Oxidation of Metal−EDTA Complexes by TiO₂ Photocatalysis

Abstract

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.

Effect of scavengers on the photocatalytic digestion of organic matter in water samples assisted by TiO2 in suspension for the voltammetric determination of heavy metals

“…8 In contrast, there is a retarding effect of Cd(II) ions on the EDTA degradation, or better, a slower decomposition of the complex CdH 2 Y. This fact was already reported in literature under similar experimental conditions 19 and may be associated with a lower propensity of the adduct to approach the TiO 2 surface vicinity where the photochemical reaction takes place, or simply with a lesser degree of interaction of the ligand with the semiconductor which may be required for an effective photocatalytic effect. In support to this deduction, the effect of Cd(II) in the conventional UV photolysis of EDTA, i.e.…”

Photoelectrocatalytic Oxidation of Cu-cyanides and Cu-EDTA at TiO2 nanotube electrode

“…Thus, cyanide ions can be efficiently oxidized via the photoelectrocatalytic process. Similar effects of pH values on the TiO 2 photoelectrocatalytic oxidation of EDTA and several metal complexes of EDTA were observed [33,34]. The effect of EDTA on the photocatalytic oxidation of cyanide has been examined at different molar ratios of EDTA to cyanide [30].…”

Simultaneous destruction of Nickel (II)-EDTA with TiO2/Ti film anode and electrodeposition of nickel ions on the cathode

“…Therefore, the deposition of nickel ions on the cathode surface will be inhibited in the PEC process at high pH condition. In addition, high destruction rate of Ni-EDTA at acid condition can also partially be attributed to the increased potentials of valence band hole at low pH values in the TiO 2 photocatalytic process [30].…”