Rare Earth Element adsorption experiments onto kaolinite in water, NaCl, Na2SO4, NaHCO3 solutions: Implications on ion-adsorption REE deposits formation

In ion adsorption REE deposits, REE are adsorbed onto clay minerals. Previous experimental studies indicate that adsorption depends on the chemical and physical properties of the clay, and pH and salinity of the solution. This work presents adsorption experiments using two natural, purified kaolins with contrasting particle sizes, exposed to different ligands in low and high ionic strengths, and different pH, to assess their influence in the adsorption of REE onto kaolinite. The results reveal that: (1) REE adsorption is rapid; (2) water/dilute nitric acid experiments gave optimum adsorption; (3) high I NaCl, Na2SO4 and NaHCO3 gave least adsorption; (4) REE and Y are more adsorbed onto kaolinite in water/dilute nitric acid at lower pH, but adsorption is higher at higher pH when NaCl is present; (5) high I allowed less adsorption; and (6) Sc and Th are more effectively adsorbed than REE and Y. The data suggests inhibition of adsorption by complex ion formation in solution at high I, and in the presence of strongly binding ligands (Cl-, SO42-, CO32-). Adsorption induces no REE fraction for low I solution in nitric acid, whereas enhanced chloride complex formation for the light (L) REE results in preferential adsorption of the heavy (H) REE. Sulphate inhibits adsorption; and carbonate, at low I, inhibits LREE adsorption relative to HREE, whilst at high I kaolinite may dissolve. Low crystallinity, weathering-derived kaolinite has an order of magnitude more adsorption than processed, hydrothermal kaolin, as a result of increased surface area and adsorption sites.