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.