Graphene Oxide Quantum Dots Incorporated into a ... - ACS Publications

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Graphene Oxide Quantum Dots Incorporated into a Thin Film Nanocomposite Membrane with High Flux and Antifouling Properties for Low-Pressure Nanofiltration Chunfang Zhang, Kaifang Wei, Wenhai Zhang, Yunxiang Bai,* Yuping Sun, and Jin Gu The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, PR China S Supporting Information *

ABSTRACT: Graphene oxide quantum dots (GOQDs), novel carbon-based nanomaterials, have attracted tremendous research interest due to their unique properties associated with both graphene and quantum dots. In the present study, thin film nanocomposite (TFN) membranes comprising GOQDs dispersed within a tannic acid (TA) film were fabricated by an interfacial polymerization reaction for low-pressure nanofiltration (NF). The resultant TA/GOQDs TFN membranes had measurably smoother and more hydrophilic, negatively charged surfaces compared to the similarly formed TA thin film composite (TFC) membrane. Owing to the loose active layer structure and the combination of Donnan exclusion and steric hindrance, the TA/ GOQDs TFN membrane showed a pure water flux up to 23.33 L/m2·h (0.2 MPa), which was 1.5 times more than that of pristine TA TFC membrane, while high dye rejection to Congo red (99.8%) and methylene blue (97.6%) was kept. In addition, the TA/GOQDs TFN membrane presented better antifouling properties, which was ascribed to the favorable changes in membrane hydrophilicity, ζ-potential, and surface roughness. These results indicated the great potential of such membranes in wastewater treatment, separation, and purification in many industrial fields. KEYWORDS: graphene oxide quantum dots, nanofiltraion membrane, thin film composite, tannic acid, antifouling

1. INTRODUCTION Nanofiltration (NF), a new type of membrane separation process, has gained much attention in water softening, desalination, dye purification, and the food and paper industries.1−3 NF membrane is a type of pressure-driven membrane that has properties in between those of ultrafiltration and reverse osmosis membranes. The operating pressures are usually near 0.6 MPa (90 psi) and can be as high as 1 MPa (150 psi).4,5 The requirement of high pressure to the system usually leads to high operational cost, which limits the application of NF membranes. Recently, newgeneration low-pressure NF membranes were introduced. This kind of membrane generally has loose structure and can be operated at relative low pressure.6 Thin film composite (TFC) membrane, which deposits a thin selective separation layer on a porous support, is widely investigated and explored in the field of low-pressure NF.7 A great advantage of TFC technology is that the extremely thin layer and the porous © 2017 American Chemical Society

support can independently be optimized to meet the requirements for target application. There are several methods to fabricate TFC membranes, including interfacial polymerization,8 layer-by-layer assembly,9 and surface grafting, as well as physical coating.10,11 Among them, interfacial polymerization, achieved by the polycondensation of two monomers at the oil/ water interfaces, has been investigated and mostly employed to produce commercial TFC NF membranes. Over the past decades, water flux and solute rejection of TFC membranes have continually improved, but there are still several limitations in the NF process, it being relatively energy-intensive, lowpermeability, and fouling-prone.12,13 Therefore, the development of novel and efficient NF membranes with low operating Received: October 9, 2016 Accepted: February 28, 2017 Published: February 28, 2017 11082

DOI: 10.1021/acsami.6b12826 ACS Appl. Mater. Interfaces 2017, 9, 11082−11094

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ACS Applied Materials & Interfaces

Figure 1. Synthesis of GOQDs.

good electrical conductivity, high mobility, tunable band gaps, good biocompatibility, quantum confinement effect, and good dispersion in various solvents,25 GOQDs have attracted extensive attention from scientists and also exhibit bright promise in bioimaging devices,26 photovoltaic devices,27 lightemitting diodes,28 the environmental field,29 and fuel cells.30 GOQDs share most of the excellent features and properties of GO, which make it a potential candidate for the fabrication of composite materials with favorable properties, such as hydrophilic, mechanical, and antimicrobial properties.31 Furthermore, the particular size, shape, and edge structure give GOQDs an excellent dispersion into the polymer matrix, which will certainly generate an optimum for the application of separation and permeation. Therefore, the fabrication of a GOQDmodified membrane is an admirable attempt to achieve the optimization of multifunctional properties, leading to a substantial improvement of membrane performance. Zeng et al. reported PVDF membranes covalently functionalized with graphene oxide quantum dots for ultrafiltration with significantly enhanced bactericidal and antibiofouling performances.32 Song et al. incorporated graphene oxide quantum dots in reverse osmosis membranes for antifouling and chlorine resistance. However, to the best of our knowledge, nanocomposite membranes based on GOQDs have rarely been reported for nanofiltration.33 We report here the use of GOQDs as filler materials in TFN membranes for nanofiltration. GOQDs nanosheets were prepared by the “bottom-up” method of pyrolyzing citric acid (CA).34 Then, by incorporating GOQDs into a tannic acid (TA) layer, TA/GOQDs TFN membranes were successfully fabricated via interfacial polymerization for low-pressure NF. The TFN membranes were tested regarding their flux and separation performance and comprehensively characterized in terms of charge properties, morphology, and chemical composition of the active separation layer. In this study, TA was explored as an aqueous monomer to prepare an interfacial polymerization layer on the surface of a porous support. TA is a natural polyphenol and has antibacterial, antienzymatic, and astringent properties. Additionally, the TA thin film obtained from interfacial polymerization exhibited a relative “loose” structure,35 which could realize the separation of dyes and multivalent salts under low operating pressure. The applicability for low-pressure NF makes the TA/GOQDs TFN membranes economically attractive in terms of both manufacturing and desalination costs.

pressure and high antifouling properties is still of high importance. Nanotechnology has produced entirely new classes of nanocomposite membranes the applications of which to water treatment offer exciting and promising possibilities. Many nanocomposite membranes exhibit improved mechanical, chemical, and thermal stability, as well as improved separation, reaction, and sorption capacity.14 The interfacial polymerization of thin film nanocomposite (TFN) membranes was first developed by Jeong et al.15 They successfully embedded zeolite NaA nanoparticles into a polyamide (PA) thin film layer by interfacial polymerization, which dramatically improved the permeation properties of PA TFN membrane. MCM-41 silica nanoparticles were also incorporated into TFN membrane for water purification, showing an enhanced performance in comparison with the unmodified membrane.16 Lee et al. reported PA−TiO2 TFN membranes, which had a rejection value for MgSO4 of around 95% and a water permeation flux of 9.1 L·m−2·h−1 at 0.6 MPa.17 The recent emergence of graphene oxide (GO) has opened up a new era for developing nanocomposite membranes with high separation performance owing to its unique monatomic thickness, two-dimensional structure, high mechanical strength, and chemical inertness.18 Many studies indicated that liquid water can afford an ultrafast transport through the narrow interspace between the GO flakes, leading to a high water permeability in fluid separation.19 In addition, GO showed a clear molecular cutoff and high selectivity due to the wellarranged laminar structure and well-defined interlayer space.20 GO flakes are generally negatively charged with rich oxygencontaining functional groups, such as −COOH,21 which would be beneficial for the removal of charged solutes. Furthermore, the high negative charge density of GO could lead to excellent fouling resistance, because most foulants are also negatively charged.22 For these reasons, the preparation, microstructures, and performance of GO-based membranes for NF application have been widely studied. However, research on the combination of GO flakes with a polymer to form TFN membrane via interfacial polymerization lags behind. On the one hand, GO flakes are hard to wrap in a polymer network to form a defect-free skin layer because the size of GO flakes (0.5 μm) is generally equal to the thickness (0.2−1 μm) of thin films formed by interfacial polymerization.23 On the other hand, the accumulating of GO could not be avoided. The nonselective boundary defect would be easily formed between the GO flakes and the hosting polymer. Graphene oxide quantum dots (GOQDs), single- or fewlayer graphene with a tiny size of only several nanometers, are a new type of carbon-based nanomaterial with the unique properties associated with both GO and quantum dots.24 Due to their excellent properties, such as high specific surface area,

2. EXPERIMENTAL SECTION 2.1. Materials. Flat-sheet polyacrylonitrile (PAN) ultrafiltration (UF) membrane supplied by Lanjing Membrane Co. Ltd. (Shanghai, China) was used as a substrate. The PAN membranes were stored in fresh deionized water before using. Citric acid (CA) was purchased from Sun Chemical Technology Co. Ltd. (Shanghai, China). 11083


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Figure 2. (a) Preparation process of TA/GOQDs TFN membrane. (b) Scheme of reaction mechanism among TA, GOQDs, and IPDI. were air-dried in an oven at 60 °C for further polymerization reaction and n-hexane evaporation for 5 min. The membranes were kept in flowing tap deionization (DI) water overnight to remove excess monomers and were stored in DI water until testing. The resultant TFN membranes were designed as TA/GOQDs-x, where x indicates the concentration of GOQDs in aqueous solution. 2.4. Characterization of GOQDs and TA/GOQDs TFN Membrane. 2.4.1. Raman Spectrum. Raman spectrum of GOQDs was obtained from a Renishaw micro-Raman spectroscopy system (Renishaw in Via) at ambient temperature. The excitation wavelength is 785 nm with an argon ion laser. 2.4.2. X-ray Diffraction (XRD). XRD patterns of GOQDs were obtained from a X-ray D8 Advance instrument with Cu Kα radiation in the 2θ range from 5° to 60° at a rate of 1°/min (40 kV, 20 mA, λ = 1.540 51). 2.4.3. Fourier-Transform Infrared (FTIR). FTIR measurements were recorded using a FTLA 2000 type spectrometer for 128 scans at a resolution of 4 cm−1. The reflection and transmission mode was used for TA/GOQDs TFN membranes on a zinc−selenium/diamond plate and for GOQDs powders with the KBr pellet technique, respectively. The data were recorded in the range from 4000 to 500 cm−1 at room temperature. 2.4.4. Transmission Electron Microscopy (TEM). High-resolution TEM images of GOQDs were gathered on a JEOL 2010 (HR) transmission electron microscope at 200 keV. The samples were prepared by dispersing a small amount of GOQDs into water. Then, one drop of the above dispersion was dropped on the 300 mesh carbon-coated copper grids, followed by drying under vacuum. 2.4.5. Scanning Electron Microscopy (SEM). SEM images of the surface morphology of TA/GOQDs TFN membranes were collected using a HITACHI S4800 field emission scanning electron microscope at 1 kV. All the samples were coated with a thin layer of gold to prevent charging. 2.4.6. Atomic Force Microscopy (AFM). AFM images were collected using a Multimode 8 (Germany Brook Technology Co. Ltd.) instrument operating in tapping mode in air at room temperature

Isophorone diisocyanate (IPDI) was obtained from Aladdin Reagent Co. Ltd. (Shanghai, China). Tannic acid (TA), sodium sulfate (Na2SO4), magnesium sulfate (MgSO4 ), magnesium chloride (MgCl2), sodium chloride (NaCl), sodium dihydrogen phosphate (NaH2PO4), disodium phosphate dodecahydrate (Na2HPO4), nhexane, Congo red, methyl blue, methyl orange, methylene blue, and bovine serum albumin (BSA) were provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals were used as received without further purification. 2.2. Preparation of GOQDs. GOQDs used in this study were prepared by directly pyrolyzing CA according to the literature procedure,34 and the scheme of the synthesis process is shown in Figure 1. In a typical procedure of GOQDs preparation, 2 g of CA was put into a 5 mL beaker and heated to 200 °C using a heating mantle. During the heating process, the CA was first liquated and then the color of the liquid changed from colorless to pale yellow and finally to orange in 30 min, implying the formation of GOQDs. The obtained orange liquid for preparing GOQDs was dialyzed in a dialysis bag with a 1000 Da molecular weight cutoff against deionized water for 48 h to remove excess CA. After that, the solution in the bag was freeze-dried to obtain solid GOQDs. 2.3. Preparation of TA/GOQDs TFN Membrane. All TFN membranes were prepared on PAN UF membranes through interfacial polymerization, as shown in Figure 2a. First, a certain amount of GOQDs was dispersed in an aqueous solution of 0.3 g/L TA aqueous solution (pH was kept at 7.0 with 0.1 M phosphate buffer solution) by ultrasonication for 30 min to form mixture aqueous solution. The concentrations of GOQDs in aqueous solution were controlled at 0, 0.25, 0.5, 0.75, and 1 g/L, respectively. The PAN UF membrane taped to a glass plate was placed in the aqueous phase for approximately 20 min to ensure that GOQDs and TA monomers diffused into porous support. Then the residual solution was removed from the surface of UF membranes using filter papers. After that, the TA- and GOQDssaturated UF membrane was immersed in a solution of 2.5 g/L IPDI in n-hexane. After 10 min of reaction, the membranes were taken from organic phase. Excess organic solution was drained and membranes 11084


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Figure 3. (a) Raman spectra of as-prepared GOQDs, (b) XRD patterns of GOQDs, (c) TEM image of as-prepared GOQDs, (d) the corresponding size distribution of GOQDs shown in panel c, (e) AFM image of GOQDs, and (f) the corresponding height profile along the black line shown in panel e. to measure the thickness of GOQDs and the surface roughness of TA/ GOQDs TFN membranes. The sample of GOQDs was prepared by dropping a dilute water dispersion of GOQDs on the silicon slice, followed by drying under vacuum. The samples of the prepared membranes were prepared by fixing on a holder, and 5 × 5 μm areas were scanned by tapping mode in air at room temperature, with standard silicon tips and with a constant force of 40 N/m. 2.4.7. X-ray Photoelectron Spectroscopy (XPS). Chemical compositions of the TA/GOQDs TFN membranes surfaces were obtained by XPS (PHI-1600 X-ray photoelectron spectrometer) using Mg Ka as radiation source. The takeoff angle of the photoelectron was set at 90°. The survey spectra of the membranes were collected over the range of 0−1100 eV. High-resolution spectra of N 1s were also collected. 2.4.8. Static Contact Angle. Static contact angles of the membranes were determined by the sessile-drop method using a contact angle goniometer OCAH200. The measurements were done using 1 μL of water as the probe liquid, and the average value from five random

locations was used as the contact angle to minimize the experimental errors. 2.4.9. Surface ζ-Potential Measurement. The surface ζ-potential of the membranes was analyzed by the streaming potential method using a SurPASS electrokinetic analyzer (Anton Paar Gmb H), with 0.01 M potassium chloride (KCl) as an electrolyte solution. Each membrane was measured three times at pH 7, and average ζ-potential values were calculated to reduce the experimental errors. 2.5. Nanofiltration Performance of TA/GOQDs TFN Membranes. The performance evaluation was carried out using a crossflow NF system. The system consisted of a membrane cell, plunger pump, pressure gauge, and solution vessel. The NF membrane was loaded into the membrane cell for filtration, and the effective area of the membrane was fixed at 23.7 cm2. Each membrane was initial compacted at 0.25 MPa for 0.5 h to make sure the membrane was in the steady state. Then the pressure was maintained at 0.2 MPa with a cross-flow velocity of 40 L h−1. The pure water flux (Jw) and dye flux (Jd) was calculated using the following equation 11085


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ACS Applied Materials & Interfaces J=

V At

The surface chemistry of GOQDs was further investigated by FTIR. As shown in Figure 4, the bands emerging at 1724 and

(1)

where V (L) is the permeate volume, A (m2) is the effective area of membrane, and t (h) is the operation time. To determine the rejection properties of the membranes, the solution vessel was filled with aqueous solutions of dyes or inorganic salts. The dye and inorganic salt concentrations in feed solutions were 100 and 1000 mg/L, respectively. During the NF process, the concentrate was recirculated to the feed vessel while the permeate was collected in the permeate vessel. The rejection ratio (R) was calculated by the following equation

⎛ Cp ⎞ R = ⎜1 − ⎟ × 100% Cf ⎠ ⎝

(2)

where Cp and Cf represent the concentration of permeate and feed solutions, respectively. The dye concentrations were determined by an ultraviolet−visible spectrophotometer at the maximal absorption wavelength of the dye. Salt concentrations of solutions were measured by electrical conductivity (DDS-11A, Shanghai Leichi Instrument Co., Shanghai, China). To evaluate the long-term operational stability, the prepared membranes were tested for 20 days continuously at 0.2 MPa with a cross-flow velocity of 40 L h−1. The flux of pure water and rejection of Na2SO4 were measured every 24 h. The Na2SO4 concentration was fixed at 1000 mg/L. 2.6. Antifouling Estimation of Composite NF Membranes. In the antifouling experiments, BSA was used as a representative protein. A test solution of 0.1 g/L BSA at pH 7.0 was used, and the pressure was maintained at 0.2 MPa with a cross-flow velocity of 40 L h−1. First, pure water was filtrated through the membrane for 12 h and the steady water flux was recorded as Jw1. Then, the filtration of the BSA was continued for another 12 h and the steady flux of BSA was recorded as JBSA. After that, the used NF membranes were cleaned directly in pure water for 30 min under magnetic stirring. At last, the steady water flux of cleaned membrane (Jw2) was measured again. The total flux decline rate (FDR) and flux recovery ratio (FRR) were calculated using the following expression

⎛ J ⎞ FDR = ⎜⎜1 − BSA ⎟⎟100% Jw1 ⎠ ⎝

(3)

⎛J ⎞ FRR = ⎜⎜ w2 ⎟⎟100% ⎝ Jw1 ⎠

(4)

Figure 4. FTIR spectroscopy of GOQDs.

3300−3600 cm−1 correspond to carboxyl groups and hydroxyl groups, respectively. This confirms the existence of functional groups on the surface and at the edge of the GOQDs flakes. The hydrophilic properties of these functional groups will be beneficial in the dispersibility of GOQDs flakes in aqueous solution. More important, the hydroxyl and carboxyl groups of GOQDs can form a covalent linkage with an isocyanate group of IPDI during the interfacial polymerization process (as shown in Figure 2b), which prevents the aggregation of GOQDs flakes. Furthermore, the firmly covalent linkage could avoid the detachment of GOQDs from the separate layer, improving the stability of TA/GOQDs TFC membranes during filtration and the hydraulic cleaning process. 3.2. Membrane Characterization. 3.2.1. FTIR. Figure 5 presents the FTIR spectrum of the PAN substrate and TA/

3. RESULTS AND DISCUSSION 3.1. Characterization of GOQDs. Figure 3 displays typical characterization results of as-prepared GOQDs. As shown in Figure 3a, the Raman spectrum of the GOQDs shows a G peak at 1600 cm−1, which relates to the vibration of the sp2 hybridization of graphitic carbon, and a D peak at 1400 cm−1, which correspond to the carbon lattice distortion. The XRD (Figure 3b) patterns of GOQDs shows a wide (002) peak at 2θ = 26° corresponding to an interlayer spacing of 0.34 nm (according to the Bragg equation 2d sin θ = nλ), indicating that carbonizing CA produced graphite structures. This result is consistent with the reported result.34 The TEM image (Figure 3c) and the estimated size distribution (Figure 3d) of the GOQDs exhibit a narrow size distribution between 2 and 5 nm with an average diameter of 3.5 nm. AFM image demonstrates the topographic morphology of the GOQDs, the heights of which are mainly distributed within the range of 0.5−2 nm with an average height of 1.1 nm (Figure 3e,f), suggesting that most of the GOQDs have one to three graphene layers.

Figure 5. ART-FTIR spectrum of PAN substrate, TA TFC membrane, and TA/GOQDs TFN membranes.

GOQDs TFN membranes. For the PAN substrate, a characteristic band appeared around 2240 cm−1, corresponding to the CN stretching of the acrylonitrile unit in the PAN chains, and a band appeared at 1730 cm−1, assigned to the carboxyl group. The band at 1452 cm−1 (δC−H in CH2) is characteristics of aliphatic CH groups along the PAN backbone. For the pristine TA TFC membrane, the formation of urethane linkages between TA and IPDI can be confirmed by the appearance of two absorption bands, the one in the region between 1620 and 1640 cm−1 due to CO (amide I) stretching and the other at 1545 cm−1 attributed to N−H 11086


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hydroxyl groups of GOQDs and TA with an isocyanate group of IPDI in the interfacial polymerization.36 The appearance of −NHCO− species at 399.77 eV illustrates the reaction of carboxyl groups of GOQDs with an isocyanate group of IPDI during the interfacial polymerization.36 3.2.3. SEM. The studied membranes were characterized by SEM, and their surface morphologies are shown in Figure 7.

bending vibrations (amide II). No new absorption peak can be observed in TA/GOQDs TFN membrane. Meanwhile, the peak density of amide I and amide II displays a clear increasing tendency compared to that of the pristine TA TFC membrane, which could be attributed to the amide and urethane bonds formed by the reaction of hydroxyl and carboxyl in GOQDs with an isocyanate group in IPDI (seen in Figure 2b). These findings demonstrate that the GOQDs were successfully introduced into the TA separation layer via a covalent linkage and its content increased upon increasing the concentration of GOQDs in the aqueous phase. 3.2.2. XPS. Elemental information on the prepared membranes surface was studied by XPS. In Figure 6a, the

Figure 7. SEM images of PAN substrate, TA TFC membrane, and TA/GOQDs TFN membranes: (a) PAN substrate, (b) TA, (c) TA/ GOQDs-0.25, (d) TA/GOQDs-0.5, (e) TA/GOQDs-0.75, and (f) TA/GOQDs-1. The corresponding high-magnification SEM images were placed in the upper right corner. Figure 6. XPS spectra of TA TFC and TA/GOQDs TFN membranes (a) and high-resolution XPS spectrum of N 1s of TA/GOQDs-1 membrane surface (b).

The cross-sectional morphologies of the membrane are presented in Figure S1 (Supporting Information, SI). It can be seen from Figure 7a that a typical microporous image with a pore size of 10 nm is visible from the PAN substrate surface. A defect-free TA separation layer with surface crumples is formed on the substrates (Figure 7b) after the interfacial polymerization of TA and IPDI. By the incorporation of GOQDs, the surface crumples of TA/GOQDs TFN membranes were reduced, as shown in Figure 7c−f. This can be ascribed to the steric-hindrance effect of GOQD flakes and the delayed interfacial polymerization arising from the excess aqueous monomers concentration. During the interfacial polymerization process, the steric hindrance of the GOQDs in aqueous solution decreased the diffusion of TA and thereby retarded the formation of the TA layer. The relatively slow reaction rate of interfacial polymerization was favorable for the formation of a smooth surface.22 Also, as shown in Figure S1 (SI), the thickness of the TA/GOQDs active layer reduced slightly with the increase of GOQDs concentration in aqueous solution, further indicating that the introduction of GOQD flakes

XPS spectra for the TA TFC, TA/GOQDs-0.5, and TA/ GOQDs-1 TFN membranes reveal that the main elements on the surfaces are C, N, and O. The presence of N in the XPS spectra demonstrates the formation of the TA thin film layer on the PAN substrate by an interfacial polymerization reaction. It can also be seen from the XPS spectra that the ratios of N/C and N/O decrease with the increase of the concentration of GOQDs in the aqueous solutions. This phenomenon indicates that the content of GOQDs in the active layer of the TA/ GOQDs TFN membranes increase gradually, which is consistent with the results of the FTIR spectrum (Figure 5). The high-solution N 1s XPS spectrum of the TA/GOQDs-1 TFN membrane is shown in Figure 6b, which can be curvefitted to two peak components: −NHCOO− and −NHCO− bond bending between binding energies 396 and 404 eV. The bond of −NHCOO− at 399.84 eV indicates the reaction of the 11087


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Figure 8. Surface AFM images of PAN substrate, TA TFC membrane, and TA/GOQDs TFN membranes: (a) PAN substrate, (b) TA, (c) TA/ GOQDs-0.25, (d) TA/GOQDs-0.5, (e) TA/GOQDs-0.75, and (f) TA/GOQDs-1.

while the roughness of TA/GOQDs membranes decrease gradually with the increased concentrations of GOQDs in the aqueous phase. The Rq and Ra of pristine TA TFC membrane are 44.2 and 33.5 nm, respectively, while the Rq and Ra of TA/ GOQDs-1 TFN membrane decrease to 13 and 10.3 nm when the GOQDs concentrations is 1 g/L in the aqueous phase. This reveals that a large number of two-dimensional GOQD flakes locate at the membrane surface, resulting in a smoother morphology of the TA/GOQDs TFN membranes. The roughness of the TA/GOQDs TFN membrane in our study is much lower than that of the other TFN membranes prepared by interfacial polymerization due to the particular graphene quantum structures of GOQDs.40 It has been well-confirmed that the accumulation of contaminants in the “valleys” of the rough membrane surface will increase its fouling tendency.41 Therefore, the TA/GOQDs TFN membranes with smoother surface would possess higher antifouling ability, which will be discussed below. 3.2.5. Contact Angle. The surface hydrophilicity of TA/ GOQDs TFN membranes was evaluated by measuring the contact angle at the water−air interface. Generally, a low contact angle represents a more hydrophilic material. As shown in Figure 9, the water contact angles is 67.5° for the PAN substrate, which declines to 65.6° after the TA separation layer was deposited due to the abundant unreacted hydroxyl groups of TA. Moreover, the addition of GOQDs in the aqueous phase results in an enhancement of the surface hydrophilicity. The contact angle gradually decreases from 59.5° for TA/GOQDs0.25 TFN membrane to 41.7° for TA/GOQDs-1 TFN

retarded the formation of the TA active layer, which will be helpful to form a smoother membrane surface. Moreover, unlike many other inorganic nanoparticles, such as boehmite,37 silica,38 and alumina,39 which could be obviously observed in the TFN membrane surface, the GOQDs is well-dispersed in the polymer matrix and no obvious GOQD flakes can be found in the surface of the as-synthesized membranes, even at higher magnification (inset image in Figure 7). This phenomenon is mainly due to the particular quantum structures of GOQDs and its good dispersity in water. 3.2.4. AFM. The three-dimensional-height AFM images of the membranes are shown in Figure 8. The root-mean-square roughness (Rq) and peak-to-valley distance (Ra) of the membrane surface determined by AFM are listed in Table 1. It should be noted that the roughness of the membrane surface increased from PAN to TA TFC membrane when the interfacial polymerization reaction formed a separation layer, Table 1. Surface Roughness of PAN Substrate, TA TFC Membrane, and TA/GOQDs TFN Membranes membrane

Rq (nm)

Ra (nm)

PAN sustrate TA TA/GOQDs-0.25 TA/GOQDs-0.5 TA/GOQDs-0.75 TA/GOQDs-1

5.75 44.2 28.2 22.6 19.3 13.0

4.33 33.5 22.5 18.2 15.0 10.3 11088


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3.3. NF Performance of TA/GOQDs TFN Membranes. The NF performance of the TA/GOQDs TFN membranes was systematically investigated under 0.2 MPa. In the organic dye removal tests, we chose three negatively charged dyes and one positively charged dye as target molecules to evaluate the dye removal ability of the membranes. Some physical properties of four dyes are listed in Table 2. In Figure 11, the dye rejection Table 2. Some Physical Properties of the Dyes Used in This Study dye methyl orange methylene blue Congo red methyl blue

Figure 9. Contact angle of PAN substrate, TA TFC membranes, and TA/GOQDs TFN membranes.

membrane with an increase of GOQDs concentration from 0.25 to 1 g/L in the aqueous phase. This phenomenon can be rationalized by the high hydrophilicity of GOQDs.42 Zeng et al. had reported that, owing to the existence of hydrophilic carboxylic groups, the contact angle of pristine GOQD is only 14.6°,43 which is much lower than that of pristine TA TFC membrane in our study. Therefore, the TA/GOQDs TFN membranes with enhanced surface hydrophilicity can be obtained by increasing the GOQDs concentration in the aqueous phase. 3.2.6. ζ-Potential. During the NF process, the surface charges can dramatically influence the membrane performance due to the Donnan effects.44 Therefore, the surface ζ-potentials of the TA/GOQDs TFN membranes prepared at different concentrations of GOQDs in the aqueous phase were detected under pH 7.0, and the results are shown in Figure 10. PAN

molecular weight (g/mol)

charge property

maximal absorption wavelength (nm)

327.33

negative

464

373.90

positive

663

696.66 799.80

negative negative

491 590

Figure 11. Rejection of TA TFC and TA/GOQDs-0.5 TFN membranes for different dyes.

rate sequence of TA/GOQDs-0.5 membrane follows the order Congo red > methyl blue > methyl orange > methylene blue. The rejecting mechanism of charged NF membrane is usually explained by the combination of Donnan exclusion and sterichindrance effect.33 The ζ-potential (Figure 9) indicates that the TA/GOQDs TFN membranes are highly negatively charged due to the carboxyl groups on the edges of GOQDs. According to the Donnan exclusion theory, as water molecules transport through the membrane driven by the applied pressure, the negatively charged TA/GOQDs membrane surface will have obvious electrostatic attraction to positively charged methylene blue and significant electrostatic repulsion to negatively charged Congo red, methyl blue, and methyl orange. Thus, the TA/ GOQDs TFN membrane is predicted to have the lowest rejection rate for methylene blue, which coincides with the results in Figure 11. TA/GOQDs TFN membrane showed higher rejection for methyl blue than methyl orange because the molecular weight of methyl blue is larger than that of methyl orange (seen in Table 2), indicating that the steric hindrance effect also played an important role in the dyeremoving ability of TA/GOQDs membrane. To be specific, the rejection rate of Congo red (99.8%) was slightly higher than that of methyl blue (98.1%). Spólnik et al. have reported that Congo red in aqueous solutions has a tendency to aggregate and form supramolecular systems owing to the hydrophobic interactions between the aromatic rings of the dye.46 This might lead to the highest rejection rate of Congo red in TA/ GOQDs TFN membrane. In addition, the dyes’ rejection rate

Figure 10. Surface ζ-potential of PAN substrate, TA TFC, and TA/ GOQDs TFN membranes.

substrate exhibits a negative ζ-potential of approximately −2.1 mV. When a pure TA skin layer was deposited on the PAN substrate, the surface charge decreased to −8.37 mV. The incorporation of GOQDs into the TA skin layer resulted in a further decline of the potential charge. The surface ζ-potential value of the TA/GOQDs TFN membranes decreased from −8.37 to −18.2 mV with an increase of the GOQD concentration from 0.25 to 1 g/L in the aqueous solution. The declined surface charge can be attributed to the abundant carboxyl groups on the edges of GOQD flakes. Similar phenomenon were also reported in the case of composite membranes of GO22 and carbon nanotubes (CNTs).45 11089


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Figure 12. Schematic illustration of the separation process for TA/GOQDs TFN membrane.

Table 3. NF Performance of TA TFC and TA/GOQDs TFN Membranes methyl bluea membrane TA TA/GOQDs-0.25 TA/GOQDs-0.5 TA/GOQDs-0.75 TA/GOQDs-1.0 a

Jd (L/m2·h)c 15.6 16.3 18.0 20.53 23.01

± ± ± ± ±

0.15 0.2 0.22 0.3 0.35

Na2SO4b R (%) 93.6 97.5 98.1 98.0 97.6

± ± ± ± ±

1 0.6 0.5 0.4 0.5

Jw (L/m2·h)c 15.81 16.50 18.22 20.83 23.33

± ± ± ± ±

0.2 0.17 0.31 0.3 0.5

R (%) 54.6 66.7 65.7 62.7 57.1

± ± ± ± ±

1.1 2.2 2.5 1.9 0.7

The feeding concentration is 100 mg/L. bThe feeding concentration is 1000 mg/L. cUnder a driven pressure of 0.2 MPa.

high concentrations. Dynamic light scattering (DLS) results (Figure S2, SI) showed that the size of GOQDs in aqueous solution gradually increased with the increase of GOQD concentration, indicating the aggregation of GOQDs. This would hinder the good dispersion of GOQDs and consequently result in the formation of a defect at the interfacial polymerization layer. As shown in Figure 13, the salt rejection rate of TA/ GOQDs-0.5 membrane follows the order Na2SO4 > MgSO4 >

of TA/GOQDs-0.5 TFN membrane followed the same order as that of pristine TA TFC membrane, indicating that the incorporation of GOQDs does not change the separation mechanism of the membrane. Figure 12 shows a schematic illustration of the separation process of TA/GOQDs TFN membrane. Many studies have shown that GO can offer a smooth, frictionless surface for the fast flow of water during the filtration process. When water flows through the interspace between the GO flakes, the low friction between the water and the wall in the hydrophobic pristine graphene channels allows for a very fast water transport in GO stacks.19 GOQDs are quantum-sized derivatives of graphene and share most of the similar features and properties of GO. By integrating GOQD nanomaterials in the design of TFN membrane, it is possible to improve the performance of TFN membrane by increasing their permeation properties. Furthermore, compared to the high aspect ratio of GO, the quantum size of GOQD flakes may create a relatively lower tortuosity for the water transport route, which would also result in a high water permeation rate. On the basis of this assumption, we applied GOQDs as filler materials to synthesize a novel NF TFN membrane with high flux under low pressure. The NF peformance of the resultant TA/GOQDs TFN membranes with different GOQD loading are shown in Table 3, and as expected, the pure water flux (Jw) of TA/GOQDs TFN membranes increased as more GOQDs were added into the aqueous solution. The TA/GOQDs-1 TFN membrane showed a pure water flux up to 23.33 L/m2·h (0.2 MPa), which was 1.5 times more than that of pristine TA TFC membrane. Also, the dye rejection rates increased with the GOQD concentration in aquenous solution up to 0.5 g/L. Given that the ζ-potential of TA/GOQDs TFN membranes declined after GOQD incorporation, we can attribute the enhancement of dye rejection rates to the increase of electrostatic interaction. However, the concentration increment of GOQDs from 0.5 to 1.0 g/L led to only a slight decrease in the rejection rate. This result could be ascribed to the aggregation of GOQD flakes at

Figure 13. Salt rejection of TA TFC and TA/GOQD-0.5 TFN membrane.

NaCl > MgCl2, which shows the typical performance of negatively charged NF membrane due to the incorporation of GOQDs.22 According to the previous analysis, the main rejection mechanism of TA/GOQDs TFN membrane involves Donnan exclusion and the steric-hindrance effect. The electrostatic interaction will result in a high rejection for salts with multivalent anion and monovalent cation. Thus, TA/ GOQDs TFN membrane presents the highest selectivity for Na2SO4 and the lowest selectivity for MgCl2. Because both the hydrated radius values of Mg2+ (0.43 nm) and SO42− (0.38 nm) are larger than those of Na+ (0.36 nm) and Cl− (0.33 nm), TA/ 11090


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increases from 64.5% to 91.7%, an increase of 27.2%. The data of Table 4 indicate that the resistance to protein fouling for TA membranes was significantly improved by the incorporation of GOQDs. The greater the GOQDs content in the TA separation layer, the better the antifouling ability of the membranes. As mentioned above, the incorporation of GOQDs results in a smoother and more hydrophilic membrane surface due to the hydrophilic nature of GOQDs and the atomically smooth surface of graphene sheets. These improved surface properties reduce the adhesion of proteins on TA/GOQDs TFN membranes compared to pristine TA TFC membrane.48 Up to now, most studies have focused on GO blend membrane with fouling resistant property.19−22 However, to reduce the adhesion of foulants, only the outer layer truly contributes to the antifouling effect. The blending method uses more GO than necessary to impart antifouling properties to the membrane. Considering the environmental cost of producing nanocomposite membrane, integrating GOQDs into the polymer separation layer via interfacial polymerization is an optimum choice from the viewpoint of reducing the amount of nanomaterials used. 3.5. Long-Term Operational Stability of TA/GOQDs TFN Membranes. To investigate the stability of the prepared TA/GOQDs TFN membranes, a cross-flow permeation test was conducted continuously for 20 days to evaluate the longterm operational stability of a membrane. It can be seen from Figure 15 that the TA/GOQDs-0.5 TFN membrane shows no

GOQDs-0.5 TFN membrane shows a higher rejection for MgSO4 than for NaCl.47 In addition, the TA/GOQDs TFN membrane exhibits moderate salt rejection, which might be attributed to the loose active layer structure caused by the cross-lying construction of GOQD flake network (seen in Figure 12). This loose sturcture also endows the membrane with high water flux at an operational pressure of 0.2 MPa, rendering the membranes promising for the low-pressure NF process. 3.4. Antifouling Property of TA/GOQDs TFN Membranes. Fouling is an unavoidable problem during NF membrane applications that greatly disfavors the permeation of desired molecules, diminishes the NF performance, and ultimately shortens the lifetime of the membrane. Therefore, fouling resistance ability is crucial to NF membrane applications. In this study, we investigated the antifouling performance of TA/GOQDs TFN membranes using 0.1 g/L BSA as a foulant simulator. Three periods of water flux were recorded in Figure 14: the pure water flux (Jw1) before foulant

Figure 14. Fouling tests for TA TFC and TA/GOQDs TFN membranes under 0.2 MPa. Flux was plotted versus time for three periods: pure water flux for 12 h, 0.1 g/L BSA solution flux for 12 h, and pure water flux after hydraulic washing for 0.5 h.

solution was fed, the water flux when filtrating foulant solution (JBSA), and the water flux after foulant filtration (Jw2). It can be seen that the flux of all membranes went through a sharp decline after BSA solution was fed into the tank, suggesting that a large amount of BSA protein deposited on the membrane surface. After filtration of BSA, the membrane surface was directly washed with deionized water and the water flux showed a certain extent of recovery. The antifouling ability of the membranes is measured in terms of FDR and FRR, as calculated by eqs 3 and 4. Generally, a lower FDR and a higher FRR value mean a better antifouling property. It can be seen from Table 4 that the FDR for TA/ GOQDs-1 TFN membrane is 21.6%, which is about half of that for pristine TA TFC membrane, and the corresponding FRR

Figure 15. Water flux and Na2SO4 rejection of TA/GOQDs-0.5 TFN membrane with different operation times.

obvious degradation in Na2SO4 rejection and water flux, indicating that the as-prepared membrane incorporating GOQDs possesses good durability and long-term performance stability. Furthermore, we detected the ζ-potential of TA TFC and TA/GOQDs TFN membrane before and after operating for 20 days. The results are shown in Figure S3 (SI). No obvious change can be found, implying the stability of GOQDs on the TA/GOQDs TFN membrane surface. The covalent linkage of GOQDs with TA matrix prevents the GOQDs from leaching from the active layer of TFC membrane. 3.6. Comparison of NF Performance in This Study with That Reported in the Literature. The NF performances of some membranes reported in the literature and the TA/GOQDs TFN membranes prepared in this study are summarized in Table 5. It can be seen that under low operating pressure TA/GOQDs TFN membranes exhibited excellent water flux and comparable rejection against dye and salt. This is because GOQDs enwrapped in the polymer matrix of the separation layer will contribute to the loose active layer

Table 4. FDR and FRR of TA TFC and TA/GOQDs TFN Membranes membranes

FDR (%)

FRR (%)

TA TA/GOQDs-0.25 TA/GOQDs-0.5 TA/GOQDs-0.75 TA/GOQDs-1

55.1 38.7 27.6 27.5 21.6

64.5 80.6 83.9 89.7 91.7 11091


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ACS Applied Materials & Interfaces Table 5. Comparative Results of Different NF Membranes rejection (%) membranes

water flux (L/m2h)

Congo red

methyl blue

Na2SO4

NaCl

pressure (MPa)

refs

PEI-modified GO/PAA/PVA/GA CMCNa/PP PVA−PSf PA−SiO2/PSf HDA−TMC/PSf PA−GO/PSU dopamine−TMC/PES TA−Fe3+/PES TA/GOQDs-1

4.2 6.3 18.5 18.8 19.4 55 22 27.2 23.33

99.5 99.9 99.45 − 99 − 97 99 99.8

99.3 99.75 − − − − − 70 97.6

− − 96.5 81.2 80 87 57 10 66.7

37.8 25 69.8 43.45 40 94 − 5 17.2

0.5 0.8 0.5 0.6 0.8 2 0.2 0.2 0.2

9 49 50 51 52 53 54 55 this study

structure and the higher negative ζ-potential of the membrane surface. Meanwhile, the water molecules could pass through very fast due to the low friction of the distinctive channel formed by the stack of GOQD nanosheets, which makes the membrane exhibits a high water flux. At present, the operating pressure of most NF membranes is relatively high, which will greatly increase operational cost. The excellent separation performance of TA/GOQDs TFN membranes under low operating pressure indicates that GOQDs are promising filler materials to fabricated low-pressure TFN NF membranes.

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GOQDs TFN membranes before and after operating for 20 days (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 51085917090. Fax: +86 51085917763. ORCID

Yunxiang Bai: 0000-0001-8469-4510 Notes

The authors declare no competing financial interest.

4. CONCLUSION In summary, a novel TFN membrane containing GOQDs was fabricated by in situ interfacial polymerization for the lowpressure NF process. GOQDs with one to three graphene layers and an average dimeter of 3.5 nm were prepared and used as the filler to prepare the TFN membrane at concentrations ranging from 0 to 1.0 wt %. Morphology studies demonstrated that a good dispersion of GOQDs occurred in the TA thin film layer. With an increasing concentration of GOQDs in the aqueous phase, hydrophilicity, roughness, and ζ-potential of the TA/GOQDs TFN membranes all increased. The resultant increase in the permeate water flux under 0.2 MPa was from 15.81 to 23.33 L/m2·h, while high dye rejections were maintained (99.8% for Congo red and 97.6% for methylene blue). When compared with pristine TA TFC membranes, the TA/GOQDs TFN membranes showed enhanced permeability, suggesting that the short flow paths through GOQD nanosheets had played a role in water permeation. It is expected that the high hydrophilicity and smooth surface structure of TA/GOQDs TFN membranes introduced by GOQDs could improve the membranes’ resistance to fouling. Overall, GOQDs, with the unique combined properties of graphene and quantum dots, is a good filler to make the high-performance TFN NF membrane. It can be expected that GOQD-based TFN membranes with extraordinary filtration performance can be obtained by further optimizing the preparation conditions.

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ACKNOWLEDGMENTS Financial support was provided by the National Natural Science Foundation of China (No.21576114 and 21106053), the Industry−Academia Cooperation Innovation Fund Projects of Jiangsu Province (BY2016022-13), and the Science and Technology Projects of Wuxi (Advanced tap water treatment), P. R. China.

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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/acsami.6b12826. Cross sectional images of PAN substrate, TA TFC membrane, and TA/GOQDs TFN membranes, Size distribution of GOQDs with different concentration obtained from DLS, ζ-potential of TA TFC, TA/ 11092


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