The different effects of sulfate on the adsorption of REEs on kaolinite and ferrihydrite

Ion-adsorption rare earth deposits (IAREDs) are important source for rare earth elements (REEs). The transportation, accumulation, fractionation, and leaching of REEs in IAREDs are mainly controlled by the interactions between REEs and mineral geosorbents. The coexisting ions can significantly affect the mineral-REEs interactions, which, however, are not currently well studied. This work investigated the effects of sulfate (SO 4 2− ) on the adsorption of REEs on kaolinite and ferrihydrite, two typical mineral geosorbents in IAREDs with distinct surface properties. The obtained results indicate that, with increasing SO 4 2− concentration, the adsorption of REEs is hardly changed on kaolinite but remarkably enhanced on ferrihydrite. Additionally, SO 4 2− can particularly improve the adsorption of middle REEs on ferrihydrite. The in situ ATR-FTIR spectra results, coupled with 2D correlation spectroscopic analysis, indicate that the promoted adsorption of REEs by SO 4 2− on Fh is attributed to both the electrostatic attraction and the formation of Fe-REE-SO 4 ternary complexes. However, the negatively charged kaolinite can hardly adsorb SO 4 2− because of the strong electrostatic repulsion between them. Desorption experiments reveal that REEs can be more efficiently desorbed by NH 4 Cl from kaolinite than from ferrihydrite. The obtained results suggest that coexisting oxyanions have profound effects on the mineral-REEs interactions, which should be taken into consideration while exploring the transportation, accumulation, and fractionation of REEs in IARED.