Kinetics and mechanisms of EDTA photocatalytic degradation with TiO2

A complete study on the photocatalytic degradation of ethylenediaminetetraacetic acid (EDTA) over TiO2 has been initiated, to establish the influence of several parameters on the reaction rate, the nature of the intermediates and the kinetic regime. TiO2 (Degussa P25) suspensions containing EDTA at pH 3 at different concentrations were irradiated under near UV light. A Langmuirian behavior was observed, from which kinetic constants have been obtained. Experiments with 5.0 mM EDTA (zero order kinetic regime) were performed for 3 hours irradiation under different conditions. Under N2 bubbling, depletion of EDTA was very low. Under O2 bubbling, the concentration of EDTA decreased around 90%. However, the corresponding decrease of TOC ranged only between 4.5% and 9%. A higher TOC reduction (22% or more) was obtained by keeping the pH constant by HClO4 addition, or by hydrogen peroxide addition. Addition of 0.5 mM Fe(III) caused a dramatic increase on the initial rate of EDTA depletion and approximately a 32% TOC decrease. Analysis of the filtered solution was performed by ion chromatography and capillary electrophoresis to monitor the disappearance of EDTA and the formation of degradation products after different irradiation times. So far, glycine, ethylenediamine, formic acid, ammonium, iminodiacetic acid, oxalic acid and glyoxylic acid have been identified.