Theoretical and experimental evidences for the formation of carbonate anion radical (CO3−•) in the atmosphere

Troposphere is the lowest layer where the oxidation reactions responsible for the atmospheric self-purification of contaminants occur, which are initiated mostly by hydroxyl radicals (HO•). In this context, the objective of the present work was to carry out theoretical and experimental studies on the formation of the carbonate anion radical (CO3−•) in the atmosphere and evaluate the spontaneity of chemical reactions that occur at molecular. Experimental tests to evaluate the formation of CO3−• in the atmosphere were conducted using rainwater samples containing 5, 10, and 15 mg L−1 of sodium carbonate, and the addition of methylene blue dye was used to indicate the formation of the CO3−• by monitoring its degradation. Exposure of the samples to solar radiation led to a greater degradation of the dye in rainwater samples with higher concentrations of bicarbonate. The determination of the alkalinity of the samples revealed that the predominant chemical species in the medium was the bicarbonate anion and that the increase in its concentration promotes the formation of CO3−•. As a result of the simulation of the reactions with the radicals •OH, •OOH, and CO3−• at the level of theory MP2/6–311++G (2df,p), it was obtained that the formation of the radical CO3−• is possible (HO• + CO32− → HO− + CO3−• and HCO3− + HO• → H2O + CO3−•). The simulations were carried out in aqueous phase with implicit solvent, in the gas and showed the same trends. Furthermore, the reactions involving CO3−• with atmospheric molecules are spontaneous (ΔrG298 < 0) for electron-rich groups as H2S compounds, indicating that the radical CO3−• can react with some atmospheric molecules, and thus contribute to the atmospheric oxidation mechanisms.