Photocatalytic oxidation of Cu(II)-EDTA with illuminated TiO2: Mechanisms

Photocatalytic oxidation (PCO) using illuminated TiO2 was employed as a tre atment procedure for Cu(ll)-EDTA; heavy metal-chelates pose unique challeng es for treatment and removal processes. The appearance and concentration of reaction products and intermediates were determined with variation of pH f rom 4 to 7 at 10(-4) M Cu(II)-EDTA. From the mineralization of the EDTA (et hylenediaminetetraacetic acid), CO2 and NH4+ were formed as the major carbo n and nitrogen species, respectively. Formaldehyde, formate, acetate, oxala te, and nitrate were minor products. Low concentrations of Cu(ll)-IDA (imin odiacetic acid)were found at pH 7 only. The production ratio of CO2 to form aldehyde ranged from 4:1 to 10:1. A closed carbon mass balance was obtained with CO2, dissolved organic carbon, and adsorbed organic carbon during PCO . Lack of nitrogen recovery from total Kjeldahl nitrogen (TKN) and NO3-meas urements suggests the production of gaseous nitrogen compounds. It is propo sed that PCO reactions are initiated at either a carboxyl or amine group, d epending on pH a nd wh ether the Cu(II)-EDTA is adsorbed or in solution. At lower pH, the PCO reaction proceeded favorably via radical formation at a carboxyl group complexed to the TiO2 surface. Production of Cu(ll)-IDA and oxalate only at pH 7 suggests some contribution from reaction at an amine g roup at neutral pH.