Photo-Fenton cycle

Finally, hydrogen peroxide is decomposed by Fe " ions, which oxidize to Fe closing the photo-Fenton cycle [95]. Thus, when the degradation of organic compounds ends solubilized iron, ions turn back to the catalyst surface. 

Fig. 5.14 Schematic representation of the catalytic cycle in the photo-Fenton reaction using ferrioxalate (following Hislop and Bolton, 1999).

At low pH (ca. 3-5), H2O2 can be consumed through iron cycles associated with the oxidation of fulvic acids involving the photo-Fenton reaction (reaction 8) [84]. As previously discussed, reactions (8) and (9) may be important in humic-rich natural waters at low pH such as in black-water rivers of the southeastern United States (e.g., Suwanee River, GA), but their importance in marine waters at higher pH (typically ca. pH 8.2) is unlikely except for some organic rich coastal environments (vide supra). 

Such a reaction regenerates ferrous ion showing that only low concentration of Fe " is needed in the system [86]. Moreover, the photo-Fenton process may proceed using photons of wavelength close to 500 nm in the case of mixtures of ferric ion and hydrogen peroxide. It can also be performed by solar irradiation making it a low cost process [79, 80, 87, 88]. The photocatalytic cycle may be presented as shown in Scheme 6.6. 

Scheme 6.6 Photocatalytic cycle of iron in the photo-Fenton process...

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