Integrating oxidation and reduction processes in electrochemical wastewater treatment for contaminant removal with byproduct control

Abstract

Electrochemical technologies offer a promising approach for recalcitrant contaminants removal, but toxic halogenated byproducts from the treatment pose a critical challenge. Herein, an integrated electrochemical oxidation (EO) and reduction (ER) process was developed for both contaminant removal and byproduct control. The anodic EO achieved > 90% contaminant removal and generated > 0.6 μM THM4 and > 0.8 μM HAA5 when treating a saline wastewater. A trace amount of Br^- led to the production of reactive bromine species and the brominated byproducts. Carbonates made EO more compound-specific by scavenging halogen radicals to CO₃•^- and reduced the THM4 and HAA5 formation by 16% and 31%, respectively. The cathodic ER removed > 80% of THM4 and > 50% of HAA5 through direct reduction and H*-mediated indirect reduction pathways with the final concentrations of ∼ 0.1 μM THM4 and ∼ 0.4 μM HAA5. HAAs could achieve complete dehalogenation via ER and form the non-halogenated products. Throughout the treatment of the integrated process, phenolic contaminant was completely removed by the anodic EO with the k_obs > 0.045 min^-1, and the formed halogenated byproducts were subsequently removed by the cathodic ER to meet the national and global standards, with a total energy consumption of ∼ 4.5 kWh m^-3. The results of this study would encourage the further exploration of enhanced electrochemical wastewater treatment with minimized byproduct residues.