The role of transition metal complexes is overviewed for the iron, copper and chromium compounds, for which photoreactivity is of crucial environmental importance. Attention is paid to these complex systems, in which metal centres are photoreduced by organic matter under solar irradiation and re-oxidized by molecular oxygen, i.e. to the systems, that play a part in the environmental photocatalysis. The photoreduction is accompanied by simultaneous oxidation of organic matter, which plays a role of ligand and/or sacrificial electron donor. Under favourable conditions a complete photodegradation of the organic pollutants can be achieved. The general mechanisms are analysed and examples of different photocatalytic behaviours are presented in some details. The systems are more sophisticated due to concurrent generation of active oxygen species, such as the O 2 − , HO 2 , OH , H 2 O 2 , and HO 2 − , which are involved in the redox reactions of the photocatalytic cycling and influence the pollutant degradation. Beside the monometallic cycles, the mixed metal systems can function in nature. The cooperation between the photocatalytic cycles of two metals can abate diverse environmental pollutants, but their efficiency can be either higher or lower in comparison with that of the sum of single cycles.

Homogeneous photocatalysis by transition metal complexes in the environment

Irradiation of the [Cr(C2O4)3]3− complex or the chromate(VI)–oxalate mixture, or the ternary system composed of Cr(III), Cr(VI) and oxalate, leads to chromium photoreductions in consequence of the ligand to metal charge transfer (LMCT) excitations induced by artificial solar radiation. In the case of the Cr(III) complex, the photoreduction involves the innersphere electron transfer leading to the formation of the Cr(II) species and the C2O4 − radicals. On reacting with molecular oxygen, Cr(II) is oxidised to Cr(III) catalysing thereby the oxalate substitution reaction. Moreover, under specific conditions, Cr(II) can be also oxidised to Cr(VI). Chromate(VI) is not photoreducible, but in the presence of oxalate, or other sacrificial electron donor, the outersphere photoinduced electron transfer (PET) produces Cr(V) species and the C2O4 − radicals. This initiates a series of thermal reactions leading to the formation of Cr(III) and oxidized oxalate (CO2). In the system composed of [Cr(C2O4)3]3− and chromate(VI), the acidic medium and anoxic conditions favour the Cr(VI) photoreduction, whereas alkaline oxygenated solutions assist the Cr(VI) photoproduction. When an approximately neutral solution equilibrated with the ambient air is irradiated intermittently, Cr(VI) is consumed and/or produced, accordingly to the time sequence of exposure and dark periods. The oscillations of Cr(VI) concentrations are accompanied by continuous oxidation of oxalate, playing the role of the sacrificial electron donor. The effects of solution pH, molecular oxygen, concentrations of reagents and cations on the reaction rates were investigated. The results of this paper revealed that the Cr(III)/Cr(VI) system under environmental conditions behaves as the photocatalytic one catalysing the oxidation of oxalate or other organic matter by molecular oxygen, contributing thereby to the abatement of pollution.

Photoredox processes in the Cr(VI)–Cr(III)–oxalate system and their environmental relevance

Photosubstitution and photoredox behaviour of cyanometallates: reaction models

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.

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

The role of photocatalysis by transition metal complexes in the environment is reviewed, and its influence on composition of the environmental compartments, transport between them, and activation of the environmental self-cleaning behavior is characterized. In description of atmospheric processes, the attention is paid to coordination compounds as photocatalysts of the transfer and redox reactions of nitrogen oxides. In the case of hydrosphere and soils, various mechanisms of organic pollutant photodegradations are presented in which the iron, copper, and chromium complexes play crucial roles. Mono- or mixed-metal photocatalytic cycles are stimulated, in which the complex photoreduction is accompanied by oxidation of ligands and/or external molecules (sacrificial electron donors), whereas the sacrificial acceptor, closing of the photocatalytic cycle, is predominantly molecular oxygen. The self-cleaning mechanisms by photocatalysis and photosensitization apply also to organic matter included in microorganisms, thus they can lead as well to the environmental self-disinfection. The review shows fast progress in our understanding of the natural processes driven by the solar energy and catalyzed by transition metal complexes; treating the cycles as anti-photosynthesis is also discussed.

Adam Marchaj

Papers

Photochemical production of chromate(VI) ions from some chromium(Ill) complexes1

Photosubstitution in some cyanide complexes of chromium(III)(1)