Distinct structural behavior and transport of TiO2 nano- and nanostructured particles in sand

Environmental impact of TiO2 particles along with other widely-used nanomaterials as a new class of contaminants has recently emerged. Due to the lack of detailed information and proper understanding of their properties as a result of synthesis (nanoparticles vs nanostructured particles), there is a disagreement in interpreting their behavior and transport in the environment. In this study, we discuss the characteristic retention and breakthrough of commercially available TiO2 particles with unique nanoscale structure, i.e. well-defined nanoparticles (NPs) and nanostructured particles (NSPs), through sand under various reaction environments. NPs such as P-25 (30 nm) were completely different in terms of their physicochemical structure from NSPs, which may be more proper to be considered as micro or bulk TiO2 particles. Such observation undoubtedly evidenced an important finding that encapsulating NPs with stabilizers such as carboxymethyl cellulose abundant in the environment and changing their surface chemistry with pH and ionic strength greatly affected their dispersion in the aqueous phase and mobility through a clean quartz medium, while those strategies were not so effective for NSPs. Therefore, it is apparent that the terms NPs and NSPs should be more strictly differentiated in their use in order to avoid any confusion in assessing their mobility in the environment.