Hydroxyl radicals in anodic oxidation systems: generation, identification and quantification

Synergistic oxidation and reduction of complex pollutants in wastewater using a self-cycling piezo-photocatalytic fenton system

Simultaneous organic pollutant oxidation and heavy metal ion reduction may enhance ocean and medical wastewater treatment. Here a self-cycling piezo-photocatalytic Fenton (SPF) system was developed using an Na-Sm bimetal-doped layered ferroelectric perovskite SrBi2Nb2O9 (NS-SBNO) and Fe2 + ions. The experimental results and density functional theory calculations demonstrated that bimetal doping induced an enhanced internal electric field, improving electron-hole pair separation and transmission. NS-SBNO exhibited increased adsorption energy and electron transfer capacity of H2O, leading to H2O2 production at 497 μmol g-1 h-1 through a two-electron water oxidation reaction without any sacrificial reagent. Furthermore, compared with traditional Fenton systems, the SPF system afforded higher Fe2+/Fe3+ interconversion efficiency and hydroxyl and superoxide radical yields, highlighting the system's excellent redox capabilities. The SPF system also demonstrated enhanced oxidation and reduction of complex pollutants in wastewater [e.g., BPA, TC, 2, 4-DCP, NOR, LVX, U(VI), and Cr(VI)], demonstrating promising practical applicability.

Predicting the Fate of Bisphenol A During Electrochemical Oxidation: A Simple Semiempirical Method Based on the Concentration Profile of Hydroxyl Radicals

The efficiency of electrochemical advanced oxidation processes (EAOPs) is fundamentally governed by hydroxyl-radical (•OH) generation. While direct experimental measurements of these transient species remain complex and impractical, robust computational methods for predicting their temporal profiles are notably scarce. This work presents a semi-empirical methodology based on H2O2 measuring experiments that enables indirect •OH quantification. We employed a recently developed carbon-based electrode and the priority pollutant bisphenol A (BPA) as the model system. The system achieved 92.3% BPA degradation with 84% mineralization efficiency during 5-h electrooxidation at 15 mA/cm2. Gas chromatography/mass spectrometry (GC/MS) was used for tracking BPA and detection of intermediates. On this basis, we developed a computational model that successfully predicts temporal concentration profiles of all reactive species interacting with •OH, along with degradation kinetics across current densities (10-20 mA/cm2). By incorporating predictions from the Toxicity Estimation Software Tool (T.E.S.T.), the developed model accurately simulates time-dependent evolution of relative toxicity throughout the treatment process. The presented approach has a general character and requires rather simple experimental input to predict and optimize degradation outcome in terms of input concentration, degradation time, current density, and final toxicity. Further modifications of the model would enable widening to other EAOPs systems.

Unlocking single-atom induced electronic metal-support interactions in electrocatalytic one-electron water oxidation for wastewater purification

Electro-oxidation is a promising green technology for decentralized wastewater purification. However, its efficacy is primarily constrained by the selectivity and efficiency of hydroxyl radical (•OH) generation through one-electron water oxidation. In this study, we elucidate the mechanism of electronic metal-support interactions (EMSI) of Ni single-atoms on antimony-doped tin oxide anode (Ni/ATO) to enhance •OH production and overall water treatment efficiency. We experimentally and theoretically investigate both the structural evolution process and micro-interface mechanisms associated with the EMSI effects induced by Ni single-atoms. The optimized electronic structures in the interfacial catalysts under EMSI conditions and the co-catalytic role of Ni single-atoms synergistically facilitate selective and efficient •OH generation, resulting in over a fivefold increase in its steady-state concentration and tenfold enhancement in pseudo-first-order rate constant of sulfamethoxazole degradation compared to those on bare ATO. With the EMSI, rapid electron transfer channels were established for a marked enhancement in the adsorption, conversion, and dissociation of interfacial H2O molecules. Notably, it is revealed that Ni single-atoms serve as co-catalytic sites, exhibiting a "H-pulling effect" that is crucial for •OH generation. The Ni/ATO anode demonstrates great efficiency in degrading various refractory organic pollutants, and effectively treats real pharmaceutical wastewater with low energy consumption. Furthermore, it presents remarkable stability and adaptability, while maintaining a minimal environmental footprint during wastewater treatment processes. This work addresses the theoretical gaps between EMSI effects and co-catalysis in electro-oxidation systems, while providing a robust technological solution for wastewater purification.

Dynamic electrode reconfiguration promotes in situ electrochemical peracetic acid synthesis for selective water decontamination

In situ synthesis and activation of peracetic acid (PAA) for water decontamination is a promising way to overcome the transport and storage problems in PAA applications. Here, an in situ electrochemical PAA synthesis and activation system is constructed using RuO2-Ti "active" electrode and graphite plate as the anode and the cathode, respectively. PAA is efficiently generated at the RuO2-Ti anode with a maximum real-time concentration of ∼1020 μM and a negligible precursor loss of 2.91 % after 180 min, and can be activated at the cathode to destruct a refractory pollutant (i.e., benzoic acid (BA)) with the rate constant of 0.22-0.28 h-1, even under the interference of co-existing anions. Multiple pieces of evidence, including differential electrochemical mass spectrometry, sulfoxide probing test, and electron paramagnetic resonance spectroscopy, indicate that the oxygen-atom-transferring oxidation of CH3COO- by a high-valent ruthenium-oxo intermediate (i.e., RuO3) in situ formed through the electrode reconfiguration between RuO2 and chem-sorbed HO• mainly accounts for PAA synthesis. Acetylperoxyl radical (CH3C(O)OO•) was evidenced as the dominant species for BA degradation. This study proposes an in situ strategy to electrochemically synthesize and activate PAA for selective water decontamination and enriches the understandings of the mechanism of "active" electrode in peroxide synthesis.

Electrochemical Degradation of Venlafaxine on Platinum Electrodes: Identification of Transformation Products by LC-MS/MS and In Silico Ecotoxicity Assessment

Antidepressants are emerging contaminants that have raised global concern due to their abuse. Venlafaxine (VFX), a serotonin and norepinephrine reuptake inhibitor, can cause adverse and potentially toxic effects on aquatic organisms. Electrochemical advanced oxidation processes (EAOPs) are gaining attention as promising degradation techniques for a variety of drugs. EAOP methods proposed for VFX degradation mainly utilize boron-doped diamond (BDD) electrodes, characterized by low background current and high oxygen overpotential. However, challenges arise, including delamination from the substrate, difficulties in scaling up, and limited service life. In this study, platinum was employed as an anode for the galvanostatic degradation of VFX, due to its stability and well-established surface cleaning procedure, which ensured high reproducibility. A 0.1 M Na2SO4 solution at pH 9 was used as the supporting electrolyte, and a current density of 25 mA/cm2 was applied. After 7 h, a degradation efficiency of 94% was achieved for a 25 ppm VFX solution. The hydroxyl and sulfate radicals generated in the electrochemical system were the active species responsible for VFX degradation, which followed a first-order kinetic model with a rate constant of 0.0084 min-1. The main degradation intermediates were identified through LC-MS, including two isomers with a nominal m/z of 276 and three isomers with a nominal m/z of 294. The toxicity of the VFX degradation products was assessed by an in silico prediction model. This evaluation confirmed the sustainability of the developed method.

Degradation of microplastics by electrocoagulation technology: Combination oxidation and flocculation effects

Electrocoagulation (EC) technology features a promising prospect for coping with the formidable microplastics (MPs) pollution challenge, albeit the underlying abatement mechanism still needs to be further clarified. Accordingly, in this work, we evaluated the removal performance by EC for four typical MPs, including polyvinyl chloride (PVC), polystyrene (PS), polypropylene (PP), and polyethylene (PE). The Fourier transform infrared spectroscopies of MPs confirmed the presence of electrochemical oxidation during EC process, owing to its hydroxyl radical generation ability as demonstrated by the detected fluorescence spectroscopies and electron paramagnetic resonance results, which has been rarely reported in other works. Specifically, 21.2 ± 0.8 %, 10.8 ± 1.8 %, 15.6 ± 1.6 %, and 7.6 ± 1.4 % abatement efficiency for PVC, PS, PP, and PE, respectively, originated from the oxidation effect, and these values for flocculation effect were 77.2 ± 0.8 %, 74.0 ± 1.6 %, 70.8 ± 1.2 %, and 69.2 ± 1.2 %, successively. Many factors influence these differences, especially the MPs' hydrophilicity, as it facilitates the mass transfer efficiency between MPs (like PVC and PP) and the generated flocs or radicals. To lay a foundation for practical application, we also optimized the operation parameters, demonstrating the wise choice of pH 7 to maintain a balance between the oxidation and flocculation effect. Therefore, we believe our work provides a good reference for promoting MPs abatement efficiency and elucidating the corresponding mechanism, especially the contribution of the oxidation part by EC.

Quinones stimulate reactive oxygen species production from zero-valent iron over centimeter distances

Zero-valent iron (ZVI) has demonstrated high potential for in-situ remediation of contaminated groundwater and soils. Upon exposure to oxygen, ZVI generates reactive oxygen species (ROS). In parallel with the electron transfer mediated-reductive path, ROS plays a critical role in the oxidative degradation of organic pollutants during ZVI remediation of groundwater and soil. Yet, the efficiency is often constrained by the confined ROS production localized to the surface or immediate vicinity of ZVI particles. Here, we demonstrate that quinones significantly enhance ROS production from ZVI over centimeter-scale distances. H₂O₂ and •OH were detected over 1 cm from ZVI particles after 24 h of incubation, with production increasing alongside quinone concentration and incubation time, reaching 318.3 ± 50.0 µM and 1263.2 ± 143.5 nM at 2 mm, respectively. The broad applicability of quinone in promoting remote ROS generation was demonstrated for various ZVI materials. This remote ROS production is driven by sequential electron transfer from ZVI to quinone, long-distance electron transfer via quinone, and subsequent electron transfer from reduced quinone to oxygen. The resulting increase in ROS production amount and extended range improved ZVI remediation efficiency by 5- to 15-fold for organic pollutant degradation. These findings provide a promising strategy for enhancing ROS-mediated ZVI remediation in heterogeneous environments.

Self-augmented catabolism mediated by Se/Fe co-doped bioceramics boosts ROS storm for highly efficient antitumor therapy of bone scaffolds

The overexpression of glutathione (GSH) within the tumor microenvironment has long been considered as the major obstacle for reactive oxygen species (ROS)-based antitumor therapies. To address this challenge, a selenite (SeO32 -) and ferric ion co-doped hydroxyapatite (SF-HAP) nanohybrid was synthesized, which is then introduced into poly-L-lactic acid (PLLA) to prepare porous scaffold by selective laser sintering to continuously release Fe3+ and SeO32- ions. Of great significance is the released SeO32- catabolize GSH to generate superoxide anion (O2•-) rather than directly eliminating GSH, thereby reversing the obstacle posed by its overexpression and achieving a "waste-to-treasure" transformation. The newly generated O2•- synergizes with the hydroxyl radicals (•OH) produced by the Fenton reaction between Fe3+/2+ and hydrogen peroxide, triggering high concentration ROS storms. Collectively, the PLLA/SF-HAP scaffold mediated self-augmented catabolism dynamic progress significantly raised intracellular ROS levels, almost twice as much as PLLA/Fe-HAP scaffold, thereby effectively inducing tumor cell apoptosis. The study provides an innovative inspiration for ROS-based antitumor therapies.

Accelerated Pollutant Degradation by UV/H2O2 at the Air-Water Interface of Microdroplets

Ultraviolet light-induced homolysis of hydrogen peroxide (UV/H2O2) can generate powerful hydroxyl radicals (•OH) for sustainable water purification. However, the efficiency of the conventional bulk-phase UV/H2O2 system is limited by the low yield and utilization of •OH, in turn necessitating high UV energy input and long purification period. In this study, we present an innovative UV/H2O2 microdroplet system for enhanced pollutant degradation. The degradation of pollutants in sprayed microdroplets was accelerated by 8.5-63.3-fold compared to those in bulk water, demonstrating universal effectiveness across a range of pollutant types and diverse aqueous matrices. This enhancement stems from elevated •OH production at the air-water interface due to the enhanced UV absorbance of H2O2. The production of •OH in the microdroplet system was 45-fold higher than that in bulk water, facilitating rapid •OH-mediated pollutant degradation. Moreover, pollutants accumulate at the air-water interface, where •OH is concentrated, leading to higher utilization of •OH for mediating pollutant degradation before quenching. Our findings provide a solution to overcome the bottlenecks in •OH production and utilization, offering insights for improving the efficiency of UV/H2O2 water treatment systems.

Treatment of Produced Water Using a Pilot-Scale Advanced Electrochemical Oxidation Unit

The main goal of this study is to optimize the treatment of produced water (PW) using a pilot-scale advanced electrochemical oxidation unit. The electro-cell is outfitted with a boron-doped diamond BDD anode and gas diffusion (GDE) cathode. Synthetic PW was prepared in the laboratory following a protocol designed to closely replicate the characteristics of real PW. The PW used in this study had a total dissolved solids (TDS) concentration of 16,000 mg/L and a total organic carbon (TOC) concentration of 250 mg/L. The effect of various electrooxidation parameters on the reduction in TOC was investigated including pH (2-12), electric current (I) (50-200 mA/cm2), and airflow rate (0-4 NL/min). Response surface method RSM with a Box-Behnken design at a confidence level of 95 percent was employed to analyze the impact of the above factors, with TOC removal used as a response variable. The results revealed that the TOC level decreased by 84% from 250 to 40 mg/L in 4 h, current density of 200 mA/cm2, pH of 12, and airflow rate 2 (NL/min). The investigation verified the influential role of diverse operational factors in the treatment process. RSM showed that reducing the airflow rate and increasing pH levels and electric current significantly enhanced the TOC removal. The obtained results demonstrated profound TOC removal, confirming the substantial potential of treating PW using the electrochemical method.

Ni(0) formation on carbon felt cathode triggers redox synergetic degradation of Ni-EDTA during electrochemical treatment for Ni-plating wastewater

Ni organic complexes are widely used in industrial production, posing severe threats to the natural environment and human health. In this work, we found that Ni (0) formed by the decomplexation and reduction of typical Ni organic complexes (Ni-EDTA) on the surface of carbon felt cathode. The formed Ni(0) as a catalytic site generates atomic hydrogen (H•) with strong reductive reactivity via electron transfer. H• not only accelerates the oxygen activation to form H2O2 on carbon felt cathode, but also reacts with H2O2 to generate strong oxidative hydroxyl radicals (HO•) and singlet oxygen (1O2). Under the attack of multiple active species, 95.5 % of Ni-EDTA is removed, and the removal efficiencies of total organic carbon (TOC) and dissolved Ni reach 43.8 % and 77.1 % after 120 min of electrolysis at -2.0 V (vs. Ag/AgCl). The redox synergistic reaction, initiated by the formation of Ni(0) on the carbon felt cathode, demonstrates excellent stability in treating real Ni plating wastewater. This approach eliminates the need for additional chemical reagents and prevents the release of heavy metals. Our findings offer a novel strategy for leveraging the self-transformation of Ni organic complexes to generate various active species for electrochemical treatment of Ni plating wastewater.

Electrocatalytic oxidation of hexafluoropropylene oxide homologues in water using a boron-doped diamond electrode

Hexafluoropropylene oxide (GenX) is a kind of substitute to PFOA, which has been listed in the Stockholm Convention. In this study, GenX was attempted to be degraded using a boron-doped diamond anode in the electrochemical oxidation system. The effects of operating parameters, including current density (0.5-10 mA/cm2), initial pH (3.0-11.49), initial concentration of GenX (20-150 mg/L), electrode distances (0.5-2 cm), electrolyte types (Na2SO4, NaCl, NaNO3 and NaHCO3) and Na2SO4 electrolyte concentration (40-80 mm), on GenX were studied. GenX can almost completely be degraded under the optimal operating parameters after 180 min of electrolysis. Free radical quenching experiments were carried out to investigate the effects of hydroxyl radicals and sulphate radicals on the degradation of GenX. The degradation intermediates were identified based on the ultra-high performance liquid chromatography equipped with a tandem mass spectrometer, and the degradation mechanisms were also proposed. Finally, the toxicities of GenX and its degradation products were evaluated using the QSAR models. The novelty is that the degradation mechanisms of the high concentration GenX (100 mg/L) were elucidated based on the free radical quenching experiments and the intermediates detected, when the degradation ratio reached 100%.

Buffered Hydroxyl Radical for Photocatalytic Non-Oxidative Methane Coupling

Hydroxy radical (⋅OH) is a prestigious oxidant that allows the cleavage of strong chemical bonds of methane but is untamed, leading to over-oxidation of methane and waste of oxidants, especially at high methane conversion. Here, we managed to buffer ⋅OH in an aqueous solution of photo-irradiated Fe3+, where ⋅OH almost participates in methane oxidation. Due to the interaction between Fe3+ and SO4 2-, the electron transfer from OH- to excited-state Fe3+ for ⋅OH generation is retarded, while excessive ⋅OH is consumed by generated Fe2+ to restore Fe3+. When combined with a Ru/SrTiO3:Rh photocatalyst, the buffered ⋅OH converts methane to C2+ hydrocarbons and H2 with formation rates of 246 and 418 μmol h-1, respectively. The apparent quantum efficiency reaches 13.0±0.2 %, along with 10.2 % methane conversion and 81 % C2+ selectivity after 80 hours of reaction. Overall, this work presents a strategy for controlling active radicals for selective and efficient photocatalysis.

Promoting SO4·- and ·OH Generation from Sulfate Solution toward Efficient Electrochemical Oxidation of Organic Contaminants at a B/N-Doped Diamond Flow-Through Electrode

Electrochemical oxidation via in situ-generated reactive oxygen species (ROS) is effective for the mineralization of refractory organic pollutants. However, the oxidation performance is usually limited by the low yield and utilization efficiency of ROS. Herein, a B/N-doped diamond (BND) flow-through electrode with enhanced SO4·-/·OH generation and utilization was designed for electrochemical oxidation of organic pollutants in sulfate solution. Both its SO4·-/·OH yields and SO4·- selectivity were improved by regulating B/N doping, and the production and utilization of SO4·-/·OH were facilitated by flow-through mode. BND showed fast PFOA oxidation with kinetic constants of 2.56-4.58 h-1 at low current densities of 2.0-5.0 mA cm-2. Its energy consumption for PFOA oxidation was 2.15-6.46 kWh m-3, which was lower than those of state-of-the-art electrodes under similar conditions. The BND anode was also efficient for treating organic fluorine wastewater and coking wastewater. The superior performance was contributed by its enhanced SO4·-/·OH yields and utilization, as well as high SO4·- selectivity.

Innovative approaches to electrochemical oxidation of Bisphenol B in synthetic and complex water environments

The substitution of Bisphenol A (BPA) with Bisphenol B (BPB) has raised concerns due to BPB's increased environmental presence and its potential hazards. Despite the frequent detection in water environments, effective removal methods for BPB are still limited. This study hypothesizes that electrochemical oxidation (EO) can effectively degrade BPB and its by-products. To test this, EO was applied under various conditions, analyzing the role of anode material, current density, pH, and BPB concentration. The results revealed that BPB degradation followed pseudo-first-order kinetics, with boron-doped diamond (BDD) anode showing a rate constant 27 times higher than iridium oxide electrodes. After 180 min, BDD achieved 81.8 % mineralization of BPB. The remaining organic load was associated to easily biodegradable short-chain carboxylic acids. Additionally, the EO process was evaluated in different matrices, including drinking water, tap water, simulated municipal wastewater, and synthetic urine, to assess the impact of matrix complexity. Electrogenerated oxidants, such as hydroxyl radicals, sulfate radicals, and active chlorine, significantly enhanced BPB degradation rates in real water matrices. Energy consumption varied from 5.32 kWh m-3 in drinking water to 2.28 kWh m-3 in synthetic urine, demonstrating the role of matrix composition in EO efficiency. These findings show that EO is a promising technology for removing BPB and similar chemicals in real-world water matrices.

Electrochemical degradation of aromatic organophosphate esters: Mechanisms, toxicity changes, and ecological risk assessment

Aromatic organophosphate esters (AOPEs), including triphenyl phosphate (TPHP), tricresyl phosphate (TCP), and 2-ethylhexyl diphenyl phosphate (EHDPP), pose significant health and ecological risks. Electrochemical advanced oxidation process (EAOP) is effective in removing refractory pollutants. In this study, the degradation performance and detoxication ability of AOPEs by EAOP were investigated. Hydroxylation, oxidation, and bond cleavage products were identified as major degradation products (DPs) due to the reaction with ·OH and O₂·-. Toxicity assessments using ecological structure activity relationship (ECOSAR) model and flow cytometry (FCM) revealed the cytotoxicity and aquatic toxicity for DPs were significantly decreased. 16S rRNA gene sequencing of sediment exposure to AOPEs and DPs were applied to assess ecological toxicity, and results showed reduced bacterial richness and diversity with EHDPP and TCP, while TPHP slightly enhanced richness. AOPEs and DPs altered bacterial genera involved in carbon, nitrogen, sulfur cycling and organic compound degradation. Bacterial community assembly suggested elevated stochastic processes and reduced ecotoxicity, confirming AOPEs can be effectively detoxified by 10-min EAOP treatment. Molecular ecological network analysis indicated increased complexity and stability of bacterial communities with DPs. These findings comprehensively revealed the toxicity of AOPEs and their DPs and provided the first evidence of effective degradation and detoxification by EAOP from ecotoxicological perspective.

Unique role of Mn(II) in enhancing electro-oxidation of organic pollutants on anodes with low oxygen evolution potential at low current density

This study systematically explored the role of Mn(II) in the removal of 4-chlorophenol (4-CP) by electro-oxidation (EO) employing anodes with low oxygen evolution potential (OEP), i.e., Ti/RuO2-IrO2, Ti/Pt, and Ti/Ti4O7, as well as anodes with high OEP, namely, Ti/PbO2, Ti/SnO2, and boron-doped diamond (Si/BDD). Mn(II) significantly promoted 4-CP removal on the anodes with low OEP at fairly low current density (0.04 to 1 mA/cm2), but had minimal to negative impact on those with high OEP. Cyclic voltammetry and X-ray photoelectron spectra revealed that Mn(II) was oxidized to Mn(III), then to Mn(IV) on the anodes with low OEP, whereas its was oxidized directly to Mn(IV) on those with high OEP. Deposition of manganese oxide on the anodes with low OEP suppressed oxygen evolution reaction (OER) in EO process, but enhanced OER on those with high OEP. Quenching and spectral results consistently indicated that Mn(III) and Mn(IV) were the primary species responsible for enhancing 4-CP removal on the anodes with low OEP. These findings provide mechanistic insights into the redox transformation of Mn(II) in EO and the theoretical basis for a novel strategy to boost pollutant degradation in EO systems using low OEP anodes through coupling with the redox chemistry of manganese.

Simultaneous generation of residue-free reactive oxygen species and bacteria capture for efficient electrochemical water disinfection

Residue-free and highly efficient techniques for drinking water disinfection are urgently needed. Herein, we report an electrochemical water disinfection system equipped with atomically precise Ag28 nanoclusters (NCs) as electrode materials. The deployment of these Ag28 NCs not only provides sufficient electrosorption sites for intelligent microbe enrichment but also ensures high-efficiency dual-mode microbial killing through the in situ electrocatalytic production of residue-free reactive oxygen species (ROS) and the inherent antimicrobial activity of Ag28 NCs. Moreover, the design of the system enables a cyclical "alive microbe capture-killing-dead microbe desorption" process for continuous water disinfection. On this basis, this water disinfection system is efficient against broad-spectrum microbes (with >99.99% antimicrobial activity), durable (with a performance reduction of only 0.75% over 40 cycles and 99.90% antimicrobial efficiency for over 5 h of continuous operation), versatile (i.e., other NCs can be used), scalable (with water productivity of 213.33 L h-1 m-2), energy efficient (with a low energy consumption of 4.91 Wh m-3; 1.04 Wh m-3 without the pumping cost) and applicable for various real water samples. This study may open new avenues for global water disinfection techniques.

Towards the Development of Novel, Point-of-Care Assays for Monitoring Different Forms of Antioxidant Activity: The RoXstaTM System

(1) Background: This study set out to develop a series of simple, novel, rapid methods for assessing different forms of antioxidant activity. (2) Methods: An ABTS platform was used to engineer: (i) an electrochemical post-activation assay to assess free radical scavenging activity; (ii) an electrochemical pre-activation strategy to assesses the suppression of free radical formation; (iii) a horseradish peroxidase-mediated oxidation system to monitor hydrogen peroxide scavenging activity and (iv) a cumene peroxide-hematin system to determine the ability of samples to scavenge the mixture of organic peroxides and peroxyl and alkoxyl radicals generated in the presence of these reagents. Each assay was assessed against a panel of candidate antioxidant compounds to determine their relative activities and specificities. In addition, human semen samples were analyzed to determine how the results of these antioxidant assays correlated with semen quality. (3) Results: All 4 assays revealed dose-dependent antioxidant activity on the part of vitamin C, N-acetyl cysteine, hypotaurine, BSA, melatonin, glutathione, resveratrol and epigallocatechin gallate. The other compounds tested either completely lacked antioxidant activity or were only active in one of the assays. Using unfractionated human semen as an exemplar of biological fluids rich in antioxidants, the outputs from the individual assays were found to reflect different aspects of semen quality. When the data from all 4 assays were combined, accurate predictions were generated reflecting the importance of oxidative stress in defining semen quality as reflected by sperm count, seminal lipid aldehyde content, sperm DNA damage and free radical generation by the sperm mitochondria. (4) Conclusions: The methodologies described in this paper constitute the basis for rapid, point-of-care assessments of oxidative stress.

Electrochemical removal of antiretroviral drug - raltegravir from aquatic media: Multivariate optimization, degradation studies and transformation products characterization

Electrochemical degradation of the antiretroviral drug raltegravir was investigated using different electrode materials (platinum, glassy carbon and boron-doped diamond). After preliminary studies with the use of multivariate chemometric method, electrochemical degradation was conducted with a boron-doped diamond electrode and phosphate buffer at pH 9. To assess the role of different variable in degradation kinetics, final experiments were conducted with varying applied current densities, chloride and humic acid concentrations, and using a natural river water sample. The results showed that raltegravir degradation generally followed pseudo-first-order kinetics. The degradation rate was inhibited by the presence of humic acid, while increasing the applied current density or chloride concentration enhanced the removal of raltegravir. Degradation process performed in the river water sample followed second-order kinetics and led to almost complete degradation of raltegravir within 30 min, highlighting the impact of natural matrices on reaction kinetics. Total organic carbon analysis was utilized, showing that even rapid degradation of the parent compound did not ensure total mineralization. Additionally, the energy consumption analysis revealed that the presence of chloride ions significantly improves efficiency of the organic carbon elimination. With the use of high-resolution mass spectrometry fourteen transformation products were elucidated, and their aquatic toxicity was predicted using in silico approach. Half of the identified transformation products were found to possess higher aquatic toxic potential than the parent compound, emphasizing the necessity of the mineralization assessment.

Impacts of chloride ions on the electrochemical decomplexation and degradation of Cr(III)-EDTA: Reaction mechanisms of HO• and RCS

The removal of Cr(III)-organic complexes, encompassing both decomplexation and ligand degradation, presents significant challenges in industrial wastewater treatment. As one of the most common anions in wastewater, Cl- significantly improves the efficiency of electrochemically removing Cr(III)-organic complexes through generated reactive chlorine species (RCS). In the electrochemical chlorine (EC/Cl2) process, extensive experimentation revealed that ClO• plays a dominant role in the degradation of Cr(III)-EDTA, surpassing the effects of free chlorine, direct electrooxidation, HO•, and other RCS. Density functional theory calculations indicated that RCS, primarily Cl• and ClO•, preferentially oxidize the ligand in Cr(III)-EDTA via H-abstraction, whereas HO• trends to attack the Cr atom through electron transfer. The influential factors on the degradation efficiency of Cr(III)-EDTA, Cr(VI) yield, and total organic carbon removal in EC/Cl2 were also assessed, including Cl- concentration, current density, and pH. Real industrial wastewater was employed as a reaction matrix to evaluate the application of the EC/Cl2 process for treating Cr(III)-EDTA, accompanied by energy efficiency calculations. Additionally, a two-chamber reactor was established to simultaneously oxidize Cr(III)-EDTA at the anode and reduce Cr(VI) at the cathode. This study provided insight into developing RCS-dominated AOPs to effectively decomplex and decompose organic Cr(III)-complexes in Cl--containing industrial wastewater.

Mechanism of adsorption and targeted degradation of antimicrobial micropollutant sulfamethoxazole in aquatic environments

FHWSB as an integrated absorptive catalyst, based on Walnut shell biochar (WSB) via hydrochloric acid modification and ferrous chloride impregnation, was prepared, reacted with H2O2 to generate active free radicals •OH and •O2-, which oxidized and degraded about 80% of micro-pollutant sulfamethoxazole (SMX) from water, effectively resolving micro-pollutants' removal being inefficient because of high toxicity, persistence, and bioaccumulation in existed methods. It was clarified the specific degradation pathways and mechanisms of SMX by FHWSB synergistic H2O2 via characterization and analysis assisted DFT calculations. Furthermore, it was found that the toxicity of a series of intermediates produced by SMX degraded continued to decline, consistent with its direction of degradation via toxicological analysis. The work provides a simple and feasible strategy for the effective removal of antibiotic micro-pollutants in aquatic environments.

Photochemical Origins of Iron Flocculation in Acid Mine Drainage

Acid mine drainage (AMD) raises a global environmental concern impacting the iron cycle. Although the formation of Fe(III) minerals in AMD-impacted waters has previously been reported to be regulated by biological processes, the role of abiotic processes remains largely unknown. This study first reported that a photochemical reaction coupled with O2 significantly accelerated the formation of Fe(III) flocculates (i.e., schwertmannite) in the AMD, as evidenced by the comparison of samples from contaminated sites across different natural conditions at latitudes 24-29° N. Combined with experimental and modeling results, it is further discovered that the intramolecular oxidation of photogenerated Fe(II) with a five-coordinative pyramidal configuration (i.e., [(H2O)5Fe]2+) by O2 was the key in enhancing the photooxidation of Fe(II) in the simulated AMD. The in situ attenuated total reflectance-Fourier transform infrared spectrometry (ATR-FTIR), UV-vis spectroscopy, solvent substitution, and quantum yield analyses indicated that, acting as a precursor for flocculation, [(H2O)5Fe]2+ likely originated from both the dissolved and colloidal forms of Fe(III) through homogeneous and surface ligand-to-metal charge transfers. Density functional theory calculations and X-ray absorption spectroscopy results further suggested that the specific oxidation pathways of Fe(II) produced the highly reactive iron species and triggered the hydrolysis and formation of transient dihydroxo dimers. The proposed new pathways of Fe cycle are crucial in controlling the mobility of heavy metal anions in acidic waters and enhance the understanding of complicated iron biochemistry that is related to the fate of contaminants and nutrients.

Assessment of Ti, Ir, Ta and Ru influence on mixed metal oxide electrodes for photoelectrochemical generation of persulfate: Impact on sulfamethoxazole degradation

The presence of persulfate (S2O82-) in decontamination processes favors the oxidation of organic pollutants due to its strong oxidation power. In this research we study the photoelectrochemical generation of persulfate using five mixed metal oxides electrodes (MMO) with different compositions and its effect on the degradation of sulfamethoxazole antibiotic (SMX) by photoelectrocatalysis (PEC) and electro-oxidation (EO). By PEC, all anodes generated a higher concentration of S2O82- than those not exposed to light. The high S2O82-concentration obtained by PEC was 0.150 mM using MMO[Ti/Ir/Ta] in a solution with Na2SO4 100 mM applying a current density of 2 mA/cm2. On the other hand, the maximum concentration obtained was 0.250 mM at 30 min of electrolysis for MMO[Ti/Ir/Ta] using Na2SO4 50 mM and applying current density of 5 mA/cm2. S2O82-production by EO was between 0.005 and 0.089 mM. It is observed that MMO based in Ta2O5 showed the best S2O82- production. The effect of S2O82- electro-generation (using the anode with the highest and the anode with the lowest S2O82- production) on the degradation of sulfamethoxazole by PEC and EO was studied using the experimental conditions with the best production of this oxidant. MMO[Ti/Ir/Ta] and MMO[Ti/Ru] were used as anodes, and it was observed that by PEC, 100% of SMX was degraded after 30 min of electrolysis using MMO[Ti/Ir/Ta] and 60 min using MMO[Ti/Ru]. By EO, the degradation of SMX was partial, demonstrating that the electrophotocatalytic effect favors the generation of S2O82-, enhancing the degradation of SMX at short electrolysis times.

Interstitial Boron Atoms in Pd Aerogel Selectively Switch the Pathway for Glycolic Acid Synthesis from Waste Plastics

Electro-reforming of poly(ethylene terephthalate) (PET) into valuable chemicals is garnering significant attention as it opens a mild avenue for waste resource utilization. However, achieving high activity and selectivity for valuable C2 products during ethylene glycol (EG) oxidation in PET hydrolysate on Pd electrocatalysts remains challenging. The strong interaction between Pd and carbonyl (*CO) intermediates leads to undesirable over-oxidation and poisoning of Pd sites, which hinders the highly efficient C2 products production. Herein, a nonmetallic alloying strategy is employed to fabricate a Pd-boron alloy aerogel (PdB), wherein B atoms are induced to regulate the electron structure and surface oxophilicity. This approach allows a remarkable mass activity of 6.71 A mgPd -1, glycolic acid (GA) Faradaic efficiency (FE) of 93.8%, and stable 100 h cyclic electrolysis. In situ experiments and density functional theory calculations reveal the contributions of B inserted in Pd lattice on highly effective EG-to-GA conversion. Interestingly, the heightened surface oxophilicity and regulated electronic structure by B incorporation weakened *CO intermediates adsorption and enhanced hydroxyl species affinity to accelerate oxidative *OH adspecies formation, thereby synergistically avoiding over-oxidation and boosting GA synthesis. This work provides valuable insights for the rational design of high-performance electrocatalysts for GA synthesis via an oxophilic B motifs incorporation strategy.

Cobalt single-atom catalyst tailored ceramic membrane for selective removal of emerging organic contaminants

Water reuse is an effective way to solve the issues of current wastewater increments and water resource scarcity. Ultrafiltration, a promising method for water reuse, has the characteristics of low energy consumption, easy operation, and high adaptability to coupling with other water treatment processes. However, emerging organic contaminants (EOCs) in municipal wastewater cannot be effectively intercepted by ultrafiltration, which poses significant challenges to the effluent quality and sustainability of ultrafiltration process. Here, we develop a cobalt single-atom catalyst-tailored ceramic membrane (Co1-NCNT-CM) in conjunction with an activated peroxymonosulfate (PMS) system, achieving excellent EOCs degradation and anti-fouling performance. An interfacial reaction mechanism effectively mitigates membrane fouling through a repulsive interaction with natural organic matter. The generation of singlet oxygen at the Co-N3-C active sites through a catalytic pathway (PMS→PMS∗→OH∗→O∗→OO∗→1O2) exhibits selective oxidation of phenols and sulfonamides, achieving >90% removal rates. Our findings elucidate a multi-layered functional architecture within the Co1-NCNT-CM/PMS system, responsible for its superior performance in organic decontamination and membrane maintenance during secondary effluent treatment. It highlights the power of integrating Co1-NCNT-CM/PMS systems in advanced wastewater treatment frameworks, specifically for targeted EOCs removal, heralding a new direction for sustainable water management.

Electro-Fenton degradation of pefloxacin using MOFs derived Cu, N co-doped carbon as a nanocomposite catalyst

Electro-Fenton (EF) can in-situ produce H2O2 and effectively activate H2O2 to generate powerful reactive species for the destruction of contaminants under acidic conditions, however, the production of iron-containing sludge and requirement of low working pH significantly hinder its practical application. Herein, a novel Cu, N co-doped carbon (Cu-N@C) with metal organic framework (MOF) as a precursor was constructed and adopted for the elimination of pefloxacin (PEF) in the heterogeneous electro-Fenton (HEF) process. PEF could be almost completely removed within 1 h and total organic carbon (TOC) removal efficiency was 48.57% within 6 h. Meanwhile, Cu-N@C had good repeatability and environmental adaptability, it can still maintain excellent catalytic performance after 10 cycles, and it exhibited satisfactory remediation performance in simulated water matrix. In addition, the HEF process catalyzed by Cu-N@C also showed satisfactory degradation effect on other organic pollutants including atrazine, methylene blue, and chlorotetracycline. Under the action of impressed current, the HEF system could generate H2O2 in-situ, and the active species could be generated in the redox cycle of Cu0/Cu1+/Cu2+. Electron paramagnetic resonance and quenching experiments confirmed that •OH was the dominant active species in the degradation of organic compounds. The degradation process of PEF was studied by mass spectrometry analysis of intermediate products. This study provided a simple method to prepare MOF-based electrocatalyst, which exhibits promising application potential for treatment wastewater.

Optimization study of a closed-cycle low-temperature evaporation system based on mathematical model and CFD

Treating high salt and high organic matter wastewater (HHW) generated during rapid socio-economic development is a significant challenge. This study aims to optimize a closed-cycle low-temperature evaporation (CCLE) system using mathematical modelling to be adapted to industrial applications. By using mathematical modelling and computational fluid dynamics (CFD), this study investigated the operating mechanism of the system under different operating conditions. Parametric analysis shows that increasing the compressor evaporation temperature and decreasing the condensation temperature is conducive to improving the performance of the heat pump unit, thereby increasing the wastewater treatment efficiency of the system and that a smaller heat transfer coil windward area is conducive to heat and mass transfer within the humidifier. The unique characteristics of the CCLE system are identified, and the wastewater treatment process under various operating conditions is explained. These findings may provide supporting information for the treatment of HHW by the CCLE system.

Investigation of singlet oxygen and superoxide radical produced from vortex-based hydrodynamic cavitation: Mechanism and its relation to cavitation intensity

Presently, the hydroxyl radical oxidation mechanism is widely acknowledged for the degradation of organic pollutants based on hydrodynamic cavitation technology. The presence and production mechanism of other potential reactive oxygen species (ROS) in the cavitation systems are still unclear. In this paper, singlet oxygen (1O2) and superoxide radical (·O2-) were selected as the target ROS, and their generation rules and mechanism in vortex-based hydrodynamic cavitation (VBHC) were analyzed. Computational fluid dynamics (CFD) were used to simulate and analyze the intensity characteristics of VBHC, and the relationship between the generation of ROS and cavitation intensity was thoroughly revealed. The results show that the operating conditions of the device have a significant and complicated influence on the generation of 1O2 and ·O2-. When the inlet pressure reaches to 4.5 bar, it is more favorable for the generation of 1O2 and ·O2- comparing with those lower pressure. However, higher temperature (45 °C) and aeration rate (15 (L/min)/L) do not always have positive effect on the 1O2 and ·O2- productions, and their optimal parameters need to be analyzed in combination with the inlet pressure. Through quenching experiments, it is found that 1O2 is completely transformed from ·O2-, and ·O2- comes from the transformation of hydroxyl radicals and dissolved oxygen. Higher cavitation intensity is captured and shown more disperse in the vortex cavitation region, which is consistent with the larger production and stronger diffusion of 1O2 and ·O2-. This paper shed light to the generation mechanism of 1O2 and ·O2- in VBHC reactors and the relationship with cavitation intensity. The conclusion provides new ideas for the research of effective ROS in hydrodynamic cavitation process.

Unravelling the Role of Free Radicals in Photocatalysis

Free radicals are increasingly recognized as active intermediate reactive species that can participate in various redox processes, significantly influencing the mechanistic pathways of reactions. Numerous researchers have investigated the generation of one or more distinct photogenerated radicals, proposing various hypotheses to explain the reaction mechanisms. Notably, recent research has demonstrated the emergence of photogenerated radicals in innovative processes, including organic chemical reactions and the photocatalytic dissolution of precious metals. To harness the potential of these free radicals more effectively, it is imperative to consolidate and analyze the processes and action modes of these photogenerated radicals. This conceptual paper delves into the latest advancements in understanding the mechanics of photogenerated radicals.

Glycerol Electrolyzer with Graphite Anode and Cathode Produces Carbonyl Compounds and Hydrogen: Background Electrocatalysis of a "Nonparticipating" Current Collector

This study unveils a novel role of bare graphite as a catalyst in glycerol electrooxidation and hydrogen evolution reactions, challenging the prevailing notion that current collectors employed in electrolyzers are inert. Half-cell experiments elucidate the feasibility of glycerol oxidation and hydrogen production on bulk graphite electrodes at potentials exceeding 1.7 V. The investigation of varying glycerol concentrations (0.05 to 1.5 mol L-1) highlights a concentration-dependent competition between glycerol electrooxidation and oxygen evolution reactions. Employing an H-type glycerol electrolyzer, polarization curves reveal significant activation polarization attributed to the low electroactivity of the anode. Glycerol electrolysis at different concentrations yields diverse product mixtures, including formate, glycolate, glycerate, and lactate at the anode, with concurrent hydrogen generation at the cathode. The anolyte composition changes with glycerol concentration, resulting in less-oxidized compounds at higher concentrations and more oxidized compounds at lower concentrations. The cell voltage also influences the product formation selectivity, with an increased voltage favoring more oxidized compounds. The glycerol concentration also affects hydrogen production, with lower concentrations yielding higher hydrogen amounts, peaking at 3.5 V for 0.05 mol L-1. This model quantitatively illustrates graphite's contribution to current and product generation in glycerol electrolyzers, emphasizing the significance of background current and products originating from current collectors if in contact with the reactants. These results have an impact on the efficiency of the electrolyzer and raise questions regarding possible extra non-noble "nonparticipating" current collectors that could affect overall performance. This research expands our understanding of electrocatalysis on graphite surfaces with potential applications in optimizing electrolyzer configurations for enhanced efficiency and product selectivity.

Comprehensive two-dimensional liquid chromatography with high resolution mass spectrometry to investigate the photoelectrochemical degradation of environmentally relevant pharmaceuticals and their degradation products in water

The widespread presence of organic micropollutants in the environment reflects the inability of traditional wastewater treatment plants to remove them. In this context, advanced oxidation processes (AOPs) have emerged as promising quaternary wastewater treatment technologies since they efficiently degrade recalcitrant components by generating highly reactive free radicals. Nonetheless, the chemical characterization of potentially harmful byproducts is essential to avoid the contamination of natural water bodies with hazardous substances. Given the complexity of wastewater matrices, the implementation of comprehensive analytical methodologies is required. In this work, the simultaneous photoelectrochemical degradation of seven environmentally relevant pharmaceuticals and one metabolite from the EU Watch List 2020/1161 was examined in ultrapure water and simulated wastewater, achieving excellent removal efficiencies (overall >95%) after 180 min treatment. The reactor unit was linked to an online LC sample manager, allowing for automated sampling every 15 min and near real-time process monitoring. Online comprehensive two-dimensional liquid chromatography (LC × LC) coupled with high resolution mass spectrometry (HRMS) was subsequently used to tentatively identify degradation products after photoelectrochemical degradation. Two reversed-phase liquid chromatography (RPLC) columns were used: an SB-C18 column operated with 5 mM ammonium formate at pH 5.8 (1A) and methanol (1B) as the mobile phases in the first dimension and an SB-Aq column using acidified water at pH 3.1 (2A) and acetonitrile (2B) as the mobile phases in the second dimension. This resulted in a five-fold increase in peak capacity compared to one-dimensional LC while maintaining the same total analysis time of 50 min. The LC x LC method allowed the tentative identification of 12 venlafaxine, 7 trimethoprim and 10 ciprofloxacin intermediates. Subsequent toxicity predictions suggested that some of these byproducts were potentially harmful. This study presents an effective hybrid technology for the simultaneous removal of pharmaceuticals from contaminated wastewater matrices and demonstrates how multidimensional liquid chromatography techniques can be applied to better understand the degradation mechanisms after the treatment of micropollutants with AOPs.

Step-by-step guide for electrochemical generation of highly oxidizing reactive species on BDD for beginners

Selecting the ideal anodic potential conditions and corresponding limiting current density to generate reactive oxygen species, especially the hydroxyl radical (•OH), becomes a major challenge when venturing into advanced electrochemical oxidation processes. In this work, a step-by-step guide for the electrochemical generation of •OH on boron-doped diamond (BDD) for beginners is shown, in which the following steps are discussed: i) BDD activation (assuming it is new), ii) the electrochemical response of BDD (in electrolyte and ferri/ferro-cyanide), iii) Tafel plots using sampled current voltammetry to evaluate the overpotential region where •OH is mainly generated, iv) a study of radical entrapment in the overpotential region where •OH generation is predominant according to the Tafel plots, and v) finally, the previously found ideal conditions are applied in the electrochemical degradation of amoxicillin, and the instantaneous current efficiency and relative cost of the process are reported.

A solar flow photo-reactor for antibiotic removal from aquaculture effluents using TiO2/carbon quantum dots

Effluents contaminated with antibiotics must be treated before reuse or even discharge into the aquatic environment, avoiding the increase of antimicrobial resistance (AMR) - a major public health problem of the 21st century. Little is known regarding the natural solar photodegradation of antibiotics in tubular reactors operated under flow mode and even less concerning the application of photocatalysts. The use of photocatalysts is considered a promising strategy for a sustainable solar-driven removal of antibiotics from effluents. In this work, the photodegradation of two antibiotics widely used in aquaculture, namely, sulfadiazine (SDZ) and oxolinic acid (OXA), was investigated under solar flow mode in the absence and presence of carbon quantum dots (CQDs) coupled with titanium dioxide (TiO2) (4% (w/w)). The obtained results showed that TiO2/CQDs (4% (w/w)) enhanced the photodegradation of both antibiotics, which is highly beneficial for their application in the treatment of aquaculture effluents. The accumulated UV energy needed for SDZ removal using the photocatalyst was less than 4 kJ L-1 in both simulated freshwater (phosphate buffer solution (PBS)) and simulated brackish water (sea salt solution (SSS)), while for OXA less than 5 kJ L-1 and around 15 kJ L-1 were needed for removal in PBS and in SSS, respectively. Moreover, results demonstrated that the proposed photocatalytic treatment was also efficient in the elimination of OXA and SDZ antibacterial activity, either in PBS or SSS. Therefore, photocatalysis under flow mode using TiO2/CQDs constitutes a promising and sustainable treatment for antibiotics' efficient removal from aquaculture effluents.

New Insights on Designing the Next-Generation Materials for Electrochemical Synthesis of Reactive Oxidative Species Towards Efficient and Scalable Water Treatment: A Review and Perspectives

Electrochemical water remediation technologies offer several advantages and flexibility for water treatment and degradation of contaminants. These technologies generate reactive oxidative species (ROS) that degrade pollutants. For the implementation of these technologies at an industrial scale, efficient, scalable, and cost-effective in-situ ROS synthesis is necessary to degrade complex pollutant mixtures, treat large amount of contaminated water, and clean water in a reasonable amount of time and cost. These targets are directly dependent on the materials used to generate the ROS, such as electrodes and catalysts. Here, we review the key design aspects of electrocatalytic materials for efficient in-situ ROS generation. We present a mechanistic understanding of ROS generation, including their reaction pathways, and integrate this with the key design considerations of the materials and the overall electrochemical reactor/cell. This involves tunning the interfacial interactions between the electrolyte and electrode which can enhance the ROS generation rate up to ~ 40% as discussed in this review. We also summarized the current and emerging materials for water remediation cells and created a structured dataset of about 500 electrodes and 130 catalysts used for ROS generation and water treatment. A perspective on accelerating the discovery and designing of the next generation electrocatalytic materials is discussed through the application of integrated experimental and computational workflows. Overall, this article provides a comprehensive review and perspectives on designing and discovering materials for ROS synthesis, which are critical not only for successful implementation of electrochemical water remediation technologies but also for other electrochemical applications.

Challenges Relating to the Quantification of Ferryl(IV) Ion and Hydroxyl Radical Generation Rates Using Methyl Phenyl Sulfoxide (PMSO), Phthalhydrazide, and Benzoic Acid as Probe Compounds in the Homogeneous Fenton Reaction

Ferryl ion ([FeIVO]2+) has often been suggested to play a role in iron-based advanced oxidation processes (AOPs) with its presence commonly determined using the unique oxidation pathway from methyl phenyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO2). However, we show here that the oxidation products of PMSO, formed on reaction with hydroxyl radical, enhance PMSO2 formation as a result of their complexation with Fe(III) leading to the changes in the reactivity of Fe(III) species in the homogeneous Fenton reaction. As such, PMSO should be used with caution to investigate the role of [FeIVO]2+ in iron-based AOPs with these insights suggesting the need to reassess the findings of many previous studies in which this reagent was used. The other common target compounds, phthalhydrazide and hydroxybenzoic acids, were also found to modify the rate and extent of iron cycling as a result of complexation and/or redox reactions, either by the probe compound itself and/or oxidation products formed. Overall, this study highlights that these confounding effects of the aromatic probe compounds on the reactivity of iron species should be recognized if reliable mechanistic insights into iron-based AOPs are to be obtained.

Surface Processes Control the Fate of Reactive Oxidants Generated by Electrochemical Activation of Hydrogen Peroxide on Stainless-Steel Electrodes

Low-cost stainless-steel electrodes can activate hydrogen peroxide (H2O2) by converting it into a hydroxyl radical (•OH) and other reactive oxidants. At an applied potential of +0.020 V, the stainless-steel electrode produced •OH with a yield that was over an order of magnitude higher than that reported for other systems that employ iron oxides as catalysts under circumneutral pH conditions. Decreasing the applied potential at pH 8 and 9 enhanced the rate of H2O2 loss by shifting the process to a reaction mechanism that resulted in the formation of an Fe(IV) species. Significant metal leaching was only observed under acidic pH conditions (i.e., at pH <6), with the release of dissolved Fe and Cr occurring as the thickness of the passivation layer decreased. Despite the relatively high yield of •OH production under circumneutral pH conditions, most of the oxidants were scavenged by the electrode surface when contaminant concentrations comparable to those expected in drinking water sources were tested. The stainless-steel electrode efficiently removed trace organic contaminants from an authentic surface water sample without contaminating the water with Fe and Cr. With further development, stainless-steel electrodes could provide a cost-effective alternative to other H2O2 activation processes, such as those by ultraviolet light.

Electrochemical Removal of Metal-Organic Complexes in Metal Plating Wastewater: A Comparative Study of Cu-EDTA and Ni-EDTA Removal Mechanisms

Cu and Ni complexes with ethylenediaminetetraacetic acid (Cu/Ni-EDTA), which are commonly present in metal plating industry wastewaters, pose a serious threat to both the environment and human health due to their high toxicity and low biodegradability. In this study, the treatment of solutions containing either or both Cu-EDTA and Ni-EDTA using an electrochemical process is investigated under both oxidizing and reducing electrolysis conditions. Our results indicate that Cu-EDTA is decomplexed as a result of the cathodic reduction of Cu(II) with subsequent electrodeposition of Cu(0) at the cathode when the cathode potential is more negative than the reduction potential of Cu-EDTA to Cu(0). In contrast, the very negative reduction potential of Ni-EDTA to Ni(0) renders the direct reduction of EDTA-complexed Ni(II) at the cathode unimportant. The removal of Ni during the electrolysis process mainly occurs via anodic oxidation of EDTA in Ni-EDTA, with the resulting formation of low-molecular-weight organic acids and the release of Ni2+, which is subsequently deposited as Ni0 on the cathode. A kinetic model incorporating the key reactions occurring in the electrolysis process has been developed, which satisfactorily describes EDTA, Cu, Ni, and TOC removal. Overall, this study improves our understanding of the mechanism of removal of heavy metals from solution during the electrochemical advanced oxidation of metal plating wastewaters.

Highly Selective Electrocatalytic CuEDTA Reduction by MoS2 Nanosheets for Efficient Pollutant Removal and Simultaneous Electric Power Output

Electrocatalytic reduction of ethylenediamine tetraacetic acid copper (CuEDTA), a typical refractory heavy metal complexation pollutant, is an environmental benign method that operates at mild condition. Unfortunately, the selective reduction of CuEDTA is still a big challenge in cathodic process. In this work, we report a MoS2 nanosheet/graphite felt (GF) cathode, which achieves an average Faraday efficiency of 29.6% and specific removal rate (SRR) of 0.042 mol/cm2/h for CuEDTA at - 0.65 V vs SCE (saturated calomel electrode), both of which are much higher than those of the commonly reported electrooxidation technology-based removal systems. Moreover, a proof-of-concept CuEDTA/Zn battery with Zn anode and MoS2/GF cathode is demonstrated, which has bifunctions of simultaneous CuEDTA removal and energy output. This is one of the pioneer studies on the electrocatalytic reduction of heavy metal complex and CuEDTA/Zn battery, which brings new insights in developing efficient electrocatalytic reduction system for pollution control and energy output.

Realigning the melon chains in carbon nitride by rubidium ions to promote photo-reductive activities for hydrogen evolution and environmental remediation

The photocatalytic efficiency of polymeric carbon nitride (PCN) suffers from unsatisfactory charge separation because of its amorphous structure. Herein, we report a simple bottom-up method to synthesize a novel structure of rubidium ion inserted PCN (Rb-PCN), which involves the regular alignment of melon chains to endow a crystalline feature in PCN. The insertion of Rb⁺ decreased not only the N p electrons in the heptazine ring but also the plane angle of the heptazine motifs in the melon chain, which promoted the long-range periodicity and crystallinity of carbon nitride. This structurally rearranged crystalline Rb-PCN demonstrated considerably enhanced separation of charge carriers, resulting in six-fold higher photocatalytic hydrogen evolution activity than its amorphous counterpart. Furthermore, the photoexcited electrons can be efficiently trapped by O₂ to generate H₂O₂, which facilitates the production of reactive oxygen species to inactivate bacteria and degrade organic pollutants, showing great potential for use in both energy and environmental applications.

Photoelectrocatalytic selective removal of group-targeting thiol-containing heterocyclic pollutants

The removal of a class of toxic thiol-containing heterocyclic pollutants from complex water matrices has great environmental significance. In this study, a novel photoanode (Au/MIL100(Fe)/TiO₂) with dual recognition functions was designed for selective group-targeting photoelectrocatalytic removal of thiol-containing heterocyclic pollutants from various aquatic systems. The average degradation and adsorption removal efficiency of 2-mercaptobenzimidazole, 2-mercaptobenzothiazole, and 2-mercaptobenzoxazole were still above 96.7% and 13.5% after selective treatment with Au/MIL100(Fe)/TiO₂ even coexisting with 10-fold concentration of macromolecular interferents (sulfide lignin and natural organic matters) and the same concentration of micromolecular structural analogues. While they were below 71.6% and 3.9% after non-selective treatment with TiO₂. Targets in the actual system were selectively removed to 0.9 µg L^-1, which is 1/10 of that after non-selective treatment. FTIR, XPS and operando electrochemical infrared results proved that the highly specific recognition mechanism was mainly attributable to both the size screening of MIL100(Fe) toward targets and Au-S bond formed between -SH group of targets and Au of Au/MIL100(Fe)/TiO₂. •OH are the reactive oxygen species. The degradation mechanism was further investigated via excitation-emission matrix fluorescence spectroscopy and LC-MS. This study provides new guidelines for the selective group-targeting removal of toxic pollutants with characteristic functional groups from complex water matrices.

Selective Removal of Emerging Organic Contaminants from Water Using Electrogenerated Fe(IV) and Fe(V) under Near-Neutral Conditions

Fe(IV) and Fe(V) are promising oxidants for the selective removal of emerging organic contaminants (EOCs) from water under near-neutral conditions. The Fe(III)-assisted electrochemical oxidation system with a BDD anode (Fe(III)-EOS-BDD system) has been employed to generate Fe(VI), while the generation and contributions of Fe(IV) and Fe(V) have been largely ignored. Thus, we examined the feasibility and involved mechanisms of the selective degradation of EOCs in the Fe(III)-EOS-BDD system under near-neutral conditions. It was found that Fe(III) application selectively accelerated the electro-oxidation of phenolic and sulfonamide organics and made the oxidation system be resistant to interference from Cl^-, HCO₃^-, and humic acid. Several lines of evidence indicated that EOCs were decomposed via direct electron-transfer process on the BDD anode and by Fe(IV) and Fe(V) but not Fe(VI), besides HO^•. Fe(VI) was not generated until the exhaustion of EOCs. Furthermore, the overall contributions of Fe(IV) and Fe(V) to the oxidation of phenolic and sulfonamide organics were over 45%. Our results also revealed that Fe(III) was oxidized primarily by HO^• to Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system. This study advances the understanding of the roles of Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system and provides an alternative for utilizing Fe(IV) and Fe(V) under near-neutral conditions.

Electrochemical Actuation of a DyP Peroxidase: A Facile Method for Drastic Improvement of the Catalytic Performance

Dye decolorizing peroxidases (DyP) have attracted interest for applications such as dye-containing wastewater remediation and biomass processing. So far, efforts to improve operational pH ranges, activities, and stabilities have focused on site-directed mutagenesis and directed evolution strategies. Here, we show that the performance of the DyP from Bacillus subtilis can be drastically boosted without the need for complex molecular biology procedures by simply activating the enzyme electrochemically in the absence of externally added H2O2. Under these conditions, the enzyme shows specific activities toward a variety of chemically different substrates that are significantly higher than in its canonical operation. Moreover, it presents much broader pH activity profiles with the maxima shifted toward neutral to alkaline. We also show that the enzyme can be successfully immobilized on biocompatible electrodes. When actuated electrochemically, the enzymatic electrodes have two orders of magnitude higher turnover numbers than with the standard H2O2-dependent operation and preserve about 30% of the initial electrocatalytic activity after 5 days of operation-storage cycles.

An anode and cathode cooperative oxidation system constructed with Ee-GF as anode and CuFe2O4/Cu2O/Cu@EGF as cathode for the efficient removal of sulfamethoxazole

This study aimed to further improve the degradation efficiency of pollutants by electrochemical oxidation system and reduce the consumption of electric energy. A simple method of electrochemical exfoliation was used to modify graphite felt (GF) to prepare an anode material (Ee-GF) with high degradation performance. An anode and cathode cooperative oxidation system was constructed with Ee-GF as the anode and CuFe₂O₄/Cu₂O/Cu@EGF as the cathode to efficiently degrade sulfamethoxazole (SMX). Complete degradation of SMX was achieved within 30 min. Compared with anodic oxidation system alone, the degradation time of SMX was reduced by half and the energy consumption was reduced by 66.8 %. The system displayed excellent performance for the degradation of different concentrations (10-50 mg L^-1) of SMX, different pollutants, and under different water quality conditions. In addition, the system still maintained 91.7 % removal rate of SMX after ten consecutive runs. At least 12 degradation products and seven possible degradation routes of SMX were generated in the degradation process by the combined system. The eco-toxicity of degradation products of SMX was reduced after the proposed treatment. This study provided a theoretical basis for the safe, efficient, and low energy consumption removal of antibiotic wastewater.

Peroxymonosulfate Activation by Facile Fabrication of α-MnO2 for Rhodamine B Degradation: Reaction Kinetics and Mechanism

The persulfate-based advanced oxidation process has been an effective method for refractory organic pollutants' degradation in aqueous phase. Herein, α-MnO₂ with nanowire morphology was facially fabricated via a one-step hydrothermal method and successfully activated peroxymonosulfate (PMS) for Rhodamine B (RhB) degradation. Influencing factors, including the hydrothermal parameter, PMS concentration, α-MnO₂ dosage, RhB concentration, initial pH, and anions, were systematically investigated. The corresponding reaction kinetics were further fitted by the pseudo-first-order kinetic. The RhB degradation mechanism via α-MnO₂ activating PMS was proposed according to a series of quenching experiments and the UV-vis scanning spectrum. Results showed that α-MnO₂ could effectively activate PMS to degrade RhB and has good repeatability. The catalytic RhB degradation reaction was accelerated by increasing the catalyst dosage and the PMS concentration. The effective RhB degradation performance can be attributed to the high content of surface hydroxyl groups and the greater reducibility of α-MnO₂, and the contribution of different ROS (reactive oxygen species) was ¹O₂ > O2·- > SO4·- > ·OH.

Photocatalysis-self-Fenton system over edge covalently modified g-C3N4 with high mineralization of persistent organic pollutants

The Fenton process is a widely used to remedy organic wastewaters, but it has problems of adding H₂O₂, low utilization efficiency of H₂O₂ and low mineralization efficiency. Here, a new photocatalysis-self-Fenton process was exploited for the removal of persistent 4-chlorophenol (4-CP) pollutant through coupling the photocatalysis of 4-carboxyphenylboronic acid edge covalently modified g-C₃N₄ (CPBA-CN) with Fenton. In this process, H₂O₂ was in situ generated via photocatalysis over CPBA-CN, the photogenerated electrons assisted the accelerated regeneration of Fe²⁺ to improve the utilization efficiency of H₂O₂, and the photogenerated holes facilitated the enhancement of 4-CP mineralization. Under the conjugation of CPBA, the electronic structure of CN was optimized and the molecular dipole was enhanced, resulting in the deepening valence band position, accelerated electron-hole pair separation, and improved O₂ adsorption-activation. Therefore, the incremental 4-CP degradation rate in the CPBA-CN photocatalysis-self-Fenton process was approaching 0.099 min^-1, by a factor of 3.1 times compared with photocatalysis. The parallel mineralization efficiency increased to 74.6% that was 2.1 and 2.6 times than photocatalysis and Fenton, respectively. In addition, this system maintained an excellent stability in the recycle experiment and can be potentially applied in a wide range of pHs and under the coexistence of various ions. This study would provide new insights for improving Fenton process and promote further development of Fenton in organic wastewater purification.

Identifying the evolution of primary oxidation mechanisms and pollutant degradation routes in the electro-cocatalytic Fenton-like systems

Herein, electro-catalysis (EC) as the electron donor to accelerate the continuable Fe(III)/Fe(II) cycles in different inorganic peroxides (i.e., peroxymonosulfate (PMS), peroxydisulfate (PDS) and hydrogen peroxide (HP)) activation systems were established. These electro-cocatalytic Fenton-like systems exhibited an excellent degradation efficiency of sulfamethoxazole (SMX). A series of analytical and characterization methods including quenching experiments, probe experiments, and electron paramagnetic resonance spectrometry (EPR) were implemented to systematically sort out the source and yield of reactive oxygen species (ROS). A wide kind of ROS including hydroxyl radical (^•OH), singlet oxygen (¹O₂), and sulfate radical (SO₄^•-), which contributed 38%, 37%, and 24% were produced in EC/Fe(III)/PMS system, respectively. ^•OH was the dominant ROS in both EC/Fe(III)/PDS and EC/Fe(III)/HP processes. According to the analysis of SMX degradation routes and biotoxicity, abundant degradation pathways were identified in EC/Fe(III)/PMS process and lower environmental impact was achieved in EC/Fe(III)/HP process. The diversiform ROS of EC/Fe(III)/PMS system makes it exhibit greater environmental adaptability in complex water matrixes and excellent low-energy consumption performance in many organic pollutants degradation. Continuous flow treatment experiments proved that the three systems have great sustainability and practical application prospect. This work provides a strong basis for constructing suitable systems to achieve different treatment requirements.

Enhanced Direct Electron Transfer Mediated Contaminant Degradation by Fe(IV) Using a Carbon Black-Supported Fe(III)-TAML Suspension Electrode System

Iron complexes of tetra-amido macrocyclic ligands (Fe-TAML) are recognized to be effective catalysts for the degradation of a wide range of organic contaminants in homogeneous conditions with the high valent Fe(IV) and Fe(V) species generated on activation of the Fe-TAML complex by hydrogen peroxide (H₂O₂) recognized to be powerful oxidants. Electrochemical activation of Fe-TAML would appear an attractive alternative to H₂O₂ activation, especially if the Fe-TAML complex could be attached to the anode, as this would enable formation of high valent iron species at the anode and, importantly, retention of the valuable Fe-TAML complex within the reaction system. In this work, we affix Fe-TAML to the surface of carbon black particles and apply this "suspension anode" process to oxidize selected target compounds via generation of high valent iron species. We show that the overpotential for Fe(IV) formation is 0.17 V lower than the potential required to generate Fe(IV) electrochemically in homogeneous solution and also show that the stability of the Fe(IV) species is enhanced considerably compared to the homogeneous Fe-TAML case. Application of the carbon black-supported Fe-TAML suspension anode reactor to degradation of oxalate and hydroquinone with an initial pH value of 3 resulted in oxidation rate constants that were up to three times higher than could be achieved by anodic oxidation in the absence of Fe-TAML and at energy consumptions per order of removal substantially lower than could be achieved by alternate technologies.

La2CoO4+δ perovskite-mediated peroxymonosulfate activation for the efficient degradation of bisphenol A

Sulfate radical-based technology has been considered as an efficient technology to remove pharmaceuticals and personal care products (PPCPs) with heterogeneous metal-mediated catalysts for the activation of peroxymonosulfate (PMS). In this study, La₂CoO_4+δ perovskite with Ruddlesden-Popper type structure was synthesised by the sol-gel method, which was employed in PMS activation. Different characteriazation technologies were applied for the characterization of La₂CoO_4+δ , such as SEM-EDX, XRD, and XPS technologies. A common organic compound, bisphenol A (BPA), is used as a target contaminant, and the effect impactors were fully investigated and explained. The results showed that when the dosage of La₂CoO_4+δ was 0.5 g L^-1 and the concentration of PMS was 1.0 mM in neutral pH solution, about 91.1% degradation efficiency was achieved within 25 minutes. Quenching experiments were introduced in the system to verify the catalytic mechanism of PMS for the BPA degradation, proving the existence of superoxide, hydroxyl radicals and sulfate radicals, which are responsible for the catalytic degradation of BPA. Moreover, the reusability and stability of the catalyst were also conducted which showed good stability during the reaction. This work would improve the applications of A₂BO₄-type perovskites for activating PMS to degrade BPA.