Identification and characterization of sorbed lutetium species on 2-line ferrihydrite by sorption data modeling, TRLFS and EXAFS

The Lu(III) sorbed species onto synthetic hydrous ferric oxide (HFO), commonly called ferrihydrite, has been identified. Characterization of the synthetic 2-line HFO shows that its synthesis is reproducible. Potentiometric titration of freshly synthesized HFO, modeled using the constant capacity model (κ₁=0.5 F/m²) in the FITEQL code, yields a specific surface area S_a of 360±35 m²/g (N₂-BET), a site density N_d of 2.86 sites/nm² (concentration of hydroxyl groups, N_s=1.71×10^-3 mol sites/g HFO), and acidity constants pK_a1}^int=6.37 and pK_a2}^int=9.25.

Evaluation of chemical sorption data reveals the presence of two different Lu surface sorbed species, dependent on pH; a monodendate species forms at low pH and a polydentate species at pH>5. Satisfactory fits to the sorption data are obtained using a combination of monodentate and bidentate surface species. The combination of species is chosen, based on extended X-ray absorption fine structure (EXAFS) results. The sorption constants obtained from these fits are pK_s=-1.89(±0.1) and pK_s=-1.69(±0.1) for the monodentate species ≡FeOLu(H₂O)₅²⁺ for fits to the pH edge and to the isotherm at pH°5.9, respectively. A value of pK_s=3.69(±0.01) is found for the bidentate species ≡ Fe(O)₂Lu(H₂O)₅⁺ for both fits. EXAFS analysis of sorption samples prepared at 4.5<pH<8 shows that Lu is surrounded by a single first shell of 7±1 oxygen atoms, at a distance of (2.30±0.01) Â in all samples. A second coordination shell of Fe neighboring atoms at a distance of (3.38±0.01) Â is observed for sorption samples pH≥5.5. This distance is associated with the formation of a bidendate complex with bonding via edge sharing to iron octahedra. The samples prepared at pH<5.1 show no Fe shell, as expected for monodentate coordination. No evidence for surface precipitation and no noticeable difference between wet paste and dried powder samples is found.