Effects of Casting Solvents on the Morphologies, Properties, and

Effects of Casting Solvents on the Morphologies, Properties, and Performance of Polysulfone Supports and the Resultant Graphene Oxide-Embedded Thin-Fi...
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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Effects of Casting Solvents on the Morphologies, Properties, and Performance of Polysulfone Supports and the Resultant Graphene Oxide-Embedded Thin-Film Nanocomposite Nanofiltration Membranes Quanling Xie,*,†,§ Shishen Zhang,†,‡ Zhuan Hong,*,†,§ Hanjun Ma,†,‡ Chenran Liu,†,‡ and Wenyao Shao*,‡ Downloaded via MCMASTER UNIV on November 27, 2018 at 00:58:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Engineering Research Center of Marine Biological Resource Comprehensive Utilization, The Third Institute of Oceanography of the State Oceanic Administration, Xiamen 361005, China ‡ Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China § Fujian Collaborative Innovation Center for Exploitation and Utilization of Marine Biological Resources, Xiamen 361005, China S Supporting Information *

ABSTRACT: In this study, the influences of different casting solvents including N-methylpyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF) on the morphologies, properties, and performance of polysulfone (PSU) supports and their resultant graphene oxide (GO)-embedded thinfilm nanocomposite (TFN) nanofiltration (NF) membranes were systematically investigated. The influences of casting solvents on the mechanism of immersion precipitation phase-inversion process and the morphology of PSU supports were analyzed by Hansen solubility parameters. The results indicated that the physicochemical properties and performance of both PSU supports and the resultant composite NF membranes were significantly affected by the type of casting solvents. PSU support made from NMP exhibited a small surface pore size that prevented the penetration of poly(piperazine amide) (PPA) into the PSU pores, which contributed to form a defect-free active layer with excellent permeaselectivity regardless of TFC or TFN NF membranes. On the contrary, the surface pore size of PSU support made from DMF was too large to generate a dense and defect-free PPA active layer, which led to inferior rejection of the corresponding TFC or TFN membranes. After introducing an appropriate amount of GO into the aqueous phase, the nanocomposite active layer became thinner and smoother with enhanced hydrophilicity and negative charge. At a GO concentration of 40 ppm, TFN-NMP-GO-40 NF membrane exhibited excellent permeaselectivity, antifouling ability, and chlorine resistance compared with TFC-NMP membrane. Particularly, on the basis of retaining the high salt rejection (>98%) without a loss, the water flux of TFN-NMP-GO40 membrane significantly increased to 46.9 L·m−2·h−1, which was 137.9% of the value for TFC-NMP membrane. and selectivity, surface fouling, and chlorination.2 Because the top active layer first and directly contacts with the original fluid during the membrane separation process, more research efforts have been dedicated to the research of PA active layer rather than the porous support. Nevertheless, the latest studies revealed that the structure and properties of the porous support also played an important role on the final performance of TFC RO or forward osmosis (FO) membranes.2−12

1. INTRODUCTION Polysulfone (PSU) is a well-known membrane material because of its excellent chemical resistance, mechanical strength, and thermal stability. It is extensively used for producing asymmetric membranes used in microfiltration (MF) and ultrafiltration (UF) as well as for support of thinfilm composite (TFC) membranes.1 To date, TFC membranes are the most commercially successful membranes due to combination of the ultrathin polyamide (PA) selective layer and the porous sublayer. Currently, most commercial reverse osmosis (RO) and nanofiltration (NF) membranes fall into this category. Nevertheless, PA-TFC membranes still encounter great challenges, including the trade-off effect between permeability © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

September 18, 2018 November 2, 2018 November 6, 2018 November 6, 2018 DOI: 10.1021/acs.iecr.8b04515 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

2.2. Graphene Oxide Synthesis. GO was synthesized by a modified Hummers method as described in our previous report25 and in Supporting Information. 2.3. Fabrication of Polysulfone Supports. The method of immersion precipitation phase inversion was adopted to prepare PSU supports. The casting solution compositions of PSU supports are listed in Table S1. According to the type of casting solvent, the fabricated PSU supports were named SMNMP, SM-DMAc, and SM-DMF, where SM stands for supporting membrane. Briefly, PSU and PVP were successively added into the casting solvent, which was constantly stirred at 200 rpm for 12 h. Subsequently, the degassed casting solution was cast on a polyester nonwoven fabric by use of an automatic coating device with a knife gap of 150 μm. The nascent flatsheet PSU supports were immediately immersed into the pure water bath (25 ± 1 °C). An hour later, the PSU supports were transferred and stored in another water bath to guarantee complete phase inversion. 2.4. Fabrication of Composite Nanofiltration Membrane. With the self-made PSU supports as substrates, the IP method was further employed to prepare the resultant TFC or TFN membrane. Briefly, GO nanofiller at different concentrations (0−80 ppm) was added into 2.0 wt % PIP aqueous phase containing 2.0 wt % TEA as acid acceptor. The aqueous solution with various GO concentrations was poured on the top of the PSU support and held for 2 min before removal of excess aqueous solution. Subsequently, the organic solution containing 0.1% (w/v) TMC dissolved in n-hexane was poured on the PIP-saturated PSU support and held for 1 min before draining off the excess organic solution was drained off. Finally, the composite membranes were heated at 60 °C for 8 min to enhance the IP reaction. The fabricated composite membranes without GO were denoted as TFC-X and the fabricated composite membranes with incorporating GO were denoted as TFN-X-GO-Y, where X and Y referred to the casting solvent and GO concentration (parts per million, ppm) in the aqueous solution, respectively. 2.5. Characterization of Graphene Oxide. Fourier transform infrared spectroscopy (FT-IR, Bruker VERTXE 70) was used to analyze the oxygen-containing groups of GO. A transmission electron microscope (TEM, Talos F200) was used to observe the morphology. X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.154 nm, D/max-rB 12 kW Rigaku) was recorded for GO samples with a scanning rate of 10 deg·min−1. 2.6. Characterization of Polysulfone Support and Composite Nanofiltration Membranes. The viscosity of the casting solution was determined by a viscometer (Brookfiled DV-2) at 25 ± 1 °C with suitable speed and torque. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Bruker VERTXE 70) was performed to characterize the chemical groups of membrane surface. A contact-angle goniometer (Beijing Harke SPCAX3) was employed to evaluate membrane hydrophilicity. A scanning electron microscope (SEM, Zeiss Sigma) was used to record surface and cross-sectional images. An atomic force microscope (AFM, MI5500, Agilent) was used to measure the membrane roughness. The porosity (ε) of PSU support was calculated by a gravimetric method according to eq 1:26

Compared to RO membranes, the major advantage of NF membranes is high permeaselectivity under low operation pressure. Characteristics of the support, including pore size, porosity, thickness, and surface hydrophilicity, were crucial for the performance of the resultant TFC NF membrane. Ang et al.13 used poly(ethylene glycol) with different molecular weight (Mw) to adjust the physicochemical properties of PSU support. The PSU support having low surface porosity, small pore size, and suitable hydrophilicity presented the optimal separation performance. Misdan et al.14 studied the variation in physicochemical properties of supports by varying PSU concentration in the range 12−20 wt %. It was found that the structural properties of PSU support significantly affected the formation of poly(piperazine amide) (PPA) active layer. The PPA-TFC NF membrane made over a support of 15 wt % PSU concentration displayed optimal permeaselectivity. Misdan et al.15 focused on porous supports made of different materials: polysulfone (PSU), poly(ether sulfone) (PES), and poly(ether imide) (PEI). Compared to PES- and PEI-based TFC membranes, PSU-based TFC showed lower water flux and higher salt rejection due to formation of a highly crosslinked and more compact active layer. Landaburu-Aguirre et al.16 investigated the effects of nanocomposite supports on the loose PPA-TFC NF membranes. The nanocomposite support incorporated with mesoporous silica contributed to a more permeable and selective NF membrane because the carboxylic group of SBA-15 might interfere with the interfacial polymerization (IP) process. Compared to the conventional PA-TFC membranes, thinfilm nanocomposite (TFN) membranes demonstrate superior separation performance by incorporating suitable nanofiller into the PA active layer.17−19 In recent years, graphene oxide (GO) has attracted more and more attention for its application in membrane materials, due to its unique properties and outstanding dispersibility. Many reports have confirmed that the introduction of GO into the active layer is helpful to enhance separation performance.20−24 However, systematic research is still lacking on how the support characteristics influence the IP process, morphologies, properties, and performance of TFN NF membranes. In this study, the influences of casting solvents on the rheological properties, phase inversion process, morphology, and performance of PSU supports were investigated. In addition, GO nanofiller was introduced into the PPA active layer via IP process, and the influences of PSU supports made by different casting solvents on the IP process, morphology and performance of TFC and GO-embedded TFN NF membranes were studied in depth. Furthermore, GO concentration in the aqueous solution was optimized according to water flux, salt rejection, antifouling ability, and chlorination resistance.

2. EXPERIMENTAL SECTION 2.1. Materials. Polysulfone (PSU, Ultrason S 6010) and poly(vinylpoyrrolidone) (PVP) K30 and K90 were supplied by BASF. N-Methylpyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), n-hexane, triethylamine (TEA), NaClO, Na2SO4, MgSO4, NaCl, MgCl2, and HCl (36−38%) were of analytical grade and obtained from Sinopharm (Shanghai, China). Piperazine (PIP) and trimesoyl chloride (TMC) were purchased from Aladdin (Shanghai, China). Bovine serum albumin (BSA) was received from BBI Life Sciences Corp. (Shanghai, China).

ε= B

ω1 − ω2 × 100 Ald w

(1) DOI: 10.1021/acs.iecr.8b04515 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research where ω1 and ω2 refer to the wet and dry weight of PSU support (grams), respectively, A is the effective area of PSU support (square meters), dw is the water density (0.998 g· cm−3), and l is the thickness of PSU support (meters). On the basis of porosity and water permeability, the mean pore radius (rm) of PSU support was calculated according to the Guerout−Elford−Ferry equation (eq 2):26 rm =

(2.9 − 1.75ε)8ηlQ εAΔP

3.2. Effects of Casting Solvents on Phase Inversion Process and Polysulfone Supports. The relative affinity of solvent with polymer and nonsolvent strongly influences the morphology of the fabricated membrane and is commonly evaluated by Hansen solubility parameter. Hansen’s solubility parameter (δ) consists of three parts, including a dispersion force component (δd), a polar component (δp), and a hydrogen-bonding component (δh),31 and is calculated from eq 5. The difference in solubility parameters between polymer−solvent pair and nonsolvent−solvent pair is calculated as the Hansen solubility parameter distance (RHSP) from eqs 6 and 7:31

(2) −4

where η is water viscosity (8.9 × 10 Pa·s), Q is the permeate volume per unit time (cubic meters per second), and ΔP is operation pressure. 2.7. Membrane Performance. Laboratory membrane separation equipment (FlowMem-0021-HP, Xiamen Filter & Membrane Technology) was used to measure the permeaselectivities of fabricated PSU supports and resulting composite NF membranes at 0.2 and 0.6 MPa (25 ± 1 °C), respectively. To achieve a stable flux, the composite NF membranes were prepressurized at 0.6 MPa for 30 min. Retention of four kinds of salt was used to evaluate NF performance. The diffusion coefficients and Stokes radii of various ions are listed in Table S2.27 Water flux (J) and salt rejection (R) were calculated from eqs 3 and 4, respectively:

J=

V AΔt

δ 2 = (δd)2 + (δp)2 + (δ h)2

(5)

RHSP,p − s = 4(δd,p − δd,s)2 + (δp,p − δp,s)2 + (δ h,p − δ h,s)2

(6)

RHSP,ns − s = 4(δd,ns − δd,s)2 + (δp,ns − δp,s)2 + (δ h,ns − δ h,s)2 (7)

where ns, s, and p represent nonsolvent, solvent, and polymer, respectively. Table 1 lists the Hansen solubility parameters of casting solvents, polymer (PSU), and nonsolvent (water) and their

(3)

where J is water flux (liters per square meter per hour), V is permeate volume (liters), A is effective membrane area (square meters), and Δt is filtration time (hours).

Table 1. Hansen Solubility Parameters31 of Solvents, Polysulfone, and Water with Their HSP Distances

Cp yz ij zz × 100 R = jjj1 − j Cc zz{ (4) k where Cp and Cc are salt concentrations in the permeate and concentrate, respectively. The antifouling performance of composite NF membrane was assessed with BSA as the model foulant. First, the water filtration procedure was operated at 0.6 MPa for 60 min. Then the pure water was replaced by 2 g·L−1 BSA aqueous solution (pH = 5.73), and the BSA filtration procedure was recycled for another 60 min. After the fouled membrane was washed, the procedures, including water filtration and BSA filtration, were repeated. The chlorine resistance of composite NF membrane was evaluated from variation of water flux and salt rejection. The composite NF membrane samples were chlorine-treated with 2000 ppm sodium hypochlorite solution with neutral pH. The chlorine-treated membranes were rinsed and soaked in pure water before measuring the permeaselectivity.

solubility parameter (MPa1/2)

RHSP (MPa1/2)

material

δd

δp

δh

δ

p−s

ns−s

PSU water NMP DMAc DMF

19.7 15.5 18.0 16.8 17.4

8.3 16.0 12.3 11.5 13.7

8.3 42.3 7.2 10.2 11.3

22.9 47.9 23.0 22.7 24.8

5.4 6.9 7.7

35.7 32.6 31.4

RHSP values. It is well-established that the solvent power of a casting solvent toward polymer increases with decreasing RHSP,p−s. The sequence of RHSP,p−s values between PSU and casting solvents was PSU−NMP < PSU−DMAc < PSU− DMF, which indicated that NMP was the best solvent for PSU among them due to the smallest difference between their solubility parameters. The sequence of RHSP,ns−s values between nonsolvent (water) and casting solvents was water−DMF < water−DMAc < water−NMP. The low RHSP,ns−s value indicated a strong interaction between water and solvent. Compared to water−NMP pair, the increased interactions of water−DMAc and water−DMF pairs were attributed to the stronger hydrogen-bonding interaction between water and DMAc/DMF, as shown by their hydrogen-bonding parameters summarized in Table 1. The viscosity was also closely related to the Hansen solubility parameter (δ).1 As shown in Figure S4, the influence of solvent type on the viscosities of casting solution was ranked as PSU−NMP > PSU−DMAc > PSU−DMF, which was opposite to the order of RHSP,p−s values. This was consistent with the fact that the low RHSP,p−s value between PSU and solvent resulted in stronger polymer−solvent interaction32 and higher viscosity of casting solution.

3. RESULTS AND DISCUSSION 3.1. Characterization of Graphene Oxide. Besides C C bonds (1620 cm−1), the presences of oxygen-containing groups including hydroxyl (3390 cm−1), carboxyl (1732 cm−1), and epoxy (1230 and 1090 cm−1) were found in the FT-IR spectrum of GO (Figure S1). This result agreed well with previous reports.20,28 TEM images (Figure S2) showed that the fabricated GO nanosheets were transparent with a single layer. The XRD pattern of GO is shown in Figure S3. A sharp feature diffraction peak at 10.08° was attributed to the (002) plane of GO,29 which was equal to a 0.877 nm d-spacing according to the Bragg equation.30 C

DOI: 10.1021/acs.iecr.8b04515 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. SEM images of top surfaces (column 1) and cross sections (column 2, low magnification; column 3, high magnification). (a) SM-NMP; (b) SM-DMAc; (c) SM-DMF.

solution due to the strong interaction between DMF and water. As a result, the fast exchange rate between DMF and water led to the formation of a skin layer with large porous structure. Smolders et al.33 proposed that liquid−liquid demixing by means of nucleation of the polymer-lean phase could initiate macrovoid formation under certain circumstances. In the SMDMF system, an increased amount of water diffused into the casting solution initiated more nuclei formation. However, the growth of every nucleus was hindered by the surrounding nuclei because all nuclei consumed solvent. As such, the growth of macrovoids was retarded and only small pores like the spongelike structure were formed.33 SM-NMP had a skin layer with less porous structure, which could hinder the indiffusion of nonsolvent and promote the formation of macrovoids. As shown in Figure 2, the mean pore sizes of SM-NMP, SMDMAc, and SM-DMF were 50.2, 57.5, and 115.3 nm and their porosities were 44.7%, 45.5%, and 50.5%, respectively. The orders of both pore sizes and porosities were SM-DMF > SMDMAc > SM-NMP, which was consistent with SEM observations. According to Figure 3 and Table 2, the sequence of membrane roughness was SM-DMF > SM-DMAc > SM-NMP, which was opposite to the order of viscosity of casting solutions. SM-NMP exhibited the smoothest surface, as its highest viscosity of casting solution led to delayed demixing. SM-DMAc and SM-DMF showed rough surfaces with large peak-to-valley height difference due to the low viscosity of their casting solutions that resulted in instantaneous demixing.

As illustrated in Figure 1, the surface pore sizes of PSU supports could be ranked as SM-DMF > SM-DMAc > SMNMP. In addition, SM-NMP support was composed of a thin top layer consecutively followed by spongelike structure and fingerlike macrovoids in the porous sublayer, whereas SMDMAc and SM-DMF supports were composed of a thin top layer followed by spongelike structure across the overall sublayer. Furthermore, the spongelike structures of sublayers close to the top skin layer were different: the closer to the skin layer, the smaller the pore size. The viscosity of casting solution is a main rheological characteristic affecting the morphology and performance of the fabricated membrane. Generally, high viscosity of casting solution leads to delayed demixing and low viscosity of casting solution leads to instantaneous demixing. On the other hand, the interaction of polymer−solvent pair and nonsolvent− solvent pair is another important factor in deciding the final structure and performance of the fabricated membrane during the nonsolvent-induced phase separation method. For SM-NMP during the phase inversion process, NMP was slow to diffuse out from the casting solution to the water bath due to the strong interaction between NMP and PSU; meanwhile, it was also difficult for water to diffuse into the casting solution due to the weak interaction between NMP and water. Thus, the relatively slow exchange rate between NMP and water favored the formation of a skin layer with small porous structure. On the contrary, for SM-DMF, it was fast for DMF to diffuse out from the casting solution to the water bath due to the weak interaction between DMF and PSU; at the same time, it was easy for water to diffuse into the casting D

DOI: 10.1021/acs.iecr.8b04515 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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porosity and the smaller the WCA. As shown in Figure 4, the water contact angles of SM-NMP, SM-DMAc, and SM-DMF

Figure 2. Porosities and mean pore sizes of PSU supports. Figure 4. Water contact angles of PSU supports.

were 63.6° ± 2.5°, 64.9° ± 3.4° and 24.1° ± 0.5°, respectively. According to Table 2, all the PSU supports displayed surface roughness within 15 nm, whose influence on the WCA could be ignored. Meanwhile, all the supports were made from PSU. Therefore, the difference in WCA was not caused by the roughness and membrane material but were mainly caused by differences in pore size and porosity. In particular, the pore size of SM-DMF was significantly larger than that of SM-NMP and SM-DMAc, which resulted in a remarkable decrease of WCA for SM-DMF. Water flux and BSA rejection of PSU supports are shown in Figure 5. The order of water flux was SM-DMF > SM-DMAc > SM-NMP and the order of BSA rejection was SM-DMF < SMDMAc < SM-NMP, which matched very well with their pore sizes and porosities.

Figure 3. Two- and three-dimensional AFM surface topographic images of PSU supports: (a) SM-NMP; (b) SM-DMAc; (c) SMDMF.

Table 2. Roughness of Polysulfone Supports roughness (nm) PSU support

Ra

Rq

Rz

SM-NMP SM-DMAc SM-DMF

7.1 9.9 11.0

8.9 12.9 13.8

16.9 42.8 23.3

Figure 5. Water flux (left axis) and BSA rejection (right axis) of PSU supports (0.2 MPa, 25 ± 1 °C).

3.3. Characterization and Performance of Composite Nanofiltration Membranes. 3.3.1. Scanning Electron Microscopy. The morphologies of the active layers formed on different PSU supports were imaged by SEM. As shown in Figures 6−8, all the TFC and TFN membranes demonstrated a typical composite structure consisting of an ultrathin selective layer and a porous sublayer. For TFC membranes without addition of GO, the thicknesses of PPA active layers was

The water contact angle (WCA) of the porous membrane is affected not only by the material hydrophilicity but also by the other surface properties including roughness, porosity, and pore size. For porous membranes of the same material, the rougher the membrane surface, the bigger the pore size and E

DOI: 10.1021/acs.iecr.8b04515 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. Top surface (column 1) and cross-sectional (column 2) SEM images of composite NF membranes with SM-NMP as support: (a) TFC-NMP and (b) TFN-NMP-GO-40.

Figure 8. Top surface (column 1) and cross-sectional (column 2) SEM images of composite NF membranes with SM-DMF as support: (a) TFC-DMF and (b) TFN-DMF-GO-40.

Consequently, TFN membranes showed a thinner active layer compared to the corresponding TFC membranes. 3.3.2. Atomic Force Microscopy and Water Contact Angle. Figures 9−11 illustrate the surface morphology of

Figure 7. Top surface (column 1) and cross-sectional (column 2) SEM images composite NF membranes with SM-DMAc as support: (a) TFC-DMAc and (b) TFN-DMAc-GO-40.

ranked as TFC-DMF > TFC-DMAc > TFC-NMP. After introduction of 40 ppm GO in aqueous phase, TFN membranes showed the same thickness order of nanocomposite active layers as TFN-DMF-GO-40> TFN-DMAcGO-40> TFN-NMP-GO-40. Interfacial polymerization is a reaction−diffusion process. When SM-NMP, with small pore size and porosity, was used as support, the amount of PIP monomer available for IP reaction decreased, which resulted in a thinner active layer. On the contrary, when SM-DMF, with large pore size and porosity, was used as support, more PIP monomer was left in the support and the diffusion resistance of PIP molecules into the organic phase decreased, which resulted in a thicker active layer. Furthermore, the introduction of GO nanofiller would slow down the diffusion of PIP monomers into the organic phase due to the steric hindrance effect of GO nanofiller and the hydrogen bonding between PIP molecules and GO nanofiller, which led to a decrease in active layer thickness.34

Figure 9. Two- and three-dimensional AFM surface images of composite NF membranes: (a) TFC-NMP and (b) TFN-NMP-GO40.

composite membranes, and their roughness data are listed in Table 3. The roughness sequence of TFC NF membranes was TFC-DMF > TFC-DMAc > TFC-NMP, which was the same as the roughness sequence of their PSU supports. Notably, the formation of active layer was deposited on the top of PSU support via IP reaction. Thus, the roughness of active layer was closely related to the texture and roughness of PSU support.35,36 Furthermore, the physicochemical properties of PSU supports played important roles in the IP process and the formation of active layer, especially the pore sizes and porosities, which ultimately affected the roughness of active layer. For example, SM-NMP support, which showed the smallest pore size and porosity, had less PIP monomer and F

DOI: 10.1021/acs.iecr.8b04515 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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membranes decreased regardless of the PSU supports with different casting solvents. The water contact angles of composite NF membranes are given in Table 3. In comparison with TFC membranes, the WCA of the corresponding TFN membranes decreased significantly when 40 ppm GO was introduced into the aqueous phase. This was because the embedded GO in PPA active layer possessed a large amount of oxygen-containing hydrophilic functional groups.23 Generally, the hydrophilic and smooth membrane surface exhibits excellent antifouling performance. As such, GO-embedded TFN membranes were expected to show better antifouling abilities than conventional TFC membranes. 3.3.3. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. The surface chemical structures of SMNMP and its composite NF membranes were characterized by ATR-FTIR. As shown in Figure 12, SM-NMP support showed

Figure 10. Two- and three-dimensional AFM surface images of composite NF membranes: (a) TFC-DMAc and (b) TFN-DMAcGO-40.

Figure 12. ATR-FTIR spectra of SM-NMP and its composite NF membranes.

a distinct peak at 1658 cm−1 assigned to a primary amide stretch of the residual PVP in the PSU matrix.37 After IP reaction, both TFC and TFN NF membranes exhibited two new peaks at 1625 and 3410 cm−1. The former was attributed to the carbonyl stretching vibration of amide I, and the latter was ascribed to the hydroxyl stretching vibration. These changes indicated the PPA layer formed on the top surface of PSU support after IP reaction. Compared to TFC-NMP, TFNNMP-GO-40 presented a new peak at 1730 cm−1 ascribed to the carboxyl group of GO structure. Furthermore, TFN-NMPGO-40 displayed an intense absorption peak at 3410 cm−1 compared to TFC-NMP. This result indicated that GO nanofiller was successfully embedded into the PPA active layer during IP process. 3.3.4. X-ray Photoelectron Spectroscopy. According to Figure 13a, both SM-NMP support and the resultant composite NF membranes demonstrated three peaks at 530.8, 399.2, and 284.5 eV, representing the O 1s, N 1s, and C 1s regions, respectively, which were consistent with the previous report.38 Since a small amount of PVP remained in the PSU matrix, a characteristic N 1s peak could be still detected for SM-NMP. However, compared to SM-NMP support, the N 1s characteristic peak of composite NF membranes is significantly enhanced, while their S 2p characteristic peaks were significantly weakened and even disappeared. This further clarified that an ultrathin PPA

Figure 11. Two- and three-dimensional AFM surface images of composite NF membranes: (a) TFC-DMF and (b) TFN-DMF-GO40.

Table 3. Roughness and Water Contact Angles of Composite Nanofiltration Membranes roughness (nm) membrane

Ra

Rq

Rz

TFC-NMP TFN-NMP-GO-40 TFC-DMAc TFN-DMAc-GO-40 TFC-DMF TFN-DMF-GO-40

19.0 12.5 42.2 17.7 49.4 26.0

24.4 16.5 55.6 23.0 62.0 34.2

65.9 34.1 87.2 97.6 129.0 63.7

WCA (deg) 33.4 26.2 32.3 26.1 41.1 35.2

± ± ± ± ± ±

3.2 3.9 2.7 1.3 3.2 2.2

correspondingly reduced the surface roughness of TFC NF membrane. The PIP monomers diffused into the organic phase were slowed down after introduction of GO into the aqueous solution; as a result, the surface roughness of TFN NF G

DOI: 10.1021/acs.iecr.8b04515 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 13. XPS spectra of SM-NMP and its composite NF membranes.

selective layer was deposited on SM-NMP support. Figure 13 b−d shows the high resolution C 1s spectra. In comparison with SM-NMP, both TFC-NMP and TFN-NMP-GO-40 had new groups of CO and C−N, which also confirmed the formation of PPA active layer. In addition, compared to TFCNMP, TFN-NMP-GO-40 had new hydrophilic groups including −C(O)O and C−O−C, which also indicated that GO had been successfully embedded in the PPA active layer. 3.3.5. Performance of Composite Nanofiltration Membranes. The effects of casting solvents and GO concentrations on the water flux and salt rejection of composite NF membranes are illustrated in Figures 14 and 15, respectively. With increasing GO concentration, the water fluxes of both TFN-NMP and TFN-DMAc membranes increased first and Figure 15. Na2SO4 rejections of composite NF membranes (using PSU supports made with different casting solvents) as a function of GO concentration.

then decreased. The increase of water flux was mainly ascribed to the thinner active layer caused by the hindrance of PIP diffusion into the organic phase by GO. However, the water flux turned to decrease at high GO concentration because of the agglomeration of excessive GO nanosheets. The maximum water flux for TFN-NMP was 46.9 L·m−2·h−1, which was achieved at a GO concentration of 40 ppm, corresponding to 137.9% of the value for TFC-NMP. At a GO concentration of 20 ppm, the maximum water flux of TFN-DMAc was 27.9 L· m−2·h−1, which was 127.4% of the value for TFC-DMAc. Meanwhile, TFN-NMP and TFN-DMAc membranes retained the high Na2SO4 rejections (>98%) without a rejection loss when GO concentration varied from 20 to 80 ppm, indicating that the introduction of GO nanofiller did not generate membrane defects. Nevertheless, Na2SO4 rejection of TFNDMF membranes was less than 90% and decreased with

Figure 14. Water flux of composite NF membranes (using PSU supports made with different casting solvents) as a function of GO concentration. H

DOI: 10.1021/acs.iecr.8b04515 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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force to a divalent co-ion (SO42−) than to a monovalent co-ion (Cl−) due to the electrostatic repulsion effect. Meanwhile, SO42− ion has a larger Stokes radius and a lower diffusion coefficient compared to Cl− ion (Table S2). As a result, it was more difficult for SO42− ion to penetrate through the membrane, which resulted in a remarkably higher rejection of SO42− than of Cl−. Furthermore, the negatively charged NF membrane surface exhibited a stronger attraction for a divalent counterion (Mg2+) than a monovalent counterion (Na+). Consequently, Mg2+ ion more easily permeated the negatively charged NF membrane, leading to a lower rejection of MgSO4 than of Na2SO4. For salt rejection having the same monovalent co-ion (Cl−), because Mg2+ ion has a larger Stokes radius and a lower diffusion coefficient compared to Na+ ion (Table S2), the higher rejection of MgCl2 than of NaCl was mainly attributed to the size-exclusion effect hindering Mg2+ ion. The incorporation of GO into the active layer presented two opposite effects for salt rejection. On the one hand, nonporous fillers such as GO could disrupt the packing manner of the neighboring polymer chains and correspondingly increase the free volume of PPA matrix,41 which was adverse to the salt retention caused by formation of a relatively loose active layer. Meanwhile, the nanocomposite active layer exhibited a more negatively charged surface due to dissociation of carboxyl from GO, which enhanced the salt rejection because of the stronger electrostatic repulsion effect.22,24,25 As a result, TFN-NMP and TFN-DMAc membranes maintained high salt rejection, comparable to that of TFC membrane. Nevertheless, the salt rejection of TFN-DMF membranes decreased remarkably when GO concentration reached 60 ppm, because at this high GO concentration it was easier to form a defective skin layer on SM-DMF support with large pore size. 3.3.6. Antifouling Abilities. As illustrated in Figure 17, water flux showed a declining trend during the BSA filtration

increasing GO concentration. Correspondingly, their water fluxes increased with increasing GO concentration. The permeability of the composite NF membrane is affected by many factors, including the thickness and structure of active layer and the pore size and porosity of sublayer. Although SMNMP exhibited a lower water flux than SM- DMAc, TFNNMP membranes presented higher water fluxes than TFNDMAc membranes. This result confirmed that the active layer played a dominant role in the permeability of composite NF membranes, rather than the PSU support. In addition, PSU support having high water flux was not an indispensable requirement for fabricating the corresponding high-flux TFC or TFN NF membranes. In this study, PSU supports made from NMP exhibiting a small surface pore size prevented the penetration of PPA layer in the PSU pores, which resulted in formation of a defect-free active layer with high permeaselectivity regardless of TFC or TFN NF membranes. On the contrary, the surface pore size of PSU supports made from DMF was too large to form a dense and defect-free active layer, which led to inferior rejections of TFC-DMF or TFN-DMF membranes. Therefore, it was vital to choose a suitable casting solvent to optimize the performance of TFC and TFN NF membranes. As described, the nanocomposite active layers became thinner, looser, and smoother as well as more hydrophilic after incorporating an appropriate amount of GO nanofiller, which significantly reduced the filtration resistance and led to a faster water flow on the top surface with less friction.39 Moreover, GO nanofiller provided additional water channels.22 As a result, TFN-NMP-GO-40 and TFN-DMAc-GO-20 membranes demonstrated superior water fluxes over their corresponding TFC membranes. The optimum TFN membrane (TFN-NMP-GO-40) and TFC membrane (TFC-NMP) were selected for further investigation of their separation characteristics toward different inorganic salts. According to Figure 16, the salt rejections of

Figure 17. Time-dependent normalized flux of composite NF membranes. Figure 16. Salt rejection of optimal TFC-NMP and TFC-NMP-GO40 membranes.

procedure, due to irreversible BSA adsorption.42 Nevertheless, TFN-NMP membranes showed a higher normalized flux with increasing GO concentration, which indicated that the antifouling abilities of TFN-NMP membranes were improved with increasing GO concentration. In comparison with TFCNMP membranes, the membrane surfaces of TFN-NMP became more hydrophilic and negatively charged. This contributed to the formation of a regular hydration layer on the membrane surface and generated a stronger repulsive force

both TFC-NMP and TFN-NMP-GO-40 membranes decreased in the order Na2SO4 > MgSO4 > MgCl2 > NaCl, which was in good agreement with the negatively charged NF membrane characteristics. It was revealed that the co-ion valence had a stronger impact on the ion retention and transport compared to the counterion.40 The negatively charged NF membrane surface offered a greater repulsive I

DOI: 10.1021/acs.iecr.8b04515 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 18. Effects of chlorination exposure time on (a) water flux and (b) salt rejection of composite NF membranes.

to BSA. Consequently, BSA adsorption on the membrane surface was alleviated. 3.3.7. Chlorine Resistance. As illustrated in Figure 18, with increasing chlorination exposure time, both TFC-NMP and TFN-NMP membranes showed an increasing trend of normalized flux and a decreasing trend of normalized salt rejection. Furthermore, TFN-NMP membranes showed a smaller normalized flux and a larger normalized salt rejection with increasing GO concentration. This suggested that the chlorine resistance was improved with increasing GO concentration. Although the lack of amide protons in PPA was helpful to improve the chlorine resistance, chlorination still occurred at the low-abundance non-cross-linked nitrogen atoms.43 The enhanced chlorine resistance of TFN-NMP probably resulted from two aspects: (1) chemical protection against chlorination, due to chemical bonding between PPA and GO, and (2) physical protection for the underlying PPA against chlorination, due to the huge specific surface area of GO.44

membrane significantly increased to 46.9 L·m−2·h−1, which was 137.9% of the value for TFC-NMP membrane.

4. CONCLUSIONS In this study, the influences of three different casting solvents on the morphologies, properties, and performance of PSU supports and their GO-embedded TFN NF membranes were investigated in depth. Hansen solubility parameter was used to select the best casting solvent for PSU supports. The results showed that the morphologies, physicochemical properties, and performance of both PSU supports and the subsequent composite NF membranes were strongly influenced by casting solvents. PSU support with high water flux was not an indispensable requirement for fabricating the resultant highflux TFC or TFN NF membranes. Interestingly, PSU support made from NMP, exhibiting a small surface pore size, prevented the formation of PPA layer into the PSU pores, which favored the formation of a defect-free active layer with high permeaselectivity regardless of TFC or TFN NF membranes. On the contrary, the surface pore size of PSU support made from DMF was too large to form a dense and defect-free active layer, which led to inferior rejections of TFCDMF or TFN-DMF membranes. Therefore, it was vital to select the optimal casting solvent to maximize the performance of TFC and TFN NF membranes. Furthermore, the nanocomposite active layer became thinner and smoother with enhanced hydrophilicity and negative charge after incorporating an appropriate amount of GO nanofiller; as a result, TFNNMP-GO-40 membrane exhibited excellent permeaselectivity, antifouling ability, and chlorine resistance over TFC-NMP membrane. Particularly, on the basis of retaining high salt rejection without a loss, the water flux of TFN-NMP-GO-40

Quanling Xie: 0000-0002-0649-9039

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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.iecr.8b04515.

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Additional text, two tables, and four figures describing preparation and characterization of GO, compositions and viscosities of casting solutions, and key parameters of relevant ions (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.X.). *E-mail: [email protected] (Z.H.). *E-mail: [email protected] (W.S.). ORCID Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate financial support from the Scientific Research Foundation of Third Institute of Oceanography, SOA (2016036), the Science & Technology Planning Project of Xiamen City, China (3502Z20172008), Marine Economy Innovation Develepment Area Demonstration Project of Beihai (Bhsfs009), Fundamental Research Funds for the Central Universities (20720170027), and the National Natural Science Foundation of China (21736009).

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REFERENCES

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