The carbon dioxide radical anion as a reductant in water treatment and atmospheric chemistry: a review

The carbon dioxide radical anion, CO₂^●−, is a highly reactive radical species involved in the reduction of the CO₂ greenhouse gas, organic synthesis, atmospheric aerosol chemistry, and treatment of halogenated compounds. In recent years, CO₂^●− has emerged as a strong reductant, or single electron donor. Here we present techniques used to generate CO₂^●− and we discuss applications to degrading pollutants such as halogenated alkanes. The potential occurrence of such reductions in water and aqueous aerosols is discussed, notably for the degradation of perfluoroalkyl substances. In the laboratory, CO₂^●− is directly generated by either direct electrochemical reduction of CO₂ or hydrogen atom transfer of either formate salts with and without catalysts or ferrioxalate through photochemical or radiolytic processes. The CO₂^●− has an ultraviolet spectrum, and CO₂^●− vibration modes are characterized by fast kinetics using infrared and Raman spectroscopy. The second-order rate constants of the reactions of CO₂^●− with halogenated alkanes, of -1.84 ± 0.22 V, are generally slower than that of the hydrated electron, of -2.87 V, and give a negative linear relationship with energy of lower unoccupied molecular orbital, suggesting single electron transfer mechanism in reducing the halogenated compounds.