The specific role of sulfoxides in the Fe(Ⅱ)-activated persulfates processes

Sulfoxides are the primary probe compounds used to distinguish between Fe(Ⅳ) and free radicals, but whether they can serve as the quenching agents in the Fe(Ⅱ)-activated persulfate (Fe(Ⅱ)/PMS and Fe(Ⅱ)/PDS) systems remains unclear. In this study, we investigated the role of sulfoxides in the degradation of organic pollutants during the Fe(Ⅱ)/PMS and Fe(Ⅱ)/PDS systems. The results indicate that sulfoxides can enhance the degradation of 7-hydroxycoumarin in the Fe(Ⅱ)/PMS system. The promoting effect of phenyl methyl sulfoxide (PMSO) on the degradation of 7-hydroxycoumarin in the Fe(Ⅱ)/PMS system is related to the concentration of PMSO, the molar ratio of PMS/Fe(Ⅱ), and pH. The mechanism for the promoting effect of PMSO on the degradation of 7-hydroxycoumarin in the Fe(Ⅱ)/PMS systems is as follows: the generated Fe(Ⅳ) in this system can react with PMSO to generate Fe(Ⅱ), which further reacts with PMS to produce more active oxidizing species. Furthermore, the influence of PMSO on the degradation of different organic pollutants in the Fe(Ⅱ)/PMS and Fe(Ⅱ)/PDS systems is related to the properties of pollutants: the presence of PMSO exerts inhibiting effects on the degradation of some contaminants such as benzoic acid but promoting effects on the degradation of other contaminants such as such as bisphenol A, ibuprofen, phenol, and 2,4-dichlorophenol. In contrast, PMSO exhibits inhibitory effects on the degradation of different organic pollutants in the Fenton system. These results indicate that sulfoxides could not be used as the quenching agents for Fe(Ⅳ) in the oxidation systems where Fe(Ⅳ) is produced from the oxidation of Fe(Ⅱ).