Behavior of TiO2 Released from Nano-TiO2-Containing Paint and Comparison to Pristine Nano-TiO2

In the assessment of the fate and effects of engineered nanomaterials (ENM), the current focus is on studying the pristine, unaltered materials. However, ENM are incorporated into products and are released over the whole product life cycle, though mainly during the use and disposal phases. So far, released ENMs have only been characterized to a limited extent and almost nothing is known about the behavior of these materials under natural conditions. In this work we obtained material that was released from aged paint containing nano-TiO2, characterized the particulate materials, and studied their colloidal stability in media with different pH and ionic composition. A stable suspension was obtained from aged paint powder by gentle shaking in water, producing a dilute suspension of 580 mu g/L TiO2 with an average particle size of 200-300 nm. Most particles in this suspension were small pieces of paint matrix that also contained nano-TiO2. Some free nano-TiO2 particles were observed by electron microscopy, but the majority was enclosed by the organic paint binder. The pristine nano-TiO2 showed the expected colloidal behavior with increasing stability with increasing pH and strong agglomeration above the isoelectric point and settling in the presence of Ca. The released TiO2 showed very small variations in particle size, zeta potential, and colloidal stability, even in the presence of 3 mM Ca. The results show that the behavior of released ENM may not necessarily be predicted by studying the pristine materials. Additionally, effect studies need to focus more on the particles that are actually released as we can expect that the toxic effect will also be markedly different between pristine and product released materials.