Distinct toxic interactions of TiO<sub>2</sub> nanoparticles with four coexisting organochlorine contaminants on algae

<p>Engineered nanoparticles are increasingly discharged into the environment. After discharge, these nanoparticles can interact with co-existing organic contaminants, resulting in a phenomena referred to as ‘joint toxicity’. This study evaluated joint toxicities of TiO<sub>2</sub> nanoparticles (TiO<sub>2</sub>NPs) with four different (atrazine, hexachlorobenzene, pentachlorobenzene, and 3,3′,4,4′-tetrachlorobiphenyl) organochlorine contaminants (OCs) toward algae (<i>Chlorella pyrenoidosa</i>). The potential mechanisms underlying the joint toxicity were discussed, including TiO<sub>2</sub>NPs–OC interactions, effects of TiO<sub>2</sub>NPs and OCs on biophysicochemical properties of algae and effects of TiO<sub>2</sub>NPs and OCs on each other’s bioaccumulation in algae. The results indicate that coexposure led to a synergistic effect on the joint toxicity for TiO<sub>2</sub>NPs–atrazine, antagonistic effect for TiO<sub>2</sub>NPs–hexachlorobenzene and TiO<sub>2</sub>NPs–3,3',4,4'-tetrachlorobiphenyl, and an additive effect for TiO<sub>2</sub>NPs–pentachlorobenzene. There was nearly no adsorption of OCs by TiO<sub>2</sub>NPs, and the physicochemical properties of TiO<sub>2</sub>NPs were largely unaltered by the presence of OCs. However, both OCs and NPs affected the biophysicochemical properties of algal cells and thereby influenced the cell surface binding and/or internalization. TiO<sub>2</sub>NPs significantly increased the bioaccumulation of each OC. However, with the exception of atrazine, the bioaccumulation of TiO<sub>2</sub>NPs decreased when used with each OC. The distinct joint toxicity outcomes were a result of the balance between the increased toxicities of OCs (increased bioaccumulations) and the altered toxicity of TiO<sub>2</sub>NPs (bioaccumulation can either increase or decrease). These results can significantly improve our understanding of the potential environmental risks associated with NPs.</p>