Arsenic attenuation in tailings at a former Cu-W-As mine, SW Finland

Nearly half a century after mine closure, release of As from the Ylojarvi Cu-W-As mine tailings in ground-water and surface water run-off was observed. Investigations by scanning electron microscopy (SEM), electron microprobe analysis (EMPA), synchrotron-based micro-X-ray diffraction (mu-XRD), micro-X-ray absorption near edge structure (mu-XANES) and micro-extended X-ray absorption fine structure (mu-EXAFS) spectroscopy, and a sequential extraction procedure were performed to assess As attenuation mechanisms in the vadose zone of this tailings deposit. Results of SEM, EMPA, and sequential extractions indicated that the precipitation of As bearing Fe(III) (oxy) hydroxides (up to 18.4 wt.% As2O5) and Fe(III) arsenates were important secondary controls on As mobility. The mu-XRD, mu-XANES and mu-EXAFS analyses suggested that these phases correspond to poorly crystalline and disordered As-bearing precipitates, including arsenical ferrihydrite, scorodite, kankite, and hydrous ferric arsenate (HFA). The pH within 200 cm of the tailings surface averaged 5.7, conditions which favor the precipitation of ferrihydrite. Poorly crystalline Fe(III) arsenates are potentially unstable over time, and their transformation to ferrihydrite, which contributes to As uptake, has potential to increase the As adsorption capacity of the tailings. Arsenic mobility in tailings pore water at the Ylojarvi mine will depend on continued arsenopyrite oxidation, dissolution or transformation of secondary Fe(III) arsenates, and the As adsorption capacity of Fe(III) (oxy) hydroxides within this tailings deposit. (C) 2012 Elsevier Ltd. All rights reserved.