TiO 2 Nanoparticle Photoactivation and Oxidation Reactions in Freshwater and Marine Systems: The Role of Radical Scavengers

Titanium dioxide nanoparticles (TiO -NP) present in wastewater effluent are discharged into freshwater and saltwater (i.e., marine) systems. TiO -NP can be solar-driven photoactivated by ultraviolet (UV)-light producing reactive oxygen species including hydroxyl radicals (·OH). ·OH are non-selective and react with a broad range of species in water. In other studies, photoactivation of TiO -NP has been correlated with oxidative stress and ecotoxicological impacts on plant and animal biota. This study examined the photoactivation of TiO -NP in freshwater and saltwater systems, and contrasted the oxidation potential in both systems using methylene blue (MB) as a reaction probe. Maximum MB loss (51.9%, n=4; 95% confidence interval 49.4-54.5) was measured in salt-free, deionized water where ·OH scavenging was negligible; minimum MB loss (1%) was measured in saltwater due to significant ·OH scavenging, indicating the inverse correlation between MB loss and radical scavenging. A kinetic analysis of scavenging by seawater constituents indicated Cl had the greatest impact due to high concentration and high reaction rate constant. Significant loss of MB occurred in the presence of Br relative to other less aggressive scavengers present in seawater (i.e., HCO , HSO ). This result is consistent with the formation of Bromate, a strong oxidant that subsequently reacts with MB. In freshwater samples collected from different water bodies in Oklahoma (n=12), the average MB loss was 13.4%. Greater MB loss in freshwater systems relative to marine systems was due to lower ·OH scavenging by various water quality parameters. Overall, TiO -NP photoactivation in freshwater systems has the potential to cause greater oxidative stress and ecotoxicological impacts than in marine systems where ·OH scavenging is a dominant reaction.