Catherine Neel

Minerals controlling arsenic and lead solubility in an abandoned gold mine tailings

Numerous areas have been contaminated by heavy metals and metalloids due to industrial and mining activities. Studies investigating the behavior of such contaminants in the environment have identified speciation as a key factor controlling their mobility, availability and toxicity. Here we characterize 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 granulometric fractionation (sedimentation in deionized water) and then investigated using X-ray diffraction and electron microprobe analyses. Two main As-Pb-bearing minerals were clearly identified: scorodite (FeAsO4 x 2H2O) and beudantite PbFe3(AsO4)(SO4)(OH)6. Minor amounts of As and Pb were dissolved in deionized water during granulometric fractionation, indicating the possible presence of other soluble Pb-sulfates which could be some of the primary metastable products of sulfide oxidation. This dissolution also provides information about the fate of these phases in the case of intensive leaching of the tailings. Scorodite may not be considered as a relevant candidate for As on-site immobilization, because its solubility largely exceeds drinking 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 maintain low Pb concentration in waters. However, Pb dissolution in deionized water during the granulometric fractionation led to Pb concentrations much higher 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-sulfates. Accurate determination of beudantite solubility is now required to improve the Pb risk assessment on such polluted sites.

Arsenic in iron cements developed within tailings of a former metalliferous mine—Enguiales, Aveyron, France

Abstract Arsenopyrite-rich waste from a former metalliferous mine were spread out over the sloping side of a deep valley after processing. Over the past 30 a, they have been subjected to rainfall and acid water originating from the abandoned mine galleries. This intensive leaching has led to the formation of thin layers of As–Fe crusts on the tailings surface acting as a cement. X-ray diffraction and SEM coupled with EDS determined that jarosite was present in all mineral samples and could contain a small amount of As (∼5.7 wt.%). In addition EMPA and Raman microspectroscopy characterised the presence of amorphous As(V) Fe hydrates as well as rare arsenate minerals (e.g. scorodite). Raman microspectroscopy in particular identified a preponderance of goethite or hematite within the mineral framework of the tailings materials that is likely to sorb recalcitrant As species. The characterisation of the components of the tailings enable the identification of their evolution, shows the progressive decrease of their As-content and emphasises the consequences of the temporary trapping of As in the very acidic and oxidising conditions prevailing in such environments. Resinous amorphous material was identified as the richest in As with As ∼17.1 wt.%. This material evolved toward more crystallised phases (e.g. goethite, jarosite) which contained less As (3.2 wt.%

Solid speciation and mobility of potentially toxic elements from natural and contaminated soils: A combined approach

The study area (Szklary Massif, SW Poland) comprises three sites of different soil provenance: (1) natural serpentine Cambisol, (2) anthroposol situated on waste dump and (3) cultivated Inceptisol developed on glacial tills next to the dump. Potentially toxic elements (PTE) have either lithogenic or anthropogenic origins in these sites. The chemical partitioning of Co, Cr, Cu, Ni, Pb and Zn among solid forms was determined by sequential extractions coupled with direct mineral identifications (SEM, electron microprobe analysis - EMPA, and XRD). Examination of solid residues after several extraction steps was conducted in order to discuss the indirect speciation obtained by the extraction method. Total concentrations of PTE having anthropogenic origin greatly exceed those of lithogenic origin. Mobility of studied PTE is variable in the different environments except for Cr which is always mostly found in residual fractions of extractions. Cu and Pb are more mobile than Cr and Co in all soils. Zn is more stable (Cu>Pb>Ni>Co>Zn>>Cr) in the serpentine soil and cultivated epipedon (Pb>Cu>Zn>Ni>Co>>Cr) than in the anthroposol (Zn>CuPb>Ni>Co>>Cr). PTE of lithogenic origin are generally less mobile than those from anthropogenic origin except Ni which is more mobile in the serpentine soil. Nonetheless, mineral forms of metals better determine their mobility than metal origin. Identification by direct methods of the PTE mineral form was not possible for metals present at low concentrations (Cu, Pb). However, direct mineralogical examinations of the solid residues of several extractions steps improved the assessment of the PTE solid speciation and mobility, particularly for Cr, Ni and Zn.