Robust CNT-based conductive ultrafiltration membrane with tunable surface potential for in situ fouling mitigation

Robust CNT-based conductive ultrafiltration membrane with tunable surface potential for in situ fouling mitigation

Applied Surface Science 497 (2019) 143786 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locat...

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Applied Surface Science 497 (2019) 143786

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full length article

Robust CNT-based conductive ultrafiltration membrane with tunable surface potential for in situ fouling mitigation

T

Hengyang Mao, Minghui Qiu, Tianyu Zhang, Xianfu Chen, Xiaowei Da, Wenheng Jing, ⁎ Yiqun Fan State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Membrane fouling Conductive membrane Surface potential Electrostatic repulsion

A membrane filtration process assisted by an electric field is effective for removing charged foulants from the membrane surface. In this work, a robust and conductive carbon nanotube (CNT) network membrane was prepared by adding alumina nanoparticles (NPs). The interaction between the CNT and alumina were characterized. The results indicate that alumina NPs were assembled on the sidewall of CNT bundles. After sintering, the mechanical stability of the network was enhanced due to the formation of a chemical bond between the CNT and the alumina NPs. Humic acid (HA) solution purification experiments of the as-prepared membranes were carried out. The membrane potential was adjusted from 0 to −0.5 V, the membrane stationary permeance increased by 63.4% and 30.6%, when using HA and HA/Na2SO4 solutions, respectively. Simultaneously, the HA rejection performance of the membranes also increased with the applied potential. The membrane fouling mitigation and the performance improvement are ascribed to the electrostatic repulsion of the negative potential in the membrane and the negatively charged HA. The filtration performance of the CNT-based membrane was compared with other conventional membrane processes. The results further demonstrate that the performance of this reported membrane is competitive, especially with the assistance of an electric field.

1. Introduction In recent years, membrane filtration technology has drawn considerable attention for wastewater treatment due to its high efficiency and minimal environmental impact. However, the surface of the membrane is prone to fouling by concentration polarization and pore blocking. Membrane fouling results in a decrease of system efficiency, requirement of additional cleaning steps, and subsequently increase of costs. Mitigating membrane fouling has become the foremost critical challenge for membrane long term operation [1,2]. Current fouling mitigation strategies have been promoted in the membrane process including feed liquid pretreatment, optimizing the operating condition, membrane cleaning and surface modification [3]. In addition, combining other technologies with the membrane is an alternative approach to reduce fouling, such as photochemistry [4,5], ultrasound [6,7] or electric fields [8,9], etc. Membrane filtration assisted by an electric field involves applying direct current to the membrane, so the surface potential of the membrane can be negatively or positively adjusted. This method has been demonstrated to be effective for repulsing the charged foulants away from the membrane ⁎

surface [10]. The membrane in this process is normally conductive; it serves to provide separation accuracy and can be used as an electrode. The electrostatic effects or electrochemical reactions can take place directly on the membrane surface [11,12] and the energy loss can be saved compared to the use of an additional electrode [13]. Zhang et al. [14] prepared a conductive PDAAQ/rGO/PVDF membrane; the membrane fouling resistance increased by 63.5% with the application of an electric field of 1.0 V∙cm−1. They ascribed the anti-fouling performance of membrane to the synergistic effect of an electrostatic repulsive force and the in situ electro-generated H2O2 in the presence of the electric field. Similarly, Liu and coworkers [15] prepared a conductive PPy/ PVDF/SDBS membrane, and the membrane unified fouling index decreased with the application of 0.1 V electric field. A conductive membrane is the core component of the electric field-assisted process; preparing membranes with good conductivity and stability is the premise to realize anti-fouling performance. Carbon nanotube (CNT) is an excellent electrically conductive and thermally stable material, and membranes prepared with CNT also have been well-studied [16–18]. In recent research, David's group [19] synthesized a conductive PA/CNT thin film membrane and successfully

Corresponding author. E-mail address: [email protected] (Y. Fan).

https://doi.org/10.1016/j.apsusc.2019.143786 Received 26 April 2019; Received in revised form 16 August 2019; Accepted 23 August 2019 Available online 24 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic illustration of the membrane preparation mechanism and process.

applied it in the electric field-assisted process. They found that, with the application of a 1.5 V square wave, membrane flux decline was up to three times lower than control experiments. Further research [20] revealed a conductive PVA/CNT membrane, in which the change in membrane operating pressure was reduced by 51% by the application of a −5 V electric field. The fouling mitigation performance of CNTbased membranes has also been reported in other publications [21,22]; these studies are further evidence that CNT-based membranes have great anti-fouling potential in the presence of an electric field. The above-mentioned CNT-based conductive membranes are commonly prepared with an organic support. The chemical or physical actions between the CNT and the organic component are benefitted by the membrane mechanical stability. Nonetheless, reactive oxygen species such as electron-hole, the hydroxyl radical (∙OH) and H2O2 are easily generated in the electric field-assisted process [23]. These reactive oxygen species may pose a great challenge to the chemical stability of the organic components in the membrane. Alumina, due to its chemical stability and high mechanical strength, has been used widely as a support of membranes for harsh environments. Therefore, conductive membranes prepared by coating CNT on a porous alumina support provide an alternative approach for its longterm operation. However, such membranes would require a sufficient binder to enhance mechanical performance [24]. Considerable research effort has been reported to improve the mechanical properties of CNT network, such as thionyl chloride modification [25], carbon vacuum arc deposition [26] and graphene-like carbon coating [27], etc. In these aforementioned methods, covalent bonds were formed between binders and the CNT, increasing the packing density and mechanical performance of the CNT-based membranes. Quan's group [28] prepared an alumina-supported conductive CNT membrane; this membrane exhibited a good fouling mitigation performance under electrochemical assistance. The mechanical stability and stiffness of the CNT network

were improved by graphene-like carbon, generated by polyacrylonitrile pyrolysis. The above mentioned carbon-based binders are effective in enhancing CNT performance, but the low adhesion force between the CNT layer and the macroporous alumina support is still an issue. In this work, we describe a robust CNT-based conductive membrane supported by porous alumina. The membrane possesses both a high strength CNT layer and strong adhesion force between the CNT layer and the support, using nano-alumina as a binder. The CNT layer strength was enhanced due to the formation of alumina oxycarbide between the CNT bundles and nano-alumina during a carbothermal reduction process [29,30]. The adhesion force of the CNT layer and support was provided not only by the alumina oxycarbide but also by the sintering neck that formed between the two layers [31,32]. The dispersion of nano-alumina and CNT in suspension was investigated, then the mechanical performance of CNT/Al2O3 composite membranes was characterized. Finally, the optimized membrane was applied in humic acid (HA) solution treatment to study its anti-fouling performance under an electric-assisted process. 2. Experimental 2.1. Preparation of alumina/CNT suspension Boehmite sol with an alumina content of 20 mg/mL was prepared following the description in previous literature [32]. The alumina particle size was approximately 16 nm (Fig. S1) and the content of the boehmite could be adjusted by adding pure water. Then, block copolymer (F127, Sigma-Aldrich) was dissolved in boehmite sol with an F127 concentration of 2 mg/mL. After that, CNT (Sigma-Aldrich) was added to 100 mL F127-boehmite sol and the mixture was treated with ultrasonic radiation with a power of 500 W coupled with an ice bath for 10 min. 2

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Fig. 2. (a) Zeta potential of alumina particle. (b) Formation of hydrogen bonds between F127 and alumina. (c) TEM microphotograph of CNT in boehmite sol. (d) Microstructure and (e) digital camera photograph of CNT-boehmite suspension xerogel. Elemental distribution in the xerogel: (f) carbon, (g) aluminum and (h) oxygen elements.

phases of alumina were obtained using an X-ray diffractometer (MiniFlex 600, Rigaku, Japan) with Cu Kα radiation and operated at 15 mA, 40 kV, with a step size of 0. 02° and a scanning range of 20° to 80°. Thermogravimetric analysis of materials was performed with a thermogravimetric analyzer (STA449, Netzsch, Germany) at a heating rate of 3 °C/min from 20 °C to 1200 °C in an air or argon atmosphere. The chemical state of materials was detected with X-ray photoelectron spectroscopy (ESCALAB 250, Thermo, America); all the binding energies were referenced by C 1s with a peak of 284.6 eV. The morphologies of the CNT and CNT/alumina composite were examined by transmission electron microscopy (TEM) (JEM-1011, JEOL, Japan). The scratch resistance of the membranes was tested by a nano-scratch tester (NanoTest, MML, England). The membrane conductivity was tested with a four-probe meter (RTS-8, 4 Probes Tech, China). Membrane retention performance was carried out with a dextran solution in a custom-made apparatus. The dextran concentrations were 1 g/L for 40 kDa and 70 kDa, 2 g/L for 500 kDa and 1.5 g/L for 2000 kDa. The feed and permeate solutions were analyzed by gel permeation chromatography (GPC, Waters, America).

2.2. Preparation of alumina-enhanced CNT network membranes Pristine CNT network and alumina-enhanced CNT network membranes were prepared on the alumina substrate (Fig. S2) through dip coating. Then, the wet membranes were dried for 12 h at 25 ± 3 °C and further dried in an oven at 110 °C for 12 h. Subsequently, the dry membranes were sintered in a tube furnace in an argon atmosphere with a ramp of 2 °C/min and 2 h hold time at 1150 °C. The unsupported membrane material was obtained by drying and sintering of the alumina/CNT suspension at the same conditions as the asymmetric membranes. We named the network membranes following the composition of suspension. For example, the membrane prepared with a suspension of 1 mg/mL CNT and 15 mg/mL alumina was named as C1A15. 2.3. Characterization of suspension and membrane The particle size distribution and zeta potential of particles in liquid were characterized by a Zetasizer (ZS90, Malvern, England). A field emission scanning electron microscope (FESEM) (S-4800, Hitachi, Japan) with a 5 kV accelerating voltage, coupled with an X-ray energy dispersive spectrometer (EDS) (EMAX, Horiba, Japan) was employed for the morphology and elemental characterizations. The crystalline 3

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Fig. 3. (a) XRD curves of alumina sintered in argon, (b) Al 2p XPS spectra of CNT-alumina composite sintered at 1150 °C, TEM microphotograph of (c) pristine CNT and (d) CNT-alumina composite.

2.4. Fouling mitigation tests of membranes under electric assistance process

Q2 × 100 Q0

(1)

Rt =

Q0 − Q1 × 100 Q0

(2)

Rr =

Q2 − Q1 × 100 Q0

(3)

Rir =

Q0 − Q 2 × 100 Q0

(4)

FRR =

2.4.1. Electric assistance process The membrane anti-fouling performance was tested in a custommade cross-flow membrane module. This module was designed based on a three-electrode system. The membrane performance was evaluated under different potentials using an electrochemical workstation (Reference 3000, Gamry, America), with the membrane as the working electrode, a platinum electrode as the counter electrode and an Ag/ AgCl (saturated KCl) as the reference electrode. The distance between the membrane surface and the counter electrode was 1 cm. Linear sweep voltammetry (LSV) curves were carried out with a scan rate of 50 mV/s in the module to evaluate the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) potentials. During the filtration process, the membrane (working electrode) potential and the electrode current were detected on line to confirm that the system maintained good electrical contact.

2.4.3. Humic acid (HA) rejection HA (Aladdin, China) solution was prepared with a content of 200 ppm by weight. HA/Na2SO4 solution was prepared with an HA content of 200 ppm and Na2SO4 (Xilong Chemical Co., Ltd., China) content of 0.05 M. The HA rejection R (%) was calculated as follow:

2.4.2. Membrane permeance For each membrane, the pure water permeance Q0 was measured at 25 ± 2 °C with 1.5 m/s linear crossflow velocity and 1 bar pressure. Then, the stationary permeance, Q1, of an HA or HA/Na2SO4 solution was measured with applied voltage for 120 min under the same conditions. Subsequently, the membrane was washed for 10 min with distilled water. Finally, the pure water permeance was obtained again as Q2. No voltage was applied during the pure water permeance process. To study the specific contribution of membrane potential on fouling, each membrane's performance was tested at various potentials. For each combination, the flux recovery ratio (FRR), total fouling ratio (Rt), irreversible fouling ratio (Rir) and reversible fouling ratio (Rr) were calculated to evaluate the membrane fouling mitigation performance according to Eqs. (1)–(4):

R=

Cf ‐Cp Cf

× 100

(5)

where Cf and Cp are the content of HA in the feed and permeate solution, respectively. The concentration of HA in liquid was detected by an UV/Vis spectrophotometer (ND-2000C, Thermo, America) at a wavelength of 254 nm. A standard calibration curve of UV254 absorbance against the HA concentration was established using prepared solutions with HA contents of 5 ppm, 10 ppm, 40 ppm and 60 ppm. As shown in Fig. S3, the concentration of unknown HA samples ranging from 0 to 60 ppm could be obtained.

4

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Fig. 4. (a) Dextran rejection performance, (b) impedance and permeance of network membranes. (c) Diagram of nano-scratch test, (d) scratch and load curves of nano-scratch test and (e) membrane mechanical test by US.

3. Results and discussion

suspension was treated by ultrasound and the CNT bundles and alumina particles were assembled by F127. Finally, the network membranes were prepared by dip coating and a sintering process. The alumina particles filled in the space between the CNT bundles and improved the mechanical strength through carbothermal reduction. The zeta potential of nano alumina is shown in Fig. 2a; its value is seen to decrease with F127 concentration. The nano alumina is positively charged in boehmite sol to keep system stable by electric steric repulsion, while the F127 micelles are normally electrically negative in water [35,36]. Hence, it can be inferred that the changing of the zeta potential can be ascribed to the replacement of positive charge on alumina surface by the F127 micelles. As shown in Fig. 2b, it is considered that the interaction between F127 and alumina is attributed to the formation of hydrogen bonds between the –OH groups on the F127 and the alumina hydroxide colloidal particles [37]. CNT was added to the F127-boehmite gel and dispersed by ultrasound. The microphotograph of CNT in suspension was characterized by TEM. As shown in Fig. 2c, the boundary of CNT bundles is a blur and the spaces between bundles are filled. This finding is similar to our previous research

3.1. Interaction between alumina and CNT in suspension The CNT exhibited an average length of 5 μm and diameter of 6–9 nm. Owing to a high L/D ratio and weak interaction with boehmite sol, the agglomeration of CNT in the sol was difficult to avoid. There, it was necessary to employ an effective agent for dispersing CNT in aqueous boehmite sol and enhancing the interaction between the CNT and nano alumina particles. Based on the literatures, F127 can welldisperse CNT in water. F127 is a triblock copolymer with the structure of PEO-PPO-PEO; the EO groups extend into the water, while the PO groups anchor to the sidewalls of the CNT [33]. The core-shell structure with hydrophilic EO groups as shell creates a steric repulsion inhibiting the aggregation of CNT. Here, we report the F127 can not only disperse CNT in boehmite sol but also bridge the CNT with alumina. The mechanism and process of membrane preparation are shown in Fig. 1. With the dissolving of F127 in boehmite sol, micelles were formed [34] and connected with alumina particles. Then, the CNT-alumina

5

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Fig. 5. (a, b) Surface and (c) cross-section microphotograph of network membranes, (d) schematic of custom-made conductive membrane module and (e) membrane antifouling mechanism.

(C 1s = 284.6 eV). The characteristic peaks of O 1s, C 1s, Al 2s and Al 2p are observed in the full XPS spectra of boehmite and CNT-boehmite composite (Fig. S5a). In particular, it can also be found that the binding energy of Al 2p exhibited a positive shift from 73.6 eV to 75.2 eV with the existence of the CNT (Fig. S5b). As shown in Fig. 3b, the Al 2p peak in CNT-boehmite can be further divided into two peaks at 74.7 eV and 75.3 eV, which are attributed to AleO and Al-O-C bonds [28], respectively. Therefore, the positive peak shift of Al 2p may be attributed to the conduction band electrons in AleO bands transferring to the newly generated OeC bands, leading to a decrease in the outer electron cloud density of aluminum. The comparison between TEM microphotographs of the CNT and CNT-alumina composite (Fig. 3c, d) also indicate that the alumina has successfully connected to the sidewalls of the CNT bundles [41]. A similar result was also observed between the CNT and alumina on the substrate, indicating the chemical bond-based combination of network layer and substrate (Fig. S6).

[38], indicating that F127 has successfully anchored on the surface of the CNT. The suspension was dried at 110 °C for 12 h and the obtained composite xerogel was characterized by SEM and EDS to further understand the distribution of CNT and alumina. Fig. 2d and e show the xerogel with black luster; its microstructure is uniform and no obvious agglomeration can be observed by SEM. The content of the elements and distribution on the surface of xerogel are shown in Fig. 2f–h. The surface consists of carbon, aluminum and oxygen and all these elements are distributed uniformly, which demonstrates the good distribution of CNT and alumina in suspension. 3.2. Interaction between alumina and CNT in membranes The thermal properties of the CNT, alumina and F127 were characterized to investigate the sintering process of the network membranes. As shown in Fig. S4a, in air, the CNT and F127 were decomposed completely with temperature while the boehmite xerogel retained 60% of its initial mass when the temperature exceeded 550 °C. When the sintering atmosphere was switched to argon (Fig. S4b), the thermal behavior of F127 and boehmite had no obvious change while more than 85% of the CNT was preserved at 1200 °C. In this work, the sintering temperature of the network membranes was set as higher than 1000 °C. Therefore, it can be inferred that only the CNT, alumina and their composite exist in the membranes. Alumina is known to exist in many crystallographic forms, including α-Al2O3, γ-Al2O3, θ-Al2O3 and δ-Al2O3. Among these forms, α-Al2O3 is the most thermally stable while the others are metastable [39,40]. The XRD curves of alumina sintered in argon were characterized as shown in Fig. 3a. It can be seen that, after 1150 °C sintering for 2 h, nearly all alumina transformed from γ-Al2O3 into α-Al2O3. The interaction between alumina and the CNT was investigated by sintering the CNTalumina composite in argon, followed by XPS analysis. The binding energies in the XPS spectra were calibrated using a carbon contaminant

3.3. Effect of alumina content on performance of network membranes The performance of pristine CNT network membranes was greatly influenced by the CNT content in the suspensions. The detailed optimization process is listed in Figs. S7–10. The final optimized suspension was C1A0 (CNT content of 1 mg/mL), and the membrane prepared using C1A0 suspension possessed s permeance of approximately 670 L∙m−2∙h−1∙bar−1 (LMH/bar), dextran rejection of approximately 472 kDa (corresponding to a pore size of 26.9 nm) and impedance of 14.9 Ω∙cm. Nano-alumina enhanced network membranes were prepared with suspensions of C1A5, C1A10, C1A15 and C1A20. The effect of alumina content on the performance of the membranes was investigated. The rejection performance of the membranes is shown in Fig. 4a. It can be seen that the dextran rejection of the membranes increased with alumina content and the corresponding pore size decreased from 24.4 nm 6

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Fig. 6. (a) Voltage and current in membrane surface during filtration tests. (b) Linear sweep voltammetry of the conductive membrane in a HA/Na2SO4 solution. (c) Normalized membrane permeance vs. time. (d) Water permeance recovery and fouling resistance of membranes. The experiments were performed at a constant pressure of 1 bar using a 1.5 m/s crossflow velocity.

3.4. Anti-fouling performance of network membrane

at C1A5 to 19.5 nm at C1A20. Simultaneously, the membrane permeance also decreased with alumina content, as shown in Fig. 4b. This may be ascribed to the large amount of nano alumina particles filling the space between CNT bundles, reducing the pore size and increasing the liquid penetration resistance. Membrane impedance increased slightly with alumina content due to the insulative property of alumina, from 14.9 to 17.1 Ω∙cm. The interfacial adhesion between the network membranes and alumina support was examined by a nano-scratch test [42,43], as shown in Fig. 4c. The critical load (Lc) of each membrane can be obtained from the failure point in scratch curves. Reaching Lc or the failure point indicates that the complete removal of the toplayer has occurred. It can be seen in Fig. 4d that the Lc of membrane increased from 110 mN to 290 mN with alumina content increasing from 5 mg/ mL to 20 mg/mL. The adhesion force was much higher than that of other reported composite membranes [42,44], which confirms the positive effect of nano-alumina on the mechanical performance improvement of the membranes. We further characterized the membrane mechanical strength by ultrasound (US), with a power setting at 200 W. As shown in the diagram in Fig. 4e, membranes were placed half in water. After a 30 s ultrasound treatment, a large surface of top layer was peeled from C1A0. With the addition of alumina, the membrane peeling phenomenon improved considerably and no obvious delamination was observed in C1A15 and C1A20. The nano-scratch test and US treatment revealed that the membrane mechanical strength increased with alumina content, demonstrating the feasibility of our conjecture.

HA, a natural organic material, is ubiquitous in surface and ground water. HA is also commonly identified as a key solute in membrane surface fouling. It leads to the cake layer formation on membrane surface and/or the membrane pore blocking during the ultrafiltration (UF) process of drinking water [45,46]. In this work, the fouling mitigation performance of the as-prepared UF membrane combined with an electric field was investigated in 200 ppm HA solution purification. Considering that the natural surface water will contain not only organic contaminants but also dissolved salts [47]. Additional 0.05 M Na2SO4 was added into the HA solution, as a parallel experiment, to test the effect of salts on membrane filtration performance. The C1A15 membrane, with pore size of approximately 21.7 nm, water permeance of 608 LMH/bar and impedance of 16.7 Ω∙cm, was used in the anti-fouling test. The membrane surface and cross-section microstructure were characterized by SEM as shown in Fig. 5a–c. It can be seen that the membrane exhibits a uniform surface, the CNT bundles are covered with nanoparticles and the network maintains a porous structure. The separation layer with thickness of 1.2 μm is tightly bound with the alumina support, and membrane penetration is avoided due to the bridge effects of the CNT bundles [48]. Membrane fouling mitigation tests were performed in a custom-made module, as shown in Fig. 5d. The membrane surface was connected to the working electrode and its potential could be adjusted by the electrochemical workstation. During the electric field-assisted filtration process, the charged foulants could be removed away from the surface of the conductive membrane 7

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Fig. 7. The (a) UV absorbance of permeated solutions and (b) membrane HA rejection performance. (c) Particle size distribution and (d) digital camera photographs of raw HA, HA/Na2SO4 and permeated solutions for different tests. Table 1 The comparison of ultrafiltration membrane in HA solution purification.

[53] [46] [54] [55] [56] [57] [58] This work

Membrane material

HA content (mg/L)

Initial permeance (LMH/bar)

Separation process

Stable permeance (LMH/bar)

Rejection (%)

MWCNT/PANI/PVDF GO/PES Al2O3/PVDF Fe/PVDF PES GO/TiO2 PPSU/PEI CNT/Al2O3

5 10 10 20 45 50 450 200

1000 161 210 486 280 110 260 608

Conventional Conventional Conventional Conventional Conventional Conventional Conventional Conventional −0.5 V potential

150 121 140 219 106 73 80 89 145

76 81 88 75 77 87 68 87 89

In the fouling mitigation experiments, six filtration tests were conducted. We used the C1A15 UF membrane to deal with the HA solution and the HA/Na2SO4 solution with the applied potentials of 0 V, −0.2 V and −0.5 V, respectively, versus Ag/AgCl electrode. These tests were named as HA/0 V, HA/−0.2 V, HA/−0.5 V, HA/Na2SO4/0 V, HA/ Na2SO4/−0.2 V and HA/Na2SO4/−0.5 V. The membrane performance measurements were carried out as described in Section 2.4. The potential and current across the membranes are listed in Fig. 6b. When dealing with the HA/Na2SO4 solution, membranes possessing higher current than that in HA solution is ascribed to the solution conductivity improvement with the addition of salts. Fig. 6c shows the membrane filtration results; it can be seen that all pure water permeance (Q0) remained at the same value. During HA or HA/Na2SO4 solution filtration, the permeance in all tests reached an asymptotic value after approximately 40 min. In the test of HA/0 V, the membrane stationary

through electrostatic repulsion (Fig. 5e). Before the anti-fouling tests, the redox reaction on the membrane was studied. The linear sweep voltammetry (LSV) showed that, in HA solution, no hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) occurred on the membrane surface. This is due to the low conductivity of HA solution, 39.4 μs∙cm−1; at this condition, the liquid is insulative for electron transfer. Fig. 6a displays the LSV curves in HA/Na2SO4 solution; the HER and OER potentials were −0.26 V and 0.8 V, respectively, versus the Ag/AgCl electrode. No separate HA oxidation peak was detected in the measurement range, indicating that removing HA fouling through anodic oxidation is impracticable at this process. Thus, in the following filtration tests, only negative voltages were applied to the membranes. It is anticipated that the membrane fouling can be removed by the cathode membrane with an associated electric field. 8

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was prepared and its electrically-assisted fouling mitigation performance was examined during humic acid (HA) filtration. Nano-alumina particles were chosen as the active compound for their excellent performance in membrane preparation and activity with the CNT. A uniform CNT/alumina suspension was prepared using a block copolymer as a bridge. F127 is a triblock copolymer with the structure of PEOPPO-PEO. The PO groups anchored the sidewalls of the CNT via π-π bonds, while the EO groups connected with the alumina nanoparticles through hydrogen bonds. The chemical interaction between the CNT and alumina was proven by the XPS and TEM characterization. We found that, after 1150 °C sintering, Al-O-C bonds were formed in the CNT/alumina composite. The effect of alumina content on the performance of the network membranes was investigated. The results indicated that the membrane average pore size and permeance decreased while the mechanical strength and impedance increased with alumina content. In particular, the membranes prepared with C1A15 and C1A20 exhibited good mechanical strength in an ultrasound field with a power of 200 W. A membrane having the designation C1A15 was used in HA rejection to test its fouling mitigation performance combined with electrical potential. The filtration experiments revealed that the membrane stationary permeance and rejection performance increased as the membrane surface potential increased from 0 to −0.5 V. This phenomenon occurs because a higher negative potential resulted in a larger electrostatic repulsion between the membrane and the HA. Charged foulants commonly exist in wastewater, and further study of the treatment of solutions containing multiple components offers great potential.

permeance (Q1) decreased to 14.6%. With the membrane potential increasing to −0.2 V and −0.5 V, this value increased to 19.8% and 23.9%, respectively. Similar results were obtained with HA/Na2SO4 solution treatment in that the membrane anti-fouling performance increased with applied potential. With the membrane potential increasing from 0 V to −0.2 V and −0.5 V, the membrane stationary permeance increased from 15.4% to 18.5% and 20.1%, respectively. It can also be found that, with the addition of salts and operating at higher potentials, the membrane permeance (Q2) recovered more. The membrane filtration results, expressed in FRR, Rt, Rr and Rir, are shown in Fig. 6d to evaluate the specific effect of associated electric field. The FRR in HA and HA/Na2SO4 solution increased with membrane potential, indicating that the membranes are more cleanable under the negative potential. Reversible fouling, represented by Rr, is primarily ascribed to HA deposition and concentration polarization on the membrane surface. Irreversible fouling, expressed by Rir, is mainly caused by HA infiltration into the membrane pores. Rt and Rir for each test were decreased while Rr increased at higher potential. This implies that negative potentials effectively reduce HA blocking into membrane pores and weaken the interaction between HA and membrane surface. The zeta potential of HA in solution is about −53.8 mV, negatively charged, so the electrostatic repulsion between HA and cathode membrane increased with potential [21,28]. From the comparison between the membrane performance in HA and HA/Na2SO4 solution, the membranes usually possessed larger Rr and lower Rir with the presence of Na2SO4 than in HA. This is ascribed to the aggregation of HA at high salt content [49], which enhanced the formation of a cake layer on the membrane surface, thus reducing HA infiltration into the membrane pores. Total fouling, represented by Rt, in HA and HA/Na2SO4 solution, is similar at the same potential and this value decreased with the increase of potential. However, when treating the HA/Na2SO4 solution, no obvious decrease in Rt was observed with potential increase from −0.2 V to −0.5 V. This is because −0.5 V is larger than the HER potential of −0.26 V. Therefore, a large quantity of gas bubbles were generated at the membrane surface due to water electrolysis. These bubbles hindered the permeation of water and increased the permeance resistance [50]. The UV absorbance of filtrate and membranes HA rejection are shown in Fig. 7a, b. The increase of membrane rejection performance with potential is ascribed to the enhanced electrostatic repulsion between HA and the membranes [51]. In the HA/Na2SO4 solution, due to the compression of the electrical double-layer at high ionic strength, the zeta potential of HA is about −34.2 mV, lower than that in single HA solution. This leads to a decrease in electrostatic repulsion between HA and the membranes and simultaneously reducing anti-fouling and rejection performance [22,52]. The particle size distribution of HA in solutions was detected by DLS, as shown in Fig. 7c. In feed solutions, the HA aggregation leads to a comparatively wide particle size distribution in the range of dozens of nanometers to several micrometers. In the permeate solutions, its size lower than 10 nm. The HA and HA/ Na2SO4 feed solutions exhibited a brownish appearance and Tyndall effect due to high HA concentration and aggregates, as seen in Fig. 7d. After ultrafiltration, the Tyndall effect of each filtrate disappeared, indicating that the aggregated particles were rejected totally, in agreement with the particle size distribution. Simultaneously, the transparency of each solution was also greatly improved. The performance of ultrafiltration membranes in HA solution purification is listed in Table 1. It can be seen that the performance of the obtained CNT/Al2O3 membrane is competitive, especially with the assistance of the electric field. This comparison further demonstrates the benefit and efficiency of the conductive surface on the membrane fouling mitigation and performance improvement.

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