Photocatalytic oxidation of cadmium-EDTA with titanium dioxide

Ethylenediaminetetraacetic acid (EDTA) forms stable complexes with toxic metals such as cadmium. Metal-EDTA chelates are chemically stable and occur in a number of waste situations. The viability of using photocatalytic oxidation with titanium dioxide to degrade Cd-EDTA was examined at concentrations from 2 x 10(-5) to 10(-3) M at pH from 3 to 8. Initially a portion of the complex was adsorbed onto the TiO2 photocatalyst at low pH. However, independent of the degree of initial adsorption, Cd-EDTA was rapidly destroyed with little dependence on pH. Concurrently, in most cases cadmium was liberated as Cd2+ with no affiliation with organic reaction products; its fate depended on suspension pH. At low pH, Cd-aq(2+) was released into solution. Also, organic carbon was released into solution as oxidation of adsorbed EDTA occurred. At higher pH the Cd was adsorbed onto the TiO2 at adsorption equilibrium levels. Major products detected include formaldehyde, formic acid, and acetic acid. Nitrate and glyoxylic, malonic, and oxalic acids were detected, but concentrations were low. The incomplete carbon balance and the lack of nitrate production suggest the production of organic amines from the degradation of Cd-EDTA. Release of the Cd as Cd2+ occurs after mineralization of only about half of the organic carbon.