Electrochemically activated PMS and PDS: Radical oxidation versus nonradical oxidation

In this study, a comparison between electrochemically activated peroxomonosulfate (PMS) and peroxydisulfate (PDS) using carbon anodes was conducted for the first time. PMS activation was achieved using graphite (GR) and multi-walled carbon nanotube (MWCNT) anodes, which significantly promoted the degradation of organic pollutants sulfamethoxazole (SMX). The radical probing and scavenging experiments demonstrated that SO4 center dot- was the dominant reactive species (64.93% relative contribution ratio). By contrast, nonradical oxidation accounted for 95.79% relative contribution ratio to organic pollutants degradation in electrochemically activated PDS process under the identical conditions. The structure difference between PDS (-O4S-SO4-) and PMS (HO-SO4-) led to their various reactivities. The electrochemically activated PMS molecule (PMS*, acting as nonradical oxidation) had higher reactivity and lower stability than electrochemically activated PDS molecule (PDS*, acting as nonradical oxidation), thus to quickly decomposed to SO4 center dot-. Interestingly, electrochemically activated PMS (radical oxidation system) exhibited stronger resistance towards water background components than PDS (nonradical oxidation system), being suitable to treat the complicated water and wastewater containing various ions and organic compounds.