Coupled experimental and modeling approaches to reveal ion-specific migration behavior of arsenic, cadmium, and lead in soils surrounding Pb–Zn smelters

The dynamic migration mechanisms of arsenic (As), cadmium (Cd), and lead (Pb) in soils contaminated by lead-zinc (Pb-Zn) smelting during runoff infiltration remain poorly understood. This study employed batch adsorption and dynamic column experiments to investigate the migration behavior of As, Cd and Pb in acidic red soils around typical Pb-Zn mining regions. Results demonstrated that surface soils exhibited significantly higher adsorption capacities than deeper layers. As(V) preferentially bound to surface aggregates in a monolayer configuration, while Cd(II) and Pb(II) adhered through a more complex, multilayered arrangement. The surface layer (S1) had lower K_s and D value, indicating a stronger pollutant retention capacity than the intermediate (S2) and deep (S3) layers. Column experiments established a descending mobility order of Cd(II) > Pb(II) > As(V) in acidic soils. At pH levels above 6.5, the deprotonation of soil adsorption sites enhanced Cd²⁺ and Pb²⁺ adsorption and complexation with Cd(OH)₂ and Pb(OH)₂, but simultaneously increased electrostatic repulsion against HAsO₄²⁻, HAsO₃⁻, and AsO₄³⁻. A combined isothermal adsorption and non-equilibrium model effectively captured the migration trends of As, Cd, and Pb ions, though mid-migration hysteresis of Pb reduced predictive accuracy. Acidic soil chemical and mineral characteristics were instrumental in predicting As(Ⅴ) retention, whereas the retardation factors (R_f) for Cd(II) and Pb(II) were more closely associated with soil mineralogy, particularly Fe oxide content and speciation. These findings provide valuable insights into the controlling mechanisms of toxic metal migration in contaminated soils, which is crucial for developing effective remediation strategies for polluted soils in areas impacted by smelter.