Antimony contamination sources and alteration pathways of Sb mineral phases in an abandoned mining area: The role of secondary mopungite [NaSb(OH)6]

Antimony pollution caused by mining activities is a current environmental concern. This study investigates the processes involved in the Sb release and mobility in the abandoned Sb mine of Su Suergiu (SE Sardinia, Italy). Analyses of outcropping rocks, mine wastes and smelting slags by means of X-Ray Powder Diffraction, Scanning Electron Microscopy - Energy Dispersive Spectroscopy and Electron Microprobe – Wavelength Dispersive Spectroscopy provided mineralogical and compositional data which contributed to the discussion about the oxidation pathways of Sb phases. The main Sb sources are metallic Sb and Sb₂O₃ (valentinite/senarmontite), dumped in the smelting slag heaps as residues of metallurgical processes, and primary stibnite (Sb₂S₃) found in natural outcrops and mine wastes. These minerals, subjected to weathering processes, release Sb in solution where it is oxidized and remains as dissolved ${Sb\left(OH\right)}_6^{-}$. Carbonates and Na phases, like hydrate NaAl-silicate derived from metallurgical processes, influence the geochemical equilibria of the smelting slag heaps, where the precipitation of the rare mopungite, Na[Sb(OH)₆], has been observed. At Su Suergiu, mopungite originates from a dissolution-precipitation process as the last forming mineral of the oxidation pathways, limiting the Sb mobility by bonding the ${Sb\left(OH\right)}_6^{-}$ in solution. Among the detected Sb secondary phases (e.g., Sb-oxides, FeSb-oxides, etc.), mopungite is the prevalent Sb binder although it acts as a temporary sink because its stability is influenced by the hydrological regime, its solubility, and the water physicochemical parameters. Secondary Sb-bearing minerals can control the dispersion of Sb in contaminated area. At Su Suergiu the role of Fe-bearing compounds on Sb mobility is subordinate to that of mopungite due to the specific geochemical conditions related to the metallurgical activities. The relevance of this study arises from the known toxicity of Sb and from its worldwide diffuse mining, that results in the widespread occurrence of Na–Sb-rich residues produced by Sb smelting plants.