Photocatalytic Oxidation of Cu(II)−EDTA with Illuminated TiO2: Kinetics

Wastewater containing metal ions complexed with ethylenediaminetetraacetic acid (EDTA) is difficult to treat using conventional metal-removal techniques. In this work, photocatalytic oxidation kinetics of Cu(II)−EDTA using illuminated TiO 2 has been evaluated with variation of Cu(II)−EDTA concentration from 2 × 10 - 5 to 1 × 10 - 3 M at pH 6 and with variation of pH from 3 to 8 at 10 - 4 M Cu(II)−EDTA. Although the degree of adsorption varied from >50% to near zero, overall PCO rates varied only by a factor of 2, suggesting two parallel pathways: reaction on the surface and reaction with PCO-formed radicals in solution. Independent of pH, the initial degradation rate of Cu(II)−EDTA was well described by the expression, −d C /d t = k 1 C aq /(1 + k 2 C aq ) + k ads C ads, where k 1, k 2, and k ads are rate constants and C aq and C ads are aqueous and adsorbed concentrations of Cu(II)−EDTA, respectively. The success in describing the initial degradation rate of aqueous Cu(II)−EDTA by a hyperbolic rate expression suggests that [OH • ] aq is a limiting reactant at higher Cu(II)−EDTA concentrations. The initial rates of adsorbed Cu(II)−EDTA were proportional to the adsorbed Cu(II)−EDTA concentration at different suspension pH values. Little pH dependence of the total degradation rate of Cu(II)−EDTA was observed below pH 5. However, decreased destruction rates occurred above pH 5 where oxidation of Cu(II)−EDTA occurred primarily in the aqueous phase. During anaerobic photocatalysis, Cu 2+ acted as an electron scavenger, albeit inefficient, instead of oxygen. Cu 2+ that was decomplexed via PCO becomes adsorbed on the TiO 2 at higher pH.