Aqueous Hg²⁺ associates with TiO₂ nanoparticles according to particle size, changes particle agglomeration, and becomes less bioavailable to zebrafish

Engineered nanoparticles (NPs) have unique physicochemistry and potential to interact with other substances in the aqueous phase. Here, gene [metallothionein 2 (mt2)] expression changes in larval zebrafish were used to evaluate the association between aqueous Hg2+ and TiO2 (NPs and bulk particle size control) to investigate the relationship between changes in Hg2+ behavior and TiO2 size. During 24 h exposures, TiO2 agglomerates increased in size and in the presence of 25 μg Hg2+/L, greater increases in size were observed. The concentration of Hg2+ in suspension also decreased in the presence of TiO2-NPs. Mercury increased expression of mt2 in larval zebrafish, but this response was lessened when zebrafish were exposed to Hg2+ in the presence of TiO2-NPs, and which suggests that TiO2-NPs alter the bioavailability of Hg2+ to zebrafish larvae. This ameliorative effect of TiO2 was also likely due to surface binding of Hg2+ because a greater decrease in mt2 expression was observed in the presence of 1 mg/L TiO2-NPs than 1 mg/L TiO2-bulk. In conclusion, the results show that Hg2+ will associate with TiO2-NPs, TiO2-NPs that have associated Hg2+ will settle out of the aqueous phase more rapidly, and agglomerates will deliver associated Hg2+ to sediment surfaces.