Mobility and bioaccesibility of mine waste and cultivated soil in “La Preciosa II” Mine (South of Spain). 

Abstract

La Preciosa II is an abandoned copper-pyrite and chalcopyrite mine site located in the eastern part of Seville Province (South of Spain), close to Neogene materials of the Guadalquivir Basin (calcarenite and marl). Recently, a part of the mine wastes were covered with such carbonated rocks and building materials, as a reclamation purpose for agricultural activities such as growing citric trees. However, acid mine drainage is still leaching from the subsoil, discharging into a natural stream (neutral pH) and then into the Guadalquivir river. A mineralogical and geochemical study of mine wastes and cultivated soils was carried out to assess the environmental impact and the potential health risk.Several samples from the mine wastes and the cultivated soils were collected. Major and potentially toxic elements (PTE) were determined by X-ray fluorescence and the mineralogy by X-ray diffraction. Some samples were examined by scanning electron microscopy and one mine waste, and one soil samples were selected for sequential extractions and oral bioaccesibility (BARGE method).The cultivated soils were composed of quartz, calcite, feldspar, illite and traces of gypsum. The mine waste showed pyrite, quartz, magnetite, mica, amphibole, and secondary minerals such as jarosite and gypsum. The lack of calcite in mine wastes and some of the soil samples suggests that carbonate materials were completely dissolved to form gypsum, but no more neutralizing potential is present in the mine wastes.Very high concentrations of PTE were found in the mine wastes and soil samples. Some of them, such as Cu (up to 0.7 %), Zn (up to 0.2 %) and Ag (up to 50 mg/kg) could even be recovered, contributing to the circular economy. The high content in Pb (up to 0,8 %) occurred in samples with jarosite, highlighting the role of this mineral in the retention of Pb. Other very toxic elements such as As and Cd were also present.Sequential extractions showed a high mobility of Cd and Zn, and important fraction of Cu associated to sulphides, as well as Pb associated to crystalline iron oxy-hydroxides, mainly jarosite. The mobility of PTE in the soil sample was lower than in the mine wastes, showing that the amendment of carbonate rocks decreased the mobility of PTE. The oral bioaccesibility was also higher in mine wastes, but a fraction of Zn, Cu, Co and, partially, Pb is still bioaccessible in the soil sample studied.The addition of carbonates promotes the precipitation of secondary minerals and decrease the mobility and bioaccesibility of PTE. However, more efforts are necessary to remediate the mine waste dumps in la Preciosa II, focused to preserve the environment, avoid the transfer soil-plants, and consequently reduce the human health risks.