Natural Nanoparticles From Soils: Separation, Characterization and Quantification.

Being ubiquitous in soils, properties and distribution dynamics of natural nanoparticles are pertinent to ecological and human health. Their wide variation in density and particle shape is a challenge for accurate particle-size fractionation. Objectives of this study were to (1) develop a protocol to fractionate nanoparticles based on a combined approach of light scattering and centrifugation, (2) characterize particles for a wide range of soil types, and (3) relate light scattering data to particle characteristics. Six contrasting soils (Alfisol, Spodosol, Entisol, Oxisol and two Ultisols) where chosen for study. Dominant size fractions remaining in suspensions following centrifugation were determined using dynamic light scattering. Mineralogy was determined by x-ray diffraction. Particle shapes were examined using high resolution transmission electron microscope. A significant logarithmic relationship was found between centrifugation speed and particle size (R²=0.71, P<0.005). Nanoparticles (<100 nm) became dominant by fractional volume (51 to 94%) when centrifugation force reached 33, 540 × g for all soils. High variation for the six soil suspensions collected under highest separation energy related to mineral densities, pH and organic matter content. Bi-and tri-modal distributions were common within suspensions and related to density differences between organic C and hematite. Nanoparticle shape was variable and re-aggregation was evident. Light scattering analyses, UV-visible spectroscopy, and chemical analysis distinguished samples of high hematite and those without hematite. These data indicate that light scattering in conjunction with a standard centrifuge regimen provide a practical means of collecting particles within a targeted size range, compensating for natural variations among samples.