Role of dissolved Mn(III) in transformation of organic contaminants: Non-oxidative versus oxidative mechanisms

Abstract

Mn(III) is a strong oxidant for one electron transfer, which may be important in the transformation of organic contaminants during water/wastewater treatment and biogeochemical redox processes. This study explored the reaction mechanisms of dissolved Mn(III) with organics. The role of dissolved Mn(III) either as a catalyst or an oxidant in reactions with organics was recognized. Aquo and/or hydroxo (or free) Mn(III), generated from the bisulfite activated permanganate process, facilitated efficient N-dealkylation of atrazine via a β-elimination mechanism, resulting no net redox reaction. In contrast, free Mn(III) degraded 4-chlorophenol via intramolecular redox processes, the same as hydroxyl radical (OH), resulting in dechlorination,OH substitution, ring-opening and mineralization. Mn(III)-pyrophosphate compounds did not react with atrazine because complexation by pyrophosphate rendered Mn(III) unable to bond with atrazine, thus the electron and proton transfers between the reactants couldn't occur. However, it degraded 4-chlorophenol at a slower rate compared to free Mn(III), due to its reduced oxidation potential. These results showed two distinct mechanisms on the degradation of organic contaminants and the insights may be applied in natural manganese-rich environments and water treatment processes with manganese compounds.