Mechanistic insight into reactivity of sulfate radical with aromatic contaminants through single-electron transfer pathway

Removal of aromatic contaminants (ACs) in waters by sulfate radical anion (SO4−) based advanced oxidation technology has been extensively studied. Three main mechanisms have frequently been used to account for the first step of radical oxidation of ACs: radical adduct formation, hydrogen atom abstraction, and single electron transfer (SET), among which the SET pathway is the least understood. In this study, we investigated the first step of SET reactions for 76 ACs with SO4−. The result shows that the Gibbs free energy (ΔGSET∘) of the reaction increases with a decrease of the electron donating character of the substituents on the ACs. The trend was then quantitatively corroborated by a Hammett type plot, indicating that the electrostatic interaction is the driving force for the SET pathway. Further, we compared the calculated second-order rate constants (kSET) for the ACs via the SET pathway with their experimental k values, and proposed two fundamental SET reaction mechanisms based on the identified intermediates. The thermodynamic and kinetic results obtained advance the mechanistic understanding of the SET pathway of radical and non-radical bimolecular reactions, and shed light on the applicability of SO4− in ACs removal during water treatment processes.