Oxidation of metal-EDTA complexes by TiO2 photocatalysis

Ethylenediaminetetraacetic acid (EDTA), a common industrial agent for complexing metal ions in water, frequently inhibits conventional metals-removal technologies used in water treatment. This study investigated the use of TiO2 photocatalysis for the aqueous-phase oxidation of EDTA 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 widely different photocatalytic oxidation rates under equivalent conditions of pH = 4 +/- 0.1 in an aerobic system: Cu(II)-EDTA > Pb(II)-EDTA much greater than EDTA > Ni(II)-EDTA approximate to Cd(II)-EDTA approximate 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 complexes. Continuous addition of H2O2 solution during the photocatalytic treatment increased the oxidation rate for NI(II)-EDTA as did the presence of homogeneous Cu2+. The presence of Cu2+ led to rapid ligand exchange transforming the Ni(II)-EDTA into Cu(II)-EDTA, which is easily oxidized.