Electrochemical filtration for drinking water purification: A review on membrane materials, mechanisms and roles

Electrochemical filtration can not only enrich low concentrations of pollutants but also produce reactive oxygen species to interact with toxic pollutants with the assistance of a power supply, making it an effective strategy for drinking water purification. In addition, the application of electrochemical filtration facilitates the reduction of pretreatment procedures and the use of chemicals, which has outstanding potential for maximizing process simplicity and reducing operating costs, enabling the production of safe drinking water in smaller installations. In recent years, the research on electrochemical filtration has gradually increased, but there has been a lack of attention on its application in the removal of low concentrations of pollutants from low conductivity water. In this review, membrane substrates and electrocatalysts used to improve the performance of electrochemical membranes are briefly summarized. Meanwhile, the application prospects of emerging single-atom catalysts in electrochemical filtration are also presented. Thereafter, several electrochemical advanced oxidation processes coupled with membrane filtration are described, and the related working mechanisms and their advantages and shortcomings used in drinking water purification are illustrated. Finally, the roles of electrochemical filtration in drinking water purification are presented, and the main problems and future perspectives of electrochemical filtration in the removal of low concentration pollutants are discussed.

Simultaneous decontamination of arsenite and antimonite using an electrochemical CNT filter functionalized with nanoscale goethite

The co-presence of arsenic (As) and antimony (Sb) in water bodies has been commonly reported. The toxicity of As and Sb varies with different speciation. Herein, we designed a dual-functional electrochemical filter toward "one-step" detoxification and sequestration of highly toxic As(III) and Sb(III). The key to this technology is a functional anodic filter consists of nanoscale goethite and carbon nanotubes (CNT). Results showed that 97.9% As(III) and 91.9% Sb(III) transformation and 86.4% As_total and 70.1% Sb_total removal efficiency can be obtained over 2 h continuous filtration under optimized conditions. The As_total removal kinetics and efficiency enhanced with flow rate and applied voltage (e.g., the As_total removal efficiency increased from 62.9% at 0 V to 86.4% at 2.5 V). This enhancement in kinetics and efficiency can be explained by the synergistic effects of the flow-through design, plentiful exposed sorption sites, electrochemical reactivity, and nanoscale goethite. Moreover, the proposed technology works effectively across a wide pH range. Only negligible inhibition was observed in the presence of nitrate, chloride, and carbonate. Exhausted hybrid filters can be effectively regenerated by using chemical wash with NaOH solution. This study not only revealed the different adsorption behaviors of As(III) and Sb(III) on the hybrid filters, but also provided new insights into rational design of continuous-flow filters toward simultaneous decontamination of As(III) and Sb(III).

Sea urchin-like FeOOH functionalized electrochemical CNT filter for one-step arsenite decontamination

Advanced nanotechnologies for efficient arsenic decontamination remain largely underdeveloped. The most abundant inorganic arsenic species are neutrally-charged arsenate, As(III), and negatively-charged arsenite, As(V). Compared with As(V), As(III) is 60 times more toxic and more difficult to remove due to high mobility. Herein, an electrochemical filtration system was rationally designed for one-step As(III) decontamination. The key to this technology is a functional electroactive carbon nanotube (CNT) filter functionalized with sea urchin-like FeOOH. With the assistance of electric field, CNT-FeOOH anodic filter can in situ transform As(III) to less toxic As(V) while passing through. Then, as-produced As(V) could be effectively sequestrated by FeOOH. The sufficient exposed sorption sites, flow-through design, and filter's electrochemical reactivity synergistically guaranteed a rapid arsenic removal kinetic. The underlying working mechanism was unveiled based on systematic experimental investigations and theoretical calculations. The system efficacy can be adapted across a wide pH range and environmental matrixes. Exhausted CNT-FeOOH filters could be effectively regenerated by chemical washing with diluted NaOH solution. Outcomes of the present study are dedicated to provide a straightforward and effective strategy by integrating electrochemistry, nanotechnology, and membrane separation for the removal of arsenic and other similar heavy metals from water bodies.

Electrochemical filtration process for simultaneous removal of refractory organic and particulate contaminants from wastewater effluents

Versatile electrochemical reactions are effective for removing a wide range of water contaminants. This study focuses on the development and testing of bifunctional electrocatalytic filter anodes as reactive and separating media for the simultaneous removal of refractory dissolved organic and particulate contaminants from real wastewater effluents. The results show that the TiO₂ particle interlayers formed between the Ti fiber support and the top composite metal oxide catalyst layers assist in reducing filter pores to an effective size range that enables removal of most particulates within the wastewater. The double-sheet design, which is a sandwich-structured module with an internal void space for permeate, prevents filter fouling, and transmembrane pressure can be maintained at a very low level of <5 kPa during batch and continuous flow reactor operations. Substantive and simultaneous removal of dissolved organics (e.g., chromophores, fluorophores, 1,4-dioxane, chemical oxygen demand, and total organic carbon) and particulate matter (i.e., turbidity) are achieved, although removal rates and efficacies differ depending on the electric current density applied. Decolorization and particulate rejection occur swiftly and immediately, but 1,4-dioxane degradation is relatively slow and quite time-dependent. Possible 1,4-dioxane degradation pathways during electrocatalysis are also proposed. Electrochemical filtration technology shows considerable promise for use in the next generation of advanced wastewater treatment solutions.

Removal of bisphenol A by electrochemical carbon-nanotube filter: Influential factors and degradation pathway

Bisphenol A is a chemical with hazardous health effects that is largely used in the manufacture of extensively used products including adhesives, plastics, powder paints, thermal paper and paper coatings, and epoxy resin, and is reported to exist in nature in an accumulative manner. In this study, both pristine and boron-doped multiwalled carbon nanotubes (MWNTs) were employed as filtration and electrochemical filtration materials, resulting in a significant removal of bisphenol A with identical performance for both MWNTs types. It was shown that the presence of salt is not critical for the greatest contaminant removal efficiency, likely due to the vital role of other electroactive species (e.g. reactive oxygen species). Near complete removal of 1 mg L^-1 bisphenol A at 2 and 3 V of applied DC potentials was achieved, indicating that the electrochemical filtration process is voltage dependent at both 2 and 3 V. Increasing the residence time by 7.4 fold (from 2.0 to 14.9 s) resulted in a significant removal of bisphenol A and its toxic byproducts, up to 424 min of electrochemical filtration time at 3 V of applied potential. Based on these results, electrochemical filtration using MWNTs is considered a promising technology for the removal of the accumulative bisphenol A and the reduction of its hazardous effects in waters.

Electrochemical efficacy of a carboxylated multiwalled carbon nanotube filter for the removal of ibuprofen from aqueous solutions under acidic conditions

This study provides insight into the efficiency of a functionalized multiwalled carbon nanotube filter for the removal of an anti-inflammatory drug, ibuprofen, through conventional filtration and electrochemical filtration processes. A comparison was made between carboxylated multiwalled carbon nanotubes (MWNTs-COOH) and pristine multiwalled carbon nanotubes (MWNTs) in order to emphasize the enhanced performance of MWNTs-COOH for the removal of ibuprofen using an electrochemical filtration process under acidic conditions. Ibuprofen-removal trials were evaluated based on absorbance values obtained using a UV/Vis spectrophotometer, and possible degradation products were identified using liquid chromatography mass spectrometry (LC-MS). The results exhibited near complete removal of ibuprofen by MWNTs-COOH at lower applied potentials (2 V), at lower flow rates, and under acidic conditions, which can be attributed to the generation of superoxides and their active participation in simultaneous degradation of ibuprofen, and its by-products, under these conditions. At higher applied potential (3 V), the possible participation of both bulk indirect oxidation reactions, and direct electron transfer were hypothesized for the removal behavior over time (breakthrough). At 3 V under acidic conditions, near 100% removal of the target molecule was achieved and was attributed to the enhanced generation of electroactive species toward bulk chemical reactions and a possible contribution from direct electron transfer under these conditions. The degradation by-products of ibuprofen were effectively removed by allowing longer residence time during the filtration process. Moreover, the effect of temperature was studied, yet showed a non-significant effect on the overall removal process.