Mechanistic insight into manganese oxidation induced by sulfite under aerobic condition: Implication of triclosan degradation

Abstract

Various earth-rich transition metal elements such as iron and manganese have been applied to activate sulfite for the degradation of pollutants. However, the redox process of manganese and its role in pollutants removal in the Mn(II)/sulfite system remain unclear. Batch experiment was performed in this study to investigate the manganese oxidation mechanisms and triclosan (TCS) degradation in a mixture solution with Mn(II) and sulfite under aerobic conditions. About 75 % removal of TCS was achieved in the Mn(II)/sulfite (200 μM/200 μM) system at pH 6.0 within 24 h aerobic reaction, and the conversion efficiency was positively related to the concentrations of Mn(II). The addition of Mn(II) led to the rapid depletion of sulfite within the initial two minutes and the rod-shaped MnO2 was obviously generated in the subsequent reaction accompanied by the degradation of TCS. Quenching experiments revealed that manganese oxide, rather than sulfur radicals, was the major reactive oxidant for TCS degradation, and SO5•- and SO4•- were responsible for the formation of manganese oxide, directly verified by ABTS method and X-ray photoelectron spectroscopy (XPS) analysis. In addition, TCS oligomers (dimers and trimers) were mainly produced through a single electron process between manganese oxide and TCS. The results of this study proved the in-situ formation of active manganese oxide in Mn(II)/sulfite system under aerobic condition, which could induce the transformation of organic contaminants.