Trace elements in soils developed on Lotharingian limestones (Burgondy).

Abstract : Behaviour of trace elements (As, Ni, Zn, Pb, Cu and Co) in soils developed on Lotharingian (Upper Sinemurian) limestones, Pouilly-en-Auxois region, Burgondy (France), has been compared to that observed in the parental rock. Using XRD, SEM, EMPA and PIXE analyses, it is shown that trace elements are located within porous heterogeneous fluor-apatite nodules, which contain inclusions of goethite, quartz, and minor rutile. Within nodules, trace elements are not randomly distributed but associated with the goethite phase. From Eh-pH diagrams, under pH soil conditions (pH = 6), arsenic is certainly present as H2AsO4-, in solutions, i.e. with trivalent arsenic. Therefore the high arsenic concentrations, up to 8500 ppm, associated with goethite, can be due either to adsorption of the H2AsO4- species, to the precipitation of an As3+-rich phase, or to the unlikely simple Fe3+ - As3+ substitution in the goethite structure. Desorption experiments have been performed with a KH2PO4 solution on hand-picked goethite grains. This phosphate solution has been chosen to provoke phosphorus to arsenic exchange on goethite, owing to the well-known affinity of phosphorus with the goethite surface. Using a 1.33 M solution and a solid/solution weight ratio of 2.5, it is observed that less than 1The same Eh-pH considerations indicate that the other trace elements are present under the divalent state. The high concentrations observed (up to 5300 ppm for Zn, 2700 ppm for Ni), cannot be explained from adsorption considerations, since only less than 1Comparing the mineralogy as well as the major and trace elements contents of soil nodules, with those of the Lotharingian limestones indicate a probable inheritance rather than a dissolution-reprecipitation process
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Sylvain Drouin, Ary Bruand, Lydie Le Forestier, Jean-Louis Robert. Trace elements in soils developed on Lotharingian limestones (Burgondy).. Réunion des Sciences de la Terre Joint Earth Sciences Meeting, 2004, Strasbourg, France. ⟨hal-00079424⟩

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