commercial membranes were much larger at 34%−50%. Computational MD simulations indicated that the presence of MWCNT in PA results in weaker interactions between the membrane surface and BSA molecule due to the formation of (i) a stiffer PA structure resulting in lower conformity of the molecular structure against BSA, (ii) a smoother surface morphology, and (iii) an increased hydrophilicity involving the formation of an interfacial water layer. These results are important for the design and development of promising antiorganic fouling RO membranes for water treatment. KEYWORDS: reverse-osmosis (RO) membrane, antifouling, nanocomposite, carbon nanotube (CNT), polyamide (PA), molecular dynamics (MD) simulations

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INTRODUCTION Economic growth and climate change have resulted in a scarcity of clean water.1,2 Reverse osmosis (RO) has become the leading process to purify or desalinate water due to its cost efficiency, high-volume water production, and reasonably simple operation.3,4 Some of the most successful RO membranes are based on aromatic polyamide (PA) due to their excellent water permeation and salt-rejection performance. Prior to RO processes, the water source is usually pretreated by chlorination,5 ozonation,5,6 nano/micro filtration,5,6 and/or other processes5−7 to minimize fouling. However, organic matter can still reach the membrane and cause serious fouling.8,9 In this context, the formation of a fouling layer increases the flow resistance, and it reduces the permeate flux and the energetic efficiency of the entire process. Therefore, higher transmembrane pressure and frequent washing with chlorinated © 2017 American Chemical Society

water are required to maintain stable water permeation, although frequent washing promotes the degradation of the PA membranes; PA is not chemically resistant against chlorinated water.10−12 It is thus important to develop stable and effective antifouling RO membranes. There are only a few studies on antifouling performance involving fouling-release surfaces that work by using either a biomimetic approach or by releasing an antifouling substance, such as silicon oil or copper ions.13,14 However, water purification technology cannot rely on the latter method due to concerns of water contamination. Several approaches to improve the antifouling performance of RO membranes have been reported, usually by increasing the membrane surface Received: May 8, 2017 Accepted: August 25, 2017 Published: August 25, 2017 32192

DOI: 10.1021/acsami.7b06420 ACS Appl. Mater. Interfaces 2017, 9, 32192−32201

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ACS Applied Materials & Interfaces hydrophilicity. Examples include membrane modification by hydrophilic polymers,15−19 or coatings consisting of superhydrophilic nanoparticles.20 An alternative approach is to synthesize a smooth PA membrane surface to prevent the foulant deposition.21,22 Indeed, the typical ridge-and-valley morphology of PA membranes21,22 has some positive effects on permeability but also eases the fouling processes due to their relatively high roughness.21 Regarding nanocomposite membranes, the antifouling performance has been improved by modifications with graphene oxide (GO)23−25 and by the incorporation of multiwalled carbon nanotubes (MWCNTs) to PA.3,26 Antifouling performances have been evaluated by various methods that include surface studies using atomic force microscopy (AFM)20,22,27 and scanning electron microscopy (SEM),21 and by carefully monitoring the water permeation.20,22,27 Molecular dynamics (MD) simulations have become a new way to understand the fouling mechanism and the adhesion forces at the atomic level.28−30 We reported that MWCNT-PA nanocomposite membranes showed higher chlorine resistance, better antifouling properties, and similar desalination performances as compared to plain PA membranes.31 In the present study, we investigated in detail the antifouling performance of MWCNT-PA membranes against bovine serum albumin (BSA), a known model for studying organic fouling, by in situ fluorescence microscopy coupled to permeation measurements. Our experimental data were supported by MD simulations that helped to understand the fouling mechanism. An RO nanocomposite membrane prepared by adding MWCNT to PA is highly attractive because this combination improves the antifouling performance and can also decrease the time needed for membrane maintenance, save energy, and extend the membrane’s lifetime. Our findings also enable the engineering of advanced low-protein adhesion nanocomposites that will be useful in the biomedical, pharmaceutical, food processing, and organic waste fields.

Figure 1. SEM images showing the surface morphology of the RO membrane active layers. (a) MWCNT-PA nanocomposite membrane, (b) laboratory-made plain PA membrane, (c) commercial membrane A (CM-A) and (d) commercial membrane B (CM-B), and (e−h) the same membranes after a 144 h fouling period where smoother areas in (f−h) correspond to the deposited BSA foulant.

and CM-B membranes (Figure 2c,d). As previously mentioned, the presence of MWCNTs within the PA structure contributes to a decrease in the height of the surface features, with a consequent decrease in roughness. In addition, as observed by comparing Figure 2a and b, the shape of the features was also considerably affected by the presence of MWCNTs. The ridge-and-valley structures were large and numerous on both the commercial and the laboratory-made plain PA membranes with height variations ranging from 160 to 340 nm, whereas those of the MWCNT-PA membrane were smaller with maximum height variations of 140 nm (Figure 2e). From the antifouling performance standpoint, a smooth surface morphology is better,15 even though a ridge-and-valley surface contributes to the high-water permeability of a plain PA membrane. It is important to achieve a balance within the membrane structure on multiple-scales, such as the surface morphology of the microscopic ridge-and-valley structure and molecular-level smoothness, for salt rejection, permeability, and antifouling nature. During preconditioning (without BSA, NaCl solution at 0.7 MPa), the water permeate flux and salt permeability gradually decreased, whereas the NaCl rejection increased due to the simultaneous compaction under pressure and hydration of the membranes (see Figure S-1a,b). The permeate flux and salt rejection were stabilized after 60−150 h by compaction of the membrane and are shown in Table 1. These values indicate that the MWCNT-PA membrane permeate flux and salt rejection were comparable to other commercial membranes, but, remarkably, the change in water permeation during the preconditioning was different from that observed for the PA membranes (see Figure S-1). This could be related to a

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RESULTS AND DISCUSSION Morphology of Reverse-Osmosis (RO) Membranes. Figure 1a−d shows SEM images of a nanocomposite RO membrane containing approximately 15.5 wt % of MWCNT (hereinafter referred to as the “MWCNT-PA membrane”), a laboratory-made plain PA membrane, commercial RO membrane-A (CM-A), and commercial RO membrane-B (CM-B). Figure 1e−h shows SEM images of the membranes after a 144 h fouling period. The MWCNT-PA membrane with 15.5 wt % of MWCNT was used for the substantial salt rejection and permeate flux as compared to other MWCNT fractions. This amount of MWCNT was determined by thermogravimetric analysis as reported.31 Different surface features of all membranes originated from the differences in the formulation processes (e.g., the presence of MWCNTs) and reaction conditions. The presence of MWCNTs at the organic/inorganic interface during membrane synthesis affects the superficial tension, which is to a great extent responsible for the formation of the characteristic features at the surface of typical PA membranes. Atomic force microscopy (AFM) measurements confirmed the decrease in roughness (Ra) for the MWCNT-PA nanocomposite membrane. The MWCNT-PA nanocomposite (Figure 2a) and laboratory-made plain PA (Figure 2b) membranes show lower Ra values as compared to the CM-A 32193


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respectively (Table 1). CM-A was particularly affected by the decrease in the permeate flux due to the high-flow characteristics of this commercial membrane. The laboratory-made plain PA and the two commercial membranes showed a large decrease in permeability, which is a common property for PA membranes. It is noteworthy that only the MWCNT-PA membrane differed from the laboratory-made plain PA and commercial membranes, with a slight initial decrease followed by a recovery of the permeate flux within time (see below). Fluorescence Microscopy (FM) Observation. Figure 3a−d shows a series of images observed at 0, 72, and 144 h by FM on each membrane during fouling; all images are available in Figure S-2. There was no fouling at the beginning of the experiment (0 h), where the membrane was not exposed to FITC-BSA. This frame captured at 0 h is also a control to show that there is no fluorescence from the membrane, additives, acrylic cell, or the spacer. In the subsequent frames, the areas shown in green are due to the presence of FITC-BSA. For the commercial membrane, CM-A shown in Figure 3a, the BSA foulant preferentially attached to the corner between the membrane and the spacer at first, rather than on the surface of the membrane, although BSA also attached on the surface of the spacer to a small extent. It is noteworthy that the foulant deposited behind the spacer made of polypropylene was still observable due to its transparency, and the fluorescence was scattered and distorted by the spacer (Figure S-3a,b). For the CM-B shown in Figure 3b, BSA begins to attach in the middle of the objective area as protein aggregates after 24 h. When the exposure to BSA foulant solution continued, homogeneous fouling occurred as indicated by the fluorescent green signal from the entire membrane. On the laboratory-made plain PA membrane (Figure 3c), underneath the spacer, the fluorescence intensity (green) increased after 24 h, similar to the two commercial membranes, thus indicating that the occurrence of organic fouling phenomena is equivalent between the commercial membranes and the laboratory-made plain PA membrane, and we concluded that the fouling observed here is a characteristic of the PA membrane. In contrast, as shown in Figure 3d, the images of the MWCNT-PA membrane are very different from those described above. It is known that MWCNTs act as a fluorescence quencher, but because they are embedded within the PA matrix, the FITC-BSA deposited on the surface could still be observed. The results of the additional experiments shown in Figure S-4 demonstrate that the quenching effect by MWCNTs was negligible, and the fluorescence intensity of each membrane was comparable. As compared to the plain PA membrane, the fouling on the MWCNT-PA membrane area without the spacer was much lower, and the presence of BSA was confirmed only underneath or on the spacer, the fluorescence intensity of which increased over the BSA solution exposure duration. The spacer on the MWCNT-PA membrane was still not visible after 72 h; however, on the other RO membranes the spacer was already green after 24 h. These results clearly show that the MWCNTPA membrane possesses a high level of antifouling properties against BSA. Figure 3e shows the increase in the average fluorescence intensity from the membrane surface over time for the other three RO membranes, whereas the average fluorescence intensity from the MWCNT-PA membrane stayed the same over time.

Figure 2. AFM images for (a) the MWCNT-PA nanocomposite membrane and (b) a laboratory-made plain PA membrane, (c) CM-A, and (d) CM-B, where the surface morphology and height variations are observed. In (e) the profiles indicated as white dashed lines in the images are plotted with the corresponding roughness parameter from AFM data.

Table 1. Salt Rejection and Permeate Flux of the MWCNTPA Nanocomposite Membrane, Laboratory-Made Plain PA Membrane, and CM-A and CM-B before and after Initial Fouling permeate flux (m3 m−2 d−1) membrane MWCNT-PA membrane laboratory-made plain PA membrane CM-A CM-B

salt rejection (%)

before fouling

after initial fouling

99.7 99.7

0.156 0.249

0.133 0.146

99.6 99.5

0.242 0.132

0.161 0.066

decreased proneness to compaction due to the applied pressure, which would be advantageous for maintaining the permeability performance during high pressure operations. In fact, as shown in our previous work,31 at 5.0 MPa of crossflow experiments, the water permeability for MWCNT-PA nanocomposite membranes was higher than that of plain PA membranes, which would be explained by the higher compactability, and thus densification, of the plain PA membrane. In the present study, the fouling process was started by the addition of 100 ppm of fluorescein isothiocyanate-bovine serum albumin (FITC-BSA) to the MWCNT-PA membrane, laboratory-made plain PA membrane, CM-A and CM-B, decreasing the permeate flux to 0.133 m3 m−2 day−1 (15% decrease), 0.146 m3 m−2 day−1 (42% decrease), 0.161 m3 m−2 day−1 (34% decrease), and 0.066 m3 m−2 day−1 (50% decrease), 32194


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Figure 3. Snapshots of the membrane and the spacer observed under a fluorescence microscope as a function of time in a crossflow experiment at 0− 144 h on the (a) CM-A, (b) CM-B, (c) laboratory-made plain PA, and (d) MWCNT-PA nanocomposite membranes. Arrows indicate representative BSA foulant adhesion. The images were captured using the same light intensity and exposure time (196 ms). (e) Increase in fluorescence intensity from the RO membranes (the spacer was not included in the measurement) over time. (f) The normalized permeate flux, Jr(t), after the addition of BSA to the four membranes as a function of time.

Permeate Flux after the Addition of BSA. The permeate flux was measured during the fouling study. The normalized permeate flux as a function of time (Jr(t)) is given by eq 1: Jr (t ) = J(t )/J0

membranes barely changed or gradually decreased over time after the BSA addition. The Jr(t) value of the MWCNT-PA membrane increased up to 1 after the initial reduction, which means that the permeate flux recovered its initial value due to the self-cleaning of the membrane. These results thus revealed that the MWCNT-PA membrane exhibits low protein-adhesion properties against BSA deposition. This low protein-adhesion property of the MWCNT-PA membrane suggests that the BSA detachment is easier than that of the laboratory-made plain PA membrane and commercial membranes. A possible explanation is that when a critical amount of BSA is deposited on the surface of the MWCNT-PA membrane, it detaches under the shear stress of the flow turbulence. This is also supported by Figure 3a−d, where the fouling of the MWCNT-PA membrane is apparently lower, and the fluorescence intensity is less than those of the other RO membranes. The reason for the reduced fouling on the MWCNT-PA membrane is most likely due to the lower interaction between the membrane and the foulants. This phenomenon will be discussed later using MD simulations. Effects of MWCNTs in Nanocomposite on Fouling Behavior. Figure S-5 shows the images of a MWCNT-PA membrane taken by reflected white light mode and the corresponding FM image. A glass rod was used to spread the dispersion of MWCNT and m-phenylenediamine (MPD), and,

(1)

where J0 is the permeate flux before BSA addition and J(t) is the permeate flux after the BSA addition at time t. As the BSA fouling phenomenon progresses, the value of J(t) decreases. Interestingly, the BSA addition decreased the value of Jr(t) within 10 h for the MWCNT-PA membrane, laboratory-made plain PA membrane, CM-A, and CM-B to 0.85, 0.58, 0.66, and 0.50, respectively (Table 1). A high Jr(t) value (close to the original Jr = 1) indicates that the permeate flux is not greatly affected by the addition of BSA. The plot in Figure 3f shows that the MWCNT-PA membrane had the best initial antifouling behavior among all membranes. The BSA thin layer covered the surface of the membrane at the microscopic level, and this resulted in the closure of water-permeable channels and a large decrease in the Jr(t) values. Because the formation of the thin BSA layer occurs at the molecular level, the initial fouling stage is not observable by FM, but the FM can depict the further progression of foulant deposition. It is also interesting to note that the Jr(t) values of both the laboratory-made plain PA membrane and the commercial 32195


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Figure 4. Snapshots of water molecules in (a) graphene-PA composite (GPA) and (b) plain PA. Comparison of the diffusion coefficients of the PA (c,d), water (e,f), and hydrogen bond between the water and PA molecules (g,h) in the GPA and plain PA, respectively.

surface of GPA is advantageous because it lowers the structural conformity with BSA, therefore reducing their interaction.32,33 In addition to a stiffer PA structure of GPA, the important finding is that the hydrogen bonds established between water molecules and PA are localized at the surface region in GPA (4−5 nm; Figure 4g). This result suggests the presence of a water layer bound to the GPA surface. This water layer on the GPA surface is known as the interfacial water layer,34 and a previous report has pointed out the importance of this water layer in the mechanism of the reversible adsorption of BSA on surfaces.35 Indeed, interfacial water keeps the BSA and the membrane surface apart, thus avoiding their interaction. The average number of water molecules per unit area (nm−2) was calculated on the GPA model (XY plane = 123.45 Å, 126.64 Å) and the PA model (XY plane = 150.67 Å, 152.83 Å); those were 18.2 nm−2 on the GPA surface and 12.1 nm−2 on the PA surface. Although the water layer is present in both membrane models, the water layer on the GPA model is well-ordered and more compact than that on the plain PA model (Figure S-7a,b), and the water layer thus hinders the amide bonds from the surface, preventing their possible interaction with the protein. Other research groups reported that this water layer formed on modified hydrophilic surfaces,15−19 which agrees with the interfacial water layer on GPA. This result obtained by simulations is consistent with the experimental results showing that the MWCNT-PA membrane had less BSA fouling as compared to the plain PA membrane. Simulations of Foulant Deposition. Figure 5a and b shows the height map of the simulated GPA and plain PA models, respectively. GPA exhibited a smoother surface as compared to plain PA as shown by a more homogeneous height map at the molecular level. In particular, the plain PA surface has typical ridge-and-valley structures (Figure 5b). We simulated the effect of water flow on the surface of a fouled model membrane by molecular dynamics. In the case of the GPA, BSA is relatively easy to remove with a water flow (1.0 ×

in some cases, a line with a relatively low concentration of MWCNT was formed due to the stick−slip behavior of the solution (Figure S-5a). It is also noteworthy that by contrast enhancing the image (Figure S-5b), the higher fouling regions overlap with the white line regions in the optical microscope image. These results indicate that a higher MWCNT concentration in the membrane prevents the adhesion of BSA; that is, BSA preferentially attaches to the PA-rich areas. These observations would explain why both the laboratorymade plain membrane and the commercial membranes were more prone to fouling as compared to the MWCNT-PA membrane. Computer Simulations of Membrane Structures and the Interfacial Water Layer. Figure S-6 shows (a) an MD snapshot and (b) the charge density map subtracting the PA partial charge and the graphene neutral charge from three layers of graphene-PA (GPA) composite models calculated by the charge equilibration method QEq. A significant charge transfer occurred from the PA molecule around Z = 1.00 nm to the graphene molecule. A similar phenomenon was confirmed for large diameter MWCNTs (Figure S-6c,d). We therefore assumed that GPA can be used for the MD simulation as an equivalent model for simulating the MWCNT-PA membrane. The diffusion coefficients of solvated GPA and PA were studied in terms of the PA, the water molecule in the membrane, and the hydrogen bond established between water and PA (see Figure 4a,b). A comparison of Figure 4c and d shows that the addition of graphene to PA decreased the diffusion coefficient of PA from 2.88 × 10−7 to 1.44 × 10−7 cm2/s. This means that the PA structures of GPA become stiffer. Especially, this stiffness is higher at the outermost surface region (4−5 nm). Therefore, in GPA, the average diffusion coefficients of water (2.20 × 10−5 cm2/s, Figure 4e) and the hydrogen bond between water and PA (1.43 × 10−5 cm2/s, Figure 4g) are slightly lower than those values (2.29 × 10−5 cm2/s, Figure 4f and 1.47 × 10−5 cm2/s, Figure 4h) of PA. In terms of antifouling performance, a stiffer PA structure at the 32196


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layer preventing BSA adhesion on the membrane, thus providing antifouling and low protein-adhesion features. Figure 7a,b shows the schematic models of the MWCNT-PA membrane surface with a small ridge-and-valley structure, and

Figure 7. Schematic model showing the antifouling and low proteinadhesion process of (a) the MWCNT-PA nanocomposite membrane and (b) a plain PA membrane based on the surface morphology and MD simulation. Step i: Formation of the interfacial water layer (MWCNT-PA membrane). Step ii: Formation of a thin layer of BSA. Step iii: The attachment (both the MWCNT-PA membrane and the plain PA membrane) and detachment (the MWCNT-PA membrane) of BSA.

Figure 5. Two-dimension plots of the membrane surface based on aromatic rings distribution to the Z direction of (a) GPA and (b) plain PA. BSA attached on the GPA membrane (c) without water flow and (d) with water flow, and BSA on the plain PA membrane (e) without water flow and (f) with water flow. 1.0 × 10−6 nm/ps of water flow is applied for 500 ps. The white line and arrow indicate the displacement of BSA. (g) The total energy (interaction strength) between the attached BSA and membrane for the simulation time. (h) The number of hydrogen bonds between the BSA and membrane for the MD time.

the plain PA membrane surface with a large ridge-and-valley structure. The addition of BSA to the water source causes the initial fouling, which forms a thin layer of BSA. It causes a sudden drop in the Jr(t) value (Figure 7, steps i and ii). However, the initial drop with the MWCNT-PA membrane was lower due to the reduced interaction between PA and BSA, the smoother surface, and the formation of an interfacial water layer. Therefore, the foulant is easily detached from the surface of the membrane by the shear stress of the water flow as compared to a plain PA membrane (Figure 7, step iii).

10−6 nm/ps) from the surface (Figure 5c,d) as compared to plain PA (Figure 5e,f). The average of total energies (interaction strength) of BSA adsorption from 2 to 12 ns is −13 840 kcal/mol for the GPA case and −14 620 kcal/mol for the plain PA case (Figure 5g). The higher interaction strength of plain PA means that BSA attached more strongly on the uneven surface by interacting with polyamide chain branches by hydrogen bonds (Figure 5h), because of the higher conformity of the PA structure. The water permeation hindrance by the BSA attachment on the membrane is weaker in GPA due to the smaller number of hydrogen bonds established between PA in GPA and BSA (Figure 6a,b). Antifouling Mechanism and the Low Protein-Adhesion Performance of MWCNT-PA Membranes. In light of the results of our experimental studies and theoretical simulations, it was confirmed that the MWCNT-PA has (i) a stiffer structure, (ii) a smoother surface at the microscopic and molecular levels, and (iii) the formation of an interfacial water

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CONCLUSION We successfully prepared an MWCNT-PA nanocomposite RO membrane containing a high carbon nanotube content (approximately 15.5 wt %) with the permeate flux and NaCl rejection properties of 0.16 m3 m−2 day−1 and 99.7%, respectively, for a 10 mmol/L-NaCl feed concentration at 0.7 MPa, using a crossflow experimental setup. The MWCNT-PA membrane exhibited outstanding antifouling performance for the BSA model organic-foulant. MD simulations indicated that the antifouling and outstanding low protein-adhesion properties of the MWCNT-PA membrane against BSA foulant are induced by the stiff PA structure and the interfacial water layer. The smoother surface of the MWCNT-PA membrane as compared to a plain PA membrane, from the microscopic to molecular levels, also contributes to the antifouling and low protein-adhesion performance. The present MWCNT-PA nanocomposite RO membrane is promising and could be used for a wide range of applications. In addition to the desalination process, the reported technology has important potential to advance low-protein adhesion on nanocomposite membranes for the biomedical, pharmaceutical, organic waste, and food processing fields.

Figure 6. Plots of water density mapping after 12 ns of molecular dynamics with BSA on the membrane, (a) GPA and (b) plain PA. 32197


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EXPERIMENTAL SECTION

BSA Functionalization with Fluorescein. The protein foulant, BSA (IgG-Free and Protease-Free), purchased from Jackson ImmunoResearch Laboratories (West Grove, PA) was used as an organic foulant model because of its well-known fouling behavior and relatively simple chemistry. BSA was modified by covalently bonding fluorescein to the free amino groups of the lysine residues of the protein backbone to make the protein visible by fluorescence microscopy. Typically, 100 mg of FITC was dissolved in 30 mL of dimethyl sulfoxide (DMSO). This FITC solution was then slowly added to the protein solution (1.0 g of BSA was dispersed in 500 mL of a buffer solution containing sodium bicarbonate and sodium carbonate, pH 10.0). Preparation of the RO Membranes. A MWCNT-PA nanocomposite RO membrane containing approximately 15.5 wt % of MWCNT, a laboratory-made plain PA membrane, and two commercial membranes, that is, CM-A and CM-B, were carefully compared. The MWCNT-PA membrane and laboratory-made plain PA membrane were prepared by interfacial polymerization between mphenylenediamine (MPD) and trimesoyl chloride (TMC) on the porous polysulfone (PSf) substrate as described.31 For the preparation of the MWCNT-PA membrane, MPD was dissolved in the MWCNT predispersed aqueous solution purchased from KJ Specialty Paper Co., Ltd. (Shizuoka, Japan). Characterization of the RO Membranes. SEM images of each membrane’s surface were recorded in an SU8000 ultrahigh performance scanning electron microscope (Hitachi, Tokyo) operating at 1.0 kV. For the SEM observations, all of the samples were coated with platinum (approximately 1.0 nm thickness) to avoid surface charging during the observation. Atomic force microscopy (Agilent Technologies AFM 5500, Santa Clara, CA) was performed directly on each membrane, using tapping mode for topography imaging. Crossflow Test. Each membrane sample (25.0 mm diam) was loaded in a specially made transparent acrylic crossflow cell (each with 25.0 mm diameter and 0.356 mm height in the cell chamber; Figure 8a), with an effective membrane surface area of 3.46 cm2. Because of the transparency of the experimental cell, the membrane surface was observable during the fouling experiments. A 1.2 mm-thick spacer (GE Water & Process Technologies, Trevose, PA) was placed on the membrane’s surface (Figure 8b) to replicate a typical large-scale RO module. Feed spacers are generally used to keep membranes apart to allow the water flow and to create turbulence that reduces the foulant attachment and salt concentration gradient. The SEM images of the top view (Figure 8c) and side view (Figure 8d) of the spacers observed using a TM3030Plus Miniscope (Hitachi, Tokyo) show that threads in one direction (i) can touch the experimental RO membrane surface, while the threads in the other direction (ii) are fixed on direction-(i) threads without touching the membrane. The crossflow system FTU-1 (Membrane Solutions Technology, Tokyo) was used at 0.7 MPa. A schematic illustration of the experimental system is shown in Figure 8e. The water flow rate was 500 mL/min along the surface of the membrane. The water source temperature was kept at 21 ± 1 °C. A 10 mmol/L-NaCl aqueous solution was used for the initial compaction process. The NaCl rejection, permeate flux, and salt permeability were measured on the four different membranes during compaction. During the steady-state of the fouling experiments, the permeate flux was also measured. The NaCl rejection was calculated by eq 2:

R (%) =

Cf − C p Cf

× 100

Figure 8. (a) The acrylic cell used for the BSA fouling in situ observation during crossflow. (b) Visible light image of the mesh-like spacer and the RO membrane surface. SEM images of the mesh-like spacer (c) top view and (d) side view of the spacer used for the crossflow test (the numbers on the thread are correlated with each other). Gaps between the membrane and the spacer are visible. (e) The crossflow system used for the RO membrane evaluation and fouling observation. Permeate flux (J) was calculated on the basis of eq 3:

J=

ΔV AΔt

(3)

where ΔV is the volume of permeated water collected for the permeation time ΔV and A is the effective surface area of the membrane. BSA Fouling Test. First, 100 ppm of FITC-BSA was dissolved in 10 mmol/L-NaCl aqueous solution after the membrane compaction was completed in the crossflow test. The fouling tests were conducted at pH 10. The concentration of BSA and that of the NaCl aqueous solution were similar to that reported.16 The surface of the membrane was observed through the openings of the net-like spacer (Figure 8b) at the center of the acrylic crossflow cell every 24 h over the 144 h fouling period. In all of the fluorescent images of the membrane, the water was fed from the left to the right. By fluorescence microscopy, FITC-BSA deposited on the surface of the membrane was observable using blue light (λ = 490 nm). The camera used was a CMOS microscope digital eyepiece camera (MC500, Ostec, Guanzhou, China), with the exposure time of 196 ms, equipped with an epifluorescence mode using a stereomicroscope (SMZ18, Nikon, Tokyo), with a Nikon P2-EFLC green filter. The FITC-BSA solution flowing above the membrane inside the acrylic cell prevented the continuous observation of the membrane surface due to the high fluorescence of the FITC-BSA solution that surpasses the fluorescence from the attached FITC-BSA on the membrane surface. Therefore, the water feed was replaced with a 100ppm nonfluorescent BSA/10 mmol/L-NaCl aqueous solution during image acquisition to avoid BSA detachment. After imaging the membrane surface, the feedwater was restored to the 100-ppm FITCBSA/10 mmol-NaCl aqueous solution for the subsequent fouling experiments. Molecular Dynamics (MD) Simulation Conditions and Membrane Construction. Computer simulations were performed by LAMMPS code using the classical MD simulation package.36 Our target was to simulate the BSA adsorption on plain PA and three layers of graphene-PA (GPA) composite models. We assumed that a GPA

(2)

where Cf is the feed concentration and Cp is the permeate concentration of the solution. Both the Cf and the Cp values were obtained from the electrical conductivity measurements of the solutions by a portable electrical conductivity meter, ES-71 (Horiba, Kyoto, Japan), with conductivity electrodes #9382-10D and #357410C (Horiba, Kyoto, Japan) for source water and permeate water, respectively. 32198


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ACS Applied Materials & Interfaces ORCID

model can behave equivalently to the MWCNT-PA system. This assumption is based on the similar charge-transfer taking place from PA molecules to graphene as compared to the MWCNT-PA model. The BSA structure was adopted from the report by Bujacz.37 For the BSA model, a single chain was extracted from the PDB file of the protein crystal downloaded from the Protein Data Bank (PDB ID: 4F5S). The method used to construct the membrane model is similar to that reported in our previous study of MWCNT-PA composite membranes.30 Briefly, a few TMC molecules were used as seeds to create the PA membrane topology. The GPA model was built by placing the TMC molecules scattered over the three-layer graphene surfaces, followed by the polymerization of these seeds. After the polymerization, isothermal−isobaric (NPT) relaxation was achieved by maintaining the total pressure at 1 atm, while the structure was relaxed during 200 psec using 0.5 fs steps. For the GPA model, we studied the charge transfer from the PA molecules to the three-layer graphene by using a charge equilibration method (QEq).38,39 During the simulations, the SPC/E model40 was adopted for the water molecules. For the plain PA and GPA, we used the General Amber Force Field (GAFF)41 for the interaction of the molecules. Plain PA is usually described by GAFF.29,42,43 The atom charges for the plain PA were set with ANTECHAMBER 1.27 and AM1-BCC partial charges.44,45 For the GPA case, the atom charges of the PA and graphene parts were obtained by the QEq method.38 The BSA description was achieved using the CHARMM Force Field.46 The interactions between molecules were calculated by LennardJones (LJ) and Coulomb interactions with particle−particle mesh solver (PPPM).47 For all MD simulations, the time step was set to 1.0 fs, and the trajectory data were saved at 10 000-step intervals for analysis. For the calculations of the charge density maps, the mesh was set to 0.2 Å. The distance criterion for hydrogen bonds was set to ≤4.0 Å. All calculations were performed inside a unit cell with periodic conditions. To study the state of the interfacial water layer on the GPA and plain PA, we set 5200 water molecules on the membrane surface to perform MD simulations with water interactions in a NPT ensemble for 5 ns. For the fouling MD simulations, the initial states for the membrane models were set to 200 TMC molecules. After polymerization and relaxation, the plain PA models had a total of 65 165 atoms in the XY plane = (150.67 Å, 152.83 Å), and the GPA model had a total of 52 073 atoms in the XY plane = (123.45 Å, 126.64 Å). BSA was constructed with 9253 atoms, and the number of water molecules was set at 95 915 in the plain PA and 56 425 in GPA. The MD simulations with the NPT ensemble were maintained at 1 atm for 12 ns.

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Yoshihiro Takizawa: 0000-0002-3762-1877 Author Contributions ⊥

Author Contributions

Y.T. performed fouling experiments, fluorescence microscope observation, and SEM, and wrote the manuscript. S.I. and T.N. prepared the membranes. T.A. and S.T. performed MD simulations and analysis, and wrote the manuscript. R.C.-S. carried out FITC-BSA preparation, image analysis, MD analysis, and advised and discussed the results during project. N.U. performed BSA model construction. A.M.-G., J.O.-M., K.T., T.K., T.H., and M.T. advised and discussed the results during the project. M.E. proposed and supervised the project, and wrote the manuscript. All authors reviewed the approved the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Center of Innovation Program “Global Aqua Innovation Center for Improving Living Standards and Water-sustainability” from Japan Science and Technology Agency, JST. The numerical calculations were carried out on the TSUBAME2.5 supercomputer in the Tokyo Institute of Technology and Earth Simulator at the Japan Marine Science and Technology Center (JAMSTEC).

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06420. Permeate flux, salt rejection, and salt permeability of RO membranes during the compaction process; snapshots of the membrane under a fluorescence microscope as a function of time in a crossflow experiment at 0−144 h; fluorescent patterns through the transparent spacer; fluorescent patterns on the MWCNT-PA membrane and CM-A; fluorescence-microscope image of the MWCNTPA membrane; MD snapshots and charge transfer mapping of GPA and MWCNT-PA; and MD snapshots of the interfacial water formed on the GPA and plain PA (PDF)

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Y.T. and S.I. contributed equally to this work.

AUTHOR INFORMATION

Corresponding Author

*Tel.: (+81)-26-269-5588. Fax: (+81)-26-269-5667. E-mail: [email protected]. 32199


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Antiorganic Fouling and Low-Protein Adhesion on Reverse-Osmosis

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