Photoredox reactions of environmental chromium

Abstract

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.

Homogeneous photocatalysis by transition metal complexes in the environment

“…(23)- (25)). The mechanism of the photoinduced electron transfer substantiated for the Cr(VI)-alcohol system [135] has been recently documented also in other systems, containing Cr(VI) and electron donor, such as oxalate or phenol and its derivatives [136,137]. It is probably also responsible for the Cr(VI) photoreduction initiated by sunlight in natural waters that was earlier interpreted solely in terms of photochemical generation of such reducers as copper(I), superoxide (O 2 − ), or iron(II) [129,147,[162][163][164].…”

“…The oxalate complex [Cr(C 2 O 4 ) 3 ] 3− was found to undergo photoisomerization [206], photodissociation or photoreduction. The last behaviour was reported as induced by ultraviolet irradiation (254 nm) in vacuum [207], and by radiation mimicking the sunlight under environmental conditions [137].…”

“…First of them consists in thermal reduction of dissolved Cr(VI) to Cr(III) mostly by reducing agents such as S 2− , Fe(II) or some organic compounds [133]. The second pathway includes photochemical reduction of Cr(VI) in the organic matter systems [134][135][136][137].…”

“…Subsequent research [137,[192][193][194] has shown that in the Cr(III) complexes with reducible ligands (L) the LMCT transitions can induce the inner-sphere photoinduced electron transfer (PET), e.g., (27) or the back electron transfer is accompanied with the L •+ substitution.…”

“…Photocatalytic chromium cycle has been newly recognized [134][135][136][137]195]. The cycle fundamentally differs from the previously known iron or copper cycles, which are based on photochemical reduction of the metal ion and its thermal oxidation by molecular oxygen.…”