Mobility of potentially toxic metals and metalloids in soils impacted by tailings with distinct oxidation stages.

To develop effective methods to solve pollution problems caused by mine tailings in soils, the environmental availability of potentially toxic metals and metalloids (PTMM) must be adequately assessed. This involves detailed chemical and mineralogical studies in the tailing piles and impacted soils. Research carried out in tailings with various oxidation degrees in a semi-arid historical Mexican mining zone revealed differences in the mobility of As, Cd, Pb, Zn, Cu, Mn in the respective soils that were impacted by rain wash-off, aerial transport, and acid mine drainage. Sequential extractions showed that most of the PTMM were in the Mn and Fe oxyhydroxides fractions in soils impacted by oxidized tailings while a predominance in the proportion present in the organic and sulfide fractions was determined in those receiving the influence of less oxidized deposits. Zn and Cd presented the highest geoavailability in oxidized tailings. Although sulfide minerals oxidation as pyrite and arsenopyrite produce acid mine drainage, the abundance of calcite in the geology of the area increases the pH and promotes the formation of iron oxyhydroxides that retain part of the PTMM through sorption. On the other hand, although As has low mobility in soils (less than 3 % in the soluble fraction), its high total concentrations (up to 51,500 mg/kg), implies also an environmental hazard. A modification in the climate conditions of the area, such as heavy storms that may be promoted by climate change, would increase the tailings transport through wash-off,  in addition to the mobilization and soil infiltration mainly of the most geoavailable elements by dissolution.