Engineering Interface with a One-Dimensional RuO2/TiO2 Heteronanostructure in an Electrocatalytic Membrane Electrode: Toward Highly Efficient Micropollutant Decomposition

Electric field-confined synthesis of single atomic TiOxCy electrocatalytic membranes

Electrocatalysis exhibits certain benefits for water purification, but the low performance of electrodes severely hampers its utility. Here, we report a general strategy for fabricating high-performance three-dimensional (3D) porous electrodes with ultrahigh electrochemical active surface area and single-atom catalysts from earth-abundant elements. We demonstrate a binder-free dual electrospinning-electrospraying (DESP) strategy to densely distribute single atomic Ti and titanium oxycarbide (TiOxCy) sub-3-nm clusters throughout interconnected carbon nanofibers (CNs). The composite offers ultrahigh conductivity and mechanical robustness (ultrasonication resistant). The resulting TiOxCy filtration membrane exhibits record-high water purification capability with excellent permeability (~8370 liter m-2 hour-1 bar-1), energy efficiency (e.g., >99% removal of toxins within 1.25 s at 0.022 kWh·m-3 per order), and erosion resistance. The hierarchical design of the TiOxCy membrane facilitates rapid and energy-efficient electrocatalysis through both direct electron transfer and indirect reactive oxygen species (1O2, ·OH, and O2·-, etc.) oxidations. The electric field-confined DESP strategy provides a general platform for making high-performance 3D electrodes.

Synergistic roles of oxidation and self-aggregation in efficient ultrafiltration membrane fouling alleviation using a flow-through Sb-SnO2 anode during wastewater reclamation

The application of ultrafiltration (UF) in wastewater reclamation alleviates the demand for limited water supplies. However, the membrane fouling caused by the effluent organic matter (EfOM) becomes a major obstacle for UF application. In this study, a pre-oxidation strategy for UF using a Sb-SnO2 (ATO) anode in flow-through mode was proposed with the hopes to improve the performance of UF during wastewater reclamation. The results indicated that this flow-through ATO (FA) anode significantly outperformed a boron-doped diamond (BDD) anode in terms of EfOM degradation and membrane fouling control. It is noteworthy that apart from oxidation, the self-aggregation behavior of foulants was also involved in the mechanisms of membrane fouling mitigation. On the one hand, FA pre-oxidation relieved the burden of membrane fouling by decomposing the macromolecular EfOM into small molecular organic matter, and even mineralizing it. The effective destruction of unsaturated EfOM by FA pre-oxidation made a remarkable contribution to fouling mitigation due to the strong correlation between the total fouling index and UV254. On the other hand, the surface morphology of membrane and interface properties of foulants revealed the self-aggregation behavior of foulants. FA pre-oxidation made the foulants aggregate spontaneously and reduced the potential of forming a dense cake layer on the membrane surface, which was conductive for water permeation. Overall, FA pre-oxidation proved to be a feasible and chemical-free option for UF pretreatment to simultaneously produce high-quality reused water and alleviate membrane fouling during wastewater reclamation.

Contact-Electro-Catalysis-Assisted Separation via a Dancing PTFE Membrane for Fouling Control

Advanced oxidization processes (AOPs) offer promising solutions for addressing the fouling issues in membrane separation systems. However, the high energy requirements for electrical or light power in the AOPs can be a drawback. In this study, we present a contact-electro-catalysis (CEC)-based approach for controlling membrane fouling, which is stimulated by mild ultrasonic irradiation. During this process, electrons are transferred between a dancing polytetrafluoroethylene membrane and water or oxygen molecules, resulting in the formation of free radicals •OH and •O2-. These free radicals are capable of degrading or inactivating foulants, eliminating the need for additional chemical cleaners, secondary waste disposal, or external stimuli. Furthermore, the time-dependent voltage spikes/oscillations (peak, +7.8/-8.2 V) generate a nonuniform electric field that drives dielectrophoresis, effectively keeping contaminants away from the membrane surface and further enhancing the antifouling performance of the dancing membrane. Therefore, the CEC-assisted membrane separation system offers a green and effective strategy for controlling membrane fouling through mild mechanical stimulation.

Sustainable application of electrocatalytic and photo-electrocatalytic oxidation systems for water and wastewater treatment: a review

Wastewater treatment and reuse have risen as a solution to the water crisis plaguing the world. Global warming-induced climate change, population explosion and fast depletion of groundwater resources are going to exacerbate the present global water problems for the forthcoming future. In this scenario, advanced electrochemical oxidation process (EAOP) utilising electrocatalytic (EC) and photoelectrocatalytic (PEC) technologies have caught hold of the interest of the scientific community. The interest stems from the global water management plans to scale down centralised water and wastewater treatment systems to decentralised and semicentralised treatment systems for better usage efficiency and less resource wastage. In an age of rising water pollution caused by contaminants of emerging concern (CECs), EC and PEC systems were found to be capable of optimal mineralisation of these pollutants rendering them environmentally benign. The present review treads into the conventional electrochemical treatment systems to identify their drawbacks and analyses the scope of the EC and PEC to mitigate them. Probable electrode materials, potential catalysts and optimal operational conditions for such applications were also examined. The review also discusses the possible retrospective application of EC and PEC as point-of-use and point-of-entry treatment systems during the transition from conventional centralised systems to decentralised and semi-centralised water and wastewater treatment systems.

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.

Environment-friendly and efficient electrochemical degradation of sulfamethoxazole using reduced TiO2 nanotube arrays-based Ti membrane coated with Sb-SnO2

This study focused on the preparation, characterization, and sulfamethoxazole (SMX) removal performance of the SnO₂-coated reactive electrochemical membrane (REM). This REM was fabricated by loading SnO₂ on the reduced TiO₂ nanotube arrays (RTNA)-based Ti membrane (TM). Regarding the dopant for SnO₂, Sb was more effective in boosting the electrocatalytic activity than Bi, and the energy consumption for Sb-SnO₂-coated REM (TM/RTNA/ATO) was lower than Bi-SnO₂-coated REM (TM/RTNA/BTO). As for the internal layer, RTNA provided TM/RTNA/ATO with more electroactive surface areas and prolonged the service lifetime. Compared with batch mode, the SMX removal efficiency in flow-through mode was increased up to 8.4-fold. The SMX degradation performances were also affected by fluid velocity, current density, initial SMX concentration, and electrolyte concentration. The synergistic effects of ^•OH oxidation and direct electron transfer were responsible for the effective removal of SMX. TM/RTNA/ATO was proved to be stable and durable by multi-cycle and accelerated lifetime tests. Its extensive applicability was verified with high removal efficiencies of SMX in the surface water and wastewater effluent. These results demonstrate the promise of TM/RTNA/ATO for water treatment applications.

BiPO4-coated carbon microtube electrodes: preparation and characterization of their properties and electrocatalytic degradation of methylene blue

Bismuth phosphate (BiPO₄), a very attractive candidate for organics electrodegradation, harbors tremendous potential on removing contaminants from water. Here, four carbon microtube electrodes were prepared from corncob, each coated with BiPO₄ by a different method to study the electrodegradation of methylene blue (MB). A thorough insight into the composite features of four electrodes was characterized. Better reversibility and electrocatalytic activity of the fourth electrode (BCC4) prepared by digital signal generator was presented with a current density of 5.71 mA cm^-2 at a potential of 1.6 V vs Ag/AgCl. The electrochemical impedances and actual lifetime of BCC4 were 125 Ω and 833 h, respectively. The effectiveness of each kind of BiPO₄/carbon electrode was preliminarily evaluated by analyzing the actual conversion rate of the MB concentration, which confirms MB electrodegradation by the BiPO₄/carbon electrode was mainly dominated by the hydroxyl radical oxidation. The mass transfer rate was increased by carbon microtube; thereby, electrocatalysis of BiPO₄/carbon electrode increased as revealed by an increase in the MB degradation rate. The rate constants k obtained for the degradation of MB by BiPO₄/carbon electrode at 20 ℃ was 0.0046 mM^-1 s^-1, which was 11 times than that of BiPO₄. The diffusion layer was decreased by carbon microtube, resulting in MB electrodegradation rate increased. The BiPO₄ coated on the surface of the carbon microtube electrodes strengthened their electrocatalytic performance, which shed new light on effective selection of suitable carbon electrode for degradation of organics. Therefore, BiPO₄/carbon electrode could be potentially applied in the electrodegradation of organic pollutants.

Recent advances in electrocatalytic membrane for the removal of micropollutants from water and wastewater

The increasing occurrence of micropollutants in water and wastewater threatens human health and ecological security. Electrocatalytic membrane (EM), a new hybrid water treatment platform that integrates membrane separation with electrochemical technologies, has attracted extensive attention in the removal of micropollutants from water and wastewater in the past decade. Here, we systematically review the recent advances of EM for micropollutant removal from water and wastewater. The mechanisms of the EM for micropollutant removal are first introduced. Afterwards, the related membrane materials and operating conditions of the EM are summarized and analyzed. Lastly, the challenges and future prospects of the EM in research and applications are also discussed, aiming at a more efficient removal of micropollutants from water and wastewater.

A High Flux Electrochemical Filtration System Based on Electrospun Carbon Nanofiber Membrane for Efficient Tetracycline Degradation

In this work, an electrochemical filter using an electrospun carbon nanofiber membrane (ECNFM) anode fabricated by electrospinning, stabilization and carbonization was developed for the removal of antibiotic tetracycline (TC). ECNFM with 2.5 wt% terephthalic acid (PTA) carbonized at 1000 °C (ECNFM-2.5%-1000) exhibited higher tensile stress (0.75 MPa) and porosity (92.8%), more graphitic structures and lower electron transfer resistance (23.52 Ω). Under the optimal condition of applied voltage 2.0 V, pH 6.1, 0.1 mol L−1 Na2SO4, initial TC concentration 10 ppm and membrane flux 425 LMH, the TC removal efficiency of the electrochemical filter of ECNFM-2.5%-1000 reached 99.8%, and no obvious performance loss was observed after 8 h of continuous operation. The pseudo-first-order reaction rate constant in flow-through mode was 2.28 min−1, which was 10.53 times higher than that in batch mode. Meanwhile, the energy demand for 90% TC removal was only 0.017 kWh m−3. TC could be converted to intermediates with lower developmental toxicity and mutagenicity via the loss of functional groups (-CONH2, -CH3, -OH, -N(CH3)2) and ring opening reaction, which was mainly achieved by direct anodic oxidation. This study highlights the potential of ECNFM-based electrochemical filtration for efficient and economical drinking water purification.

Simultaneously enhancing degradation of refractory organics and achieving nitrogen removal by coupling denitrifying biocathode with MnOx/Ti anode

To simultaneously remove carbon and nitrogen from refractory organic wastewater, this study couples the denitrifying biocathode and MnO_x/Ti anode to oxidize refractory organic pollutants in the anode chamber and remove NO₃^--N in the cathode chamber (denitrifying biocathode-electrocatalytic reactor, DBECR). After inoculation, DBECR started up at 1.3 and 1.5 V with NO₃^--N reduction peak appearing on the cyclic voltammetry curve and increased NO₃^--N removal by approximately 90 %. Compared to the electrocatalytic reactor without inoculation (ECR), NO₃^--N removal of DBECR significantly increased from 0.09 to 0.45 kg NO₃^--N/m³ NCC/d (NCC: net cathodic compartment). NO₃^--N removal correlated well with charges/current flowing through the circuit of DBECR, further validating the presence of electrotrophic denitrifiers. Moreover, coupling of denitrifying biocathode significantly enhanced methylene blue (MB) removal in the anode chamber (0.18 ± 0.002 and 2.92 ± 0.02 g COD/m²/d for ECR and DBECR, respectively). This was because the growth of eletrotrophic denitrifiers increased the cathodic potential and thus the potential of MnO_x/Ti anode. The higher potential of MnO_x/Ti anode promoted the generation of hydroxyl radicals and consequently promoted MB removal. This study demonstrated that DBECR not only realized nitrogen removal in the cathode chamber, but also enhanced refractory organic carbon degradation in the anode chamber.