Photoredox reactions of environmental chromium

Abstract

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. AlthoughHCrO4−is more susceptible to the quenching than theCrO42−ion, and oxygen favours the reverse reaction, the reduction can be easy noticeable at air-saturated natural waters. Photoredox behaviour of[crEDTA(L)]n-is more complex: it is initiated also by photoreduction, Cr(III)hv→Cr(II), which at alkaline medium and a large excess ofO2over 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[crEDTA(L)]n-complexes the conditions can be adjusted under which theCr(VI)/Cr(III)forms are able to mediate theO2oxidation of organic matter in the environmental photocatalytic cycle.