<p>Antimony (Sb) contamination from mining runoff threatens agricultural soil quality and groundwater safety, yet effective stabilization strategies based on synergistic mineral-organic composites remain underexplored. In this study, a ferrihydrite-functionalized biochar–montmorillonite composite (Fe-BMC) was developed and evaluated for Sb immobilization under repeated Sb-contaminated water application in a soil column. Among all tested amendments, Fe-BMC exhibited the highest stabilization efficiency, reducing cumulative Sb in leachates by 44.9% and surface EDTA-extractable Sb by 41.6% relative to the unamended control (<i>p</i> &lt; 0.05). Sequential extraction revealed substantial redistribution of Sb into Fe-associated (17.7%), organic-bound (13.3%), and residual (61.3%) fractions, indicating conversion of mobile Sb into more stable forms. Post-experiment spectroscopic analyses showed strengthened Fe-O and Fe-OH vibrational features, and attenuation of oxygenated carbon signals, consistent with the involvement of ferrihydrite-derived reactive sites and the mineral-organic interface in Sb retention. These results suggest that Sb stabilization was primarily associated with interfacial complexation at ferrihydrite-derived Fe(III) (oxyhydr)oxide sites, with additional contributions from oxygen-containing functional groups within the composite matrix. The superior performance of Fe-BMC is attributed to the combined effects of ferrihydrite-derived surface reactivity, the hierarchical porosity of biochar, and the structural stability of montmorillonite. Collectively, Fe-BMC substantially reduced mobility and bioavailability of Sb, demonstrating strong potential as a composite amendment for Sb immobilization in contaminated soils.</p> Graphical Abstract <p></p>