Arsenate Accumulation, Distribution, and Toxicity Associated with Titanium Dioxide Nanoparticles in <i>Daphnia magna</i>

Titanium dioxide nanoparticles (nano-TiO<sub>2</sub>) are widely used in consumer products. Nano-TiO<sub>2</sub> dispersion could, however, interact with metals and modify their behavior and bioavailability in aquatic environments. In this study, we characterized and examined arsenate (As­(V)) accumulation, distribution, and toxicity in Daphnia magna in the presence of nano-TiO<sub>2</sub>. Nano-TiO<sub>2</sub> acts as a positive carrier, significantly facilitating D. magna’s ability to uptake As­(V). As nano-TiO<sub>2</sub> concentrations increased from 2 to 20 mg-Ti/L, total <i>As</i> increased by a factor of 2.3 to 9.8 compared to the uptake from the dissolved phase. This is also supported by significant correlations between arsenic (<i>As</i>) and titanium (<i>Ti</i>) signal intensities at concentrations of 2.0 mg-Ti/L nano-TiO<sub>2</sub> (<i>R</i> = 0.676, <i>P</i> < 0.01) and 20.0 mg-Ti/L nano-TiO<sub>2</sub> (<i>R</i> = 0.776, <i>P</i> < 0.01), as determined by LA-ICP-MS. Even though <i>As</i> accumulation increased with increasing nano-TiO<sub>2</sub> concentrations in D. magna, As­(V) toxicity associated with nano-TiO<sub>2</sub> exhibited a dual effect. Compared to the control, the increased <i>As</i> was mainly distributed in BDM (biologically detoxified metal), but <i>Ti</i> was mainly distributed in MSF (metal-sensitive fractions) with increasing nano-TiO<sub>2</sub> levels. Differences in subcellular distribution demonstrated that adsorbed As­(V) carried by nano-TiO<sub>2</sub> could dissociate itself and be transported separately, which results in increased toxicity at higher nano-TiO<sub>2</sub> concentrations. Decreased As­(V) toxicity associated with lower nano-TiO<sub>2</sub> concentrations results from unaffected <i>As</i> levels in MSFs (when compared to the control), where several <i>As</i> components continued to be adsorbed by nano-TiO<sub>2</sub>. Therefore, more attention should be paid to the potential influence of nano-TiO<sub>2</sub> on bioavailability and toxicity of cocontaminants.