Rare earth element adsorption experiments onto kaolinite in water, NaCl, Na 2 SO 4 and NaHCO 3 solutions: Implications on ion-adsorption REE deposits formation

In ion adsorption rare earth element (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 on the pH and salinity of the solution. This work presents adsorption experiments using two natural, purified kaolins with contrasting particle sizes exposed to different ligands at low and high ionic strengths (I) and at different pH levels to assess their influence on 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 demonstrated the least adsorption; (4) REE and yttrium 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) scandium and thorium are more effectively adsorbed than REE and yttrium. The data suggest 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 solutions in nitric acid, whereas enhanced chloride complex formation for light REE results in the preferential adsorption of heavy REE. sulphate inhibits adsorption, and carbonate at low I inhibits light REE adsorption relative to heavy REE, whereas at high I kaolinite may dissolve. Low-crystallinity, weathering-derived kaolinite demonstrates an order of magnitude more adsorption than processed, hydrothermal kaolin as a result of its increased surface area and adsorption sites.