The influence of solution matrix on the photocatalytic degradation of TNT in TiO2 slurries

Previous work had demonstrated that TNT, a widespread hazardous waste, is effectively mineralized in aerated TiO2 slurries using near-UV radiation. However, prior work failed to evaluate the reaction under conditions likely to be encountered in field applications. The present study was undertaken to examine the impact of pH and the presence of inorganic ions and organic acids commonly found in natural waters on rates of TiO2 photocatalyzed TNT transformation and mineralization. Raising the pH slightly increased the rate of TNT transformation, primarily as a result of an increased rate of TNT photolysis, but significantly reduced rates of mineralization due to increased electrostatic repulsion between the catalyst surface and anionic TNT intermediates. The presence of inorganic anions did not substantially hinder TNT transformation at alkaline pH, but mineralization rates were diminished when the anion either adsorbed strongly to the photocatalyst or was an effective hydroxyl radical scavenger. Addition of humic acid to the solution matrix did not alter the net rate of TNT transformation, but did decrease the overall decay rate of dissolved organic carbon. The propensity of humic acid to transform TNT through sensitized photocatalysis, and the importance of this reaction in overall compound degradation, were also assessed. Data indicated that humic acids can serve to sensitize TiO2 photocatalytic degradation of TNT, but under the aerated solution conditions of this work rates of humic sensitized photolysis were higher than rates of sensitized photocatalysis.