The effect of atmospheric organic compounds on the Fe-catalyzed S(IV) autoxidation in aqueous solution

Laboratory experiments were conducted with real atmospheric aerosol particles as well as with synthetic solutions under dark conditions, to simulate some of the chemical features of aerosols. In solutions obtained by the leaching of aerosols (size range D-ae: 0.4-1.6 mu m) that contained sufficient amounts of transition metal ions (e.g. Fe) and organic species (e.g. oxalate), S(IV) oxidation rates were significantly lower than those expected from the Fe-catalyzed S(IV) autoxidation in Milli-Q water. The results suggest that oxalate is responsible for much of the observed inhibition. Acetate and formate also inhibit the reaction, but to a much lesser extent. Oxalate has a strong inhibiting effect on the Fe-catalyzed S(IV) autoxidation at all investigated pH values (2.8, 3.7 and 4.5). It was established that Fe(III)-oxalato complexes affect the redox cycling of Fe(II)/Fe(III) and that the observed decrease of the reaction rate is caused by the reduced amount of catalytically active Fe(III) due to the complexation with oxalate. For the system Fe-S(IV)-O-2-oxalate at initial pH 3.7 the reaction rate was calculated using exponential simplification to account for oxalate influence on the amount of free Fe(III) by the following equation: -r(S(IV)) = k . [S(IV)] . [Fe(III)] . e(-b.[Ox]).