Potentially toxic metals in ultramafic mining materials: Identification of the main bearing and reactive phases

International audience; Modeling the geochemical behaviour of metals in spoil materials is prerequisite to the rehabilitation of lateritic nickel mining sites to avoid environmental contamination. The global aim of this work was to assess the different parameters controlling the release of Ni and other trace metals (Co, Cu, Cr and Mn) from model materials generated by mining activities in nickeliferous laterites from Goaís State (Brazil). This work was undertaken as a first part in a geochemical modeling project and consisted in the characterisation of the bearing phases and sources of such metals in representative materials from the mine. Ores and spoils had similar mineralogical compositions: i) mainly smectites and talc in garnierites and ii) goethite and hematite in limonites; we therefore concentrated our analyses on the purest materials. Garnierite was richer in Ni and poorer in Cr than limonite. In the first one, the richest phase in Ni was smectite (Fe: 8.8 at.%; Al: 3.3 at.%; Mg: 1.8 at.%; Cr: 0.5 at.%; Ni: 1.2 at.%) whereas chromiferous spinels contained high concentrations of Cr (Fe: 9.6 at.%; Al: 17.6 at.%; Mg: 4.1 at.%; Cr: 17.6 at.%). In Limonite, Ni and Cr were mainly borne by goethite (Fe: 37.6 at.%; Al: 1.8 at.%; Cr: 0.2 at.%; Ni: 0.5 at.%) and chromiferous spinels for Cr. Fine microscopy and spectroscopy allowed us to observe the structure of the minerals in both samples as well as the metal distribution in these different mineral phases. We then focused on metal lability and partitioning in the different compartments revealed by the mineralogical study. In the garnierite, exchangeable Ni (10% of total Ni) was mainly located between the layers of smectite as outer sphere complexes, and was thus easily available. Chromium, either located as octahedral or tetrahedral substitution in the smectites of the garnierite, or sequestered in chromiferous spinel lattices, was poorly available in both cases. In the typical limonite, both Ni and Cr were part of the goethite lattice but most of the Cr was associated with chromiferous spinel, which could be a primary source of Cr(III). The mobility of the Ni and Cr found in goethite was low. However, limonite presented very high exchangeable Cr(VI) contents, (2% of total Cr) in the form of inner-sphere complexes at the goethite surface. Cr(VI) is probably formed through Cr(III) oxidation by Mn oxides. Now that the reactive phases are identified and characterised, further work will model the reactivity of model bearing phases of Ni and Cr and compare the geochemical simulation with actual mobility data.