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Heavy metals-laden industrial wastewater represents both a threat to ecosystems and human health and, in some instances, a potential source of valuable metals however the presence of organic ligands that bind the metals in heavy metal complexes (HMCs) renders metal removal (and, where appropriate, recovery) difficult. Electrochemical-based oxidation and reduction processes represent a potentially promising means of degrading the organic ligands and reducing their ability to retain the metals in solution. In this state-of-the-art review, we provide a comprehensive overview of the current status on use of electrochemical redox technologies for organic ligand degradation and subsequent heavy metal removal and recovery from industrial wastewaters. The principles and degradation mechanism of common organic ligands by various types of electrochemical redox technologies are discussed in this review and consideration given to recent progress in electrode materials synthesis, cell architecture, and operation of electrochemical redox systems. Furthermore, we highlight the current challenges in application of electrochemical redox technologies for treatment of HMC-containing wastewaters including (i) limited understanding of the chemical composition of industrial wastewaters, (ii) constrained mass transfer process affecting the direct/indirect electron transfer, (iii) absence of approaches to convert recovered metal into high-value-added products, and (iv) restricted semi-or full-industrial-scale application of these technologies. Potential strategies for improvement are accordingly provided to guide efforts in addressing these challenges in future research., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Electrochemical advanced oxidation processes (EAOP) have shown great potential for the abatement of complexed heavy metals, such as metal-EDTA complexes, in recent studies. While removal of metal-EDTA complexes has been extensively examined in bench-scale reactors, much less attention has been given to the efficacy of this process at larger scale. In this study, we utilize a 72 L pilot-scale continuous flow system comprised of six serpentine flow channels and 90 pairs of flow-through electrodes for the degradation of Ni-EDTA complexes and removal of Ni from solution. The influence of a range of key operating parameters including flow rate, current density and initial Ni-EDTA concentration on rate and extent of Ni-EDTA degradation and Ni removal were examined. Our results showed that at a feed flow rate of 36 L h -1 , current density of 5 mA cm -2 and initial Ni-EDTA concentration of 1 mM, the pilot-scale system achieved 74 % total Ni removal, 78 % total EDTA removal and 40 % TOC removal with energy consumption of 13.6 kWh m -3 order -1 and energy efficiency of 7.9 g kWh -1 for total Ni removal. A mechanistically-based kinetic model, which was developed in our previous bench-scale study, provides a satisfactory description of the experimental results obtained in the pilot-scale unit. Long term operation of the pilot-scale unit resulted in corrosion of PbO 2 anode along with inorganic scaling as well as organic fouling on the PbO 2 surface resulting in an obvious decline in Ni-EDTA degradation. Overall, the results of this study suggest that large scale anodic oxidation of wastewaters containing metal-organic complexes is an effective means of degrading organic ligands thereby enabling removal of the metal at the cathode. However, additional efforts are required to enhance the durability of the anode material and reduce material costs and energy consumption., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

The peroxone process (O 3 /H 2 O 2 ) is reported to be a more effective process than the ozonation process due to an increased rate of generation of hydroxyl radicals ( • OH) and inhibition of bromate (BrO 3 - ) formation which is otherwise formed on ozonation of bromide containing waters. However, the trade-off between the H 2 O 2 dosage required for minimization of BrO 3 - formation and effective pollutant removal has not been clearly delineated. In this study, employing experimental investigations as well as chemical modelling, we show that the concentration of H 2 O 2 required to achieve maximum pollutant removal may not be the same as that required for minimization of BrO 3 - formation. At the H 2 O 2 dosage required to minimize BrO 3 - formation (<10 µg/L), only pollutants with high to moderate reactivity towards O 3 and • OH are effectively removed. For pollutants with low reactivity towards O 3 / • OH, high O 3 (O 3 :DOC>>1 g/g) and high H 2 O 2 dosages (O 3 :H 2 O 2 ∼1 (g/g)) are required for minimizing BrO 3 - formation along with effective pollutant removal which may result in a very high residual of H 2 O 2 in the effluent, causing secondary pollution. On balance, we conclude that the peroxone process is not effective for the removal of low reactivity micropollutants if minimization of BrO 3 - formation is also required., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Metal-organic complexes, especially those of ethylenediaminetetraacetic acid (EDTA) with metals such as copper (Cu) and nickel (Ni) (denoted here as Cu-EDTA and Ni-EDTA), are common contaminants in wastewaters from chemical and plating industries. In this study, a multi-electrode (ME) system using a two-chamber reactor and two pairs of electrodes is proposed for simultaneous electrochemical oxidation of a wastewater containing both Cu-EDTA and Ni-EDTA complexes as well as separation and selective recovery of Cu and Ni onto two different cathodes via electrodeposition. Our results demonstrate that the ME system successfully achieved 90% EDTA removal, 99% solid Cu recovery at the Cu recovery cathode and 56% Ni recovery (33.3% on the Ni recovery cathode and 22.6% in the solution) after a four-hour operation. The system further achieved 85.5% Ni recovery after consecutive five cycles of operation for 20 h. While Cu removal was mainly driven by the direct reduction of EDTA-complexed Cu(II) at the cathode, oxidation of EDTA within the Ni-EDTA complex at the anode was a prerequisite for Ni removal. The oxidation of metal-bound EDTA and free EDTA was driven by • OH and direct electron transfer on the PbO 2 anode surface and graphite anode, respectively. We further show that ME system performs well for all pH conditions, treatment of real wastewaters as well as wastewaters containing other metals ions (Cr and Zn) along with Cu/Ni. The separation efficiency of Cu and Ni is dependent on applied electrode potential as well as nature and concentration of binding ligand present with comparatively lower separation efficiency achieved in the presence of weaker binding capacity and/or at lower ligand concentration and lower applied electrode potential. As such, some optimization of electrode potential is required depending on the nature/concentration of ligands in the wastewaters. Overall, this study provides new insights into the design and operation of EAOP technology for effective organic abatement and metal recovery from wastewaters containing mixtures of various metal-organic complexes., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Dissolved organic matter (DOM) driven-photochemical processes play an important role in the redox cycling of trace metals and attenuation of organic contaminants in estuarine and coastal ecosystems. In this study, we evaluate the effect of Cu on 4-carboxybenzophenone (CBBP) and Suwannee River natural organic matter (SRNOM)-photosensitized degradation of seven target contaminants (TCs) including phenols and amines under pH conditions and salt concentrations typical of those encountered in estuarine and coastal waters. Our results show that trace amounts of Cu(II) (25 -500 nM) induce strong inhibition of the photosensitized degradation of all TCs in solutions containing CBBP. The influence of TCs on the photo-formation of Cu(I) and the decrease in the lifetime of transformation intermediates of contaminants (TC •+ / TC • (-H) ) in the presence of Cu(I) indicated that the inhibition effect of Cu was mainly due to the reduction of TC •+ / TC • (-H) by the photo-produced Cu(I). The inhibitory effect of Cu on the photodegradation of TCs decreased with the increase in Cl - concentration since less reactive Cu(I)-Cl complexes dominate at high Cl - concentrations. The impact of Cu on the SRNOM-sensitized degradation of TCs is less pronounced compared to that observed in CBBP solution since the redox active moieties present in SRNOM competes with Cu(I) to reduce TC •+ / TC • (-H) . A detailed mathematical model is developed to describe the photodegradation of contaminants and Cu redox transformations in irradiated SRNOM and CBBP solutions., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)

Catalyst deactivation is an ongoing concern for industrial application of catalytic ozonation processes. In this study, we systematically investigated the performance of a catalytic ozonation process employing Fe 2 O 3 /Al 2 O 3 •SiO 2 catalyst for the treatment of coal chemical industry (CCI) wastewater using pilot-scale and laboratory-scale systems. Our results show that the activity of the Fe 2 O 3 /Al 2 O 3 •SiO 2 catalyst for organic contaminant removal deteriorated over time due to formation of a dense and thin carbonaceous layer on the Fe 2 O 3 catalyst surface. EPR and fluorescence imaging analysis confirm that the passivation layer essentially inhibited the O 3 -catalyst interaction thereby minimizing formation of surficial • OH and associated oxidation of organic contaminants on the catalyst surface. Calcination was demonstrated to be effective in restoring the activity of the catalyst since the carbonaceous layer could be efficiently combusted during calcination to re-establish the surficial • OH-mediated oxidation process. The combustion of the carbonaceous layer and restoration of the Fe layer on the surface on calcination was confirmed based on SEM-EDX, FTIR and thermogravimetric analysis. Cost analysis indicates that regeneration using calcination is economically viable compared to catalyst replacement. The results of this study are expected to pave the way for developing appropriate regeneration techniques for deactivated catalysts and optimising the catalyst synthesis procedure., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)

With the development of our society, the desire to recover valuable metal resources from metal-containing wastewaters or natural water bodies is becoming increasingly stronger nowadays. To overcome the limitations of single techniques, coupling technologies with synergistic effects are attracting increasing attention regarding metal resource recovery from water with particular interest in electrochemical coupling technologies in view of the advantages of electrochemical methods. This state-of-the-art review comprehensively presented the mechanisms and performance of electrochemical coupling systems for metal recovery from water. To give a clear overview of current research trends, technologies coupled with electrochemical processes can be categorized into six main types: electrochemical techniques, membrane modules, adsorption/extraction techniques, sonication technologies, energy supply techniques and others. The electrochemical coupling system has shown synergistic advantages (e.g., improving metal recovery efficiency, reducing energy consumption) over single technologies. We then discuss the remaining challenges, present corresponding solutions, and put forward future directions for current electrochemical coupled systems towards metal recovery. This review is conducive to broadening the potential applications of electrochemical coupling processes for metal recovery and sustainable water treatment., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

As(III) removal from groundwaters is challenging because of its neutral charge and low surface affinity under circumneutral pH conditions. In this work, we investigate the influence of Ca 2+ and Mg 2+ on the removal of As(III) by a redox active polyvinylferrocene (PVF) functionalized electrode in a modified double potential step chronoamperometry (DPSC) setup. In the absence of divalent cations, nearly 90% As(III) removal is achieved over ten continuous cycles by single-pass DPSC, even in the presence of competing anions, however the presence of divalent cations at concentrations ≥ 1.25 mM significantly inhibits As(III) removal. The divalent cations enhance arsenic removal in the first (removal) step but suppress electrode regeneration in the 2 nd step. Our results suggest that Ca 2+ /Mg 2+ either acts as a bridge between the electrode surface and As anions or the sorption of Ca 2+ /Mg 2+ increases the positive charge on the electrode surface thereby facilitating As(V) sorption. We show that effective electrode regeneration can be achieved using an NaOH wash however the overall complexity of the process increases. Overall, we conclude that the influence of divalent cations on As removal by electro-sorption processes needs to be taken into consideration for application of this technology for real groundwater treatment., (Copyright © 2021 Elsevier B.V. All rights reserved.)

In this work, a novel Ni-doped PbO 2 anode (Ni-PbO 2 ) was prepared via a co-electrodeposition method and used to remove Ni-ethylenediaminetetraacetic acid (Ni-EDTA) from solutions typical of electroless nickel plating wastewater. Compared with a pure PbO 2 electrode, Ni doping increased the oxygen evolution potential as well as the reactive surface area and reactive site concentration and reduced the electron transfer resistance thereby resulting in superior Ni-EDTA degradation performance. The 1% Ni-doped PbO 2 electrode exhibited the best electrochemical oxidation activity with a Ni-EDTA removal efficiency of 96.5 ± 1.2%, a Ni removal efficiency of 52.1 ± 1.4% and an energy consumption of 2.6 kWh m - 3 . Further investigations revealed that 1% Ni doping enhanced both direct oxidation and hydroxyl radical mediated oxidation processes involved in Ni-EDTA degradation. A mechanism for Ni-EDTA degradation is proposed based on the identified products. The free nickel ion concentration initially increased as a result of the degradation of Ni-EDTA complexes and subsequently decreased as a consequence of nickel electrodeposition on the cathode surface. Further characterization of the cathode deposits by X-ray diffraction and X-ray photoelectron spectra indicated that the deposition products were a mixture of Ni 0 , NiO and Ni(OH) 2 with elemental Ni accounting for roughly 80% of the deposited nickel. Results of this study pave the way for the application of anodic oxidation processes for efficient degradation of Ni-containing complexes and recovery of Ni from nickel-containing wastewaters., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The heterogeneous catalytic ozonation process is a promising treatment option for high salinity reverse osmosis concentrate (ROC) however the influence of salts on the catalyst performance is not well understood. In this work, we investigate the effect of salts on the performance of the catalytic ozonation process for treatment of synthetic ROC using a commercially available Fe-loaded Al 2 O 3 catalyst. Our results show that the presence of salts influences the rate and extent of degradation of organic compounds present in the synthetic ROC when subjected to the heterogeneous catalytic ozonation process. Scavenging of aqueous O 3 by chloride ions and/or transformation of organics (particularly humics) to more hydrophobic form as a result of charge shielding between adjacent functional groups and/or intramolecular binding by cations inhibits the bulk oxidation of organics to a measurable extent. While the scavenging of aqueous hydroxyl radicals at the salt concentrations investigated here was minimal, the accumulation of chloride ions in the electric double layer near the catalyst surface, particularly when pH< pH pzc , results in more significant scavenging of surface associated hydroxyl radicals. Overall, the presence of salts (particularly chloride ions) has a significant influence on the performance of both conventional and catalytic ozonation processes with some scope to mitigate this effect through appropriate choice of catalyst., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Cu-based Fenton systems have been recognized as a promising suite of technologies for the treatment of industrial wastewaters due to their high catalytic oxidation capacity. Rapid progress regarding Cu Fenton systems has been made not only in fundamental mechanistic aspects of these systems but also with regard to applications over the past decade. Based on available literature, this review synthesizes the recent advances regarding both the understanding and applications of Cu-based Fenton processes for industrial wastewater treatment. Cu-based catalysts that are essential to the effectiveness of use of Cu Fenton reactions for oxidation of target species are mainly classified into two types: (i) Cu complexes with organic or inorganic ligands, and (ii) Cu composites with inorganic materials. Performance of the Cu-based catalysts for the removal of organic pollutants in industrial wastewaters are reviewed, with the key operating parameters illustrated. Furthermore, the roles of Cu complexes and composites in both homogeneous and heterogeneous Cu-Fenton systems are critically examined with particular focus on the mechanisms involved. Perspectives and future efforts needed for Cu-based Fenton systems using Cu complexes and composites for industrial wastewater treatment are presented., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Recent studies have demonstrated that the rate at which Fe(II)-Fe(III) oxyhydroxide systems catalyze the reduction of reducible contaminants, such as 4-chloronitrobenzene, is well correlated to their thermodynamic reduction potential. Here we confirm this effect in the presence of Fe(III) oxyhydroxide phases not previously assessed, namely ferrihydrite and nano-goethite, as well as Fe(III) oxyhydroxide phases previously examined. In addition, silicate is found to decrease the extent of Fe(II) sorption to the Fe(III) oxyhydroxide surface, increasing the reduction potential of the Fe(II)-Fe(III) oxyhydroxide suspension and, accordingly, decreasing the rate of 4-chloronitrobenzene reduction. A linear relationship between the reduction potential of the Fe(II)-Fe(III) oxyhydroxide suspensions and the reduction rate of 4-chloronitrobenzene (normalized to surface area and concentration of sorbed Fe(II)) was obtained in the presence and absence of silicate. However, when ferrihydrite was doped with Si (through co-precipitation) the reduction of 4-chloronitrobenzene was much slower than predicted from its reduction potential. The results obtained have significant implications to the likely effectiveness of naturally occurring contaminant degradation processes involving Fe(II) and Fe(III) oxyhydroxides in groundwater environments containing high concentrations of silicate, or other species which compete with Fe(II) for sorption sites., (Copyright © 2016 Elsevier B.V. All rights reserved.)

In this study sodium dithionite (NaS2O4) and sodium borohydride (NaBH4) were employed as reducing agents for the synthesis of nanosized iron-based particles. The particles formed using NaBH4 (denoted nFe(BH4)) principally contained (as expected) Fe(0) according to XAS and XRD analyses while the particles synthesized using NaS2O4, (denoted nFe(S2O4)) were dominated by the mixed Fe(II)/Fe(III) mineral magnetite (Fe3O4) though with possible presence of Fe(0). The ability of both particles to reduce trichloroethylene (TCE) under analogous conditions demonstrated remarkable differences with nFe(BH4) resulting in complete reduction of 1.5mM of TCE in 2h while nFe(S2O4) were unable to effect complete reduction of TCE in 120 h. Moreover, acetylene was the major reaction product formed in the presence of nFe(S2O4) while the major reaction product formed following reaction with nFe(BH4) was ethylene, which was further reduced to ethane as the reaction proceeded. Considering that effective Pd reduction to Pd(0) requires the presence of Fe(0), this is consistent with our finding that Fe(0) is not the dominant phase formed when employing dithionite as a reducing agent under the conditions employed in this study., (Copyright © 2015 Elsevier B.V. All rights reserved.)