Distinct toxic interactions of TiO2 nanoparticles with four coexisting organochlorine contaminants on algae

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 TiO2 nanoparticles (TiO(2)NPs) with four different (atrazine, hexachlorobenzene, pentachlorobenzene, and 3,3,4,4-tetrachlorobiphenyl) organochlorine contaminants (OCs) toward algae (Chlorella pyrenoidosa). The potential mechanisms underlying the joint toxicity were discussed, including TiO(2)NPs-OC interactions, effects of TiO(2)NPs and OCs on biophysicochemical properties of algae and effects of TiO(2)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(2)NPs-atrazine, antagonistic effect for TiO(2)NPs-hexachlorobenzene and TiO(2)NPs-3,3',4,4'-tetrachlorobiphenyl, and an additive effect for TiO(2)NPs-pentachlorobenzene. There was nearly no adsorption of OCs by TiO(2)NPs, and the physicochemical properties of TiO(2)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(2)NPs significantly increased the bioaccumulation of each OC. However, with the exception of atrazine, the bioaccumulation of TiO(2)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(2)NPs (bioaccumulation can either increase or decrease). These results can significantly improve our understanding of the potential environmental risks associated with NPs.