Photochemical oxidation of Cr(III) to Cr(VI) in the presence of Fe(III): Influence of Fe(III) concentration and UV wavelength.

The reduction of Cr(VI) to Cr(III) is key to lowering environmental toxicity and mobility, but the reverse process remains less understood. We investigated Cr(III) oxidation mechanisms across various pH levels and light wavelengths (185, 254, and 358 nm) in the presence of Fe(III). At pH 3.0 under 358 nm light, Cr(VI) production peaked at 11.65 μM, driven by photo-reactive Fe(OH)²⁺ producing •OH radicals. While Fe(III) generally promotes Cr(III) oxidation, concentrations above 0.5 mM inhibited the process. Oxidation was most intense under 185 nm light, generating up to 217 μM of Cr(VI), due to Fe(III) and water photolysis. At 254 and 358 nm, •OH was solely produced by Fe(III) photolysis, where •OH oxidized Fe(II), which then reduced Cr(VI), slowing Cr(III) oxidation. Short-wavelength, high-energy light significantly enhances Cr(III) oxidation. Under such UV exposure in the atmosphere, Cr(III)-containing aerosols and particles may undergo harmful transformations, potentially entering ecosystems via acidic deposition and posing health risks.