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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.

Treatability data on actual waste show that titaniabased photocatalysis can be used to remove silver ions from black and white photoprocessing waste. The silver ion is reduced to its metallic form producing particles comparable in size to the TiO 2 catalyst particles. The mass of silver recovered approaches three times that of the titania. Thiosulfate (the predominant chemical in spent fixer) plays a complicated role in the process of silver ion reduction: (1) as a hole scavenger, it can increase the silver reduction rate; (2) as the chemical that stabilizes silver ion in solution, thiosulfate hinders photocatalysis when present at high concentration. Metallic silver can be separated from titania by the physical process of sonication. We also show that sunlight can be used directly to power the photo-electrochemical silver removal process.

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