(166f) Advanced Oxidation Processes; the Role of the Redox Couple Fe+3/Fe+2

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 2+ activity due to precipitation (Figure 1).

References:

Andreozzi R., Caprio V., Insola A., Marotta R.: Advanced Oxidation Processes (AOP) for water purification and recovery, Catalysis Today, 53, 51-59, 1999

Babay P.A., Emilio C.A., Ferreyra R.E., Gautier E.A., Gettar R.T., Litter M.I.: Kinetics and mechanisms of EDTA photocatalytic degradation with TiO2, Water Science and Technology, Vol. 44 No 5, 179-185, 2001

Faust C.B. and Hoigné J.: Photolysis of Fe(III)-Hydroxy complexes as sources of OH radicals in clouds, fog and rain, Atmospheric environment Vol. 24A, No.1, 77-89, 1990

Gangl W., Zelenka J., Letonja P., Siebenhofer M., Marr R.: Electrochemical and photochemical Oxidation of Ethylendiaminetetraacetic acid, AIChE Annual Meeting Cincinnati (Proceedings CD ROM), 2005

Lockhart B.H. and Blakeley V.R.: Aerobic Photodegradation of Fe(III)-(Ethylendinitrilo)tetraacetate (Ferric EDTA), Environmental Science & Technology, Volume 9, 12, 1035-1938, 1975