Co-adsorption of phosphate and rare earth elements (REEs) on goethite induce middle REE-enriched fractionation

As prominent geochemical tracers, rare earth elements (REEs) can be scavenged by Fe (hydr)oxides that are ubiquitous in supergene environments, profoundly altering the enrichment and fractionation behaviors of REEs. In light of the particle-reactive nature, high affinity for adsorption onto Fe (hydr)oxides, as well as strong complexation with REEs, phosphate probably affects the mobility and fractionation of REEs on Fe (hydr)oxides. This issue has yet been poorly understood. Herein, the co-adsorption of phosphate and REEs on goethite was studied at different phosphate concentrations and pH. The batch adsorption results show that phosphate shifted the pH adsorption edge of REEs on goethite to lower values, and thus enhanced the REE adsorption. Such positive effect became more pronounced with the increase of phosphate concentration. Interestingly, the presence of adsorbed phosphate altered the fractionation patterns of REEs, from heavy REE (HREE, Ho-Lu)-enriched pattern to a middle REE (MREE, Sm-Dy)-enriched one, which was intensified with the increase of phosphate concentration, especially at acidic pH. The adsorption geometries and energies were further investigated through in situ attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) combined with two-dimensional correlation analysis (2D-COS), density functional theory (DFT) and surface complexation model (SCM). The enhancement of REE adsorption by phosphate is ascribed to the formation of ternary inner-sphere complexes, where phosphate served as a bridge between REEs and goethite surface. At pH 5, phosphate formed mono-protonated bidentate binuclear phosphate complexes (Fe₂O₂POOH, MBB). Then, REEs (e.g., Sm) were bonded to the oxygen atom of phosphate by displacing an H atom in the MBB phosphate complexes, and ultimately forming monodentate ternary complexes (MT, Fe₂O₂POOSm). But at pH 7, the MT complexes were initially formed and then transformed into bidentate ternary complexes (Fe₂O₂PO₂Sm, BT), where Sm atom was concurrently bonded to two oxygen atoms in the phosphate complexes. These adsorption configurations were verified by DFT computations, as the adsorbed phosphate decreased the adsorption energy of REEs on goethite to form ternary complexes. Moreover, MREEs (e.g., Sm) formed the more stable ternary complexes than light REEs (LREEs, e.g., La) and HREEs (e.g., Lu). Thus, the formation of ternary complexes accounts for the phosphate-induced enrichment of MREEs. This study elucidates the molecular-level mechanisms governing the co-adsorption of phosphate and REEs on goethite and holds significant geochemical implications, including a novel perspective in understanding the enrichment patterns of MREEs in natural terrestrial waters.