The effects of electrophoresis, bubbles and electroosmosis for conductive membrane performance in the electro-filtration process

In situ cleaning of foulants by gas scouring on the membrane-electrode in an electro-membrane bioreactor

Low-maintenance membrane cleaning is essential for the stable operation of membrane bioreactors. This work proposes an in-situ electrical-cleaning method using an electro-MBR. When the applied bias was transiently increased, the membrane flux recovered rapidly because of the scouring effect from gas evolution reactions. The exfoliation of the cake layer induced by gas scouring played a major role in mitigating membrane fouling, recovering the transmembrane pressure (TMP) by 88.6 % under optimal conditions. Membrane modules did not require replacement during the operation period due to the efficacy of electrical cleaning, with the TMP varying between 37.5 % and 62.5 % of the ultimate pressure requiring change of the membrane modules. Despite the increase in power consumption of 0.66 Wh·m-3 due to the additional applied bias, there was no need for chemical additives or manual maintenance. Therefore, the electrical cleaning method enhanced the service life and reduced the maintenance costs of the electro-MBR.

Design and characterization of an electrochemically-modulated membrane chromatography device

Membrane separations offer a compelling alternative to traditional chromatographic methods by overcoming mass transport limitations. We introduce an additional degree of freedom in modulating membrane chromatography by using metalized membranes in a potential-driven process. Investigating the impact of a gold coating on membrane characteristics, the sputtered gold layer enhances the surface conductivity with stable electrochemical behavior. However, this comes at the expense of reduced permeability, wettability, and static binding capacity (∼ 474 µg g-1 of maleic acid). The designed device displayed a homogenous flow distribution, and the membrane electrodes exhibit predominantly capacitive behavior during potential application. Modulating the electrical potential during the adsorption and desorption phase strongly influenced the binding and elution behavior of anion-exchange membranes. Switching potentials between ±1.0 V vs. Ag/AgCl induces desorption, confirming the process principle. Elution efficiency reaches up to 58 % at -1.0 V vs. Ag/AgCl in the desorption phase without any alteration of the mobile phase. Increasing the potential perturbation ranging from +1.0 V to -1.0 V vs. Ag/AgCl resulted in reduced peak width and improved elution behavior, demonstrating the feasibility of electrochemically-modulated membrane chromatography. The developed process has great potential as a gentle and sustainable separation step in the biotechnological and chemical industry.

Electrically Redox-Active Membrane with Switchable Selectivity to Contaminants for Water Purification

Membrane technology provides an attractive approach for water purification but faces significant challenges in separating small molecules due to its lack of satisfactory permselectivity. In this study, a polypyrrole-based active membrane with a switchable multi-affinity that simultaneously separates small ionic and organic contaminants from water was created. Unlike conventional passive membranes, the designed membrane exhibits a good single-pass filtration efficiency (>99%, taking 1-naphthylamine and Pb2+ as examples) and high permeability (227 L/m2/h). Applying a reversible potential can release the captured substances from the membrane, thus enabling membrane regeneration and self-cleaning without the need for additives. Advanced characterizations reveal that potential switching alters the orientation of the doped amphipathic molecules with the self-alignment of the hydrophobic alkyl chains or the disordered sulfonate anions to capture the target organic molecules or ions via hydrophobic or electrostatic interactions, respectively. The designed smart membrane holds great promise for controllable molecular separation and water purification.

Hyperoxidation active species produced by seawater electro membrane reactor assisted electrolytic cell system for simultaneous decyanation and carbon removal

Herein, a novel triple-layered heterojunction catalytic cathode membrane (PVDF/rGO/TFe/MnO2, TMOHccm) was reported and applied in seawater electro membrane reactor assisted electrolytic cell system (SEMR-EC), achieving increased properties for cyanide wastewater treatment. Hydrophilic TMOHccm exhibits higher electrochemical activity (qT* 1.11 C cm-2, qo* 0.03 C cm-2), indicating excellent electron transfer efficiency. Further analysis shows a one-electron redox cycle of exposed transition metal oxides (TMOs) on rGO support mediated oxygen reduction reaction (ORR) process, and calculated results of density functional theory (DFT) demonstrates positive Bader charge (72 |e|) of synthesized catalyst. The developed SEMR-EC was implemented in intermittent-stream operation for treating cyanide wastewater, the system achieved optimized decyanation and carbon removal performance (CN- 100%, TOC 88.49%). Hyperoxidation active species produced SEMR-EC including hydroxyl, sulfate, and reactive chlorine species (RCS) was confirmed. The proposed mechanistic explanation indicated multiple removal pathways relevant to cyanide, organic matter, and iron were elucidated, and the engineering applications prospects were highlighted by cost (5.61 $) and benefit (Ce 399.26 mW m-2 $-1, EFe 248.11 g kWh-1) analysis of the system.

A critical review on classifications, characteristics, and applications of electrically conductive membranes for toxic pollutant removal from water: Comparison between composite and inorganic electrically conductive membranes

Research efforts have recently been directed at developing electrically conductive membranes (EMs) for pressure-driven membrane separation processes to remove effectively the highly toxic pollutants from water. EMs serve as both the filter and the electrode during filtration. With the assistance of a power supply, EMs can considerably improve the toxic pollutant removal efficiency and even realize chemical degradation to reduce their toxicity. Organic-inorganic composite EMs and inorganic EMs show remarkable differences in characteristics, removal mechanisms, and application situations. Understanding their differences is highly important to guide the future design of EMs for specific pollutant removal from water. However, reviews concerning the differences between composite and inorganic EMs are still lacking. In this review, we summarize the classifications, fabrication techniques, and characteristics of composite and inorganic EMs. We also elaborate on the removal mechanisms and performances of EMs toward recalcitrant organic pollutants and toxic inorganic ions in water. The comparison between composite and inorganic EMs is emphasized particularly in terms of the membrane characteristics (pore size, permeability, and electrical conductivity), application situations, and underlying removal mechanisms. Finally, the energy consumption and durability of EMs are evaluated, and future perspectives are presented.

Dielectrophoresis-Based Universal Membrane Antifouling Strategy toward Colloidal Foulants

Membrane fouling compromises the benefits of membrane technology, leading to its performance deterioration and incremental cost. Coupling with an electric field has been attractive but is limited by the electrical dependence of the electrophoresis (EP) mechanism and undesired faradic reactions. This study reports a universal dielectrophoresis-based (DEP) membrane antifouling strategy for electronegative, electropositive, and neutral colloidal foulants, which depends on the particle polarizability rather than its charge. The porous Ni@PVDF model electroconductive membrane was fabricated to construct a nonuniform electric field inducing DEP, while applying a low voltage avoided side electrochemical reactions. For electronegative SiO₂(-) and electropositive Al₂O₃(+) particles with a lower relative permittivity than the medium water (78), the membrane permeability all remarkably increased by 90.1% under AC/DC (±1.0 V) fields. By contrast, serious membrane fouling occurred for the BaTiO₃ colloids with a higher relative permittivity (∼2000). Notably, the permittivity of nearly all colloids in wastewater treatment is much less than that of water, which makes the dielectrophoresis-based antifouling strategy universal. The theoretical simulation systematically analyzed the forces on particles including DEP, EP, and others, indicating that the formed protected area on the membrane pore wall by DEP forces prevented the irreversible membrane blockage of colloids and facilitated loose cake layer formation for alleviating membrane fouling. In brief, this work reported a hopeful concept for dielectrophoresis-based membrane antifouling and verified its antifouling mechanism.

Defective MOFs-based electrocatalytic self-cleaning membrane for wastewater reclamation: Enhanced antibiotics removal, membrane fouling control and mechanisms

In order to resolve the poor antibiotics rejection and serious fouling of ultrafiltration (UF) membrane during municipal wastewater reclamation, a novel anodic membrane (defective UiO-66 (D-UiO-66)/Graphite/Polyvinylidene fluoride (PVDF)) with high pure water flux (596.1 L•h ^- 1•m ^- 2•bar^-1) was fabricated by incorporating defective zirconium based metal-organic framework (D-UiO-66) and conductive graphite particles into PVDF matrix and applied in the coupling of electro-oxidation and membrane filtration process. Compared to the other anodic membranes (i.e., Graphite/PVDF and UiO-66/Graphite/PVDF), D-UiO-66/Graphite/PVDF possesses superior anti-fouling and self-cleaning abilities (flux recovery=100%, model foulant: bovine serum albumin) in both intermittent and continuous supply of electric field under current density of 0.01 mA/cm²; moreover, efficient antibiotics (tetracycline, norfloxacin, tylosin and sulfamethoxazole) removal (> 96.6%) and bactericidal efficiency against E. coli and S. aureus (100%) were achieved simultaneously without the addition of chemical reagents due to the higher electrocatalytic activity of anodic membrane for oxidation of pollutants by •OH and •O₂^- free radicals. Three degradation pathways of antibiotics were proposed and the self-cleaning mechanism of membrane was dominated by the synergy of the partial mineralization and the reduced fouling potential of foulants after oxidation as revealed by the increase in hydrophilicity, and decrease in negative charge and molecular weight. The fabricated membrane also presents excellent electrochemical stability, separation and self-cleaning performance for treatment of municipal secondary effluent during long-term filtration with low electric energy consumption, which is promising in wastewater reclamation.

Visualization of Electrochemically Accessible Sites in Flow-through Mode for Maximizing Available Active Area toward Superior Electrocatalytic Ammonia Oxidation

Active chlorine species-mediated electrocatalytic oxidation is a promising strategy for ammonia removal in decentralized wastewater treatment. Flow-through electrodes (FTEs) provide an ideal platform for this strategy because of enhanced mass transport and sufficient electrochemically accessible sites. However, limited insight into spatial distribution of electrochemically accessible sites within FTEs inhibits the improvement of reactor efficiency and the reduction of FTE costs. Herein, a microfluidic-based electrochemical system is developed for the operando observation of microspatial reactions within pore channels, which reveals that reactions occur only in the surface layer of the electrode thickness. To further quantify the spatial distribution, finite element simulations demonstrate that over 75.0% of the current is accumulated in the 20.0% thickness of the electrode surface. Based on these findings, a gradient-coated method for the active layer was proposed and applied to a Ti/RuO₂ porous electrode with an optimized pore diameter of ∼25 μm, whose electrochemically accessible surface area was 381.7 times that of the planar electrode while alleviating bubble entrapment. The optimized reactor enables complete ammonia removal with an energy consumption of 60.4 kWh kg^-1 N, which was 24.2% and 39.9% less than those with pore diameters of ∼3 μm and ∼90 μm, respectively.

Mitigating membrane fouling by coupling coagulation and the electrokinetic effect in a novel electrocoagulation membrane cathode reactor

Membrane reactors with efficient antifouling and low maintenance are desirable for distributed membrane water treatment. In this study, a novel membrane separation reactor with an Al anode and a conductive membrane as the cathode was built to develop a chemical-free method for mitigating membrane fouling via electrocoagulation coupled with the electrokinetic effect. The electrostatic repulsion between humic acid (HA) and the membrane cathode reduced the adhesion of HA foulants on the membrane, thereby contributing to antifouling in the initial stage. Electrocoagulation and polarization induced by the electric field enlarged the HA-Al flocs, which prevented membrane pore blocking and facilitated the formation of a porous cake layer, thereby leading to a high water flux of the electrocoagulation membrane cathode reactor (ECMCR) in the stable stage. The bubbles from hydrogen evolution on the membrane cathode scoured the HA foulants and washed out the dense cake layer, thereby playing an important role in membrane fouling mitigation. Compared with membrane filtration, the membrane cathode reactor, membrane anode reactor, and HA removal of the ECMCR increased by 9.6, 8.3, and 2.8 times, respectively, whereas the transmembrane pressure decreased by 84.6%, 21.5%, and 63.0%, respectively. The synergy of electrocoagulation and the electrokinetic effect provides the ECMCR with a feasible method of antifouling and improved effluent quality with low maintenance.

Systematic Design of a Flow-Through Titanium Electrode-Based Device with Strong Oil Droplet Rejection Property for Superior Oil-in-Water Emulsion Separation Performance

Oily wastewater treatment has been restricted by the existence of stable oil-in-water (O/W) emulsions containing micrometer-sized oil droplets. However, the strong adhesion and stacking of emulsified oil droplets on the surface of current separation media cause serious fouling of the treatment unit and the rapid decline of treatment efficiency. Herein, a novel flow-through titanium (Ti) electrode-based filtration device with remarkable oil droplet rejection property was well designed for the continuously separating O/W emulsion. In contrast to the pristine Ti foam, the permeance of the TiO₂ nanoarray-coated Ti foam (NATF) increased from 2538 to 4364 L m^-2 h^-1 bar^-1 through gravity-driven flow. Further, more than ∼70% permeability can be maintained after 6 h of O/W emulsion filtration using the current device, the value of which was markedly higher than that of conventional oil/water separation filters (less than 5%). According to the results of wettability test, the super-oil-repellent surface endowed by this nanoarray structure primarily avoided the formation of a compact oil fouling layer. When the voltage was applied, accompanied by the electrophoresis effect, redistribution of surfactant molecules on the surface of oil droplets induced by an electric field made them readily captured by the microbubbles continuously generated from the electrode, thereby rapidly migrating these bubble-adhered oil droplets far from the filtration medium.