Light and Iron(III)-induced oxidation of chromium(III) in the presence of organic acids and manganese(II) in simulated atmospheric water

In atmospheric waters, oxidation of Cr(III) by Mn(III/II) species has been considered to be the major pathway for Cr(III) to become Cr(VI), but the finding of light-induced Cr(III) oxidation in the presence of Fe(III) implicates a new possible pathway. This study further tests the occurrence of this pathway in simulated atmospheric water containing organic acids and Mn(II). Our results showed that in simulated atmospheric water, the light and Fe(III)-induced Cr(III) oxidation occurred in the presence of the organic acids of low Fe(III) photoreductivity (LFPR) such as acetate and succinate and in the presence of very low levels of the organic acids of high Fe(III) photoreductivity (HFPR) such as citrate and tartarate (e.g., [Citrate]0 : [Cr(III)]0< 1 : 10). This oxidation became very limited in the presence of very high levels of the LFPR acids (e.g., [Acetate]0 : [Cr(III)]0> 7.5 : 1) because of competition of the acids with Cr(III) for •OH radicals as well as in the presence of mild levels of the HFPR acids (e.g., [Citrate]0:[Cr(III)]0> 1 : 10) because of rapid reduction of the formed Cr(VI) by organic radicals from photolysis of complexes of Fe(III) with these acids. This oxidation was substantially limited at pH 2.5 or 4.5 in the presence of the organic acids. The presence of Mn(II) led to decreases in the Cr(III) oxidation presumably resulting from the competition of Mn(II) with Cr(III) for •OH radicals. This study thus suggests that the light and Fe(III)-induced Cr(III) oxidation could be a potential important pathway for Cr(III) to be converted to toxic Cr(VI) in atmospheric waters where organic acids such as acetate are frequently predominant.