Abstract
Rare earth elements (REE) speciation in laterites are commonly estimated from sequential extraction procedure (SEP). In this study, the selectivity of a 5-steps SEP protocol was evaluated on individual synthetic Ce-material and applied to a well-characterized lateritic profile developing over a granite bedrock (Madagascar). The synthetic Ce-materials are representative of laterite minerals: Ce-doped oxyhydroxides and kaolinite, cerianite, Ce-rhabdophane and Ce-bastnäsite. The morphology, specific surface area, Ce sorption concentrations were thoroughly characterized prior to SEP. X-Ray absorption spectroscopy on synthetic material indicates that Ce(III) dominates apart for cerianite and Mn-oxide. The evaluation of the 5-steps SEP scheme shows that reagent selectivity is generally good, but the use of hydroxylamine hydrochloride is strongly discouraged. First, distinction between Ce sorbed at the surface of Mn- and amorphous or crystalline Fe-oxyhydroxides remains ambiguous. Second, Ce-rhabdophane (phosphate) and Ce-bastnäsite (carbonate) dissolve partially with this reagent. These experimental results combined with previous mineralogical and geochemical characterization were fundamental to interpret SEP results in the laterite profile from Madagascar. In the three laterite samples from the A-, B- and C-horizons, it is proposed that REE mainly distribute in authigenic phosphates and Fe- and (Mn-)oxyhydroxides. In the oxidized B-horizon, there is a significant LREE/HREE and Ce(IV)/Ce(III) fractionation between laterite minerals. While LREE-Ce is mainly concentrated in phosphates (alunite-jarosite supergroup), it is proposed that HREE and Ce(IV) uptake is dominated by oxyhydroxides. In the A- and C-horizons, the LREE and HREE speciations are similar, indicating no significant LREE/HREE fractionation in laterite minerals and organic matter. In all horizons, the REE ion exchangeable is negligible, although kaolinite is a main mineral of the A- and B-horizons.
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