Effects of oxalate on Fe-catalyzed photooxidation of dissolved sulfur dioxide in atmospheric water

Fe-catalyzed photochemical oxidation of dissolved sulfur dioxide (SO2), S(IV), in the presence of oxalate has been studied under the conditions typical for acidified atmospheric water. The Fe(III)-catalyzed oxidation of S(IV) is first order with respect to S(IV) concentration in the absence of oxalate. The presence of oxalate strongly inhibits Fe(III)-catalyzed S(IV) oxidation due to the formation of Fe(III)–oxalato complexes both in the dark and under UV–visible light. However, under UV–visible light irradiation, the generation of photooxidants, such as H2O2 and OH radicals, by photochemical/chemical cycling of Fe(III)/Fe(II)–oxalato complexes, leads to the photochemical oxidation of S(IV). The initial oxidation rate increases with oxalate and Fe(III) concentration in the concentration ranges studied. The rate of Fe(III)-catalyzed photochemical oxidation of S(IV) in the presence of oxalate increases with increasing pH value of reaction solution and with decreasing irradiation wavelength. The concentrations of oxalate and other organic ligands are sufficiently large to significantly change the rates of Fe-catalyzed oxidation of dissolved SO2 in atmospheric water droplets during both day- and night time.