Photochemical formation of carbonate radical and its reaction with dissolved organic matters

The carbonate radical (CO₃^•−) is a strong oxidative radical that is generated via the reactions of HCO₃⁻/CO₃²⁻ with hydroxyl radical (HO^•) or triplet states of dissolved organic matter (³DOM^∗) in sunlit surface water. The bimolecular reaction rate constants of CO₃^•− with various DOM isolates (k_{CO3^•−,DOM}) were calculated as 15–239 (mg of C/L)⁻¹ s⁻¹ and were correlate to the bulk DOM properties, such as the content of phenolic moieties, the specific UV absorbance (SUVA), the E2/E3 value, and the fluorescence index (FI). The spectroscopic E2/E3 values was found to strongly correlated (R² = 0.93) with k_{CO3^•−,DOM}, and an empirical equation was established. Our results also demonstrate that CO₃^•− is involved in the photobleaching of dissolved organic matter (DOM) and in particular reacts with electron-donor moieties, leading to faster decay rates at long wavelengths of UV–vis absorption. Furthermore, a model was developed to calculate the steady-state concentrations of CO₃^•− during DOM photobleaching. These results allow us to estimate the reactivity of DOM with CO₃^•− and to evaluate the role of CO₃^•− in sunlit surface water. It will also allow a better assessment of the concentration and utilization of CO₃^•− during the application of advanced oxidation processes.