Quantitative measurements of hydroxyl radicals generated by irradiated titanium dioxide nanoparticle suspensions

Abstract

ABSTRACTIncreased use of titanium dioxide (TiO2) nanoparticles in different applications has increased risk for adverse environmental implications based on an elevated likelihood of organism exposure. Anatase TiO2 is photoactive with exposure to ultraviolet light. TiO2 nanoparticle exposure to UV-A radiation in aquatic environments generates hydroxyl radical species, which may ultimately be responsible increased organism toxicity. The present research demonstrates that the rate of radical generation heavily depends on exposure conditions, particularly the presence of natural organic matter (NOM). Environmentally relevant concentrations of TiO2 nanoparticles were co-exposed to increasing NOM amounts (measured as concentration of dissolved organic carbon (DOC)) and UV-A intensities. Hydroxyl radical generation rate was determined using fluorescence spectroscopy. Radical generation rate was positively correlated to increases in TiO2 concentration and UV-A intensity, and negatively correlated to increased DOC concentration. Nanoparticle aggregation over time and decrease in light transmission from NOM had negligible contributions to the generation rate. This suggests the decreased radical generation rate is a result of radical quenching by NOM functionalities. D. magna toxicity to hydroxyl radicals is also demonstrated to decreased following the addition of DOC. These results correlate with the rate generation data, indicating that DOC provides rate attenuation that is protection to organisms. These conclusions demonstrate the importance considering exposure conditions during TiO2 toxicity testing, and during TiO2 waste management and regulatory decisions.