The overlooked carbonate radical in micropollutant degradation: An insight into hydration interaction

Carbonate radical (CO3∙-) is an abundant reactive oxygen species in sunlit surface waters and radical-based treatment systems. The roles of this kosmotropic radical in these aquatic systems have been often overlooked presumably due to its low redox potential and ambiguous solvation mechanism. In this study, CO3∙--mediated oxidation of a frequently detected micropollutant, ibuprofen, was investigated. The results showed that the H-atom abstraction (HAA) pathway dominated the degradation processes regardless of the protonation state. However, without explicitly considering the solvent molecules, the calculated reaction rate constant (k, 1.09 × 107 M−1 s−1) was overestimated by an order of magnitude. With the explicit inclusion of the hydration shell, the calculated k value (5.26 × 105 M−1 s−1) agreed well with the measured one (7.89 × 105 M−1 s−1). Considerable charge transfer was found from the CO3 center to the solvation shells. In addition, the contour plot of CO3∙- contribution to overall elimination of various micropollutants was also constructed. The surface with the contribution greater than 10% includes many micropollutants with electron-rich groups such as thioethers, anilines and phenolates, demonstrating that the role of CO3∙- cannot be overlooked in most sunlit surface waters. These results yield mechanistic insights to the CO3∙--mediated degradation of micropollutants, providing practical guidance for their removal in both natural water systems and engineered water treatment processes.