Photochemistry of the [FeIII(edta)(H2O)]− and [FeIII(edta)(OH)]2− complexes in presence of environmentally relevant species

The study shows that the *[FeII(edta)(H2O]− or *[FeII(edta)(OH)]2− LMCT excited state can undergo self-quenching generating the [(H2O)(edta)FeII(μ-OHx)FeIII(edta)]x−4 millisecond intermediate, the fate of which depends on the availability of molecular oxygen. In the presence of O2 or another external electron acceptor the excited state undergoes oxidative quenching, whereas the presence of an external electron donor leads to its reductive quenching. In the former case, the EDTA undergoes oxidation by the external electron acceptor in reaction catalysed by the excited Fe(III) complex, which changes its coordination sphere to [FeIII(ed3a)]. Thus, with reference to the Fe(III) complex the process can be classified as photosubstitution, whereas with reference to the oxidized electron donors it can be identified as oxidation photocatalysed by the Fe(III) complex.The reductive quenching results in reduction of Fe(III) in the excited complex accompanied by oxidation of EDTA and/or external electron acceptor; thus the process can be identified as photoreduction of the Fe(III) complex.Numerical analysis of the [Fe(edta)(H2O]− and [Fe(edta)(OH)]2− absorption spectra leads to conclusion that there are two different LMCT excited states, which differ in location of the unpaired electron (on N or O atom). Reactive decay of these excited states leads, however, to generation of analogous products, although with different quantum yields.The Fe(III) photocatalytic cycle can be driven by sunlight and in the aerated media plays a crucial role in abatement of the pollutants, which have either electron donor or electron acceptor character. Among others, the photocatalytic cycle contributes to abate one of the most noxious pollutants, i.e. chromate(VI).