Cr(VI) genesis and dynamics in Ferralsols developed from ultramafic rocks: The case of Niquelandia, Brazil
2013
Garnier, Jeremie | Quantin, Cecile | Guimaraes, Edi Mendes | Vantelon, Delphine | Montarges-Pelletier, Emmanuelle | Becquer, Thierry | IG GMP ICC Ctr ; Universidade de Brasilia = University of Brasilia [Brasília] (UnB) | Interactions et dynamique des environnements de surface (IDES) ; Université Paris-Sud - Paris 11 (UP11)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS) | Géosciences Paris Sud (GEOPS) ; Université Paris-Sud - Paris 11 (UP11)-Centre National de la Recherche Scientifique (CNRS) | Synchrotron SOLEIL (SSOLEIL) ; Centre National de la Recherche Scientifique (CNRS) | Laboratoire Environnement et Minéralurgie (LEM) ; Institut National Polytechnique de Lorraine (INPL)-Centre National de la Recherche Scientifique (CNRS) | Ecologie fonctionnelle et biogéochimie des sols et des agro-écosystèmes (UMR Eco&Sols) ; Centre de Coopération Internationale en Recherche Agronomique pour le Développement (Cirad)-Institut de Recherche pour le Développement (IRD)-Institut National de la Recherche Agronomique (INRA)-Centre international d'études supérieures en sciences agronomiques (Montpellier SupAgro)-Institut national d’études supérieures agronomiques de Montpellier (Montpellier SupAgro) | CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnologico, Brazil) [475124/2006-5]; French Ministry of National Education and Research
Despite high chromium concentrations in soils developed on ultramafic rocks, Cr availability is generally low, as Cr-bearing minerals are considered stable in supergene conditions. However, KH2PO4 extractions have shown high hexavalent chromium availability in Ferralsols developed on the ultramafic massif of Niquelandia (Brazil). A study combining mineralogical, geochemical, mu SXRF and XANES approaches was performed to assess the solid speciation of Cr and to understand its genesis. Chromium is mostly present in the trivalent form and is included in chromites and Fe-oxides. Nevertheless, a large amount of Cr(VI) is associated with the clay-sized fraction in the mineral horizons. Mn-oxides, the only natural Cr(III) oxidants, are also present in these horizons. Microscale investigations revealed the close association of Cr(VI) with Mn-oxides and strongly suggest that Mn-oxides can oxidize Cr(III) into Cr(VI). The XANES analyses show the occurrence of Cr(VI), which is removed by KH2PO4 extraction, demonstrating that the Cr(VI) is completely sorbed onto the soil matrix, i.e., Fe-oxides. Finally, the high mobility of Cr(VI), associated with the finest Fe-oxide particles mobilized during runoff following rainy events, may have harmful environmental consequences. This approach of combining direct and indirect observations allows us to characterize the total extractable Cr(VI) pool of these soils and to give key information on its mobility and localization.
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