Kinetics and mechanisms of EDTA photocatalytic degradation withTiO2under different experimental conditions

Abstract

The photocatalytic degradationof EDTA overTiO2has been analyzed to establish the influence of oxidants on the reaction rate, the nature of the intermediates and the kinetic regime. Degussa P-25 suspensions containing EDTA at initial pH 3 in different concentrations were irradiated under near UV light. A Langmuirian behavior was observed.O2at saturation concentrations was found to be crucial for EDTA degradation. The rapid depletion of EDTA was not accompanied by a corresponding TOC decrease, indicating formation of refractory intermediates. An enhancement in TOC reduction could be achieved by keeping pH constant or by hydrogen peroxide addition. Addition of Fe(III) caused a remarkable increase on the initial rate of EDTA consumption and also on TOC decrease. Changes in both parameters clearly increased under the simultaneous addition of Fe(III) andH2O2, until limiting values.Some of the possible intermediates of EDTA degradation were evaluated in the filtered solution. So far, glycine, ethylenediamine, ammonium, formaldehyde, and formic, iminodiacetic, oxalic, oxamic, glycolic and glyoxylic acids have been identified in different proportions, depending on the experimental conditions. Different degradationpathways are proposed. Inthe presence of Fe(III), photo-Fenton reactions would contribute also to the degradation process.