Integrating oxidation and reduction processes in electrochemical wastewater treatment for contaminant removal with byproduct control
Kaichao Yang, Zhen He
Abstract
• Electrochemical oxidation and reduction are integrated for wastewater treatment. • EO achieves contaminants removal but generates toxic halogenated byproducts. • Halide ions and carbonates affect EO process by forming different reactive species. • Byproducts were removed by cathodic ER process through reductive dehalogenation. • Dehalogenation is achieved by direct reduction and H*-mediated indirect reduction. 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 CO 3 • - 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.