Cation regulation on electro-synthesis of peroxydisulfate from sulfate-containing wastewater via outer-sphere pathway.

Peroxydisulfate (PDS) is widely used in environmental engineering and chemical industry due to its high oxidation potential and excellent universality. Electrochemical synthesis of PDS using sulfate-containing wastewater can effectively realize the conversion of pollutants into high-value-added products. Nevertheless, the underlying mechanism of typical monovalent cations in sulfate-containing wastewater on PDS formation remains under discussion, resulting in a lack of targeted regulation guidelines. This work investigated the effects of different cations only through the outer-sphere oxidation pathway of electrochemical PDS synthesis. In the dilute electrolyte with NH4SCN added, the yield and Faradaic efficiency of PDS synthesis both followed the order of K+ > NH4+ > Na+. Experimental and theoretical studies revealed that this trend could be attributed to the fact that K+ is more prone to form clusters with ·OH and delocalize its spin density, thereby increasing the conversion probability of ·OH to SO4-·. This work uncovers the atomic-scale mechanism of cation regulation of ·OH affecting outer-sphere reactions, and provides a theoretical basis for improving the economic efficiency of sulfate-containing wastewater resource utilization through ion regulation.