Photoredox reactions of environmental chromium
The role of direct photochemical reactions of Cr(VI) and Cr(III) species in the environmental degradation of organic matter was investigated by means of Cr(VI)/alcohol and Cr(III)/EDTA model systems under conditions mimicking the environmental ones. Photoreduction of Cr(VI) proceeds through reductive quenching of chromate(VI) excited within UV-A by an alcohol molecule. Although is more susceptible to the quenching than the ion, and oxygen favours the reverse reaction, the reduction can be easy noticeable at air-saturated natural waters. Photoredox behaviour of is more complex: it is initiated also by photoreduction, Cr(III) Cr(II), which at alkaline medium and a large excess of over Cr(II) is followed by the Cr(II) Cr(VI) oxidation. Both the photoreductions are accompanied by oxidation of organic matter. Due to the substitutional lability of the complexes the conditions can be adjusted under which the forms are able to mediate the oxidation of organic matter in the environmental photocatalytic cycle.
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