Next Article in Journal
Simulation of a Reverse Electrodialysis–Absorption Refrigeration Integration System for the Efficient Recovery of Low-Grade Waste Heat
Next Article in Special Issue
Circular Economy Applied to Sludge Minimization: The STAR Project
Previous Article in Journal
Recycling Lithium-Ion Batteries—Technologies, Environmental, Human Health, and Economic Issues—Mini-Systematic Literature Review
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Membranes
Volume 15
Issue 1
10.3390/membranes15010001
membranes-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Electro-Conductive Modification of Polyvinylidene Fluoride Membrane for Electrified Wastewater Treatment: Optimization and Antifouling Performance

by
Jinzhuo Shi
1,2,
Yisong Hu
1,2,*,
Songhua Li
1,2,
Wenqian Xiao
1,2,
Yuan Yang
1,2 and
Jiayuan Ji
3,*
1
Shaanxi Key Laboratory of Environmental Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China
2
Key Laboratory of Northwest Water Resource, Environment and Ecology, Ministry of Education, Xi’an University of Architecture and Technology, Xi’an 710055, China
3
Institute for Future Initiatives, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, Japan
*
Authors to whom correspondence should be addressed.
Membranes 2025, 15(1), 1; https://doi.org/10.3390/membranes15010001
Submission received: 22 November 2024 / Revised: 17 December 2024 / Accepted: 22 December 2024 / Published: 24 December 2024
(This article belongs to the Special Issue Green Membrane Technologies: Advancements in Materials and Energy Efficiency for Water Treatment)
Browse Figures
Versions Notes

Abstract

Electro-conductive membranes coupled with a low-voltage electric field can enhance pollutant removal and mitigate membrane fouling, demonstrating significant potential for electrified wastewater treatment. However, efficient fabrication of conductive membranes poses challenges. An in situ oxidative polymerization approach was applied to prepare PVDF-based conductive membranes (PVDF-CMs) and response surface methodology (RSM) was adopted to optimize modification conditions enhancing membrane performance. The anti-fouling property of the conductive membranes was analyzed using model pollutants. The results indicate that when the concentrations of the pyrrole, BVIMBF4, and FeCl3·6H2O are 0.9 mol/L, 4.8 mmol, and 0.8 mol/L, respectively, the electrical resistance of the PVDF-CM is 93 Ω/sq with the water contact angle of 31°, demonstrating good conductivity and hydrophilicity. Batch membrane filtration experiments coupled with negative voltage indicated that when an external voltage of 2.0 V is applied, membrane fouling rates for the conductive membrane filtering BSA and SA solutions are reduced by 17.7% and 17.2%, respectively, compared to the control (0 V). When an external voltage of 0.5 V is applied, the membrane fouling rate for the conductive membrane filtering HA solution is reduced by 72.6%. This study provides a valuable reference for the efficient preparation of conductive membranes for cost-effective wastewater treatment.

1. Introduction

Membrane separation technology has several advantages of high-quality effluent production, small footprint, and low energy consumption, making it an essential option in water/wastewater treatment [1,2]. Nowadays, pressure-driven membrane processes, such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), have been widely utilized in drinking water purification and wastewater treatment [3,4]. Among such membrane-based processes, MF and UF are widely used in wastewater treatment because of their high membrane surface porosity and permeability. However, challenges still exist in the application of membrane technologies, such as membrane fouling and low selectivity [5,6,7]. Therefore, the development of novel membrane materials with improved selectivity and antifouling capabilities is necessary. The electrically conductive membrane (also named conductive membrane) represents a rapidly developing field and shows great potential in wastewater treatment [8,9,10,11].
With the assistance of an external electric field, the conductive membrane acts as a working electrode and filtration membrane, achieving the dual effects of physical separation of pollutants and electrochemical degradation simultaneously. Moreover, the clogging of membrane pores and the development of a cake layer can be efficiently prevented by electrochemical redox reactions, electrostatic repulsion, and produced gas scouring between electro-conductive membranes and contaminants, hence mitigating membrane fouling [12,13,14,15]. In electrochemical membrane bioreactors (EMBRs), conductive membranes are directly used as electrodes; they have gained much attention for excellent antifouling performance and space-saving characteristics. A series of approaches such as surface coating, chemical polymerization, and blending modification have been employed to fabricate or modify the conductive membranes that can serve as anodes or cathodes [16,17,18], facilitating in situ control of electrochemical reactions to achieve cost-effective membrane fouling control [19,20,21].
In recent years, electrically conductive polymers have attracted significant attention from researchers because of their capacity to perform reversible redox reactions, which result in modifications to conductivity, color, and volume during functional material fabrication [22]. Electrically conductive polymers have the potential to be used in many applications, including membrane separation [23], supercapacitors [24], electrochemical sensors [25], as well as mechanical actuators [26]. Polymers like polypyrrole, polyacetylene polyaniline, and polythiophene are ideal organic materials for tunable separation membranes, offering biocompatibility, cost-effectiveness, and good compatibility. Among electrically conductive polymers, polypyrrole is one of the most studied materials due to its excellent conductivity (with conductivity ranging from 0.1 to 7500 S/cm), simple preparation methods, low density, and wide application prospects [27,28]. Recent studies have shown that electrically conductive polymer membranes exhibit improved permeation performance, antifouling properties, and separation of charged pollutants. Benson et al. utilized an electrodeposition technique to integrate electrically conductive polyaniline onto nonwoven carbon nanotube (CNT) fabric [29]. The resulting high-strength conductive membranes demonstrated rapid ion adsorption and impressive specific capacitances, which render them ideal for low-energy desalination through capacitive deionization. Dudchenko et al. developed electrically conductive membranes (2500 S/m) composed of cross-linked polyvinylalcohol (PVA) and carboxylated multi-walled carbon nanotubes (PVA-CNT-COOH) using a sequential deposition and cross-linking method [30]. The effects of applied extern voltage on membrane fouling when filtering alginic acid and poly(ethylene) oxide solutions were studied, and the membrane fouling rate was decreased by 51% at −5 V compared with the control group. However, it remains a challenge to optimize the modification conditions to prepare high-quality conductive membranes with high conductivity, hydrophilicity, and porous structure [31] under the circumstance of increasing attention being paid to electrified wastewater treatment in nowadays.
In this study, the in situ oxidative polymerization method was employed to prepare polyvinylidene fluoride based conductive membranes (PVDF-CMs). To achieve high conductivity and hydrophilicity of the prepared PVDF-CMs, response surface methodology (RSM) was utilized to explore the relationship between the modification conditions and membrane properties including electrical resistance and water contact angle. The preparation conditions for the PVDF-CMs were optimized through multiple quadratic regression equations and variance analysis. The control PVDF membrane and PVDF-CMs were characterized using four-probe electrical resistance meter, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), static contact angle measurements, as well as an electrochemical workstation. To verify the antifouling performance of the PVDF-CM, batch filtration experiments were conducted using natural organic pollutants such as bovine serum albumin (BSA), sodium alginate (SA), and humic acid (HA) to analyze the filtration behaviors and antifouling properties. This study provided new insights into the preparation of cost-effective and highly conductive membranes for electrified wastewater treatment.

2. Materials and Methods

2.1. Materials

The supporting material for preparing conductive membrane is a commercial polyvinylidene fluoride (PVDF) membrane (Chongqing Jichuang Technology Co., Ltd., Chongqing, China), with a pore size of 0.45 μm. The pyrrole and 1-vinyl-3-butylimidazolium tetrafluoroborate (BVIMBF4) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China, with the structural formula shown in Table 1. Hexahydrate ferric chloride (FeCl3·6H2O), ethanol (C2H5OH), sodium hydroxide (NaOH), hydrochloric acid (HCl), and sodium chloride (NaCl) were acquired from Tianjin Kemiou Chemical Reagent Co., Ltd., Tianjin, China. Bovine serum albumin (BSA, Biochemical reagent) and sodium alginate (SA, Chemically Pure) as well as humic acid (HA, Biological reagent) were obtained from China National Pharmaceutical Group Chemical Reagents Co., Ltd., Shanghai, China. Humic acid (HA, Biological reagent) was obtained from Shanghai Zhanyun Chemical Co., Ltd., Shanghai, China. The average molecular weights of BSA, SA, and HA are 68 kDa, 160 kDa, and 3.5 kDa, respectively.

2.2. Preparation and Characterization of Conductive Membrane

2.2.1. Modification of the Conductive Membrane

In situ oxidative polymerization was employed to prepare conductive membranes [32]. The PVDF-based membrane was first soaked in pure water for more than 24 h, then removed and dried in the air. A certain concentration of pyrrole (with ethanol as the solvent) was prepared in 20 mL and thoroughly mixed with a specific amount of BVIMBF4. The pre-treated PVDF membrane was immersed in the prepared pyrrole solution for 30 min (flipping every 5 min), then transferred to the FeCl3·6H2O solution for a reaction lasting 30 min (conducted at low temperatures). The membrane surface was washed sequentially with 0.1 mol/L hydrochloric acid, ethanol, and pure water to remove any residuals. Finally, the modified membrane (namely PVDF-CM) was dried in air for 30 min.
Response surface methodology (RSM) is a prevalent tool that statistically observes the relationship (or response) between experimental indicators and various factors for process optimization [33]. As presented in Table 2, a three-factor and three-level Box-Behnken design (BBD) was conducted using Design-Expert 12 software (12.0.3.0), and a second-order RSM was utilized for regression fitting of the experiments to optimize modification conditions of the conductive membrane. The concentrations of pyrrole (A), BVIMBF4 (B), and FeCl3·6H2O (C) were selected as the response variables, with electrical resistance and water contact angle as the response values. The experiment data were fitted using a second-order polynomial equation, as shown in Equation (1), after conducting a total of 17 experiments.
Y = b 0 + ∑ i = 1 k b i X i + ∑ i = 1 k b i i X i 2 + ∑ i = 1 k ∑ j > 1 k b i j X i Y j
where Y is the response value of the conductive membrane; b0 is the constant term; bi is the regression coefficient for linear effects; bii is the regression coefficient for quadratic effects; bij is the regression coefficient for interaction effects; and Xi is the coded experimental variable. The coefficient of determination R2 was used to evaluate the extent of interaction among experimental factors affecting the observed response values, with values ranging from 0 to 1 [34]. If the R2 value is close to 1, the model can predict the response better.

2.2.2. Characterization of the Conductive Membrane

Scanning electron microscopy (SEM; Quanta650FEG, FEI, Hillsboro, OR, USA) was used to analyze the surface morphology of the PVDF membrane and PVDF-CM, and energy-dispersive X-ray spectroscopy (EDX; Quanta650FEG, FEI, Hillsboro, OR, USA) was employed to determine the elemental content. Fourier transform infrared spectroscopy (FTIR; Nicolet iS50, Thermo, Waltham, MA, USA) was utilized to characterize the functional groups of the PVDF membrane and the PVDF-CM. Four-probe electrical resistance meter (HPS2523, HAIERPA, Changzhou, China) was used to measure the electrical resistance of the conductive membranes, with each membrane measured more than 10 times and the average values reported as the final results. The static water contact angle of pure water on both the PVDF and the PVDF-CM was determined using a contact angle goniometer (SL200A, Kino, Shanghai, China), with each membrane measured three times, and the mean value was reported as the final result. A liquid zeta potential meter (ZS90, Malvern, Malvern City, Worcs, UK) was used to measure the zeta potential of model pollutants. An electrochemical workstation (PAR2273, Ametec, San Diego, CA, USA) was used to characterize the electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) curves of the conductive membrane, with an electrolyte of 0.1 mol/L NaCl solution, a reference electrode of Ag/AgCl, and a counter electrode of platinum electrode. EIS parameters were listed as follows: amplitude of 5 mV and frequency ranging from 0.01 to 100,000 Hz. CV parameters were shown below: a scan range of −2 to 2 V and a scan speed of 0.05 V/s for 4 cycles.

2.3. Anti-Fouling Performance of Conductive Membranes

For the pure water flux measurement, the pump speed was adjusted from low to high levels, and the transmembrane pressure (TMP) was recorded at different pump speeds. Pure water was filtered through the membrane for 4 min, followed by a 1 min pause. Permeate volume under each pump speed was measured three times with the average value used, and Equation (2) was employed to calculate the flux.
J = V A t
where J is the water flux (LMH), V is the volume of the permeate for a certain measurement time (L), A is the membrane area (m2), and t is the measurement time (h).
A membrane filtration device was used to assess the filtration performance of conductive membranes, as shown in Figure 1. Influent feeding and effluent extraction were achieved with the assistance of two peristaltic pumps. The TMP evolution during the operation was recorded based on an online pressure sensor. The effective filtration area of the flat-sheet PVDF-CM module is 40 cm2 in total. Conductive membranes were responsible for the cathodes and graphite plates were used as the anodes, with a distance of 1.5 cm between them. A DC power supply (eTM-305P, TONGMENG, Dongguan, China) was used to generate electrical potentials of 0 V, 0.5 V, 1.0 V, and 2.0 V for the experiment. BSA, SA, and HA were selected as the model pollutants at a concentration of 100 mg/L. The pH values of BSA and SA solutions prepared are in a range of 6.5 to 7.5. The preparation of HA solution involved stirring it magnetically for 24 h in a 0.01 mol/L NaOH solution, and then diluting it with a 0.01 mol/L HCl solution to reach pH 7. The solution temperature was maintained at 25 °C. In the batch filtration experiments, the initial flux was set to 36 LMH, effluent samples were taken every 20 min, and the concentrations of polysaccharides, proteins, and humic substances were analyzed using the anthrone-sulfuric acid method [35], the Folin–Lowry method [36], and UV254 absorbance measurement, respectively.
The permeability of the membranes was used to assess their filtration performance, which was calculated based on Equation (3):
Permeability = Flux/TMP
where Flux and TMP is the flux (LMH) and transmembrane pressure (bar) in membrane filtration, with the unit of the permeability expressed as LMH/bar.
The rejection performance of the membranes for BSA, SA, and HA filtration was calculated using Equation (4):
R = (1 − cb/ca) × 100%
where R is the rejection rate (%) and cb and ca are the concentrations of pollutants in the permeate and feed, respectively (mg/L).

3. Results and Discussion

3.1. Optimization of Conductive Membrane Preparation Methods

3.1.1. Regression Model and Variance Analysis

The RSM was utilized to optimize experimental conditions, facilitating the selection of the best experimental scheme. A total of 17 designed experiments were conducted in no fixed order to reduce external influences on the experimental results. Table 3 summarizes the 17 experimental schemes and the response values of the conductive membrane.
To obtain an accurate and effective mathematical model, multi-variable nonlinear fitting analysis was performed based on the results from Table 3. The variance analysis for the regression model is listed in Table S1. Additionally, a multiple regression fitting analysis was conducted to establish second-order polynomial regression equations for the target functions of electrical resistance (Y1) and water contact angle (Y2) in relation to the variables, namely the concentrations of pyrrole (A), BVIMBF4 (B), and FeCl3·6H2O (C), as shown in Equations (5) and (6).
Y1 = 431.47 − 214.13A −
+ 263.56B − 1517.53C − 19.19AB + 257.07AC − 163.88BC + 96.40A2 + 283.71B2 + 1236.42C2
(R12 = 0.9907)
Y2 = 38.97 − 6.68A − 24.44B − 7.08C + 1.46AB + 8.01AC + 0.43BC + 7.71A2 − 19.43B2 + 3.88C2
(R22 = 0.8276)
where Y1 is electrical resistance (Ω/sq), Y2 is the water contact angle (°), and A, B, and C are the encoded forms of the variables, displayed in singular form to show the influence of each factor. The interaction terms of AB, AC, and BC represent the interactions between the variables, while A2, B2, and C2 denote the squared terms of each factor [37]. The corrected R2 values for the average values predicted by Equations (5) and (6) are 0.98 and 0.61, respectively, whereas the R2 values of the models are 0.99 and 0.83, as shown in Figure 2. The signal-to-noise ratios for Y1 and Y2 models are 24.59 and 8.70, respectively, both greater than 4, indicating that the model has accurate predictive capabilities for the experimental data [38,39].
A smaller probability value (p value) can indicate the significance of the corresponding coefficients [40]. From Table S1, the ANOVA shows that the p-values for both Y1 and Y2 models are less than 0.05, indicating that the RSM model analysis is statistically significant. The lack of fit for Y1 and Y2 is 0.22 and 0.91, respectively, both greater than 0.05 (p > 0.05), indicating that the model can precisely replicate the modification conditions of the conductive membrane [41].

3.1.2. Data Analysis and Optimization

Based on the ANOVA table and model selection, three-dimensional response surface plots were drawn to analyze the interaction effects of pyrrole, BVIMBF4, and FeCl3·6H2O on the electrical resistance and water contact angle of the prepared conductive membrane, as shown in Figure 3. The electrical resistance of the conductive membrane initially decreases and then increases with the addition of BVIMBF4, displaying a parabolic response surface, as shown in Figure 3a. The lowest electrical resistance of 365 Ω/sq is achieved when the BVIMBF4 concentration is 3.2 mmol because the BVIMBF4, which possesses excellent ionic characteristics, provides some proton sources as a green reaction medium with their anions doped into the molecular chains [42]. The larger molecular weight and volume favor the stretching of the polypyrrole molecular chains, thereby improving conductivity. However, excessive addition of BVIMBF4, due to its viscosity, can hinder the diffusion of the pyrrole, leading to reduced polymerization and increased electrical resistance. When the amount of BVIMBF4 is held constant, the electrical resistance exhibits a similar pattern of first declining and then increasing with the addition of pyrrole, with the lowest electrical resistance occurring at a pyrrole concentration of 0.81 mol/L. This is attributed to the increased content of polypyrrole on the membrane surface, enhancing conductivity. However, further increase in pyrrole concentration does not significantly improve conductivity. The electrical resistance also exhibits a tendency of lowering and subsequently increasing with variations in FeCl3·6H2O concentration under constant concentrations of pyrrole and BVIMBF4, reaching a minimum electrical resistance of 196 Ω/sq near 0.7 mol/L.
From Figure 3b, it can be observed that the water contact angle of the conductive membrane decreases overall with the increase in BVIMBF4. A higher BVIMBF4 content results in a greater quantity of BVIMBF4 doping into the pyrrole, which improves the membranes’ hydrophilicity and lowers the water contact angle. The effects of pyrrole and FeCl3·6H2O concentrations on the water contact angle seem to be minimal, resulting in a relatively flat response surface. This is due to a layer of polypyrrole adhering to the membrane surface, and when the concentration exceeds a certain level, it has little impact on the hydrophilicity of the membrane.
Using the Design-Expert software (12.0.3.0) for data processing and analysis, the significance of the three factors on electrical resistance and water contact angle as well as the application of the conductive membrane in EMBR was evaluated, resulting in the optimal design for preparing conductive membranes, which maintains high conductivity while achieving high hydrophilicity. The specific results are shown in Figure S1.
According to the optimized predictions, the best conditions for preparing the conductive membrane are pyrrole, BVIMBF4, and FeCl3·6H2O concentrations of 0.84 mol/L, 0.87 mol/L, and 4.84 mmol, which yield an electrical resistance of 73.17 Ω/sq and a water contact angle of 23.08°. To facilitate experimental manipulation, pyrrole, BVIMBF4, and FeCl3·6H2O concentrations of 0.90 mol/L, 4.80 mmol, and 0.80 mol/L were selected. To verify that the optimal values of the model are accurate, three sets of validation experiments were conducted under these experimental conditions, with results listed in Table 4. In the actual experiments, the electrical resistance and water contact angle of the conductive membrane were 93 Ω/sq and 31 °, respectively, which are in good accord with the theoretical predictions of the model. Therefore, it can be concluded that the best conditions obtained through response surface optimization are highly reliable and have practical reference value.

3.2. Performance of the Conductive Membranes

3.2.1. Permeability and Physicochemical Properties

The PVDF-CM prepared by in situ oxidative polymerization showed a notable visual change. The surface color of the PVDF-CM changed from white to a very deep and uniform black, indicating that polypyrrole has polymerized on the surface of PVDF membrane. Figure 4a presents FTIR analysis of the PVDF membrane and PVDF-CM. Compared to the PVDF membrane, PVDF-CM exhibits an absorption peak at 1711 cm−1 corresponding to the N-H bond on the pyrrole ring, and a peak at 1238 cm−1 for the stretching vibrations of C-N in the pyrrole ring. The range of 900 to 650 cm−1 shows out-of-plane bending vibrations of the N-H bond. The absorption peak for the C=N bond of the imidazole ring in the BVIMBF4 appears between 1711 and 1238 cm−1. Most of the absorption peaks between 1000 and 500 cm−1 are due to various in-plane bending vibrations of C-H in the BVIMBF4, while the peak at 1016 cm−1 corresponds to the stretching vibrations of the B-F bond.
Figure 4b illustrates the water contact angles of the PVDF membrane and PVDF-CM, examining the hydrophilicity of the membranes. When the water contact angle is less than 90°, it means that the material is hydrophilic, with a smaller contact angle reflecting greater hydrophilicity. The water contact angle of the PVDF membrane is 76.17°, while that of the PVDF-CM is 31°. Polymerization of polypyrrole on the membrane surface has been shown to enhance the hydrophilicity of PVDF-CM. Current understanding of the hydrophilicity of membrane bioreactor (MBR) membranes suggested that membranes with better hydrophilicity exhibit better anti-fouling capability [43].
To investigate the formation and adhesion of the polymers on membrane surface, a microscopic comparative analysis of the surface morphology of PVDF membrane and PVDF-CM was conducted. Figure 4c shows that the PVDF membrane has a relatively rough and uneven surface, with a fairly uniform pore structure. In contrast, Figure 4d shows that PVDF-CM exhibits a relatively uniform microstructure, indicating that the addition of polypyrrole has influenced the membrane morphology, leading to the formation of coral-like clusters on the membrane surface with some pore coverage. The pure water flux tests (Figure 5) showing a decrease in membrane permeability confirm this finding. The presence of C, F, and O elements was found in both PVDF membrane and PVDF-CM after analyzing the element contents using EDX. However, due to the deposition and adhesion of polypyrrole and minor residuals on the membrane surface, elements N and Cl were found to be increased in PVDF-CM, while the reduction in C and F contents was attributed to the deposition of polypyrrole.
Figure 5 shows the relationship between TMP and pure water flux for PVDF membrane and PVDF-CM. Within a certain flux range, the permeability of pristine PVDF membrane is between 1703.02 and 2383.67 LMH/bar, with a fitted average value of 2118.13 LMH/bar. In contrast, the permeability of the PVDF-CM ranges from 591 to 1088.62 LMH/bar, with a fitted mean value of 832.29 LMH/bar. It indicates that the permeability decreases by 60.7% after membrane modification. This reduction is primarily due to the dense layer of polypyrrole formed on the PVDF membrane surface, which may partially block the membrane pores and lead to decreased permeability [44,45].

3.2.2. Electrochemical Performance of Conductive Membranes

Figure 6a shows the change in electrical resistance of the PVDF-CM in air over 30 d. The initial electrical resistance was 93 Ω/sq, and with prolonged exposure time the electrical resistance gradually increased, indicating a decrease in conductivity. After 30 days, the electrical resistance reached 138.7 Ω/sq, but the conductive membrane still exhibited good conductivity compared with previous studies [32,46,47].
Figure 6b,c display the EIS (Nyquist plots) and CV curves of the PVDF membrane and PVDF-CM, respectively. The impedance of the PVDF membrane shows a semicircular arc in the low-frequency range (Figure 6b), demonstrating that electron transfer is hindered due to the non-conductive nature of the PVDF material. On the contrary, the impedance of the PVDF-CM approaches a linear trend in the low-frequency region, indicating a significant enhancement in the electron transfer rate at the membrane interface due to the attachment of a conductive polypyrrole layer on the membrane surface, which facilitates electron transfer [48].
Figure 6c illustrates the CV curves of the membranes, showcasing the redox characteristics before and after conductive modification. Within the scanning range, the CV curve of PVDF membrane appear flattened and nearly closed, with a current range of -0.4 to 0.6 mA at the same voltage, indicating stable redox characteristics of the PVDF membrane. On the other hand, the CV curve of the PVDF-CM is broader compared to that of the PVDF membrane, indicating that no significant electrochemical reactions occurred within the scanning range of the conductive material in the membrane. This suggests that the PVDF-CM has stable oxidative current characteristics, with a current range of −0.9 to 1.0 mA, demonstrating a higher current density than the PVDF membrane, which is in line with EIS results [49]. Therefore, the PVDF-CM prepared with polypyrrole as the conductive layer demonstrated good stability, conductivity, and relatively stable redox characteristics.

3.3. Anti-Fouling Performance of Conductive Membranes

The experimental results of the PVDF membrane and PVDF-CM in BSA filtration are shown in Figure 7a at a constant flux filtration mode (36 LMH). The changes in TMP and membrane permeability over filtration time (4 h) are illustrated in Figure 7a. During the 4 h filtration, the TMP of the PVDF membrane remained stable at around 1.7 kPa. When applying external voltages of 0V, 0.5 V, 1.0 V, and 2.0 V, the TMP growth rates for PVDF-CM were 8.35, 8.55, 7.43, and 6.87 kPa/h, respectively. The fouling rate of the modified membrane increased mainly due to the attachment of a polypyrrole layer, which partially blocked the membrane pores and reduced permeability. After applying a 2.0 V electric field, the fouling rate of the PVDF-CM was the lowest, decreasing by 17.72% compared to 0 V.
Figure 8a provides SEM images supporting this observation, showing that the surface of the PVDF membrane had fewer attached foulants, while the PVDF-CM at 0 V had a denser accumulation of contaminants. This phenomenon is likely caused by the changes in surface morphology, structure, and porosity before and after membrane modification. After applying an external electric field, the contaminants are more loosely attached and less dense on the membrane surface of the PVDF-CM, primarily due to the negatively charged protein molecules, which enhanced the electrostatic repulsive force between the membrane and the contaminants as the voltage increases within a certain range. This change in movement direction and material morphology effectively reduced the accumulation of contaminants on the membrane surface and within its pores [50]. However, at 0.5 V, the fouling rate slightly increased, possibly because of the accumulation of contaminants around the membrane at low potential. Previous research showed that contaminants are typically negatively charged, and applying a negative potential on the conductive membrane is beneficial for controlling membrane fouling [51,52]. However, in this study, the conductive membrane acted as a cathode, and the pollution worsened when applying 0.5 V, challenging the conventional understanding that negative potentials alleviate membrane fouling. Similar observations were noted by Ying et al. in an AnEMBR study, showing that applying 0.5 V to a conductive ceramic membrane also led to increased fouling [10].
The retention performance of the membranes is illustrated in Figure 7a3. PVDF exhibits almost no retention effect on proteins, with the retention rate of PVDF-CM at 0 V being relatively low (approximately 25.7%). However, with the application of external voltage, the initial retention rate during filtration was higher, subsequently declining but still remaining at a relatively high level. The average retention rates of proteins by the PVDF-CM at voltages of 0 V, 0.5 V, 1.0 V, and 2.0 V were 28.90%, 46.05%, 40.72%, and 36.01%, respectively. The membrane surface with a dense fouling layer effectively retains pollutants, leading to its highest retention rate at 0.5 V.
The experimental results of the PVDF membrane and PVDF-CM in SA filtration are shown in Figure 7b. At 0 V, PVDF-CM showed a rapid increase in TMP during the first 2 h of filtration, but after 4 h, the TMP became similar to that of the PVDF membrane, with a membrane fouling rate of 8.15 kPa/h. After applying external voltage, the TMP growth rates for the PVDF-CM at 0.5 V, 1.0 V, and 2.0 V were 6.90, 7.35, and 6.75 kPa/h, respectively, low compared to both PVDF and PVDF-CM at 0 V. As the voltage increased, membrane fouling rate declined more significantly, primarily because the pollutants carry a negative charge. The increased voltage enhances the electrostatic repulsive force between the membrane and the pollutants. When applying 2.0 V, the membrane fouling rate decreased by 17.18% compared to 0 V. From the SEM images in Figure 8b, it can be observed that the fouling layer with adhered pollutants on the surface of PVDF membrane and PVDF-CM at 0 V was denser, while the accumulated pollutants were obviously reduced after applying external voltage. The retention performance of the membranes is shown in Figure 7b3. The average retention rate of PVDF-CM at 0 V is 90.56%, while that of PVDF membrane is 78.50%. However, after applying external voltage, the retention rates of PVDF-CM for SA decreased. The average retention rates at external voltages of 0.5 V, 1.0 V, and 2.0 V were 57.99%, 78.52%, and 60.63%, respectively. The decrease in the retention effect after applying an external electric field may be attributed to the direct oxidation on the anode surface by hydroxyl radicals generated through physical adsorption or active oxygen through chemical adsorption, as well as the indirect oxidation by oxidants produced at the anode that diffuse into the solution [53]. This process can oxidize large molecular SA into smaller molecules (or degradation products), making it easier for them to transfer within the well-mixed solution and to pass through the membrane pores, resulting in a higher concentration of polysaccharides in the permeate.
The experimental results of the PVDF membrane and PVDF-CM in HA filtration are presented in Figure 7c. The TMP growth rate for PVDF membrane is 1.23 kPa/h, while the TMP growth rates for PVDF-CM at external voltages of 0 V, 0.5 V, 1.0 V, and 2.0 V are 5.83, 1.40, 4.85, and 6.98 kPa/h, respectively. Similar to BSA filtration, the PVDF membrane has fewer contaminants attached to its surface, whereas the PVDF-CM membrane has a denser layer of contaminants. This is primarily due to changes in the surface morphology, structure, and porosity of the modified membrane. At 0.5 V, the membrane fouling rate is similar to that of PVDF membrane, which represents a reduction of 76% compared to 0 V. However, at 2.0 V, the membrane fouling rate increases. It is reported that HA molecules easily undergo dipolarization under a higher electric field, causing the distribution of positive and negative charges within the molecules to become polarized at both ends. This leads to aggregation and a stronger affinity between the molecules and the membrane [54]. The SEM images in Figure 8c help explain this phenomenon; at 0.5 V, the membrane surface has a fouling layer that is looser than at 0 V, while the fouling layer at 2.0 V is denser. The membrane retention performance is shown in Figure 7c3. The average retention rate of PVDF membrane is 23.51%. The average retention rates of PVDF-CM at external voltages of 0 − 2.0 V are 83.14%, 62.80%, 78.65%, and 84.21%, respectively. After conductive modification, the PVDF-CM exhibits improved retention performance for HA pollutants, although retention performance decreases with the application of voltage. As the external voltage increases, the retention performance of the membrane gradually improves.

3.4. Implication of This Study

In this study, the preparation of the PVDF-CM was accomplished via an in situ oxidative polymerization approach, achieving both good conductivity (93 Ω/sq) and hydrophilicity (water contact angle of 31°) under optimized electro-conductive modification conditions. PVDF-CM demonstrates excellent anti-fouling and retention capabilities assisted by a low-voltage electric field. Moreover, the specific concentrations of pyrrole, BVIMBF4, and FeCl3·6H2O used during the synthesis highlight the importance of material composition in the performance of conductive polymers. This may lead to further optimization of the desired electrochemical and physical properties of the materials, providing a reference for further exploration of the synthesis and applications of conductive polymers.
It is worth noting that the modified conductive membrane exhibits a phenomenon of “permeability–selectivity trade-off”, where the retention performance of the conductive membrane has improved, but at the cost of reducing permeation performance substantially. In future research, it is considered to add pore-forming agents when preparing the conductive membrane aiming to increase membrane porosity and pore size and enhance membrane permeability [55,56]. In this study, the antifouling properties of the cathodic conductive membrane were explored, while it is of great interest to apply the conductive membrane as an anode to verify the anodic oxidation characteristics in mitigating membrane fouling and prolonging filtration duration [13], thereby further reducing the costs associated with membrane chemical cleaning practice.

4. Conclusions

In this study, an in situ oxidative polymerization approach was adopted to prepare PVDF-CM, and the RSM method was applied for optimizing the modification conditions. The prepared PVDF-CM exhibited favorable properties in terms of conductivity, permeability, stability, electrochemical characteristics, as well as anti-fouling performance. The following are the main conclusions:
(1)
After optimization, the best conditions for preparing PVDF-CM were pyrrole concentration of 0.9 mol/L, BVIMBF4 concentration of 4.80 mmol, and FeCl3·6H2O concentration of 0.80 mol/L. The optimized PVDF-CM exhibited an electrical resistance of 93 Ω/sq and a water contact angle of 31°.
(2)
The permeability of the control PVDF membrane was 2118.13 LMH/bar, while the permeability of PVDF-CM decreased to 832.29 LMH/bar, indicating a substantial reduction in permeability after modification.
(3)
During batch filtration experiments, the application of an electric field at 2 V significantly reduced membrane fouling rates for BSA and SA, with a reduction of 17.72% and 17.18%, respectively. For HA, the membrane fouling rate decreased by 72.6% at 0.5 V.
In the future, the trade-off between permeability and selectivity of conductive membranes needs to be further investigated, and incorporating porogenic materials while ensuring that membrane conductivity and stability remain high can be a promising strategy.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/membranes15010001/s1, Figure S1: The optimal reaction conditions; Figure S2: The zeta potential of model pollutants.; Table S1: ANOVA for the response surface of electrical resistance and contact angle.

Author Contributions

Investigation, data curation, writing—original draft preparation, J.S.; funding acquisition, supervision, writing—review and editing, Y.H.; writing—review and editing, S.L.; writing—review and editing, W.X.; writing—review and editing, Y.Y.; supervision, writing—review and editing, J.J. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by National Key Research and Development Program of China (grant no. 2022YFC3203102), and Xi’an University of Architecture and Technology Special Program for Cultivation of Frontier Interdisciplinary Fields (grant no. X20240113).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

Jiayuan Ji acknowledges the support from Hirose Foundation.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Bai, L.M.; Wu, H.Y.; Ding, J.W.; Ding, A.; Zhang, X.Y.; Ren, N.Q.; Li, G.B.; Liang, H. Cellulose nanocrystal-blended polyethersulfone membranes for enhanced removal of natural organic matter and alleviation of membrane fouling. Chem. Eng. J. 2020, 382, 11. [Google Scholar] [CrossRef]
  2. Pan, Z.L.; Song, C.W.; Li, L.; Wang, H.; Pan, Y.Q.; Wan, C.L.; Li, J.X.; Wang, T.H.; Feng, X.S. Membrane technology coupled with electrochemical advanced oxidation processes for organic wastewater treatment: Recent advances and future prospects. Chem. Eng. J. 2019, 376, 19. [Google Scholar] [CrossRef]
  3. Efligenir, A.; Déon, S.; Fievet, P.; Druart, C.; Morin-Crini, N.; Crini, G. Decontamination of polluted discharge waters from surface treatment industries by pressure-driven membranes: Removal performances and environmental impact. Chem. Eng. J. 2014, 258, 309–319. [Google Scholar] [CrossRef]
  4. Xu, R.; Qin, W.; Zhang, B.; Wang, X.M.; Li, T.Y.; Zhang, Y.; Wen, X.H. Nanofiltration in pilot scale for wastewater reclamation: Long-term performance and membrane biofouling characteristics. Chem. Eng. J. 2020, 395, 9. [Google Scholar] [CrossRef]
  5. Shahid, M.K.; Kashif, A.; Rout, P.R.; Aslam, M.; Fuwad, A.; Choi, Y.; Banu, R.; Park, J.H.; Kumar, G. A brief review of anaerobic membrane bioreactors emphasizing recent advancements, fouling issues and future perspectives. J. Environ. Manag. 2020, 270, 12. [Google Scholar] [CrossRef]
  6. Krzeminski, P.; Leverette, L.; Malamis, S.; Katsou, E. Membrane bioreactors—A review on recent developments in energy reduction, fouling control, novel configurations, LCA and market prospects. J. Membr. Sci. 2016, 527, 207–227. [Google Scholar] [CrossRef]
  7. Arvind, K.C.A.; Barbara, B.; Schneider, E.S.; Tomi, M.; Mathias, E.; Volkan, F.; Sarah, G. Modification of Polyacrylonitrile Ultrafiltration Membranes to Enhance the Adsorption of Cations and Anions. Membranes 2022, 12, 580. [Google Scholar] [CrossRef] [PubMed]
  8. Liao, X.J.; Lim, Y.J.; Khayet, M.; Liao, Y.; Yao, L.; Zhao, Y.L.; Razaqpur, A.G. Applications of electrically conductive membranes in water treatment via membrane distillation: Joule heating, membrane fouling/scaling/wetting mitigation and monitoring. Water Res. 2023, 244, 22. [Google Scholar] [CrossRef] [PubMed]
  9. Zhang, Z.X.; Huang, G.H.; Li, Y.P.; Chen, X.J.; Yao, Y.; Ren, S.J.; Li, M.N.; Wu, Y.W.; An, C.J. Electrically conductive inorganic membranes: A review on principles, characteristics and applications. Chem. Eng. J. 2022, 427, 19. [Google Scholar] [CrossRef]
  10. Ying, X.B.; Huang, J.J.; Shen, D.S.; Feng, H.J.; Jia, Y.F.; Guo, Q.Q. Fouling behaviors are different at various negative potentials in electrochemical anaerobic membrane bioreactors with conductive ceramic membranes. Sci. Total Environ. 2021, 761, 10. [Google Scholar] [CrossRef] [PubMed]
  11. Liu, L.F.; Zhao, F.; Liu, J.D.; Yang, F.L. Preparation of highly conductive cathodic membrane with graphene (oxide)/PPy and the membrane antifouling property in filtrating yeast suspensions in EMBR. J. Membr. Sci. 2013, 437, 99–107. [Google Scholar] [CrossRef]
  12. Fan, X.F.; Zhao, H.M.; Quan, X.; Liu, Y.M.; Chen, S. Nanocarbon-based membrane filtration integrated with electric field driving for effective membrane fouling mitigation. Water Res. 2016, 88, 285–292. [Google Scholar] [CrossRef] [PubMed]
  13. Yu, Q.L.; Zhang, Y.B. Fouling-resistant biofilter of an anaerobic electrochemical membrane reactor. Nat. Commun. 2019, 10, 10. [Google Scholar] [CrossRef]
  14. Hu, Y.S.; Wang, J.S.; Shi, J.Z.; Yang, Y.; Ji, J.Y.; Chen, R. A review of electro-conductive membrane enabled electrochemical anaerobic membrane bioreactor process for low-carbon wastewater treatment. J. Environ. Chem. Eng. 2024, 12, 14. [Google Scholar] [CrossRef]
  15. Roy, D.; Drogui, P.; Rahni, M.; Lemay, J.F.; Tyagi, R.D. Effect of cathode material and charge loading on the nitrification performance and bacterial community in leachate treating Electro-MBRs. Water Res. 2020, 182, 115990. [Google Scholar] [CrossRef] [PubMed]
  16. Shakeri, A.; Salehi, H.; Rastgar, M. Antifouling electrically conductive membrane for forward osmosis prepared by polyaniline/graphene nanocomposite. J. Water Process Eng. 2019, 32, 100932. [Google Scholar] [CrossRef]
  17. Xing, C.Y.; Zhao, M.M.; Zhao, L.P.; You, J.C.; Cao, X.J.; Li, Y.J. Ionic liquid modified poly(vinylidene fluoride): Crystalline structures, miscibility, and physical properties. Polym. Chem. 2013, 4, 5726–5734. [Google Scholar] [CrossRef]
  18. Song, H.C.; Shao, J.H.; He, Y.L.; Liu, B.; Zhong, X.Q. Natural organic matter removal and flux decline with PEG-TiO2-doped PVDF membranes by integration of ultrafiltration with photocatalysis. J. Membr. Sci. 2012, 405, 48–56. [Google Scholar] [CrossRef]
  19. Radjenovic, J.; Sedlak, D.L. Challenges and Opportunities for Electrochemical Processes as Next-Generation Technologies for the Treatment of Contaminated Water. Environ. Sci. Technol. 2015, 49, 11292–11302. [Google Scholar] [CrossRef]
  20. Zhao, K.; Su, F.; Gu, K.H.; Qi, J.; Liu, R.P.; Hu, C.Z. Antifouling potential and microbial characterization of an electrochemical anaerobic membrane bioreactor utilizing membrane cathode and iron anode. Bioresour. Technol. 2021, 334, 8. [Google Scholar] [CrossRef]
  21. Jia, T.Z.; Feng, R.; Cui, C.; Chen, Q.; Cseri, L.; Zhou, R.F.; Szekely, G.; Cao, X.L.; Sun, S.P. Conductive nanofiltration membranes via in situ PEDOT-polymerization for electro-assisted membrane fouling mitigation. Water Res. 2024, 252, 10. [Google Scholar] [CrossRef]
  22. Sui, J.; Travas-Sejdic, J.; Chu, S.Y.; Li, K.C.; Kilmartin, P.A. The actuation behavior and stability of p-toluene sulfonate doped polypyrrole films formed at different deposition current densities. J. Appli. Polym. Sci. 2009, 111, 876–882. [Google Scholar] [CrossRef]
  23. Arroyo, J.; Akieh-Pirkanniemi, M.; Lisak, G.; Latonen, R.M.; Bobacka, J. Electrochemically controlled transport of anions across polypyrrole-based membranes. J. Membr. Sci. 2019, 581, 50–57. [Google Scholar] [CrossRef]
  24. Zhou, C.; Zhang, Y.W.; Li, Y.Y.; Liu, J.P. Construction of High-Capacitance 3D CoO@Polypyrrole Nanowire Array Electrode for Aqueous Asymmetric Supercapacitor. Nano Lett. 2013, 13, 2078–2085. [Google Scholar] [CrossRef]
  25. Chen, J.S.; Liu, J.F.; Thundat, T.; Zeng, H.B. Polypyrrole-Doped Conductive Supramolecular Elastomer with Stretchability, Rapid Self-Healing, and Adhesive Property for Flexible Electronic Sensors. ACS Appl. Mater. Interfaces 2019, 11, 18720–18729. [Google Scholar] [CrossRef] [PubMed]
  26. Park, S.J.; Cho, M.S.; Nam, J.D.; Kim, I.H.; Choi, H.R.; Koo, J.C.; Lee, Y. The linear stretching actuation behavior of polypyrrole nanorod in AAO template. Sens. Actuat. B-Chem. 2009, 135, 592–596. [Google Scholar] [CrossRef]
  27. Long, Y.Z.; Li, M.M.; Gu, C.Z.; Wan, M.X.; Duvail, J.L.; Liu, Z.W.; Fan, Z.Y. Recent advances in synthesis, physical properties and applications of conducting polymer nanotubes and nanofibers. Prog. Polym. Sci. 2011, 36, 1415–1442. [Google Scholar] [CrossRef]
  28. Wang, L.X.; Li, X.G.; Yang, Y.L. Preparation, properties and applications of polypyrroles. React. Funct. Poly. 2001, 47, 125–139. [Google Scholar] [CrossRef]
  29. Benson, J.; Kovalenko, I.; Boukhalfa, S.; Lashmore, D.; Sanghadasa, M.; Yushin, G. Multifunctional CNT-Polymer Composites for Ultra-Tough Structural Supercapacitors and Desalination Devices. Adv. Mater. 2013, 25, 6625–6632. [Google Scholar] [CrossRef] [PubMed]
  30. Dudchenko, A.V.; Rolf, J.; Russell, K.; Duan, W.Y.; Jassby, D. Organic fouling inhibition on electrically conducting carbon nanotube-polyvinyl alcohol composite ultrafiltration membranes. J. Membr. Sci. 2014, 468, 1–10. [Google Scholar] [CrossRef]
  31. Ahmed, F.; Lalia, B.S.; Kochkodan, V.; Hilal, N.; Hashaikeh, R. Electrically conductive polymeric membranes for fouling prevention and detection: A review. Desalination 2016, 391, 1–15. [Google Scholar] [CrossRef]
  32. Shen, Q.; Xing, D.Y.; Sun, F.Y.; Dong, W.Y.; Shang, W.T.; Zhao, L.Y.; Dong, Z.J. Facile pyrrole oxidative polymerization with ionic liquid to modify ceramic membrane for sustainable fouling control. J. Environ. Chem. Eng. 2021, 9, 9. [Google Scholar] [CrossRef]
  33. Choi, J.; Cho, H.; Choi, Y.; Lee, S. Combination of computational fluid dynamics and design of experiments to optimize modules for direct contact membrane distillation. Desalination 2022, 524, 15. [Google Scholar] [CrossRef]
  34. Liu HLChiou, Y.R. Optimal decolorization efficiency of Reactive Red 239 by UV/ZnO photocatalytic process. J. Chin. Inst. Chem. Eng. 2006, 37, 289–298. [Google Scholar] [CrossRef]
  35. Gaudy, A.F. Colorimetric Determination of Protein and Carbohydrate. Ind. Water Wastes 1962, 7, 17–22. [Google Scholar]
  36. The State Environmental Protection Administration the Water and Wastewater Monitoring Analysis Method Editorial Board. Water and Wastewater Monitoring Analysis Method, 4th ed.; China Environmental Science Press: Beijing, China, 2002. [Google Scholar]
  37. Jankovic, A.; Chaudhary, G.; Goia, F. Designing the design of experiments (DOE)-An investigation on the influence of different factorial designs on the characterization of complex systems. Energy Build. 2021, 250, 17. [Google Scholar] [CrossRef]
  38. Jang, H.; Park, S.J.; Kim, J. Response surface methodology to investigate the effects of operational parameters on membrane fouling and organic matter rejection in hard-shell encased hollow-fiber membrane. Chemosphere 2022, 287, 9. [Google Scholar] [CrossRef]
  39. Tibi, F.; Park, S.J.; Kim, J. Improvement of Membrane Distillation Using PVDF Membrane Incorporated with TiO2 Modified by Silane and Optimization of Fabricating Conditions. Membranes 2021, 11, 18. [Google Scholar] [CrossRef]
  40. Milano, J.; Ong, H.C.; Masjuki, H.H.; Silitonga, A.S.; Chen, W.H.; Kusumo, F.; Dharma, S.; Sebayang, A.H. Optimization of biodiesel production by microwave irradiation-assisted transesterification for waste cooking oil-Calophyllum inophyllum oil via response surface methodology. Energy Conv. Manag. 2018, 158, 400–415. [Google Scholar] [CrossRef]
  41. Noshadi, I.; Amin, N.A.S.; Parnas, R.S. Continuous production of biodiesel from waste cooking oil in a reactive distillation column catalyzed by solid heteropolyacid: Optimization using response surface methodology (RSM). Fuel 2012, 94, 156–164. [Google Scholar] [CrossRef]
  42. Ni, C.J.; Zheng, X.; Zhang, Y.; Zhang, X.W.; Li, Y.J. Multifunctional porous materials with simultaneous high water flux, antifouling and antibacterial performances from ionic liquid grafted polyethersulfone. Polymer 2021, 212, 10. [Google Scholar] [CrossRef]
  43. Meng, F.; Chae, S.-R.; Drews, A.; Kraume, M.; Shin, H.-S.; Yang, F. Recent advances in membrane bioreactors (MBRs): Membrane fouling and membrane material. Water Res. 2009, 43, 1489–1512. [Google Scholar] [CrossRef]
  44. Salehi, E.; Madaeni, S. Influence of conductive surface on adsorption behavior of ultrafiltration membrane. Appl. Surf. Sci. 2010, 256, 3010–3017. [Google Scholar] [CrossRef]
  45. Sun, J.Q.; Wang, G.G.; Zhang, H.; Zhang, B.; Hu, C.Z. Facile fabrication of a conductive polypyrrole membrane for anti-fouling enhancement by electrical repulsion and in situ oxidation. Chemosphere 2021, 270, 9. [Google Scholar] [CrossRef] [PubMed]
  46. Liu, H.; Huang, Q.L.; Wang, Y.F.; Xiao, C.F. PTFE conductive membrane for EVMD process and the application of electro-catalysis. Sep. Purif. Technol. 2017, 187, 327–333. [Google Scholar] [CrossRef]
  47. Huang, J.; He, Y.R.; Shen, Z.H. Joule heating membrane distillation enhancement with multi-level thermal concentration and heat recovery. Energy Conv. Manag. 2021, 238, 11. [Google Scholar] [CrossRef]
  48. Meng, Y.T.; Zhang, L.X.; Xing, R.R.; Huang, H.; Qu, Y.; Jiao, T.F.; Zhou, J.X.; Peng, Q.M. Facile preparation and electrochemical characterization of self-assembled core-shell diamond-polypyrrole nanocomposites. Colloid Surf. A-Physicochem. Eng. Asp. 2018, 555, 787–794. [Google Scholar] [CrossRef]
  49. Zhang, Y.Z.; Wang, T.; Meng, J.J.; Lei, J.; Zheng, X.; Wang, Y.F.; Zhang, J.; Cao, X.; Li, X.L.; Qiu, X.P.; et al. A novel conductive composite membrane with polypyrrole (PPy) and stainless-steel mesh: Fabrication, performance, and anti-fouling mechanism. J. Membr. Sci. 2021, 621, 10. [Google Scholar] [CrossRef]
  50. Akamatsu, K.; Yoshida, Y.; Suzaki, T.; Sakai, Y.; Nagamoto, H.; Nakao, S. Development of a membrane-carbon cloth assembly for submerged membrane bioreactors to apply an intermittent electric field for fouling suppression. Sep. Purif. Technol. 2012, 88, 202–207. [Google Scholar] [CrossRef]
  51. Liu, J.D.; Liu, L.F.; Gao, B.; Yang, F.L. Integration of bio-electrochemical cell in membrane bioreactor for membrane cathode fouling reduction through electricity generation. J. Membr. Sci. 2013, 430, 196–202. [Google Scholar] [CrossRef]
  52. Yang, Y.; Qiao, S.; Jin, R.F.; Zhou, J.T.; Quan, X. Novel Anaerobic Electrochemical Membrane Bioreactor with a CNTs Hollow Fiber Membrane Cathode to Mitigate Membrane Fouling and Enhance Energy Recovery. Environ. Sci. Technol. 2019, 53, 1014–1021. [Google Scholar] [CrossRef] [PubMed]
  53. Salvestrini, S.; Fenti, A.; Chianese, S.; Iovino, P.; Musmarra, D. Electro-Oxidation of Humic Acids Using Platinum Electrodes: An Experimental Approach and Kinetic Modelling. Water 2020, 12, 2250. [Google Scholar] [CrossRef]
  54. Hu, C.; Li, M.; Sun, J.; Liu, R.; Liu, H.; Qu, J. NOM fouling resistance in response to electric field during electro-ultrafiltration: Significance of molecular polarity and weight. J. Colloid Interf. Sci. 2019, 539, 11–18. [Google Scholar] [CrossRef] [PubMed]
  55. Aryanti, N.; Nafiunisa, A.; Adina, A.R.; Kusworo, T.D. Synthesis, characterization and anti-fouling properties of poly[vinylidene fluoride]-incorporated SiO2, TiO2, ZrO2 nanoparticle-LiCl pore former ultrafiltration membranes. Case Stud. Chem. Environ. Eng. 2024, 9, 100664. [Google Scholar] [CrossRef]
  56. Ho, C.-C.; Su, J.F. Boosting permeation and separation characteristics of polyethersulfone ultrafiltration membranes by structure modification via dual-PVP pore formers. Polymer 2022, 241, 124560. [Google Scholar] [CrossRef]
Membranes 15 00001 g001
Figure 1. Schematic diagrams of the experimental setup (a); membrane module (b).
Figure 1. Schematic diagrams of the experimental setup (a); membrane module (b).
Membranes 15 00001 g001
Membranes 15 00001 g002
Figure 2. Actual values from experiments and RSM-predicted values for (a) electrical resistance and (b) water contact angle. (note: The color changes in the image represent variations in data values across different areas. Specifically, a gradient from green to yellow to red indicates a decrease in values, while the speed of the color transition reflects changes in slope).
Figure 2. Actual values from experiments and RSM-predicted values for (a) electrical resistance and (b) water contact angle. (note: The color changes in the image represent variations in data values across different areas. Specifically, a gradient from green to yellow to red indicates a decrease in values, while the speed of the color transition reflects changes in slope).
Membranes 15 00001 g002
Membranes 15 00001 g003
Figure 3. RSM diagrams of response values optimized for the PVDF-CM: (a) electrical resistance, (b) water contact angle. (note: The color changes in the image represent variations in data values across different areas. Specifically, a gradient from green to yellow to red indicates a decrease in values, while the speed of the color transition reflects changes in slope).
Figure 3. RSM diagrams of response values optimized for the PVDF-CM: (a) electrical resistance, (b) water contact angle. (note: The color changes in the image represent variations in data values across different areas. Specifically, a gradient from green to yellow to red indicates a decrease in values, while the speed of the color transition reflects changes in slope).
Membranes 15 00001 g003
Membranes 15 00001 g004
Figure 4. Structural and chemical composition analysis of PVDF and PVDF-CM. (a) FTIR, (b) water contact angle; SEM-EDX analysis of the PVDF (c) and PVDF-CM (d).
Figure 4. Structural and chemical composition analysis of PVDF and PVDF-CM. (a) FTIR, (b) water contact angle; SEM-EDX analysis of the PVDF (c) and PVDF-CM (d).
Membranes 15 00001 g004
Membranes 15 00001 g005
Figure 5. The filtration performance of PVDF and PVDF-CM in pure water.
Figure 5. The filtration performance of PVDF and PVDF-CM in pure water.
Membranes 15 00001 g005
Membranes 15 00001 g006
Figure 6. The conductivity of PVDF-CM changes with time in air (a); EIS (b) and CV (c) of PVDF membrane and PVDF-CM.
Figure 6. The conductivity of PVDF-CM changes with time in air (a); EIS (b) and CV (c) of PVDF membrane and PVDF-CM.
Membranes 15 00001 g006
Membranes 15 00001 g007
Figure 7. The variations of TMP, permeability, and pollutants retention with filtration time using PVDF membrane and PVDF-CM under varied external voltages filtering model pollutants: (a) BSA solution: TMP (a1); Permeability (a2); Retention (a3); (b) SA solution: TMP (b1); Permeability (b2); Retention (b3); and (c) HA solution: TMP (c1); Permeability (c2); Retention (c3).
Figure 7. The variations of TMP, permeability, and pollutants retention with filtration time using PVDF membrane and PVDF-CM under varied external voltages filtering model pollutants: (a) BSA solution: TMP (a1); Permeability (a2); Retention (a3); (b) SA solution: TMP (b1); Permeability (b2); Retention (b3); and (c) HA solution: TMP (c1); Permeability (c2); Retention (c3).
Membranes 15 00001 g007
Membranes 15 00001 g008
Figure 8. SEM images of the PVDF membrane and PVDF-CM fouled at different voltages by BSA (a), SA (b) and HA (c).
Figure 8. SEM images of the PVDF membrane and PVDF-CM fouled at different voltages by BSA (a), SA (b) and HA (c).
Membranes 15 00001 g008
Table 1. The main chemicals used in this work.
Table 1. The main chemicals used in this work.
NameMolecular FormulaStructural Formula
PyrroleC4H5NMembranes 15 00001 i001
1-vinyl-3-butylimidazolium tetrafluoroborateC9H5BF4N2Membranes 15 00001 i002
Polypyrrole(C4H5N)nMembranes 15 00001 i003
Table 2. Three-variable and three-level design using response surface methodology.
Table 2. Three-variable and three-level design using response surface methodology.
FactorsParametersUnitsCode
−101
APyrrolemol/L0.60.81.0
BBVIMBF4mmol1.63.24.8
CFeCl3·6H2Omol/L0.20.61.0
Table 3. Experimental design by the RSM-BBD and the obtained results.
Table 3. Experimental design by the RSM-BBD and the obtained results.
Operating FactorsResults
RunA: Pyrrole (mol/L)B: BVIMBF4
(mmol/L)		C: FeCl3·6H2O
(mol/L)		Electrical Resistance (Ω/sq)Water Contact Angle (°)
10.83.20.6594.0755.61
20.83.20.6397.6058.42
30.63.20.23464.6782.66
414.80.6365.4029.97
50.83.20.6304.0035.03
613.21458.0047.80
70.64.80.6884.7340.45
80.83.20.6196.4039.22
90.63.21281.0748.00
1013.20.22613.3350.40
110.61.60.6902.7354.18
1211.60.6444.8039.02
130.83.20.6365.1339.90
140.84.81172.0023.70
150.81.60.23144.6751.14
160.84.80.23572.6733.39
170.81.61268.4040.48
Table 4. A comparison of the optimal conditions for both models and experiments.
Table 4. A comparison of the optimal conditions for both models and experiments.
ParameterPredicted Optimum ValueExperimental Optimum Value
Electrical resistance (Ω/sq)73.1793
Water contact angle (°)23.0831
Pyrrole (mol/L)0.870.9
BVIMBF4 (mmol)4.844.8
FeCl3·6H2O (mol/L)0.840.8
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Shi, J.; Hu, Y.; Li, S.; Xiao, W.; Yang, Y.; Ji, J. Electro-Conductive Modification of Polyvinylidene Fluoride Membrane for Electrified Wastewater Treatment: Optimization and Antifouling Performance. Membranes 2025, 15, 1. https://doi.org/10.3390/membranes15010001

AMA Style

Shi J, Hu Y, Li S, Xiao W, Yang Y, Ji J. Electro-Conductive Modification of Polyvinylidene Fluoride Membrane for Electrified Wastewater Treatment: Optimization and Antifouling Performance. Membranes. 2025; 15(1):1. https://doi.org/10.3390/membranes15010001

Chicago/Turabian Style

Shi, Jinzhuo, Yisong Hu, Songhua Li, Wenqian Xiao, Yuan Yang, and Jiayuan Ji. 2025. "Electro-Conductive Modification of Polyvinylidene Fluoride Membrane for Electrified Wastewater Treatment: Optimization and Antifouling Performance" Membranes 15, no. 1: 1. https://doi.org/10.3390/membranes15010001

APA Style

Shi, J., Hu, Y., Li, S., Xiao, W., Yang, Y., & Ji, J. (2025). Electro-Conductive Modification of Polyvinylidene Fluoride Membrane for Electrified Wastewater Treatment: Optimization and Antifouling Performance. Membranes, 15(1), 1. https://doi.org/10.3390/membranes15010001

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop