The occurrence of perchlorate during drinking water electrolysis using BDD anodes

Challenges and contribution of electrochemical driven by-products in tannery wastewater treatment: Optimization, detection and distribution of reactive oxidation species

Electrochemical oxidation (EO) is an excellent approach for the treatment of persistent pollutant from synthesistic and real wastewater than conventional wastewater treatment processes. Chloride and sulfate salts generally used and present in natural wastewater that affect the EO process. In this research, the effect of electrolyte concentration on active sulfate (SO42⁻) species (HSO4⁻, SO4•⁻ and S2O82⁻) formation, chlorinated by-products distribution (ClO4-, ClO3-, Cl2), and tannery effluent degradation have been examined while using graphite electrodes. A full factorial design was used to optimize the three independent factors, namely: initial pH (pHo): 3-11, current (I): 1-3 A, and electrolysis time (t): 20-110 min for the responses of chemical oxygen demand (COD) and chromium (Cr) removal. Under the optimum treatment conditions of 3 A current, 90 min electrolysis time, 600 mg L-1 Na2SO4 concentration and pHo of 7, more than 88% COD and 90% Cr removal were achieved under optimal conditions. Qualitative and quantitative analysis confirmed the formation and distribution of various reactive oxidation species and a plausible mechanism was discussed. EO processes yielded almost total mineralization due to the synergistic action of generated active chlorine, sulfate species and hydroxyl radicals. A relatively higher amount of ClO3⁻ was occurred that sign the efficient •OH generation in sulfate mediated EO because the ClO3⁻ formation is certainly associated to •OH concentration. Overall results demonstrate that sulfate enriched electrolyte systems are helpful for EO of hazardous organic pollutants.

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

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

MMO-induced batch and pilot-scale electro-oxidation treatment of municipal wastewater

The present research aimed to explore the durability of MMO electrodes through electro-oxidation (EO) in purifying secondary treated actual sewage wastewater using batch and pilot-scale setups. The main aim is to inactivate bacteria in sewage treatment plants before they are released into the environment, thus contaminating water and soil. Process parameters such as current density (j), NaCl dose (n), and treatment time (t) were optimized using response surface methodology in a lab-scale EO reactor under batch conditions. The results showed that optimization of current density at 5.90 mA/cm2 and NaCl concentration at 1.31 g/L led to 93.90% of bacterial inactivation (Q1) within 8 min of treatment and 0.48 kWh/m3 energy consumption (Q2). Biological analysis was conducted to validate bacterial cell destruction and count coliform bacteria in the EO-treated sewage wastewater. XRD, cyclic voltammetry studies, and FE-SEM/EDS analysis were done to confirm the MMO anode's durability and stability after 100 recycles. The study prioritized bacterial inactivation along with organic matter degradation. Besides that, a small pilot-scale study on the actual sewage wastewater with a volume of 10-50 L was done in batch mode under previously optimized conditions to analyze the efficacy of the MMO anodes in terms of bacterial inactivation.

Comparative study of MMO and BDD anodes for electrochemical degradation of diuron in methanol medium

Treating emerging pollutants at low concentrations presents significant challenges in terms of degradation efficiency. Anodic oxidation using active and non-active electrodes shows great potential for wastewater treatment. Thus, this study compared the efficiency of a commercial mixed metal oxide anode (MMO: Ti/Ti0.7Ru0.3O2) and a boron-doped diamond anode (BDD) for the electrochemical oxidation of diuron in methanol, in chloride and sulfate media. The MMO anode achieved diuron removal rates of 94.9% and 92.8% in chloride and sulfate media, respectively, with pseudo-first-order kinetic constants of 0.0177 and 0.0143 min-1. The BDD anode demonstrated slightly higher removal rates, achieving 96.2% in sulfate medium and 96.9% in chloride medium, with respective kinetic constants of 0.0193 min⁻1 and 0.0177 min⁻1. Increasing the current density enhanced diuron removal by up to 15% for both electrodes; however, excessively high current densities led to increased energy consumption due to side reactions. The present of water had antagonistic effects, resulting in removal rates of 91.1% for chloride media using the BDD anode; and 87.4% and 90.4% in sulfate media with MMO and BDD anodes, respectively. The MMO anode in chloride medium did not show significant difference in the degradation percentage, reaching 96% of diuron removals. The degradation mechanism was proposed based on the detection of various by-products. The primary reactions observed during the oxidation of diuron in methanol involved chlorine substitution in the aromatic ring and dealkylation. These processes generated several intermediates and by-products at low concentrations, ultimately leading to high diuron removal.

Integrating redox-electrodialysis and electrosorption for the removal of ultra-short- to long-chain PFAS

A major challenge in per- and polyfluoroalkyl substances (PFAS) remediation has been their structural and chemical diversity, ranging from ultra-short to long-chain compounds, which amplifies the operational complexity of water treatment and purification. Here, we present an electrochemical strategy to remove PFAS from ultra-short to long-chain PFAS within a single process. A redox-polymer electrodialysis (redox-polymer ED) system leverages a water-soluble redox polymer with inexpensive nanofiltration membranes, facilitating the treatment of varied chain lengths of PFAS without membrane fouling. Our approach combines both ion migration by electrodialysis (for PFAS with chain lengths ≤C4) and electrosorption strategies (for PFAS with chain lengths ≥C6) to eliminate approximately 90% of ultra-short-, short-chain, and long-chain PFAS. At the same time, we achieve continuous desalination of the source water down to potable water level. The redox-polymer ED exhibits remarkable PFAS removal in real source water scenarios, including from matrices with 10,000 times higher salt concentrations, as well as secondary effluents from wastewaters. Additionally, the removed PFAS is mineralized with a defluorination performance between 76-100% by electrochemical oxidation, highlighting the viability of integrating the separation step with a reactive degradation process.

Managing PFAS exhausted Ion-exchange resins through effective regeneration/electrochemical process

This study proposes an integrated approach that combines ion-exchange (IX) and electrochemical technologies to tackle problems associated with PFAS contamination. Our investigation centers on evaluating the recovery and efficiency of IX/electrochemical systems in the presence of five different salts, spanning dosages from 0.1 % to 8 %. The outcomes reveal a slight superiority for NaCl within the regeneration system, with sulfate and bicarbonate also showing comparable efficacy. Notably, the introduction of chloride ion (Cl-) into the electrochemical system results in substantial generation of undesirable chlorate (ClO3-) and perchlorate (ClO4-) by-products, accounting for ∼18 % and ∼81 % of the consumed Cl-, respectively. Several agents, including H2O2, KI, and Na2S2O3, exhibited effective mitigation of ClO3- and ClO4- formation. However, only H2O2 demonstrated a favorable influence on the degradation and defluorination of PFOA. The addition of 0.8 M H2O2 resulted in the near-complete removal of ClO3- and ClO4-, accompanied by 1.3 and 2.2-fold enhancements in the degradation and defluorination of PFOA, respectively. Furthermore, a comparative analysis of different salts in the electrochemical system reveals that Cl- and OH- ions exhibit slower performance, possibly due to competitive interactions with PFOA on the anode's reactive sites. In contrast, sulfate and bicarbonate salts consistently demonstrate robust decomposition efficiencies. Despite the notable enhancement in IX regeneration efficacy facilitated by the presence of methanol, particularly for PFAS-specific resins, this enhancement comes at the cost of reduced electrochemical decomposition of all PFAS. The average decay rate ratio of all PFAS in the presence of 50 % methanol, compared to its absence, falls within the range of 0.11-0.39. In conclusion, the use of 1 % Na2SO4 salt stands out as a favorable option for the integrated IX/electrochemical process. This choice not only eliminates the need to introduce an additional chemical (e.g., H2O2) into the wastewater stream, but also ensures both satisfactory regeneration recovery and efficiency in the decomposition process through electrochemical treatment.

Efficient removal of 2,4,6-trinitrotoluene (TNT) from industrial/military wastewater using anodic oxidation on boron-doped diamond electrodes

With growing public concern about water quality particular focus should be placed on organic micropollutants, which are harmful to the environment and people. Hence, the objective of this research is to enhance the security and resilience of water resources by developing an efficient system for reclaiming industrial/military wastewater and protecting recipients from the toxic and cancerogenic explosive compound-2,4,6-trinitrotoluene (TNT), which has been widely distributed in the environment. This research used an anodic oxidation (AO) process on a boron-doped diamond (BDD) electrode for the TNT removal from artificial and real-life matrices: marine water and treated wastewater. During experiments, TNT concentrations were significantly decreased, reaching the anodic degradation efficiency of above 92% within two hours and > 99.9% after six hours of environmental sample treatment. The presented results show the great potential of AO performed on BDD anodes for full-scale application in the industry and military sectors for TNT removal.

New diamond coatings for a safer electrolytic disinfection

In this work, a new coating of boron-doped diamond ultra-nanocrystalline (U-NBDD), tailored to prevent massive formation of perchlorates during disinfection, is evaluated as electrode for the reclaiming of treated secondary wastewater by the electrochemically assisted disinfection process. Results obtained are compared to those obtained by using a standard electrode (STD) that was evaluated as a standard in previous research showing outstanding performance for this application. First tests were carried out to evaluate the chlorine speciation obtained after the electrolysis of synthetic chloride solutions at two different ranges of current densities. Concentrations of hypochlorite obtained using the U-NBDD anode at 25 mA cm-2 were 1.5-fold higher, outperforming STD anode; however, at 300 mA cm-2, an overturn on the behavior of anodes occurs where the amount of hypochlorite produced on STD anode was 1.5-fold higher. Importantly, at low current density the formation of chlorates and perchlorates is null using U-NBDD. Then, the disinfection of the real effluent of the secondary clarifier of a municipal wastewater treatment facility is assessed, where inactivation of Escherichia coli is achieved at low charge applied per volume electrolyzed (0.08 A h L-1) at 25 mA cm-2 using the U-NBDD. These findings demonstrate the appropriateness of the strategy followed in this work to obtain safer electro-disinfection technologies for the reclaiming of treated wastewater.

Chlorine-mediated electrochemical advanced oxidation process for ammonia removal: Mechanisms, characteristics and expectation

Chlorine-Mediated Electrochemical Advanced Oxidation (Cl-EAO) technology is a promising approach for ammonia removal from wastewater due to its numerous advantages, including small infrastructure, short processing time, easy operation, high security, and high nitrogen selectivity. This paper provides a review of the ammonia oxidation mechanisms, characteristics, and anticipated applications of Cl-EAO technology. The mechanisms of ammonia oxidation encompass breakpoint chlorination and chlorine radical oxidation, although the contributions of active chlorine, Cl, and ClO remain uncertain. This study critically examines the limitations of existing research and suggests that a combination of determining free radical concentration and simulating a kinetic model would help elucidate the contributions of active chlorine, Cl, and ClO to ammonia oxidation. Furthermore, this review comprehensively summarizes the characteristics of ammonia oxidation, including kinetic properties, influencing factors, products, and electrodes. The amalgamation of Cl-EAO technology with photocatalytic and concentration technologies has the potential to enhance ammonia oxidation efficiency. Future research should concentrate on clarifying the contributions of active chlorine, Cl, and ClO to ammonia oxidation, the production of chloramines and other byproducts, and the development of more efficient anodes for the Cl-EAO process. The main objective of this review is to enhance the understanding of the Cl-EAO process. The findings presented herein contribute to the advancement of Cl-EAO technology and provide a foundation for future studies in this field.

A Review of PFAS Destruction Technologies

Per- and polyfluoroalkyl substances (PFASs) are a family of highly toxic emerging contaminants that have caught the attention of both the public and private sectors due to their adverse health impacts on society. The scientific community has been laboriously working on two fronts: (1) adapting already existing and effective technologies in destroying organic contaminants for PFAS remediation and (2) developing new technologies to remediate PFAS. A common characteristic in both areas is the separation/removal of PFASs from other contaminants or media, followed by destruction. The widely adopted separation technologies can remove PFASs from being in contact with humans; however, they remain in the environment and continue to pose health risks. On the other hand, the destructive technologies discussed here can effectively destroy PFAS compounds and fully address society's urgent need to remediate this harmful family of chemical compounds. This review reports and compare widely accepted as well as emerging PFAS destruction technologies. Some of the technologies presented in this review are still under development at the lab scale, while others have already been tested in the field.

Comparison of homogeneous and heterogeneous electrochemical advanced oxidation processes for treatment of textile industry wastewater

This study aimed at understanding the influence of the generation of oxidants in a heterogeneous way at boron-doped diamond (BDD) anode (anodic oxidation (AO)) or homogeneously in the bulk (electro-Fenton (EF)) during treatment of a textile industry wastewater. Both processes achieved high TOC removal. A yield of 95 % was obtained by combining EF with BDD anode during 6 h of treatment. The EF process was found to be faster and more efficient for discoloration of the effluent, whereas AO was more effective to limit the formation of degradation by-products in the bulk. An advantage of AO was to treat this alkaline effluent without any pH adjustment. Operating these processes under current limitation allowed optimizing energy consumption in both cases. However, using BDD anode led to the formation of very high concentration of ClO₃^-/ClO₄^- from Cl^- oxidation (even at low current density), which appears as a key challenge for treatment of such effluent by AO. By comparison, EF with Pt anode strongly reduced the formation of ClO₃^-/ClO₄^-. Operating EF at low current density even maintained these concentrations below 0.5 % of the initial Cl^- concentration. A trade-off should be considered between TOC removal and formation of toxic chlorinated by-products.

Electrochemical Oxidation of Landfill Leachate after Biological Treatment by Electro-Fenton System with Corroding Electrode of Iron

Electrochemical oxidation of landfill leachate after biological treatment by a novel electrochemical system, which was constructed by introducing a corroding electrode of iron (Fe_c) between a boron-doped diamond (BDD) anode and carbon felt (CF) cathode (named as BDD–Fe_c–CF), was investigated in the present study. Response surface methodology (RSM) with Box–Behnken (BBD) statistical experiment design was applied to optimize the experimental conditions. Effects of variables including current density, electrolytic time and pH on chemical oxygen demand (COD) and ammonia nitrogen (NH₃-N) removal efficiency were analyzed. Results showed that electrolytic time was more important than current density and pH for both COD and NH₃-N degradation. Based on analysis of variance (ANOVA) under the optimum conditions (current density of 25 mA·cm⁻², electrolytic time of 9 h and pH of 11), the removal efficiencies for COD and NH₃-N were 81.3% and 99.8%, respectively. In the BDD–Fe_c–CF system, organic pollutants were oxidized by electrochemical and Fenton oxidation under acidic conditions. Under alkaline conditions, coagulation by Fe(OH)₃ and oxidation by Fe(VI) have great contribution on organic compounds degradation. What is more, species of organic compounds before and after electrochemical treatment were analyzed by GC–MS, with 56 kinds components detected before treatment and only 16 kinds left after treatment. These results demonstrated that electrochemical oxidation by the BDD–Fe_c–CF system has great potential for the advanced treatment of landfill leachate.

Tertiary treatment of a mixture of composting and landfill leachates using electrochemical processes

The study investigated the treatment efficiency of coupled electrocoagulation (EC) and electrooxidation (EO) processes for landfill leachate treatment in batch and continuous mode. The EC process (iron anode and graphite cathode) at 18.2 mA/cm² for 2.5 min resulted in COD, turbidity, total phosphorus, total coliforms and fecal coliforms removal of 58.1, 72.9, 98.5, 97.9, and 97.2% respectively. Under the same operating conditions, the coupled EC/EO (Ti-Pt anode, bipolar iron electrode, and graphite cathode) processes showed that the COD, turbidity, total phosphorus, total coliforms, and fecal coliforms removal of 56.5%, 78.3%, 96.3%, 97.2% and fecal coliforms 72.7%, respectively. The energy costs associated with the EC and EC/EO were 0.11 and 0.25 $/m³, respectively. Compared to the batch configuration, the continuous configuration of EC resulted in similar processing performance. However, the EC/EO process resulted in the production of chlorates, perchlorates, and trihalomethanes as by-products. Moreover, the continuous process slightly increases the pH and ammonia concentration of the leachate and also resulted in the metallic sludge production with an average dryness of 4.2%. The toxicity tests determined that the treated effluent was not toxic to Rainbow trout and Daphnia.

Research trends in the development of anodes for electrochemical oxidation of wastewater

The review focuses on the recent development in anode materials and their synthesis approach, focusing on their compatibility for treating actual industrial wastewater, improving selectivity, electrocatalytic activity, stability at higher concentration, and thereby reducing the mineralization cost for organic pollutant degradation. The advancement in sol–gel technique, including the Pechini method, is discussed in the first section. A separate discussion related to the selection of the electrodeposition method and its deciding parameters is also included. Furthermore, the effect of using advanced heating approaches, including microwave and laser deposition synthesis, is also discussed. Next, a separate discussion is provided on using different types of anode materials and their effect on active •OH radical generation, activity, and electrode stability in direct and indirect oxidation and future aspects. The effect of using different synthesis approaches, additives, and doping is discussed separately for each anode. Graphene, carbon nanotubes (CNTs), and metal doping enhance the number of active sites, electrochemical activity, and mineralization current efficiency (MCE) of the anode. While, microwave or laser heating approaches were proved to be an effective, cheaper, and fast alternative to conventional heating. The electrodeposition and nonaqueous solvent synthesis were convenient and environment-friendly techniques for conductive metallic and polymeric film deposition.

A critical review on the electrosorption of organic compounds in aqueous effluent - Influencing factors and engineering considerations

Despite being an old process from the end of the 19^th century, electrosorption has attracted renewed attention in recent years because of its unique properties and advantages compared to other separation technologies and due to the concomitant development of new porous electrode materials. Electrosorption offer the advantage to separate the pollutants from wastewater with the possibility of selectively adsorbing and desorbing the targeted compounds. A comprehensive review of electrosorption is provided with particular attention given to the electrosorption of organic compounds, unlike existing capacitive deionization review papers that only focus on inorganic salts. The background and principle of electrosorption are first presented, while the influence of the main parameters (e.g., electrode materials, electrode potential, physico-chemistry of the electrolyte solutions, type of compounds, co-sorption effect, reactor design, etc.) is then detailed and the modeling and engineering aspects are discussed. Finally, the main output and future prospects about recovery studies and combination between electro-sorption/desorption and degradation processes are given. This review particularly highlights that carbon-based materials have been mostly employed (85% of studies) as porous electrode in organics electrosorption, while existing studies lack of electrode stability and durability tests in real conditions. These electrodes have been implemented in a fixed-bed reactor design most of the time (43% of studies) due to enhanced mass transport. Moreover, the electrode potential is a major criterion: it should be applied in the non-faradaic domain otherwise unwanted reactions can easily occur, especially the corrosion of carbon from 0.21 V/standard hydrogen electrode or the water oxidation/reduction. Furthermore, there is lack of studies performed with actual effluents and without addition of supporting electrolyte, which is crucial for testing the real efficiency of the process. The associated predictive model will be required by considering the matrix effect along with transport phenomena and physico-chemical characteristics of targeted organic compounds.

On-Demand Atomic Hydrogen Provision by Exposing Electron-Rich Cobalt Sites in an Open-Framework Structure toward Superior Electrocatalytic Nitrate Conversion to Dinitrogen

Electrocatalytic nitrate (NO₃^-) reduction to N₂ via atomic hydrogen (H*) is a promising approach for advanced water treatment. However, the reduction rate and N₂ selectivity are hindered by slow mass transfer and H* provision-utilization mismatch, respectively. Herein, we report an open-framework cathode bearing electron-rich Co sites with extraordinary H* provision performance, which was validated by electron spin resonance (ESR) and cyclic voltammetry (CV) tests. Benefiting from its abundant channels, NO₃^- has a greater opportunity to be efficiently transferred to the vicinity of the Co active sites. Owing to the enhanced mass transfer and on-demand H* provision, the nitrate removal efficiency and N₂ selectivity of the proposed cathode were 100 and 97.89%, respectively, superior to those of noble metal-based electrodes. In addition, in situ differential electrochemical mass spectrometry (DEMS) indicated that ultrafast *NO₂^- to *NO reduction and highly selective *NO to *N₂O or *N transformation played crucial roles during the NO₃^- reduction process. Moreover, the proposed electrochemical system can achieve remarkable N₂ selectivity without the additional Cl^- supply, thus avoiding the formation of chlorinated byproducts, which are usually observed in conventional electrochemical nitrate reduction processes. Environmentally, energy conservation and negligible byproduct release ensure its practicability for use in nitrate remediation.

Interpretation of high perchlorate generated during electrochemical disinfection in presence of chloride at BDD anodes

Perchlorate is a disinfection by-product (DBP) of serious health concern. Herein, the long sought mechanism of high perchlorate production during electrochemical disinfection at boron-doped diamond (BDD) anode in the presence of chloride was elucidated. The generated perchlorate at BDD during electrochemical disinfection (in 10 mM NaCl) in 60 min reached 0.125 mM, which was 830 times higher than the EPA standard. In contrast, perchlorate at PbO₂ and SnO₂ anodes was below the detection limit. Further experiments employing NaClO₃ revealed that the conversion ratio from ClO₃^- to ClO₄^- in 10 h at BDD (98%) was considerably higher than PbO₂ (13%) and SnO₂ (12%). Such significant difference among anodes was fully interpreted with a two-step mechanism. The first step is essential to produce ·ClO₃ by oxidizing ClO₃^- at electrodes. Otherwise, the conversion to perchlorate would be impossible even with excessive ·OH, which was verified with the photocatalysis process. The second step is the perchlorate generation with radical reaction between ·ClO₃ and ·OH, where the primary role of ·OH was substantiated by scavenging test. Interestingly, the capability of perchlorate production was correlated with free ·OH instead of the total amount of ·OH. Despite the similar abilities of electron transfer between anodes and ClO₃^-, much higher free ·OH exists at BDD anode than at PbO₂ and SnO₂ anodes through chronoamperometry experiments and work function characterization, which reasonably provides interpretation of high perchlorate production at BDD anode.

Graphene-based sponges for electrochemical degradation of persistent organic contaminants

Graphene-based sponges doped with atomic nitrogen and boron were applied for the electrochemical degradation of persistent organic contaminants in one-pass, flow-through mode, and in a low-conductivity supporting electrolyte. The B-doped anode and N-doped cathode was capable of >90% contaminant removal at the geometric anodic current density of 173 A m^-2. The electrochemical degradation of contaminants was achieved via the direct electron transfer, the anodically formed O₃, and by the OH^• radicals formed by the decomposition of H₂O₂ produced at the cathode. The identified transformation products of iopromide show that the anodic cleavage of all three C-I bonds at the aromatic ring was preferential over scissions at the alkyl side chains, suggesting a determining role of the π- π interactions with the graphene surface. In the presence of 20 mM sodium chloride (NaCl), the current efficiency for chlorine production was <0.04%, and there was no chlorate and perchlorate formation, demonstrating a very low electrocatalytic activity of the graphene-based sponge anode towards chloride. Graphene-based sponges were produced using a low-cost, bottom-up method that allows easy introduction of dopants and functionalization of the reduced graphene oxide coating, and thus tailoring of the material for the removal of specific contaminants.

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.

Electrochemical degradation of per- and poly-fluoroalkyl substances using boron-doped diamond electrodes

Electrochemical degradation using boron-doped diamond (BDD) electrodes has been proven to be a promising technique for the treatment of water contaminated with per- and poly-fluoroalkyl substances (PFAS). Various studies have demonstrated that the extent of PFAS degradation is influenced by the composition of samples and electrochemical conditions. This study evaluated the significance of several factors, such as the current density, initial concentration of PFAS, concentration of electrolyte, treatment time, and their interactions on the degradation of PFAS. A 2⁴ factorial design was applied to determine the effects of the investigated factors on the degradation of perfluorooctanoic acid (PFOA) and generation of fluoride in spiked water. The best-performing conditions were then applied to the degradation of PFAS in wastewater samples. The results revealed that current density and time were the most important factors for PFOA degradation. In contrast, a high initial concentration of electrolyte had no significant impact on the degradation of PFOA, whereas it decreased the generation of F^-. The experimental design model indicated that the treatment of spiked water under a current density higher than 14 mA cm^-2 for 3-4 h could degrade PFOA with an efficiency of up to 100% and generate an F^- fraction of approximately 40-50%. The observed high PFOA degradation and a low concentration of PFAS degradation products indicated that the mineralization of PFOA was effective. Under the obtained best conditions, the degradation of PFOA in wastewater samples was 44-70%. The degradation efficiency for other PFAS in these samples was 65-80% for perfluorooctane sulfonic acid (PFOS) and 42-52% for 6-2 fluorotelomer sulfonate (6-2 FTSA). The presence of high total organic carbon (TOC) and chloride contents was found to be an important factor affecting the efficiency of PFAS electrochemical degradation in wastewater samples. The current study indicates that the tested method can effectively degrade PFAS in both water and wastewater and suggests that increasing the treatment time is needed to account for the presence of other oxidizable matrices.

Novel Ti/RuO2IrO2 anode to reduce the dangerousness of antibiotic polluted urines by Fenton-based processes

The treatment of hospital wastewater is very complex, so treating polluted human urine is a significant challenge. Here, we tested a novel MMO-Ti/RuO₂IrO₂ electrode to reduce the ecotoxicity risk of hospital urines contaminated with antibiotics. This electrode was used as the anode in electro-Fenton (EF) and photoelectro-Fenton (PhEF) processes. The results were compared with those obtained using the boron-doped diamond (BDD) anode, as well as those obtained by a conventional Fenton oxidation. In order to analyze the performance of the processes, the treatments were evaluated on the subject of Penicilin G (PenG) removal, toxicity (using a standardized method with Vibrio Fisheri), and antibiotic activity (Enterococcus faecalis as the target bacterium). The results reveal that PenG degrades in the following order: Fenton < EF < PhEF. The best results are found for the MMO-PhEF, which completely removed PenG, decreased 96% of toxicity, and completely removed antibiotic activity. Besides, for comparison, tests were performed with BDD, and results point out the higher convenience of the new electrode in terms of acceptable use of energy because the effluents generated can be further degraded in an urban wastewater treatment plant. Because of that, MMO-RuO₂-IrO₂ emerges as a promising cost-effective material for the pre-treatment of hospital urine effluents.

Photoelectrocatalytic degradation of 2,4-dichlorophenol in a TiO2 nanotube-coated disc flow reactor

Photoelectrocatalytic (PEC) water treatment is a promising technology for organic pollution abatement. Much of the prior research focused on material discovery and optimization. However, challenges exist in scaling-up PEC processes and are associated with designing reactors with effective light irradiation on electrode surfaces and, simultaneously, efficient electrode configurations. We design and demonstrate key reactor design principles, which influence reaction mechanisms, for a reactor using a TiO₂ nanotube-coated disc flow reactor. Degradation of organochlorinated 2,4-dichlorophenol was studied as representative carcinogenic micropollutant. The synergistic photoelectrocatalytic process showed 5-fold faster degradation kinetics than solely electrocatalytic treatment or a greater than 2-fold enhancement over photocatalysis alone. Applicability of photoelectrocatalytic treatment was demonstrated over a wide range of micropollutant concentrations with almost complete abatement even at concentrations up to 25 mg L^-1 of 2,4-dichlorophenol. Mechanistically, the increase in applied current density efficiency for degradation of 2,4-dichlorophenol was due to stabilization of charge carriers and higher oxidants production rates in the PEC system. Carboxylic acids were identified as the main by-products formed from cleavage of the phenolic ring moieties in 2,4-dichlorophenol. However, very importantly we achieved dehalogenation photoelectrocatalysis with evidence of chlorine heteroatoms released as innocuous chloride anions. Overall, this research demonstrates the importance of PEC reactor design and how properly orientated TiO₂ nanotube-coated disc flow reactors leverage both novel material designs and reactor architectures to achieve pollutant degradation.

Efficient ammonia removal and toxic chlorate control by using BiVO4/WO3 heterojunction photoanode in a self-driven PEC-chlorine system

The efficient removal of ammonia is a difficult issue in wastewater treatment because ammonia is easily converted to nitrate instead of N₂. The oxidation of ammonia by chlorine radical (Cl) is recognized as an effective method. However, the massive generation of toxic byproducts chlorate and nitrate pose great risk for its practical application due to the excessive oxidation capacity of hydroxyl radical. Herein, we propose a novel method to selectively generate Cl for efficient ammonia removal using BiVO₄/WO₃ photoanode in a self-driven photoelectrocatalytic (PEC) system. Cl was predominantly produced by regulating the valence band edge of WO₃ though modifying BiVO₄, which tuned the moderate oxidative force of hole to reduce OH generation and thereby inhibited the formation of chlorate and nitrate. The self-driven ammonia degradation was achieved by employing BiVO₄/WO₃ and Si photovoltaic cells as composite photoanodes to improve light-absorption and electron-hole separation, thus enhancing Cl production. These results showed that 10 mg L^-1 of ammonia-N was completely removed (99.3 %) in 120 min with 80.1 % of total nitrogen removal. Toxic byproducts chlorate and nitrate were inhibited by 79.3 % and 31 %, respectively, compared to WO₃. This work provides new insights to develop efficient, energy-saving and environment-friendly method for ammonia pollution treatment.

Carbon nanoarchitectures as high-performance electrodes for the electrochemical oxidation of landfill leachate

Nanomaterials and assemblies of the aforementioned into complex architectures constitute an opportunity to design efficient and selective solutions to widespread and emerging environmental issues. The limited disposal of organic matter in modern landfills generates extremely concentrated leachates characterised by high concentrations of refractory compounds. Conventional biochemical treatment methods are unsuitable, while advanced treatment, such coagulation, reverse osmosis and ultrafiltration can be very costly and generate additional waste. Electrochemical oxidation is an established technique to efficiently mineralise a plethora of recalcitrant pollutants, however the selectivity and efficiency of the process are strongly related to the anode material. For this reason, a nanoarchitectured carbon material has been designed and synthesised to improve the capability of the anode towards the adsorption and decomposition of pollutants. Instead of simple nanostructures, intelligently engineered nanomaterials can come in handy for more efficient advanced treatment techniques. In this study, a carbon nanoarchitecture comprising boron-doped vertically aligned graphene walls (BCNWs) were grown on a boron-doped diamond (BDD) interfacial layer. The results show how the peculiar maze-like morphology and the concurrence of different carbon hybridisations resulted in a higher current exchange density. The BDD performed better for the removal of NH₄⁺ while the BCNW-only sample exhibited a faster deactivation. The BDD/BCNW nanoarchitecture resulted in an enhanced COD removal and a NH₄⁺ removal similar to that of BDD, without the intermediate production of NO₂^- and NO₃^-.

Chlorinated Byproduct Formation during the Electrochemical Advanced Oxidation Process at Magnéli Phase Ti4O7 Electrodes

This research investigated chlorinated byproduct formation at Ti₄O₇ anodes. Resorcinol was used as a model organic compound representative of reactive phenolic groups in natural organic matter and industrial phenolic contaminants and was oxidized in the presence of NaCl (0-5 mM). Resorcinol mineralization was >68% in the presence and absence of NaCl at 3.1 V/SHE (residence time = 13 s). Results indicated that ∼4.3% of the initial chloride was converted to inorganic byproducts (free Cl₂, ClO₂^-, ClO₃^-) in the absence of resorcinol, and this value decreased to <0.8% in the presence of resorcinol. Perchlorate formation rates from chlorate oxidation were 115-371 mol m^-2 h^-1, approximately two orders of magnitude lower than reported values for boron-doped diamond anodes. Liquid chromatography-mass spectroscopy detected two chlorinated organic products. Multichlorinated alcohol compounds (C₃H₂Cl₄O and C₃H₄Cl₄O) at 2.5 V/SHE and a monochlorinated phenolic compound (C₈H₇O₄Cl) at 3.1 V/SHE were proposed as possible structures. Density functional theory calculations estimated that the proposed alcohol products were resistant to direct oxidation at 2.5 V/SHE, and the C₈H₇O₄Cl compound was likely a transient intermediate. Chlorinated byproducts should be carefully monitored during electrochemical advanced oxidation processes, and multibarrier treatment approaches are likely necessary to prevent halogenated byproducts in the treated water.

Testing the use of cells equipped with solid polymer electrolytes for electro-disinfection

This work focuses on disinfection of water using electrolysis with boron doped diamond (BDD) coatings and faces this challenge by comparing the performance of two different cells manufactured by CONDIAS GmbH (Izehoe, Germany): CONDIACELL® ECWP and CabECO cells. They are both equipped with diamond electrodes, but the mechanical design is completely different, varying not only by geometry but also by the flow conditions. ECWP is a flow-through cell with perforated electrodes while the CabECO cell is a zero-gap cell with a proton exchange membrane as a solid polymer electrolyte (SPE) separating the anode and cathode. At 0.02 Ah dm^-3 both cells attain around 3-5 logs pathogen removal, but design and sizing parameters give an advantage to the CabECO: it can minimize the production of chlorates and perchlorates when operating in a single-pass mode, which becomes a really remarkable point. In this paper, we report tests in which we demonstrate this outstanding performance and we also explain the differences observed in the two cells operating with the same water.

Enhanced perfluorooctanoic acid degradation by electrochemical activation of peroxymonosulfate in aqueous solution

Perfluorooctanoic acid (PFOA) was efficiently decomposed at Ti/SnO₂-Sb anode via peroxymonosulfate (PMS) activation. PFOA degradation followed both pseudo-zero-order (0-30 min) and pseudo-first-order (30-120 min) kinetics. The pseudo-first-order kinetics constant could increase to 0.0484 min^-1 (3.84 times higher than that without PMS) during 30-120 min electrolysis. The inhibited performance in radical scavengers implied both sulfate radical (SO₄^•-) and hydroxyl radical (•OH) contributed to PFOA degradation. The •OH quantitative detection experiments demonstrated that SO₄^•- formed from PMS activation could promote •OH generation (from 0.12 mM to 0.24 mM). Electron spin resonance (ESR) tests further proved that SO₄^•- and •OH were generated during PFOA degradation. According to linear sweep voltammetry (LSV) analyses, the oxygen evolution potential (OEP) value of Ti/SnO₂-Sb electrode increased from 1.59 V to 1.72 V (vs SCE) via PMS addition, indicating the inhibited oxygen evolution which was beneficial for the reactive species formation (i.e. •OH, SO₄^•-). On the basis of intermediates verification and mass balance of carbon and fluorine, PFOA was proposed to be oxidized into short-chain perfluorocarboxylic acids mainly by •OH and SO₄^•-.

Electrooxidation of short and long chain perfluorocarboxylic acids using boron doped diamond electrodes

This study investigates electrooxidation of short (C3-C6) and long (C7-C-18) chain perfluorocarboxylic acids (PFCAs) including perfluorooctane sulfonate (PFOA) using Si/BDD electrode. The effect of operational parameters (supporting electrolyte type, applied current density, and initial pH) were explored for PFOA removal. At the optimized conditions, 74% TOC removal and 37% defluorination ratio were gained for 10 mg L^-1 of PFOA solution which evidences that the shorter chain PFCAs were formed. The PFOA degradation pathway followed one direct electron transfer from PFOA molecule to anode surface. Then two different degradation pathways were proposed. The first proposed degradation mechanism involved the reaction of perfluoroheptyl radical and hydroxyl radical, the release of HF and hydrolysis. The second mechanism involved the reaction between perfluoroheptyl radical and O₂, formation of C₇F₁₅O and perfluorohexyl radical with releasing COF₂. The removal of short- (C3-C6) and long-chain PFCAs (C7-C18) was also characterized. More than 95% of removal efficiency was gained for all long-chain PFCAs, excluding C7. The removal ratios of short-chain PFCAs (C3-C6) were 39%, 41%, 66% and 70% for C3, C4, C5 and C6, respectively. Contrary to long-chain PFCAs, chain-length dependence for short-chain PFCAs were observed. Defluorination ratio of short-chain PFCAs was only 45% signifying that defluorination partially occurred. Water matrix did not significantly affect the degradation of short-chain PFCAs in deionized water (DI), river water and secondary effluent of a wastewater treatment plant (WWTP). In contrast, defluorination ratio of long-chain PFCAs was noticeably affected by water matrix with the order of DI water > WWTP effluent > river water.

Application of mixed metal oxide anode for the electro-oxidation/disinfection of synthetic urine: Potential of harnessing molecular hydrogen generation

The efficacy of electro-oxidation has been checked for the deterioration of synthetic urine (SU) using mixed metal oxide anode along with the potential of harnessing the commercially useful byproduct i.e. molecular hydrogen gas. The results from batch have been used to execute the scale-up studies for the continuous electro-oxidation treatment of SU in a photovoltaic driven reactor. The effect of different operational variables like pH, time, current density and N/Cl ratio on process efficiency was evaluated in terms of %COD removal and specific energy consumption using response surface methodology. The results showed that 87.25% removal in COD and 85.88% in TOC were achieved in 8.8 h. The complete deactivation of E. coli spiked synthetic urine wastewater was achieved in 45 min only. The main strength lies in the demonstration of the significant reduction in treatment time to 6 h by incorporating dual effect i.e. Photo-electrocatalysis. The anode used was proven to be stable and effective even after 100 recycles (207.5 h). The intermediates formed during the treatment process were analyzed through LC-MS. The techno-economic analysis for the proposed technology under optimized conditions was calculated to be 0.85 $/kg of COD removed.

Effect of reactor configuration on the kinetics and nitrogen byproduct selectivity of urea electrolysis using a boron doped diamond electrode

Electrochemical systems have emerged as an advantageous approach for decentralized management of source-separated urine with the possibility of recovering or removing nutrients and generating energy. In this study, the kinetics and byproduct selectivity of the electrolytic removal of urea were investigated using a boron doped diamond working electrode under varied operational conditions with a primary focus on comparing undivided and divided reactors. The urea removal rate in the undivided and divided reactors was similar, but the divided reactor had an increased required cell voltage needed to maintain the equivalent current density. The current efficiency was similar for 0.1, 0.25, and 0.5 A (33.3, 83.3, 167 mA/cm²), suggesting no interference from competing reactions at higher potentials. In a divided reactor, increasing the anolyte pH reduced the urea removal rate presumably from hydroxyl radical scavenging by hydroxide. Further, for all divided reactor experiments, the final pH was less than 1, suggesting that the transport of protons across the ion exchange membrane to the cathode was slower than the oxidation reactions producing protons. The nitrogen byproduct selectivity was markedly different in the undivided and divided reactors. In both reactors, nitrate (NO₃^-) formed as the main byproduct at the anode, but in the undivided reactor it was reduced at the stainless steel cathode to ammonia. In the presence of 1 M chloride, the urea removal kinetics improved from the generation of reactive chlorine species, and the byproduct selectivity was shifted away from NO₃^- to presumably chloramines and N₂. Overall, these results indicate that the electrochemical reactor configuration should be carefully considered depending on the desired outcome of treating source-separated urine (e.g., nitrogen recovery, H₂ generation).

Advanced Oxidation Processes for the Removal of Antibiotics from Water. An Overview

In this work, the application of advanced oxidation processes (AOPs) for the removal of antibiotics from water has been reviewed. The present concern about water has been exposed, and the main problems derived from the presence of emerging pollutants have been analyzed. Photolysis processes, ozone-based AOPs including ozonation, O3/UV, O3/H2O2, and O3/H2O2/UV, hydrogen peroxide-based methods (i.e., H2O2/UV, Fenton, Fenton-like, hetero-Fenton, and photo-Fenton), heterogeneous photocatalysis (TiO2/UV and TiO2/H2O2/UV systems), and sonochemical and electrooxidative AOPs have been reviewed. The main challenges and prospects of AOPs, as well as some recommendations for the improvement of AOPs aimed at the removal of antibiotics from wastewaters, are pointed out.

The beneficial effect of cathodic hydrogen peroxide generation on mitigating chlorinated by-product formation during water treatment by an electro-peroxone process

The formation of chlorinated by-products is a major concern associated with electrochemical water treatment processes. This study investigated the formation of chlorinated by-products during surface water treatment by a newly developed electrochemical advanced oxidation process (EAOP), the electro-peroxone (E-peroxone) process, which couples ozonation with in situ electro-generation of hydrogen peroxide (H₂O₂) from cathodic oxygen reduction. Due to the enhanced ozone (O₃) conversion to hydroxyl radicals (•OH) by electro-generated H₂O₂, the E-peroxone process considerably accelerated the abatement of ozone-refractory micropollutants such as clofibric acid and chloramphenicol in the selected surface water compared to conventional ozonation. In addition, the cathodically generated H₂O₂ effectively quenched hypochlorous acid (HOCl) derived from the anodic oxidation of chloride in the surface water. Therefore, the formation of trichloromethane (TCM) and chloroacetic acids (CAAs) from the reactions of HOCl with dissolved organic matter (DOM) was insignificant during the E-peroxone process, and similar levels of TCM and CAAs were generally observed in the conventional ozonation and E-peroxone treated water. In contrast, considerable amounts of HOCl could be generated from the anodic oxidation of chloride and then accumulated in the surface water during conventional electrolysis process, which resulted in significantly higher concentrations of TCM and CAAs in the electrolysis treated water. The results of this study suggest that the E-peroxone process can overcome the major limitation of conventional electrochemical processes and provide an effective and safe EAOP alternative for micropollutant abatement during water treatment.