Mechanisms of Interaction between Persulfate and Soil Constituents: Activation, Free Radical Formation, Conversion, and Identification.

Persulfate-based in situ chemical oxidation (ISCO) for soil remediation has received great attention in recent years. However, the mechanisms of interaction between persulfate (PS) and soil constituents are not fully understood. In this study, PS decomposition, activation, free radical formation and conversion processes in 10 different soils were examined. The results showed that soil organic matter (SOM) was the dominant factor affecting PS decomposition in soil, but Fe/Mn-oxides were mainly responsible for PS decomposition when SOM was removed. Electron paramagnetic resonance (EPR) spectroscopy analysis showed that sulfate radicals (SO4•-) and hydroxyl radicals (•OH) generated from PS decomposition subsequently react with SOM to produce alkyl-like radicals (R•), and this process is dependent on SOM content. R• and SO4•-/•OH radicals predominated in soil with high and low SOM, respectively, and all three radicals coexist in soil with medium SOM. Chemical probe analysis further identified the types of radicals, and R• can reductively degrade hexachloroethane in high SOM soil, while SO4•- and •OH oxidatively degrade phenol in low SOM soil. These findings provide valuable information for PS-ISCO, and new insight into the role of SOM in the remediation of contaminated soil.