S(IV) Autoxidation in Atmospheric Liquid Water: The Role of Fe(II) and the Effect of Oxalate

The reactivity of dissolved iron compounds towards different pollutants and photooxidants in atmospheric liquid water depends upon the oxidation state and speciation of iron. Our measurements of the oxidation state of dissolved iron eluted from aerosol particles (Dae: 0.4–1.6 m) collected in the urban atmosphere of Ljubljana showed that a large fraction of the iron content is present as Fe(II). The concentration ratio [Fe(II)]/[Fe(III)] varied between 0.9 and 3.1. The kinetics of S(IV) autoxidation catalyzed by Fe(II) under the conditions representative for acidified atmospheric liquid water and the influence of oxalate on this reaction under dark conditions was investigated. The reaction rate is the same if Fe(II) or Fe(III) is used as a catalyst under the condition that Fe(II) can be oxidized in Fe(III), which is the catalytically active species. Oxalate has a strong inhibiting effect on the S(IV) autoxidation in the presence of Fe(II). The reaction is autocatalytic with an induction period, that increases with higher concentrations of oxalate. The inhibiting effect of oxalate differs according to whether iron is initially in the Fe(II) or Fe(III) state. However, in both cases the inhibition by oxalate is a result of the formation of complexes with the catalyst.