Influences of anion concentration and valence on dispersion and aggregation of titanium dioxide nanoparticles in aqueous solutions.

Dispersion and aggregation of nanoparticles in aqueous solutions are important factors for safe application of nanoparticles. In this study, dispersion and aggregation of nano-TiO2 in aqueous solutions containing various anions were investigated. The influences of anion concentration and valence on the aggregation size, zeta potential and aggregation kinetics were individually investigated. Results showed that the zeta potential decreased from 19.8 to -41.4mV when PO43- concentration was increased from 0 to 50mg/L, while the corresponding average size of nano-TiO2 particles decreased from 613.2 to 540.3nm. Both SO42- and NO3- enhanced aggregation of nano-TiO2 in solution. As SO42- concentration was increased from 0 to 500mg/L, the zeta potential decreased from 19.8 to 1.4mV, and aggregate sizes increased from 613.2 to 961.3nm. The trend for NO3- fluctuation was similar to that for SO42- although the range of variation for NO3- was relatively narrow. SO42- and NO3- accelerated the aggregation rapidly, while PO43- did so slowly. These findings facilitate the understanding of aggregation and dispersion mechanisms of nano-TiO2 in aqueous solutions in the presence of anions of interest.