Conductive thin film nanocomposite forward osmosis membrane (TFN-FO) blended with carbon nanoparticles for membrane fouling control

Science of the Total Environment 697 (2019) 134050

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

Conductive thin film nanocomposite forward osmosis membrane (TFNFO) blended with carbon nanoparticles for membrane fouling control Xiaotong Xu, Hanmin Zhang ⁎, Mingchuan Yu, Yuezhu Wang, Tianyu Gao, Fenglin Yang Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, No.2 Linggong Road, Dalian 116024, China

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

• Synthesis of a conductive TFN-FO membrane with good electrical conductivity. • The conductive TFN-FO membrane shows superior anti-pollution performance. • Five different charged contaminants are investigated with positive and negative electric field applied in TFN-FO membranes.

a r t i c l e

i n f o

Article history: Received 28 April 2019 Received in revised form 23 June 2019 Accepted 21 August 2019 Available online 22 August 2019 Editor: Baoliang Chen Keywords: Conductive forward osmosis membrane Carbon Anti-fouling Electrostatic repulsion

a b s t r a c t Membrane fouling in forward osmosis (FO) significantly affects water flux and membrane life, which restricts the further development of FO. In this work, carbon nanoparticles were blended in polyethersulfone (PES) to prepare a conductive thin film nanocomposite (TFN) FO membrane to control the membrane fouling in FO processes. The membrane containing 4 wt% carbon exhibited an optimum performance with water flux of 14.0 and 17.2 LMH for FO (active layer for FS) and PRO (active layer for DS) modes, respectively, using DI water as feed solution and 1 M NaCl as draw solution and electrical conductivity of 170.1 mS/m. Dynamic antifouling experiments showed that, compared with no voltage applied, the water flux decline of surface charged TFN-FO membrane was significantly retarded. For CaSO4, BSA and LYS as model contaminants, the water fluxes were improved by 31%, 13% and 7% under the voltages of +1.7 V, −1.7 V and +1.7 V, respectively. Moreover, the charged membrane is more effective in relieving the initial membrane fouling, and contaminant-contaminant interactions mechanism dominates the formation of further membrane fouling processes. Therefore, for contaminants with different charge conditions, customizing membrane surface charges is a feasible and promising approach for controlling membrane fouling in situ method. © 2019 Published by Elsevier B.V.

1. Introduction

⁎ Corresponding author. E-mail address: [email protected] (H. Zhang).

https://doi.org/10.1016/j.scitotenv.2019.134050 0048-9697/© 2019 Published by Elsevier B.V.

As an emerging membrane process, forward Osmosis (FO), which can produce both clean energy and water driven by the osmotic pressure difference across a semi-permeable membrane, has gained wider

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attention in many applications such as desalination, power generation, food processing, and wastewater treatment (Akther et al., 2015; Chung et al., 2012; Lutchmiah et al., 2014). Compared to pressure driven membrane process (such as microfiltration (MF) and ultrafiltration (UF)), the membrane fouling of the FO is light due to the lack of hydraulic pressure. However, membrane fouling is an inevitable problem in all membrane processes, resulting in a reduction in osmotic pressure across the membrane (Boo et al., 2012) and loss of water flux (Akther et al., 2015). Moreover, membrane fouling problems still affect FO membrane life and increase energy consumption during water treatment (Bell et al., 2017). It has been proved that applying a direct electric field during membrane filtration can generate electrostatic repulsion between the membrane surface and the contaminants with charge, thus effectively relieving the membrane fouling (Busalmen and de Sanchez, 2001; van der Borden et al., 2005). Two different coupling configurations have been reported for coupling the applied electric field in a membrane filtration system (Li et al., 2018). The first is to place electrodes on both sides of the insulating membrane to apply an electric field (Shang et al., 2010; Park, 2006). The other is to directly use a conductive membrane modified with a conductive material as a working electrode, and a conductive material as a counter electrode (S. Wang et al., 2015; Geng and Chen, 2016; Wu et al., 2014; Duan et al., 2016). Because of its simpler design and smaller resistance between the electrodes (Li et al., 2018), the second way attracted more researchers' attention in recent years. UF membrane. At the same time, a positive potential was applied and organic pollution was exacerbated. The mechanism of electrostatic interaction between membrane surface charge and contaminants was demonstrated (Zhang and Vecitis, 2014). Wang et al. also demonstrated that as a cathode, the conductive CNT/PVDF membrane exhibited significant antifouling properties in the presence of an external electric field (S. Wang et al., 2015). Research on conductive FO membranes has also received extensive attention. Liu et al. study used conductive porous carbon paper instead of the conventional non-woven fabric to prepare a conductive thin film composite FO (TFC-FO) membrane. In the presence of an externally applied negative voltage (2.0 V), good organic and microbial stain resistance were exhibited (Liu et al., 2016). However, thicker carbon paper substrates inevitably increase internal concentration polarization. Fan et al. proposed a self-sustaining carbon nanotube hollow fiber scaffold polyamide film composite (CNT TFCFO) membrane. As a cathode material, the effect of electrochemical assist on organic fouling, biofouling and gypsum scaling was also investigated at low DC voltages (2.0 V) (Fan et al., 2018). Nonetheless, complex preparation processes limit its further development and application. Carbon nanoparticle, as a kind of carbon material, is a potential material for use in membrane preparation because of its ultra-small sizes, high chemical inertness, antifouling characteristics and good electrical conductivity (Zhao and Chung, 2018; Zhao et al., 2017). In this study, a conductive TFN-FO membrane was prepared by blending low-cost carbon powder with good permeability and electrical conductivity. CaSO4, bovine serum albumin (BSA) and lysozyme (LYS) are commonly used as model contaminants, representing inorganic contaminants, negatively charged proteins and positively charged proteins,

Table 1 Composition of casting solution. FO membrane

TFC TFN-1 TFN-2 TFN-3 TFN-4 TFN-5

Composition of casting solution PES (wt%)

PEG (wt%)

NMP (wt%)

Carbon (wt%)

13 13 13 13 13 13

13 13 13 13 13 13

74 73 72 71 70 69

0 1 2 3 4 5

respectively (Liu et al., 2018). Amphoteric surfactants and polyethylene glycol (PEG) were used as amphoteric and neutral-charged contaminants, respectively. At the same time, the membrane was used as a cathode and anode, respectively, and 0.2 V, 1.0 V and 1.7 V DC voltages were applied to comprehensively analyze the effect of electrochemical assist on different charged contaminants. 2. Experimental section 2.1. Materials and reagents Poly(ether sulfone) (PES, molecular weight 58,000 g/mol, Ultrason E6020, BASF Co., Germany), polyethylene glycol 300 (PEG, Mn = 300 g/mol, Aladdin) and 1-methyl-2-pyrrolidinone (NMP, N99.7%, Kermel) were used to fabrication support layer. A thin polyamide (PA) film as active layer was synthesized by using 1,3-phenylenediamine (MPD, N99.5%, Aladdin) and 1,3,5-benzenetricarbonyl trichloride (TMC, N98%, Energy chemical) with n-hexane (Kermel) as solvent. Carbon (99.5%, 30 nm) was supplied by Aladdin. CaSO4·2H2O (Kermel), BSA (Aladdin), LYS (Aladdin), and dodecyl ethoxysulfonyl betaine (Beijing Institute of Chemical Technology) were chosen as model contaminants. NaCl was purchased from Sinopharm. The chemicals were used as received without any further purification. 2.2. Membrane fabrication 2.2.1. Preparation of support Six different support layers were prepared using non-solvent induced phase inversion through the immersion precipitation method as shown in Table 1, named as TFC and TFN membranes for without and with carbon nanoparticles, respectively. The ratio of PES to total casting solution constant was kept at 13 wt% (Zhao et al., 2011). In detail, to make a casting solution, the PES polymer after drying in vacuum overnight (for moisture removal) was mixed with PEG-300, NMP, and carbon. The dope after overnight degassing was cast onto a glass plate with a casting knife set at a gate height of 170 μm. The as-cast membrane was quickly immersed in a room temperature water coagulation bath and kept 24 h in DI water to ensure complete precipitation. 2.2.2. Fabrication of polyamide active layer The ultrathin polyamide (PA) active layers of the TFC and TFN membranes were fabricated on the top surface of the substrates which were based on the interfacial polymerization reaction between MPD and TMC monomers. First, the substrate was immersed in a 2 wt% MPD aqueous solution for 1 min. Then, the excess MPD solution was carefully removed from the membrane surface. Next, the MPD saturated substrate was soaked in a 0.1 wt% TMC/n-hexane for 30 s. Finally, the TFN membrane fabricated as such was cured in oven at 60 °C for 5 min before being stored in DI water. 2.3. Evaluation of membranes intrinsic properties The surface morphology of TFN support layer and PA layers are characterized by field emission scanning electron microscopy (FESEM, NOVA NanoSEM, FEI Company) after freeze-dried and cryo-fractured in liquid nitrogen. Energy-dispersive X-ray spectroscopy (EDS) was also carried out to perform elemental analysis of the support layers. Before imaging, samples were dried in a vacuum desiccator at 60 °C for 12 h. The membranes were coated with Pt using a sputter coater before SEM imaging. The chemical structure of the membranes was also analyzed using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR, 6700, Thermo Fisher). Porosity was determined gravimetrically. The wet membrane was placed in a vacuum oven at 60 °C for 24 h and recorded as dry sample weight. The membrane was maintained in distilled water for 24 h and

X. Xu et al. / Science of the Total Environment 697 (2019) 134050

then measured after mopping superficial water with filter paper recorded as wet sample weight. The porosity of membrane was calculated from the two weights using Eq. (1) Zheng et al. (2006) as εð%Þ ¼

mw −md  100 Ad

ð1Þ

where ε is the porosity of membrane, mw the wet sample weight (g), md the dry sample weight (g), A the square of the membrane (cm2) and d is the thickness of the membrane (mm). Membrane thickness is measured by a micrometer. In order to minimize experimental error, each membrane was measured three times and calculated average. Water contact angle measurements were carried with a Contact Angle Goniometer (POWEREACH JC2000D1) to observe the hydrophilicity of the membrane substrates and PA active layers. For each of the samples, the contact angle was measured in three random locations and the average values were reported. In order to evaluate the mechanical strength of the prepared support layer, rectangular stripes (10 mm × 40 mm) were carefully cut from each of the cast membranes and stretched by a stretching machine (Electromechanical Universal Testing Machine, CMT 8502) at room temperature. The surface morphology of the support layer was also studied using an atomic force microscope (AFM, PARK, XE-70) technique with a scan area of 5 μm × 5 μm. The film roughness values in this study are expressed as root mean square roughness (Rq), average roughness (Ra), and average peak-to-valley depth (Rz). The electrical resistance of the membrane was measured using digital multimeter (MASTECH MS8050). The membrane surface conductivity was then calculated using Eq. (2) (Ho et al., 2018). ρ¼

l wtR

ð2Þ

where ρ is the membrane surface conductivity (S/cm), l is the distance between clips (cm), w is the membrane width (1 cm), t is the membrane thickness (cm), and R is the membrane electrical resistance (Ω). 2.4. Forward osmosis performance tests Membrane FO performance was evaluated using a cross-flow FO system with a rectangular unit (2 × 8 cm) at an operating temperature of 25 °C (Fig. S1). 1 M (mol/L) NaCl aqueous solution as a draw solution (DS), while DI water was used as the feed solution (FS). FO performance was evaluated in FO (active layer for FS) and PRO (active layer for DS) modes. The FO water flux (Jw, L m−2 h −1 or LMH) and the reverse salt flux (Js, g m−2 h −1 or gMH) were calculated by the following equations (Emadzadeh et al., 2014; Kwon et al., 2017). Jw ¼

ΔV Δt  Am

ð3Þ

Js ¼

C t V t −C 0 V 0 Δt  Am

ð4Þ

3

Table 2 EDS results of surface for TFC and TFN substrates. Substrate

TFC TFN-1 TFN-2 TFN-3 TFN-4 TFN-5

Element Weight (%) C

O

S

Total

75.36 78.47 79.61 80.58 81.29 81.95

18.65 16.08 15.34 14.58 14.30 13.88

5.99 5.45 5.05 4.84 4.41 4.17

100 100 100 100 100 100

contaminant solution, such as CaSO4·2H2O (1 g/L), BSA (200 mg/L), LYS (200 mg/L), dodecyl ethoxysulfonyl betaine (200 mg/L), and PEG300 (200 mg/L), while the draw solution is a 1 mol/L (M) NaCl solution in DI. At the same time, different voltages (±0.2 V, 1.0 V and 1.7 V) was also applied to the TFN FO membrane in the bacterial suspension under the same conditions (Liu et al., 2016). The value of ±1.7 V was determined after a limited range of tests and provided “best” membrane performance without any gas release. During application of the electric field, the membrane worked as cathode and a stainless steel wire mesh anode (the distance of 15 mm). The voltage was supplied by an outer DC power. 3. Result and discussion 3.1. Membrane characterization 3.1.1. EDS&FTIR The presence of carbon is investigated by EDS analysis which confirms the existence of carbon on the top surface of the nanocomposite membrane (Razmjou et al., 2011). As shown in Table 2, as the carbon powder content in the casting solution increases, the carbon content of the membrane increases. PES itself has carbon. It is proved that the carbon powder is successfully blended into the support layer by the increase in the carbon content. Fig. 1 shows the ATR-FTIR spectra of uncoated support and PAcoated membranes, respectively, with TFC and TFN-4. The spectrum of the composite samples is composed of bands attributed to both PA film and PES substrate (Bui et al., 2011). Peaks in all ATR-FTIR spectra, at 1474 cm−1 (C_C aromatic ring stretching), 1231 cm−1 (Aryl-OAryl C\\O stretch), 1483 cm−1 (the benzene ring and C\\C bond

where ΔV(L) is the volume of water permeated through the membrane in a given period of time Δt (h), Am is the effective membrane surface area (m2). Ct (g/L) and Vt (L) are the salt concentration (g/L) and the volume of the feed solution (L) at the end of the FO test, respectively. Their corresponding values at the start of the experiment are C0 (g/L) and V0 (L). The salt concentration is determined by measuring its conductivity. 2.5. Membrane anti-fouling experiments The fouling of the membrane was evaluated on the same laboratory scale in the FO mode. In this continuous fouling test, the feed is a

Fig. 1. ATR-FTIR spectra of TFC and TFN-4 FO membranes.

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roughness (Ma et al., 2018). However, since the surface of the PES substrates was uniformly covered by the PA active layer, the water contact angle of the upper surface of the TFN FO membranes mainly depended on the nature of the PA. As shown in Fig. 2, the surface contact angles of the TFC and TFN FO membranes fluctuated between 50° and 55° without a significant change tendency. Membrane fouling was alleviated by increasing the hydrophilicity of the membrane material, mainly because the surface of the hydrophilic membrane formed a hydration layer, which alleviated the adsorption deposition of pollutants on the membrane surface, thereby reducing membrane fouling. When the PA layer faces the feed solution (FO mode), the hydrophobicity of the support layer does not exacerbate membrane fouling.

Fig. 2. Comparison of water contact angles of PA films and PES substrates surface for the different carbon loading contents. Contact angle measurements were conducted for each support layer at three different points. Mean values are presented with their standard deviations.

stretch), 1151 cm−1 (SO2 symmetric stretch) and 1294 cm−1, 1325 cm−1 (S_O stretching and C-SO2-C asymmetric stretching) are characteristic of the PES substrate. The new peaks appearing in the composite membrane are characteristic of the polyamide coating, such as peaks at 1661 cm−1 (-C=O stretching vibration of amide I bond), which inferred successful formation of the PA active layer over the support layers. Table S1 demonstrates a summary of probable assignments of FTIR bands for the PES-PA composite membrane surface. 3.1.2. Water contact angle The hydrophilicity of the substrate is indicated by the surface water contact angle and the results are mentioned in Fig. 2. As the carbon content increases, the water contact angle has a quick increase, and then the increase gradually becomes slower (Emadzadeh et al., 2014). The pure PES substrate has a water contact angle of about 67° but the water contact angle increases to 85° by carbon weight of 5 wt%. The increase of water contact angle maybe caused by the hydrophobic additive (Park et al., 2015; Ziari et al., 2019) and the different surface

3.1.3. Mechanical strength, porosity and thickness of the prepared substrates The structure and intrinsic properties of the substrate significantly affect its permeability and separation (Guillen et al., 2011). Therefore, the thicknesses, porosity and mechanical strength of the substrate were tested seriously and then presented in Fig. 3. It shows that with the increasing amount of carbon, the overall thickness of the substrates and mechanical strength increase, however, the porosities reduce simultaneously (Guillen et al., 2011). The change in substrate properties may be caused by different interactions speed between solvent (NMP) and non-solvent (water) during phase inversion (Emadzadeh et al., 2014; Ismail and Hassan, 2007). It should be noted that the total dope concentration mixed with carbon is significantly higher than that of PES (only) (Ho et al., 2018). On the other hand, compared to the pure PES dope solution, the presence of hydrophobic carbon nanoparticles in the dope solution is not beneficial to diffusion of water from water coagulation bath to the cast polymer membrane. These all lead to the decrements in overall porosity (Emadzadeh et al., 2014). During immersion precipitation, carbon nanoparticles could plug pores at higher concentrations and consequently hinder the phase exchange process, may be leading to a lower porosity (Hu et al., 2018; Rahimpour et al., 2008). The pores of the FO membrane support layer are mainly derived from the dissolution of solvent (NMP) and pore former (PEG-300) in the phase exchange process. More dissolution leads to increased porosity and reduced thickness (Mousavi and Zadhoush, 2017). Therefore, as the carbon content increases, the percentage of NMP and PEG-300 in the casting solution decreases and results in a smaller porosity and a larger thickness. Further, both lower porosity and greater thickness

Fig. 3. Mechanical strength, porosity and thickness of the TFC and TFN substrates.

X. Xu et al. / Science of the Total Environment 697 (2019) 134050

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Fig. 4. SEM images of substrates (a–f) and active layer (g–l) morphologies of TFC and TFN FO membranes.

Fig. 5. AFM images of substrate surface morphology of (a) TFC, (b) TFN-1, (c) TFN-2, (d) TFN-3, (e) TFN-4, and (f) TFN-5 membranes.

result in greater mechanical strength. Consistent with the experimental results in Fig. 3, the maximum mechanical strength of the TFN-5 membrane was 2.5 MPa. As shown in Fig. 4, SEM images show top surface morphologies of different substrates (a–f), and PA active layers (g–l) for TFC and TFN FO membranes. No particles can be observed on the surface of pure PES membrane in Fig. 2(a) (Huang et al., 2012). It can be found out that more particles of carbon nanoparticles (small white spots on SEM images) are formed at the top surface of the substrate as carbon loading is increased, keeping consistent with the research of Emadzadeh et al. (2014). Meanwhile, these existing particles increase the roughness of the substrates, which follow a similar trend with AFM characterization results (Fig. 5). Fig. S2 provides the SEM images of TFC and TFN membranes cross section. It can be seen that the FO membranes are composed of a dense skin layer and a loose porous support layer. Fig. 5 reports the three-dimensional height of the membranes containing different amounts of carbon, while Table 3 reports the values of Ra, Rq and Rz for all treatments in order to better understand the roughness variations. In the range of the scan areas 5 μm × 5 μm, for TFC, TFN-1, TFN-2, TFN-3, TFN-4 and TFN-5 membranes, the mean surface roughness (Ra) is found to be 14 nm, 22 nm, 28 nm, 31 nm, 35 nm and 44 nm, respectively. The intrinsic properties of the substrate not only play a key role in structure, but also impact the conformation of the polyamide active layer subsequently formed via interfacial polymerization (Xu et al., 2017), to further influence the separation and permeability of the membrane (Shokrollahzadeh and Tajik, 2018). The SEM images of the PA

layer of both the TFC and TFN membranes are illustrated in Fig. 4(g–l). The TFC and TFN membranes exhibited ridge-valley surface morphology, which demonstrated the successful formation of the PA layer on the surface of two kinds of substrates (Shokrollahzadeh and Tajik, 2018). However, when the amount of the carbon nanoparticle added reaches 3 wt%, the scar-like structure gradually appears. The surface of the membrane was fully covered by a scar-like structure, as the additive content is further increased to 5 wt%. This is because, compared with the smooth surface, more MPD is retained on the surface of the rough support layer, reacting with TMC and forming a scar-like morphology (Zuo et al., 2017).

3.2. The FO performance of TFC and TFN membranes The water flux and the reverse salt flux of the TFC and TFN membranes in FO were measured in the conventional cross-flow FO setup Table 3 Surface roughness of membrane substrates via AFM analysis. Substrate

Ra

Rq

Rz

TFC TFN-1 TFN-2 TFN-3 TFN-4 TFN-5

13.77 22.34 27.91 31.20 35.28 44.03

17.83 28.21 34.28 40.23 44.06 50.11

62.22 88.97 102.25 137.44 137.61 154.76

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Fig. 6. FO performance of TFC and TFN membranes: (a) water flux; (b) reverse salt flux.

using DI water as feed solution and 1 M NaCl as draw solution. The membrane performance was evaluated in two typical membrane orientations (FO and PRO) in triplicate and the results were summarized in Fig. 6. The average water flux in the PRO mode is generally higher than the average water flux in the FO mode. This may be affected by the harmful diluted internal concentration polarization (ICP) effect, thus reducing the transmembrane effective osmotic pressure in FO mode (Tang et al., 2010). The experimental results show that the higher the concentration of carbon, the poorer the hydrophilicity of the membrane substrate and the lower the porosity, so the permeability of the membrane get worse (Rastgar et al., 2018). The TFN-4 still maintained a water flux of 14.0 and 17.2 LMH in the FO and PRO modes, respectively. The reverse salt flux obviously decreased from 14 gMH (FO mode) and 15 gMH (PRO) for TFC FO membrane to 5 gMH (FO mode) and 6 gMH (PRO) for TFN-4 FO membrane. It can be seen that although TFN-4 membrane has lower water flux than TFN-1~3 due to the addition of carbon nanoparticles, it is similar to that of the commercial TFC-FO membrane in the literature (6–17 LMH for FO mode), with 1 M NaCl as the draw solution and DI as the feed solution, as listed in Table 4. 3.3. The electrical properties of the TFN membrane The electrical properties of the TFN membrane are important to the use of external potential for fouling control. Compared with pure PES membrane of 6.9 mS/m, the membrane conductivity is enhanced with the increase of the additive carbon content from 0 to 5 wt%. The addition of more conductive carbon nanoparticles forms continuous electron paths across the membrane matrix, so membrane conductivity increases (Ho et al., 2018). So here the conductivity is significantly improved when the carbon content reaches 4 wt%. Thinking of the poor mechanical properties of TFC and TFN-1 membranes, while TFN-5

membrane with a high contact angle and low flux, they were discarded. Considering water flux and reverse salt flux of TFN-2~4 membranes being not much different, the membrane with 4% carbon powder added (TFN-4) with a better conductivity was selected for the further antifouling test in the next section (Table 5). 3.4. Effects of different contaminants on the performance of TFN-FO and electrically charged TFN-FO membrane Membrane fouling is a significant challenge in FO process. Therefore, the fouling resistance to five contaminants with different charged condition was explored on the TFN-FO membrane with or without voltage application. It is worth noting that the degree of fouling can be appraised by tendencies of water flux decline. Rapidly decreasing curve indicates severe membrane fouling. In order to define appropriate values of voltage, prior to the anti-fouling test, the feed solution was subjected to DI to study the water decomposition of the applied voltage system. The results show that 1.7 V is the highest voltage that does not produce electrolyzed water under the experimental conditions. Considering that the electrolyzed water process is a waste of energy, three voltage gradients of 0.2 V, 1.0 V and 1.7 V were carried out in the next studies. To evaluate the electrical-assisted contamination resistance on the TFN-FO membrane, uncharged PEG (Fig. 7(a)) and amphoteric surfactants (Fig. 7(b)) were firstly selected as model contaminants. Even if voltages up to ±1.7 V were applied to the membrane, the water fluxes were reduced by about 10% for PEG as a contaminant, which is substantially consistent with the case of no-voltage applied. The water flux is always in 90% of initial value under the seven conditions of TFN-FO membrane, proving that the application of an electric field has no significant effect on PFG, an uncharged contaminant. It had also been suggested (Zhang and Vecitis, 2014) that the application of a voltage to the UF system reduced the structural rate constant of the PEG. Since

Table 4 The water flux of commercial HTI TFC FO membranes. Membrane note TFN-4 FO HTI TFC HTI TFC HTI TFC HTI TFC HTI TFC HTI TFC HTI TFC HTI TFC HTI TFC

Membrane orientation

Feed solution

Draw solution

Water flux (LMH)

reverse salt flux (gMH)

FO FO FO FO FO FO FO FO FO FO

DI DI DI DI DI DI DI DI DI DI

1 M NaCl 1 M NaCl 1 M NaCl 1 M NaCl 1 M NaCl 0.61 M NaCl 1 M NaCl 1 M NaCl 1 M NaCl 2 M NaCl

14.0 6.4 17 15 15 12.7 14.3 6.73 15 18

5 2.6 8.8 4 8 8.5 5.4 19.2 11 2.6

References This work (Stillman et al., 2014) (Boo et al., 2015) (Ren and McCutcheon, 2014) (Kwon et al., 2019) (Islam et al., 2019) (Chun et al., 2018) (Chen and Lee, 2018) (Z. Wang et al., 2015) (Guo et al., 2014)

X. Xu et al. / Science of the Total Environment 697 (2019) 134050 Table 5 The electrical properties of the TFN membrane. TFN FO membranes

TFC

TFN-1

TFN-2

TFN-3

TFN-4

TFN-5

Conductivity (mS/m)

6.9

43.7

57.2

74.3

170.1

482.0

the fouling constants had low absolute values, there was no significant visual improvement in permeability. But the mechanism is still unclear. Then for amphoteric surfactant, a surfactant has both ionic and cationic properties. To my best knowledge, the amphoteric surfactant, twocharged contamination, was first considered in the study of applying an electric field to control membrane fouling. Under the open circuit condition, the water flux was 78% after 8 h. When an electric field of the same magnitude was applied, the decline of water flux has a similar downward trend regardless of whether the voltage is positive or negative (about 80% of the initial water flux, for 0.2 V voltage applied), and both were superior to the no-power source. As the applied voltage increased to ±1.0 V and ±1.7 V, the water flux stabilized at 84% and 89% of the initial water flux after 8 h of operation. And the higher the applied voltage value, the more significant the effect. Because the contaminants have both positive and negative charges at the same time, the electrostatic force is not obvious. However, due to the existence of the Donnan balance (Lv et al., 2018), the amphoteric surfactant is not easy to adsorb on the surface of the membrane. The increase in applied voltage leads to more membrane surface charge, resulting in a stronger Donnan effect, and the mitigation of membrane fouling is more pronounced. Therefore, the application of an electric field has a positive effect on membrane fouling caused by amphoteric pollutants. CaSO4 is one of the typical components of inorganic membrane fouling (Fan et al., 2018), which is owing to crystallization of sparingly soluble mineral salts when the salt concentration exceeds saturation (Liu and Mi, 2012). The formation of this fouling during FO can be induced by two different mechanisms: heterogeneous or surface crystallization and homogeneous crystallization. The former means that the crystal grows directly on the surface of the membrane, and the latter denotes that crystals are formed in the bulk solution and then deposited on the membrane surface. It has been widely confirmed that membrane surface properties significantly affect the formation of fouling on

7

membrane surface (Mi and Elimelech, 2013). As expected, the application of an electric field has a significant effect on CaSO4. As shown in Fig. 7(c), the membrane permeation flux of the filtered CaSO4 decreased to 74% of the initial flux after 8 h without applying an electric field, indicating that the membrane flux loss was severe due to membrane fouling. The carboxyl group of the PA active layer binds to calcium ions, resulting in the TFN-FO membrane being more vulnerable to inorganic fouling (Wang et al., 2016). When a positive potential of 0.2 V was applied, the tendency of the water flux to decrease was somewhat relieved. Further increasing from +1.0 V (80%) to +1.7 V (97%) potential, the membrane flux loss is significantly reduced, because the electrostatic repulsion between the positively charged membrane and Ca2+ is enhanced to prevent Ca2+ from adsorbing to the membrane surface. Therefore more CaSO4 crystal formed in the bulk solution, rather than grew directly on the surface of the membrane, slowing down the formation of the filter cake layer. Further alleviate membrane fouling and retard the downward trend of water flux. To further demonstrate the effect of electrostatic interaction on membrane fouling trends the membrane was acted as a cathode, a significant flux drop occurs. Ca2+ is carried to the surface of the negatively charged membrane. And as the voltage increased from −0.2 V to −1.7 V, the increase in electrostatic activity caused the membrane permeation flux to drop sharply to 52% (−1.7 V). The crystallization behavior of CaSO4 on the surface of the membrane is intensified, showing more serious membrane fouling. It can be seen that the effect of the applied electric field on the inorganic components in the membrane fouling is evident. Organic pollution is an important part of membrane fouling, so antipollution tests are further carried out on BSA and LYS, as shown in Fig. 7 (c) and (d). Study has suggested that the main organic fouling reduction mechanism is a potentially induced surface charge that increases the Derjaguin-Landau-Verwey-Overbeek (DLVO) energy barrier and reduces the collision efficiency of charged organic materials with the membrane surface (Zhang and Vecitis, 2014). The results of Masse et al. show that the mechanism of membrane fouling by different polymer solutions is different (Masse et al., 1988), the formation of protein membrane fouling can be divided into two parts: initial organic adhesion which is mainly decided by interfacial interactions between the organic molecules and clean membrane surface, and subsequent organic

Fig. 7. Comparison of PEG (a), Amphoteric surfactant (b), CaSO4·2H2O (c), BSA (d) and LYS (e). (PEG, Amphoteric surfactant, BSA and LYS: 200 mg/L, CaSO4·2H2O: 1 g/L. FO conditions: the temperature of 25 °C for both feed (DI) and draw solution (1 M NaCl).)

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adhesion governed by interfacial interactions between already deposited and newly deposited organics (Motsa et al., 2015). In a nutshell, membrane-contaminant-contaminant interactions mechanism dominates the formation of organic membrane fouling processes (Mi and Elimelech, 2010). The physicochemical properties of the membrane surface are one of the key factors affecting the adsorption of proteins on membranes (Miao et al., 2017). Hence, it is considered effective to introduce charge into the surface of the membrane to control membrane fouling. As we all know, BSA is negatively charged, while LYS is positively charged (Liu et al., 2018). When the contaminants have the same charge as the membrane, there is almost no loss of water flux. The BSA and LYS water fluxes are maintained at 98% at −1.7 V and +1.7 V, respectively. It shows that there is electrostatic repulsion between the membrane and protein contaminants with the same charge as the contaminants, preventing the protein from adsorbing to the membrane surface, thereby reducing membrane fouling and slowing down the water flux decline. When the opposite charge is applied, the membrane surface adsorbs more contaminants and the water flux deteriorates significantly, for 87% and 91%, respectively. For BSA, the water flux decreased rapidly for the first 2 h when +1.0 and +1.7 V was applied, due to the increased electrostatic force between the membrane and the contaminant. At the same time, there is a similar trend for LYS. Subsequent flux change tendency is similar to the absence of an applied electric field, demonstrating that the interaction between contaminants and contaminants dominates the further development of the protein contamination layer. Therefore, the effect of applying an electric field to the membrane to alleviate the increase in membrane fouling by improving the adsorption of the initial protein on the membrane surface is remarkable. Therefore, this study provides a reference for the control of different types of charged contaminants. SEM images of the contaminated membrane surface and the cleaned membrane surface were supplemented, respectively (Fig. S3) placed in the supporting information. It can be seen that the application of an electric field can alleviate membrane fouling while at the same time allowing the membrane to recover to a better state after cleaning. 4. Conclusion In the present study, a conductive TFN-FO positive osmosis membrane prepared by blending conductive carbon nanoparticles in a support layer, showed good permeability, rejection and antifouling performance. The carbon nanoparticles for 4 wt% were dispersed in the support layer to form a continuous electron path and have satisfactory electrical conductivity (170.1 mS/m). The water fluxes were still 97%, 98% and 98% of initial values after 8 h for CaSO4, BSA and LYS in feed solution (with 1 M NaCl as draw solution), under the voltages of +1.7 V, −1.7 V, and +1.7 V, respectively. Compared with no-voltage applied, their water fluxes were improved by 31%, 13% and 7%, respectively, attributed to the electrostatic force between the membrane and the contaminants. It is indicated that the anti-pollution performance of surface charged membrane depends on the nature of the contaminant. The membrane pollution reduces when the contaminant and the membrane surface have the same charge, which in turn makes it worse. Acknowledgements This work was financially supported by National Natural Science Foundation of China (NSFC, No. 51278079) and Dalian Science and Technology Bureau (No. 2018J12SN075). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.134050.

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