OXIDATION OF METAL-EDTA COMPLEXES BY TIO2 PHOTOCATALYSIS

Ethylenediaminetetraacetic acid (EDTA), a common industrial agent for complexing metal ions in water, frequently inhibits conventional metal s-removal technologies used in water treatment. This study investigate d the use of TiO2 photocatalysis for the aqueous-phase oxidation of ED TA and several metal complexes of EDTA. Reactions were performed at 0. 1 wt % loading of Degussa P-25 TiO2, a solute concentration of 0.8 mM and at a constant pH. The different metal-EDTA complexes exhibited wid ely different photocatalytic oxidation rates under equivalent conditio ns of pH = 4 +/- 0.1 in an aerobic system: Cu(II)-EDTA > Pb(II)-EDTA m uch greater than EDTA > Ni(II)-EDTA approximate to Cd(II)-EDTA approxi mate to Zn(II)-EDTA >>> Cr(III)-EDTA. Photoefficiency based on the Cu( II)-EDTA initial rate is nearly 60%. The rates of total organic carbon (TOC) removal and formaldehyde generation during photocatalytic EDTA oxidation indicate similarities to electrochemical oxidations of EDTA. Several means were explored to enhance the oxidation of NI(II)-EDTA, whose behavior was taken to represent that of the slowly oxidizing com plexes. Continuous addition of H2O2 solution during the photocatalytic treatment increased the oxidation rate for NI(II)-EDTA as did the pre sence of homogeneous Cu2+. The presence of Cu2+ led to rapid ligand ex change transforming the Ni(II)-EDTA into Cu(II)-EDTA, which is easily oxidized.