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

Molecular composition and formation mechanism of chlorinated organic compounds in biological waste leachate treated by electrochemical oxidation with a boron-doped diamond anode

The use of electrochemical oxidation with boron-doped diamond (BDD) as an anode has been demonstrated to be an effective means of removing dissolved organic matter (DOM) from biologically treated waste leachate. However, in the presence of chloride ions, undesired chlorine evolution occurs on the anode; this forms chlorinated DOM, mostly of unknown molecular composition. We investigate the molecular composition and formation mechanism of chlorinated DOM during electrochemical oxidation process of biologically treated leachate DOM. At a current density of 8 mA/cm2, after 120 min of electrolysis, 479 unknown chlorinated DOMs were detected in the treated effluent, comprising 21.55% of the total. The unknown species are dominated by oxygen-rich, highly unsaturated structures, and exhibit higher oxidation degrees, lower unsaturation, and lower aromaticity compared to the removed nonchlorinated DOM. An additional 43.63 mg/L of known chlorinated DOM species, predominantly dichloroacetic and trichloroacetic acids, also accumulate in the treated effluent. Introducing hydroxyl radicals (HO•) to the anode surface forms reactive chlorine species including chlorine radical (Cl•), dichlorine radical (Cl2•-), and hypochlorous acid/hypochlorite (HOCl/OCl-); the concentration of HOCl/OCl- reaches 529.2 mg/L. These species react with reduced and aromatic dissolved organic matter via reaction pathways such as chlorine substitution for hydrogen (Cl+H-) and the HOCl addition reaction (HO+Cl+) to generate unknown chlorinated DOM species; the known chlorinated DOM are formed afterward via ring opening and dealkylation pathways. Our results provide a theory for the prevention and control of chlorinated DOM during treatment of chlorine-laden organic wastewater by an electrochemical oxidation system with a boron-doped diamond anode.

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

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

Engineering decentralized electrodisinfection to sustain consistent chlorine generation under varying drinking water chloride content

In situ electrochlorination can offer an efficient and feasible solution to enable decentralized water disinfection. Unfortunately, there has been only a limited number of studies exploring single-pass flow cell systems with representative flowrates used at household level, particularly under varying chloride concentrations. This work aims to assess anode materials in a single pass and examine the impact of cross velocity, current density, and chloride concentration on various responses such as chlorine production and energy consumption. The primary objective is to determine whether the flow cell can achieve desirable chlorine levels under consistent operation while chloride content of water varies. Chlorine (Cl2/HOCl/OCl-), chlorine dioxide (ClO2) production, and toxic oxyanions (ClO3 -, ClO4 -) were assessed in a single pass setup utilizing different representative anodes including Ti/RuO2, Ti/IrO2, and Boron-doped diamond. Among these materials, the Ti/RuO2 anode emerged as the most suitable for effective chlorine generation while minimizing the formation of ClO3 - and ClO4 -. The performance of in situ electrochlorination using the Ti/RuO2 anode in the flow cell revealed that cross velocity exerted the most significant influence on chlorine generation, while chloride content and current density primarily impacted energy consumption. Optimization of the operating parameters illustrated that stable chlorine concentrations ranging from 2 to 4 mg L-1 could be maintained even with significant fluctuations in chloride concentration from 50 to 250 mg L-1, resulting in a daily energy consumption of less than 0.07 kWh per treated volume of 634 L (i.e., < 0.11 Wh L-1). These experimental findings hold promise for advancing electrodisinfection systems to higher technological readiness level.

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.

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.

Advances on electrochemical disinfection research: Mechanisms, influencing factors and applications

Disinfection, a vital barrier against pathogenic microorganisms, is crucial in halting the spread of waterborne diseases. Electrochemical methods have been extensively researched and implemented for the inactivation of pathogenic microorganisms from water and wastewater, primarily owing to their simplicity, efficiency, and eco-friendliness. This review succinctly outlined the core mechanisms of electrochemical disinfection (ED) and systematically examined the factors influencing its efficacy, including anode materials, system conditions, and target species. Additionally, the practical application of ED in water and wastewater treatment was comprehensively reviewed. Case studies involving various scenarios such as drinking water, hospital wastewater, black water, rainwater, and ballast water provided concrete instances of the expansive utility of ED. Finally, coupling ED with other technologies and the resulting synergies were introduced as pivotal foundations for subsequent engineering advancements.

Bibliometric analysis of electrochemical disinfection: current status and development trend from 2002 to 2022

The removal of waterborne pathogens from water is critical in preventing the spread of waterborne diseases. Electrochemical methods have been extensively researched and implemented for disinfection, primarily owing to their simplicity, efficiency, and eco-friendliness. Thus, it is essential to conduct a review about the research progress and hotspots on this promising technique. In this paper, we provided a comprehensive bibliometric analysis to systematically study and analyze the current status, hotspots, and trends in electrochemical disinfection research from 2002 to 2022. This study analyzed literature related to electrochemical disinfection or electrochemical sterilization published in the Web of Science database from 2002 to 2022 using CiteSpace and Biblioshiny R language software packages. The analysis focused on the visualization and assessment of annual publication volume, discipline and journal distribution, collaborative networks, highly cited papers, and keywords to systematically understand the current status and trends of electrochemical disinfection. The results showed that between 2002 and 2022, 1171 publications related to electrochemical disinfection were published, with an exponential increase in the cumulative number of publications (y=17.518e0.2147x, R2= 0.9788). The publications covered 76 disciplines with many articles published in high-impact journals. However, the research power was characterized by a large number of scattered research efforts and insufficient cooperation, indicating the need for further innovative collaboration. The citation analysis and keyword analysis suggest that future development in this field may focus on optimizing electrode materials, investigating the disinfection performance of ·OH based systems, optimizing conditions for actual wastewater treatment, and reducing energy consumption to promote practical applications.

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.

Changes of dissolved organic matter fractions and formation of oxidation byproducts during electrochemical treatment of landfill leachates: Development of spectroscopic indicators for process optimization

Electrochemical oxidation (EO) is an attractive option for treatment of dissolved organic matter (DOM) in landfill leachate but concerns remain over the energy efficiency and formation of oxidation byproducts ClO₃^- and ClO₄^-. In this study, EO treatment of landfill leachates was carried out using representative active and nonactive anode materials, cell configurations and current densities. Size exclusion chromatograms coupled with 2D synchronous and asynchronous correlation analysis showed that the sensitivity of DOM fractions to EO degradation was dependent on the anode material. The nonactive boron-doped diamond (BDD) anode demonstrated the best performance for DOM oxidation. The humic acid-like fraction (HA, 2.5-20 kDa) predominated the visible absorbance of landfill leachates at λ ≥400 nm, and it generally had the highest reaction rates except the occurrence of the pH-induced denaturation and precipitation of the proteinaceous biopolymer fraction (BP, >20 kDa). During the EO treatment of landfill leachate with BDD anode, the UV absorbance spectra of landfill leachates at wavelengths <400 nm were affected by the formation of free chlorine. Instead, the decrease of Abs420 was found to be a good indicator of the shift of the oxidation from predominantly HA fraction to the proteinaceous BP fraction. The behavior of the Abs420 parameter was also indicative of the transition from the energy-efficient oxidation of DOM to the dominance of side reactions of chlorine evolution and the subsequent formation of ClO₃^- and ClO₄^-. These findings suggest that the EO treatment of landfill leachate can be optimized by adjusting the current density with feedback signals from the online monitoring of Abs420, to achieve a trade-off between degradation of DOM and control of ClO₃^- and ClO₄^-.

Water disinfection by the UVA/electro-Fenton process under near neutral conditions: Performance and mechanisms

An efficient and thorough water disinfection is critical for human health. In this study, UVA-LEDs, nitrilotriacetic acid (NTA) and a boron-doped diamond anode were respectively used as the UVA source, the iron chelator and the anode for the UVA/electro-Fenton (E-Fenton) reaction to treat wastewater. The disinfection performance of the UVA/E-Fenton had been investigated. The mechanisms of the E. coli inactivation had been clarified. The results showed that complete disinfection (about 5.6-log removal) could be achieved within 50 min at a certain condition due to the synergistic effort of the UVA, anodic oxidation and the electro-Fenton. The quenching experiments and the electron paramagnetic resonance (EPR) detection indicated that •OH, •O₂^- and ¹O₂ play important roles for inactivating E. coli. The results of SEM images and genomic DNA electrophoresis suggested that both the cell structure and the DNA had been thoroughly destroyed during the UVA/E-Fenton process. Increasing the UVA irradiation, oxygen bubbling could improve the disinfection rate, while it also would increase the energy consumption. The appropriate Fe and NTA ratio was 1:2 to realize an efficient Fenton reaction under near neutral condition. Complete disinfection was also achieved within 50 min when it used for treating real wastewater. Thus, the UVA/E-Fenton process is a satisfied way for water disinfection.