The Application of Electrochemical Methods in Water Treatment

Water resources form the cornerstone of sustainable development in human society. Over the past century, the accelerating pace of industrialization and urbanization has resulted in the discharge of significant pollutant loads stemming from anthropogenic activities. Simultaneously, the diversification of societal needs has prompted the widespread development and release of numerous synthetic compounds into aquatic environments, thereby increasing the complexity of water pollution and presenting formidable challenges for effective treatment. Against this backdrop, the advancement of wastewater treatment technologies becomes imperative for reducing contaminant concentrations to levels below ecologically acceptable thresholds, an essential prerequisite for safeguarding ecosystem health and ensuring the protection of public health.

For conventional pollutants commonly found in domestic wastewater, such as suspended solids, organic matter, nitrogen, and phosphorus, mature biochemical treatment technologies have been widely adopted in engineering practice, demonstrating robust performance and operational reliability [1]. However, the growing prevalence of emerging contaminants (ECs) in recent years has introduced substantial new challenges to the field of water treatment. ECs comprise a diverse array of substances, including persistent organic pollutants (POPs), endocrine-disrupting compounds (EDCs), antimicrobials, antibiotics, microplastics, and nanoparticles. Of particular concern are pharmaceuticals and personal care products (PPCPs), which represent a class of recalcitrant organic pollutants characterized by high chemical stability and low biodegradability. Even at trace concentrations, these substances can pose significant risks to both ecosystems and human health. Studies have shown that conventional wastewater treatment processes are often inadequate for the effective removal of many ECs, with residual contaminants frequently detected in secondary effluents or wasted sludge, thereby entering natural water bodies and exacerbating ecological risks [2]. As a result, the development of efficient, sustainable, and scalable treatment technologies specifically targeting these persistent and hazardous compounds has become a critical and urgent priority.

In response to these challenges, researchers have explored a broad range of advanced pollutant removal strategies, including membrane separation, adsorption, chemical oxidation, and electrochemical techniques. Among these, electrochemical technologies have garnered increasing attention due to their high removal efficiency, operational simplicity, and precise process control. Particularly noteworthy is the integration of electrochemical and biological processes within bioelectrochemical systems (BESs), which offers a promising pathway to overcome limitations commonly associated with conventional electrochemical methods, such as high energy consumption and the generation of undesirable byproducts. This synergistic approach not only mitigates inherent drawbacks but also broadens the applicability of electrochemical treatment to complex and variable wastewater matrices [3].

Against this backdrop, the Special Issue “Applications of Electrochemical Methods in Water Treatment (AEMWT)” was launched in the journal Water, with the objective of showcasing recent advances in electrochemical technologies and electron transfer mechanisms relevant to water treatment. This collection features cutting-edge research that bridges fundamental theoretical understanding with practical applications, addressing the urgent global need for effective wastewater treatment and the sustainable management of related solid waste.

This Special Issue presents a selection of exemplary studies that collectively underscore the versatility and potential of various technologies, especially the electrochemical technologies in tackling both conventional and emerging contaminants. Hong et al. developed an advanced oxidation system that integrates an air-diffusion cathode with ultraviolet (UV) irradiation. In this system, electrochemically generated hydrogen peroxide is activated by UV light, enabling the efficient mineralization of organic pollutants in lake water. The system exhibited excellent operational stability over extended periods and significantly enhanced the microbial safety of the treated water, offering a practical and scalable solution for water purification in regions experiencing water scarcity [contribution 8]. Guo et al. investigated the impact of different electron donor types on the microbial community structure and functional dynamics of electrode biofilms in BESs. It demonstrated that simple organic substrates (e.g., sodium acetate and glucose) rendered microbial communities more sensitive to electrode polarity, whereas complex substrates, such as actual domestic wastewater, promoted higher microbial diversity and community stability, with stronger spatial responses to electrode positioning. These findings provide important insights for optimizing the design and operational parameters of BESs [contribution 2]. Yao et al. proposed a hybrid denitrification system that couples solid-phase heterotrophic denitrification with electrochemical hydrogen autotrophic denitrification. Using polycaprolactone as a solid carbon source, the system achieved effective removal of nitrate from groundwater. Electrical stimulation was found to significantly enrich dominant denitrifying and electroactive microorganisms, with the reduction step from nitric oxide to nitrous oxide identified as the rate-limiting stage. This integrated approach offers an innovative strategy for mitigating nitrate pollution in groundwater environments [contribution 3]. Besides, other emerging studies have also provided unique insights into wastewater treatment and resource recovery applications. Lu et al. investigated the ozonation (O₃/H₂O₂) degradation of atrazine in phosphate buffer (pH 7), achieving a 92.59% removal rate under optimal conditions (25 °C, 20 mol/L O₃, 20 mol/L H₂O₂). Mechanism analysis revealed a synergistic 1:1 oxidation by HO∙ and O₃ under acidic conditions, with HO∙ playing the dominant role. Kinetics followed a pseudo-second-order model across various temperatures, pH, and H₂O₂ concentrations [contribution 1]. Wang et al. revealed that the addition of biocompatible high-mesh metal materials (Ni, Cu, stainless steel) significantly enhanced methane production from waste activated sludge by up to 61% (77.52 mL gVSS⁻¹) via promoting microbial cooperation and enriching syntrophic bacteria and methanogens, with electron transfer on metal surfaces further improving efficiency [contribution 4]. Ouyang et al. prepared rabbit manure biochar (RBC) at different pyrolysis temperatures (400–600 °C) and found that RBC600 exhibited the highest catalytic activity for activating peroxymonosulfate (PMS), degrading 93.38% of rhodamine B within 60 min via both free-radical (SO₄⁻·, ·OH) and non-radical (dominant ¹O₂) pathways, while maintaining preferable reusability over five cycles, enabling waste reuse and efficient organic wastewater treatment [contribution 5]. Huang et al. developed a co-pyrolysis method for Fenton sludge and pomelo peel that significantly enhanced biochar surface area and aromaticity, effectively immobilizing heavy metals by converting them into stable fractions and reducing leachability, thereby lowering ecological risk from considerable pollution to moderate or clean levels [contribution 6]. Guo et al. demonstrated that UV365 was the most effective wavelength in generating reactive species (³DOM*, ¹O₂, and ∙OH) from dissolved organic matter (HA, FA, EfOM), achieving the highest formation rates, steady-state concentrations, and quantum yields, thereby enhancing understanding of DOM photochemistry in aquatic environments [contribution 7]. Hou et al. demonstrated that the heterotrophic nitrification–aerobic denitrification strain JQ1004 efficiently removed nitrogen primarily through assimilation (54.61%) and heterotrophic nitrification–aerobic denitrification, with maximum degradation rates of 7.93 (ammonia) and 4.08 mg/(L·h) (nitrate), and revealed a potential direct ammonia oxidation pathway (dominated by highly transcribed amoA) alongside high oxygen tolerance and pH sensitivity of key functional genes [contribution 9]. Zhang et al. prepared N and KHCO₃ co-modified biochar from rapeseed straw, which exhibited a high tetracycline adsorption capacity of 604.71 mg/g through multiple mechanisms including pore filling, π–π interaction, and hydrogen bonding, demonstrating great potential for antibiotic removal from water [contribution 10].

The contributions to the AEMWT Special Issue collectively reflect both significant advancements and enduring challenges in the application of electrochemical technologies for water treatment. These studies not only highlight important technological breakthroughs but also contribute to the establishment of a solid theoretical foundation and practical framework to guide future research and engineering implementation in this rapidly evolving field.

We extend our sincere gratitude to all the authors whose contributions to this Special Issue exemplify intellectual rigor and a deep commitment to advancing the field. Their works have significantly expanded the frontiers of knowledge in electrochemical water treatment. We are equally grateful to the expert reviewers for their meticulous evaluations and scholarly insights, which have been instrumental in ensuring the academic integrity and high quality of this publication. Special appreciation is also due to the Water editorial team for their steadfast support and seamless coordination, which enabled the successful and timely release of this issue. It is through this collective effort that we have assembled a forward-looking and practically impactful body of work.

In conclusion, the AEMWT Special Issue marks a significant milestone in the application of electrochemical technologies to address contemporary water pollution challenges. As researchers dedicated to this field, we are both encouraged and inspired by the achievements presented herein. We warmly invite colleagues around the world to join in fostering innovation and collaboration aimed at advancing electrochemical water treatment toward broader implementation and technological maturity.