Abstract
The clay minerals, kaolinite and halloysite are dominant hosts for rare earth elements (REEs) in the weathered crust elution-deposited REE deposits. However, the accumulation of REEs on kaolinite and halloysite has not been systematically compared. In this study, the adsorption of complete REE series on kaolinite and halloysite was investigated under different pH levels and ionic strengths, which was linked to the enrichment and fractionation of REEs within the clay fraction of a typical weathered crust elution-deposited REE deposit. At low ionic strength, the adsorption of REEs on kaolinite and halloysite increased with increase in pH, with some noticeable fluctuation observed on halloysite at high pH. All the REEs were adsorbed to a similar extent without apparent fractionation, except for the slight enrichment of heavy REEs (HREEs) at high pH. At high ionic strength, REE adsorption exhibited a linear increase with the increase in pH, particularly at high pH, with HREEs being preferentially adsorbed. Compared to halloysite, kaolinite possessed a higher specific surface area (SSA) normalized adsorption capacity towards REEs. These observed adsorption characteristics could possibly explain the distribution of REEs in the clay fraction along the REE deposit. The decrease of ion-exchangeable REE content with depth was significant for kaolinite, while the REE fractionation was ascribed to the selective adsorption of HREEs on both kaolinite and halloysite. The enrichment and fractionation mechanism of REEs on kaolinite and halloysite were also discussed in terms of the surface chemistry and morphology of the clay minerals and the variations of chemical properties across the REE group.
Published Version
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