The Effects of Oxyanion Adsorption on Reactive Oxygen Species Generation by Titanium Dioxide

The growing use of nano titanium dioxide (TiO2) in consumer and agricultural products has accelerated its introduction into terrestrial environments, where its impact has not been documented extensively. TiO2 toxicity arises primarily from its ability to photochemically generate reactive oxygen species (ROS), including hydrogen peroxide (H2O2). While common ligands in soil porewaters can either hinder or enhance the degradation of organic contaminants by TiO2, their effects on ROS production by TiO2 have not been understood clearly. The objective of this study was to assess the effect of phosphate (P) and nitrate on UV-irradiated anatase, nano-TiO2. Accordingly, H2O2-generation kinetics experiments were conducted in UV-irradiated TiO2 under environmentally relevant concentrations of the ligands (0, 50, 100, and 250 mu M) and pH values (4.00 +/- 0.02 and 8.00 +/- 0.02) from 0-100 min. Under all conditions, H2O2 grew logarithmically and reached between 5.38 and 22.98 mu M after 100 min. At pH 4.00 +/- 0.02, H2O2 production was suppressed by P but not by nitrate. Conversely, at pH 8.00 +/- 0.02, nitrate did not affect H2O2 concentration while P increased it. Non-specific, minimal adsorption of nitrate prevented interference with the photoreactivity of TiO2. Due to the pH-dependent behavior of suspended TiO2 and H2O2 degradation rates, specific adsorption of P on TiO2 impeded its ability to produce H2O2 photochemically at pH 4.00 +/- 0.02 but amplified it at pH 8.00 +/- 0.02.