A new method for separation, characterization, and quantification of natural nanoparticles from soils

Properties and distribution dynamics of nanoparticles are pertinent to ecological and human health. Wide variation in particle density confounds their accurate separation for study. The objective of this study was to develop a fine-particle separation protocol requiring no constant density assumption. Six soils with contrasting properties were chosen for study. Size distributions of suspended particles following centrifugation were assessed using dynamic light scattering. Mineralogy was determined by X-ray diffraction. Particle characteristics were examined using transmission electron microscopy. A significant pooled logarithmic relationship was found between centrifugation speed and particle-size distribution. Nanoparticles (<100 nm) became dominant by fractional volume at widely differing separation energies among the six soils. However, relations were strong between separation energies and fractional volume for individual soils. Bi- and tri-modal-size distributions within suspensions were related to density differences of soil components. Re-aggregation at high separation energy was apparent for the tropical soils. Light-scattering, UV–Visible absorbance, and chemical analyses distinguished samples of high hematite content from those without hematite. These results indicate that dynamic light scattering in conjunction with a standard centrifuge regimen provides a practical means of collecting colloidal particles within a targeted size range, compensating for natural variations among the soil samples.