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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. Although HCrO 4 − is more susceptible to the quenching than the CrO 4 2− 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) hν −→ Cr(II), which at alkaline medium and a large excess of O 2 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 [CrEDTA(L)] n− complexes the conditions can be adjusted under which the Cr(VI)/Cr(III) forms are able to mediate the O 2 oxidation of organic matter in the environmental photocatalytic cycle.

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