Interactions between nano-TiO2 particles and algal cells at moderate particle concentration

Nano-sized titanium dioxide (nano-TiO2) has wide industrial applications and therefore considerable chances of exposure are created for human beings and ecosystems. To better understand the interactions between nano-TiO2 and aquatic organisms, we first studied TiO2 uptake by algae exemplified by Pseudokirchneriella subcapitata. P. subcapitata were exposed to nano-TiO2 in a series of concentrations and at various pH. TiO2 uptake was quantified using a sedimentation curve analysis technique. After exposure of algae to TiO2, the variation of zeta potential was measured and the morphology of algae-TiO2 aggregate was observed with scanning electron microscopy and the optical microscopy. The steady-state TiO2 uptake was found to be pH-dependent and the isotherms can be described well by Freundlich model. TiO2 deposited on algal surfaces causes the shift of pHzpc of TiO2-covered algae from that of algae toward that of TiO2. The attraction between TiO2-covered algal cells induces the agglomeration of algae and TiO2 and thus the formation of algae-TiO2 aggregates in the size of 12 to 50 μm. The 2-D fractal dimension of the aggregates is pH-dependent and ranges from 1.31 to 1.67. The theoretical analysis of the Gibbs energy of interaction indicates that both TiO2 uptake by algae and the formation of algae-TiO2 aggregate are influenced by the interaction between TiO2 particles.