Chlorine species evolution during electrochlorination on boron-doped diamond anodes: In-situ electrogeneration of Cl2, Cl2O and ClO2

Oxidation of organics in water by active chlorine performed in microfluidic electrochemical reactors: a new way to improve the performances of the process

Wastewater polluted by organics can be treated by using electro-generated active chlorine, even if this promising route presents some important drawbacks such as the production of chlorinated by-products. Here, for the first time, this process was studied in a microfluidic electrochemical reactor with a very small inter-electrode distance (145 μm) using a water solution of NaCl and phenol and a BDD anode. The potential production of chloroacetic acids, chlorophenols, carboxylic acids, chlorate and perchlorate was carefully evaluated. It was shown, for the first time, up to our knowledge, that the use of the microfluidic device allows to perform the treatment under a continuous mode and to achieve higher current efficiencies and a lower generation of some important by-products such as chlorate and perchlorate. As an example, the use of the microfluidic apparatus equipped with an Ag cathode allowed to achieve a high removal of total organic carbon (about 76%) coupled with a current efficiency of 17% and the production of a small amount of chlorate (about 30 ppm) and no perchlorate. The effect of many parameters (namely, flow rate, current density and nature of cathode) was also investigated.

A combined experimental and computational approach to unravel degradation mechanisms in electrochemical wastewater treatment

Electrochemical wastewater treatment is a promising technique to remove recalcitrant pollutants from wastewater. However, the complexity of elucidating the underlying degradation mechanisms hinders its optimisation not only from a techno-economic perspective, as it is desirable to maximise removal efficiencies at low energy and chemical requirements, but also in environmental terms, as the generation of toxic by-products is an ongoing challenge. In this work, we propose a novel combined experimental and computational approach to (i) estimate the contribution of radical and non-radical mechanisms as well as their synergistic effects during electrochemical oxidation and (ii) identify the optimal conditions that promote specific degradation pathways. As a case study, the distribution of the degradation mechanisms involved in the removal of benzoic acid (BA) via boron-doped diamond (BDD) anodes was elucidated and analysed as a function of several operating parameters, i.e., the initial sulfate and nitrate content of the wastewater and the current applied. Subsequently, a multivariate optimisation study was conducted, where the influence of the electrode nature was investigated for two commercial BDD electrodes and a customised silver-decorated BDD electrode. Optimal conditions were identified for each degradation mechanism as well as for the overall BA degradation rate constant. BDD selection was found to be the most influential factor favouring any mechanism (i.e., 52-85% contribution), given that properties such as its boron doping and the presence of electrodeposited silver could dramatically affect the reactions taking place. In particular, decorating the BDD surface with silver microparticles significantly enhanced BA degradation via sulfate radicals, whereas direct oxidation, reactive oxygen species and radical synergistic effects were promoted when using a commercial BDD material with higher boron content and on a silicon substrate. Consequently, by simplifying the identification and quantification of underlying mechanisms, our approach facilitates the elucidation of the most suitable degradation route for a given electrochemical wastewater treatment together with its optimal operating conditions.

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.

Novel Synthesis Pathways for Highly Oxidative Iron Species: Generation, Stability, and Treatment Applications of Ferrate(IV/V/VI)

Difficulties arise related to the economy-of-scale and practicability in applying conventional water treatment technologies to small and remote systems. A promising oxidation technology better suited for these applications is that of electro-oxidation (EO), whereby contaminants are degraded via direct, advanced, and/or electrosynthesized oxidant-mediated reactions. One species of oxidants of particular interest includes ferrates (Fe(VI)/(V)/(IV)), where only recently has their circumneutral synthesis been demonstrated, using high oxygen overpotential (HOP) electrodes, namely boron-doped diamond (BDD). In this study, the generation of ferrates using various HOP electrodes (BDD, NAT/Ni-Sb-SnO2, and AT/Sb-SnO2) was investigated. Ferrate synthesis was pursued in a current density range of 5-15 mA cm-2 and initial Fe3+ concentrations of 10-15 mM. Faradaic efficiencies ranged from 11-23%, depending on operating conditions, with BDD and NAT significantly outperforming AT electrodes. Speciation tests revealed that NAT synthesizes both ferrate(IV/V) and ferrate(VI), while the BDD and AT electrodes synthesized only ferrate(IV/V) species. A number of organic scavenger probes were used to test the relative reactivity, including nitrobenzene, carbamazepine, and fluconazole, whereby ferrate(IV/V) was significantly more oxidative than ferrate(VI). Finally, the ferrate(VI) synthesis mechanism by NAT electrolysis was elucidated, where coproduction of ozone was found to be a key phenomenon for Fe3+ oxidation to ferrate(VI).

A Bipolar Membrane-Integrated Electrochlorination Process for Highly Efficient Ammonium Removal in Mature Landfill Leachate: The Importance of ClO• Generation

Electrochemical oxidation has been demonstrated to be a useful method for removing biorefractory organic pollutants in mature landfill leachate but suffers from low efficiency in eliminating ammonium because of its resistance to being oxidized by HO• or free chlorine (FC) at decreased pH. Here, we propose a new bipolar membrane-electrochlorination (BPM-EC) process to address this issue. We found that the BPM-EC system was significantly superior to both the undivided and divided reactors with monopolar membranes in terms of elevated rate of ammonium removal, attenuated generation of byproducts (e.g., nitrate and chloramines), increased Faradaic efficiency, and decreased energy consumption. Mechanistic studies revealed that the integration of BPM was helpful in creating alkaline environments in the vicinity of the anode, which facilitated production of surface-bound HO• and FC and eventually promoted in situ generation of ClO•, a crucial reactive species mainly responsible for accelerating ammonium oxidation and selective transformation to nitrogen. The efficacy of BPM-EC in treating landfill leachates with different ammonium concentrations was verified under batch and continuous-flow conditions. A kinetic model that incorporates the key parameters was developed, which can successfully predict the optimal number of BPM-EC reactors (e.g., 2 and 5 for leachates containing 589.4 ± 5.5 and 1258.1 ± 9.6 mg L-1 NH4+-N, respectively) necessary for complete removal of ammonium. These findings reveal that the BPM-EC process shows promise in treating ammonium-containing wastewater, with advantages that include effectiveness, adaptability, and flexibility.

Self-adaptive amorphous CoOxCly electrocatalyst for sustainable chlorine evolution in acidic brine

Electrochemical chlorine evolution reaction is of central importance in the chlor-alkali industry, but the chlorine evolution anode is largely limited by water oxidation side reaction and corrosion-induced performance decay in strong acids. Here we present an amorphous CoOxCly catalyst that has been deposited in situ in an acidic saline electrolyte containing Co2+ and Cl- ions to adapt to the given electrochemical condition and exhibits ~100% chlorine evolution selectivity with an overpotential of ~0.1 V at 10 mA cm-2 and high stability over 500 h. In situ spectroscopic studies and theoretical calculations reveal that the electrochemical introduction of Cl- prevents the Co sites from charging to a higher oxidation state thus suppressing the O-O bond formation for oxygen evolution. Consequently, the chlorine evolution selectivity has been enhanced on the Cl-constrained Co-O* sites via the Volmer-Heyrovsky pathway. This study provides fundamental insights into how the reactant Cl- itself can work as a promoter toward enhancing chlorine evolution in acidic brine.

Pulsed versus direct current electrochemical co-catalytic peroxymonosulfate-based system: Elevated degradation and energy efficiency with enhanced oxidation mechanisms

In this work, the pulsed electrochemical (PE) system was investigated to activate peroxymonosulfate (PMS) with the addition of Fe(III) to achieve efficient degradation of sulfamethoxazole (SMX) with reduced energy consumption, in comparison with the direct current (DC) electrochemical system. The operational conditions of PE/PMS/Fe(III) system were optimized as 4 kHz pulse frequency, 50% duty cycle, and pH 3, at which 67.6% reduction of energy consumption and enhanced degradation performance were achieved compared to the DC/PMS/Fe(III) system. Results of electron paramagnetic resonance spectroscopy analysis and quenching and chemical probe experiment revealed the presence of ^•OH, SO₄^•-, and ¹O₂ in the system, with ^•OH being the dominant role. The concentrations of these active species were averagely 15 ± 1% higher in the PE/PMS/Fe(III) system than those of the DC/PMS/Fe(III) system. Identification of SMX byproducts was achieved based on high resolution mass spectrometry analysis to predict the degradation pathways. The SMX byproducts could eventually be eliminated by the PE/PMS/Fe(III) system with extended treatment time. Overall, the PE/PMS/Fe(III) system was demonstrated with high energy and degradation performance, and is appear to be an robust strategy for practical treatment of wastewater.

Toxicological aspect of water treated by chlorine-based advanced oxidation processes: A review

As well established in the literature, residual toxicity is an important parameter for evaluating the sanitary and environmental safety of water treatment processes, and this parameter becomes even more crucial when chlorine-based processes are applied for water treatment. Eliminating initial toxicity or preventing its increase after water treatment remains a huge challenge mainly due to the formation of highly toxic disinfection by-products (DBPs) that stem from the degradation of organic contaminants or the interaction of the chlorine-based oxidants with different matrix components. In this review, we present a comprehensive discussion regarding the toxicological aspects of water treated using chlorine-based advanced oxidation processes (AOPs) and the recent findings related to the factors influencing toxicity, and provide directions for future research in the area. The review begins by shedding light on the advances made in the application of free chlorine AOPs and the findings from studies conducted using electrochemical technologies based on free chlorine generation. We then delve into the insights and contributions brought to the fore regarding the application of NH₂Cl- and ClO₂-based treatment processes. Finally, we broaden our discussion by evaluating the toxicological assays and predictive models employed in the study of residual toxicity and provide an overview of the findings reported to date on this subject matter, while giving useful insights and directions for future research on the topic.

Insight into electrochemically boosted trace Co(II)-PMS catalytic process: Sustainable Co(IV)/Co(III)/Co(II) cycling and side reaction blocking

A novel homogeneous electrocatalytic system was constructed by current-assisted trace Co(II) activating PMS (ECP) to remove reactive blue 19 (RB19). More than 93 % of RB19 was rapidly removed with only a trace dose, and the PMS was 98.35 % utilized during the reaction. By exploring the active species and analyzing the PMS consumption, it was found that current strongly accelerated the Co(III)/Co(II) redox cycle by providing electrons to Co(III), and inhibited the side reaction thus improving the PMS utilization. Electric energy per order was very low, only 0.26 kWh·m³. Radicals (SO₄^•-) and non-radicals (Co(III), Co(IV) and ¹O₂) participated in ECP system, in which SO₄^•- was dominant. By excluding the other three precursors (PMS, •OH and O₂^•-), the side reaction product SO₅^•- was identified as the source of ¹O₂ in ECP system. Combining chelating agent EDTA and chemical probe PMSO, Co(IV) was considered formed by single and double charge transfer. Five degradation pathways of RB19 were proposed using mass spectrometry and DFT calculation. The ecotoxicity and mutagenicity of RB19 and its transformation products were predicted using software simulation. These studies provided an interesting insights into the synergistic Co(II)-PMS systems and offered a new strategy for electrochemical processes.

Opportunities for in situ electro-regeneration of organic contaminant-laden carbonaceous adsorbents

Adsorptive separation technologies have proven to be effective on organic contaminant removal in aqueous water. However, the breakthrough of contaminants is inevitable and can be at relatively low bed volumes, which makes the regeneration of spent adsorbents an urgent need. Electrochemically induced regeneration processes are given special attention and may provide ease of operation through in situ regeneration avoiding (i) removal and transport adsorbents, and (ii) avoiding use of hazardous chemicals (i.e., organic solvents, acids, or bases). Therefore, this review article critically evaluates the fundamental aspects of in situ electro-regeneration for spent carbons, and later discusses specific examples related to the treatment of emerging contaminants (such as per- and polyfluoroalkyl substances or PFAS). The fundamental concepts of electrochemically driven processes are comprehensively defined and addressed in terms of (i) adsorbent characteristics, (ii) contaminant properties, (iii) adsorption/regeneration driving operational parameters and conditions, and (iv) the competitive effects of water matrices. Additionally, future research needs and challenges to enhance understanding of in situ electro-regeneration applications for organic contaminants (specifically PFAS)-laden adsorbents are identified and outlined as a future key perspective.

Evaluating UV254 absorbance reductions in landfill leachate for municipal sewage co-treatment through timed UV/electrooxidation

Landfill leachate contains dissolved organic matter (DOM) exhibiting high ultraviolet absorbance at 254 nm (UVA₂₅₄). The UVA₂₅₄ limits leachate co-treatment with municipal sewage by hindering the downstream UV disinfection efficiency at wastewater treatment plants. Here, we alleviated the UVA₂₅₄ by timing the radiation in a UV/electrooxidation (UV/EO) process to accelerate reactive species formation. At 200 A·m^-2, the UV radiation was delayed by 10 min to accumulate 21 mg·L^-1 as Cl₂, which enhanced the initial radical formation rate by 5.25 times compared with a simultaneous UV/EO. The timed operation increased the steady-state concentrations of ClO^• by 700 times to 4.11 × 10^-14 M and reduced the leachate UVA₂₅₄ by 78.2% after 60 min. We identified that aromatic formulas with low oxygen content were susceptible to UV/EO from Fourier-transform ion cyclotron resonance mass spectrometry analysis. The toxicity of the treated leachate and generated byproducts was assessed through specific oxygen uptake rates (SOUR) and developmental assays with Platynereis dumerilii. After quenching the residual chlorine, leachate co-treatment at 3.5% v/v presented minimal toxicological risk. Our findings provide operational insights for applying UV/EO in high UVA₂₅₄ matrices such as landfill leachate.

Electrochemical activation of persulfate by Al-doped blue TiO2 nanotubes for the multipath degradation of atrazine

The combination of electrolysis and persulfate activation (E/PDS) is a cost-effective method for the treatment of refractory organics. However, persulfate is difficult to be activated into radicals at the anode, resulting in insufficient electro-activation efficiency. Herein, Al doped blue TiO₂ nanotube electrodes (Al-bTNT) were first employed as cost-effective anode materials to fully activate PDS to radicals. In E/PDS, the kinetic constant of atrazine removal by Al-bTNT (0.048 min^-1) substantially outperformed the other anodes, including the blue TiO₂ nanotube electrodes (bTNT) (0.024 min^-1), Ti₄O₇ (0.02 min^-1), and B doped diamond (BDD) anodes (0.023 min^-1). The Al-bTNT-E/PDS exhibited a low energy consumption (E_EO = 0.72 kWh m^-3) and a high mineralization rate. Based on the results of electron paramagnetic resonance, quenching experiments, and probe experiments, we propose that atrazine degrades in the Al-bTNT-E/PDS system mainly via a novel radical pathway that involves both·OH and SO₄·^- and the generated SO₄·^- is responsible for the enhanced removal rate. The oxygen vacancies (V_O) generated from interstitial Al may serve as the active sites to adsorb and dissociate the persulfate molecules based on extensive characterizations. The attempt at soil-washing wastewater disposal indicated the synergistic system possessed good potential for future practical application.

Investigation of CuCoFe-LDH as an efficient and stable catalyst for the degradation of acetaminophen in heterogeneous electro-Fenton system: Key operating parameters, mechanisms and pathways

Pharmaceuticals, as anthropogenic pollutants in a wide range of water sources, generally require specific treatment methods for degradation. A trimetallic layered double hydroxide (CuCoFe-LDH) was successfully fabricated by coprecipitation and applied as a novel heterogeneous electro-Fenton (EF) catalyst for the degradation of acetaminophen (ACT) from aqueous environments. The EF experiments showed that the CuCoFe-LDH/EF process achieved 100% of ACT degradation efficiency within 60 min at pH = 5, catalyst dosage of 0.50 g/L, current density of 10 mA/cm² and initial ACT concentration of 20 mg/L. An impressive (>80%) mineralization of ACT was obtained over a wide pH range (pH 3-9) after 180 min. Meanwhile, the role of ·OH and O₂^.- were certified by radical quenching experiments and electron paramagnetic resonance (EPR) analysis. Through mechanism exploration, the coexistence of Cu and Co on Fe-based LDHs can accelerate the interfacial electron transfer and promote the formation of the reactive oxygen species (ROS), thus facilitating the EF process. Furthermore, the degradation by-products and possible degradation pathways of ACT in the CuCoFe-LDH/EF process were proposed. The reusability test and the treatment of various typical organic pollutants experiments indicated that the CuCoFe-LDH/EF process has excellent stability and broad application prospects. This work provides a valuable reference for the treatment of pharmaceuticals by the heterogeneous EF process in a wide range of pH.

Chlorine Dioxide: Friend or Foe for Cell Biomolecules? A Chemical Approach

This review examines the role of chlorine dioxide (ClO₂) on inorganic compounds and cell biomolecules. As a disinfectant also present in drinking water, ClO₂ helps to destroy bacteria, viruses, and some parasites. The Environmental Protection Agency EPA regulates the maximum concentration of chlorine dioxide in drinking water to be no more than 0.8 ppm. In any case, human consumption must be strictly regulated since, given its highly reactive nature, it can react with and oxidize many of the inorganic compounds found in natural waters. Simultaneously, chlorine dioxide reacts with natural organic matter in water, including humic and fulvic acids, forming oxidized organic compounds such as aldehydes and carboxylic acids, and rapidly oxidizes phenolic compounds, amines, amino acids, peptides, and proteins, as well as the nicotinamide adenine dinucleotide NADH, responsible for electron and proton exchange and energy production in all cells. The influence of ClO₂ on biomolecules is derived from its interference with redox processes, modifying the electrochemical balances in mitochondrial and cell membranes. This discourages its use on an individual basis and without specialized monitoring by health professionals.

Deep oxidation of norfloxacin by the electrochemical enhanced heterogeneous catalytic oxidation: The role of electric field and reaction optimization

In this study, electrochemical (EC_G-G: graphite anode and cathode, EC_I-G: iron anode and graphite cathode) enhanced heterogeneous activation of peroxymonosulfate (PMS) by CoFe₂O₄ nanoparticles for the degradation of norfloxacin (NOR) in water was investigated. Although a higher NOR removal efficiency was achieved in EC_I-G/CoFe₂O₄/PMS system, the generation of Fe³⁺ had resulted in the deposition of iron mud and affect the recovery of CoFe₂O₄. Under the optimum reaction conditions of CoFe₂O₄/PMS system, the final removal efficiency of NOR did not show significant difference in EC_G-G/CoFe₂O₄/PMS system (96.0%) and CoFe₂O₄/PMS system (95.5%), but the value of apparent rate constant significantly increased in EC_G-G/CoFe₂O₄/PMS system (0.21 min^-1) compared with CoFe₂O₄/PMS system (0.11 min^-1). Similar NOR degradation pathways were obtained in these two systems, and the TOC removal efficiency in EC_G-G/CoFe₂O₄/PMS system (28.8%) is almost as low as CoFe₂O₄/PMS system (26.0%). Therefore, it can be proposed that the applied electric field through active electrodes can accelerate the reaction of heterogeneous catalytic oxidation, but does not participate much in NOR degradation. However, the TOC removal efficiency (30 min) could be reached 68.7% as the mass ratio of PMS to CoFe₂O₄ increased to 5:1 (250 mg L^-1: 50 mg L^-1). The EC_G-G/CoFe₂O₄/PMS system is a promising low-cost technique for efficient mineralization of antibiotics in wastewater.

Valorization of UWWTP effluents for ammonium recovery and MC elimination by advanced AOPs

The main objective of this study was to generate ready-to-use revalorized irrigation water for fertilization from urban wastewater treatment plant (UWWTP) effluents. The focus was on controlled retention of NH₄⁺ and microcontaminants (MC), using nanofiltration. Retentates generated were treated by solar photo-Fenton at circumneutral pH using Ethylenediamine-N, N'-disuccinic acid (EDDS) iron complexing agent. Solar photo-Fenton degradation efficacy was compared with electrooxidation processes as anodic oxidation, solar-assisted anodic oxidation, electro-Fenton and solar photoelectro Fenton. Finally, phytotoxicity and acute toxicity tests were performed to demonstrate the potentially safe reuse of treated wastewater for crop irrigation. Nanofiltration was able to produce a ready-to-use permeate stream containing recovered NH₄⁺. (valuable nutrient). Solar photo-Fenton treatment at circumneutral pH would only be of interest for rapid degradation of contaminants at less than 1 mg/L in nanofiltration retentates. Other alternative tertiary treatments, such as electrooxidation processes, are a promising alternative when a high concentration of MC requires longer process times. Anodic oxidation was demonstrated to be able to eliminate >80% of microcontaminants and solar-assisted anodic oxidation significantly reduced the electricity consumption. Electro-Fenton processes were the least efficient of the processes tested. Phytotoxicity results showed that irrigation with the permeates reduced germination, root development was mainly promoted and shoot development was positive only at low retention rate (concentration factor = 2). Acute and chronic Daphnia magna toxicity studies demonstrated that the permeate volumes should be diluted at least 50% before direct reuse for crop irrigation.

Sensor Data Fusion as an Alternative for Monitoring Chlorate in Electrochlorination Applications

As chlorate concentrations have been found to be harmful to human and animal health, governments are increasingly demanding strict control of the chlorate concentration in drinking water. Since there are no chlorate sensors available, the current solution is sampling and laboratory analysis. This is costly and time consuming. The aim of this work was to investigate Sensor Data Fusion (SDF) as an alternative approach, with a focus on chlorate formation in the electrochlorination process, and design an observer for the real-time estimation of chlorate. The pH, temperature and UV-a absorption were measured in real time. A reduced-order nonlinear model was derived, and it was found to be detectable. An Extended Kalman Filter (EKF), based on this model, was then used to estimate the chlorate formation. The EKF algorithm was verified experimentally and was found to be capable of accurately estimating chlorate concentrations in real time. Electrochlorination is an emerging and efficient method of disinfecting drinking water. Soft sensing of chlorate concentrations, as proposed in this paper, may help to better control and manage the process of electrochlorination.

Membrane and Electrochemical Based Technologies for the Decontamination of Exploitable Streams Produced by Thermochemical Processing of Contaminated Biomass

Phytoremediation is an emerging concept for contaminated soil restoration via the use of resilient plants that can absorb soil contaminants. The harvested contaminated biomass can be thermochemically converted to energy carriers/chemicals, linking soil decontamination with biomass-to-energy and aligning with circular economy principles. Two thermochemical conversion steps of contaminated biomass, both used for contaminated biomass treatment/exploitation, are considered: Supercritical Water Gasification and Fast Pyrolysis. For the former, the vast majority of contaminants are transferred into liquid and gaseous effluents, and thus the application of purification steps is necessary prior to further processing. In Fast Pyrolysis, contaminants are mainly retained in the solid phase, but a part appears in the liquid phase due to fine solids entrainment. Contaminants include heavy metals, particulate matter, and hydrogen sulfide. The purified streams allow the in-process re-use of water for the Super Critical Water Gasification, the sulfur-free catalytic conversion of the fuel-rich gaseous stream of the same process into liquid fuels and recovery of an exploitable bio-oil rich stream from the Fast Pyrolysis. Considering the fundamental importance of purification/decontamination to exploit the aforementioned streams in an integrated context, a review of available such technologies is conducted, and options are shortlisted. Technologies of choice include polymeric-based membrane gas absorption for desulfurization, electrooxidation/electrocoagulation for the liquid product of Supercritical Water Gasification and microfiltration via ceramic membranes for fine solids removal from the Fast Pyrolysis bio-oil. Challenges, risks, and suitable strategies to implement these options in the context of biomass-to-energy conversion are discussed and recommendations are made.

Production of Chlorine Dioxide Using Hydrogen Peroxide and Chlorates

Chlorine dioxide was produced by the reduction of chlorate with hydrogen peroxide in strongly acidic media. To avoid reaction interference during measuring procedures, UV spectra were acquired to monitor the chlorate reduction. This reduction led to the formation of chlorine dioxide and notable concentrations of chlorite and hypochlorous acid/chlorine, suggesting that the hydrogen peroxide:chlorate ratio is important. Once chlorates are transformed to chlorine dioxide, the surplus hydrogen peroxide promoted the further reaction of the chlorinated species down to less-important species. Moreover, chlorine dioxide was stripped with the outlet gas flow. A linear relationship was established between the amount of limiting reagent consumed and the maximum height of the absorption peak at 360 nm after testing with different ratios of hydrogen peroxide and chlorate, allowing calculations of the maximum amount of chlorine dioxide formed. To verify the reproducibility of the method, a test with four replicates was conducted in a hydrogen peroxide/chlorate solution where chlorine dioxide reduction was not promoted due to the presence of surplus chlorate in the reaction medium after the test. Results confirmed the efficient formation of this oxidant, with maximum concentrations of 8.0 ± 0.33 mmol L−1 in 400–450 min and a conversion percentage of 97.6%. Standard deviations of 0.14–0.49 mmol L−1 were obtained during oxidation (3.6–6.5% of the average), indicating good reproducibility.

Comparison of the degradation of multiple amine-containing pharmaceuticals during electroindirect oxidation and electrochlorination processes in continuous system

The degradation of pharmaceuticals by electrochemical oxidation (EO) in simulated wastewater containing multiple pharmaceuticals was compared between batch and continuous reactors. Despite the excellent efficiencies achieved in batch experiments, the practical/large-scale applications of EO-degrading amine-containing pharmaceuticals has not yet been accomplished. This paper presents the results of continuous experiments with one of the most promising electrochemical configurations of Pt/Ti electrodes before proceeding to application. In the continuous electrooxidation system (without chloride), direct oxidation on the electrode surface and oxidation by hydroxyl radicals were the main pathways. Due to their short lifespans, the radicals could not be transferred to the bulk solution, and the removal of pharmaceuticals followed the order of sulfamethoxazole (SMX) > paracetamol (PAR) > diclofenac (DIC). In the electrochlorination system (with chloride), oxidation by residual chlorine was the main pathway. The removal of pharmaceuticals followed the order of sulfamethoxazole (SMX) > diclofenac (DIC) > paracetamol (PAR). High SMX removal was realized because of the high reaction rate of SMX with free chlorine. Among the pharmaceuticals, PAR had the lowest removal because it is a neutral species with a low mass transfer rate without the attraction of electrostatic force. These results are consistent with the predictions from our previous batch-scale study, which showed that the reaction rate of dissociated compounds could be increased by the addition of electrostatic force. Furthermore, multiple coexisting pharmaceuticals, such as SMX and PAR or DIC, may form dimers that can be transferred to complex structures and cause higher toxicity.

Removal of persistent organic contaminants from wastewater using a hybrid electrochemical-granular activated carbon (GAC) system

A three-dimensional (3D) electrochemical flow-through reactor equipped with GAC packed bed, polarized by the electric field, was evaluated for the removal of persistent organic contaminants from real sewage effluent. The performance of the reactor was investigated for 27 consecutive runs at two anodic current densities, i.e., low current density (LCD) of 15 A m^-2, and high current density (HCD) of 100 A m^-2. In the HCD experiments, the adsorption ability of saturated GAC was increased, mainly due to the increase in the mesoporosity of GAC. A synergy between electrosorption/adsorption on GAC and electrooxidation was observed in terms of the removal of all target pollutants. DEET presented the highest synergy, ranging from 40% to 57%, followed by iopromide (22-46%), carbamazepine (15-34%) and diatrizoate (4-30%). The addition of GAC decreased the concentrations of toxic chlorate and perchlorate by 2-fold and 10-fold, respectively, due to their electrosorption on GAC. Also, 3D electrochemical system yielded lower concentrations of adsorbable organic iodide (AOI) and adsorbable organic chlorine (AOCl). Thus, addition of low amounts of GAC in electrochemical systems may be a low-cost and simple way of minimizing the formation and final effluent concentrations of toxic halogenated byproducts.

Cartap removal from simulated water matrices by fluidized-bed Fenton process: optimization of process parameters

Cartap is a thiocarbamate pesticide widely-used to protect rice crops, one of the most mass-produced cereals worldwide. Effluents containing cartap pose serious environment and health risks due to the acute toxicity of this emerging contaminant. This work evaluates the capabilities of the Fenton process to efficiently remove cartap from water matrices. Process parameters such as hydrogen peroxide dosage, ferrous ion concentration and operating pH were optimized using Box-Behnken design. Results showed complete cartap removal with Fenton oxidation in a fluidized-bed reactor while eliminating sludge generation during treatment. Fluidized-bed Fenton process had improved reduction in chemical oxygen demand and total organic carbon due to the contribution of heterogeneous Fenton catalysis to the overall degradation of cartap species compared to conventional Fenton in a batch reactor. Furthermore, competitive reactions and scavenging effects in complex natural water matrices were simulated with the use of inorganic ions such as nitrate, chloride, and phosphate. Results demonstrated the detrimental effect of phosphate ions on Fenton oxidation due to the precipitation of soluble catalysts as iron phosphates, which stops the catalytic Fenton cycle and thus the production of oxidants for contaminant degradation.

Electrochemically-active carbon nanotube coatings for biofouling mitigation: Cleaning kinetics and energy consumption for cathodic and anodic regimes

Biofouling is a major obstacle in engineered systems exposed to aqueous conditions. Many attempts have been made to engineer the surface properties of materials to render them resistant to biofouling. These modifications typically rely on passive antimicrobial or anti-adhesive surface coatings that prevent the deposition of bacteria or inactivate them once they reach the surface. However, no surface modification strategy completely prevents biofilm formation, and, over time, surfaces will be fouled and require cleaning. In this work, we demonstrate the capacity of electrochemical carbon nanotube coatings in dispersing biofilms formed on the surface. A systematic analysis of the biofilm removal kinetics in function of applied current density is made to identify the optimal current conditions needed for efficient surface cleaning. Operating the electrochemically active surface as a cathode produces superior results compared to when it is operated as an anode. Specifically, the 5.00 A m^-2 and 2.50 A m^-2 cathodic conditions produced rapid cleaning, with complete biofilm dispersal after 2 min of operation. Surface cleaning is attributed to the generation of microbubbles on the surface that scours the surface to remove the adhered biofilm. Energy consumption analyses indicate that the 2.50 A m^-2 cathodic condition offers the best combination of cleaning kinetics and energy consumption achieving 99% biofilm removal at an energy cost of ~$ 0.0318 m^-2. This approach can be competitive compared to the current chemical cleaning strategies, while offering an opportunity for a more sustainable and integrated approach for biofouling management in engineered systems.

Electrocoagulation-electrooxidation for mitigating trace organic compounds in model drinking water sources

In-situ water treatment can be accomplished using electrochemical treatments such as electrocoagulation (EC), which generates coagulants, and electrooxidation (EO), which generates oxidants (e.g., free chlorine and reactive oxygen species) via electrolysis using boron-doped diamond electrodes. In sequential EC-EO, EC can remove oxidant scavengers present in dissolved organic carbon (DOC), thereby improving the efficacy of downstream oxidation via EO. This study evaluated sequential EC-EO (and each process independently for comparison) for mitigating the trace organic compounds (TOrCs) acyclovir (ACY), trimethoprim (TMP), and benzyldimethyldecylammonium chloride (BAC-C10) in model groundwaters and surface waters. EO-only removed greater than 70% of ACY and TMP but negligible BAC-C10 in model groundwaters. In model surface waters, EO-only removed ∼55-75% BAC-C10, but had less removal for ACY and TMP (∼20-55%), primarily due to DOC interference. Sequential-EC-EO was investigated to better gauge the potential process improvement due to the addition of EC ahead of EO. EC removed 74 ± 7% DOC from model surface water and improved downstream EO treatment relative to EO-only by a factor of 3.4 for ACY, 1.7 for TMP, and 1.4 for BAC-C10. When treating model groundwater, EC-EO resulted in no improvement compared to EO-only for ACY and TMP. BAC-C10 removal was attributed to the particle separation step between EC and EO rather than electrochemical inputs. EO-only treatment was more energy efficient for model groundwater compared to model surface waters based on electrical energy per order (E_EO) values. Sequential EC-EO further improved the energy efficiency for treating model river water.

Rational Surface and Interfacial Engineering of IrO2 /TiO2 Nanosheet Arrays toward High-Performance Chlorine Evolution Electrocatalysis and Practical Environmental Remediation

The chlorine evolution reaction (CER) is a critical and commercially valuable electrochemical reaction in industrial-scale utilization, including the Chlor-alkali industry, seawater electrolysis, and saline wastewater treatment. Aiming at boosting CER electrocatalysis, hybrid IrO₂ /TiO₂ nanosheet arrays (NSAs) with rational surface and interfacial tuning strategies are proposed in this study. The IrO₂ /TiO₂ NSAs exhibit superb CER electrocatalytic activity with a low overpotential (44 mV) at 10 mA cm^-2 , low Tafel slope of 40 mV dec^-1 , high CER selectivity (95.8%), and long-term durability, outperforming most of the existing counterparts. The boosting mechanism is proposed that the aerophobic/hydrophilic surface engineering and interfacial electronic structure tuning of IrO₂ /TiO₂ are beneficial for the Cl^- mass-transfer, Cl₂ release, and Volmer-Heyvrosky kinetics during the CER. Practical saline wastewater treatment by using the IrO₂ /TiO₂ NSAs electrode is further conducted, demonstrating it has a higher p-nitrophenol degradation ratio (95.10% in 60 min) than that of other electrodes. The rational surface and interfacial engineering of IrO₂ /TiO₂ NSAs can open up a new way to design highly efficient electrocatalysts for industrial application and environmental remediation.

Electro-Fenton beyond the Degradation of Organics: Treatment of Thiosalts in Contaminated Mine Water

Electro-Fenton (EF) is an emerging technology with well-known outstanding oxidation power; yet, its application to the treatment of inorganic contaminants has been largely disregarded. Thiosalts are contaminants of emerging concern in mine water, responsible for delayed acidity in natural waterways. In this study, EF was used to treat thiosalts in synthetic and real mine water. Thiosulfate (S₂O₃^2-) solutions were first used to optimize the main parameters affecting the process, namely, the current density (2.08-6.25 mA cm^-2), temperature (4 vs 20 °C), and S₂O₃^2- concentration (0.25-2 g L^-1). S₂O₃^2- was almost completely removed in 2 h of treatment at 6.25 mA cm^-2, while temperature played no important role in the process efficiency. The optimal conditions were then applied to treat a real sample of contaminated mine water, resulting in complete S₂O₃^2- and S₄O₆^2- oxidation to SO₄^2- in 90 min at 6.25 mA cm^-2 (95% removal in only 60 min). The reaction mechanisms were investigated in detail based on the quantification of the main degradation byproducts. This study opens new possibilities for EF application to the treatment of thiosalt-contaminated mine water and other oxidizable inorganic-impacted wastewaters.

Mechanism of N,N-dimethylformamide electrochemical oxidation using a Ti/RuO2-IrO2 electrode

The compound N,N-dimethylformamide (DMF) is a widely used industrial chemical and a common environmental contaminant that has been found to be harmful to human health. In this study, electrochemical oxidation was adopted for the degradation of DMF. The effects of four kinds of electrodes on the removal rates of DMF and total organic carbon were compared, and based on the result, the Ti/RuO₂–IrO₂ electrode was selected as the operating electrode. The effects of three independent factors (current density, pH, and NaCl proportion) on the DMF degradation were investigated through single-factor experiments, and the experimental results were optimized by response surface methodology. The optimal experimental conditions were obtained as follows: current density = 47 mA cm⁻², pH = 5.5, and NaCl proportion = 15%. The electrochemical oxidation of 50 mg L⁻¹ DMF was performed under the optimal conditions; the degradation rate was 97.2% after 7 h, and the reaction followed the pseudo-first-order kinetic model. The degradation products under optimal conditions and chlorine-free conditions were analyzed, and four degradation pathways were proposed. The DMF degradation was more thorough under optimal conditions.

DEMS as an emerging technology was used to investigate the degradation mechanism of DMF.

Nickel ferrite nanoenabled graphene oxide (NiFe2O4@GO) as photoactive nanocomposites for water treatment

Nanocomposite materials can enhance the capabilities of water treatment processes such as photocatalysis. In this work, novel light-driven nanocatalysts were synthesized by using nickel ferrite (NiFe₂O₄) to nanoenable graphene oxide (GO) substrates. GO is an emerging 2D nanomaterial with high conductivity and adsorption properties. Moreover, the electric properties of GO improve photocatalytic performance by promoting charge carrier separation. Results of the characterization of the nickel ferrite nanoenabled graphene oxide (NiFe₂O₄@GO) nanocomposites demonstrate that homogeneous and stable photocatalysts were produced. The as-synthesized nanocatalysts enabled complete decolorization of the colored water matrix in short irradiation times of 150 min using minimal catalyst loading at 0.5 g L^-1. The selective hook and destroy mechanism reduced the competitive effect of co-existing ions in solution. Furthermore, the use of specific scavengers helped to elucidate the degradation mechanisms of organic dye methylene blue by NiFe₂O₄@GO nanocomposites.

Photocatalytic treatment of natural waters. Reality or hype? The case of cyanotoxins remediation

This review compiles recent advances and challenges in the photocatalytic treatment of natural water by analyzing the remediation of cyanotoxins. The review frames the treatment need based on the occurrence, geographical distribution, and legislation of cyanotoxins in drinking water while highlighting the underestimated global risk of cyanotoxins. Next, the fundamental principles of photocatalytic treatment for remediating cyanotoxins and the complex degradation pathway for the most widespread cyanotoxins are presented. The state-of-the-art and recent advances on photocatalytic treatment processes are critically discussed, especially the modification strategies involving TiO₂ and the primary operational conditions that determine the scalability and integration of photocatalytic reactors. The relevance of light sources and light delivery strategies are shown, with emphasis on novel biomimicry materials design. Thereafter, the seldomly-addressed role of water-matrix components is thoroughly and critically explored by including natural organic matter and inorganic species to provide future directions in designing highly efficient strategies and scalable reactors.

Polycyclic Aromatic Hydrocarbons: Occurrence, Electroanalysis, Challenges, and Future Outlooks

The past several decades have seen increasing concern regarding the wide distribution of polycyclic aromatic hydrocarbons (PAHs) in environmental matrices. Primary toxicological data show PAHs' persistent characteristics and possible toxicity effects. Because of this pressing global issue, electroanalytical methods have been introduced. These methods are effective for PAH determination in environmental waters, even outclassing sophisticated analytical techniques such as chromatography, conventional spectrophotometry, fluorescence, and capillary electrophoresis. Herein, the literature published on PAHs is reviewed and discussed with special regard to PAH occurrence. Moreover, the recent developments in electrochemical sensors for PAH determination and the challenges and future outlooks in this field, are also presented.

Development of a treatment train for the remediation of a hazardous industrial waste landfill leachate: A big challenge

This study focuses on the development of a treatment train for a leachate from a hazardous industrial waste landfill (HIWL) previously treated by: (i) catalytic oxidation with hydrogen peroxide (H₂O₂) for sulphide and sulphite conversion into oxidized sulphur species, including sulphate, and (ii) chemical precipitation of sulphate as barite. The complete treatment line counted on four more stages: (iii) 1^st biological oxidation for removal of biodegradable organic compounds and nitrogen species, (iv) coagulation with ferric chloride (coagulant dose of 100 mg Fe L^-1, pH 2.8) for removal of a fraction of recalcitrant organics and suspended solids, (v) photo-Fenton oxidation using ultraviolet A (UVA) radiation (PF-UVA) (pH 2.8, initial total dissolved iron content of 140 mg L^-1, treatment time of ~4 h) for recalcitrant organics degradation and biodegradability improvement, and (vi) 2^nd biological oxidation for removal of the biodegradable organic matter resulting from the PF-UVA process. The use of anodic oxidation or photoelectro-Fenton processes in stage (v) demonstrated to be unfeasible. A chemical oxygen demand (COD) below 1000 mg O₂ L^-1, a common limit imposed by municipal wastewater treatment plants (MWWTPs) to effluents discharged into the municipal sewer, was achieved after a feasible treatment time (~4 h) using the multistep approach. The remediation of the HIWL leachate proved to be a big challenge.

Effective treatment of greywater via green wall biofiltration and electrochemical disinfection

Low energy and cost solutions are needed to combat raising water needs in urbanised areas and produce high quality recycled water. In this study, we investigated key processes that drive a unique greywater treatment train consisting of a passive green wall biofiltration system followed by disinfection using a Boron-doped diamond (BDD) electrode with a solid polymer electrolyte (SPE). In both systems, the treatment was performed without any additional chemicals and pollutants of concern were monitored for process evaluation. The green wall system removed over 90% of turbidity, apparent colour, chemical oxygen demand, total organic carbon, and biological oxygen demand, and 1 log of E. coli and total coliforms, mostly through biological processes. The green wall effluent met several proposed greywater reuse guidelines, except for E. coli and total coliform treatment (below 10 MPN/100 mL). Further disinfection of treated greywater (contained 28 mg/L Cl¯ and electrical conductivity (EC) of 181.3 µS/cm) by electrolysis at current density 25 mA/cm² inactivated over 3.5 logs of both E. coli and total coliforms, in 10 - 15 min of electrolysis, resulting in recycled water with less than 2 MPN/100 mL. A synergistic effect between electrochemically-generated free chlorines and reactive oxygen species contributed to the inactivation process. Although the treated water contained diluted chloride and had low EC, estimated energy consumption was just 0.63 - 0.83 kWh/m³. This is the first study to show the effectiveness of a low energy and a low cost greywater treatment train that combines green urban infrastructure with BDD electrochemical treatment process with SPE, offering a reliable and an environmentally-friendly method for greywater reuse.

Enhanced decomposition of H2O2 by molybdenum disulfide in a Fenton-like process for abatement of organic micropollutants

Accelerating the rate-limiting step of Fe³⁺/Fe²⁺ conversion is a major challenge for H₂O₂ decomposition in conventional Fenton process. In this study, the catalytic mechanism of H₂O₂ by molybdenum disulfide (MoS₂) nanoparticles and Fe³⁺ ions was revealed and the abatement of organic micropollutants was investigated. The presence of both MoS₂ and Fe³⁺ can efficiently decompose H₂O₂. Reaction system of H₂O₂/MoS₂/Fe³⁺ is found to remove most of the tested pollutants by over 80% (except 65.9% for carbamazepine) within 60 min at pH of 3.0. Effective pH range of this reaction system can be extended to pH of 5.0. Adding MoS₂ to Fe³⁺/H₂O₂ system promotes the Fe³⁺/Fe²⁺ cycle and improves the reaction rate between Fe³⁺ and H₂O₂. The formation of Mo⁶⁺ ions and Mo⁶⁺ peroxo-complexes is beneficial to H₂O₂ decomposition and pollutant degradation. Electron paramagnetic resonance (EPR) measurements and quenching experiments confirm the important role of hydroxyl radicals in H₂O₂/MoS₂/Fe³⁺ system. Chloride ions (Cl^-) promote degradation, while bicarbonate ions (HCO₃^-) inhibit degradation. As H₂O₂ concentration increases from nil to 1.0 mM, the value of total EE/O decreases from 0.083 to 0.003 kWh L^-1, and the most energy efficient condition is determined. This study provides a new pathway for efficient decomposition of H₂O₂ by Fe³⁺ ions in an extended pH range, which is considered a facile and promising strategy for wastewater treatment.

Decreased formation of disinfection by-products during electrochemical leachate oxidation and their post-removal by electro-adsorption

Membrane electrochemical reactor (MER) is an effective treatment system to remove recalcitrant compounds in landfill leachate but harmful byproducts can be formed during the treatment. Herein, we have investigated the formation of total halogenated organics and the associated toxicity as a side effect of leachate treatment by the MER. Those byproducts are represented by the term "disinfection byproducts, or DBPs", because of the similar formation mechanisms. Several DBP groups were detected during the leachate treatment; however, the amount of DBP generated in the MER only accounted for 19.1 ± 4.5% of that in a membrane-less electrooxidation system (control). Likewise, the total toxicity value in the MER effluent was 26.6 × 10^-3, only 15.1% of that in the control system. While trihalomethanes dominated mass concentration by 84.1% in the MER, haloacetonitriles contributed to majority of the additive toxicity due to their higher toxicity index. Increasing the initial leachate pH from 9.5 to 13 could increase the DBP concentration by 2.18 times because of less removal of humic acids. A high initial ammonia concentration of 6000 mg L^-1 resulted in the increased DBP formation by 146.8%, compared to that with 2500 mg L^-1, due to the increased formation of nitrogenous DBPs. A higher current density of 30 mA cm^-2 doubled the DBP formation because of a faster reaction rate and a higher solution temperature. The extended treatment time caused trihalomethanes to continue forming DBP and degradation of most DBPs to some extent. With removing 67.5% of DBP mass concentration and 74.4% of the additive toxicity, the GAC-electrode system was shown more effective than GAC adsorption alone in remediating DBP from the MER effluent.