Evaluation of selectivity of sequential extraction procedure applied to REE speciation in laterite

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