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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.31 No.2 pp.101-119
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2021.31.2.101

Recent Progress in Conductive Polymer-based Membranes

Shinyoung Park*, Rajkumar Patel**†
*Nano Science and Engineering, Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, 85 Songdogwahak-ro, Yeonsu-gu, Incheon 03722, South Korea
**Energy and Environmental Science and Engineering (EESE), Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, 85 Songdogwahak-ro, Yeonsu-gu, Incheon 03722, South Korea
†Corresponding author(e-mail: rajkumar@yonsei.ac.kr, http://orcid.org/0000-0002-3820-141X)
March 12, 2021 ; April 22, 2021 ; April 23, 2021

Abstract


The demand for clean water is virtually present in all modern human societies even as our society has developed increasingly more advanced and sophisticated technologies to improve human life. However, as global climate change begins to show more dramatic effects in many regions in the world, the demand for a cheap, effective way to treat wastewater or to remove harmful bacteria, microbes, viruses, and other solvents detrimental to human health has continued to remain present and remains as important as ever. Well-established synthetic membranes composed of polyaniline (PANI), polyvinylidene fluoride (PVDF), and others have been extensively studied to gather information regarding the characteristics and performance of the membrane, but recent studies have shown that making these synthetic membranes conductive to electrical current by doping the membrane with another material or incorporating conductive materials onto the surface of the membrane, such as allotropes of carbon, have shown to increase the performance of these membranes by allowing the adjustability of pore size, improving antifouling and making the antibacterial property better. In this review, modern electrically conductive membranes are compared to conventional membranes and their performance improvements under electric fields are discussed, as well as their potential in water filtration and wastewater treatment applications.


conducting polymer , polypyrrole (PPy) , PANI , PVDF , membrane

전도성 고분자 분리막의 최근 연구동향

박 신 영*, 라즈쿠마 파텔**†
*연세대학교 언더우드국제대학 융합과학공학부 나노과학공학
**연세대학교 언더우드국제대학 융합과학공학부 에너지환경과학공학

초록


깨끗한 물에 대한 수요는 우리 사회가 인간의 삶을 개선하기 위해 점점 더 진보되고 수준 높은 기술을 개발함에도 불구하고 모든 현대 사회에 존재한다. 그러나 지구 기후 변화가 전 세계 여러 지역에서 더욱 극적인 영향을 미치기 시작하면서 폐수를 처리하거나 인체 건강에 해로운 박테리아, 미생물, 바이러스 및 기타 용매를 제거하기 위해 저렴하고 효과적인 방법에 대한 요구가 계속되고 있고 그 어느 때보다 중요하다. 폴리아닐린(PANI), 폴리(비닐리덴 플루오라이드)(PVDF) 등으로 구성 되어 있는 합성막은 잘 구축되어 있고 막의 특성과 성능에 관한 정보를 수집하기 위해 광범위하게 연구되었지만 최근 연구에 따르면 이러한 합성막을 전류에 전도성 있게 만드는 것으로 나타났다. 다른 물질로 막을 도핑하거나 탄소 동소체와 같은 전 도성 물질을 막 표면에 통합함으로써 기공 크기의 조정 가능성, 더 나은 방오성과 항균성을 허용함으로써 이러한 막의 성능 을 증가시키는 것으로 나타났다. 본 총설에서는 현대의 전기 전도성 막을 기존 막과 비교하고 전기장 하에서의 성능 향상과 물 여과 및 폐수 처리 응용 분야에서의 잠재력에 대해 논의한다.


    1. Introduction

    To meet the needs of fresh water free from most contaminants around the globe, scientists have studied synthetic membranes for microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF), as well as for various water treatment applications. Some of the membranes found to be more conventional in this area of application are membranes fabricated from PANI, PDVF, or even polyethersulfone (PES)[1-10]. However, through the fabrication of composite membranes that have electrically conducting material incorporated onto or into the structure of the membrane through chemical polymerization, the performance capacities of these membranes for water filtration were found to be increased because of the potential of using electric charge to negatively charge the membrane or to even adjust the characteristics of the membrane itself. Examples of electrically conductive materials that could be polymerized or attached to the membrane include carbon nanotubes (CNTs), layers of graphene (Gr), and polypyrrole (PPy) nanoparticles[11-20]. Such membrane modifications can be done to both flat sheet and hollow fiber membranes to improve their performance, and it was found that applying a direct current (DC) voltage to these membranes had a significant impact on their performance, allowing for better antifouling and antibacterial properties. Negatively charging the membranes allowed the membrane to repel many contaminants found in the feed solution, many of which are normally negatively charged, leading to lesser levels of fouling. Applying an electrical potential to some membranes have the capability detach bacteria from the surface of the membrane as a result of an increased antibacterial property, whether it be dead or live bacteria, with membrane stability improvements a very real possibility.

    These composite membranes capable of being under an electric field can be fabricated through various means, such as non-solvent induced phase separation, electro-polymerization, vapor phase polymerization, in situ chemical oxidative polymerization, blending, spinning, and more. Both hollow fiber and flat plate membranes can be made to be electrically conductive, and both types of membranes are subject to have their properties and performance altered under certain electrical potentials. In this review article, the fabrication methods, various property changes and performance enhancements of electrically conductive membranes under a given electrical potential are discussed, as well as morphology changes to the membrane in making the membranes electrically conductive. The schematic diagram of a conducting polymer membrane is presented in Fig. 1, and Table 1 summarizes membrane performance.

    2. Conducting Polymer

    2.1. Polypyrrole

    For the purposes of selective separation and the reduction of pore blockage, a membrane made from polypyrrole- dodecylbenezene sulfone (PPy-DBS) was fabricated featuring in situ tuning of pores through the application of external redox potential[21]. The structure of a PPy-DBS membrane is represented in Fig. 2. Vapor phase polymerization of PPy inside pores of a membrane made from polyvinylidene fluoride (PVDF) was used to fabricate the ultrafiltration (UF) membrane that was electrically responsive. Initially, a pristine PVDF membrane was dipped for 5 min in a 50 g/L FeCl3/ ethanol solution, after which underwent gaseous polymerization to form the PPy/DBS membrane. Scanning electron microscopy (SEM) results showed that PPy vapor can infiltrate and polymerize with the PVDF membrane, as the membrane had a 3D network containing many pores and channels for its structure. Polymerization resulted in 3D growth that was cauliflower shaped. The PPy-DBS membrane exhibited pores having diameters of 70~88 nm, with the water flux measuring 158.02 L/m2 h bar. The contact angle of the PPy-DBS membrane was 25.28°, indicating good hydrophilicity, which is good for antifouling. Referring to atomic force microscopy (AFM) observations, there was an increase in the roughness of the surface for the membrane that was reduced, meaning that the surface area increased as well. This occurs as the PPy-DBS membrane is relaxed conformationally as the PPy network accepts Na+ ions into its structure. Conversely, the PPy volume decreases when oxidized as the Na+ ions are removed. The pore size of the membranes can be adjusted by applying an oxidation voltage to enlarge the pores and by applying a reduction voltage to shrink the pore size after enlargement. Furthermore, there was a 21.91% specific flux increase after the application of voltages and backwashing, with great potential for recycling. There was also a decrease of humic acid distribution present in the permeate that shows the improved sieving effect of membrane pores that are smaller when negative voltage is applied. Pore tuning can help with reducing membrane fouling during the cleaning procedure, switching between the macropore mode for cleaning and the micropore mode for filtration. When the PPy-DBS membrane was reduced at -0.7 V then backwashed for 1 h, it had a specific flux that was 21.9% higher compared to the membrane that experienced no reduction. Even after continuous pollution, the membrane’s specific flux was higher by 10% compared to the non-reduced membrane.

    There was an enhancement of antifouling performance of the polyvinylidene fluoride (PVDF) membrane after it was polymerized with PPy through vapor phase polymerization because of four principal reasons[22]. First, there was an increase in hydrophilicity; second, a reduction of membrane roughness; third, better rejection of foulants and better effluent properties because of smaller pore sizes; and fourth, greater membrane surface negative charge to prevent foulants that were negatively charged to be adsorbed onto the surface. After the pristine PVDF membranes were fabricated using the method of phase inversion that was induced by immersion precipitation, a FeCl3 aqueous solution having a 200 g/L concentration was made to act as an oxidant. The membranes were then pretreated with the solution and subsequently dried and then moved inside the polymerization chamber. After a 2 h-long exposure to PPy vapor, vapor-phase polymerization took place in a single- chamber. After washing with tap water, the modified membranes were immersed in sodium dodecyl benzene sulfonate (SDBS) solution (1 g) for a day, after which the membranes were washed with tap water again. The membrane that was modified, or the PPy-SDBS membrane, exhibited a surface that was dark and uniform. SEM and AFM imaging provided observations of the PPy-SDBS membrane having a microstructure containing fissures that was relatively uniform, whereas the pristine PVDF membrane had a surface that was relatively rough and had regions that were not even. A fine particle design for the membrane was shown post-modification, unlike the pristine membrane where uniform pores were observed. The PPy-SDBS membrane had a reduced roughness of 12.68 ± 3.34 nm from 18.23 ± 0.78 nm of the pristine membrane, as well as a lower pure water flux of 569.48 ± 150.82 L/m2 h compared to 3393.26 ± 222.99 L/m2 h for the pristine membrane because of the blockage of pores caused by PPy. As for the contact angle, the PPy-SDBS membrane was much more hydrophilic than the pristine membrane, having an angle of 27.53° compared to 58.63°. The PPy-SBDS membrane exhibited a greater permeate flux of 17.72 L/m2 h than the pristine membrane. In addition, the pristine membrane had a much greater unified membrane fouling index (UMFI), an index used to reflect the fouling rate and cleaning results of membranes, over three cycles, implying greater rapid fouling of the membrane than the modified membrane, which had a slope with a less steep gradient.

    A PVDF membrane was modified with SBDS as the dopant and PPy as the conductive additive for blending modification[23]. The macropores were observed to have reduced in size, and the modified membrane displayed an improved antifouling property and a clearer effluent compared to the unmodified membrane. Moreover, foulant retention improved after the application of a 0.1 V electric field. Through the method of phase inversion that was induced by immersion precipitation, the modified PVDF membrane was fabricated. Referring to surface morphology through SEM imaging, the microstructure with the roughest surface was the membrane having PPy. There was an increase in roughness after the addition of PPy because particles were formed and accumulated on the surface of the membrane. Regarding the pristine membrane, there were many large pores resembling fingers, and these pores had a wide size distribution, with some pores going through the entire cross-sectional area. The PPy membrane had sponge-like pores that were typical along with a minor pore size increase. The studied PPy-SDBS composite membrane also displayed a typical sponge structure. The calculations of average porosity were 47.08% for the pristine membrane, 66.44% for the PPy-SDBS membrane, and 66.44% for the PPy membrane. For zeta potential measurements, as PPy is positively charged and SDBS negatively charged, the positive charge on the surface of the PPy-SDBS membrane went down because of the neutralization effect of the negatively charged SDBS on the positively charged PPy. However, the modified membranes exhibited more of a hydrophobic nature compared to the pristine membrane as observed by the sharp contact angle increase. Looking at UMFI measurements, the PPy-SDBS membrane had a lower UMFI compared to the PPy and pristine membranes, with the pristine membrane having the highest UMFI, but exhibited an even lower slope when an electric field was applied. This implied the slowest fouling condition for the PPy-SDBS membrane under an electric field.

    A hollow fiber membrane made with a polysulfone and PPy blend was fabricated that displayed excellent potential for self-cleaning[24]. PPy has a bactericidal property from its charged backbone, but not many studies reported on PPy blending into polymeric membranes and the potential it shows for antibiofouling and the antibacterial property during separation. PPy loading into the membrane allowed for better membrane permeability, hydrophilicity, tensile strength, and zeta potential. The benefit of this PPy modification to the hollow fiber membrane is that it is scalable and simple to avoid biofouling in the long-term performance of the membrane. PPy particles were prepared by utilizing ferric chloride as an oxidant to polymerize pyrrole. To make the hollow fiber unit, first a polymer dope was formed by dissolving 18 wt% polysulfone in N,N-dimethyl formamide (DMF) at 250 rpm and 60°C for 4 h. Then, 0, 0.25, 0.5, and 1 wt% PPy particle concentrations were incorporated into the polymer dope. The polymer dope was then sonicated for a period of 3 h, allowing the solution to become homogeneous. The hollow fiber was spun at 25°C using DI water as the bore fluid, and the products underwent phase inversion by placing them in DI water standing up for a period of 30 h. Looking at SEM imaging of the hollow fiber cross-section, a spongy layer was observed between macrovoids that appeared from both the inside and outside. As the PPy concentration increased, the width of the finger-like pores increased, giving the fiber a more porous structure. Additionally, the roughness of the surface is very important to the membrane property of anti-biofouling. As seen in Fig. 3, using AFM, a dramatic decrease of surface roughness was seen as PPy concentration increased (120 nm, 0 wt%; 18 nm, 0.25 wt%; 8 nm, 0.5 wt%; 3 nm, 1 wt%). Membrane fouling is promoted when there are valleys and high peaks present on the surface as there is greater space for bacterial accumulation, leading to the clogging of the valleys. As the PPy concentration increases, however, the membranes are seen to be smoother in feature with the gradual disappearance of valleys. At 1 wt% PPy concentration, the hollow fiber membrane exhibited an outstanding antibacterial property as cytoplasmic materials were released from damage to the cell membrane. Additionally, at the same concentration, no viable E. coli or S. aureus bacterial colony was detected, suggesting that this PPy concentration is good enough to reach absolute disinfection.

    There were two nanocomposites that were synthesized chemically through the deposition of PPy onto the raw or oxidized form of multiwalled carbon nanotubes (MWCNTs), called either PPy-raw MWCNTs or PPy-oxidized MWCNTs[25]. MWCNTs have a high surface area and are known to significantly improve permeability in the preparation of membranes. In situ chemical oxidative polymerization was done for their preparation, and varying concentrations of these nanocomposites were applied when fabricating thin film nanocomposite (TFN) membranes for nanofiltration. Membranes with PPy-oxidized MWCNT incorporation were found to have high fluxes, and membranes with PPy-raw MWCNT incorporation revealed better permeation. The oxidation polymerization technique was used for the fabrication MWCNTs nanocomposites coated with PPy. This technique was conducted twice, once with the raw MWCNTs and once with the oxidized MWCNTs. For membrane fabrication, after supports made from polysulfone were prepared, interfacial polymerization was applied to create the PIP polyamide layer. Looking at SEM imaging, there was a great similarity between the two nanocomposites that were fabricated. Membrane surfaces were observed to be nodular, rough, and firmly globular. Both membranes exhibited high Na2SO4 solution rejection percentages of about 96~98%. Adding PPy-MWCNTs to the membrane had the effect of improving rejection efficiency, since the pristine membrane only had 95.7% rejection. For membrane fouling, PPy-oxidized MWCNT membranes exhibited the most fouling with relatively rougher surfaces.

    A variety of membrane characteristics were studied after the successful fabrication of polypyrrole-carbon nanotube/polyethersulfone (PPy-CNT/PES) composite membranes through chemically polymerizing pyrrole onto the substrate of CNT/PES[26]. The new membrane displayed a decreased electric resistance of 450 ± 40 Ω cm-1 with a pure water flux of around 136.2 ± 5.1 L/m2 h bar, while the flux recovery rate showed an increase compared to a PES membrane that was pristine to 76.38% from 68.18%. This flux recovery rate was even increased to 81.78% after the application of a 1 V/cm electric field, indicating its potential in antifouling. After the preparation of the pure PES membrane through phase inversion, the surface of the membrane was modified with PPy and CNT by first dispersing CNTs in DI water at a 0.1mg/ml concentration with the dispersant being dodecyl benzene sulfonate (SDBS). After the suspension of CNTs was sonicated for 2 h at 120 W, 10 mL of it was deposited onto the PES membrane by applying a pressure of 1 bar, forming a ~2 μm-thick CNT layer. Next, a 0.2 M 40 mL pyrrole solution was applied to the membrane by dipping for a period of 1 h, after which the membrane was taken out and placed in a 0.5 M 40 mL FeCl3 solution acting as an oxidant medium where chemical polymerization occurred, forming on the surface thin conductive layers. This composite membrane was then placed in water for 12 h minimum to remove any adhering oxidizers and excess monomers. First, looking at the FE-SEM images of the pristine PES, PPy/PES, and PPy-CNT/PES membranes and comparing them to each other, the pristine PES membrane first displayed a selective compact and thinner upper layer with lots of asymmetric pores shaped like fingers from the quick phase transformation. However, for the PPy/PES membrane, these pores were partly covered after PPy was polymerized onto the surface. The PPy/PES membrane exhibited more resistance with a decrease in permeability compared to the pristine PES membrane because PPy particles went through the skin layer through the pores on the surface after uniform distribution on the surface. For the PPy-CNT/PES membrane, lots of PPy-CNT on the surface of the membrane formed a structure resembling a net. CNT surfaces had a PPy layer grown on it, and that layer was able to link the PES membrane with the CNTs. The layer of CNT provided the membrane a high level of conductivity. Through X-ray photoelectron spectroscopy (XPS) surface elemental composition characterization, more nitrogen was detected for the PPy/ PES membrane and the PPy-CNT/PES membrane than the pristine PES membrane. Because of the hydrophobicity exhibited by CNTs, the CNT/PES membrane had a 123.6° contact angle, which was modified to 80.3° for the PPy-CNT/PES membrane, improving its capability for antifouling as the hydrophilic membrane surface can form a hydrate layer, preventing contaminants from contacting the surface of the membrane.

    2.2. Polyaniline

    A new PANI membrane that can be modified electrically was fabricated using poly (2-acrylamido-2-methyl- 1-propanesulfonic acid) (PAMPSA) as a polymer acid to form PANI-PAMPSA membranes through in-situ polymerization[27]. Normally, conventional membranes are not able to modify their properties once they have been fabricated, but through acid doping, PANI membranes can potentially be modified in-situ. The PANI-PAMPSA membranes exhibited many beneficial properties, including leach resistance to acid, good tensile strength, a level of conductivity three times higher than that of a post cast doped PANI membrane, stability regarding in-filtration performance, and even characteristics exhibiting the removal of foulants under electrical potential. A novel method of fabricating the PANI-PAMPSA membranes was proposed, with polymer acid (PA) incorporation into PANI by first starting with oxidizing aniline on the PA template. Using the PANI-PA complex obtained through this method, the membrane will be prepared without the need for processes of dedoping and redoping. It is hypothesized that incorporated acid dopants will form a bond that is stronger than that of PANI that is fabricated and then undergoes secondary doping. After PANI-PAMPSA and PANI-HCl complexes were prepared using aniline, acid solution, and DI water with ammonium sulfate, the non-solvent induced phase separation (NIPS) technique was used to fabricate the PANI-PAMPSA membrane with the membrane forming through immersion precipitation. However, the PANI-HCl membrane was not able to be made the same way; instead, PANI-EB, or undoped PANI, was first developed by using ammonia to dedope the PANI-HCl complex. After casting of the membrane, immersion membrane pieces in a 0.1 M solution of HCl were used to dope the PANI-EB for a day, creating the final PANI-HCl doped membrane. Typical of PANI prepared in a solution that is highly acidic, PANI-EB exhibited a granular morphology, while conversely a fibrous network was seen for the PANI-PAMPSA complex in powder form. This difference can be attributed to the dopants, as PAMPSA and -SO3 groups showed a strong interaction with each other and possible hydrogen bonds formed between PANI and PAMPSA. For the membranes themselves, the PANI-HCl membrane showed a less porous and tighter structure than the PANI-PAMPSA membrane, with the PANI-HCl membrane having a denser skin top layer, a region of transition, and a backing layer that was relatively porous. It may be suggested that larger dopants had the effect of causing more intermolecular spacing in PANI, expanding pore structures in the membrane. PAMPSA can form a double-stranded complex with PANI through strong interlinking as it features a flexible backbone to adapt to PANI and its rigid conjugated structure. The powder form of the PANI-PAMPSA complex exhibited an electrical conductivity of 1.2 × 10-1 S cm-1, compared to a value of only 3.0 × 10-4 S cm-1 for PAMPSA that underwent secondary doping. The higher value for the PANI-PAMPSA complex that was formed through in-situ polymerization suggests a PAMPSA-incorporated PANI structure with the potential of charge transfer through the polymer chain.

    The preparation of polyaniline (PANI) membranes that were electrically conductive through the NIPS method produced a membrane that reached a 15.5 L/m2 pure water flux measurement[28]. This membrane was utilized in an electrofiltration cell to see how well the membrane was able to prevent fouling of bovine serum albumin (BSA) when an external voltage was applied. The performance of the membrane in respect to antifouling was better when the voltage increased to 1 V from 0 V. Based on calculations grounded in Derjaguin- Landau-Verywey-Overbeek (XDLVO) theory, adding an external voltage had the effect of increasing the membrane’s surface potential, improving the electrostatic repulsion between the membrane and the foulants, making for a much looser fouling layer. This had the effect of lowering hydraulic resistance overall. For synthesis, 7.5 g of PANI polymer was slowly added to the mixture containing 2.4 g of 4-methyl piperidine (4-MP) and N-methyl-2-pyrrolidone (NMP) to create a 15 wt% PANI solution. This mixture was then stirred continuously, forming a homogenous solution. After air bubbles were removed, a NIPS process with the non-solvent being water was used for membrane preparation. The solution to be cast was spread 250 μm thick a fabric tiled on a glass plate that was smooth with the help of a film applicator that was adjustable. After exposure to air, it was immersed for phase separation in a coagulation bath that was at 25°C. After a minimum 24 h soak in DI water to remove any leftover solvent, the PANI membrane was then doped by immersing it in a 0.15 M dodecylbenzene sulfonic acid (DBSA) solution for 10 h at 60°C. The doped PANI membrane was observed to be thicker with a thickness of 125.5 ± 2.1 μm compared with the 111.6 ± 3.1 μm-thick undoped PANI membrane. The potential main reason for this is that incorporating DBSA dopants inside the backbone of PANI elongated and pushed apart the polymer chains allowing the membrane to swell and the PANI to change conformation. The contact angle decreased to 38.9 ± 3.7° from 60.9 ± 1.3° post acid doping, as well as the surface zeta potential going down to -55.0 ± 4.8 mV from -40.3 ± 0.7 mV after doping with DBSA. Ultimately, the doped PANI membrane was changed to a semiconductor from an insulator as the undoped PANI membrane did not conduct electricity while the doped PANI membrane had a surface electrical conductivity of 2.20 × 10-4 S/cm. Compared to the undoped PANI membrane, the PANI membrane that was doped with DBSA had a much lower flux decline rate during BSA solution filtration. The reason for this is that doping with DBSA increased the membrane hydrophilicity, lowering the water contact angle and the surface potential, enhancing protein molecule resistance. Under dead-end filtration, with the application of 1 V of external voltage, the normalized water flux of the 30 mL BSA solution went down to ~60% for the doped membrane and 22% for the undoped membrane.

    Poly(L-lactic acid) (PLLA) membranes are very prone to fouling because it is hydrophobic[29]. On the other hand, PANI can be used for membrane fabrication because it is hydrophilic, easily available, and electrically conductive. In this study, the NIPS method was used to create PLLA composite membranes with PANI nanoparticle incorporation, which had the effect of improving the permeability and hydrophilicity of the membrane, as well as the antifouling property. This new composite membrane incorporated with 1 wt% of PANI even showed better results for antifouling and separation performance when compared to other composite membranes. Both composite and pristine PLLA membranes were fabricated through the method of immersion precipitation. First, 0, 0.5, 1, 1,5, and 2 wt% of PANI were sonicated in the 1,4-dioxane/N-methyl-2-pyrrolidone (DX/NMP) solvent mixture. Then, 1 wt% of Tween 80 and 15% of PLLA was added and violently stirred at 80°C for 12 h. After any air bubbles that were trapped were removed, the viscosity of the solution was measured, followed by casting done on a glass plate with a blade thinning the solution to 100 μm. After a 20 s evaporation period in air, phase inversion took place after the film was quickly immersed in DI water. To get rid of any solvents left over, DI water was used to wash the membrane, and the membrane was also air-dried as well. Looking at SEM images, as the PANI content increased, pores shaped like fingers fully appeared from the uppermost layer to the lowest layer, reducing pores that were closed and better interconnecting the membrane structure. Adding PANI also had the effect of increasing density and pore size, with 2 wt% PANI in corporation increasing mean surface pore size to 34.2 nm from 7.6 nm and membrane porosity to 73.9% from 57.5% compared to no PANI incorporation. Regarding surface contact angle, there was a gradual decrease up to 1 wt% of PANI from 0 wt%, with a minor increase when more PANI was added past 1 wt% of PANI. As the concentration of PANI increased, it was observed that there was a sharp decline in casting solution viscosity. This decline increased the rate of diffusion between the nonsolvent and solvent at the time of the phase separation process, leading to more porosity in the membrane. Additionally, a static adsorption test displayed that the composite membrane had a minimum BSA foulant adsorption value of 62.9 μg/cm2 at 1 wt% of PANI. Adsorption decreased as the concentration of PANI increased between 0.5 and 1 wt%, and the adsorption increased as the concentration of PANI increased past 1 wt% up to a value of 2 wt%. The flux of the membranes showed a similar trend. Ultimately, PANI nanoparticles had the effect of improving the antifouling property of the membrane by lessening the adsorption and deposition of foulants on the surface of the membrane.

    Electrically conducting membranes (ECMs) made from a PANI-CNT composite that is anodically stable and highly conductive for ultrafiltration was created using electro-polymerization of aniline in acidic conditions with CNT as the substrate[30]. Fig. 4 represents the surface morphology and structure of ECMs. These ECMs had the advantages of having better surface hydrophilicity, better stability in anodic conditions, and better electrical conductivity. Fig. 5 shows PANI-CNT ECM surfaces created using various current densities. Pressure deposition was used to deposit a PVA : CNT 1 : 3 ratio in terms of concentration on a PS-35 acting as a support to fabricate PVA-CNT membranes. On the other hand, electrochemical polymerization took place for the fabrication of PANI-CNT composite ECMs. Looking at AFM results, the PANI-CNT ECMs with the sulfuric acid dopant had the lowest roughness of 20 ± 2 nm, and contact angle measurements indicated that all of the PANI-CNT ECMs were more hydrophilic than the PVA-CNT ECM, with the greatest hydrophilicity found in the PANI-CNT ECM doped with sulfuric acid. The acid dopant significantly impacted the overall porosity and flux of the PANI-CNT ECMs. When the dopant was sulfuric acid, the membrane exhibited a greater amount of porosity and higher flux, similar to the PVA-CNT membrane, compared to when the dopant was oxalic and hydrochloric acid. Regarding electrical conductivity, the PANI-CNT membrane with the sulfuric acid dopant had the greatest electrical conductivity value at 8928 ± 307 S m-1. As PANI is able to bridge gaps between nearby CNTs effectively after it has been electropolymerized onto the network of CNTs, this may be the reason why a decrease in electrical resistance is observed. Based on electrical resistance, physical characterization, flux and molecular weight cut off (MWCO) results, the PANI-CNT ECMs doped with sulfuric acid were determined to be the best options for applications in water treatment. Fig. 6 displays the stability of the PANI-CNT membrane under anodic conditions. Additionally, ECMs with a sulfuric acid dopant displayed the highest stability as even after 18,000 ppm/h of exposure the increase of electrical resistance was at 27 ± 2.3%. Fig. 7 represents the electrochemical cleaning of a membrane that was fouled under differing conditions.

    In another study, graphene (Gr) preparation was put together with making the Gr layer itself of a membrane surface, in which the membrane was a PANI membrane[ 31]. Fig. 8 represents the procedure for fabricating the PANI membrane that was electrically conductive. Through the investigation of permeation flux of the suspension of yeast that was used, the membrane’s antifouling performance was observed with there being a 109% increase in the total permeation flux when 1 V of DC was applied. Fig. 9 represents an illustration of the setup for electrofiltration. Also, under 1 V of DC voltage, the Gr/PANI membrane average flux was about 1.4 times higher compared to the pure PANI membrane. To initially fabricate the PANI membrane, the NIPS method was used. The prepared membrane casting solution was cast onto non-woven fabric and evaporated, after which it was placed in, and later rinsed by DI water. Then, the Gr/PANI membrane was prepared through the one-step electrochemical method, where the peeling graphite anode was the source of Gr nanosheets that were highly conductive. Fig. 10 represents the process of electrical repulsion in electrofiltration. When the voltage of the power supply was set to 10 V, Gr was removed off the graphite foil, and using electrophoresis, Gr was assembled on top of the PANI membrane. It was confirmed that electrochemical exfoliation was used to obtain Gr, and the π-π interaction was the reason Gr was able to combine stably with the PANI membrane. Fig. 11 represents SEM images under varying magnification, water contact angles, and Gr/PANI, Gr/PANI-0 V, and Gr/PANI-1 V membrane pore sizes. Looking at SEM imaging, there was an observation of a smaller pore size on the membrane surface when looking at the top surface of the membrane. Observing the membrane surface more closely, the Gr seemed to be aggregated on the surface of the membrane in a manner that was random. Increasing the magnification even more revealed a Gr flake structure, proving the existence of Gr aggregation and that the Gr coverage of the surface was random. Using energy dispersive X-ray spectroscopy (EDX) analysis, it could be proved that Gr assembly with random distribution on the membrane surface took place as there was an increase of the Gr/PANI membrane’s surface carbon content. AFM results indicated a more rugged ridge-andvalley morphology for the Gr/PANI membrane because there was randomly distributed Gr aggregation for the membrane surface. The rms roughness went up to ~35.4 nm and the average roughness went up ~32.9 nm for the Gr/PANI membrane. Additionally, there was a significant decrease in the Gr/PANI membrane’s pore size and a narrowing of the pore size distribution after Gr assembly on the membrane surface. Referring to EDX and SEM analysis, Gr of smaller size have the potential to go into large pore channels and assemble on pore walls. A conductivity increase was recorded from the PANI membrane to the Gr/PANI membrane as the conductivity of the Gr/PANI membrane was 1.51 S/m compared to 0.97 S/m of the Gr/PANI membrane. Membrane antifouling performance was observed when there was an increase to 179 L/m2 h from 87 L/m2 h regarding the average flux for three cycles after the application of a DC electric field of 1 V. The presence of the DC electric field seemed to result in better antifouling performance for the Gr/PANI membrane compared to the PANI membrane. In addition, it was found that a stability improvement for the Gr/PANI membrane could be made through the application of an electric field at low intensity. Regarding the membrane rejection ratio, raising the voltage within a low range of DC did not have a significant effect. For total permeation flux, DC power had an effect of increasing the total permeation flux of the Gr/PANI membrane by 109% compared to having the same membrane not having DC power.

    2.3. Others

    Three coupling systems with a control system absent of a microbial fuel cell (MFC) were involved in the study how energy allocation affects membrane fouling, each with a different concentration of mixed liquid suspended solids (MLSS)[32]. One control membrane bioreactor (CMBR) and three MFC-membrane bioreactor (MBR)s were constructed. Various MLSS containing MBRs were utilized to investigate ow the MFC-MFR coupling systems under different loads were affected in regard to membrane fouling by how energy was distributed. A greater tendency for loosely bound extracellular polymeric substances (LB-EPS) and soluble microbial products (SMP) to result in irreversible fouling of the membrane was reported in experiments on membrane filtration with a working electric field and microbial contaminants. This irreversible fouling, however, was able to be reduced primarily if the number of SMP decreased in the presence of an electric field. The efficiencies in chemical oxygen demand (COD) are important to evaluate the system energy and performance of wastewater treatment systems. All system COD removal efficiencies in the low-loading stage reached levels greater than 85%, meaning that the wastewater was treated effectively. However, while the CMBR had a 96.20% efficiency, the efficiency for MFC-MBR when MLSS = 8000 mg/L (EMBR-8000) was 3.55% lower than that of CMBR, inferring lower removal efficiencies as the MLSS level increased. For the medium-loading stage, degradation rates of EMBR-5000 and CMBR went slightly down compared to the low-loading stage, while these rates went slightly up for EBMR-12000 and EMBR-8000 compared to the low-loading stage. The highest energy utilization was found for CMBR at 0.94 g-COD⋅d-1, and with increasing sludge concentration, less energy was present in the MBRs. In addition, the coupling system with MLSS = 8000 mg/L had the best degradation of organic matter.

    When an electrical potential was applied to a conducting CNT-PVA composite membrane for ultrafiltration, there was a noticeable impact of live bacteria detachment compared to particles and dead bacteria[33]. Fig. 12 displays representative images of the surfaces of membranes having E. coli deposition that were taken after various conditions were applied. According to image analysis, when the membrane acted as an anode or cathode, percentages of dead bacteria were 67 ± 3.6 and 32 ± 2.1%, respectively, after the application of a 1.5 V potential to the pair of the membrane and counter electrode. Low hydrogen peroxide (HP) concentrations were produced through oxygen electroreduction when the low electrical potentials were applied, leading to a decrease in microbial cell viability and an increase in cellular permeability. Fig. 13 represents the microparticle detachment ratio as well as E. coli cells and dead live assay analysis of the membranes. The prevention of bacterial attachment on the surface of the membrane can be attributed to exposure to electrochemically produced HP in low concentrations. CNT-PVA membranes were fabricated through pressure deposition onto a PS-35 support, and bacterial colonies were able to be harvested from a petri dish on a daily basis. Fig. 14 represents membrane SEM images post-detachment. SEM images showed a smooth membrane made up of CNTs that were intertwined, with 60~50% average porosity and a 100~150 nm pore size range. Thanks to the percolating CNT network that was porous, the PVA-CNT layer had a 2400 S/m electrical conductivity. Bacteria deposition coverage was around 4.5 ± 0.23%, with microparticle deposition coverage being around 26 ± 0.45%. For both dead and live bacteria, their deposition coefficient trends were almost the same. Fig. 15 represents the effects of the electrical potential on hydrogen peroxide production, cell membrane integrity, and suspended cell viability. When a 1.5 V electrical potential was applied, 86 ± 0.21% of the bacteria deposited came off the membrane on average, while at 1.0 V, detachment was only at 71 ± 0.22%. The difference regarding the detachment of bacteria showed that high enough electrical potentials, such as at 1~1.5 V, can have the effect of preventing live bacteria attachment to a surface that is electrically charged following initial deposition. The substance 5-cyano-2,3-ditolyltetrazolium chloride (CTC) was used as a fluorescent indicator, and results from CTC show that there was a lowering of cell viability upon the application of 1 and 1.5 V potential, with the maximum 61 ± 1.5% cellular viability decrease for 1.5 V. However, there were no observations indicating a significant viability decrease at 0.5 V compared to the control.

    A DC membrane coupling technology can be used for the control of membrane fouling, mechanisms to alleviate fouling, and performances for membrane filtration regarding the membrane coupling process with a DC electric field[34]. The configuration of this technology was reviewed in this article. In the traditional coupling configuration, the membrane experiences an electric field across it, having one electrode located on each of the two faces of the membrane. Membrane materials that are common, like PES or PVDF, are near insulated, not allowing for the direct application of an electric field on such materials. The insulating property of the membrane is a limiting factor, and to overcome the resistance that arises from it, an external electrical field that is strong enough to overcome the resistance may be used. In the modified coupling configuration, the membrane has been modified by using conductive materials, such as carbon or metal, and can function as a negative electrode.

    The application of an electric field (EF) had a significant effect of improving membrane flux for natural organic matter (NOM) electro-ultrafiltration[35]. This increase was even more noticeable for NOM that were more hydrophilic and had a greater molecular weight. The electro-ultrafiltration flux increased up to 20% for the hydrophilic molecules after the voltage was applied. Specifically, for humic acid (HA), a type of NOM, as the strength of the EF increased, there was an initial increase of membrane flux as well because of negatively charged HA molecules electrokinetically migrating but decreased when the field strength bypassed the critical EF. As hydrophobic compounds with high molecular weight are known to be major causes for membrane fouling, the fouling extent is dependent on the material of the membrane as well as the characteristics of the NOM. The novelty of this study was that it was the first to investigate NOM molecule polarization in the process of electro-ultrafiltration and their effect on the fouling resistance of membranes made of PVDF. The molecular weight and molecular polarity of NOM molecules was considered during electro-ultrafiltration. An ultrafiltration membrane composed of PDVF was used for the study. DC power for the voltage was used, and the PDVF membrane was located between the cathode and anode, with the membrane much closer to the cathode and at the cell bottom. AFM measurements displayed information about the cake layer that was formed as a result of membrane fouling, with it being dense when EF strength was high and relatively loose when EF strength was low. There was maximum roughness of the membrane when the EF was at 0.5 V/cm. The NOM observed in this study were classified into fulvic acid (FA), HA, and hydrophilic substances (HI). The flux increase for HI electro-ultrafiltration was observed to be more noticeable than the less hydrophilic HA and FA. During the process of electro-UF, HA with smaller molecular weights was able to pass through the membrane more easily. Under a low EF strength, the primary mechanism for antifouling in electro-UF can be considered to be the electromigration of the negatively charged HA molecules. In contrast, when there was high EF strength, HA molecules were more likely to have polarization that was highly developed, resulting in the easier aggregation of HA molecules when EF strength was high. Regarding pH effects, as the water pH increased, there was an increase in membrane flux, but a decrease in HA rejection. For the electro-UF of the 1 mM NaCl HA solution at the 0.5 V/cm critical EF, it resulted in the maximum flux of 0.83 for that solution, whereas a 0.88 flux measurement was achieved for the electro-UF of the 100 mL NaCl HA solution at the critical EF measuring 4 V/cm.

    A cross-linking method with sequential deposition was used to fabricate thin films that were electrically conductive and robust, which were made out of cross-linked poly(vinyl alcohol) and carboxylated multi-walled carbon nanotubes (PVA-CNT-COOH)[36]. These membranes were placed inside of an electrofiltration cell and were tested to see what effects applying electrical potentials had on membrane fouling when alginic acid (AA) was present in high concentration. At moderate applied fields and cell potentials of 9~15 V/cm and 3~5 V respectively, there is the potential to significantly reduce the fouling of the UF membrane caused by AA. To prepare the membranes, pressure deposition was used on a PS-35 membrane having a 0.57 g/m2 CNT-COOH concentration to deposit PVA-CNT-COOH (w/w) solution in a 3 : 1 ratio. After immersion in a crosslinking solution of glutaraldehyde and hydrochloric acid, which acted as the cross linker and the catalyst respectively, the membranes were heated and dried. Robust and very smooth surfaces were found on conductive thin films that were composed of CNF-COOH and PVA. The PVA-CNT-COOH layer’s overall thickness was found to be about 400 nm. This layer had an electrical conductivity measurement of 2412 ± 97 S/m, with the CNT-COOH percolating network influencing this measurement.

    3. Conclusion

    In summary, composite membranes designed to be electrically conductive by doping, by incorporating a substance that conducts electricity onto the surface of the membrane, or by using other means were found to conduct electricity successfully. Under a given electric potential, these membranes displayed beneficial property changes, such as being able to tune membrane pores electrically, mitigating biofouling, having better antibacterial properties, and possibly more. Because of these findings, applying a DC voltage to electrically conductive membranes are an appealing consideration to improve upon conventional membranes in applications of wastewater treatment, desalination, water purification, and more.

    Figures

    MEMBRANE_JOURNAL-31-2-101_F1.gif

    Schematic diagram of a conducting polymer membrane.

    MEMBRANE_JOURNAL-31-2-101_F2.gif

    Structure of polypyrrole-dodecylbenzene sulfonate (PPy-DBS).

    MEMBRANE_JOURNAL-31-2-101_F3.gif

    Surface roughness profile of (a1) M0, (b1) M1, (c1) M2, (d1) M3 (Reproduced with permission from Mukherjee et al., 24 Copyright 2018, John Wiley and Sons).

    MEMBRANE_JOURNAL-31-2-101_F4.gif

    ECM structure and surface morphology. (Top) Cross-sectional images of ECMs. (Middle) Top-view of ECM surface. (Bottom) ECM surface topography by atomic force microscopy. Scale bars for all SEM images are 2 μm (Reproduced with permission from Duan et al., 30, Copyright 2016, American Chemical Society).

    MEMBRANE_JOURNAL-31-2-101_F5.gif

    Representative images of PANI-CNT ECM surfaces fabricated using different current densities. Membranes were electropolymerized using sulfuric acid as the acid dopant. Scale bars for all SEM images are 500 nm. (A) Bare CNT, (B) PANI-CNT 0.25 mA cm-2, (C) PANI-CNT 0.9 mA cm-2, (D) PANI-CNT 1.5 mA cm-2 (Reproduced with permission from Duan et al., 30, Copyright 2016, American Chemical Society).

    MEMBRANE_JOURNAL-31-2-101_F6.gif

    PANI-CNT membrane stability under anodic conditions. (A) CV scans of PANI-CNT and PVA-CNT membranes (20 cycles); scan rate was 0.01 V/s in a 10 mM NaClO4 solution. PVA-CNT voltammogram is shown in red, PANI-CNT in black. The total surface area of the anode was 4 cm2. (B) Normalized current changes when a constant anodic potential of 2, 2.5, 2.8, 3, and 5 V (vs Ag/AgCl reference electrode) was applied to PANI-CNT ECMs and constant anodic potentials of 3 and 5 V (vs Ag/AgCl reference electrode) were applied to PVA-CNT ECMs; the reaction time was 10 min in a PBS buffer (10 mM Na2HPO4, 1.8 mM KH2PO4). The total surface area of the membrane anode was 8 cm2. (C) Normalized current changes when a constant anodic potential of 2.5, 2.8 V (vs Ag/AgCl reference electrode) was applied to PANI-CNT ECMs and constant anodic potentials of 2.8 V (vs Ag/AgCl reference electrode) were applied to PVA-CNT ECMs; the reaction time was 180 min in a PBS buffer (10 mM Na2HPO4, 1.8 mM KH2PO4). The total surface area of the membrane anode was 8 cm2. (D) Changes in resistance for PVA-CNT (black line) and PANI-CNT (red line) ECMs after applying 1, 2, and 3 V (vs Ag/ AgCl) to the membrane’s surface for 10 min in 80 mL of PBS buffer (10 mM Na2HPO4, 1.8 mM KH2PO4). (E) Images of the ECMs after exposure to 3.5 V (vs Ag/AgCl) for 20 min. (F) TOC and TNb degradation percentage of PVA-CNT and PANI-CNT ECMs after applying 1, 2, and 3 V (vs Ag/AgCl) to the membrane’s surface for 20 min in 40 mL of PBS buffer (10 mM Na2HPO4, 1.8 mM KH2PO4) (Reproduced with permission from Duan et al., 30, Copyright 2016, American Chemical Society).

    MEMBRANE_JOURNAL-31-2-101_F7.gif

    Electrochemical cleaning of fouled membrane (A) PANI-CNT ECM flux recovery under different flow conditions. The blue line represents the initial flux of the unfouled membrane; the gray line represents the fouled membrane flux at crossflow conditions, and the black line represents membrane flux after 5 min backwashing cycles with permeate (orange circles mark the backwashing events). (B) Flux recovery at three cycles of continuous BSA fouling and electrochemical cleaning at 3 V for PANI-CNT ECMs (black) and PVA-CNT ECMs (red). Initially membranes were fouled until flux reached about 30% of initial flux. (C) SEM images of membrane surface. (1) Membrane fouled by 100 ppm BSA. (2) BSA fouled ECM cleaned by back washing. (3) BSA fouled ECM cleaned by applying 3 V (membrane as anode) to the membrane surface. Scale bars for SEM images are 2 μm (Reproduced with permission from Duan et al., 30, Copyright 2016, American Chemical Society).

    MEMBRANE_JOURNAL-31-2-101_F8.gif

    Schematic of the fabrication procedure for the conductive PANI membrane (Reproduced with permission from Li et al., 31, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-31-2-101_F9.gif

    Schematic drawing of the electrofiltration setup (Reproduced with permission from Li et al., 31, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-31-2-101_F10.gif

    Schematic of the electrical repulsion process in electrofiltration (Reproduced with permission from Li et al., 31, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-31-2-101_F11.gif

    (a) SEM images of the Gr/PANI, Gr/PANI-0V, and Gr/PANI-1V membranes, 500× magnification. (b) SEM images, 20,000× magnification. (c) Water contact angles of the Gr/PANI, Gr/PANI-0V, and Gr/PANI-1V membranes. (d) Pore sizes of the Gr/PANI, Gr/PANI-0V, and Gr/ PANI-1V membranes (Reproduced with permission from Li et al., 31, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-31-2-101_F12.gif

    Representative images of membrane surfaces with deposited E. coli cells taken after (A-D) deposition and (E-H) detachment. (A and E) Control (no voltage); (B and F) 1.5 V, membrane as anode; (C and G) 1.5 V, membrane as cathode; and (D and H) microparticles at 1.5 V, membrane as anode (similar results with membrane as cathode) (Reproduced with permission from Ronen et al., 33, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-31-2-101_F13.gif

    Detachment ratio of microparticles, live and dead E. coli cells from membranes during detachment experiments and analysis of dead live assay of the membranes (dead bacteria were stained by PI, live bacteria were stained by Syto9). Applied potential ranged between 0 and 1.5 V. In the X-axis, (+) indicates the membrane was positively charged (anode) and (-) indicates the membrane was negatively charged (cathode). The indicated applied potential refers to the overall potential applied to the membrane and counter electrode (whole flow cell potential) (Reproduced with permission from Ronen et al., 33, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-31-2-101_F14.gif

    SEM images of the membranes after detachment phase. (A and D) control with no applied potential; (B and E) 1.5 V, membrane as anode; (C and F) 1.5 V, membrane as cathode. Top panel scale bar 5 μm, lower panel scale bar 2 μm. Circles indicate damaged bacteria on the membrane surface (Reproduced with permission from Ronen et al., 33, Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-31-2-101_F15.gif

    Impact of electrical potential on suspended cell viability, cell membrane integrity, and hydrogen peroxide production. (A) Hydrogen peroxide concentration, PI uptake, and E. coli cellular viability as detected by CTC staining over a range of applied potential after 30 min. (B) PI uptake and cellular viability as detected by CTC staining of E. coli as a function of hydrogen peroxide in the solution (Reproduced with permission from Ronen et al., 33, Copyright 2020, American Chemical Society).

    Tables

    Summary of Conducting Polymer Membrane

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