Mechanistic insights into carbonate radical-driven reactions: Selectivity and the hydrogen atom abstraction route

Carbonate radical (CO₃^•) is inevitably produced in advanced oxidation processes (AOPs) when addressing real-world aqueous environments, yet it often goes unnoticed due to its relatively lower reactivity. In this study, we emphasized the pivotal role of CO₃^• in targeting the elimination of contaminants by contrasting it with conventional reactive oxygen species (ROSs) and assessing the removal of sulfamethazine (SMT). Similar to singlet oxygen (¹O₂), CO₃^• shows a preference for electron-rich organic compounds. In addition, hydrogen atom abstraction (HAA) was determined as the primary pathway in CO₃^•-driven reactions, with a lower free energy barrier (∆G^‡) compared to the addition process, while single electron transfer (SET) was found to be thermodynamically unfavorable in all selected aromatics with varying substituents, using DFT calculations. The H atoms within amino groups (NH₂ and NH) were shown to be the most susceptible to abstraction by CO₃^•, which is more facile than hydroxyl radical (^•OH) due to the shorter NH bond cleavage length. Finally, the degradation intermediates of SMT by CO₃^• were identified, with SO₂ extraction, the cleavage of SN and CN bonds, and nitration/nitrosation of NH₂ groups being the main degradation pathways. The results from this study are expected to set the stage for the large-scale utilization of CO₃^• and advance our understanding of its reaction characteristics.