How to distinguish natural versus engineered nanomaterials: insights from the analysis of TiO2 and CeO2 in soils

Increased amounts of engineered nanomaterials (ENMs) such as TiO₂ and CeO₂ are being released into soils and sediments, increasing the need for detection and quantification of these ENMs. However, Ti- and Ce-rich particles are naturally present in soils and sediments at concentrations typically much higher than the estimated ENM concentrations. Therefore, a precise knowledge of the properties of natural nanomaterials (NNMs) is a prerequisite to develop strategies to detect ENMs in environmental systems. Characterization of the physicochemical properties of NNMs and ENMs is often complicated by their heteroaggregation and interaction with larger particles. This study evaluates six different extractants for soil NNM extraction in term of recovery and disaggregation to primary particles. The extracted NNMs were characterized for hydrodynamic diameter and zeta potential by dynamic light scattering, size-based elemental distribution by field flow fractionation coupled with inductively coupled plasma-mass spectroscopy, and morphology by transmission electron microscopy. The extracted NNM concentrations increased following the order CH₃COOH-NaCl-ultrapure water (UPW) < UPW ~ NaCl-UPW < Na₂CO₃ < Na₄P₂O₇ < NaCl-Na₄P₂O₇. Na₄P₂O₇ was the most efficient extractant which resulted in the release of primary NNMs from microaggregates. Although sodium carbonate extracted relatively high NNM concentrations, the extracted NNMs occur mainly as aggregates of primary NNMs. Ultrapure water, sodium chloride and acetic acid resulted in poor NNMs extraction and broad size distributions. Elemental ratios illustrate that Ti is associated with Nb, Ta, and V and that Ce is associated with the rare earth elements (e.g., La, Eu, Y, Ho, Er, Tm, and Yb). The findings of this study provide key properties of NNMs (e.g., size, size distribution and elemental ratios) that can be used as fingerprints to differentiate ENMs (e.g., TiO₂ and CeO₂) from NNMs in complex environmental media.