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Advanced Oxidation Processes; the Role of the Redox Couple Fe+3/Fe+2

Wolfgang Gangl, Julia Zelenka, Peter Letonja, Matthäus Siebenhofer, and Rolf Marr. Department of Chemical Engineering and Environmental Technology, Graz University of Technology, Inffeldgasse 25/C/II, Graz, A-8010, Austria

Advanced oxidation processes (AOP) increasingly contribute to wastewater treatment. AOPs focus on persistent, poorly biodegradable substances and substances with mutagenic or carcinogenic properties. The hydroxyl radical (OH-radical) is a preferred oxidizer for degradation and mineralization of persistent substances. OH-radicals can be generated by irradiation of photocatalysts (e.g. TiO2), photolysis of H2O2 or O3and by the Fentons reagent (Fe2+/H2O2) [Andreozzi et al. 1999]. Faust and Hoigné (1990) reported the generation of OH radicals from aerated Fe(III)-hydroxycomplex Fe(OH)2+. In latter process Fe3+. is reduced to Fe2+ which itself is inactive in OH-radical production. The photochemical degradation mechanism of Fe3+. chelated EDTA was investigated by Lockhart and Blakeley (1975). Kinetics of TOC depletion and EDTA degradation was reported (Gangl et al., 2005). Because of its powerful role the influence of the redox couple Fe3+ and Fe2+ on EDTA degradation and mineralisation (TOC depletion) under UV irradiation (Photolysis) was then subject of detailed analysis. Investigation was accomplished in an aerated UV reactor (800 ml) equipped with a 150 W Hg medium pressure lamp and a 15 W Hg low pressure lamp. All experiments were carried out with a 1.34 mM EDTA solution. Temperature, Fe2+-concentration and the pH-value was varied. The rate of mineralization passes a maximum within a pH-range of 3 < pH < 5. Limited mineralization is observed at pH 6, indicating loss of Fe(OH)2+ activity due to precipitation (Figure 1).


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