Competitive photocatalytic oxidation of Cu(II)-EDTA and Cd(II)-EDTA with illuminated TiO2

Competitive photocatalytic oxidation (PCO) of mixtures of Cu(Il)-EDTA and C d(ll)-EDTA was studied with variation of molar ratio of these two complexes (I x 10(-4):0, 8 x 10(-5): 2 x 10(-5), 5 x 10(-5):5 x 10(-5), 2 x 10(-5):8 x 10(-5), 0:1 x 10(-4) M) and in the pH range of 4-8. PCO rates for each c ompound can be described using a combined aqueous + adsorbed pathway: -dC/d t = k(1)C(aq)/(l+ k(2)C(aq)) + k(ads)C(ads). This expression is valid under both noncompetitive and competitive conditions. Differences in rates under competition result from differences in the partitioning of the two species between the TiO2 surface and the aqueous phase. Total initial complex degr adation rates (r(Pi)), obtained by summation of the total destruction rates for CU(II)-EDTA and Cd(Il)-EDTA, were relatively constant at pH 4 and 5 fo r all ratios. At these pH values, contribution of adsorbed pathways to r(Pi ) was important, and rates were similar to those of the aqueous phase pathw ays. From pH 6 to 8, the degree of adsorption, and thus the adsorbed pathwa y rate, diminished. Through the adsorbed pathway, no difference in rate con stants was found between Cu(ll)-EDTA and Cd(II)-EDTA; Cd(Il)-EDTA is somewh at more reactive through the aqueous phase pathway.