The fate of titanium dioxide nanoparticles in the water compartment

Conference poster, 2009

Environmental risk of nanoparticles has been assessed in several studies. Titanium dioxide has been identified as one of the potentially most problematic nanoparticles. It has been shown that a large amount of the titanium dioxide nanoparticles may end up in the water compartment. These risk assessments constitute first estimates and several fate mechanisms are not taken into account. In this study, we propose a risk model that includes the two fate mechanisms sedimentation and aggregation. The sedimentation rate of titanium dioxide nanoparticles is determined using simple laws of motion. Still water is assumed, which gives optimal conditions for sedimentation. It is shown that despite the optimal conditions for sedimentation, the sedimentation rates of particles with a diameter of 100 nm and 1000 nm are about 8 years and 1 month, respectively. One month can be regarded as a long time from a risk perspective, and thus sedimentation is shown not to be an important factor for the removal of titanium dioxide nanoparticles from the water compartment. However, preliminary results show that aggregation can reduce the predicted environmental concentration significantly in a short time. Hence, equilibrium concentrations could be several orders of magnitude lower than what has been indicated in earlier studies. The aggregation of titanium dioxide nanoparticles is modeled using Smoluchowski kinetics and the DLVO theory, which are commonly used in colloid chemistry. The aggregation is shown to depend mainly on the pH of the water compartment and the zero point charge of the particles.