Effects of iron, manganese, and aluminum oxides on soil cadmium distribution coefficient: A multi-scale analysis...

Cadmium (Cd) mobility in soils is controlled by reactive mineral phases and environmental chemistry. However, the effect of metal oxides on Cd adsorption in soil and the interactions among oxides and between oxides and environmental factors are still unclear. We compile a cross-regional dataset and develop an interpretable gradient-boosted model (XGBoost) with SHAP attributions to quantify the main and interactive effects of iron, manganese, and aluminum oxides (Fe_ox, Mn_ox, Al_ox) on the soil-water distribution coefficient (lnK_d). We find: (i) the XGBoost model performs well (R²=0.737, RMSE=0.930); (ii) Fe_ox, Mn_ox, Al_ox, soil pH (pH_soil), cation exchange capacity (CEC), and initial concentration (C₀) are key factors influencing Cd adsorption in soil; (iii) Mn_ox exerts a consistently near-monotonic contribution; (iv) Fe_ox is hump-shaped-positive at low-moderate levels with diminishing/negative returns at higher contents; (v) Al_ox generally suppresses Cd partitioning; and (vi) further investigation of interactions shows Mn-Fe synergy at high contents and Fe-Al antagonism. Conditional modulation by pH_soil, CEC, and C₀ exhibits interval dependence. These results emphasize the roles of reactive site quality, accessibility, and saturation in Cd immobilization and inform oxide-targeted remediation strategies.