Kinetic Modeling of the Electro-Fenton Process: Quantification of Reactive Oxygen Species Generation

Catalyst durability in electrocatalytic H2O2 production: key factors and challenges

On-demand electrocatalytic hydrogen peroxide (H2O2) production is a significant technological advancement that offers a promising alternative to the traditional anthraquinone process. This approach leverages electrocatalysts for the selective reduction of oxygen through a two-electron transfer mechanism (ORR-2e-), holding great promise for delivering a sustainable and economically efficient means of H2O2 production. However, the harsh operating conditions during the electrochemical H2O2 production lead to the degradation of both structural integrity and catalytic efficacy in these materials. Here, we systematically examine the design strategies and materials typically utilized in the electroproduction of H2O2 in acidic environments. We delve into the prevalent reactor conditions and scrutinize the factors contributing to catalyst deactivation. Additionally, we propose standardised benchmarking protocols aimed at evaluating catalyst stability under such rigorous conditions. To this end, we advocate for the adoption of three distinct accelerated stress tests to comprehensively assess catalyst performance and durability.

Electrocatalysts for Inorganic and Organic Waste Nitrogen Conversion

Anthropogenic activities have disrupted the natural nitrogen cycle, increasing the level of nitrogen contaminants in water. Nitrogen contaminants are harmful to humans and the environment. This motivates research on advanced and decarbonized treatment technologies that are capable of removing or valorizing nitrogen waste found in water. In this context, the electrocatalytic conversion of inorganic- and organic-based nitrogen compounds has emerged as an important approach that is capable of upconverting waste nitrogen into valuable compounds. This approach differs from state-of-the-art wastewater treatment, which primarily converts inorganic nitrogen to dinitrogen, and organic nitrogen is sent to landfills. Here, we review recent efforts related to electrocatalytic conversion of inorganic- and organic-based nitrogen waste. Specifically, we detail the role that electrocatalyst design (alloys, defects, morphology, and faceting) plays in the promotion of high-activity and high-selectivity electrocatalysts. We also discuss the impact of wastewater constituents. Finally, we discuss the critical product analyses required to ensure that the reported performance is accurate.

Kinetic Modeling Assisted Analysis of Vitamin C-Mediated Copper Redox Transformations in Aqueous Solutions

The kinetics of oxidation of micromolar concentrations of ascorbic acid (AA) catalyzed by Cu(II) in solutions representative of biological and environmental aqueous systems has been investigated in both the presence and absence of oxygen. The results reveal that the reaction between AA and Cu(II) is a relatively complex set of redox processes whereby Cu(II) initially oxidizes AA yielding the intermediate ascorbate radical (A•-) and Cu(I). The rate constant for this reaction was determined to have a lower limit of 2.2 × 104 M-1 s-1. Oxygen was found to play a critical role in mediating the Cu(II)/Cu(I) redox cycle and the oxidation reactions of AA and its oxidized forms. Among these processes, the oxidation of the ascorbate radical by molecular oxygen was identified to play a key role in the consumption of ascorbic acid, despite being a slow reaction. The rate constant for this reaction (A • - + O 2 → DHA + O 2 • - ) was determined for the first time with a calculated value of 54 ± 8 M-1 s-1. The kinetic model developed satisfactorily describes the Cu/AA/O2 system over a range of conditions including different concentrations of NaCl (0.2 and 0.7 M) and pH (7.4 and 8.1). Appropriate adjustments to the rate constant for the reaction between Cu(I) and O2 were found to account for the influence of the chloride ions and pH on the kinetics of the process. Additionally, the presence of Cu(III) as the primary oxidant resulting from the interaction between Cu(I) and H2O2 in the Cu(II)/AA system was confirmed, along with the coexistence of HO•, possibly due to an equilibrium established between Cu(III) and HO•.

Waste coconut shell-derived carbon monolith as an efficient binder-free cathode for electrochemical advanced oxidation treatment of endocrine-disrupting compounds

Discharge of endocrine-disrupting compounds such as methylparaben (MePa) into natural water bodies deteriorates the aquatic ecosystem. In this regard, electrochemical oxidation (EO) and electro-Fenton (EF) processes are acknowledged as effective methods to eliminate biorecalcitrant compounds from different wastewater matrices. In these systems, the H2O2-producing ability of carbon-based cathodes is put to advantage for producing homogenous hydroxyl radicals by simulating Fenton's reaction, which dramatically augments the contaminant removal efficiency. However, commercial carbon based cathodes are not economically affordable, especially for voluminous treatment. Hence in the present work, waste-derived carbonised coconut shell (CCS) monolith was employed as a cathode in EO and EF treatment of MePa. Almost the entire MePa with initial concentration of 10 mg/L was removed in 60 min by EO and 45 min by EF process at neutral pH, applied current density of 7.5 mA/cm2, NaCl concentration of 1.0 g/L and 10 mg/L of Fe2O3 dosing. The MePa removal efficiency of the CCS cathode-fitted system after 60 min was better than the commercial graphite plate and Ti-based mixed metal oxide employing system due to higher H2O2 electrosynthesis (H2O2 = 9.0 ± 0.6 mg/L after 60 min). Moreover, the same setup was used for treating 10 mg/L of MePa-spiked real sewage and demonstrated MePa and total organic carbon removal efficiency of 80.16 ± 2.31% and 37.42 ± 3.50%, respectively, in 45 min. Further, the CCS-mediated EF treatment achieved >90% removal of MePa for eight continuous batch cycles and recorded a current density drop of just 0.23% per cycle. The degradation pathway and toxicity assessment of the intermediates using the Ecological Structure Activity Relationships (ECOSAR) tool supported the eco-friendliness of the current treatment scheme.

Electrochemical degradation of favipiravir (anti-viral) drug from aqueous solution: optimization of operating parameters using the response surface method

The aim of the current study is to investigate the efficacy of the electro-Fenton process in the degradation of favipiravir drugs from aqueous solutions, which has increased in use as a result of the COVID-19 pandemic. The Response Surface Methodology (RSM) was developed using a Central Composite Design (CCD) in which five independent variables, including Fe2+ concentration, current density, initial FVP concentration, pH, and reaction time, were coded with high and low levels, and the maximum removal percentage of FVP (97.8%) and COD (91.65%) were determined as responses. In the EF process, 530 mg/L H2O2 was produced in-situ by cathodic reduction of O2 in aqueous solution and thus FVP has been successfully oxidized through hydroxyl radicals. The H2O2/Fe2+ ratio was determined to be 0.51 under optimum conditions. At the end of the experiment, the maximum energy consumption was found to be 2.12 kWh per g COD. The FVP was completely mineralized in a very short time by the EF process, according to the LC-MS/MS examination. The EF process followed the pseudo first-order kinetic model with the rate constants of 0.023, 0.016 and 0.006 1/min for pH 2, 3 and 4, respectively. According to the findings of this study, the electro-Fenton process is an effective method for removing FVP from aqueous solutions. To the authors' knowledge, this is the first study to show the degradation and optimum conditions of FVP in aqueous solution using the electro-Fenton (EF) process.

An optimized electro-fenton pretreatment for the degradation and mineralization of a mixture of ofloxacin, norfloxacin, and ciprofloxacin

The electro-Fenton process (EFP) is a powerful advanced oxidation process beneficial to treating recalcitrant contaminants, and there has been a continuing interest in combining this technology to enhance the efficiency of conventional wastewater treatment processes. In this work, an optimized EFP process is performed as pretreatment for the degradation and mineralization of three blank fluoroquinolones (FQs) drugs: ofloxacin (OFL), norfloxacin (NOR), and ciprofloxacin (CIP). The optimization of the experiment was carried out using a Box-Behnken experimental design. Faster and complete degradation of the drugs mixture was achieved in 90 min with 61.12 ± 2.0% of mineralization in 180 min, under the optimized conditions: j = 244.0 mA cm-2, [Fe2+] = 0.31 mM, and [FQs] = 87.0 mg L-1. Furthermore, a low toxicity effluent was obtained in 90 min of the experiment, according to bioassay toxicity with Vibrio fischeri. Five short-chain carboxylic acids, including oxalic, maleic, oxamic, formic, and fumaric acids, were detected and quantified, in addition to F- and NO3- inorganic ions. The inhibition of the reactive oxygen species with scavenger proof was also evaluated in this paper.

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.

Critical Review on the Mechanisms of Fe2+ Regeneration in the Electro-Fenton Process: Fundamentals and Boosting Strategies

This review presents an exhaustive overview on the mechanisms of Fe³⁺ cathodic reduction within the context of the electro-Fenton (EF) process. Different strategies developed to improve the reduction rate are discussed, dividing them into two categories that regard the mechanistic feature that is promoted: electron transfer control and mass transport control. Boosting the Fe³⁺ conversion to Fe²⁺ via electron transfer control includes: (i) the formation of a series of active sites in both carbon- and metal-based materials and (ii) the use of other emerging strategies such as single-atom catalysis or confinement effects. Concerning the enhancement of Fe²⁺ regeneration by mass transport control, the main routes involve the application of magnetic fields, pulse electrolysis, interfacial Joule heating effects, and photoirradiation. Finally, challenges are singled out, and future prospects are described. This review aims to clarify the Fe³⁺/Fe²⁺ cycling process in the EF process, eventually providing essential ideas for smart design of highly effective systems for wastewater treatment and valorization at an industrial scale.

In-situ Electrochemical Synthesis of H2O2 for p-nitrophenol Degradation Utilizing a Flow-through Three-dimensional Activated Carbon Cathode with Regeneration Capabilities

The growing ubiquity of recalcitrant organic contaminants in the aqueous environment poses risks to effective and efficient water treatment and reuse. A novel three-dimensional (3D) electrochemical flow-through reactor employing activated carbon (AC) encased in a stainless-steel (SS) mesh as a cathode is proposed for the removal and degradation of a model recalcitrant contaminant p-nitrophenol (PNP), a toxic compound that is not easily biodegradable or naturally photolyzed, can accumulate and lead to adverse environmental health outcomes, and is one of the more frequently detected pollutants in the environment. As a stable 3D electrode, granular AC supported by a SS mesh frame as a cathode is hypothesized to 1) electrogenerate H2O2 via a 2-electron oxygen reduction reaction on the AC surface, 2) initiate decomposition of this electrogenerated H2O2 to form hydroxyl radicals on catalytic sites of the AC surface 3) remove PNP molecules from the waste stream via adsorption, and 4) co-locate the PNP contaminant on the carbon surface to allow for oxidation by formed hydroxyl radicals. Additionally, this design is utilized to electrochemically regenerate the AC within the cathode that is significantly saturated with PNP to allow for environmentally friendly and economic reuse of this material. Under flow conditions with optimized parameters, the 3D AC electrode is nearly 20% more effective than traditional adsorption in removing PNP. 30 grams of AC within the 3D electrode can remove 100% of the PNP compound and 92% of TOC under flow. The carbon within the 3D cathode can be electrochemically regenerated in the proposed flow system and design thereby increasing the adsorptive capacity by 60%. Moreover, in combination with continuous electrochemical treatment, the total PNP removal is enhanced by 115% over adsorption. It is anticipated this platform holds great promises to eliminate analogous contaminants as well as mixtures.

Transformation of microplastics by oxidative water and wastewater treatment processes: A critical review

Microplastics (MPs) are contaminants of emerging concern that accumulate in various environments, where they pose threats to both the ecosystem and public health. Since MPs have been detected in drinking water resources and wastewater effluents, more efficient treatment is needed at wastewater treatment plants (WWTPs) and drinking water treatment plants (DWTPs). This review discusses the potential of biological, photochemical, Fenton (-like) systems, ozonation, and other oxidation processes in the treatment of MPs in terms of their indicators of oxidation such as mass loss and surface oxidation. The oxidation processes were further analyzed in terms of limitations and environmental implications. Most previous studies examining MPs degradation using conventional treatments-such as UV disinfection, ozonation, and chlorination-employed significantly higher doses than the common doses applied in DWTPs and WWTPs. Owing to such dose gaps, the oxidative transformation of MPs observed in many previous studies are not likely to occur under practical conditions. Some novel oxidation processes showed promising MPs treatment efficiencies, while many of them have not yet been applied on a larger scale due to high costs and the lack of extensive basic research. Health and environmental impacts related to the discharge of oxidized MPs in effluents should be considered carefully in different aspects: the role as vectors of external pollutants, release of organic compounds (including organic byproducts from oxidation) and fragmentation into smaller particles as MPs circulate in the ecosystem as well as the possibility of bioaccumulation. Future research should also focus on ways to incorporate developed oxidation processes in DWTPs and WWTPs to mitigate MPs contamination.

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

Anodic oxidation has emerged as a promising treatment technology for the removal of a broad range of organic pollutants from wastewaters. Hydroxyl radicals are the primary species generated in anodic oxidation systems to oxidize organics. In this review, the methods of identifying hydroxyl radicals and the existing debates and misunderstandings regarding the validity of experimental results are discussed. Consideration is given to the methods of quantification of hydroxyl radicals in anodic oxidation systems with particular attention to approaches used to compare the electrochemical performance of different anodes. In addition, we describe recent progress in understanding the mechanisms of hydroxyl radical generation at the surface of most commonly used anodes and the utilization of hydroxyl radical in typical electrochemical reactors. This review shows that the key challenges facing anodic oxidation technology are related to i) the elimination of mistakes in identifying hydroxyl radicals, ii) the establishment of an effective hydroxyl radical quantification method, iii) the development of cost effective anode materials with high corrosion resistance and high electrochemical activity and iv) the optimization of electrochemical reactor design to maximise the utilization efficiency of hydroxyl radicals.

An efficient simultaneous degradation of sulfamethoxazole and trimethoprim by photoelectro-Fenton process under non-modified pH using a natural citric acid source: study of biodegradability, ecotoxicity, and antibacterial activity

In this work, the use of natural organic wastes (orange and lemon peels) as sources of citric acid was evaluated along with the application of the photoelectro-Fenton (PEF) system under non-modified pH as a novel alternative to degrade a complex mixture of pharmaceuticals: sulfamethoxazole (SMX-7.90 × 10^-5 mol/L) and trimethoprim (TMP-6.89 × 10^-5 mol/L). The system was equipped with a carbon felt air diffusion cathode (GDE) and a Ti/IrO₂ anode doped with SnO₂ (DSA). A 3.6 × 10^-5 mol/L solution of commercial citric acid was used as a reference. The pharmaceuticals' evolution in the mixture was followed by high-performance liquid chromatography (HPLC). The addition of natural products showed an efficient simultaneous degradation of the antibiotics (100% of SMX and TMP at 45 min and 90 min, respectively) similar to the performance produced by adding the commercial citric acid to the PEF system. Moreover, the addition of natural products allowed for an increment of biodegradability (100% removal of TOC by a modified Zahn Wellens test) and a decrease in ecotoxicity (0% in the bioassay with D. Magna) of the treated solutions. The antibacterial activity was eliminated after only 45 min of treatment, suggesting that the degradation by-products do not represent a significant risk to human health or the environment in general. Results suggest that, because of the efficient formation of Fe-citrate complexes, the PEF could be enhanced by the addition of natural organic wastes as a sustainable alternative ecological system for water contaminated pharmaceuticals. Additionally, the potential of reusing natural organic wastes has been exposed, contributing to an improved low-cost PEF by decreasing the environmental contamination produced by this type of waste.

Hydroxyl Radical Production via a Reaction of Electrochemically Generated Hydrogen Peroxide and Atomic Hydrogen: An Effective Process for Contaminant Oxidation?

An electrochemical advanced oxidation process (EAOP) is demonstrated with a catalytic cathode capable of simultaneously catalyzing the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) with resultant in situ generation of atomic hydrogen (H*) and hydrogen peroxide (H₂O₂). A palladium-coated carbon-PTFE gas diffusion electrode (Pd/C GDE) was used as a catalytic cathode with hydroxyl radical (^•OH) formed as a result of the reaction of electrogenerated H* with H₂O₂. As both the HER and ORR can be induced to occur at the same cathode, the H*/GDE process results in more effective degradation of organic contaminants than can be achieved by a conventional H*/H₂O₂ process involving direct addition of H₂O₂. At circumneutral pH, 82.7% of added formate was degraded after 2 h treatment at an applied potential of -1.0 V vs Ag/AgCl with relatively low concentrations of generated H₂O₂ remaining in the solution. We also show that H* and H₂O₂ (and thus ^•OH) can be electrogenerated effectively over a wide range of pH (3.2-7.0). These results suggest that by in situ generation of H* and H₂O₂, the H*/GDE process is able to produce significant amounts of ^•OH without external chemical addition and thus offers an alternative method for abatement of aqueous organic contaminants.

Coupling electrode aeration and hydroxylamine for the enhanced Electro-Fenton degradation of organic contaminant: Improving H2O2 generation, Fe3+/Fe2+ cycle and N2 selectivity

To improve H2O2 generation and Fe3+/Fe2+ cycle simultaneously for enhancing Electro-Fenton performance, the electrode aeration (EA) and hydroxylamine sulfate (HA) were coupled. With dimethyl phthalate (DMP) as main target contaminant, combination of HA and EA greatly accelerated the degradation of DMP and exhibited a synergy in the pH of 2.0-6.9 through promoting the key reactions, including electrochemical two-electron reduction of O2 into H2O2 and redox cycles of Fe3+/Fe2+, which then improved the generation of hydroxyl radicals (·OH). The coupling EA and HA reduced the use of HA and converted most of HA into environment-friendly N2 (60.1-62.1% of HA products), while HA/solution aeration(SA) system consumed HA rapidly and the generated N2 only accounted for 5.8-6.7% of HA products. Furthermore, compared with HA/SA and EA Electro-Fenton systems, enhancement degree of DMP degradation in HA/EA Electro-Fenton process was higher in actual waterbody than in ultrapure water. The coupling EA and HA in the Electro-Fenton process could solve the low Fe3+/Fe2+ cycle efficiency and low H2O2 production simultaneously, and improve the N2 selectivity of HA transformation, which advanced its application in practical environmental remediation.

Hydroxylamine promoted Fe(III) reduction in H2O2/soil systems for phenol degradation

Production of hydroxyl radicals (•OH) upon the oxidation of solid Fe(II) by O₂ or H₂O₂ in soils and sediments has been confirmed, which benefits in situ remediation of contaminants. However, Fe(III) reduction by H₂O₂ is rate-limiting. Accelerating the Fe(III)/Fe(II) cycle could improve the efficiency of remediation. This study intended to use hydroxylamine to promote Fe(III)/Fe(II) cycle during 100 g/L soil oxidation by H₂O₂ for phenol degradation. The removal of phenol was 76% in 3 h during soil oxidation with 1 mM H₂O₂ in the presence of 1 mM hydroxylamine but was negligible in the absence of hydroxylamine. Fe(III) in the soil was reduced to 0.21 mM Fe(II) by 1 mM hydroxylamine in 30 min. The accelerated cycle of Fe(III)/Fe(II) in the soil by hydroxylamine could effectively decompose H₂O₂ to produced •OH, which was responsible for the effective enhancement of phenol degradation during soil oxidation. Under the conditions of 1 mM H₂O₂ and 100 g/L soil, the pseudo-first-order kinetic constant of phenol degradation increased proportionally from 0.0453 to 0.0844 min^-1 with the increase of hydroxylamine concentrations from 0.5 to 1 mM. The kinetic constant also increased from 0.0041 to 0.0111 min^-1 with H₂O₂ concentration increased from 0.5 to 2 mM, while it decreased from 0.0100 to 0.0051 min^-1 with soil dosage increased from 20 to 200 g/L. In addition, column experiments showed that phenol (10 mg/L) degradation ratio kept at about 48.7% with feeding 2 mM hydroxylamine and 2 mM H₂O₂ at 0.025 PV/min. Column experiments suggested an optional application of hydroxylamine and H₂O₂ for in situ remediation. The output of this study provides guidance and optional strategies to enhance contaminant degradation during soil oxidation.

The principle of detailed balancing, the iron-catalyzed disproportionation of hydrogen peroxide, and the Fenton reaction

The iron-catalyzed disproportionation of H₂O₂ has been investigated for over a century, as has been its ability to induce the oxidation of other species present in the system (Fenton reaction). The mechanisms of these reactions have been under consideration at least since 1932. Unfortunately, little or no attention has been paid to ensuring the conformity of the proposed mechanisms and rate constants with the constraints of the principle of detailed balancing. Here we identify more than 200 publications having mechanisms that violate the principle of detailed balancing. These violations occur through the use of incorrect values for certain rate constants, the use of incorrect forms of the rate laws for certain steps in the mechanisms, and the inclusion of illegal loops. A core mechanism for the iron-catalyzed decomposition of H₂O₂ is proposed that is consistent with the principle of detailed balancing and includes both the one-electron oxidation of H₂O₂ by Fe(III) and the Fe(II) reduction of HO₂˙.

Removal of Bromocresol Green from aqueous solution by electro-Fenton and electro-Fenton-like processes with different catalysts: laboratory and kinetic model investigation

This study presents the removal of triarylmethane dye Bromocresol Green from aqueous solution by the electro-Fenton process. As catalysts five different cations were used: Fe²⁺, Ce³⁺, Ni²⁺, Mn²⁺, and Co²⁺. They play crucial roles in the whole process because they react with H₂O₂ producing hydroxyl radicals that are capable of breaking down dye molecules. Based on this, a comparison of catalytic activity of these cations in the electro-Fenton process is made for Bromocresol Green degradation. A simple and universal kinetic model is also applied to study the catalytic activity of investigated catalysts. Due to its multidimensionality it is fitted to experimental data using a genetic algorithm. The procedure of fitting using a genetic algorithm is thoroughly described and demonstrated. The activity of utilized catalysts is compared based on both experimental and model data revealing that for Bromocresol Green removal all alternative catalysts (Ni²⁺, Co²⁺, Ce³⁺, Mn²⁺) are better than the typical one (Fe²⁺, 51.83% degradation). The best catalyst is Co²⁺ with 78.35% degradation efficiency. Moreover, the adopted kinetic model proved its universality and outlined different interactions between catalysts and dye molecules.

Simultaneous Galvanic Generation of Fe2+ Catalyst and Spontaneous Energy Release in the Galvano-Fenton Technique: A Numerical Investigation of Phenol’s Oxidation and Energy Production and Saving

The present paper investigates the potential of the Galvano-Fenton process as an advanced technique in terms of the simultaneous oxidation of a model pollutant, phenol, and the energy release and saving as compared to conventional electrochemical techniques, namely, Fenton, Fenton-like, and Electro-Fenton. A numerical model describing the electrochemical, electrolytic, and phenol’s mineralization reactions is presented. Simulations are conducted to predict the kinetics of ferrous and ferric ions, radicals’ formation, and phenol degradation along with released power. Parametric analysis and comparisons are also performed between the basic configuration of the Galvano-Fenton process and its upgraded version integrating a pre-immersion stage of the electrodes in the electrolyte equivalent to 25% of the total experiment’s duration. The ratio of the initial concentration of H2O2 to the concentration of the released/added Fe2+ catalyst is varied from 10 to 30. The effect of phenol concentration is inspected over the range of 0.188 to 10 mg/L as well. Compared to conventional Fenton-based techniques, the Galvano-Fenton process demonstrated a higher performance by reaching 1.34% of degradation efficiency per released J. This is associated with the generation of hydroxyl radicals of 0.047 nM/released J with initial concentrations of hydrogen peroxide and phenol of 0.187 mM and 2 µM, respectively. Moreover, the integration of the pre-immersion stage allowed the overcoming the barrier of the null degradation rate at the initial instant.

Arsenic immobilization as crystalline scorodite by gas-diffusion electrocrystallization

Gas-diffusion electrocrystallization (GDEx) is demonstrated as an effective process for the immobilization of arsenic into stable scorodite.

Effect of surface properties of activated carbon fiber cathode on mineralization of antibiotic cefalexin by electro-Fenton and photoelectro-Fenton treatments: Mineralization, kinetics and oxidation products

Solutions of 200 mg L^-1 cefalexin (CLX), an antibiotic with high usage frequency and biodegradation resistance, have been comparatively degraded by electro-Fenton (EF) and photoelectro-Fenton (PEF) processes using two kinds of activated carbon fiber (ACF) cathodes with different physical properties. These two ACFs shared similar pore volumes and pore diameters but varied BET surface areas, which were confirmed to be 0.5210 cm³ g^-1, 2.26 nm and 921 m² g^-1 for ACF1, while 0.6508 cm³ g^-1, 2.16 nm and 1206 m² g^-1 for ACF2, respectively. Their oxidation abilities were comparatively assessed in terms of degradation kinetics and mineralization rates, which increased in the order: ACF1-EF < ACF2-EF < ACF1-PEF < ACF2-PEF. These results confirmed the superiority of ACF with higher surface area, which was correlated to faster H₂O₂ and OH accumulation in more reaction sites provided. After 120 min electrolysis, ACF1 exhibited 1510 μM H₂O₂ and 37 μM OH accumulation, while ACF2 generated 1934 μM H₂O₂ and 85 μM OH. Moreover, ACF cathode with more developed pore structure also revealed faster formation of degradation by-products like inorganic ions (NH₄⁺ and NO₃^- ions) and short-chain carboxylic acids (acetic, formic, oxamic and oxalic acids), as well as enhanced removal for partial acids. In order to gain a deeper understanding of degradation mechanisms for ACF2-PEF system, evolutions of six aromatic by-products generated from sulfoxidation, hydroxylation and decarboxylation were confirmed by UPLC-QTOF-MS/MS determination. Based on the above identifications of the degradation intermediates, a plausible reaction pathway for CLX removal was proposed.

Advanced oxidation of antihypertensives losartan and valsartan by photo-electro-Fenton at near-neutral pH using natural organic acids and a dimensional stable anode-gas diffusion electrode (DSA-GDE) system under light emission diode (LED) lighting

In this work photo-electro-Fenton (PEF) processes using a dimensionally stable anode-gas diffusion electrode (DSA-GDE) system under light emission diodes (LED)-type radiation were used in the degradation of the angiotensin-II-receptor antagonists (ARA II), valsartan (VAL), and losartan (LOS), which are used in the treatment of hypertension diseases, and are considered among the emerging contaminants (ECs). Organic acids as citric, tartaric, and oxalic acids were used as complexing agents of iron ions in order to maintain the performance of the Fenton reaction at near-neutral pH value. The results show that at 3.42 mA/cm² after 90 min of electro-Fenton (EF) treatment, degradation of 70% of VAL and 100% of LOS were observed. Total degradation of VAL and LOS was reached with a PEF process at the same time with mineralization of 30%. When citric and tartaric acids were used instead of oxalic acid, similar results were obtained, i.e., total degradation of both compounds, LOS and VAL, after 90 min of treatment. The degradation performance can be attributed to the increase of the initial dissolved iron in the system, facilitating the Fe³⁺/Fe²⁺ turnover in the catalytic photo-Fenton reaction and consequently, hydroxyl radical (^•OH) production. In addition, the increased photo-activity of the complexes can be associated with their high capability to complex Fe³⁺ and to promote ligand-to-metal charge transfer, which is of key importance to feed Fe²⁺ to the Fenton process. The results show that the system evaluated was more efficient to eliminate sartan family compounds using LED lighting in comparison with traditional UV-A lamps used in this kind of work. Moreover, three transformation products of VAL degradation and two transformation products of LOS degradation were identified by high-resolution mass spectrometry (HRMS) using hybrid quadrupole-time-of-flight (QTOF) MS and, at the end of the PEF system, the several organic compounds accumulated and no mineralized were effectively treated in a subsequent aerobic biological system.

Active chlorine mediated ammonia oxidation revisited: Reaction mechanism, kinetic modelling and implications

Ammonia nitrogen removal from wastewaters has gained much attention in recent decades as a result of the environmental problems associated with discharge of excessive amounts of this critical nutrient including eutrophication of receiving waters, generation of offensive odours as a result of organism decay and complications associated with the disinfection of water supplies. While removal via biological processes represents the principal means by which a reduction in dissolved nitrogen concentrations is achieved, an electrochemical advanced oxidation process has been proposed as a potentially effective alternate means of removing ammonia from wastewaters with the removal associated with the in situ generation of oxidants (particularly active chlorine) at the anode. Here we describe the influence of key factors on the rate and extent of ammonia nitrogen removal in an electrochemical cell with a Ti/IrO₂-RuO₂ anode and Ti cathode. The rate of ammonia removal was found to be dependent on both current density and initial chloride concentration with ∼95% ammonia removed from a 20 mM Cl^- solution within approximately 40 min at a current density of 3 mA cm^-2, resulting in an energy consumption of 126 kWh kg^-1 NH₄⁺-N. Additionally, we show that by-products formation is effectively suppressed during the electrolysis process. A mechanistically-based kinetic model incorporating the key processes operating in the ammonia electro-oxidation process was developed with particular attention given to (i) anodic generation of active chlorine and other chloride-related by-products, (ii) active chlorine mediated ammonia oxidation. The results demonstrate that the electrochemical advanced oxidation process is a promising technology for treatment of ammonia-containing wastewaters with advantages including simplicity, safety and effectiveness.

Current advances and trends in electro-Fenton process using heterogeneous catalysts - A review

Over the last decades, advanced oxidation processes have often been used alone, or combined with other techniques, for remediation of ground and surface water pollutants. The application of heterogeneous catalysis to electrochemical advanced oxidation processes is especially useful due to its efficiency and environmental safety. Among those processes, electro-Fenton stands out as the one in which heterogeneous catalysis has been broadly applied. Thus, this review has introduced an up-to-date collation of the current knowledge of the heterogeneous electro-Fenton process, highlighting recent advances in the use of different catalysts such as iron minerals (pyrite, magnetite or goethite), prepared catalysts by the load of metals in inorganic and organic materials, nanoparticles, and the inclusion of catalysts on the cathode. The effects of physical-chemical parameters as well as the mechanisms involved are critically assessed. Finally, although the utilization of this process to remediation of wastewater overwhelmingly outnumber other utilities, several applications have been described in the context of regeneration of adsorbent or the remediation of soils as clear examples of the feasibility of the electro-Fenton process to solve different environmental problems.

The synergistic effect of nickel-iron-foam and tripolyphosphate for enhancing the electro-Fenton process at circum-neutral pH

A composite nickel-iron-foam (Ni-Fe-F) electrode was used as a cathode in the electro-Fenton (EF) process at circum-neutral pH in the presence of sodium tripolyphosphate (TPP) as supporting electrolyte. It was found that phenol degradation was dramatically improved by the synergistic effect of Ni-Fe-F and TPP, reaching 100% removal in 40 min, with k_app = (8.90 ± 0.12) × 10^-2 min^-1, which was about 18 times higher than that of Ni-Fe-F with sulfate as conventional electrolyte at pH 3.00 (k_app = (5.00 ± 0.14) × 10^-3 min^-1). A (75.00 ± 1.67)% mineralization yield was attained after 4-h treatment time. Ni-Fe-F proved capable of providing the Fe²⁺ ions necessary to catalyze the Fenton's reaction via a controlled chemical/electrochemical redox process. In addition, Ni-Fe-F promoted the chemical and electrochemical generation of H₂O₂. With respect to TPP, its chelation with Fe ions prevented iron precipitation at neutral and higher pH values, extending the pH range of the Fenton's reaction. Furthermore, the TPP ligand promoted the activation of molecular O₂ for the chemical production of OH, enhancing the process efficiency. By overcoming these common limitations of conventional EF in K₂SO₄ electrolyte, the Ni-Fe-F/TPP system represents a more sustainable alternative for practical application of EF. A degradation pathway for phenol mineralization with homogeneous and heterogeneous OH produced by the EF Ni-Fe-F/TPP system is proposed based on the identification of the oxidation by-products.

Review of the methods for determination of reactive oxygen species and suggestion for their application in advanced oxidation induced by dielectric barrier discharges

Advanced oxidation processes (AOPs) particularly non-thermal plasmas based on electrical discharges have been widely investigated for water and wastewater treatment. Dielectric barrier discharges (DBDs) generate large amounts of selective and non-selective reactive oxygen species (ROS) such as ozone, hydrogen peroxide, atomic oxygen, superoxide molecular anions and hydroxyl radicals, having been proved to be efficient for water decontamination among various forms of electrical discharge systems. The detection and quantification methods of these oxygen species in non-thermal plasmas have been reviewed. However, their application in dielectric barrier discharge has not been well studied. It is therefore imperative to summarise the various detection and quantification methods for oxygen-based species determination in AOPs, aqueous systems and non-thermal plasma processes. Thereafter, reviewed methods are suggested for the determination of ROS in DBD configurations to understand the consumption trend of these oxidants during treatment of water effluents and to evaluate the performance of the treatment reactor configuration towards the degradation of targeted pollutants.

Advanced oxidation processes for the removal of natural organic matter from drinking water sources: A comprehensive review

Natural organic matter (NOM), a key component in aquatic environments, is a complex matrix of organic substances characterized by its fluctuating amounts in water and variable molecular and chemical properties, leading to various interaction schemes with the biogeosphere and hydrologic cycle. These factors, along with the increasing amounts of NOM in surface and ground waters, make the effort of removing naturally-occurring organics from drinking water supplies, and also from municipal wastewater effluents, a challenging task requiring the development of highly efficient and versatile water treatment technologies. Advanced oxidation processes (AOPs) received an increasing amount of attention from researchers around the world, especially during the last decade. The related processes were frequently reported to be among the most suitable water treatment technologies to remove NOM from drinking water supplies and mitigate the formation of disinfection by products (DBPs). Thus, the present work overviews recent research and development studies conducted on the application of AOPs to degrade NOM including UV and/or ozone-based applications, different Fenton processes and various heterogeneous catalytic and photocatalytic oxidative processes. Other non-conventional AOPs such as ultrasonication, ionizing radiation and plasma technologies were also reported. Furthermore, since AOPs are unlikely to achieve complete oxidation of NOM, integration schemes with other water treatment technologies were presented including membrane filtration, adsorption and others processes.

Effects of Good's Buffers and pH on the Structural Transformation of Zero Valent Iron and the Oxidative Degradation of Contaminants

The presence of Good's buffers caused rapid ZVI corrosion and a dramatic release of Fe(II) leading to the Fe(II)-catalyzed transformation of ferrihydrite to lepidocrocite and/or the direct formation of lepidocrocite from the oxidation of Fe(II) in the pH range 4.0-6.2. In comparison, in the absence of Good's buffers, elution of Fe(II) was insignificant with ferrihydrite being the only Fe(III) oxyhydroxide detected following the oxidative transformation of ZVI. The rapid ZVI corrosion in the presence of Good's buffer is possibly due to either (i) disruption of the Fe oxide surface layer as a result of attack by Good's buffers and/or (ii) interaction of Good's buffer with the outer Fe oxide surface and surface-associated Fe(II)/Fe(III) causing the Fe oxide surface layers to be more porous with both these processes facilitating continuous O₂ access to the Fe(0) core and allowing the diffusion of Fe atoms outward. Our results further show that the deprotonated forms of Good's buffers and the surface charge of the Fe oxides formed at the ZVI surface strongly affect the sorption of the target compound (i.e., formate) and hence the oxidation of these compounds via surface-associated Fe(II)-mediated heterogeneous Fenton processes.

Electrochemical advanced oxidation processes for Staphylococcus aureus disinfection in municipal WWTP effluents

This paper presents the Staphylococcus aureus inactivation in a simulated wastewater treatment plant effluent by different electrochemical techniques, including the photo-electro-Fenton process. S. aureus, dissolved organic carbon (DOC), total oxidants and H₂O₂ concentrations, as well as pH, were monitored during the assays. An electrolytic cell, including a UVA lamp, a gas diffusion electrode (GDE) as cathode and an IrO₂ anode, was used to conduct the experiments under galvanostatic conditions (20 mA). Low inactivation (-0.4) and low DOC removal were achieved within 120 min when applying the GDE-IrO₂ system, in which bacteria disinfection was caused by the generated H₂O₂. When light was combined with GDE-IrO₂, the process efficiency noticeably increased (-3.7 log inactivation) due to the synergistic effect between UVA and H₂O₂. Introducing iron (5 mg L^-1 Fe²⁺) into the system also produced higher disinfection and DOC mineralization. The electro-Fenton process (GDE-IrO₂+Fe²⁺) led to a bacterial reduction of -0.9 log units and DOC reduction of 14%, while with the photo-electro-Fenton process (GDE-IrO₂+UVA + Fe²⁺) -5.2 units of bacteria and 26% of DOC were removed. Increasing the current intensity (20 mA, 30 mA and 40 mA) in the photo-electro-Fenton system increased H₂O₂ production and, consequently, augmented the bacterial inactivation (-5.2 log, -6.2 log and -6.5 log, respectively). However, mineralization extent slightly increased or remained practically the same. When comparing the influence of Fe²⁺ and Fe³⁺ on photo-electro-Fenton, similar S. aureus inactivation was observed, while DOC removal was higher with Fe²⁺ (31%) than with Fe³⁺ (19%). Finally, by testing the system with a Ti anode, the direct anodic oxidation contribution of the IrO₂ anode was identified as negligible.