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 >> EDTA > Ni(II)−EDTA ≈ Cd(II)−EDTA ≈
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.
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