Formation and degradation mechanisms of CX3R-type oxidation by-products during cobalt catalyzed peroxymonosulfate oxidation: The roles of Co3+ and SO4•-

Sulfate radical (SO4 center dot-)-based advanced oxidation processes (AOPs) attract increasing attention in the control of micropollutants. However, SO4 center dot- can react with other chemicals present in water and result in undesired oxidation by-products (OBPs) generation. The formation and degradation mechanisms of CX3R-type OBPs during cobalt catalyzed peroxymonosulfate (Co2+/PMS) oxidation were investigated. In the formation of CX3R-type OBPs, both Co3+ and SO4 center dot- could convert chloride to free chlorine that then reacted with natural organic matter, leading to the formation of CX3R-type OBPs. The concentrations of trichloromethane, chloral hydrate, dichloroacetonitrile, dichloroacetamide and trichloroacetamide after 15 min reaction were 9.8, 3.9, 1.2, 5.9 and 22.3 nM, respectively. Compared to SO4 center dot-, Co3+ played a more significant role in the CX3R-type OBP formation and calculated toxicity values of CX3R-type OBPs. CX3R-type OBPs could not only be formed but also be degraded at the same time during Co2+/PMS oxidation. As for the degradation of CX3R-type OBPs, both Co3+ and SO4 center dot- could transform CX3R-type OBPs to chloride. Compared to Co3+, SO4 center dot- played a more important role in the degradation of CX3R-type OBPs and the conversion from chloride to final by-product chlorate. The adverse effects that results from Co3+ need more attention in SO4 center dot--based AOPs application.