Effect of phosphate on the sorption and fractionation of rare earth ions on kaolinite surface and its geochemical significance

Kaolinite (Kln) is ubiquitous in weathering crust, rivers, groundwater, and oceans, and plays an important role in controlling the migration of rare earth elements (REEs). Phosphate forms strong complexes with REEs in aqueous environments, but it remains unclear whether phosphate influences the sorption and fractionation of REEs onto Kln. In this study, the characteristics of the sorption of phosphate, REEs, and phosphate plus REEs onto Kln and the mechanisms of these processes were investigated through batch sorption experiments and model-based computation. Under both weakly acidic and neutral conditions, the sorption isotherms of phosphate and REEs, respectively, were well fitted with a Langmuir model, suggesting that they underwent monolayer sorption onto Kln. The sorption capacity of Kln increased with pH. At pH 4.0, the sorption mechanism of phosphate involved electrostatic attraction and surface complexation, whereas that of REEs mainly involved electrostatic attraction without apparent fractionation. Although pre-sorption of phosphate generated more binding sites on Kln, most REEs were sorbed via electrostatic attraction, which did not affect the fractionation of REEs into light REEs (LREEs) and heavy REEs (HREEs). At pH 6.5, the sorption mechanism of phosphate was only surface complexation, whereas that of REEs involved electrostatic attraction and surface complexation, with slight fractionation. Pre-sorbed phosphate formed ternary surface complexes with REEs, thereby increasing sorption of HREEs, which had a greater complexation ability than LREEs. The above-described results account for the reported effect of phosphate on the enrichment and fractionation of REEs in the supergene environment, and thus improve our understanding of the transfer characteristics of REEs on Earth's surface.