Improved extraction efficiency of natural nanomaterials in soils to facilitate their characterization using a...

Characterization of natural nanomaterial (NNM) physicochemical properties - such as size, size distribution, elemental composition and elemental ratios - is often hindered by lack of methods to disperse NNMs from environmental samples. This study evaluates the effect of extractant composition, pH, and ionic strength on soil NNM extraction in term of recovery and release of primary particles/small aggregate sizes (i.e., <200 nm). The extracted NNMs were characterized for hydrodynamic diameter and zeta potential by dynamic light scattering and laser Doppler electrophoresis, natural organic matter desorption by UV-Vis spectroscopy, element composition by inductively coupled plasma-mass spectroscopy (ICP-MS), size based elemental distribution by field flow fractionation coupled to ICP-MS, and morphology by transmission electron microscopy. The extracted NNM concentrations increased following the order of NaOH ≤ Na₂CO₃ < Na₂C₂O₄ < Na₄P₂O₇. Na₄P₂O₇ was the most efficient extractant and results in 2-12 folds higher NNM extraction than other extractants. The Na₄P₂O₇ extracted NNMs exhibited narrower size distribution with smaller modal size relative to NaOH, Na₂CO₃, Na₂C₂O₄ extracted NNMs. Thus, Na₄P₂O₇ enhances the extraction of primary NNMs and/or smaller NNM aggregates (i.e., size <200 nm). Na₄P₂O₇ promote soil microaggregates breakup and release of NNMs by reducing free multivalent cation concentration in soil pore water by forming metal-phosphate complexes and by enhancing NNM surface charge via phosphate sorption on NNM surfaces. Additionally, the extracted NNM concentrations increased with the increase in extractant concentration and pH, except at 100 mM where the high ionic strength might have induced NNM aggregation. The improved NNM-extraction will improve the overall understanding of the physicochemical properties of NNMs in environmental systems. This study presents the key properties of NNMs that can be used as background information to differentiate engineered nanomaterials (ENMs) from NNMs in complex environmental media.