Degradation of 2,4-dichlorophenol by CuO-activated peroxydisulfate: Importance of surface-bound radicals and reaction kinetics

Peroxydisulfate (PDS, S2O82-) is a promising oxidant for water treatment and contaminated groundwater remediation. It requires activation to generate sulfate radical (SO4-) and hydroxyl radical (OH) for indirect oxidation of organic pollutants. Recently, efforts were devoted to developing PDS activation systems for direct oxidation of organic pollutants without producing radicals. However, the mechanism was still ambiguous and the kinetics was either not quantified or empirical in nature. In this research, we examined the activation of PDS by CuO for the degradation of 2,4-dichlorophenol (2,4-DCP). Dual-compound control experiments, radical scavenging tests and electron paramagnetic resonance (EPR) studies showed that surface-bound OH generated from the adsorbed PDS was the main reactive species responsible for the degradation of 2,4-DCP. A kinetic model considering the important reaction steps, including the adsorption of PDS onto CuO, activation of adsorbed PDS to form surface-bound SO4- and then surface-bound OH, and degradation of 2,4-DCP by surface-bound OH, was developed to better elucidate the reaction kinetics. The results suggested that the overall reaction kinetics of 2,4-DCP degradation was regulated by the adsorption of PDS onto CuO and the electron transfer between surface Cu and adsorbed PDS to form surface-bound SO4-. The developed kinetic model could serve as a framework to characterize other persulfate oxidation systems relying on surface-bound radicals.