Effect of colloidal particle size on physicochemical properties and aggregation behaviors of two alkaline soils

Abstract. Soil colloidal particles are the most active components of all, and they also vary in elemental composition and environmental behaviors with the particle size. The purposes of the present study are to clarify how particle size affects the physiochemical properties and aggregation kinetics of soil colloids, and to further reveal the underlying mechanisms. Soil colloidal particles from two alkaline soils—Lou soil and cinnamon soil were subdivided into three ranges: d < 2 μm, d < 1 μm and d < 100 nm. The organic and inorganic carbon contents, clay mineralogy, surface electrochemical properties, including surface functional groups and zeta potentials, were characterized. Through time-resolved light scattering technique, the aggregation kinetics of soil colloidal fractions were investigated, and their critical coagulation concentrations (CCCs) were determined. With decreasing colloidal particle diameter, the total carbon content, organic carbon, organic functional groups content and illite content all increased. The absolute zeta potential values and the charge variability decreased with decreasing particle diameter. The CCC values of Lou soil and cinnamon soil colloids followed the descending order of d < 100 nm, d < 1 μm, d < 2 μm. Compared with the course factions (d < 1 μm and d < 2 μm), soil nanoparticles were more abundant in organic carbon and more stable clay minerals (d < 100 nm), thus they exhibited strongest colloidal suspension stability. The differences in organic matter contents and clay mineralogy are the fundamental reasons for the differences in colloidal suspension stability behind the size effects of Lou soil and cinnamon soil colloids. The present study revealed the size effects of two alkaline soil colloids on carbon content, clay minerals, surface properties and suspension stability, emphasizing that soil nanoparticles are prone to be more stably dispersed instead of being aggregated. These findings can provide references for in-depth understanding of the environmental behaviors of the heterogeneous soil organic-mineral complexes.