Minerals controlling arsenic and lead solubility in an abandoned gold minetailings

Numerous areas have been contaminated by heavy metals and metalloids due to industrial and mining activities. Studies investigating the behavior of su ch contaminants in the environment have identified speciation as a key fact or controlling their mobility, availability and toxicity. Here we character ize As- and Pb-bearing phases resulting from the oxidation of sulfide-rich tailings of a former gold mine (La Petite Faye, France) in order to assess the risk for water quality. Elements were first pre-concentrated by granulo metric fractionation (sedimentation in deionized water) and then investigat ed using X-ray diffraction and electron microprobe analyses. Two main As-Pb -bearing minerals were clearly identified: scorodite (FeAsO4 . 2H(2)O) and beudantite PbFe3(AsO4)(SO4)(OH)(6). Minor amounts of As and Pb were dissolv ed in deionized water during granulometric fractionation, indicating the po ssible presence of other soluble Pb-sulfates which could be some of the pri mary metastable products of sulfide oxidation. This dissolution also provid es information about the fate of these phases in the case of intensive leac hing of the tailings. Scorodite may not be considered as a relevant candida te for As on-site immobilization, because its solubility largely exceeds dr inking water standards whatever the pH. Since beudantite solubility has not yet been determined, an estimation of its solubility product was obtained using the Gibbs free energy of formation of plumbojarosite [Pb0.5Fe3(SO4)(2 )(OH)(6)]. This estimation suggests that beudantite should efficiently main tain low Pb concentration in waters. However, Pb dissolution in deionized w ater during the granulometric fractionation led to Pb concentrations much h igher than the French and US drinking water standards (2.4 x 10(-7) mol l(- 1)), which may be due to dissolution of the suspected metastable Pb-sulfate s. Accurate determination of beudantite solubility is now required to impro ve the Pb risk assessment on such polluted sites. (C) 2000 Elsevier Science S.A. All rights reserved.