Particle-size and element distributions of soil colloids: Implications for colloid transport

Transport of colloids (diameter < 1 mu m) through soil has implications for both horizon development and contaminant mobility. Colloid physical properties (size, shape), mineralogy, and surface chemistry are known to influence transport and deposition. Sedimentation field-flow fractionation-inductively coupled plasma-mass spectrometry (Sd FFF-ICP-MS) was used to examine size and element composition distributions of colloids collected from the profile of a texture-contrast soil located in South Australia. The morphology of the colloids was also examined by transmission electron microscopy (TEM). Colloids collected from the soil matrix, the source of mobile colloids, had significant differences in the size distributions among the three horizons sampled. These differences were consistent with long-term soil formation processes. Colloids mobilized by rainfall, which were collected from overland flow and infiltration through the soil profile, all showed very similar size distributions. This is consistent with the presence of preferential flow paths and suggests that colloids, and colloid-associated contaminants, can be transported rapidly through the vadose zone with minimal interaction with the soil matrix. The Sd FFF-ICP-MS analysis showed variation in element ratios (Fe/Al, Mg/Al), which were used to detect changes in surface coatings and mineralogy over the colloid size range. The study demonstrated the utility of Sd FFF-ICP-MS for examining the influence of colloid size on element composition and on elucidating colloid transport processes in soils.