Photoredox behaviour of the Cr–EDTA complex and its environmental aspects

Polychromatic radiation from a mercury lamp or light mimicking the sunlight (λ≥300 nm) is able to initiate an innersphere electron transfer in the [CrIIIEDTA(OH)]2− complex, leading to formation of a Cr(II) centre and the EDTA radical. Under defined conditions (pH ≥ 7, O2 in excess over the Cr(II) concentration) the photoreduction is followed by thermal multi-step electron transfer resulting in the Cr(II)→Cr(VI) oxidation by molecular oxygen, although EDTA is known to enhance the Cr(VI) reduction. Electron donors, present in the system, enlarge the efficiency of Cr(VI) production. These are also able to take part in the photoinduced electron transfer to CrO42−/HCrO4− in excited states, when pH reaches corresponding level. As Cr(VI) production is accompanied by decrease in pH, and its reduction with an opposite effect, the processes proceed according to an electron shuttle mechanism. Their progress under natural conditions seems attainable, because all conditions needed to the photocatalytic oxidation of organic pollutants by O2, with oscillatory recovery of the Cr(III) and Cr(VI) species can be provided.