Role of titanium dioxide nanoparticles in the elevated uptake and retention of cadmium and zinc in Daphnia magna

Titanium dioxide nanoparticles (nano-TiO ₂) are now widely applied in consumer products, and the dispersion of nano-TiO ₂ may adsorb metals and modify their behavior and bioavailability in the aquatic environment. In the present study, the aqueous uptake, dietary assimilation efficiency (AE), and efflux rate constant (k _e) of two toxic metals (cadmium-Cd, and zinc-Zn) adsorbed on nano-TiO ₂ in a freshwater zooplankton Daphnia magna were quantified. The biokinetics was then compared to daphnids that were exposed only to dissolved metals as controls. The aqueous uptake of Cd and Zn involved an initial rapid uptake and then an apparent saturation, and the uptake of metals was accompanied by an ingestion of nano-TiO ₂. The AEs of Cd and Zn adsorbed on nano-TiO ₂ were 24.6 ± 2.4-44.5 ± 3.7% and 30.4 ± 3.4-51.8 ± 5.0%, respectively, and decreased with increasing concentrations of nano-TiO ₂. Furthermore, the difference between the AEs of Cd and Zn indicated that the desorption of Cd and Zn from nano-TiO ₂ may have occurred within the gut of daphnids. With the use of algae as carrier, the AEs of Cd and Zn adsorbed on nano-TiO ₂ were significantly higher than those of Cd and Zn directly from nano-TiO ₂. The efflux rate constants of Cd and Zn adsorbed on nano-TiO ₂ in the zooplankton were significantly lower than those of Cd and Zn not adsorbed on nano-TiO ₂. Our study shows that the uptake and retention of toxic metals is enhanced when they are adsorbed on nano-TiO ₂, and suggests more attention be paid to the potential influences of nano-TiO ₂ on the bioavailability and toxicity of other contaminants.