TiO₂-Assisted photocatalysis of lead-EDTA

Several industrial and remedial processes create aqueous wastes containing complexed heavy metal species. TiO₂-assisted photocatalysis was evaluated to address contamination problems from Pb-EDTA at both stoichiometric and non-stoichiometric Pb(II)/EDTA ratios. Pb-EDTA destruction occurred rapidly. At higher pH, lead became adsorbed onto the TiO₂ as complexed intermediates. Continued PCO saw reappearance of the lead in solution, primarily as Pb²⁺. Stoichiometric Pb-EDTA studies showed no noticeable pH effects even though Pb-EDTA adsorption depends strongly on pH. This suggests that initial complex adsorption is not required for its destruction and that Pb-EDTA photocatalysis occurs both at the TiO₂ surface and in the aqueous phase. Production of CO₂ and formaldehyde during photocatalysis suggests initial attack at an acetate group on the complex. Acetate and formate were found as minor intermediates. Nitrogen-containing products were ammonium and nitrate. For EDTA>Pb(II) studies, smaller amounts of CO₂ compared with stoichiometric Pb-EDTA were found, suggesting that attack on the acetate group is promoted by lead complexation. Pb(II)>EDTA studies showed decreasing CO₂ production with increasing pH, resulting from a shift in adsorbed Pb-EDTA speciation, or increased adsorption of free lead at higher pH, decreasing the available surface sites for OH· production.