Reactive Electrochemical Membranes Research Articles

Flow-through inactivation of pathogenic Escherichia coli by titanium suboxide reactive electrochemical membranes: Unlocking the coupled mechanics of electrochemical and subcellular alterations

Limited mass transfer controlled by diffusion and unexplored disinfection mechanisms at the subcellular alternation respond to different electrochemical reactions always obstacles the electrochemical disinfection decentralized water treatment. Here, a large-scale reactive electrode membrane composed of titanium suboxide (TiSO-REM) was pioneered for pathogen inactivation and mechanism exploration. Complete bacteria inactivation (>7.1 log reduction) was achieved after 20 min of electrolysis at 1440 L m−2 h−1 (LMH) and 15 mA cm−2. Electrostatic adsorption facilitated the interaction of pathogens with oxidants in the electrode boundary layer by enhancing the adhesion of Escherichia coli at anode surface, resulting in 25 % improvement in removal. Subcellular alternation of waterborne pathogens depended on specific electrochemical mechanisms. Direct electron transfer (DET) unbalanced intracellular metabolism by excessive intracellular reactive oxygen species accumulation. While more violent electro-generated •OH annihilated the whole cells with disorderly alternation which also effectively reduced the risk of bacterial regeneration with no bacterial resuscitation. The study also provides a universal model to develop sustainable technologies for pathogen inactivation and precise regulation of inactivation efficiency from a mechanistic perspective for a wide range of engineering applications like water disinfection and bio-safety control.

Enhanced oxidation of organic pollutants by regulating the interior reaction region of reactive electrochemical membranes

Reactive electrochemical membrane (REM) emerges as an attractive strategy for the elimination of refractory organic pollutants that exist in wastewater. However, the limited reaction sites in traditional REMs greatly hinder its practical application. Herein, a feed-through coating methodology was developed to realize the uniform loading of SnO2-Sb catalysts on the interior surface of a REM. The uniformly coated REM (Unif-REM) exhibited 2.4 times higher reaction kinetics (0.29 min–1) than that of surface coated REM (Surf-REM) for the degradation of 2 mM 4-chlorophenol (4-CP), rendering an energy consumption as low as 0.016 kWh gTOC–1. The fast degradation of various emerging contaminants, e.g., sulfamethoxazole (SMX), ofloxacin (OFLX), and tetracycline (TC), also confirms its superior oxidation capability. Besides, the Unif-REM exhibited good performance in generating hydroxyl radicals (•OH) and a relatively long service lifetime. The simulation of spatial current distribution demonstrates that the interior reaction region in the Unif-REM channels can be drastically extended, thereby maximizing the surface coupling of mass diffusion and electron transfer. This study offers an in-depth look at the spatially confined reactions in REM and provides a reference for the design of electrochemical systems with economically efficient water purification.

Unveiling the spatially confined oxidation processes in reactive electrochemical membranes

Electrocatalytic oxidation offers opportunities for sustainable environmental remediation, but it is often hampered by the slow mass transfer and short lives of electro-generated radicals. Here, we achieve a four times higher kinetic constant (18.9 min−1) for the oxidation of 4-chlorophenol on the reactive electrochemical membrane by reducing the pore size from 105 to 7 μm, with the predominate mechanism shifting from hydroxyl radical oxidation to direct electron transfer. More interestingly, such an enhancement effect is largely dependent on the molecular structure and its sensitivity to the direct electron transfer process. The spatial distributions of reactant and hydroxyl radicals are visualized via multiphysics simulation, revealing the compressed diffusion layer and restricted hydroxyl radical generation in the microchannels. This study demonstrates that both the reaction kinetics and the electron transfer pathway can be effectively regulated by the spatial confinement effect, which sheds light on the design of cost-effective electrochemical platforms for water purification and chemical synthesis.

Carbon-based electrocatalytic dual-membrane system bolsters singlet oxygen production for ultrafast water decontamination

Reactive electrochemical membranes (REMs) are promising technologies in treating pharmaceuticals and personal care products (PPCPs) in water. Herein, a novel carbon-based electrocatalytic dual-membrane system was designed to exploit the whole redox process of electrodes, in which the membrane cathode and anode were formed by carbon fibers doped with Fe and metal organic frameworks derived SnO2, respectively. Propranolol (PRO) was used as a representative of PPCPs. The system bolstered singlet oxygen (1O2) production by the synergy of two REM electrodes, further improving the removal rate constant of PRO compared with single-electrode-dominant modes. 97.5 ± 1.7% of PRO removal was achieved in a single-pass electro-filtration at a residence time of ∼2.9 s. The generation of 1O2 and its reaction with pollutants were systematically and thoroughly explored via experiments coupled with theoretical calculation. The toxicity of the decomposition products was predicted to be reduced compared with PRO. These findings suggested that the carbon-based electrocatalytic dual-membrane system could effectively promote 1O2 production for ultrafast catalytic oxidation of PRO, providing a cost-effective solution for the development of an efficient and stable technology for PPCPs removal.

Mechanistic Insight into the Spatial Scale of Nonuniform Oxidation of Micropollutants in Reactive Electrochemical Membranes for Water Purification

Mechanistic Insight into the Spatial Scale of Nonuniform Oxidation of Micropollutants in Reactive Electrochemical Membranes for Water Purification

Recent advances in sub-stoichiometric TiO2 as reactive electrochemical membranes (REM) for bio-refractory pollutants removal: A critical review

Water stress must be addressed by developing cost-effective wastewater treatment technologies. Reactive electrochemical membranes (REM) have gained popularity in this scenario due to the benefits of both electrochemical advanced oxidation processes (EAOP) and membrane filtration processes (MFP) in reducing mass transport limitations and fouling issues. In this context, this review aims to consolidate recent advances in REM, focusing on sub-stoichiometric titanium oxide-based REM (TiSO-REM), mainly Ti4O7, the most promising ceramic material due to its high conductivity and ability to generate •OH. The primary fabrication methods for TiSO porous anodes are discussed, resulting in a significant reminder of their electrocatalytic, porosimetric and physicochemical intrinsic properties. In addition to the real advantages of using this material in flow-through configurations, which allow convection-enhanced mass transport and provide a high electrochemical surface area. The latest progresses in TiSO-REM are presented, with specific emphasis on understanding the main operating parameters in performance efficiency: the determinative key role among pollutant nature, matrix effect and anode in removal mechanisms, and where the anode is in relation to the cathode in the reactor design. These points are intended to identify process limitations and challenges to be overcome when implementing various reactor configurations until the scaling-up. All these aspects are reviewed critically in order to provide future research directions for the industrial advancement of this technique, which has proven to be more energy-efficient and therefore economically feasible than alternative EAOP.

Electrochemical Oxidation of Organic Pollutants in Aqueous Solution Using a Ti4O7 Particle Anode.

Anodes based on substoichiometric titanium oxide (Ti4O7) are among the most effective for the anodic oxidation of organic pollutants in aqueous solutions. Such electrodes can be made in the form of semipermeable porous structures called reactive electrochemical membranes (REMs). Recent work has shown that REMs with large pore sizes (0.5-2 mm) are highly efficient (comparable or superior to boron-doped diamond (BDD) anodes) and can be used to oxidize a wide range of contaminants. In this work, for the first time, a Ti4O7 particle anode (with a granule size of 1-3 mm and forming pores of 0.2-1 mm) was used for the oxidation of benzoic, maleic and oxalic acids and hydroquinone in aqueous solutions with an initial COD of 600 mg/L. The results demonstrated that a high instantaneous current efficiency (ICE) of about 40% and a high removal degree of more than 99% can be achieved. The Ti4O7 anode showed good stability after 108 operating hours at 36 mA/cm2.

Electrochemical activation of peroxymonosulfate by 3D printed blue TiO2 nanotube arrays reactive electrochemical membrane for efficient degradation of acetaminophen

Electrocatalytic technology (EC) is a promising strategy for peroxymonosulfate (PMS) activation due to its environmental safety and energy conservation. Herein, 3D printed blue TiO2 nanotube arrays (3DP-BTNAs) reactive electrochemical membranes (REM) were designed as anode and cathode in EC and EC/PMS systems. EC/PMS system displayed superior performance for acetaminophen (ACT) degradation and detoxication. The ACT degradation in EC/PMS system was 3.70 times faster than that in EC system. Quenching experiments suggested that singlet oxygen (1O2) mediated nonradical pathway was responsible for ACT degradation in EC/PMS system, while hydroxyl radical (•OH) was the predominant reactive oxygen species (ROS) in EC system. Our study first provided direct evidence that PMS activation on cathode was mainly triggered by superoxide radical (O2•-). PMS was reduced by O2•- derived from O2 reduction on cathode, and subsequently converted into ROS for organics oxidation. However, PMS tended to adsorb on anode and formed 3DP-BTNAs-PMS* complex for organics degradation. The influencing factors (e.g., PMS dosage, flow rate) on ACT degradation in EC/PMS system were evaluated, and the obtained optimal kinetic constant and lowest energy consumption were 0.135 min−1 and 0.27 Wh/L, respectively. In addition, eight degradation products were identified and possible degradation pathways of ACT were proposed to be the cleavage of C-N bonds and benzene ring. This study is dedicated to the rational design of highly porous REM material by 3DP technology for wastewater treatment and the in-depth mechanism of PMS activation over BTNAs electrode in EC/PMS system.

Efficient reactive electrochemical carbon membranes for organic pollutants removal from aqueous solution: A study of multi-physics modeling and simulation approach

A novel application of three metal oxide catalysts CuOx, Fe2O3, and CoO onto coal-based carbon membrane (CM) in the enhancement of electrochemical removal of high concentration organic pollutants is presented. Their morphology, structure, and elemental composition were characterized. Analysis of their performance in the treatment of highly concentrated Rhodamine B (RhB), Rose Bengal, and phenol pollutants with respective concentrations of 600 mg/l, 637 mg/l, and 50 mg/l presented excellent removal efficiency (all >97 %). A study of the degradation mechanism of pollutants at the membrane surface was approached through multi-physics modeling and simulation of the reaction kinetics of complex indirect RhB oxidation chemistry occurring at the copper oxide membrane surface which involved oxidation of resistant intermediates. The results gave a degradation efficiency of 99.82 % after 60 s of reaction at the CM-CuOx surface which is very close to the experimental results. An understanding of the pollutants concentration distribution across the electrocatalytic filtration system was achieved through simulation of the transport, diffusion, and convection of degraded products through the membrane walls. No fouling was observed for RhB treatment and a new insight into the interactions between pollutants molecules and reactive membrane surfaces was brought by the mean of Monte-Carlo simulations of their adsorption mechanisms. Finally, a comparison with other Electrochemical Advanced Oxidation Processes (EAOPs), revealed that the membranes fabricated in this work possessed significant advantages both in terms of permeability and removal efficiency.

Membrane Fouling and Electrochemical Regeneration at a PbO2-Reactive Electrochemical Membrane: Study on Experiment and Mechanism.

Membrane fouling and regeneration are the key issues for the application of membrane separation (MS) technology. Reactive electrochemical membranes (REMs) exhibited high, stable permeate flux and the function of chemical-free electrochemical regeneration. This study fabricated a micro-filtration REM characterized by a PbO2 layer (PbO2-REM) to investigate the electro-triggered anti-fouling and regeneration progress within REMs. The PbO2-REM exhibited a three-dimensional porous structure with a few branch-like micro-pores. The PbO2-REM could alleviate Humic acid (HA) and Bisphenol A (BPA) fouling through electrochemical degradation combined with bubble migration, which achieved the best anti-fouling performance at current density of 4 mA cm−2 with 99.2% BPA removal. Regeneration in the electro-backwash (e-BW) mode was found as eight times that in the forward wash and full flux recovery was achieved at a current density of 3 mA cm−2. EIS and simulation study also confirmed complete regeneration by e-BW, which was ascribed to the air–water wash formed by bubble migration and flow. Repeated regeneration tests showed that PbO2-REM was stable for more than five cycles, indicating its high durability for practical uses. Mechanism analysis assisted by finite element simulation illustrated that the high catalytic PbO2 layer plays an important role in antifouling and regeneration.

Enhanced electrochemical oxidation of tetracycline and atrazine on SnO2 reactive electrochemical membranes by low-toxic bismuth, cerium doping

SnO2-Sb electrodes are widely used in electrochemical advanced oxidation processes for water treatment; however, the release of highly toxic Sb3+/Sb5+ has aroused environmental concern. This study systematically investigated the potential of Bi and Ce elements as substitutes to fabricate doped-SnO2 reactive electrochemical membrane (REM) for water purification. Compared with the control (SnO2-Sb-REM), SnO2-Bi-REM showed a higher direct electron transfer (DET) rate and •OH yield due to its larger electroactive surface area. The higher oxygen evolution potential of SnO2-Ce-REM enhanced selectivity for •OH generation. Although both SnO2-Bi-REM and SnO2-Ce-REM exhibited higher electrooxidation of tetracycline hydrochloride (TCH) and atrazine (ATZ) than SnO2-Sb-REM, only the service lifespan of SnO2-Bi-REM can meet the requirements of practical applications. Mechanistic studies of the degradation pathways indicated that the rapid degradation of TCH on SnO2-Bi-REM was mainly contributed by the DET process, while the transformation of intermediates involved the reactions with •OH. In comparison, aqueous •OH played an important role in the degradation of refractory ATZ. Moreover, a SnO2-Bi-REM system demonstrated ∼99% removal of trace TCH and ATZ from wastewater effluent in single-pass operation at a low electrical efficiency per log order removal (EE/O, 0.71 kWh m−3). To this end, the SnO2-Bi-REM is demonstrated to be a promising electrode to replace SnO2-Sb-REM for efficient and sustainable water purification.

Enhanced Electrochemical Oxidation of Tetracycline and Atrazine on Sno2 Reactive Electrochemical Membranes by Low-Toxic Bismuth, Cerium Doping

Enhanced Electrochemical Oxidation of Tetracycline and Atrazine on Sno2 Reactive Electrochemical Membranes by Low-Toxic Bismuth, Cerium Doping

Insight into the rapid elimination of low-concentration antibiotics from natural waters using tandem multilevel reactive electrochemical membranes: Role of direct electron transfer and hydroxyl radical oxidation

Herein, we reported a tandem multilevel reactive electrochemical membrane (REM) system was promising for the rapid and complete removal of trace antibiotics from natural waters. Results indicate that a four-stage REM module-in-series system achieved steady over 98% removal of model antibiotic norfloxacin (NOR, 100 μg·L−1) from wastewater treatment plant final effluent and surface water with a residence time of 5.4 s, and the electric energy consumption was only around 0.007–0.011 kWh·m−3. As for the oxidation mechanism, direct electron transfer (DET) oxidation process played an important role in NOR rapid oxidation, enabling the REM system to tolerate various •OH scavenges in natural waters, including natural organic matters, Cl- and HCO3-, even at very high concentration levels. Meanwhile, •OH-mediated indirect oxidation process promotes the oxidation and mineralization of NOR. Although the DET-dominated oxidation mechanism makes the REM system cannot achieve the complete mineralization of NOR with residence times of few seconds, the antibacterial activity from NOR was completely eliminated. This REM system featured effective removal performance of trace contaminants with low energy cost and was tolerant to complex waster matrix, suggesting that it could be a powerful supplementary step for wastewater/water treatment.

A Simple 1D Convection-Diffusion Model of Oxalic Acid Oxidation Using Reactive Electrochemical Membrane.

In recent years, electrochemical methods utilizing reactive electrochemical membranes (REM) have been recognized as the most promising technologies for the removal of organic pollutants from water. In this paper, we propose a 1D convection-diffusion-reaction model concerning the transport and oxidation of oxalic acid (OA) and oxygen evolution in the flow-through electrochemical oxidation system with REM. It allows the determination of unknown parameters of the system by treatment of experimental data and predicts the behavior of the electrolysis setup. There is a good agreement in calculated and experimental data at different transmembrane pressures and initial concentrations of OA. The model provides an understanding of the processes occurring in the system and gives the concentration, current density, potential, and overpotential distributions in REM. The dispersion coefficient was determined as a fitting parameter and it is in good agreement with literary data for similar REMs. It is shown that the oxygen evolution reaction plays an important role in the process even under the kinetic limit, and its contribution decreases with increasing total organic carbon flux through the REM.

Current progress in electrochemical anodic-oxidation of pharmaceuticals: Mechanisms, influencing factors, and new technique

Various pharmaceuticals have been detected in natural water and wastewater bodies, causing threats to water ecosystem and human health. Although electrochemical anodic-oxidation (EAO) has been shown to be efficient for pharmaceuticals degradation from aqueous solution, it still has a distinct need to apply EAO technology for pharmaceuticals removal rationally. This review provides the most recent progress on the mechanisms, influencing factors, and new technique of EAO for pharmaceuticals degradation. The mechanism and superiority of EAO were analyzed. Major influencing factors (e.g., electrode materials, electrochemical reactor, applied current density, anode-cathode distance, electrolyte type and concentration, initial solution pH value, and initial pharmaceuticals concentration) were discussed on the removal of pharmaceuticals. The latest development of reactive electrochemical membranes (REM) was regarded as an emerging EAO technique, and it was also highlighted. This work revealed that the EAO of pharmaceuticals has extraordinary application prospects in the field of water and wastewater treatment.

Modeling the Formation of Gas Bubbles inside the Pores of Reactive Electrochemical Membranes in the Process of the Anodic Oxidation of Organic Compounds.

The use of reactive electrochemical membranes (REM) in flow-through mode during the anodic oxidation of organic compounds makes it possible to overcome the limitations of plate anodes: in the case of REM, the area of the electrochemically active surface is several orders of magnitude larger, and the delivery of organic compounds to the reaction zone is controlled by convective flow rather than diffusion. The main problem with REM is the formation of fouling and gas bubbles in the pores, which leads to a decrease in the efficiency of the process because the hydraulic resistance increases and the electrochemically active surface is shielded. This work aims to study the processes underlying the reduction in the efficiency of anodic oxidation, and in particular the formation of gas bubbles and the recharge of the REM pore surface at a current density exceeding the limiting kinetic value. We propose a simple one-dimensional non-stationary model of the transport of diluted species during the anodic oxidation of paracetamol using REM to describe the above effects. The processing of the experimental data was carried out. It was found that the absolute value of the zeta potential of the pore surface decreases with time, which leads to a decrease in the permeate flux due to a reduction in the electroosmotic flow. It was shown that in the solution that does not contain organic components, gas bubbles form faster and occupy a larger pore fraction than in the case of the presence of paracetamol; with an increase in the paracetamol concentration, the gas fraction decreases. This behavior is due to a decrease in the generation of oxygen during the recombination reaction of the hydroxyl radicals, which are consumed in the oxidation reaction of the organic compounds. Because the presence of bubbles increases the hydraulic resistance, the residence time of paracetamol—and consequently its degradation degree—increases, but the productivity goes down. The model has predictive power and, after simple calibration, can be used to predict the performance of REM anodic oxidation systems.

Energy-efficient for advanced oxidation of bio-treated landfill leachate effluent by reactive electrochemical membranes (REMs): Laboratory and pilot scale studies

This study for the first time investigated the advanced treatment of bio-treated landfill leachate effluent using a novel reactive electrochemical membrane (REM) technology at the laboratory and pilot scales. At the laboratory scale, RuO2-Ir-REM, Ti4O7-REM, and β-PbO2-REM featured similar properties in pore size and water flux. Although RuO2-Ir-REM holds more reactive sites than the other two REMs, β-PbO2-REM and Ti4O7-REM featured higher oxidation ability than RuO2-Ir-REM, causing their high yield of hydroxyl radical. Consequently, β-PbO2-REM and Ti4O7-REM performed better than RuO2-Ir-REM, which removed total organic carbon and ammonia nitrogen by 70%–76% and 100%, respectively, after 45 minutes of treatment. Fluorescence spectroscopy analysis showed that humic acid-like substances were oxidized by the REM treatment. Using the β-PbO2-REM in the lab-scale setup with the solutions circulated, we observed a greater removal of chemical oxygen demand (COD) at a higher applied current or a faster water flux. The pilot system with four large size of β-PbO2-REMs modules in series was developed based on the lab-scale setup, which steadily treated landfill leachate in compliance with the disposal regulations of China, at an energy consumption of 3.6 kWh/m3. Also, a single-pass REM can effectively prevent the transformation of chloride to chlorate and perchlorate. Our study showed REM technology is a powerful and promising process for the advanced treatment of landfill leachate.

A 2D Convection-Diffusion Model of Anodic Oxidation of Organic Compounds Mediated by Hydroxyl Radicals Using Porous Reactive Electrochemical Membrane.

In recent years, electrochemical methods utilizing reactive electrochemical membranes (REM) have been considered as a promising technology for efficient degradation and mineralization of organic compounds in natural, industrial and municipal wastewaters. In this paper, we propose a two-dimensional (2D) convection-diffusion-reaction model concerning the transport and reaction of organic species with hydroxyl radicals generated at a TiOx REM operated in flow-through mode. It allows the determination of unknown parameters of the system by treatment of experimental data and predicts the behavior of the electrolysis setup. There is a good agreement in the calculated and experimental degradation rate of a model pollutant at different permeate fluxes and current densities. The model also provides an understanding of the current density distribution over an electrically heterogeneous surface and its effect on the distribution profile of hydroxyl radicals and diluted species. It was shown that the percentage of the removal of paracetamol increases with decreasing the pore radius and/or increasing the porosity. The effect becomes more pronounced as the current density increases. The model highlights how convection, diffusion and reaction limitations have to be taken into consideration for understanding the effectiveness of the process.

Mechanistic Investigation of Haloacetic Acid Reduction Using Carbon-Ti4O7 Composite Reactive Electrochemical Membranes.

Carbon-Ti4O7 composite reactive electrochemical membranes (REMs) were studied for adsorption and electrochemical reduction of haloacetic acids (HAAs). Powder activated carbon (PAC) or multiwalled carbon nanotubes (MWCNTs) were used in these composites. Results from flow-through adsorption experiments with dibromoacetic acid (DBAA) as a model HAA were interpreted with a transport model. It was estimated that ∼46% of C in the MWCNT-REM and ∼10% of C in the PAC-REM participated in adsorption reactions. Electrochemical reduction of 1 mg L-1 DBAA in 10 mM KH2PO4/K2HPO4 at -1.5 V/SHE (hydraulic residence time, ∼11 s) resulted in 73, 94, and 96% DBAA reduction for Ti4O7, PAC-Ti4O7, and MWCNT-Ti4O7 REMs, respectively. The reactive-transport model yielded kobs values between 9.16 and 33.3 min-1, which were 2 to 4 orders of magnitude higher than previously reported. PAC-Ti4O7 REM was tested with tap water spiked with 0.11 mg L-1 of nine different HAAs in a similar reduction experiment. The results indicated that all HAAs were reduced to <20 μg L-1. Moreover, the total combined concentration of five regulated HAAs was lower than the regulatory limit (60 μg L-1). Density functional theory simulations suggest that a direct electron transfer reaction was the probable rate-determining step for HAA reduction.

Modeling electrochemical oxidation and reduction of sulfamethoxazole using electrocatalytic reactive electrochemical membranes

In this research, degradation of the antibiotic sulfamethoxazole (SMX) was studied using electrochemical reduction and oxidation in single pass, flow-through mode using porous titanium suboxide (Ti4O7) reactive electrochemical membranes (REMs) and Pd-Cu doped Ti4O7 REMs (Pd-Cu/Ti4O7 REMs). Electrochemical reduction of SMX increased from 3.8 ± 0.3% for the Ti4O7 REM to 96.1 ± 3.9% for the Pd-Cu/Ti4O7 REM at −1.14 V/SHE and at a permeate flux of 300 L m−2 h−1 (LMH) (liquid residence time: ∼1.8 s). By contrast, electrochemical oxidation using Ti4O7 REMs achieved 95.7 ± 1.0% removal of SMX at 2.03 V/SHE and a permeate flux of 300 LMH (liquid residence time: ∼9.0 s) without the catalyst addition. We developed a reactive transport mathematical model and calibrated it to the SMX experimental data. The calibrated model predicted SMX permeate concentrations at fixed potentials and as a function of permeate flux. Based on products from SMX reduction, we proposed that SMX was reduced by a hydrogen atom transfer reaction that was mediated by the Pd-Cu/Ti4O7 REM. Toxicity tests indicated that electrochemical oxidation/reduction lowered solution toxicity. The results of this work indicate that a tandem electrochemical reduction/oxidation approach using the REM-based technology is a potential treatment strategy for sulfonamide-contaminated pharmaceutical wastewater.