Ferrioxalate Photochemistry

Ferrioxalate complexes are thought to hold a major portion of Fe(III) in atmospheric waters [167]. Although such complexes are widely used as chemical actinometers [206] and have been the subject of numerous experimental investigations [207-218], the exact primary step in ferrioxalate photochemistry is stiU controversial. Two different versions of the ferrioxalate reaction mechanism have been proposed following the excitation of the complex [219]. One possibility is an intramolecular electron transfer from the oxalate ligand to the center ion Fe(in) and the formation of a long lived radical complex (19) or the formation of a C204 radical (20) [213, 215]  

Another option is the sequential cleavage of the Fe(III)-0 bond between irrai and one oxalate ligand and its C-C bond which produces a biradical complex or two 

At first glance, the results appear quite scattered. The values obtained under conditions of chemical ferrioxalate actinometry represent the upper boundary of the reported values, which mostly agree with each other. Between 250 and 350 nm the quantum yields are fairly constant around 1.25. Ferrioxalate actinometry is performed under standardized conditions using millimolar concentrations of ferrioxalate (and above millimolar at A 436 nm) and an acidic pH (0.05 M H2SO4) of about 1.2 [206]. Other measurements have been carried out at lower initial Fe(III) concentrations as well as different Fe(III) to oxalate ratios and different pH values these mostly result in lower Fe(II)-quantum yields. Some investigations discriminating between individual complexes of Fe(III) and oxalate have been performed, while others did not provide an analysis of the individual complexes and are thus valid only for their respective complex-mixtures. However, all measurements with initial Fe(HI) concentrations below millimolar result in lower quantum yields. It is therefore desirable to characterize systemically any possible effects of initial Fe(III) complex concentration, speciation, and other experimental conditions on the ferrioxalate quantum yield to be able to interpret reported differences. 

At initial Fe(III) concentrations higher than 2 x 10 M, quantum yields of 

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Photooxidation of coordinated oxalate has been known since the earliest studies of transition metal photochemistry (42). In these reactions oxalate ligand is photooxidized to CO2, and up to two metal centers are reduced by one electron (e.g. ferrioxalate). We wondered whether the oxalate ligand could be a two-electron photoreductant, by simultaneous or rapid sequential electron transfer, with metals prone to 2e redox processes. Application of this concept to l6e square planar d complexes, Equation 15, was attractive because it should produce solvated I4e metal complexes that are inorganic analogues of... 

Owing to the different and distinct absorption properties of the individual auxiliary oxidants or photocatalysts, the photo-initiated AOPs presented in Fig. 5-15 must be utilized at specific spectral bands covering the VUV, UV-C, UV-B, UV-A and parts of the visible range of the electromagnetic spectrum. This is outlined in Fig. 5-16. The photo-Fenton process using Fe(III) oxalate is probably the most favorable for solar photochemistry, since the quantum yield 0 is high (cf Tab. 6-4), and ferrioxalate absorbs up to X of 500 nm. 

By 1830, the photochemistry in solution of complexes such as ferrioxalate and ferricyanide was known, and in the following decades most of simple inorganic compounds were explored. Summing up the knowledge available by 1920, Plotnikov concluded that the photoactivity depended on the change in the electron crmfigura-tion that occurred as a cmisequence of light absorption, and not only the free electrons... 

A prerequisite to simulate the impact of iron complex photochemistry in atmospheric aqueous systems is the characterization of its efficiency. Figure 10 presents an overview of quantum yield measurements in the ferrioxalate system as a function of wavelength. 

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