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

Photocatalytic oxidation (PCO) using illuminated TiO2 was employed as a treatment procedure for Cu(ll)-EDTA; heavy metal-chelates pose unique challenges for treatment and removal processes. The appearance and concentration of reaction products and intermediates were determined with variation of pH from 4 to 7 at 10(-4) M Cu(II)-EDTA. From the mineralization of the EDTA (ethylenediaminetetraacetic acid), CO2 and NH4+ were formed as the major carbon and nitrogen species, respectively. Formaldehyde, formate, acetate, oxalate, and nitrate were minor products. Low concentrations of Cu(ll)-IDA (iminodiacetic acid)were found at pH 7 only. The production ratio of CO2 to formaldehyde 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-measurements suggests the production of gaseous nitrogen compounds. It is proposed that PCO reactions are initiated at either a carboxyl or amine group, depending 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 group at neutral pH.