The role of cassiterite controlling arsenic mobility in an abandoned stanniferous tailings impoundment at Llallagua, Bolivia

Abstract

The surface water contamination by potentially toxic elements (PTE) leached from mine tailings is a major environmental concern. However, the formation of insoluble solid phases can control the mobility of PTE, with subsequent decrease of the risk that tailings suppose to the environment. We characterized the tailings from a tin inactive mine in Llallagua, Bolivia in order to assess the risk for surface water quality. These tailings contain high concentrations of PTE, with up to 94,344mg/kg Fe, 9135mg/kg Sn, 4606mg/kg As, 1362mg/kg Cu, 1220mg/kg Zn, 955mg/kg Pb and 151mg/kg Cd. Oxidation of sulfide minerals in these tailings generates acid leachates (pH=2.5–3.5), rich in SO42− and dissolved PTE, thereby releasing contaminants to the surface waters. Nevertheless, the concentrations of dissolved Sn, As and Pb in acid leachates are low (Sn<0.01mg/L; As=0.25–2.55mg/L; Pb<0.05mg/L). This indicates that, for the most part, Sn, As and Pb are being retained by the solid phases in the impoundment, so that these elements are not reaching the surface waters. Fe-bearing cassiterite—an insoluble and weathering-resistant oxide mineral—is abundant in the studied tailing deposits; it should be the main solid phase controlling Sn and As mobility in the impoundment. Additionally, jarosite and plumbojarosite, identified among the secondary minerals, could also play an important role controlling the mobility of As and Pb. Taking into account (a) the low solubility constants of cassiterite (Ksp=10−64.2), jarosite (Ksp=10−11) and plumbojarosite (Ksp=10−28.66), and (b) the stability of these minerals under acidic conditions, we can conclude that they control the long-term fate of Sn, As and Pb in the studied tailings.