Natural TiO₂-Nanoparticles in Soils: A Review on Current and Potential Extraction Methods

The monitoring of anthropogenic TiO₂-nanoparticles in soils is challenged by the knowledge gap on their characteristics of the large natural TiO₂-nanoparticle pool. Currently, no efficient method is available for characterizing natural TiO₂-nanoparticles in soils without an extraction procedure. Considering the reported diversity of extraction methods, the following article reviews and discusses their potential for TiO₂ from soils, focusing on the selectivity and the applicability to complex samples. It is imperative to develop a preparative step reducing analytical interferences and producing a stable colloidal dispersion. It is suggested that an oxidative treatment, followed by alkaline conditioning and the application of dispersive agents, achieve such task. This enables the further separation and characterization through size or surface-based separation (i.e., hydrodynamic fractionation methods, filtration or sequential centrifugation). Meanwhile, cloud point extraction, gel electrophoresis, and electrophoretic deposition have been studied on various nanoparticles but not on TiO₂-nanoparticles. Furthermore, industrially applied methods in, for example, kaolin processing (flotation and flocculation) are interesting but require further improvements on terms of selectivity and applicability to soil samples. Overall, none of the current extraction methods is sufficient toward TiO₂; however, further optimization or combination of orthogonal techniques could help reaching a fair selectivity toward TiO₂.