Sulfur dioxide oxidation in atmospheric water: role of iron(II) and effect of ligands

Laboratory experiments were conducted with fogwater collected in the urban area of Dübendorf, Switzerland, and with synthetic solutions designed to simulate some of the chemical features of fogwater. In fogwater that contained higher concentrations of organic pollutants (i.e., DOC), S(IV) oxidation rates were significantly lower than those expected for Fe-catalyzed S(IV) oxidation by oxygen. It is suggested that organic species that can scavenge radical intermediates (e.g., acetate, formate), form complexes with Fe(III) (e.g., oxalate), or form adducts with S(IV) (e.g., formaldehyde) are responsible for the observed inhibition. Fe(III) reduction was also observed in filtered fog samples in the dark. Pseudo-first-order rate constants for the Fe(III) reduction ranged between 1.7 x 10⁻⁴ and 3.8 x 10⁻⁵ s⁻¹ and did not seem to depend on pH for pH values between 2.9 and 4.2.