Humic Acid-Mediated Enhancement of Stibnite Oxidative Surface Weathering and Antimony Mobilization Under Photochemical Conditions and Variable pH

Stibnite (Sb2S3), the predominant antimony-bearing sulfide mineral, represents a major natural source of antimony (Sb) in mining-impacted environments. Although humic acid (HA) is ubiquitous in mine wastes and surface waters, its influence on the oxidative transformation and mobilization of Sb from Sb2S3 under photochemical conditions remains poorly understood. Here, the coupled effects of HA, controlled ultraviolet irradiation and pH on Sb2S3 weathering and Sb mobilization were systematically investigated through batch dissolution experiments combined with hydroxyl radical (·OH) determination, elemental sulfur (S0) analysis and complementary solid-phase characterization. Sb release increased with increasing pH and was further enhanced by HA, particularly under circumneutral and alkaline conditions. Controlled UV irradiation further increased Sb mobilization under the investigated conditions, with a more pronounced effect in HA-containing systems. The enhanced Sb mobilization was accompanied by increased ·OH production and greater S0 accumulation, indicating increased oxidative activity during weathering. SEM, EDS and FTIR analyses further revealed progressive surface morphological and compositional changes, whereas XRD showed that the bulk crystalline structure of Sb2S3 was largely preserved. XPS revealed the formation of oxidized Sb- and S-containing surface species, providing evidence for progressive surface chemical transformation. These results indicate that HA and controlled photochemical conditions were associated with enhanced Sb mobilization and oxidative transformation at the Sb2S3-solution interface, while the overall weathering remained predominantly surface-confined. The findings highlight the importance of considering natural organic matter, photochemical conditions and pH together when evaluating Sb mobility from sulfide minerals in mining-impacted environments.