The influence of oxalate on Fe-catalyzed S(IV) oxidation by oxygen in aqueous solution

This study demonstrates that oxalate has a strong inhibiting effect on Fe-catalyzed S(IV) oxidation by oxygen in aqueous solution. While the pseudo-first order rate constant of S(IV) oxidation was determined to be 1.6 x 10(3) M-1 s(-1) in experiments without oxalate, the oxidation of S(IV) was totally inhibited at a molar concentration ratio of iron:oxalate = 1:5 at an oxalate concentration of 4 mu M. Under these conditions, the Fe(II)/Fe(III) ratio remained nearly constant during the observed reaction time. The determined rate constants were independent of the initial oxidation state of iron. However, with increasing concentrations of oxalate, a longer induction period is observed for experiments with iron initially in the Fe(II) oxidation state.