Quantum chemical investigations of the decomposition of the peroxydisulfate ion to sulfate radicals

Abstract

Decomposition of the peroxydisulfate ion (S2O82−, PS) to sulfate radicals (SO4−) is a key step in sulfate radical-based advanced oxidation processes (SR-AOPs). In this study, quantum chemical investigations and calculation of activation energy were performed to reveal the decomposition process of S2O82− at the atomic level. Furthermore, the impacts of several main co-existing components (CCs), including inorganic cations, inorganic anions and organic molecules in wastewater, on the activation energy were evaluated. The results revealed that 3 steps were involved in the S2O82− decomposition: 1) the distance between two subgroups of the ground state of S2O82− (PSS0) increases from ∼0.15 to ∼0.215 nm driven by exogenous energy; 2) a transition occurs for S2O82− from PSS0 to the lowest triplet state (PST1); and 3) two subgroups of PST1 separate from each other spontaneously. The energy needed in the first step was demonstrated to be the activation energy. Moreover, two possible routes by which CCs affect the transition from PSS0 to PST1 were proposed. After ascertaining which route would be taken by different types of CCs, various methods of calculation were used to determine their impacts on S2O82− decomposition, i.e. promotion effect for H+, NH4+, Ca2+, Cl− and Br−, and inhibition effect for Na+, K+, Mg2+, NO3−, CO32−, SO42−, and all studied organic molecules, including acetic acid, benzene, bromoethane, 1-propanol and 4-chlorotoluene. The present study provided novel insights into the decomposition of S2O82− to SO4−, and emphasized the effects of main co-existing inorganic ions and organic molecules in wastewater on SR-AOPs.