Colloid aggregation dynamics govern the partitioning and transport of heavy metal(loid)s at smelting site

Smelting sites are commonly plagued by heavy metal(loid)s co-contamination. Functioning as a critical vector for the transport of heavy metal(loid)s, the aggregation behavior of soil colloids profoundly affects the fractionation and transport of heavy metal(loid)s at the soil-groundwater system. This study explored the controlling mechanism of pH, coexisting ion types, and ionic strength on the colloid stability and the colloidization potential of heavy metal(loid)s by leaching experiment, colloidal aggregation kinetics, and xDLVO calculation. The transition from stable dispersion to rapid aggregation occurred at CCC of 19.3, 2.7, and 0.3 mM for Na⁺, Ca²⁺, and Al³⁺. High-valence cations significantly reduced the critical coagulation concentration by compressing the electric double layer, promoting colloid aggregation. Specifically, the fractionation of heavy metal(loid)s (Zn, Pb, As, Cd) between colloidal and truly dissolved phases exhibited distinct pH-dependent and ion-specific effects. When the Na⁺ concentration increased to 50 mM, the colloidal concentrations of Zn, Pb, As and Cd decreased by 5.2%, 20.0%, 22.1% and 24.9% respectively. High ionic strength and high-valence cations weaken the colloidization effect of heavy metal(loid)s mainly by compressing the electric double layer and reducing the repulsive energy barrier, thereby promoting colloid aggregation and sedimentation, with additional contribution from surface charge neutralization and competitive adsorption on colloidal surfaces. This study systematically clarified the controlling mechanism of colloid aggregation behavior on the colloidization of heavy metal(loid)s, providing theoretical support for the prediction of heavy metal(loid)s migration and the formulation of remediation strategies in contaminated sites.