Electrolysis-assisted Mn(II)/sulfite process for organic contaminant degradation at near-neutral pH

Abstract

Manganese-catalyzed sulfite activation (i.e., Mn(II)/sulfite) has emerged as an advanced oxidation process to produce sulfate radical (SO4•-) for water treatment. However, to maintain the catalytic activity of Mn(II) ion, solution acidity has to be kept below pH 4, which is difficult to maintain in practice. Moreover, Mn(II)/sulfite reaction is a strongly oxygen-dependent process, and purging air into reaction solution is another extra cost. To solve the above issues, we devised to implement electrolysis into Mn(II)/sulfite (i.e., electro/Mn(II)/sulfite process) for organic compound (bisphenol A, BPA) oxidation. It was revealed that, under near-neutral conditions (pH 6), the removal rate of 10 μM BPA was increased from 46.3%, by Mn(II)/sulfite process, to 94.2% by electro/Mn(II)/sulfite process. The enhancement of BPA removal after implementation of electrolysis to Mn(II)/sulfite process was investigated, and concluded to be a result of several pathways. In detail, the produced oxygen from water electrolysis, direct sulfite oxidation on anode, and local acidic pH at anode vicinity together play a role in promoting SO4•- production and, therefore, contaminant removal. Radical-scavenging assays confirmed the dominant role of SO4•- in electro/Mn(II)/sulfite process.