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 feme oxide (HFO), commonly called fenihydrite, 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 (K-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 x 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 drop FeOLu(H2O)(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 drop Fe(O)(2)Lu(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) Angstrom in all samples. A second coordination shell of Fe neighboring atoms at a distance of (3.38 +/- 0.01) Angstrom is observed for sorption samples pH greater than or equal to 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.