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 (κ 1 = 0.5 F/m 2 ) in the FITEQL code, yields a specific surface area S a of 360 ± 35 m 2 /g (N 2 -BET), a site density N d of 2.86 sites/nm 2 (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(H 2 O) 5 2+ 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) 2 Lu(H 2 O) 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) Å in all samples. A second coordination shell of Fe neighboring atoms at a distance of (3.38 ± 0.01) Å 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.

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