Alternative assessment of nano-TiO₂ sedimentation under different conditions based on sedimentation efficiency at quasi-stable state

The predictable significant increase in manufacture and use of engineered nanoparticles (ENPs) will cause their inevitable release into environment, and the potential harmful effects of ENPs have been confirmed. As representative ENPs, sedimentation behavior of nano-titanium dioxide (n-TiO₂) should be better understood to control its environmental risk. In this study, an experimental methodology was established to set the sampling area and sampling time of n-TiO₂ sedimentation. In addition, we defined a quasi-stable state and a precise index, i.e., sedimentation efficiency (SE) at this state, to describe the n-TiO₂ sedimentation behavior. Both alternative concentration determination and conventional size measurement were applied to evaluate the sedimentation behavior of n-TiO₂ with fulvic acid. Results showed that the sedimentation behavior described by SE was more precise and in disagreement with those predicted by particle size. Moreover, sedimentation experiments with salicylic acid (SA), under an electric field and different water temperatures or with sulfosalicylic acid under light irradiation were also performed. When the total organic carbon concentration of SA, the voltage of working electrodes, and water temperature increased, or the wavelength of light source decreased, the SE of n-TiO₂ increased and n-TiO₂ showed a tendency to settle in water. These findings might be important for deepening the understanding of n-TiO₂ environmental behavior and exploring sedimentation behavior of other ENPs.