Functional graphene oxide membrane preparation for ...

Science of the Total Environment 628–629 (2018) 261–270

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Functional graphene oxide membrane preparation for organics/inorganic salts mixture separation aiming at advanced treatment of refractory wastewater Jing-Long Han a,b, Xue Xia a, Muhammad Rizwan Haider a,b, Wen-Li Jiang a,b, Yu Tao c, Mei-Jun Liu a, Hong-cheng Wang a,b, Yang-Cheng Ding a,b, Ya-Nan Hou a,d, Hao-Yi Cheng a, Ai-Jie Wang a,d,⁎ a

Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK d State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, PR China b

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Organics/inorganic salts mixed waste produced when RO/NF treating refractory wastewater. • Mixed waste hinder water reuse and resourcelization of inorganic salts • Functional GO membrane was prepared, organics/inorganic salts mixed waste was avoided. • Influence of supporting layer, GO number and preparing pressure was investigated. • GO used was 14.4 mg/m2, material cost was no longer limited factor for GO membrane.

a r t i c l e

i n f o

Article history: Received 4 January 2018 Received in revised form 4 February 2018 Accepted 4 February 2018 Available online xxxx Editor: D. Barcelo Keywords: GO membrane Refractory wastewater advanced treatment Organics/inorganic salts mixed concentrated waste Supporting layer

a b s t r a c t Some refractory organic matters or soluble microbial products remained in the effluents of refractory organic wastewater after biological secondary treatment and need an advanced treatment before final disposal. Graphene oxide (GO) was known to have potential to be the next generation membrane material. The functional organics/inorganic salts separation GO membrane preparation and application in wastewater advanced treatment could reduce energy or chemicals consumption and avoid organics/inorganic salts mixed concentrate waste problems after nanofiltration or reverse osmosis. In this study, we developed a novelty GO membrane aiming at advanced purification of organic matters in the secondary effluents of refractory organic wastewater and avoiding the organics/inorganic salts mixed concentrate waste problem. The influence of preparation conditions including pore size of support membrane, the number of GO layers and the applied pressure was investigated. It was found that for organics/inorganic salts mixture separation membrane preparation, the rejection and flux would achieve balance for the support membrane at a pore size of ~0.1 μm and the number of GO layers of has an optimization value (~10 layers). A higher assemble pressure (~10 bar) contributed to the acquisition of a higher rejection efficiency and lower roughness membrane. This as prepared GO membrane was applied to practical secondary effluent of a chemical synthesis pharmaceuticals wastewater. A good organic matter rejection efficiency (76%) and limited salt separation

⁎ Corresponding author at: Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China. E-mail address: [email protected] (A.-J. Wang).

https://doi.org/10.1016/j.scitotenv.2018.02.043 0048-9697/© 2018 Elsevier B.V. All rights reserved.

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J.-L. Han et al. / Science of the Total Environment 628–629 (2018) 261–270

(b14%) was finally obtained. These results can promote the practical application of GO membrane and the resourcelized treatment of industrial wastewater. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Water security is threating nearly 80% of the world's population (Vörösmarty et al., 2000; Vörösmarty et al., 2010). One of the crises for human society is the absence of clean water (Vörösmarty et al., 2000; Vörösmarty et al., 2010), disorderly discharge of refractory organic wastewater is one of the important causes of water pollution (Ranade and Bhandari, 2014). In some developing countries, agricultural and industrial consuming too much clean fresh water, and the water leaved for the regular running of local ecosystem was far from inadequate. That caused many environmental problems (Vörösmarty et al., 2000; Vörösmarty et al., 2010). To the inland areas, wastewater reuse can be a realistic way to ease these problems. During the production process of pharmaceuticals (Gadipelly et al., 2014), textiles (Holkar et al., 2016), petroleum refinery (Diya'uddeen et al., 2011), coal chemical industry (Ji et al., 2016; Pal and Kumar, 2013), leather (Lofrano et al., 2013), pulp-paper (Ashrafi et al., 2015), pesticide (Oller et al., 2011) and some other chemical synthesis industries (Ranade and Bhandari, 2014), a large amount of clean fresh water was consumed, and refractory organic wastewater was also produced. For economic reasons, the treatment methods of these wastewater are still dominated by biological treatment (Ashrafi et al., 2015; Diya'uddeen et al., 2011; Gadipelly et al., 2014; Holkar et al., 2016; Ji et al., 2016; Lofrano et al., 2013; Oller et al., 2011; Pal and Kumar, 2013; Ranade and Bhandari, 2014). However, limited by hydraulic retention time, biodegradable potential, introduction of microbial products and the huge fluctuations of in inlet wastewater quality and emission volume, a certain amount of organic matters often leaved in the effluent after biological treatment (Laspidou and Rittmann, 2002; Oller et al., 2011; Sheng et al., 2010; Shon et al., 2006; Xie et al., 2016). These organic matters often include the original refractory components (Oller et al., 2011), bio-converted self-polymerized products and soluble microbial products (Azami et al., 2012; Laspidou and Rittmann, 2002; Sheng et al., 2010). In the follow-up disinfection process, some of them tend to be converted to disinfection by-products (Doederer et al., 2014). These disinfection by-products (Doederer et al., 2014) and residual refractory organic matters discharged to the environment will bring water pollution problems and other environmental risks (Jiang et al., 2013). So, these residual refractory organic matters often require advanced treatment (tertiary treatment) (Oller et al., 2011). The existing popular advanced treatment technology includes advanced oxidation [19, 20] and membrane filtration (Li et al., 2011) process. The advanced oxidation technology is usually effective, but tends to have high consumption to chemicals (Deng and Englehardt, 2006; Oturan and Aaron, 2014) or energy (Ternes et al., 2003). Other problems can be chemical sludge from Fenton process (Neyens and Baeyens, 2003), residual ozone from ozone oxidation process causing air pollution (Altmann et al., 2014) and other toxic by-products problems (Pignatello et al., 2006; Wang et al., 2013). All of these limit the application of these technologies. Nanofiltration and reverse osmosis (NF/RO) technology can achieve water reuse, but tend to have a high energy consumption (Werber et al., 2016) and serious membrane fouling (Zhao and Yu, 2014) problems. Polyamides (PA) and other organic membrane have limited antioxidant capacity, making the fouling control and membrane cleaning process difficult (Werber et al., 2016). These hinder the application of membrane filtration technology in wastewater advanced treatment process (Werber et al., 2016). In addition, the concentrated waste generate after NF/RO (Dialynas et al., 2008; Van der Bruggen et al., 2003). As the inorganic salts content of refractory wastewater is

often high (Perez-Gonzalez et al., 2012), the concentrated waste tended be an organics/inorganic salts mixed state that usually exceeded the direct emission standards (Mickley, 2001). Organics/inorganic salts mixed concentrate waste often led to the follow-up concentrated water evaporation process heavy fouling (Rautenbach and Mellis, 1995) and cause the crystallize of inorganic salt difficult to carry on (Perez-Gonzalez et al., 2012) that due to the coordination effect of organic matter to inorganic (Song et al., 2014). These affect the resourcelization of salts and the final mineralization of organic matters (Pignatello et al., 2006). So the organic and inorganic matters should be separated, treated or resource utilized respectively (Rautenbach and Mellis, 1995). Graphene oxide (GO) was known to have potential to be the next generation of membrane material (2015; Mi, 2014; You et al., 2015; Zhao et al., 2015). It showed good chemical stability (Choi et al., 2013; Dreyer et al., 2010), fine rejection efficiency (Joshi et al., 2014; Li et al., 2013; Morelos-Gomez et al., 2017; Radha et al., 2016; Sun et al., 2016) and have potential to be prepared in a relative low price (Hummers and Offeman, 1958; Joshi et al., 2015; Marcano et al., 2010). GO and GO like materials also shows a wide range of applications in other areas of the environment (Li et al., 2017; Yao et al., 2017a; Yao et al., 2017b; Yin et al., 2017). A great amount of attention has been paid to obtain desalination GO membrane (Cheng et al., 2016; McGuinness et al., 2015; Radha et al., 2016; Werber et al., 2016; You et al., 2015) that have high flux and rejection efficiency (Abraham et al., 2017; Chen et al., 2017; Kidambi et al., 2017; Morelos-Gomez et al., 2017; Wang et al., 2017) or solve the problem GO layer swelling in aqueous solution (Abraham et al., 2017; Zheng et al., 2017). The research related to organics/inorganic salts separation GO membrane preparation and application was still lacking. Since the bio-converted self-polymerized organic products and soluble microbial products of refractory wastewater secondary effluent tend to have high molecular weight (Shon et al., 2006; Xie et al., 2012; Yang et al., 2013), it is possible to separate these organic matters from salts and water. If GO membrane can be prepared as the targeted functional membrane and used in advanced treatment of refractory wastewater, it could contribute to solve the high chemical (Deng and Englehardt, 2006; Oturan and Aaron, 2014) or energy (Ternes et al., 2003; Werber et al., 2016) consumption problem, and the organics/inorganic salts mixed concentrated waste problem (Perez-Gonzalez et al., 2012; Rautenbach and Mellis, 1995). These widely existed in the current advanced treatment process to refractory wastewater. However, the GO membrane preparation study aiming at refractory wastewater advanced treatment was relatively fewer (Lee et al., 2013). The influence of supporting membrane pore size, GO layer number (Kim et al., 2013) and preparing pressure (Tang et al., 2014; Tsou et al., 2015) on the organics/inorganic salts separation property and other characteristics of GO membrane was not explored enough. In this study, efforts were made to prepare a GO membrane that capable to organic matters and salts separation, avoiding organics/ inorganic salts mixed concentrate waste problem and promote the resourcelization of inorganic concentrate waste during membrane filtration. The influence of GO layer number, support layer material and pore size, and GO nanosheets assemble pressure to the flux, rejection and robustness of prepared membrane was investigated. The optimized GO membrane had a COD rejection efficiency of 76%, and salts rejection b 14% to a real secondary effluent of refractory pharmaceutical wastewater. Results of this study opened a new approach for GO membrane applying to the advanced purification of wastewater.


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2016). After the GO nanosheets was filtrated on the membrane surface, it was thermal dried at 90 °C or not for comparison.

2. Materials and methods 2.1. Chemicals Dopamine hydrochloride (C8H11NO2·HCl), Tris(hydroxymethyl) amino-methane (Tris, ≥99.8%), Poly(ethylene glycol) (PEG, Aldrich) were purchased from Sigma-Aldrich, USA. KMnO4 (≥99.5%), H2O2 (30%), H2SO4 (≥95–98%), NaNO3 (≥99.0%), NaHCO3 (≥99.5%), CaCl2 (≥96.0%), NaCl (≥99.5%), HCl (36.0–38.0%), NaOH (≥97.0%), and KCl (≥99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. MgSO4·7H2O (≥99.0%), Alizarin red S (≥80.0%) and chromotropic acid disodium salt dehydrate (≥98%) were purchased from Aladdin Industrial Corporation, China. All reagents were of analytical grade or above and used as received. 2.2. GO preparation GO was prepared from graphite using the Hummers' method (Han et al., 2016; Hummers and Offeman, 1958; Marcano et al., 2010). GO nanosheets' characterization results could be found in the previous work (Han et al., 2016). The prepared GO nanosheets were dispersed in pure water, stored at 4 °C and the exterior of reagent bottle was covered by opaque barrier (Han et al., 2016). 2.3. Description of GO membrane fabrication A modified method was developed to prepare the GO membrane (Scheme 1). Poly(vinylidene fluoride) (PVDF) microfiltration membranes with an average pore diameter of 0.08, 0.1, 2.0 μm (Shanghai SINAP Membrane Tech, Shanghai, China; RisingSun Membrane Tech, Beijing, Co., Ltd., China; Shanghai Minglie Science Technology Co., Ltd., Shanghai, China), the polyethersulfone (PES) microfiltration membrane (RisingSun Membrane Tech, Beijing, Co., Ltd., China) with an average pore diameter of 0.1 μm and the poly(tetra fluoroethylene) (PTFE) microfiltration membrane with an average pore diameter of 0.45 μm (RisingSun Membrane Tech, Beijing, Co., Ltd., China) was used as supporting membrane. The GO membrane was fabricated by filtrating a certain amounts of GO (calculated by 0.144 μg/cm2 can be approximated as a layer, as the GO to 1 nm a layer and with a density ~1.05 g/cm3) nanosheets on the membrane surface using a modified SEPA CF II membrane cell system (Sterlitech Corporation, USA). To enhance the firmness of GO layer during preparing, the supporting membrane surface was treated in polydopamine solution (2 g/L) (Han et al.,

2.4. Membrane flux and rejection performance testing The water permeability, rejection performance of the membrane was tested on the same modified SEPA CF II membrane cell system (Sterlitech Corporation, USA). Ultrapure water was used for membrane compaction and water permeability measurement. The flux before a certain time was recorded by a computer by getting the balance (CP4102, OHAUS corporation) data. All of these tests were conducted at a trans-membrane pressure (TMP) of 0.50 MPa and a cross-flow velocity of 0.11 m/s (for flux) or 0.44 m/s (for rejection efficiency). The temperature of the feed solution was 25 ± 1 °C. Rejection performance was characterized by solutions of Poly(ethylene glycol) 3350 (PEG3350, Aldrich), MgSO4 (25 mM), NaCl (25 mM). Alizarin red S (10 mg/L, Aladdin Industrial Corporation, China.), chromotropic acid disodium (10 mg/L, Aladdin Industrial Corporation, China.). The concentrations of PEG were tested following Hach TOC (COD) method (goodness of fit N 0.999). Salt concentrations were determined by the conductivity meter (TetraCon®325,WTW)·The concentrations of the Alizarin red S and chromotropic acid disodium salt were measured using a spectrophotometric method with a T6 spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.) at wavelengths of 261and 233 nm (An et al., 2011), respectively. 2.5. Field emission scanning electron microscope (FESEM) test The structure of GO membrane surfaces was observed using a field emission scanning electron microscope (FESEM, Hitachi SU8020, Japan). The samples were dried in an oven at 45 °C and sprayed with Au for 10 s under fine vacuum conditions, prior to observation. 2.6. Zeta potential tests The zeta potential of the GO membrane surfaces was determined using a SurPASS electrokinetic analyzer with an adjustable gap cell (with cell height of 100 ± 10 μm, Anton Paar GmbH, Austria) (Buksek et al., 2010; Han et al., 2016). Measurements were carried out in a 1 mM KCl solution at 25 ± 1 °C. The pH was set in the range of 4–10 by adding HCl or KOH. Prior to the measurement, the samples were equilibrated with the testing solution for at least 45 min (Ben-Sasson

Pressure drivened GO self-assembly

Cross-flow water, GO dispersion introduced GO functional layer Supporting membrane Filtrate circulated

Scheme 1. The self-assembly process schematic of GO membrane.

All AFM (NanoSCcope IIIa) data was obtained with contact mode to get more meticulous data. A soft probe (DNP-S10, D-fc, average spring const. 0.06 N/m, Bruker Nano Inc.) was select for experiment to avoid scratching the membrane surface. 3. Results and discussion 3.1. Membrane preparation and GO layer numbers regulation In the experiment, the GO membrane was prepared by a modified filtration method (Scheme 1), where, cross-flow water was introduced to enhance the uniformity of self-assembled GO layers. Compared to the traditional phase inversion methods for organic membrane preparation, the filtration method for GO membrane could coat GO active layer to different configurations of membrane modules easily, while avoiding the use of organic solvents and improving the environmental friendliness of the preparation process. The amount of GO required to use for a certain layer number was calculated by a surface density as about 0.144 μg·cm−2 for single-layer GO (as assuming a GO density of ~1.05 g·L−1 (Hu and Mi, 2013) and single layer GO thickness of ~1 nm (Han et al., 2016) as determined by atomic force microscopy (AFM)). In order to compare the effects of different preparation conditions to the property of GO membrane, MgSO4 rejection efficiency was selected as an indicator, as it can be tested conveniently. MgSO4 rejection efficiency has a good correlation with organic matter (Bowen et al., 1997). Pre-experiments found that the GO membrane with MgSO4 rejection efficiency of 20.5%, proved to have good rejection efficiency of negative charged organic matter (94.2% to Alizarin Red S). Too higher or too lower MgSO4 rejection efficiency was harmful for the separation of organic matter and salts. So the ideal MgSO4 rejection efficiency of GO membrane was selected between 20%–30%. GO membrane was first prepared without thermal drying posttreatment process. It was found that when the number of GO layers was around ~100, the flux and salt rejection was at relatively balanced state (Fig. 1a). But when the number of GO layers increased to ~250, the flux was decreased from 7.8 to 3.1 L·m−2·h·bar (Fig. 1a), and the MgSO4 rejection efficiency was only increased slightly (from 27.2% to 32.8%). This might indicate that the 2D interlayer nano-channels were too large for hydrated ions and water molecules separation when GO membrane was prepared without thermal drying post-treatments. Thus, enhancing the number of GO layers did not decisively improve the rejection. But, as the flow around distance of water molecules increases rapidly, the water permeability dropped sharply. The ~100 layer GO membrane without thermal drying posttreatment was balanced and adequate in the flux and rejection efficiency (Fig. 1a). However, from the viewpoint of robustness and price, it might not be a good choice. The bonding force between GO layers was weak and there was an urgent threat of GO exfoliation from supporting layer during the long-term use of the membrane without thermal drying post-treatment. In fact, we detected the loss of the GO layer for the membrane prepared without thermal drying posttreatments when tested by NaOH/SDS cleaning (West et al., 2014). Moreover, excessive use of GO nanosheets would require additional material resulting in higher membrane price and affecting its commercial application. The thermal drying post-treatment can dehydrate the GO membrane, consequently decreasing the layer spacing. The interaction between the adjacent layers could be enhanced quickly, so the stability of GO membrane also increased (An et al., 2011). At the same time, as the separation efficiency of 2D interlayer nano-channels increased obviously after thermal drying post-treatment (Nair et al., 2012), the

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et al., 2014). The original Helmholtz-Smoluchowski approach was applied to calculate the zeta potential (Buksek et al., 2010).

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Membranes with different GO layers after dry Fig. 1. The influence of GO layer numbers to the water permeability and rejection efficiency of GO membranes. The membrane was made by assemble GO nanosheets on 0.08 μm PVDF supporting membrane. (a) The GO membrane prepared without thermal dry. (b) The GO membrane prepared after thermal dry.

number of GO layers needed to achieve a certain rejection efficiency can be reduced significantly. It was found that after thermal drying post-treatment, only ~10 GO layers were required to achieve a comparable rejection efficiency to the ~100 layer GO membrane without thermal drying post-treatment (Fig. 1b). However, when the number of GO layers increased to ~50, its salt rejection efficiency increased sharply and became no longer suitable for the separation of organic matter and inorganic salts. An interesting point is that when the number of GO layers increased again to ~100 and its MgSO4 rejection efficiency maintained at about 60% like ~50 layers GO membrane (Fig. 1b). Which also showed that the higher rejection efficiency could not be obtained by simply increasing the number of GO layers. That might be due to the swelling of GO interlayer distance that cannot separate hydrated ions and water molecules effectively. By balance consideration the rejection, robustness and materials cost of membrane, we chose a ~10 layer GO membrane with thermal drying post-treatment as the organics/inorganic salts separation membrane for advanced purification of wastewater. 3.2. Supporting membrane influence on the property GO membrane The GO membrane prepared in this study can be regarded as a new kinds of thin-film composite (TFC) membrane with a supporting layer and GO active separation layer that was comparable to traditional thin polyamide (PA) layer. It has been reported that the pore size and material of the supporting membrane have an important influence on the properties of PA membrane (Ehsan Yakavalangi et al., 2017; Tiraferri


et al., 2011). Similar to the supporting membrane function in PA membrane, the pore size and characteristics of the support layer of GO membrane could also have some effect on the uniformity of the GO layer, thereby affecting the final rejection efficiency and the flux. So the influence of supporting layer was investigated. As the membrane was aimed at the advanced purification of secondary effluent, the supporting membrane should have a uniform pore diameter, good pressure resistance and oxidation resistance for fouling control. Furthermore, the flux of supporting membrane after thermal drying post-treatment should not significantly decrease, as the thermal drying post-treatment seemed inevitable to obtain a higher robustness GO layer. It was found that after thermal drying the supporting membranes of 0.03 μm pore diameter polyether sulfone (PES) membrane and the 0.08 μm polyvinylidene fluoride (PVDF) membrane had a sharp decline in water permeability (Fig. 2a). However, the 0.1 μm and 1.0 μm PVDF membranes exhibited better thermal drying resistance, that was deemed suitable for the preparation of organics/inorganic salts separation membrane (Fig. 2a). The following experimental results showed that the GO membranes prepared with 0.1 μm PES or 0.08 μm PVDF membrane as supporting layer, had a good rejection of MgSO4 but their flux was too low to use (Fig. 2b). However, the membranes prepared with 0.1 μm or 1.0 μm PVDF membrane as the supporting layer had a relatively balanced flux and rejection efficiency (Fig. 2b). Unlike increase in flux with the increase in pore size of supporting layer from 0.08 to 0.1 μm, there was no significant improvement observed in the flux when pore size increased from 0.1 μm to 1.0 μm (Fig. 2b). It indicated that if flux of the

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support membrane was higher than 50 L·m−2·h−2·bar after thermal drying treatment, the supporting layer was no longer the main limiting factor for the final flux of GO membrane. When the rejection efficiency of GO membrane prepared on PES and PVDF supporting layers was compared (Fig. 2b), it appeared that the support layer actually affected the rejection of GO membrane, even it has been known that the rejection efficiency of the membrane depends on the active layer rather than the support layer. One of the possible reasons was that GO and PES layers could form stronger hydrogen bonding, thereby promoting the formation of a narrower 2D separation nanochannel on the surface of PES. The other possible reason was that supporting layer posed a certain retention function after thermal drying post-treatment process, as the flux of support membrane dropped sharply, skin layer of supporting membrane became denser. It was also found that the rejection of GO membrane fabricated on 1.0 μm PVDF support membrane was lower than that of GO membrane fabricated on 0.1 μm PVDF support membrane (Fig. 2b). The possible reason was that the homogeneity of GO active layer decreased on the 1.0 μm PVDF support membrane. Some parts of GO layer become too thinner to obtain good rejection efficiency. Compared to the traditional organic active layer, GO layer have a better antioxidant capacity. That property could be important for the membrane fouling control and to deal with the complex and diverse environment which GO membranes may encounter during actual use. In order to maintain and effectively play out the antioxidant capacity of GO layer, the better antioxidant supporting membrane should be chosen to be attached together with GO layers. As the PVDF had a better oxidation resistance compared to the PES, the PVDF supporting layer becomes a better choice. Poly tetrafluoroethylene (PTFE) membrane has a better oxidation resistance and chemical stability than PVDF membrane, certainly. However, it was found that the PTFE membrane had a limited pressure bearing capacity (Sun et al., 2005), which originate from the creep of PTFE as the lack of intermolecular hydrogen bonds. The flux attenuation could be observed even though the transmembrane pressure (TMP) decreased to 1.0 bar. Therefore, the PTFE membrane was not very suitable for GO membrane preparation. By overall balance the flux and rejection efficiency of the obtained GO membrane, 0.1 μm PVDF membrane was finally chosen as the support layer for advanced purification of wastewater.

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Fig. 2. The influence of supporting layer to the water permeability and rejection efficiency of GO membranes. The membrane was made by assemble ~10 layers GO on supporting membrane. (a) The GO membrane prepared on different supporting membrane. (b) Supporting membrane water permeability after hot dry.

Self-assembly pressure (Tang et al., 2014; Tsou et al., 2015) affects the interlayer spacing and the overall structure of GO layer, thereby affecting the water permeability and rejection efficiency. So the selfassemble pressure was also examined in this study. It was found that the GO membrane prepared at a low self-assembly pressure (1 bar) had a higher flux than that of prepared at the high self-assembly pressure (10.0 bar) (Fig. 3a). One possible reason the larger and more irregular surface area of the membrane obtained at lower self-assembly pressure (Fig. 3b–d). It resulted a larger water flow area, so the water permeability increased. On the other hand, at a lower self-assembly pressure, GO layer tend to have a larger interlayer 2D nano-channels, that could permit a larger volume of water. However, rejection efficiency of the membrane prepared at lower self-assembly pressure was noticeably declined (Fig. 3a), which might due to a larger 2D nano-channel, that was harder to separate the hydrate ions and water molecules. Too lower inorganic salts' rejection efficiency tends to lead a low organic matter rejection efficiency. Between balance of water flux and rejection efficiency, it was believed that the rejection rate of membrane should be treated as the more important property for a specific target. From this perspective, a high assembly pressure (10 bar) can be selected to for a better rejection efficiency. A higher assembly pressure could also lead to a GO membrane that with a smaller layer spacing and the stronger interlayer interaction force, resulting in a more robust membrane that will last longer during

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the long-term operation. Actually, it was found that the membrane prepared at assembly pressure of 1.0 bar was very easy to be scratched in wet state, even with gentle slide of finger.

Membranes preapred at different self-assemble pressure for GO nanosheets

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Assembly pressure can also affect surface roughness (Fig. 3b–d), which has a positive correlation with membrane fouling. Membrane fouling was the main threat during filtration process (Boussu et al., 2005; Hobbs et al., 2006; Hoek et al., 2003). A lower roughness membrane surface can lead to a lower interaction force between membrane and foulants (Hoek et al., 2003), hence less foulants was likely to accumulate on membrane surface. During cross-flow filtration or cleaning process, the foulants in the “valley” of rough surface (Fig. 3b) are harder to be removed from the membrane surface. So preparing a membrane with lower roughness and avoid “valley clogging” was important for fouling control. Fig. 3b–d revealed that when GO was prepared in a higher self-assemble pressure, the membrane tended to have a lower roughness. Therefore, a higher self-assembly pressure was important for membrane fouling control. In summary, after considering the flux, rejection efficiency and the robustness of membrane during use, 10.0 bar was chosen as selfassembly pressure for GO layer. 3.4. The characterization of GO membrane after optimization After preparation parameters optimization, surface morphology, retention characteristics and zeta potential of the optimized GO membrane were tested. The results of FESEM showed that GO nanosheets could cover the pores of the PVDF supporting membrane surface well (Fig. 4), and the cross-sectional images show that the GO layer was thin and dense (Fig. 4). Rejection efficiency test revealed that GO membrane had better rejection to the negatively charged organic matters than the electrically neutral polyethylene glycol (PEG) (Fig. 5a–b). That could originate from the higher surface electronegativity of GO membrane (~70.7 mV, pH 7.2) (Fig. 5c). This strongly negative charged active layer tends to have a strong electric repulsion for negative charged organic matter (Bandini and Vezzani, 2003), so their passage through the membrane was much lower than electrically neutral water molecules. This kind of membrane could be suitable for the rejection of residual organic matters in secondary effluent as they tend to be generally negatively charged. Reasons could be that the positively charged amino group (when pH = 7.0) can be removed by microbes more easily (Nielsen et al., 1992), while the negatively charged carboxyl group, phosphoric acid group and sulfonic acid groups (when pH = 7.0) containing macromolecules commonly existed in secondary effluents (Jarusutthirak and Amy, 2007; Sheng et al., 2010). It was noticed that the surface of GO membrane was very negatively charged (~70.7 mV, pH 7.2) (Fig. 5c). The origin of this zeta potential is an interesting and important topic. At first, people tend to think it is the dissociation of carboxyl groups (Li et al., 2008), lead to this so negative zeta potential. However, this explanation seemed not sufficient, as the content of carboxyl groups on GO could be b3% (Han et al., 2016) and the amount of carboxyl groups dissociated under neutral conditions was limited (pKa = ~3.5, predicted by ACD/Labs). In addition, it was reported that zeta potential curve of GO did not change like the trend of dissociation curve of carboxyl groups with the change of pH (Han et al., 2016; Hu and Mi, 2013). That made people to search other possible reasons. George Bepete et al. found that OH– can be adsorbed on graphene flakes (Bepete et al., 2017). As the presentence of nonoxidation regions on GO, so they can also adsorb OH– and lead to negatively charged surface. The other possible reason was that the absorbed ·OH on the nanosheets during the preparation of GO (Yang et al., 2014) can easily take off (Zhang et al., 2012) with the form of OH−. But, still be adsorbed and tethered to the GO surface, as the big-π bond of GO nonFig. 3. The influence applied pressure to the characteristics of GO membranes. The membrane was made by assemble ~10 layers GO on 0.1 μm PVDF supporting membrane. (a) The influence of applied self-assemble pressure to the water permeability and rejection efficiency of GO membranes. (b)–(d) The influence of applied self-assemble pressure to the surface roughness of GO membranes.


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500 nm

Fig. 4. FESEM image of the optimized GO membrane. The membrane was prepared by assemble ~10 layers GO on 0.1 μm PVDF supporting membrane.

oxidation regions that lose the electron have an attraction to negatively charged •OH. Experiments also confirmed that the prepared GO membrane had a low rejection efficiency for NaCl and MgSO4 (Fig. 5d), which was important for the separation of organic and inorganic salts, and avoiding the problem of organics/inorganic salts mixed concentrated wastewater problem. 3.5. GO membrane application to actual secondary effluents of refractory organic wastewater A secondary effluent of chemical synthesis pharmaceuticals wastewater in province of Shandong, China was selected as the test wastewater. Main products of the factory were cephalosporin, anti-tumor drugs, cardiovascular and cerebrovascular drugs. Their wastewater treatment station mainly uses biological methods. The key treatment section was hydrolysis acidification pool, UASB and A/O pool. The chemical oxygen demand (COD) of secondary effluent was about ~180mgO2/L. The effluents were discharged to the sewer and from where they were transported to local municipal sewage treatment plant previously. However, because these secondary effluent has the refractory nature, the sewage treatment plant often attributed the situation that their COD in final effluent difficult to reach the emission standards to the receiving of secondary effluent from pharmaceutical factory. The factory wastewater treatment station was facing an increasing pressure to reduce COD level of the effluents. A series of pilot experiments were carried out to solve these problems. Fenton oxidation process showed a serious hydrogen peroxide residues problem, so the effluent of COD tended to exceed 50mgO2/L. And the COD value after ozone oxidation

was not significant changed. In addition, the traditional RO membrane filtration process faced with serious membrane fouling problem, high operating costs and high evaporation costs problem to the organics/inorganic salts mixed concentrated waste (Perez-Gonzalez et al., 2012). And the inorganic salts of organics/inorganic salts mixed concentrated waste was hard to be resourcelized as the organics matter prevent the crystallization of inorganic salts (Song et al., 2014). Under these situations, GO membrane was used for separation of organics/inorganic salts of secondary effluent as an advanced purification step. The experiment results revealed that COD of the effluents can be reduced from 176 to 42mgO2/L (76%, Fig. 6a) (five times concentration ratio) while the conductivity test results revealed that GO membrane rejection efficiency of inorganic salts was low (13.2%) (Fig. 6a). At the same time, it was found that the appearance of filtrated water changed from dark yellow to basically colorless state (Fig. 6b). Organics/inorganic salts mixed concentrated waste problem can be avoided. And the organic concentrated water tends to be degraded and mineralized easily. There were still some technical problems for GO membrane application to water desalination process (Abraham et al., 2017; Koh and Lively, 2015; You et al., 2015; Zheng et al., 2017). But its application to the treatment of refractory organic wastewater exhibited a good prospect (Goh and Ismail, 2017). The oxidation resistance (Dreyer et al., 2010), surface electronegativity, good retention efficiency of organic matter properties of GO membrane can be its unique advantages in the application process. In addition, the material cost can no longer be a problem as amount of GO used can be reduced to 14.4 mg/m2 (~10 layer of GO). GO membrane could have a comparable price even compared with traditional organic membrane.

268


Rejection efficiency(%)

100

(a)

80 60 40 20 0

PEG 3350 Alizarin Red S Chromotropic acid disodium

Differents orginic matters +

H

O n

H

O

Na

Na

-

O O

+

+

Na O O S O

(b)

O

O

-

O

S

-

S

O

O O

OH OH OH OH

PEG3350

Alizarin red S

Chromotropic acid disodium

Zeta Potential(mV)

(c) -40 -50 -60 -70 4.0

6.0

8.0

10.0

Apparent rejection efficiency(%)

pH

(d) 30

20

10

0 NaCl

MgSO4

Different salts Fig. 5. The rejection efficiency and zeta potential of optimized GO membrane. (a) Rejection to typical organic matters. (a) The structural formula of typical organic matters. (c) Rejection to salts, (d) The zeta potential of GO membrane (n = 4). The membrane was prepared by assemble ~10 layers GO on 0.1 μm PVDF supporting membrane.

4. Conclusions According to the requirement of organic matter/inorganic salts separation during advanced purification of refractory organic wastewater, GO layer number, support membrane (materials and pore size) and GO self-assembly pressure were studied. The results revealed that ~10 layers GO (14.4 mg/m2) membrane with thermal drying post-

treatment process can get a comparable rejection efficiency as of ~100 layer GO membrane. Moreover, it was found that 0.1 μm PVDF membrane was a suitable supporting layer for the preparation of the organics/inorganic salts separation GO membrane. A low self-assembly pressure (1.0 bar) for GO nanosheets may result in a higher membrane flux, but it would also lead to a rough surface, low rejection efficiency and easy shedding GO layer. Increasing self-assembly pressure to


(a) COD Conductivity

6

120 4

80 40

2

0

0

Secondary effluent

Conductivity (mScm)

COD (mgO2·L-1)

Acknowledgments

8

160

Filtered water

Water samples

(b)

Fig. 6. The GO membrane rejection efficiency to actual secondary effluent of a chemical synthesis pharmaceuticals wastewater. (a) Rejection to organic matters and salts. (b) Apparent color change before and after GO membrane advanced purification. The membrane was prepared by assemble ~10 layers GO on 0.1 μm PVDF supporting membrane.

10.0 bar contributed to solve these problems. When the GO membrane was applied to the actual secondary effluents of refractory pharmaceutical wastewater, it was found that COD can remarkably reduce (76%) while the rejection efficiency to salts was relatively low (13.2%), the organics/inorganic salts mixed concentrated waste can be avoided. This research provides a new way for advanced treatment of refractory wastewater, and shows a new perspective to utilize advantages of GO membrane to solve the problems during water purification process.

Abbreviations GO NF RO PA COD PEG PVDF PES PTFE TMP TOC AFM UASB A/O

graphene oxide nanofiltration reverse osmosis polyamide chemical oxygen demand poly(ethylene glycol) poly(vinylidene fluoride) polyethersulfone poly(tetra fluoroethylene) trans-membrane pressure total organic carbon atomic force microscopy up-flow anaerobic sludge bed anoxic/oxic process

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This work was supported by the Major Science and Technology Program for Water Pollution Control and Treatment, China (2014ZX07204005), funding sources also included the Natural Science Foundation for the Youth (21507150). The authors would like to thank Prof. Tao Zhang, Prof. Wei-Guang Li, Dr. Yong Wang and Dr. Li-Hui Yang for their kind help and useful discussion. Competing interests The authors declare no competing financial interests. References Abraham, J., Vasu, K.S., Williams, C.D., Gopinadhan, K., Su, Y., Cherian, C.T., et al., 2017. Tunable sieving of ions using graphene oxide membranes. Nat. Nanotechnol. 12, 546–550. Altmann, J., Ruhl, A.S., Zietzschmann, F., Jekel, M., 2014. Direct comparison of ozonation and adsorption onto powdered activated carbon for micropollutant removal in advanced wastewater treatment. Water Res. 55, 185–193. An, Z., Compton, O.C., Putz, K.W., Brinson, L.C., Nguyen, S.T., 2011. Bio-inspired borate cross-linking in ultra-stiff graphene oxide thin films. Adv. Mater. 23, 3842–3846. Ashrafi, O., Yerushalmi, L., Haghighat, F., 2015. Wastewater treatment in the pulp-andpaper industry: a review of treatment processes and the associated greenhouse gas emission. J. Environ. Manag. 158, 146–157. Azami, H., Sarrafzadeh, M.H., Mehrnia, M.R., 2012. Soluble microbial products (SMPs) release in activated sludge systems: a review. Iran. J. Environ. Health Sci. Eng. 9. Bandini, S., Vezzani, D., 2003. Nanofiltration modeling: the role of dielectric exclusion in membrane characterization. Chem. Eng. Sci. 58, 3303–3326. Ben-Sasson, M., Lu, X., Bar-Zeev, E., Zodrow, K.R., Nejati, S., Qi, G., et al., 2014. In situ formation of silver nanoparticles on thin-film composite reverse osmosis membranes for biofouling mitigation. Water Res. 62, 260–270. Bepete, G., Anglaret, E., Ortolani, L., Morandi, V., Huang, K., Penicaud, A., et al., 2017. Surfactant-free single-layer graphene in water. Nat. Chem. 9, 347–352. Boussu, K., Van der Bruggen, B., Volodin, A., Snauwaert, J., Van Haesendonck, C., Vandecasteele, C., 2005. Roughness and hydrophobicity studies of nanofiltration membranes using different modes of AFM. J. Colloid Interface Sci. 286, 632–638. Bowen, W.R., Mohammad, A.W., Hilal, N., 1997. Characterisation of nanofiltration membranes for predictive purposes - use of salts, uncharged solutes and atomic force microscopy. J. Membr. Sci. 126, 91–105. Buksek, H., Luxbacher, T., Zeta Potential, Petrinic I., 2010. Determination of polymeric materials using two differently designed measuring cells of an electrokinetic analyzer. Acta Chim. Slov. 57, 700–706. Chen, L., Shi, G., Shen, J., Peng, B., Zhang, B., Wang, Y., et al., 2017. Ion sieving in graphene oxide membranes via cationic control of interlayer spacing. Nature. 550, 380–383. Cheng, C., Jiang, G., Garvey, C.J., Wang, Y., Simon, G.P., Liu, J.Z., et al., 2016. Ion transport in complex layered graphene-based membranes with tuneable interlayer spacing. Sci. Adv. 2. Choi, W., Choi, J., Bang, J., Lee, J.H., 2013. Layer-by-layer assembly of graphene oxide nanosheets on polyamide membranes for durable reverse-osmosis applications. ACS Appl. Mater. Interfaces 5, 12510–12519. Deng, Y., Englehardt, J.D., 2006. Treatment of landfill leachate by the Fenton process. Water Res. 40, 3683–3694. Dialynas, E., Mantzavinos, D., Diamadopoulos, E., 2008. Advanced treatment of the reverse osmosis concentrate produced during reclamation of municipal wastewater. Water Res. 42, 4603–4608. Diya'uddeen, B.H., Daud, W.M.A.W., Abdul Aziz, A.R., 2011. Treatment technologies for petroleum refinery effluents: a review. Process Saf. Environ. Prot. 89, 95–105. Doederer, K., Gernjak, W., Weinberg, H.S., Farre, M.J., 2014. Factors affecting the formation of disinfection by-products during chlorination and chloramination of secondary effluent for the production of high quality recycled water. Water Res. 48, 218–228. Dreyer, D.R., Park, S., Bielawski, C.W., Ruoff, R.S., 2010. The chemistry of graphene oxide. Chem. Soc. Rev. 39, 228–240. Ehsan Yakavalangi, M., Rimaz, S., Vatanpour, V., 2017. Effect of surface properties of polysulfone support on the performance of thin film composite polyamide reverse osmosis membranes. J. Appl. Polym. Sci. 134. Gadipelly, C., Pérez-González, A., Yadav, G.D., Ortiz, I., Ibáñez, R., Rathod, V.K., et al., 2014. Pharmaceutical industry wastewater: review of the technologies for water treatment and reuse. Ind. Eng. Chem. Res. 53, 11571–11592. Goh, P.S., Ismail, A.F., 2017. A review on inorganic membranes for desalination and wastewater treatment. Desalination https://doi.org/10.1016/j.desal.2017.07.023 (in press). Han, J.L., Xia, X., Tao, Y., Yun, H., Hou, Y.N., Zhao, C.W., et al., 2016. Shielding membrane surface carboxyl groups by covalent-binding graphene oxide to improve anti-fouling property and the simultaneous promotion of flux. Water Res. 102, 619–628. Hobbs, C., Taylor, J., Hong, S., 2006. Effect of surface roughness on fouling of RO and NF membranes during filtration of a high organic surficial groundwater. J. Water Supply Res. Technol. AQUA 55, 559. Hoek, E.M.V., Bhattacharjee, S., Elimelech, M., 2003. Effect of membrane surface roughness on colloid-membrane DLVO interactions. Langmuir 19, 4836–4847.

270


Holkar, C.R., Jadhav, A.J., Pinjari, D.V., Mahamuni, N.M., Pandit, A.B., 2016. A critical review on textile wastewater treatments: possible approaches. J. Environ. Manag. 182, 351–366. Hu, M., Mi, B., 2013. Enabling graphene oxide nanosheets as water separation membranes. Environ. Sci. Technol. 47, 3715–3723. Hummers, W.S., Offeman, R.E., 1958. Preparation of graphitic oxide. J. Am. Chem. Soc. 80, 1339. Jarusutthirak, C., Amy, G., 2007. Understanding soluble microbial products (SMP) as a component of effluent organic matter (EfOM). Water Res. 41, 2787–2793. Ji, Q., Tabassum, S., Hena, S., Silva, C.G., Yu, G., Zhang, Z., 2016. A review on the coal gasification wastewater treatment technologies: past, present and future outlook. J. Clean. Prod. 126, 38–55. Jiang, J.-Q., Zhou, Z., Sharma, V.K., 2013. Occurrence, transportation, monitoring and treatment of emerging micro-pollutants in waste water—a review from global views. Microchem. J. 110, 292–300. Joshi, R.K., Carbone, P., Wang, F.C., Kravets, V.G., Su, Y., Grigorieva, I.V., et al., 2014. Precise and ultrafast molecular sieving through graphene oxide membranes. Science 343, 752–754. Joshi, R.K., Alwarappan, S., Yoshimura, M., Sahajwalla, V., Nishina, Y., 2015. Graphene oxide: the new membrane material. Appl. Mater. Today 1, 1–12. Kidambi, P.R., Jang, D., Idrobo, J.C., Boutilier, M.S.H., Wang, L., Kong, J., et al., 2017. Nanoporous atomically thin graphene membranes for desalting and dialysis applications. Adv. Mater. 29, 33. Kim, H.W., Yoon, H.W., Yoon, S.-M., Yoo, B.M., Ahn, B.K., Cho, Y.H., et al., 2013. Selective gas transport through few-layered graphene and graphene oxide membranes. Science 342, 91–95. Koh, D.Y., Lively, R.P., 2015. Nanoporous graphene: membranes at the limit. Nat. Nanotechnol. 10, 385–386. Laspidou, C.S., Rittmann, B.E., 2002. A unified theory for extracellular polymeric substances, soluble microbial products, and active and inert biomass. Water Res. 36, 2711–2720. Lee, J., Chae, H.-R., Won, Y.J., Lee, K., Lee, C.-H., Lee, H.H., et al., 2013. Graphene oxide nanoplatelets composite membrane with hydrophilic and antifouling properties for wastewater treatment. J. Membr. Sci. 448, 223–230. Li, D., Muller, M.B., Gilje, S., Kaner, R.B., Wallace, G.G., 2008. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 3, 101–105. Li, N.N., Fane, A.G., Ho, W.W., Matsuura, T., 2011. Advanced Membrane Technology and Applications. John Wiley & Sons. Li, H., Song, Z., Zhang, X., Huang, Y., Li, S., Mao, Y., et al., 2013. Ultrathin, molecular-sieving graphene oxide membranes for selective hydrogen separation. Science 342, 95–98. Li, J., Wang, X., Wang, H., Wang, S., Hayat, T., Alsaedi, A., et al., 2017. Functionalization of biomass carbonaceous aerogels and their application as electrode materials for electro-enhanced recovery of metal ions. Environ. Sci.: Nano 4, 1114–1123. Lofrano, G., Meric, S., Zengin, G.E., Orhon, D., 2013. Chemical and biological treatment technologies for leather tannery chemicals and wastewaters: a review. Sci. Total Environ. 461-462, 265–281. Marcano, D.C., Kosynkin, D.V., Berlin, J.M., Sinitskii, A., Sun, Z.Z., Slesarev, A., et al., 2010. Improved synthesis of graphene oxide. ACS Nano 4, 4806–4814. McGuinness, N.B., Garvey, M., Whelan, A., John, H., Zhao, C., Zhang, G., et al., 2015. Nanotechnology Solutions for Global Water Challenges. 1206 pp. 375–411. Mi, B.X., 2014. Graphene oxide membranes for ionic and molecular sieving. Science 343, 740–742. Mickley, M., 2001. Membrane Concentrate Disposal: Practices and Regulation: US Department of the Interior. Bureau of Reclamation, Technical Service Center, Water Treatment Engineering and Research Group Boulder, Colo. Morelos-Gomez, A., Cruz-Silva, R., Muramatsu, H., Ortiz-Medina, J., Araki, T., Fukuyo, T., et al., 2017. Effective NaCl and dye rejection of hybrid graphene oxide/graphene layered membranes. Nat. Nanotechnol. 12, 1083–1088. Nair, R.R., Wu, H.A., Jayaram, P.N., Grigorieva, I.V., Geim, A.K., 2012. Unimpeded permeation of water through helium-leak-tight graphene-based membranes. Science 335, 442–444. Neyens, E., Baeyens, J., 2003. A review of classic Fenton's peroxidation as an advanced oxidation technique. J. Hazard. Mater. 98, 33–50. Nielsen, P.H., Raunkjaer, K., Norsker, N.H., Jensen, N.A., Hvitved-Jacobsen, T., 1992. Transformation of wastewater in sewer systems—a review. Water Sci. Technol. 25, 17–31. Oller, I., Malato, S., Sanchez-Perez, J.A., 2011. Combination of advanced oxidation processes and biological treatments for wastewater decontamination—a review. Sci. Total Environ. 409, 4141–4166. Oturan, M.A., Aaron, J.-J., 2014. Advanced oxidation processes in water/wastewater treatment: principles and applications. A review. Crit. Rev. Environ. Sci. Technol. 44, 2577–2641. Pal, P., Kumar, R., 2013. Treatment of coke wastewater: a critical review for developing sustainable management strategies. Sep. Purif. Rev. 43, 89–123. Perez-Gonzalez, A., Urtiaga, A.M., Ibanez, R., Ortiz, I., 2012. State of the art and review on the treatment technologies of water reverse osmosis concentrates. Water Res. 46, 267–283. Pignatello, J.J., Oliveros, E., Advanced Oxidation, MacKay A., 2006. Processes for organic contaminant destruction based on the Fenton reaction and related chemistry. Crit. Rev. Environ. Sci. Technol. 36, 1–84. Radha, B., Esfandiar, A., Wang, F.C., Rooney, A.P., Gopinadhan, K., Keerthi, A., et al., 2016. Molecular transport through capillaries made with atomic-scale precision. Nature 538, 222–225. Ranade, V.V., Bhandari, V.M., 2014. Industrial Wastewater Treatment, Recycling and Reuse: Butterworth-Heinemann.

Rautenbach, R., Mellis, R., 1995. Hybrid processes involving membranes for the treatment of highly organic/inorganic contaminated waste water. Desalination 101, 105–113. Sheng, G.P., Yu, H.Q., Li, X.Y., 2010. Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: a review. Biotechnol. Adv. 28, 882–894. Shon, H.K., Vigneswaran, S., Snyder, S.A., 2006. Effluent organic matter (EfOM) in wastewater: constituents, effects, and treatment. Crit. Rev. Environ. Sci. Technol. 36, 327–374. Song, Y., Dai, Y., Hu, Q., Yu, X., Qian, F., 2014. Effects of three kinds of organic acids on phosphorus recovery by magnesium ammonium phosphate (MAP) crystallization from synthetic swine wastewater. Chemosphere 101, 41–48. Sun, H., Cooke, R.S., Bates, W.D., Wynne, K.J., 2005. Supercritical CO2 processing and annealing of polytetrafluoroethylene (PTFE) and modified PTFE for enhancement of crystallinity and creep resistance. Polymer 46, 8872–8882. Sun, P., Wang, K., Zhu, H., 2016. Recent developments in graphene-based membranes: structure, mass-transport mechanism and potential applications. Adv. Mater. 12, 2287–2310. Tang, Y.P., Paul, D.R., Chung, T.S., 2014. Free-standing graphene oxide thin films assembled by a pressurized ultrafiltration method for dehydration of ethanol. J. Membr. Sci. 458, 199–208. Ternes, T.A., Stuber, J., Herrmann, N., McDowell, D., Ried, A., Kampmann, M., et al., 2003. Ozonation: a tool for removal of pharmaceuticals, contrast media and musk fragrances from wastewater? Water Res. 37, 1976–1982. Tiraferri, A., Yip, N.Y., Phillip, W.A., Schiffman, J.D., Elimelech, M., 2011. Relating performance of thin-film composite forward osmosis membranes to support layer formation and structure. J. Membr. Sci. 367, 340–352. Tsou, C.-H., An, Q.-F., Lo, S.-C., De Guzman, M., Hung, W.-S., Hu, C.-C., et al., 2015. Effect of microstructure of graphene oxide fabricated through different self-assembly techniques on 1-butanol dehydration. J. Membr. Sci. 477, 93–100. Van der Bruggen, B., Lejon, L., Vandecasteele, C., 2003. Reuse, treatment, and discharge of the concentrate of pressure-driven membrane processes. Environ. Sci. Technol. 37, 3733–3738. Vörösmarty, C.J., Green, P., Salisbury, J., Lammers, R.B., 2000. Global water resources: vulnerability from climate change and population growth. Science 289, 284–288. Vörösmarty, C.J., McIntyre, P.B., Gessner, M.O., Dudgeon, D., Prusevich, A., Green, P., et al., 2010. Global threats to human water security and river biodiversity. Nature 467, 555–561. Wang, Y.J., Yu, J.W., Han, P., Sha, J., Li, T., An, W., et al., 2013. Advanced oxidation of bromide-containing drinking water: a balance between bromate and trihalomethane formation control. J. Environ. Sci. (China) 25, 2169–2176. Wang, L., Boutilier, M.S.H., Kidambi, P.R., Jang, D., Hadjiconstantinou, N.G., Karnik, R., 2017. Fundamental transport mechanisms, fabrication and potential applications of nanoporous atomically thin membranes. Nat. Nanotechnol. 12, 509–522. Werber, J.R., Osuji, C.O., Elimelech, M., 2016. Materials for next-generation desalination and water purification membranes. Nat. Rev. Mater. 1, 16018. West, S., Horn, H., Hijnen, W.A.M., Castillo, C., Wagner, M., 2014. Confocal laser scanning microscopy as a tool to validate the efficiency of membrane cleaning procedures to remove biofilms. Sep. Purif. Technol. 122, 402–411. Xie, W.M., Ni, B.J., Seviour, T., Sheng, G.P., Yu, H.Q., 2012. Characterization of autotrophic and heterotrophic soluble microbial product (SMP) fractions from activated sludge. Water Res. 46, 6210–6217. Xie, W.M., Ni, B.J., Sheng, G.P., Seviour, T., Yu, H.Q., 2016. Quantification and kinetic characterization of soluble microbial products from municipal wastewater treatment plants. Water Res. 88, 703–710. Yang, W., Li, X., Pan, B., Lv, L., Zhang, W., 2013. Effective removal of effluent organic matter (EfOM) from bio-treated coking wastewater by a recyclable aminated hyper-crosslinked polymer. Water Res. 47, 4730–4738. Yang, L., Zhang, R., Liu, B., Wang, J., Wang, S., Han, M.Y., et al., 2014. Pi-conjugated carbon radicals at graphene oxide to initiate ultrastrong chemiluminescence. Angew. Chem. Int. Ed. Eng. 53, 10109–10113. Yao, W., Wang, J., Wang, P., Wang, X., Yu, S., Zou, Y., et al., 2017a. Synergistic coagulation of GO and secondary adsorption of heavy metal ions on Ca/Al layered double hydroxides. Environ. Pollut. 229, 827–836. Yao, W., Yu, S., Wang, J., Zou, Y., Lu, S., Ai, Y., et al., 2017b. Enhanced removal of methyl orange on calcined glycerol-modified nanocrystallined Mg/Al layered double hydroxides. Chem. Eng. J. 307, 476–486. Yin, L., Wang, P., Wen, T., Yu, S., Wang, X., Hayat, T., et al., 2017. Synthesis of layered titanate nanowires at low temperature and their application in efficient removal of U (VI). Environ. Pollut. 226, 125–134. You, Y., Sahajwalla, V., Yoshimura, M., Joshi, R.K., 2015. Graphene and graphene oxide for desalination. Nanoscale 8, 117–119. Zhang, W., Wang, C., Li, Z., Lu, Z., Li, Y., Yin, J.J., et al., 2012. Unraveling stress-induced toxicity properties of graphene oxide and the underlying mechanism. Adv. Mater. 24, 5391–5397. Zhao, D., Yu, S., 2014. A review of recent advance in fouling mitigation of NF/RO membranes in water treatment: pretreatment, membrane modification, and chemical cleaning. Desalin. Water Treat. 55, 870–891. Zhao, X., Zhang, P., Chen, Y., Su, Z., Wei, G., 2015. Recent advances in the fabrication and structure-specific applications of graphene-based inorganic hybrid membranes. Nano 7, 5080–5093. Zheng, S., Tu, Q., Urban, J.J., Li, S., Mi, B., 2017. Swelling of graphene oxide membranes in aqueous solution: characterization of interlayer spacing and insight into water transport mechanisms. ACS Nano 11, 6440–6450.