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

Increasing amounts of engineered nanomaterials such as TiO2 and CeO2 are released into air, waters, soils, and sediments. However, assessing the human-made origin of those nanomaterials is rather difficult because Ti- and Ce-rich particles are naturally present in soils and sediments at concentrations typically much higher than estimated concentrations of engineered nanomaterials. In addition, analysis is complicated by the interactions and aggregation of nanoparticles with environmental particles. Therefore, more knowledge on the properties of natural nanomaterials is needed to distinguish engineered nanomaterials in natural systems. Here, we extracted soil nanomaterials with six extractants and compared recovery and disaggregation to primary particles. Nanomaterials 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. Results show that nanomaterial concentrations increased from CH3COOH-NaCl-water (lowest), to water or NaCl-water, Na2CO3, Na4P2O7, and NaCl-Na4P2O7 (highest). Na4P2O7 was the most efficient extractant that induced the release of primary nanomaterials from microaggregates. Although sodium carbonate extracted relatively high concentrations of nanomaterials, the extracted nanomaterials occurred mainly as aggregates of primary nanomaterials. Ultrapure water, sodium chloride and acetic acid resulted in poor nanomaterial extraction and broad size distributions. Elemental ratios illustrate that Ti is associated with Nb, Ta, and V, and that Ce is associated with rare earth elements such as La, Eu, Y, Ho, Er, Tm, and Yb. Our findings indicate that size, size distribution, and elemental ratios can be used as fingerprints to differentiate engineered nanomaterials such as TiO2 and CeO2 from natural nanomaterials in complex media.