Kinetic and mechanistic insights into the evolution of sulfur-centered radicals in transition-metal-activated bisulfite processes

Transition-metal-activated S(IV) processes hold excellent application potential for decontamination. However, the underlying redox chemistry remains largely obscure, impeding the advancement and practical implementation of these processes. Hence, there is an urgent need to explore the intricate redox chemistry to facilitate the development and application of transition-metal-activated S(IV) processes. In this study, we selected the Ce(IV)/S(IV) process as a representative system and employed kinetic modeling to explore the evolution of sulfur-centered radicals (i.e., SO₃^•−, SO₅^•−, and SO₄^•−) under acidic conditions. The oxidation of S(IV) by various oxidants (i.e., Ce(IV), SO₅^•−, and SO₄^•−) could produce SO₃^•−, which was regarded as a critical sulfur-centered radical for SO₄^•− production. Despite the efficient transformation of SO₅^•− into SO₄^•−, 95.0% of the generated SO₄^•− was rapidly consumed by S(IV) due to its high reactivity towards S(IV). The yield and utilization of SO₄^•− were further investigated using kinetic modeling and experimental methods. The yield of SO₄^•− was independent of target compounds (TCs), whereas the utilization of SO₄^•− by TCs depended on the captured capacity of TCs for SO₄^•−. Moreover, controlling a low dosage of S(IV) significantly improved both the yield and utilization of SO₄^•− in the Ce(IV)/S(IV) process. This work provides valuable insights into the redox chemistry and kinetic behavior of transition-metal-activated S(IV) processes, thereby guiding the development and application of transition-metal-activated S(IV) processes in decontamination.