Mechanisms of Sb(III) Photooxidation by the Excitation of Organic Fe(III) Complexes

Organic Fe(III) complexes are widely distributed in the aqueous environment, which can efficiently generate free radicals under light illumination, playing a significant role in heavy metal speciation. However, the potential importance of the photooxidation of Sb(III) by organic Fe(III) complexes remains unclear. Therefore, the photooxidation mechanisms of Sb(III) were comprehensively investigated in Fe(III)-oxalate, Fe(III)-citrate and Fe(III)fulvic acid (FA) solutions by kinetic measurements and modeling. Rapid photooxidation of Sb(III) was observed in an Fe(III)-oxalate solution over the pH range of 3 to 7. The addition of tert-butyl alcohol (TBA) as an center dot OH scavenger quenched the Sb(III) oxidation, suggesting that center dot OH is an important oxidant for Sb(III). However, the incomplete quenching of Sb(III) oxidation indicated the existence of other oxidants, presumably an Fe(IV) species in irradiated Fe(III)-oxalate solution. In acidic solutions, center dot OH may be formed by the reaction of Fe-II(C2O4) with H2O2, but a hypothetical Fe(III) species may be generated by the reaction of Fe-II(C2O4)(2)(2-) with H2O2 at higher pH. Kinetic modeling provides a quantitative explanation of the results. Evidence for the existence of center dot OH and hypothetical Fe(IV) was also observed in an irradiated Fe(III)-citrate and Fe(III)-FA system. This study demonstrated an important pathway of Sb(III) oxidation in surface waters.