Role of surfactants in accelerating or retarding persulfate decomposition

Abstract

When surfactants are coupled with persulfate-based ISCO (in situ chemical oxidation) to facilitate the remediation of soil-sorbed hydrophobic organic contaminants or non-aqueous phase liquids, surfactant selection is critical in avoiding excessive nonproductive consumption of persulfate and ensuring proper longevity of persulfate. In this study, the kinetics and mechanisms of the decomposition of persulfate in the presence of simple organic compounds containing primary basic building blocks of nonionic and anionic surfactants were first examined to elucidate the impact of individual building blocks to persulfate decomposition. Further kinetic and mechanistic studies were then carried out in the presence of representative surfactants from the four families of surfactants frequently applied in subsurface remediation. The results indicate that alkyl polyoxyethylene nonionic surfactants significantly accelerated initial persulfate decomposition due to radical-chain processes induced by the oxyethylene functional groups, whereas alkylphenol polyoxyethylene nonionic surfactants retarded initial persulfate decomposition by quenching the radical-chain processes by the aromatic rings. On the other hand, alkyl sulfonate anionic surfactants and alkylbenzene sulfonate surfactants retarded initial persulfate decomposition to some extent; Coulomb repulsion of SO4−/S2O82− and surfactant anion/radical, and entry of surfactant radical anion from the pseudo-aqueous phase into the pseudo-micellar phase may contribute to retarding initial persulfate decomposition. Overall, the results offer insightful structure-based guidance in selecting proper surfactants for surfactant-enhanced persulfate-based ISCO.