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Sulfate radical (SO 4 ·- )-based advanced oxidation processes (AOPs) attract increasing attention in the control of micropollutants. However, SO 4 ·- can react with other chemicals present in water and result in undesired oxidation by-products (OBPs) generation. The formation and degradation mechanisms of CX 3 R-type OBPs during cobalt catalyzed peroxymonosulfate (Co 2+ /PMS) oxidation were investigated. In the formation of CX 3 R-type OBPs, both Co 3+ and SO 4 ·- could convert chloride to free chlorine that then reacted with natural organic matter, leading to the formation of CX 3 R-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 SO 4 ·- , Co 3+ played a more significant role in the CX 3 R-type OBP formation and calculated toxicity values of CX 3 R-type OBPs. CX 3 R-type OBPs could not only be formed but also be degraded at the same time during Co 2+ /PMS oxidation. As for the degradation of CX 3 R-type OBPs, both Co 3+ and SO 4 ·- could transform CX 3 R-type OBPs to chloride. Compared to Co 3+ , SO 4 ·- played a more important role in the degradation of CX 3 R-type OBPs and the conversion from chloride to final by-product chlorate. The adverse effects that results from Co 3+ need more attention in SO 4 ·- -based AOPs application., (Copyright © 2020 Elsevier B.V. All rights reserved.)