Soil nanoparticles and agricultural ligands remobilize cadmium sulfide colloids through composition-dependent heteroaggregation in saturated porous media.

Cadmium sulfide (CdS) formation can attenuate dissolved Cd mobility in reducing or sulfur-enriched agricultural soils, yet the remobilization risk of CdS nanoparticles (CdS-NPs) by natural soil nanoparticles and agricultural ligands remains poorly understood. Here, we show that soil-derived nanoparticles (SNPs) and common ligands can remobilize CdS as colloidal particles rather than dissolved Cd2 +, creating a potentially overlooked pathway for Cd transport in saturated porous media. Saturated quartz sand column experiments, effluent speciation, colloidal characterization, fluorescence microscopy, FTIR spectroscopy, XDLVO calculations, and two-site kinetic attachment modeling were integrated to determine how nanoparticle size, soil-derived composition, humic acid, and phosphate regulate CdS transport. Smaller SNPs promoted CdS breakthrough by forming smaller, more negatively charged, and more stable CdS-nanoparticle heteroaggregates. Nanoparticle composition exerted contrasting effects: organic matter-rich black soil nanoparticles and paddy soil nanoparticles enhanced CdS mobility, with black soil nanoparticles producing the highest transport percentage of 58.27%, whereas loessial soil nanoparticles reduced CdS transport to 35.45% by promoting aggregation and retention, likely through Ca-mediated cation bridging. Humic acid and phosphate further increased CdS-nanoparticle transport, with humic acid showing the stronger effect by enhancing colloidal stability and increasing interaction energy barriers. XDLVO calculations and kinetic attachment modeling linked enhanced CdS breakthrough to higher repulsive energy barriers and lower irreversible retention. These findings demonstrate that naturally occurring soil nanoparticles and agricultural ligands can convert immobilized CdS into mobile colloidal carriers, highlighting an important colloid-facilitated pathway for Cd migration and groundwater contamination risk in agricultural soils.