Quantitative prediction and mechanistic insights into Cu(II) transport in kaolinite-coated sand columns affected by ferrihydrite nanoparticles

Colloidal particles play a significant role in the transport of heavy metals in soil, but quantitative prediction of the impact of colloids on their migration and bioavailability remains challenging. This study examines the impact of ferrihydrite nanoparticles (Fh-NPs) in influencing the transport and spatial distribution of Cu(II) within a kaolinite-coated sand column. When Fh-NPs are pre-adsorbed on kaolinite or co-migrate with Cu(II), they hinder the migration of Cu(II) in kaolinite-coated sand columns. The observed retardation factor of Cu(II) with Fh-NPs was 1.19–1.52 times higher than in their absence, indicating a notable increase in Cu(II) retention. Calculations of Cu(II) retention (0.0042–0.0166 mg g⁻¹ at pH 3.0–5.5) closely matched experimental values (0.0044–0.0170 mg g⁻¹), underscoring the reliability of the theoretical model. Our study also showed that the adsorption mechanism of Cu(II) on both kaolinite and Fh-NPs surfaces involved the formation of bidentate binuclear inner-sphere complexes. The shorter Cu–Fe interatomic distance, compared to Cu–Al/Si, accounted for the enhanced bonding stability of Cu(II) with Fh-NPs. This study quantitatively examined colloidal-mediated Cu(II) transport in saturated porous media, elucidating its underlying mechanisms through molecular interactions. The findings provided critical insights for enhancing contaminants transport modeling, improving risk assessment, and developing targeted remediation strategies.