Sources and dispersion of antimony at the abandoned mine of Su Suergiu (SE Sardinia)

Antimony (Sb) pollution linked to present and past mining activities is a current environmental concern. Typical Sb concentrations in natural unpolluted waters are less than 1 μg/L, but can increase up to 103104 μg/L near mining and smelting sites. This study investigates the processes involved in the Sb release and dispersion in the abandoned Sb mine area of Su Suergiu (SE Sardinia). For this purpose, mineralogical (XRPD) and minerochemical (SEMEDS) analyses have been performed on outcropping rocks, mine wastes and foundry slags; moreover, the Sb contents in waters draining the mine area and surroundings have been determined. Waters flowing at Su Suergiu show slightly alkaline pH and oxidizing conditions, and their chemical composition is Ca(Na)sulfate. Waters of adits, spring and streams sampled in the mine area show variable Sb content, between 48 and 4,020 μg/L (median: 323 μg/L). The drainage waters collected downstream the foundry slag heaps reaches 104 μg/L of Sb concentration, indicating that the residues of Sb metallurgy, especially metallic Sb and Sb2O3 (valentinite/sénarmontite) dumped in the foundry slag heaps, are the main Sb pollution sources at Su Suergiu. Of particular interest is the presence of mopungite, NaSb(OH)6, that acts as a temporary, highly soluble sink for Sb. Waters draining the mine area flow into the Riu Ciurixedda stream, which in turn flows into the Flumendosa River, a major regional source of water for agricultural and domestic uses. After the Riu Ciurixedda confluence, the Sb concentration in the Flumendosa waters increases above drinking water limits set by the World Health Organization (20 μg/L) and European Community (5 μg/L). Results show that Sb(V), stable under oxidizing conditions as dissolved Sb(OH)6 in a wide range of pH, prevails over the less mobile Sb(III) in the studied waters, explaining Sb dispersion up to several kilometers downstream the mine area, where natural Sb attenuation occurred mainly by dilution.