Quantitative Profiling of Elementary Reaction Steps in Sulfate Radical-Based Treatment Processes: A Two-Loop Self-Consistent Approach

Advanced oxidation processes involve radical-induced reactions to degrade organic contaminants, and understanding their reaction mechanisms is essential for optimizing and controlling treatment performance. However, the transient nature and complex kinetics of radical systems, characterized by highly branched pathways, make full quantitative mechanism elucidation challenging. Here, the decay kinetics of sulfate and carbonate radicals (SO₄^•- and CO₃^•-) were investigated through direct testing of mechanistic hypotheses against observations from time-resolved spectroscopy with a two-loop self-consistent approach. We found that four elementary reactions (SO₄^•- self-termination and reactions with persulfate PS, H₂O, and hydroxyl radical ^•OH) dominate the decay kinetics of SO₄^•- in UV/PS systems. In the presence of HCO₃^- near neutral solutions, the kinetics of CO₃^•- can be described by its self-termination reaction alone. For comparison, in UV/H₂O₂/HCO₃^- systems, two elementary reactions (CO₃^•- self-termination and its reaction with H₂O₂) dictate the chemical dynamics of CO₃^•-. This differential involvement results in 2 orders of magnitude discrepancy in the CO₃^•- lifetime (9.07 ms and 70.2 μs, respectively) between the two systems. For the first time, direct evidence was presented to discern the elementary reactions involved and quantify the contribution of each elementary reaction to overall radical consumption.