Effects of Citric Acid and Fe(III) on Cr(III) Photo-Oxidation in an Acidic Solution.

Citric acid and iron ions occur naturally, and these substances exist ubiquitously in natural waters. Previous study had shown that light-induced oxidation of aqueous Cr(III) in the presence of Fe(III) would be one of the potential pathways for the conversions of Cr(III) to toxic Cr(VI). However, effects of citric acids on Cr(III) photo-oxidization in an acidic solution with Fe(III) ions are unclear. In the study, oxidation of Cr(III) in a system with various concentrations of Fe(III) and citric acids was conducted at pH 2, 3, and 4 under UV light irradiation. Influences of aerobic conditions on Cr(III) photo-oxidation were also examined under an O2 or N2 atmosphere. The results showed that, in the absence of citric acids, Cr(III) could be photo-oxidized to Cr(VI) with Fe(III) ions because the photolysis of Fe(OH)2+ could produce OH radicals, a strong oxidant for Cr(III). Cr(III) photo-oxidation decreased with an increase in pH, due probably to the productions of Fe hydroxide precipitates at a higher pH, i.e., pH 4. With the addition of citric acid, Cr(III) photo-oxidation was greatly inhibited because of the formations of citrate-Fe(III) complexes, leading to a decrease in free Fe(III), a precursor for the productions of OH radicals. O2 was found an important component in the enhancement of Cr(III) oxidation because the OH radical productions were favorable in an aerobatic environment.