ROLE OF REDUCTION IN THE PHOTOCATALYTIC DEGRADATION OF TNT

The reduction of TNT by electrochemical and photoelectrochemical techn iques has been investigated to obtain insight into the role of reducti ve transformations in a photocatalytic degradation process. TNT was ob served to be labile to reductive transformation by a platinum electrod e at electrochemical potentials commensurate with the flat band potent ial of TiO2 and aminodinitrotoluene species were detected as early red uction products. Oxygen did not influence the rate of reductive TNT tr ansformation, but byproducts were more stable in aerated than deaerate d conditions. Photocatalytic reduction of TNT was analyzed using CdS a s a chromophore coupled to TiO2 and visible light excitation so that d irect photolysis of TNT was precluded. Under deaerated conditions, red uction of TNT occurred through both direct and sensitized mechanisms, predominantly forming aminodinitrotoluene compounds as relatively stab le byproducts. Photocatalytic transformation and mineralization of TNT was also examined with particulate films of TiO2 immobilized on optic ally transparent electrodes. Varying levels of positive bias were appl ied to the photocatalyst in order to decrease the availability of phot oexcited electrons. The rate of TNT degradation was found to decrease with increasing positive bias, indicating that conduction band electro ns facilitate overall compound degradation. Results from studies with TiO2 thin films support the proposal that photocatalytic TNT destructi on proceeds through oxidative pathways, where molecular oxygen acceler ates byproduct degradation, and reductive pathways in which byproduct degradation is retarded by oxygen.