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Trivalent metal cation cross-linked graphene oxide membranes for NOM removal in water treatment
Journal of Membrane Science 542 (2017) 31–40
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Trivalent metal cation cross-linked graphene oxide membranes for NOM removal in water treatment ⁎
Ting Liua, Bing Yanga, Nigel Grahamb, Wenzheng Yub, , Kening Suna, a b
MARK
⁎
School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene oxide membrane Water treatment Trivalent cation crosslinking Natural organic matter Membrane fouling
This paper summarizes the development and testing of novel graphene oxide (GO) membranes for the removal of natural organic matter (NOM) from raw water sources used for drinking water supply. In this work, two trivalent cations, Al3+ and Fe3+, were employed as crosslinking agents to stack GO nanosheets layer by layer on a PVDF membrane support, in order to fabricate a suitable GO membrane. The trivalent cations greatly improve the bonding strength between the GO nanosheets through electrostatic forces and coordination bonds, and thus enhance the stability of the GO membrane; the integrity of the membrane in a range of solutions could be maintained for over a month. The initial interlayer spacing of GO nanosheets (0.80 nm) could be increased up to 0.86–0.95 nm by changing the Al3+ or Fe3+ ion concentration. It was found that the GO membrane flux ranged between 79 and 902 LMH/MPa when treating three representative NOM solutions and a real surface water. A relatively low thickness of the GO layer induced a higher flux of the GO membrane when prepared with the same cation concentration, while increasing the cation concentration resulted in a decline in flux. The flux of the Fe3+ cross-linked GO membrane was approximately 1.1–2.3 times that of the Al3+ cross-linked GO membrane, while both cation-modified GO membranes achieved a similar separation efficiency of the organic contaminants. The study has demonstrated a facile approach to the fabrication of a novel, stable GO membrane employing Al3+/ Fe3+ ions as crosslinking species, in order to utilize the excellent properties of GO and produce a GO membrane with a high flux and organic removal performance for water treatment.
1. Introduction The continuing pressure on water resources and increasing demand for drinking water supplies worldwide can be alleviated by the development of new water treatment technologies which are more economical and efficient in producing clean and safe water from contaminated sources [1–3]. Membrane separation is recognized as a versatile and effective technology that has been used widely for water treatment at many scales and in many applications throughout the world. In particular, ultrafiltration (UF) and nanofiltration (NF) are utilized most frequently because of their ability to achieve a high degree of contaminant removal, mainly particulates and microorganisms [4,5]. However, the most significant technical limitation of these processes is fouling of the membrane, which can reduce the flux and service life, and increase energy consumption owing to the need for greater applied pressures [6]. Membrane fouling inevitably occurs when a cake layer forms on the membrane surface and/or the membrane pores are blocked by the contaminants in water, causing an increase in filtration resistance; this ⁎
Corresponding authors. E-mail addresses: [email protected] (W. Yu), [email protected] (K. Sun).
http://dx.doi.org/10.1016/j.memsci.2017.07.061 Received 1 June 2017; Received in revised form 28 July 2017; Accepted 29 July 2017 Available online 01 August 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
can lead to a decrease in flux of the membrane and an increase in the operating costs. The membrane fouling rate and degree are closely related to the hydrophobic/hydrophilic nature of the membrane surface, whereby contaminant accumulation (fouling) is enhanced with membrane surfaces that have greater hydrophobicity [7]. Therefore, modification of the membrane surface in order to improve its hydrophilicity (decrease its hydrophobicity) is a potential means of reducing the deposition of contaminants and thus mitigate the membrane fouling. Some previous studies have reported that the coating of super-hydrophilic nanomaterials on the membrane surface can greatly enhance the hydrophilicity and anti-fouling ability of the membrane [6,8]. Graphene oxide (GO) is a relatively novel type of nanomaterial, which is attracting considerable attention in the field of membranebased water treatment research because of its high degree of hydrophilicity and other unique properties [9]. GO nanosheets are graphene sheets which have been oxidized by strong oxidants and then functionalized with carboxyl, hydroxyl, and epoxide groups [10]. The high density of oxygenated groups offers a great potential to improve the performance of GO functionalized membranes by increasing their
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Fig. 1. Characterization of GO: Raman spectrum (A), FTIR spectrum (B), XRD pattern (C) and TEM image (D) of GO nanosheets.
stability in aqueous solution. In previous studies, the most commonly used method for reducing the instability/disintegration of GO membrane in solution was to use a crosslinking agent to connect the GO nanosheets, such as multivalent cations (Mg2+, Ca2+, Al3+) [21–23], modified graphene oxide (aminated-GO [24], partially reduced GO [25]) or high-molecular polymers (including 1,3,5-benzenetricarbonyl trichloride polyallylamine [26], polyvinyl alcohol and polymethyl methacrylate [19]). Among these methods that have been considered, the use of multivalent cations as a crosslinking agent is a preferable alternative owing to their low-cost and facile operation in the membrane synthesis. The present study provides new information and a greater understanding of the role of two common trivalent cation crosslinking agents, Al3+ and Fe3+, in GO membrane fabrication and in the performance of organic fouling control of the GO membranes. In this investigation GO nanosheets were stacked on a support membrane to reduce membrane fouling by producing a more hydrophilic surface. The trivalent cations Al3+ and Fe3+ were compared as crosslinking agents to enhance the stability of the GO nanosheets, which were stacked on the support membrane in the solution. The two cation-modified GO membranes were evaluated to explore the mechanisms of flux improvement and fouling mitigation. In particular, the effects of GO mass and Al3+ and Fe3+ cation concentration on NOM removal efficiency and water flux were examined in this study.
hydrophilicity, chemical stability, and antimicrobial and antifouling properties [11,12]. In addition, the large horizontal to vertical ratio of the GO nanosheet enables it to possess a remarkable stacking property, which benefits the fabrication of GO functionalized membranes [13]. There are two main GO-based membrane fabrication approaches, which are either the blending of GO into a polymer matrix to form various nanocomposite membranes, or the direct loading of GO on to the surface of a polymer membrane as a support layer. When the GO is blended into the polymer matrix, such as polyvinylidene fluoride (PVDF), polysulfone and polyether sulfone [14–17], the important properties of hydrophilicity and high specific surface area of GO cannot be fully utilized because the GO is not directly exposed to foulants and the interactions between GO and foulants are limited [18]. In contrast, directly loading the GO on the surface of the membrane by deposition represents a more efficient approach to the preparation of the GO-based membrane, and which benefits fouling control. The two-dimensional channels formed by the stacking of the GO nanosheets are selective, allowing water to pass through whilst intercepting pollutants in the water. When the organic pollutant molecules and/or ions diffuse into the channels of the GO membrane from the edge of the GO nanosheets, they can be retained by the narrow interlayer channels which are formed by hydrogen bonding and electrostatic interactions. Furthermore, the selectivity of the GO membrane also arises from the electrostatic interaction between charged pollutants and the highly (negatively) charged GO [19]. These channels have large slip lengths because of their nanoscale size and the presence of hydrophobic regions on the GO nanosheet, similar to the capillary phenomenon which causes a larger water flux [20]. Since the GO nanosheets have a good dispersity in water owing to their hydrophilic properties and surface charge, the GO layers that are deposited on the surface of the polymer support membrane can readily disintegrate in water [21]. Therefore, the most significant challenge for the development and application of a GO membrane is to improve its
2. Materials and methods 2.1. GO preparation GO was prepared using the modified Hummers method [27,28]. Natural flake graphite was oxidized in a mixture of H2SO4, NaNO3 and KMnO4. Then hydrochloric acid solution was used to wash the product which was obtained from filtration of the solution after the oxidation 32
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Fig. 2. Cross-sectional SEM images of the GO membrane (A); XRD intensity spectra for different cation-GO membranes (B); variation of contact angle for membranes prepared with different cation concentrations (C); element analysis and EDX mapping (Al distribution pattern at 5 k×) for Al3+-GO membrane with 0.001 M Al3+ (D). 2 mg GO mass (440 mg/m2) for all membranes.
2.2. Membrane fabrication
reaction was completed. The GO solid was placed in hydrochloric acid solution for washing and then centrifuged at 4000 g-force, and impurities were removed after 4–6 cycles of washing. Subsequently, the acid was replaced by deionized water and the centrifugal speed increased to 8000 g-force to wash out the residual acid in the GO suspension. The pure multilayer GO was exfoliated into GO nanosheets under ultrasonic vibration and after freeze-drying the GO powder was obtained.
To fabricate the GO membrane, we used a commercial PVDF membrane (Shanghai Mosu Technology Co., Ltd., China) as the support layer. Before the fabrication, the PVDF membrane was pre-conditioned by filtering 200 mL DI water, and then the GO was loaded onto the membrane surface by pressure filtration to produce a pure GO membrane. In order to obtain a stable and suitable membrane, either Al3+ or Fe3+ (at concentrations of 0.001 M，0.01 M or 0.1 M) was added as a 33
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Fig. 3. XPS spectra of GO membranes prepared with Al3+/Fe3+ at different ion concentrations.
membrane. These were evaluated using a commercial pressure filtration system (Millipore, Amicon 8400, USA) employing nitrogen gas as the means for obtaining a stable applied pressure (0.1 MPa). A digital balance (ML3002, Mettler-Toledo International Inc., USA) was connected to a computer to automatically record the weight of the filtered water in real time by the data acquisition software. DI water (Millipore Milli-Q) was used to test the water flux of GO membranes. Three different kinds of model NOM solutions, namely bovine serum albumin (BSA), humic acid and alginate (sodium salt, SA), at a concentration of 10 mg/L were used as feed solutions to test the flux, rejection rate and the fouling resistance of the GO membrane; samples of surface water from the South Long River, Beijing, China were also used as feed water in this study. Before each test the newly fabricated GO membrane was pre-conditioned by filtering DI water for ten minutes to ensure there was no residual Al3+/Fe3+ on the surface of the GO membrane. The TOC concentration of samples of all feed waters and permeates were determined by total organic carbon (TOC) analyzer (TOC-VCPH, Shimadzu, Japan), and the UV254 absorbance of humic acid solution and surface water were analyzed by UV–visible spectrophotometer (U3010, Hitachi, Japan) at a wavelength of 254 nm. Size exclusion chromatography (SEC) was used to determine the apparent molecular weight (MW) distribution of UV-active substances. SEC was performed using HPLC (Perkin Elmer, USA), with a Series 200 pump, a BIOSEPSEC-S3000 column (Phenomenex, UK) (7.8 mm×300 mm), and a Security Guard column fixed with a GFC-3000 disc 4 mm (ID). The injection volume of water samples was 100 μL and the UV–visible detector operated at a wavelength of 254 nm with autosampler. The fouled GO membranes were cleaned and reused for two filtration cycles to investigate their performance in terms of flux recovery. In the cleaning procedure, the fouled GO membranes after the filtration process were rinsed moderately with DI water for about 20 s and then ultrasonically cleaned in DI water for 3 min. Values for the flux recovery ratio (FRR
crosslinking agent to 100 mL deionized water with 1 mg or 2 mg GO (corresponding to GO loadings of 220 mg/m2 or 440 mg/m2). Finally, residual unbound Al3+ or Fe3+ was removed from the synthesized GO membrane by filtering DI water for ten minutes. 2.3. Characterization of GO nanosheets and membranes The physical and chemical properties of GO nanosheets and the structure of GO membranes were evaluated by several characterization techniques. These properties have close relationships with the performance of the GO membrane. Scanning electron microscopy (SEM, QUANTA FEG 250, FEI, USA) provided microscopic images after being sputter-coated with aurum on the GO membrane's surface and cross section, while the presence and distribution of Al3+/Fe3+ in the GO membrane was mapped using energy-dispersive X-ray spectroscopy (EDX, X-Max, Oxford Instruments, UK). High-resolution transmission electron microscopy (HRTEM, JEOL, JEM-2100F, Japan) revealed the individual GO nanosheets and the folds. Contact angle measurements were conducted to indicate membrane hydrophilicity. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, America) spectra were collected to identify the relative abundance of the different functional groups. X-ray diffraction (XRD, Rigaku UltimaIV, Cu-Κα radiation, 45 kV, 50 mA, Japan) characterization was used to explore the inter-nanosheet spacing of GO membranes. Fourier transform infrared spectroscopy (FT-IR, BRUKER, ALPHA-P, USA) was used to identify the presence of oxygen containing functional groups on the GO. 2.4. Flux and rejection tests Water flux, normalized flux and rejection rate (removal efficiency) are important parameters to evaluate the fouling resistance of the GO 34
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Fig. 4. Variation of membrane flux (LMH–litres m−2 h−1) with Al3+ and Fe3+ concentration (2 mg GO mass) in different NOM solutions: BSA (A), HA (B), alginate (C) and surface water (D).
process. The Raman spectrum of the GO nanosheets (Fig. 1A) shows two peaks at 1350 cm−1 (D peak) and 1593 cm−1 (G peak), respectively. The D peak represents a defect of the carbon atomic lattice, which indicates the addition of oxygen-containing functional groups, while the G peak represents in-plane stretching vibration of sp2 hybridization of the graphitic carbon. Furthermore, the FTIR spectrum (Fig. 1B) suggests that the GO nanosheets contain a stretching vibration of sp2 C˭C (1625 cm−1), −C˭O stretching vibration (1726 cm−1), stretching vibration of C−O (1055 cm−1), stretching vibration of C−OH (1226 cm−1) and deformation vibration of O−H (1382 cm−1) [29]. The XRD pattern (Fig. 1C) shows that there is a sharp peak at around 10 degrees and a small peak at around 43.1 degrees, which corresponds well with the crystal phase of graphene oxide. The HRTEM image (Fig. 1D) exhibits a uniform and typical wrinkled nanosheets structure of GO.
Table 1 Variation of normalized flux J/J0 (J0 is initial flux) after 30-min filtration with NOM solutions for cation-modified GO membranes (various Al3+/Fe3+ concentrations, and GO mass of 1 mg and 2 mg). Membrane (Cation concn, GO mass)
J/J0 of BSA
J/J0 of HA
J/J0 of SA
J/J0 of Surface water
0.001 M Al3+, 1 mg 0.01 M Al3+, 1 mg 0.1 M Al3+, 1 mg 0.001 M Fe3+, 1 mg 0.01 M Fe3+, 1 mg 0.1 M Fe3+, 1 mg 0.001 M Al3+, 2 mg 0.01 M Al3+, 2 mg 0.1 M Al3+, 2 mg 0.001 M Fe3+, 2 mg 0.01 M Fe3+, 2 mg 0.1 M Fe3+, 2 mg
0.322 0.350 0.365 0.200 0.233 0.274 0.333 0.423 0.445 0.316 0.334 0.386
0.698 0.723 0.806 0.690 0.724 0. 738 0.768 0.786 0.810 0.628 0.662 0.681
0.516 0.567 0.779 0.667 0.698 0.710 0.855 0.861 0.820 0.808 0.764 0.712
0.785 0.756 0.744 0.725 0.679 0.723 0.891 0.841 0.821 0.766 0.821 0.829
3.2. Characteristics of GO membrane SEM images of the cross-section of the GO membrane (Fig. 2A) show that the GO layer with a certain degree of folds completely covered the surface of the PVDF membrane support and bonded closely with it. The average thickness of the GO layer was estimated to be in the range of 120–150 nm. The nano-scale GO sheets were deposited layer by layer on the surface of the PVDF membrane support, with interspaces of a similar nano-scale between the layers, which provide channels for water flow and tortuous surfaces for the retention of NOM contaminants in the water. The change of average interlayer spacing between the GO nanosheets caused by intercalation of the two trivalent metal ions was confirmed by the XRD patterns (Fig. 2B). While the XRD peak of the original GO membrane indicated an interlayer spacing of ~0.80 nm (from Bragg's Law), the peaks for the metal ion cross-linked GO membranes shifted to lower θ values, which indicated that the
%), reversible fouling (%) and irreversible fouling (%) of the membranes were calculated.
3. Results and discussion 3.1. Properties of GO nanosheets The physicochemical properties of GO nanosheets were determined by several techniques, as the properties directly affect the synthesis process and filtration performance of the GO membrane. The GO nanosheets were fabricated by introducing a large quantity of oxygencontaining functional groups into the graphite sheets, and then the GO nanosheets were evenly separated in the water by an ultrasound 35
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Fig. 5. NOM removal (% change in UV254) for HA solution by GO membrane at different cation concentrations: (A) Al3+ and (B) Fe3+；NOM removal (% change in UV254) for surface water by GO membrane: (C) Al3+ and (D) Fe3+.
corresponding interlayer spacing increased to ~0.86 nm (Al3+) and ~0.95 nm (Fe3+), respectively. Hence, the XRD patterns of the cation cross-linked GO membrane demonstrated a significant increase in the layer-to-layer distance compared to that of the original GO membrane, which therefore influences the water flux of the membrane. Comparing the effect of the two trivalent cations, the Fe3+ cross-linking appeared to produce a larger interlayer spacing compared to that of Al3+, which could be attributed to their relative atomic size; Fe3+ has an ionic radius of 0.0645 nm, compared to 0.0535 nm for Al3+. It was also evident that the membranes prepared with different metal ion concentrations, specifically 0.001 and 0.1 M, did not have significantly different θ values. Although a large number of oxygen-containing functional groups on the GO nanosheets are believed to give the membrane surface a high degree of hydrophilicity, the change in hydrophilicity with cation incorporation was measured in terms of contact angle. It was observed that the incorporation of Al3+ and Fe3+ ions during preparation had some beneficial effects on the hydrophilicity of the GO membrane. The contact angle of the GO membrane showed a decreasing trend with increase in cation concentration, but there were differences between the extent of the changes and the cation (Fig. 2C). The results showed that Fe3+ had a greater impact on hydrophilicity of the GO membrane compared to Al3+ at the same concentration. The presence and distribution of Al3+/Fe3+ in the GO membrane were indicated by EDX mapping. Fig. 2D presents the EDX image of Al3+-GO membrane with 0.001 M Al3+. The peak at 1.487 keV demonstrated the presence of Al, and the right-hand image showed that the Al was uniformly distributed in the Al3+-GO membrane. The other EDX mappings of Al3+/Fe3+ cross-linked GO membranes at different ion concentrations can be seen in the supporting information (Fig. S1). In addition, the stability of the
GO membrane incorporated with either Al3+ or Fe3+ was investigated in DI water and two different model NOM solutions, these being: 50 mg/L SA solution and 50 mg/L BSA solution. The results showed that the GO membrane with Al3+ and Fe3+ was stable in the three solutions for at least two weeks (Fig. S2). A series of GO leaching tests for all the membranes was conducted using TOC analysis, and the results indicated that the extent of GO leaching was lower than 0.1%, which confirmed the stability of the Al3+/Fe3+-modified GO membranes. The interactions between the GO nanosheets and the cations Al3+ or 3+ Fe , include the electrostatic attractive force between the negative charge of the GO's oxygen functional groups and Al3+/Fe3+ cations (producing charge reduction/neutralization), and coordination bonds between the cation and the oxygen functional groups, such as the epoxy and hydroxyl groups. As graphene oxide is believed to have a large number of epoxy groups, its exposure to Lewis acidic metal ions may lead to ringopening of the epoxide to create C-OH moieties, together with the carbonyl groups [21]. XPS spectra of the original GO membrane and the GO membranes prepared with 0.1 M Al3+/Fe3+, together with a comparison of different component (bond types) proportions at different concentrations of Al3+/Fe3+, are shown in Fig. 3. It can be seen that with increasing cation incorporation there was a decrease in the percentage of C–O bonds and a corresponding increase in C-C/C˭C bonds [30]. The results suggested that increasing the concentration of the cation could facilitate the reduction of carbon-oxygen bonds, which mainly relates to the ring-opening of the epoxide. It was deduced that the spacing between the GO nanosheets could be reduced and thus produce narrower nanochannels for water flow when the cations with a higher concentration diffuse into the inner structure of the GO nanosheets.
36
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Fig. 6. NOM removal (% change in TOC) for, (A) BSA model water, (B) HA model water, (C) SA model water, (D) river/surface water, with different cation-modified GO membranes.
3.3. Effect of thickness of GO membrane on the flux
crosslinked with the same ion (Al3+ or Fe3+), the initial flux ratio between GO membranes with 0.001 M Al3+ and 0.1 M Al3+ ranged from 1.8 to 2.9, while that of GO membranes with 0.001 M Fe3+ and 0.1 M Fe3+ ranged from 1.6 to 2.8, which indicated that the ion concentration had a significant effect on the initial flux of the GO membrane. Moreover, the GO membrane cross-linked with Fe3+ had a higher water flux compared to Al3+ at each cation concentration and for the three NOM solutions and surface water. These results are consistent with the XRD data, which showed that the spacing between the Fe3+ cross-linked GO nanosheets was larger than that for Al3+. Furthermore, for the four types of feed water, the difference in the fluxes between the Fe3+ and Al3+ GO membranes was generally more significant at the lowest ion concentration (0.001 M), whilst at the higher concentration of cations the stronger binding force among the GO nanosheets, and consequential reduction in the layer spacing and corresponding water flux, was less marked between the two cations.
As shown in the Supporting information (Fig. S3) the average flux of the GO membrane with DI water decreased systematically with increase of the thickness, i.e. the mass of GO loaded on the PVDF supporting layer, at a specific cation concentration. It is likely that the increase of flow channel length and resistance of the water flow (reduction in water flux for constant applied pressure) is proportional to the increase in layers of GO nanosheets. From the results of the flux test for the alginate solution, shown in Fig. S15, the membrane with a thin GO layer has a more favorable performance in respect of initial flux, while a thick GO layer showed less reduction in flux with operating time, indicating a greater resistance to fouling [31]. In order to achieve the best performance of the membrane, both flux and fouling resistance need to be considered and therefore an optimal GO mass loading is required to match the particular operating conditions. The variations of flux and normalized flux of the GO membrane with different thickness (Fig. S15) showed that the membrane had an optimal performance in terms of initial flux and low-fouling behaviour when the mass loading of GO was 1 mg (220 mg/m2) or 2 mg (440 mg/m2). In the subsequent experiments, these two loadings were used to evaluate further the treatment performance of the membrane at different cation concentrations.
3.5. Membrane flux variations for different feed waters Filtration experiments involving the cation-modified GO membranes with different concentrations of Al3+ and Fe3+ and different membrane thicknesses (GO layers) were conducted with the four types of feed water, namely BSA, HA, alginate and river/surface water. The results summarized in Figs. S5-12 show that variations of the flux and normalized flux were different during the filtration process for the different types of organic matter in water. In general, the GO membrane with Fe3+ had a better performance than Al3+ in terms of flux at the same thickness of GO layer. In contrast, the normalized flux decline of the GO membrane with Fe3+ was similar to that with Al3+, under the same conditions for all the filtration experiments, which suggests that
3.4. Effect of ion species and concentration on the flux variation of cationmodified GO membranes For the filtration experiments, a concentration of 10 mg/L of BSA, HA, alginate solutions and real surface water was used as the feed water. In Fig. 4, the initial flux of the GO membrane is shown for different feed waters and cation concentrations. For GO membranes 37
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Fig. 7. Molecular weight distributions of feed and treated waters with cation-modified GO membranes (2 mg GO): BSA model water (A, B), HA model water (C, D), river/surface water (E, F).
surface water (Figs. S5 and S6). In addition, Table 1 summarizes the normalized flux J/J0 after 30-min filtration for different cation-modified GO membranes. The data show that increasing the thickness of GO layer (greater GO mass), and greater cation content, moderated the reduction in normalized flux, and indicated a greater fouling resistance. In addition to the filtration experiments with model NOM feed solutions, the flux and normalized flux variation of the cation-modified GO membranes when treating the surface water are shown in Figs. S11 and S12. In general, the results were similar to those for the model solutions. For the same cation concentration and GO loading, the GO membrane cross-linked with Fe3+ achieved a higher flux compared with Al3+ at the two GO loadings. Also, while a higher initial flux was obtained at the lower GO loading of 1 mg, the normalized flux decline was greater than for the GO loading of 2 mg. As evident for the model
both types of cation-modified GO membrane had an almost equal ability to reduce membrane fouling. After the first filtration operation, the fouled membranes were cleaned and reused for two further filtration cycles. Values for the flux recovery of the GO membranes crosslinked with Al3+ or Fe3+ are given in the Supporting information (Fig. S13), which indicated a relatively high recovery ratio of the GO membranes, especially when treating HA and SA. Values for the flux recovery ratio (%), reversible fouling (%) and irreversible fouling (%) of the membranes for the first cycle were calculated and shown in Table S2. A more detailed comparison of the normalized flux decline of GO membranes with Al3+ and Fe3+ confirmed that the presence of BSA, which is a representative proteinaceous NOM, tends to contribute to more severe membrane fouling compared to the other NOM solutions or 38
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NOM solutions, the normalized flux decline of the cation-modified GO membranes were similar, at the same GO loading, and the decline was relatively minor.
membrane was cross-linked with Fe3+.
3.6. NOM removal efficiency
This study has investigated the preparation and performance of a novel and stable GO membrane for NOM removal in water treatment. The GO membrane was synthesized facilely by applying a trivalent metal cation, either Fe3+ or Al3+, to cross-link GO nanosheets, superimposed onto a PVDF UF membrane supporting layer. The hydrophilicity of the cation-modified GO increased with the amount of cation incorporated. It was found that the GO membrane cross-linked with Fe3+ exhibited a greater flux and NOM removal efficiency compared with the GO membrane cross-linked with Al3+. The results have highlighted several advantages of the cation-modified GO membrane prepared in this study. Firstly, the material cost of the novel membrane is likely to be low since both the GO material and Fe3+/Al3+ salts are readily available and inexpensive. Secondly, the preparation method for the cation-modified GO membrane is relatively straight-forward and should not present significant challenges to the realization of largescale membrane production. Thirdly, the Fe3+/Al3+ cross-linked GO membrane demonstrated an excellent performance in terms of flux, NOM removal and organic fouling control, especially for the GO membrane cross-linked with Fe3+.
4. Conclusions
Humic substances, as a principal component of NOM, are commonly quantified in water by determining the UV absorbance at 254 nm (UV254). In this work, the removal efficiency of humic substances in the HA solution and surface water by the cation-modified GO membranes were determined by the change in UV254. The average UV254 value of the feed HA solution was 0.691 cm−1, and decreased to 0.003 cm−1 after GO membrane filtration. For the HA solution, therefore, the GO membrane had an excellent UV254 removal efficiency of 99.6% with both Fe3+ and Al3+ cross-linked cations as shown in Fig. 5A and B. In contrast, the average UV254 value of the river/surface water was 0.392 cm−1 and decreased to approximately 0.101 cm−1 after GO membrane filtration. In this case, the UV254 removal efficiency was in the range of 70–80%, which was lower than that of the HA solution probably due to the more complex composition of the real water. It was also noted that the Fe3+ cross-linked GO membrane had a slightly greater removal efficiency than the Al3+ cross-linked GO membrane, for both the HA solution and surface water. Furthermore, the removal efficiency of the organic matter was determined in terms of the TOC of the feed and permeate waters. As shown in Fig. 6, the cation-modified GO membranes had a high treatment performance in terms of TOC removal of 90–95% for the three model NOM solutions, and 60–70% for the surface water. As indicated previously by the UV254 values, the Fe3+ modified GO membrane was slightly more effective in terms of TOC removal than the GO membrane with Al3+. It was also found that the TOC removal was less variable (< 1% for the model NOM solutions, and < 3% for the river/surface water) when the Fe3+ ion concentration changed from 0.001 to 0.1 M (i.e. removal efficiency was high even at the lowest cation concentration of 0.001), compared to the GO membranes with Al3+; for the Al3+modified GO membrane, an increasing cation concentration enhanced the TOC rejection. The results suggest that Fe3+ has a superior ability to combine with surface hydroxyl groups on the GO nanosheets compared to Al3+ (each Fe3+ can bind to 6 hydroxyl groups, while Al3+ can bind to 4–6 hydroxyl groups), so the combination of Fe3+ and GO is more stable. It is speculated that at low Al3+ concentrations, partial regions of the GO nanosheet were not cross-linked by the Al3+ and thus influenced the uniformity of the channel sizes, allowing NOM contaminants to pass through larger nanochannels. At higher Al3+concentrations, the uniformity of the nanochannels within the GO layer is enhanced, and the rejection efficiency of NOM contaminants in the feed water by the GO membrane increased accordingly. In addition to the measurement of UV254 and TOC, the removal of organic matter was investigated in terms of the molecular weight (MW) distribution, as determined by SEC, to further evaluate the performance of the GO membranes. Fig. 7 shows that the GO membranes provided an almost complete removal of the BSA as indicated by the MW range between 10 kDa and 100 kDa, regardless of the type or concentration of the cross-linking cation. For the HA solution, the GO membranes also had an excellent rejection efficiency of the HA, which has a smaller MW range of 1 kDa~10 kDa, but a small fraction of HA of about 2 kDa was evident as a residual after treatment. For the surface water, the results shown in Fig. 7E and F indicated that a large quantity of the humic acidtype compounds were removed, which was consistent with the results of the HA model solution. However, the surface water comprised a substantial quantity of small MW organic components and the organic matter with MW lower than 2 kDa were difficult to be removed by the GO membrane. By comparing the SEC results of the permeate water from the 2 mg GO membranes (Fig. 7) and 1 mg GO membranes (Fig. S16), it was evident that the thicker GO layer was more effective in the rejection of organic molecules over 1000 Da, especially when the GO
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51308043) and Marie Curie International Incoming Fellowship (FP7-PEOPLE-2012-IIF-328867). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2017.07.061. References [1] C.J. Vorosmarty, P.B. McIntyre, M.O. Gessner, D. Dudgeon, A. Prusevich, P. Green, S. Glidden, S.E. Bunn, C.A. Sullivan, C.R. Liermann, P.M. Davies, Global threats to human water security and river biodiversity, Nature 467 (2010) 555–561. [2] Y. Jiang, D. Liu, M. Cho, S.S. Lee, F. Zhang, P. Biswas, J.D. Fortner, In situ photocatalytic synthesis of Ag nanoparticles (nAg) by crumpled graphene oxide composite membranes for filtration and disinfection applications, Environ. Sci. Technol. 50 (2016) 2514–2521. [3] M. Hu, B. Mi, Enabling graphene oxide nanosheets as water separation membranes, Environ. Sci. Technol. 47 (2013) 3715–3723. [4] L. Bai, H. Liang, J. Crittenden, F. Qu, A. Ding, J. Ma, X. Du, S. Guo, G. Li, Surface modification of UF membranes with functionalized MWCNTs to control membrane fouling by NOM fractions, J. Membr. Sci. 492 (2015) 400–411. [5] Y. Zhang, S. Zhang, T.S. Chung, Nanometric Graphene oxide framework membranes with enhanced heavy metal removal via nanofiltration, Environ. Sci. Technol. 49 (2015) 10235–10242. [6] X. Fu, T. Maruyama, T. Sotani, H. Matsuyama, Effect of surface morphology on membrane fouling by humic acid with the use of cellulose acetate butyrate hollow fiber membranes, J. Membr. Sci. 320 (2008) 483–491. [7] F. Meng, A. Drews, R. Mehrez, V. Iversen, M. Ernst, F. Yang, M. Jekel, M. Kraume, Occurrence, source, and fate of dissolved organic matter (DOM) in a pilot-scale membrane bioreactor, Environ. Sci. Technol. 43 (2009) 8821–8826. [8] I. Sawada, R. Fachrul, T. Ito, Y. Ohmukai, T. Maruyama, H. Matsuyama, Development of a hydrophilic polymer membrane containing silver nanoparticles with both organic antifouling and antibacterial properties, J. Membr. Sci. 387–388 (2012) 1–6. [9] M. Hu, B. Mi, Layer-by-layer assembly of graphene oxide membranes via electrostatic interaction, J. Membr. Sci. 469 (2014) 80–87. [10] Y. Jiang, W.N. Wang, D. Liu, Y. Nie, W. Li, J. Wu, F. Zhang, P. Biswas, J.D. Fortner, Engineered crumpled graphene oxide nanocomposite membrane assemblies for advanced water treatment processes, Environ. Sci. Technol. 49 (2015) 6846–6854. [11] D.A. Dikin, S. Stankovich, E.J. Zimney, R.D. Piner, G.H. Dommett, G. Evmenenko, S.T. Nguyen, R.S. Ruoff, Preparation and characterization of graphene oxide paper, Nature 448 (2007) 457–460. [12] M. Koinuma, C. Ogata, Y. Kamei, K. Hatakeyama, H. Tateishi, Y. Watanabe, T. Taniguchi, K. Gezuhara, S. Hayami, A. Funatsu, M. Sakata, Y. Kuwahara, S. Kurihara, Y. Matsumoto, Photochemical engineering of graphene oxide nanosheets, J. Phys. Chem. C 116 (2012) 19822–19827.
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Trivalent metal cation cross-linked graphene oxide membranes for NOM removal in water treatment ⁎
Ting Liua, Bing Yanga, Nigel Grahamb, Wenzheng Yub, , Kening Suna, a b
MARK
⁎
School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene oxide membrane Water treatment Trivalent cation crosslinking Natural organic matter Membrane fouling
This paper summarizes the development and testing of novel graphene oxide (GO) membranes for the removal of natural organic matter (NOM) from raw water sources used for drinking water supply. In this work, two trivalent cations, Al3+ and Fe3+, were employed as crosslinking agents to stack GO nanosheets layer by layer on a PVDF membrane support, in order to fabricate a suitable GO membrane. The trivalent cations greatly improve the bonding strength between the GO nanosheets through electrostatic forces and coordination bonds, and thus enhance the stability of the GO membrane; the integrity of the membrane in a range of solutions could be maintained for over a month. The initial interlayer spacing of GO nanosheets (0.80 nm) could be increased up to 0.86–0.95 nm by changing the Al3+ or Fe3+ ion concentration. It was found that the GO membrane flux ranged between 79 and 902 LMH/MPa when treating three representative NOM solutions and a real surface water. A relatively low thickness of the GO layer induced a higher flux of the GO membrane when prepared with the same cation concentration, while increasing the cation concentration resulted in a decline in flux. The flux of the Fe3+ cross-linked GO membrane was approximately 1.1–2.3 times that of the Al3+ cross-linked GO membrane, while both cation-modified GO membranes achieved a similar separation efficiency of the organic contaminants. The study has demonstrated a facile approach to the fabrication of a novel, stable GO membrane employing Al3+/ Fe3+ ions as crosslinking species, in order to utilize the excellent properties of GO and produce a GO membrane with a high flux and organic removal performance for water treatment.
1. Introduction The continuing pressure on water resources and increasing demand for drinking water supplies worldwide can be alleviated by the development of new water treatment technologies which are more economical and efficient in producing clean and safe water from contaminated sources [1–3]. Membrane separation is recognized as a versatile and effective technology that has been used widely for water treatment at many scales and in many applications throughout the world. In particular, ultrafiltration (UF) and nanofiltration (NF) are utilized most frequently because of their ability to achieve a high degree of contaminant removal, mainly particulates and microorganisms [4,5]. However, the most significant technical limitation of these processes is fouling of the membrane, which can reduce the flux and service life, and increase energy consumption owing to the need for greater applied pressures [6]. Membrane fouling inevitably occurs when a cake layer forms on the membrane surface and/or the membrane pores are blocked by the contaminants in water, causing an increase in filtration resistance; this ⁎
Corresponding authors. E-mail addresses: [email protected] (W. Yu), [email protected] (K. Sun).
http://dx.doi.org/10.1016/j.memsci.2017.07.061 Received 1 June 2017; Received in revised form 28 July 2017; Accepted 29 July 2017 Available online 01 August 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
can lead to a decrease in flux of the membrane and an increase in the operating costs. The membrane fouling rate and degree are closely related to the hydrophobic/hydrophilic nature of the membrane surface, whereby contaminant accumulation (fouling) is enhanced with membrane surfaces that have greater hydrophobicity [7]. Therefore, modification of the membrane surface in order to improve its hydrophilicity (decrease its hydrophobicity) is a potential means of reducing the deposition of contaminants and thus mitigate the membrane fouling. Some previous studies have reported that the coating of super-hydrophilic nanomaterials on the membrane surface can greatly enhance the hydrophilicity and anti-fouling ability of the membrane [6,8]. Graphene oxide (GO) is a relatively novel type of nanomaterial, which is attracting considerable attention in the field of membranebased water treatment research because of its high degree of hydrophilicity and other unique properties [9]. GO nanosheets are graphene sheets which have been oxidized by strong oxidants and then functionalized with carboxyl, hydroxyl, and epoxide groups [10]. The high density of oxygenated groups offers a great potential to improve the performance of GO functionalized membranes by increasing their
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Fig. 1. Characterization of GO: Raman spectrum (A), FTIR spectrum (B), XRD pattern (C) and TEM image (D) of GO nanosheets.
stability in aqueous solution. In previous studies, the most commonly used method for reducing the instability/disintegration of GO membrane in solution was to use a crosslinking agent to connect the GO nanosheets, such as multivalent cations (Mg2+, Ca2+, Al3+) [21–23], modified graphene oxide (aminated-GO [24], partially reduced GO [25]) or high-molecular polymers (including 1,3,5-benzenetricarbonyl trichloride polyallylamine [26], polyvinyl alcohol and polymethyl methacrylate [19]). Among these methods that have been considered, the use of multivalent cations as a crosslinking agent is a preferable alternative owing to their low-cost and facile operation in the membrane synthesis. The present study provides new information and a greater understanding of the role of two common trivalent cation crosslinking agents, Al3+ and Fe3+, in GO membrane fabrication and in the performance of organic fouling control of the GO membranes. In this investigation GO nanosheets were stacked on a support membrane to reduce membrane fouling by producing a more hydrophilic surface. The trivalent cations Al3+ and Fe3+ were compared as crosslinking agents to enhance the stability of the GO nanosheets, which were stacked on the support membrane in the solution. The two cation-modified GO membranes were evaluated to explore the mechanisms of flux improvement and fouling mitigation. In particular, the effects of GO mass and Al3+ and Fe3+ cation concentration on NOM removal efficiency and water flux were examined in this study.
hydrophilicity, chemical stability, and antimicrobial and antifouling properties [11,12]. In addition, the large horizontal to vertical ratio of the GO nanosheet enables it to possess a remarkable stacking property, which benefits the fabrication of GO functionalized membranes [13]. There are two main GO-based membrane fabrication approaches, which are either the blending of GO into a polymer matrix to form various nanocomposite membranes, or the direct loading of GO on to the surface of a polymer membrane as a support layer. When the GO is blended into the polymer matrix, such as polyvinylidene fluoride (PVDF), polysulfone and polyether sulfone [14–17], the important properties of hydrophilicity and high specific surface area of GO cannot be fully utilized because the GO is not directly exposed to foulants and the interactions between GO and foulants are limited [18]. In contrast, directly loading the GO on the surface of the membrane by deposition represents a more efficient approach to the preparation of the GO-based membrane, and which benefits fouling control. The two-dimensional channels formed by the stacking of the GO nanosheets are selective, allowing water to pass through whilst intercepting pollutants in the water. When the organic pollutant molecules and/or ions diffuse into the channels of the GO membrane from the edge of the GO nanosheets, they can be retained by the narrow interlayer channels which are formed by hydrogen bonding and electrostatic interactions. Furthermore, the selectivity of the GO membrane also arises from the electrostatic interaction between charged pollutants and the highly (negatively) charged GO [19]. These channels have large slip lengths because of their nanoscale size and the presence of hydrophobic regions on the GO nanosheet, similar to the capillary phenomenon which causes a larger water flux [20]. Since the GO nanosheets have a good dispersity in water owing to their hydrophilic properties and surface charge, the GO layers that are deposited on the surface of the polymer support membrane can readily disintegrate in water [21]. Therefore, the most significant challenge for the development and application of a GO membrane is to improve its
2. Materials and methods 2.1. GO preparation GO was prepared using the modified Hummers method [27,28]. Natural flake graphite was oxidized in a mixture of H2SO4, NaNO3 and KMnO4. Then hydrochloric acid solution was used to wash the product which was obtained from filtration of the solution after the oxidation 32
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Fig. 2. Cross-sectional SEM images of the GO membrane (A); XRD intensity spectra for different cation-GO membranes (B); variation of contact angle for membranes prepared with different cation concentrations (C); element analysis and EDX mapping (Al distribution pattern at 5 k×) for Al3+-GO membrane with 0.001 M Al3+ (D). 2 mg GO mass (440 mg/m2) for all membranes.
2.2. Membrane fabrication
reaction was completed. The GO solid was placed in hydrochloric acid solution for washing and then centrifuged at 4000 g-force, and impurities were removed after 4–6 cycles of washing. Subsequently, the acid was replaced by deionized water and the centrifugal speed increased to 8000 g-force to wash out the residual acid in the GO suspension. The pure multilayer GO was exfoliated into GO nanosheets under ultrasonic vibration and after freeze-drying the GO powder was obtained.
To fabricate the GO membrane, we used a commercial PVDF membrane (Shanghai Mosu Technology Co., Ltd., China) as the support layer. Before the fabrication, the PVDF membrane was pre-conditioned by filtering 200 mL DI water, and then the GO was loaded onto the membrane surface by pressure filtration to produce a pure GO membrane. In order to obtain a stable and suitable membrane, either Al3+ or Fe3+ (at concentrations of 0.001 M，0.01 M or 0.1 M) was added as a 33
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Fig. 3. XPS spectra of GO membranes prepared with Al3+/Fe3+ at different ion concentrations.
membrane. These were evaluated using a commercial pressure filtration system (Millipore, Amicon 8400, USA) employing nitrogen gas as the means for obtaining a stable applied pressure (0.1 MPa). A digital balance (ML3002, Mettler-Toledo International Inc., USA) was connected to a computer to automatically record the weight of the filtered water in real time by the data acquisition software. DI water (Millipore Milli-Q) was used to test the water flux of GO membranes. Three different kinds of model NOM solutions, namely bovine serum albumin (BSA), humic acid and alginate (sodium salt, SA), at a concentration of 10 mg/L were used as feed solutions to test the flux, rejection rate and the fouling resistance of the GO membrane; samples of surface water from the South Long River, Beijing, China were also used as feed water in this study. Before each test the newly fabricated GO membrane was pre-conditioned by filtering DI water for ten minutes to ensure there was no residual Al3+/Fe3+ on the surface of the GO membrane. The TOC concentration of samples of all feed waters and permeates were determined by total organic carbon (TOC) analyzer (TOC-VCPH, Shimadzu, Japan), and the UV254 absorbance of humic acid solution and surface water were analyzed by UV–visible spectrophotometer (U3010, Hitachi, Japan) at a wavelength of 254 nm. Size exclusion chromatography (SEC) was used to determine the apparent molecular weight (MW) distribution of UV-active substances. SEC was performed using HPLC (Perkin Elmer, USA), with a Series 200 pump, a BIOSEPSEC-S3000 column (Phenomenex, UK) (7.8 mm×300 mm), and a Security Guard column fixed with a GFC-3000 disc 4 mm (ID). The injection volume of water samples was 100 μL and the UV–visible detector operated at a wavelength of 254 nm with autosampler. The fouled GO membranes were cleaned and reused for two filtration cycles to investigate their performance in terms of flux recovery. In the cleaning procedure, the fouled GO membranes after the filtration process were rinsed moderately with DI water for about 20 s and then ultrasonically cleaned in DI water for 3 min. Values for the flux recovery ratio (FRR
crosslinking agent to 100 mL deionized water with 1 mg or 2 mg GO (corresponding to GO loadings of 220 mg/m2 or 440 mg/m2). Finally, residual unbound Al3+ or Fe3+ was removed from the synthesized GO membrane by filtering DI water for ten minutes. 2.3. Characterization of GO nanosheets and membranes The physical and chemical properties of GO nanosheets and the structure of GO membranes were evaluated by several characterization techniques. These properties have close relationships with the performance of the GO membrane. Scanning electron microscopy (SEM, QUANTA FEG 250, FEI, USA) provided microscopic images after being sputter-coated with aurum on the GO membrane's surface and cross section, while the presence and distribution of Al3+/Fe3+ in the GO membrane was mapped using energy-dispersive X-ray spectroscopy (EDX, X-Max, Oxford Instruments, UK). High-resolution transmission electron microscopy (HRTEM, JEOL, JEM-2100F, Japan) revealed the individual GO nanosheets and the folds. Contact angle measurements were conducted to indicate membrane hydrophilicity. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, America) spectra were collected to identify the relative abundance of the different functional groups. X-ray diffraction (XRD, Rigaku UltimaIV, Cu-Κα radiation, 45 kV, 50 mA, Japan) characterization was used to explore the inter-nanosheet spacing of GO membranes. Fourier transform infrared spectroscopy (FT-IR, BRUKER, ALPHA-P, USA) was used to identify the presence of oxygen containing functional groups on the GO. 2.4. Flux and rejection tests Water flux, normalized flux and rejection rate (removal efficiency) are important parameters to evaluate the fouling resistance of the GO 34
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Fig. 4. Variation of membrane flux (LMH–litres m−2 h−1) with Al3+ and Fe3+ concentration (2 mg GO mass) in different NOM solutions: BSA (A), HA (B), alginate (C) and surface water (D).
process. The Raman spectrum of the GO nanosheets (Fig. 1A) shows two peaks at 1350 cm−1 (D peak) and 1593 cm−1 (G peak), respectively. The D peak represents a defect of the carbon atomic lattice, which indicates the addition of oxygen-containing functional groups, while the G peak represents in-plane stretching vibration of sp2 hybridization of the graphitic carbon. Furthermore, the FTIR spectrum (Fig. 1B) suggests that the GO nanosheets contain a stretching vibration of sp2 C˭C (1625 cm−1), −C˭O stretching vibration (1726 cm−1), stretching vibration of C−O (1055 cm−1), stretching vibration of C−OH (1226 cm−1) and deformation vibration of O−H (1382 cm−1) [29]. The XRD pattern (Fig. 1C) shows that there is a sharp peak at around 10 degrees and a small peak at around 43.1 degrees, which corresponds well with the crystal phase of graphene oxide. The HRTEM image (Fig. 1D) exhibits a uniform and typical wrinkled nanosheets structure of GO.
Table 1 Variation of normalized flux J/J0 (J0 is initial flux) after 30-min filtration with NOM solutions for cation-modified GO membranes (various Al3+/Fe3+ concentrations, and GO mass of 1 mg and 2 mg). Membrane (Cation concn, GO mass)
J/J0 of BSA
J/J0 of HA
J/J0 of SA
J/J0 of Surface water
0.001 M Al3+, 1 mg 0.01 M Al3+, 1 mg 0.1 M Al3+, 1 mg 0.001 M Fe3+, 1 mg 0.01 M Fe3+, 1 mg 0.1 M Fe3+, 1 mg 0.001 M Al3+, 2 mg 0.01 M Al3+, 2 mg 0.1 M Al3+, 2 mg 0.001 M Fe3+, 2 mg 0.01 M Fe3+, 2 mg 0.1 M Fe3+, 2 mg
0.322 0.350 0.365 0.200 0.233 0.274 0.333 0.423 0.445 0.316 0.334 0.386
0.698 0.723 0.806 0.690 0.724 0. 738 0.768 0.786 0.810 0.628 0.662 0.681
0.516 0.567 0.779 0.667 0.698 0.710 0.855 0.861 0.820 0.808 0.764 0.712
0.785 0.756 0.744 0.725 0.679 0.723 0.891 0.841 0.821 0.766 0.821 0.829
3.2. Characteristics of GO membrane SEM images of the cross-section of the GO membrane (Fig. 2A) show that the GO layer with a certain degree of folds completely covered the surface of the PVDF membrane support and bonded closely with it. The average thickness of the GO layer was estimated to be in the range of 120–150 nm. The nano-scale GO sheets were deposited layer by layer on the surface of the PVDF membrane support, with interspaces of a similar nano-scale between the layers, which provide channels for water flow and tortuous surfaces for the retention of NOM contaminants in the water. The change of average interlayer spacing between the GO nanosheets caused by intercalation of the two trivalent metal ions was confirmed by the XRD patterns (Fig. 2B). While the XRD peak of the original GO membrane indicated an interlayer spacing of ~0.80 nm (from Bragg's Law), the peaks for the metal ion cross-linked GO membranes shifted to lower θ values, which indicated that the
%), reversible fouling (%) and irreversible fouling (%) of the membranes were calculated.
3. Results and discussion 3.1. Properties of GO nanosheets The physicochemical properties of GO nanosheets were determined by several techniques, as the properties directly affect the synthesis process and filtration performance of the GO membrane. The GO nanosheets were fabricated by introducing a large quantity of oxygencontaining functional groups into the graphite sheets, and then the GO nanosheets were evenly separated in the water by an ultrasound 35
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Fig. 5. NOM removal (% change in UV254) for HA solution by GO membrane at different cation concentrations: (A) Al3+ and (B) Fe3+；NOM removal (% change in UV254) for surface water by GO membrane: (C) Al3+ and (D) Fe3+.
corresponding interlayer spacing increased to ~0.86 nm (Al3+) and ~0.95 nm (Fe3+), respectively. Hence, the XRD patterns of the cation cross-linked GO membrane demonstrated a significant increase in the layer-to-layer distance compared to that of the original GO membrane, which therefore influences the water flux of the membrane. Comparing the effect of the two trivalent cations, the Fe3+ cross-linking appeared to produce a larger interlayer spacing compared to that of Al3+, which could be attributed to their relative atomic size; Fe3+ has an ionic radius of 0.0645 nm, compared to 0.0535 nm for Al3+. It was also evident that the membranes prepared with different metal ion concentrations, specifically 0.001 and 0.1 M, did not have significantly different θ values. Although a large number of oxygen-containing functional groups on the GO nanosheets are believed to give the membrane surface a high degree of hydrophilicity, the change in hydrophilicity with cation incorporation was measured in terms of contact angle. It was observed that the incorporation of Al3+ and Fe3+ ions during preparation had some beneficial effects on the hydrophilicity of the GO membrane. The contact angle of the GO membrane showed a decreasing trend with increase in cation concentration, but there were differences between the extent of the changes and the cation (Fig. 2C). The results showed that Fe3+ had a greater impact on hydrophilicity of the GO membrane compared to Al3+ at the same concentration. The presence and distribution of Al3+/Fe3+ in the GO membrane were indicated by EDX mapping. Fig. 2D presents the EDX image of Al3+-GO membrane with 0.001 M Al3+. The peak at 1.487 keV demonstrated the presence of Al, and the right-hand image showed that the Al was uniformly distributed in the Al3+-GO membrane. The other EDX mappings of Al3+/Fe3+ cross-linked GO membranes at different ion concentrations can be seen in the supporting information (Fig. S1). In addition, the stability of the
GO membrane incorporated with either Al3+ or Fe3+ was investigated in DI water and two different model NOM solutions, these being: 50 mg/L SA solution and 50 mg/L BSA solution. The results showed that the GO membrane with Al3+ and Fe3+ was stable in the three solutions for at least two weeks (Fig. S2). A series of GO leaching tests for all the membranes was conducted using TOC analysis, and the results indicated that the extent of GO leaching was lower than 0.1%, which confirmed the stability of the Al3+/Fe3+-modified GO membranes. The interactions between the GO nanosheets and the cations Al3+ or 3+ Fe , include the electrostatic attractive force between the negative charge of the GO's oxygen functional groups and Al3+/Fe3+ cations (producing charge reduction/neutralization), and coordination bonds between the cation and the oxygen functional groups, such as the epoxy and hydroxyl groups. As graphene oxide is believed to have a large number of epoxy groups, its exposure to Lewis acidic metal ions may lead to ringopening of the epoxide to create C-OH moieties, together with the carbonyl groups [21]. XPS spectra of the original GO membrane and the GO membranes prepared with 0.1 M Al3+/Fe3+, together with a comparison of different component (bond types) proportions at different concentrations of Al3+/Fe3+, are shown in Fig. 3. It can be seen that with increasing cation incorporation there was a decrease in the percentage of C–O bonds and a corresponding increase in C-C/C˭C bonds [30]. The results suggested that increasing the concentration of the cation could facilitate the reduction of carbon-oxygen bonds, which mainly relates to the ring-opening of the epoxide. It was deduced that the spacing between the GO nanosheets could be reduced and thus produce narrower nanochannels for water flow when the cations with a higher concentration diffuse into the inner structure of the GO nanosheets.
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Fig. 6. NOM removal (% change in TOC) for, (A) BSA model water, (B) HA model water, (C) SA model water, (D) river/surface water, with different cation-modified GO membranes.
3.3. Effect of thickness of GO membrane on the flux
crosslinked with the same ion (Al3+ or Fe3+), the initial flux ratio between GO membranes with 0.001 M Al3+ and 0.1 M Al3+ ranged from 1.8 to 2.9, while that of GO membranes with 0.001 M Fe3+ and 0.1 M Fe3+ ranged from 1.6 to 2.8, which indicated that the ion concentration had a significant effect on the initial flux of the GO membrane. Moreover, the GO membrane cross-linked with Fe3+ had a higher water flux compared to Al3+ at each cation concentration and for the three NOM solutions and surface water. These results are consistent with the XRD data, which showed that the spacing between the Fe3+ cross-linked GO nanosheets was larger than that for Al3+. Furthermore, for the four types of feed water, the difference in the fluxes between the Fe3+ and Al3+ GO membranes was generally more significant at the lowest ion concentration (0.001 M), whilst at the higher concentration of cations the stronger binding force among the GO nanosheets, and consequential reduction in the layer spacing and corresponding water flux, was less marked between the two cations.
As shown in the Supporting information (Fig. S3) the average flux of the GO membrane with DI water decreased systematically with increase of the thickness, i.e. the mass of GO loaded on the PVDF supporting layer, at a specific cation concentration. It is likely that the increase of flow channel length and resistance of the water flow (reduction in water flux for constant applied pressure) is proportional to the increase in layers of GO nanosheets. From the results of the flux test for the alginate solution, shown in Fig. S15, the membrane with a thin GO layer has a more favorable performance in respect of initial flux, while a thick GO layer showed less reduction in flux with operating time, indicating a greater resistance to fouling [31]. In order to achieve the best performance of the membrane, both flux and fouling resistance need to be considered and therefore an optimal GO mass loading is required to match the particular operating conditions. The variations of flux and normalized flux of the GO membrane with different thickness (Fig. S15) showed that the membrane had an optimal performance in terms of initial flux and low-fouling behaviour when the mass loading of GO was 1 mg (220 mg/m2) or 2 mg (440 mg/m2). In the subsequent experiments, these two loadings were used to evaluate further the treatment performance of the membrane at different cation concentrations.
3.5. Membrane flux variations for different feed waters Filtration experiments involving the cation-modified GO membranes with different concentrations of Al3+ and Fe3+ and different membrane thicknesses (GO layers) were conducted with the four types of feed water, namely BSA, HA, alginate and river/surface water. The results summarized in Figs. S5-12 show that variations of the flux and normalized flux were different during the filtration process for the different types of organic matter in water. In general, the GO membrane with Fe3+ had a better performance than Al3+ in terms of flux at the same thickness of GO layer. In contrast, the normalized flux decline of the GO membrane with Fe3+ was similar to that with Al3+, under the same conditions for all the filtration experiments, which suggests that
3.4. Effect of ion species and concentration on the flux variation of cationmodified GO membranes For the filtration experiments, a concentration of 10 mg/L of BSA, HA, alginate solutions and real surface water was used as the feed water. In Fig. 4, the initial flux of the GO membrane is shown for different feed waters and cation concentrations. For GO membranes 37
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Fig. 7. Molecular weight distributions of feed and treated waters with cation-modified GO membranes (2 mg GO): BSA model water (A, B), HA model water (C, D), river/surface water (E, F).
surface water (Figs. S5 and S6). In addition, Table 1 summarizes the normalized flux J/J0 after 30-min filtration for different cation-modified GO membranes. The data show that increasing the thickness of GO layer (greater GO mass), and greater cation content, moderated the reduction in normalized flux, and indicated a greater fouling resistance. In addition to the filtration experiments with model NOM feed solutions, the flux and normalized flux variation of the cation-modified GO membranes when treating the surface water are shown in Figs. S11 and S12. In general, the results were similar to those for the model solutions. For the same cation concentration and GO loading, the GO membrane cross-linked with Fe3+ achieved a higher flux compared with Al3+ at the two GO loadings. Also, while a higher initial flux was obtained at the lower GO loading of 1 mg, the normalized flux decline was greater than for the GO loading of 2 mg. As evident for the model
both types of cation-modified GO membrane had an almost equal ability to reduce membrane fouling. After the first filtration operation, the fouled membranes were cleaned and reused for two further filtration cycles. Values for the flux recovery of the GO membranes crosslinked with Al3+ or Fe3+ are given in the Supporting information (Fig. S13), which indicated a relatively high recovery ratio of the GO membranes, especially when treating HA and SA. Values for the flux recovery ratio (%), reversible fouling (%) and irreversible fouling (%) of the membranes for the first cycle were calculated and shown in Table S2. A more detailed comparison of the normalized flux decline of GO membranes with Al3+ and Fe3+ confirmed that the presence of BSA, which is a representative proteinaceous NOM, tends to contribute to more severe membrane fouling compared to the other NOM solutions or 38
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NOM solutions, the normalized flux decline of the cation-modified GO membranes were similar, at the same GO loading, and the decline was relatively minor.
membrane was cross-linked with Fe3+.
3.6. NOM removal efficiency
This study has investigated the preparation and performance of a novel and stable GO membrane for NOM removal in water treatment. The GO membrane was synthesized facilely by applying a trivalent metal cation, either Fe3+ or Al3+, to cross-link GO nanosheets, superimposed onto a PVDF UF membrane supporting layer. The hydrophilicity of the cation-modified GO increased with the amount of cation incorporated. It was found that the GO membrane cross-linked with Fe3+ exhibited a greater flux and NOM removal efficiency compared with the GO membrane cross-linked with Al3+. The results have highlighted several advantages of the cation-modified GO membrane prepared in this study. Firstly, the material cost of the novel membrane is likely to be low since both the GO material and Fe3+/Al3+ salts are readily available and inexpensive. Secondly, the preparation method for the cation-modified GO membrane is relatively straight-forward and should not present significant challenges to the realization of largescale membrane production. Thirdly, the Fe3+/Al3+ cross-linked GO membrane demonstrated an excellent performance in terms of flux, NOM removal and organic fouling control, especially for the GO membrane cross-linked with Fe3+.
4. Conclusions
Humic substances, as a principal component of NOM, are commonly quantified in water by determining the UV absorbance at 254 nm (UV254). In this work, the removal efficiency of humic substances in the HA solution and surface water by the cation-modified GO membranes were determined by the change in UV254. The average UV254 value of the feed HA solution was 0.691 cm−1, and decreased to 0.003 cm−1 after GO membrane filtration. For the HA solution, therefore, the GO membrane had an excellent UV254 removal efficiency of 99.6% with both Fe3+ and Al3+ cross-linked cations as shown in Fig. 5A and B. In contrast, the average UV254 value of the river/surface water was 0.392 cm−1 and decreased to approximately 0.101 cm−1 after GO membrane filtration. In this case, the UV254 removal efficiency was in the range of 70–80%, which was lower than that of the HA solution probably due to the more complex composition of the real water. It was also noted that the Fe3+ cross-linked GO membrane had a slightly greater removal efficiency than the Al3+ cross-linked GO membrane, for both the HA solution and surface water. Furthermore, the removal efficiency of the organic matter was determined in terms of the TOC of the feed and permeate waters. As shown in Fig. 6, the cation-modified GO membranes had a high treatment performance in terms of TOC removal of 90–95% for the three model NOM solutions, and 60–70% for the surface water. As indicated previously by the UV254 values, the Fe3+ modified GO membrane was slightly more effective in terms of TOC removal than the GO membrane with Al3+. It was also found that the TOC removal was less variable (< 1% for the model NOM solutions, and < 3% for the river/surface water) when the Fe3+ ion concentration changed from 0.001 to 0.1 M (i.e. removal efficiency was high even at the lowest cation concentration of 0.001), compared to the GO membranes with Al3+; for the Al3+modified GO membrane, an increasing cation concentration enhanced the TOC rejection. The results suggest that Fe3+ has a superior ability to combine with surface hydroxyl groups on the GO nanosheets compared to Al3+ (each Fe3+ can bind to 6 hydroxyl groups, while Al3+ can bind to 4–6 hydroxyl groups), so the combination of Fe3+ and GO is more stable. It is speculated that at low Al3+ concentrations, partial regions of the GO nanosheet were not cross-linked by the Al3+ and thus influenced the uniformity of the channel sizes, allowing NOM contaminants to pass through larger nanochannels. At higher Al3+concentrations, the uniformity of the nanochannels within the GO layer is enhanced, and the rejection efficiency of NOM contaminants in the feed water by the GO membrane increased accordingly. In addition to the measurement of UV254 and TOC, the removal of organic matter was investigated in terms of the molecular weight (MW) distribution, as determined by SEC, to further evaluate the performance of the GO membranes. Fig. 7 shows that the GO membranes provided an almost complete removal of the BSA as indicated by the MW range between 10 kDa and 100 kDa, regardless of the type or concentration of the cross-linking cation. For the HA solution, the GO membranes also had an excellent rejection efficiency of the HA, which has a smaller MW range of 1 kDa~10 kDa, but a small fraction of HA of about 2 kDa was evident as a residual after treatment. For the surface water, the results shown in Fig. 7E and F indicated that a large quantity of the humic acidtype compounds were removed, which was consistent with the results of the HA model solution. However, the surface water comprised a substantial quantity of small MW organic components and the organic matter with MW lower than 2 kDa were difficult to be removed by the GO membrane. By comparing the SEC results of the permeate water from the 2 mg GO membranes (Fig. 7) and 1 mg GO membranes (Fig. S16), it was evident that the thicker GO layer was more effective in the rejection of organic molecules over 1000 Da, especially when the GO
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