Groundwater arsenic and antimony mobility from an antimony mining area: Controls of sulfide oxidation, carbonate and silicate weathering, and secondary mineral precipitation

Sulfide mineral oxidation has been recognized as the key driver of arsenic (As) and antimony (Sb) mobility in mining-impacted groundwater. However, the role of carbonate and silicate weathering and secondary mineral precipitation in this process remain unknown. A comprehensive geochemical study of groundwater was conducted in an Sb-mining area, Hunan, China, with samples collected from aquifers of the Xikuangshan Formation (D₃x), the Shetianqiao Formation (D₃s ), and the Lower Carboniferous Formation (C₁y). Results show co-enrichment of dissolved As and Sb with concentrations reaching up to 28.8 and 22.1 mg/L, respectively. The significant positive correlation between SO₄^2- and As or Sb concentrations, coupled with the similarity of δ³⁴S-SO₄ to δ³⁴S signature of sulfide minerals (e.g., arsenopyrite and stibnite), indicate sulfide mineral oxidation as the primary mobilization mechanism. The significantly higher SO₄^2- concentrations support more extensive sulfide mineral oxidation in the D₃s aquifer than those in the D₃x and C₁y aquifers, which was responsible for its significantly higher As and Sb concentrations. The SO₄^2-/Σ⁺ against Ca²⁺/Σ⁺ cross plot suggests that, in addition to sulfide mineral oxidation, silicate weathering was more prevalent in the D₃s groundwater, which may contribute to enhance As and Sb mobility. However, carbonate dissolution triggered by sulfide mineral oxidation dominated in the C₁y groundwater with significantly higher Ca²⁺/Σ⁺, favoring the precipitation of pharmacolite (CaHAsO₄:2H₂O) and Ca₂Sb₂O₇, which acted as important sinks for dissolved As and Sb. This study highlights that, in addition to sulfide mineral oxidation, the carbonate and silicate weathering and precipitation of As and Sb-bearing minerals are also pivotal in influencing the As and Sb mobility in groundwater from a mining area.