DOM concentration and mineralogical heterogeneity jointly regulate antimony redistribution in secondary iron mineral assemblages from acid mine drainage.

Antimony (Sb) mobility in acid mine drainage (AMD) is strongly regulated by interactions with secondary Fe minerals and dissolved organic matter (DOM), yet how mineralogical heterogeneity modifies DOM-induced Sb redistribution remains poorly understood. Here, we investigated how DOM concentration and composition regulate Sb(V) retention and mobilization in a naturally derived multicomponent secondary iron mineral assemblage (nmSIM) and representative single Fe minerals, using humic acid (HA), fulvic acid (FA) and L-tryptophan (L-Trp) as representative DOM types. By integrating Sb retention/release measurements with Fe dissolution, mineralogical characterization and surface spectroscopic analyses, we identified a concentration-dependent shift in the relative importance of retention and mobilization processes. At relatively low DOM concentrations, HA and FA generally enhanced Sb(V) retention, whereas at higher concentrations they promoted Fe dissolution and Sb mobilization; L-Trp exerted substantially weaker and less systematic effects. Notably, nmSIM did not consistently exhibit lower initial Sb release than individual minerals. Instead, under HA and FA perturbation, it displayed a distinct temporal response, with relatively high initial Sb mobilization followed by attenuation or stabilization of dissolved Sb, whereas several single-mineral systems showed more sustained release. Mineralogical and surface-chemical evidence indicates that DOM-promoted dissolution of relatively reactive Fe-bearing phases was accompanied by phase evolution toward more crystalline Fe-(oxyhydr)oxide-rich surfaces. The concurrent decline or stabilization of dissolved Sb despite continued Fe release supports the involvement of newly generated Fe-bearing interfaces in subsequent partial Sb re-immobilization, although their mineral-specific contribution cannot be quantitatively resolved by the present data. Overall, the results reveal a two-level regulatory framework in which DOM concentration and composition govern the initial balance between Sb retention and mobilization, whereas mineralogical heterogeneity modulates the subsequent fate of mobilized Sb through coupled dissolution, phase transformation and partial re-immobilization. This framework cautions against directly extrapolating single-mineral behavior to heterogeneous AMD systems and provides a mechanistic basis for assessing Sb mobility and retention under DOM perturbation.