Size-dependent colloidal stability and transport behaviors of typical zonal soil colloids

Abstract

Colloidal particles, the finest constituents of soil, originate from complex physicochemical interactions among clay minerals, metal oxides, and organic matter. Their environmental behavior plays a critical role in soil structure formation, solute transport, and soil genesis. As particle size decreases, changes occur in the composition of organic matter and clay minerals, as well as in surface properties and aggregation-migration behaviors. However, studies addressing the size-dependent surface characteristics and environmental behaviors of soil colloids remain limited. In this study, typical zonal soils from China—Phaeozem (black soil), Cambisol (brown soil), Luvisol (yellow-brown earth), and Ferralsol (latosol)—were selected. Colloidal fractions with diameters of d < 2000 nm, d < 1000 nm, d < 500 nm, and d < 100 nm were extracted through ultrasonic dispersion combined with high-speed centrifugation, and their composition, surface properties, and aggregation/migration behaviors were systematically analyzed. The results showed that, as particle size decreased, colloidal particles from Phaeozem, Cambisol, and Luvisol exhibited reductions in both diameter and thickness, while Ferralsol colloids showed minimal changes. The absolute value of the zeta potential initially increased and then decreased, indicating reduced surface charge variability. Soil organic carbon was predominantly enriched in finer fractions, particularly in nanocolloids (d < 100 nm), which were characterized by a higher density of surface functional groups, greater aromaticity, and lower hydrophilicity. The critical coagulation concentration (CCC) of Phaeozem colloids decreased with decreasing particle size, while Cambisol colloids exhibited an initial decrease followed by an increase. In contrast, the CCC of Luvisol and Ferralsol colloids increased as particle size decreased. These variations in aggregation behavior were attributed to the balance between electrostatic repulsion and van der Waals attraction. The highest mobility was observed for the d < 500 nm fractions of Phaeozem, Cambisol, and Luvisol colloids, and for the d < 1000 nm fraction of Ferralsol colloids. Conversely, the weakest mobility was found in the nanocolloidal fractions of all soils, likely due to their lower absolute value of the zeta potential and higher densities of surface functional groups. The critical particle diameters controlling size effects were determined to be 500 nm for Phaeozem, Cambisol, and Luvisol colloids, and 1000 nm for Ferralsol colloids. These findings provide a theoretical foundation for more accurately assessing the environmental behavior of soil colloids and their roles in pollutant transport processes.