High bioconcentration of titanium dioxide nanoparticles in Daphnia magna determined by kinetic approach

The environmental risk assessments of titanium dioxide nanoparticles (TiO₂ NPs) have drawn wide attention and one of the required critical information is the bioconcentration potentials of these nanoparticles in aquatic organisms. In the present study, the bioconcentration of six commercially available TiO₂ NPs with different sizes and surface properties were quantified in a freshwater cladoceran Daphnia magna using kinetic modeling approach. We first calculated the uptake rate constant (k_u) and depuration rate constant (k_e) of TiO₂ NPs and then employed a first-order kinetic model to predict the bioconcentration factors (BCF) at different TiO₂ NPs concentrations. Both the k_u and k_e of TiO₂ NPs were significantly affected by the exposure concentration and the nanoparticle property. The predicted BCF values in D. magna of six TiO₂ NPs ranged from 2.40 × 10⁵ L/kg to 1.52 × 10⁶ L/kg, and had no clear correlation with the exposure concentration. Large nominal size resulted in a lower BCF of TiO₂ NPs at lower exposure concentration. Higher hydrophobicity and Al(OH)₃ coating also resulted in a higher BCF. All the six TiO₂ NPs in this study were therefore considered very bioaccumulative. More attention should be paid to bioconcentration in the environmental risk assessments of TiO₂ NPs, and the physicochemical properties of TiO₂ NPs should be taken into account.