Iron Fe ions in photocatalytic processes

In this regard, Fe ions have been commonly used in advanced oxidation processes in what is known as the Fenton s reaction. This involves the reaction between hydrogen peroxide H2O2 and ferric (Fe ) and ferrous (Fe ) ions to produce hydroxyl radicals HO and perhydroxyl radical HO. Fe ions form a complex with H2O2 (see reaction (17) below). This complex further decomposes to produce Fe ions and perhydroxyl radicals HO2 (reaction (18)). Fe ions are then reoxidized to Fe by reacting with 

The above reactions (1-3) can occur in the dark or under near UV or visible light (A = 436 nm) (Zepp et ah, 1992). When the solution is irradiated, the rate of hydroxyl radical HO formation is accelerated by the decomposition of H2O2 with radiation of less than 360 nm (Pignatello et al., 1999). The decomposition of Fe —OOrf is also accelerated with wavelengths lower than 313 nm (Cermenati et al., 1997 Domenech et al., 2004). 

The perhydroxyl radicals HO can further react with either Fe to produce more H2O2 (reaction (4)) or Fe to produce molecular oxygen O2 and a proton H (reaction (5)). 

Additionally, reaction (6) can also take place in this system if H2O2 is in excess. Moreover, hydroxyl radicals can also be trapped by excess of ferrous ions (reaction (7)). Thus, despite the advantages such as commercial availability of the oxidant, no mass transfer problems, and formation of hydroxyl radicals from H2O2, this process presents several serious drawbacks. One of the most important one is that H2O2 has to be continuously added in controlled amoimts as a source of hydroxyl radicals (Domenech et al., 2004). 

on the other hand, has been used directly in PC processes as a dopant in semiconductors, in particular for Ti02. The results seem to be somewhat contradictory nonetheless. These doped catalysts have been tested in the PC reactions of short-chain carboxylic acids such as maleic, formic, and oxalic acids, among others. 

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