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

Abstract

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.

Sorption of Eu(III) onto titanium dioxide: Measurements and modeling

“…In the experiments with 0.1 g L À1 TiO 2 and [Eu] or [Lu] = 1.1 Â 10 À6 mol L À1 and 1.1 Â 10 À7 mol L À1 (squares and circles in Fig. 3), a clear increase in Eu/Lu sorption from almost 0 to about 100% within a range of about 2 pH units is observed, similar to previous studies on metal ion sorption onto oxide surface [6,13,23,24,28,47,48]. Furthermore, the sorption behavior of the two lanthanides, appears nearly identical, allowing the use of one or the other for subsequent studies.…”

“…No attempt to account for the asymmetry of the shell was included according to its moderate character. The interatomic distances found are significantly shorter than those reported for aquo ions, 2.31 Å for Lu [48,50] and 2.43 Å for Eu [51]. This fact together with the insignificant ionic strength effect on the sorption (see Section 2.2.1) both may be regarded as an indication of the formation of inner-sphere surface complexes and Table 4 for details.…”

Lanthanoid behaviour in an acidic landscape

“…Surface complexation constants were obtained from the Lu data set of Dardenne et al (2001) and recalculated to the other lanthanoids by use of the LFER of Dzombak and Morel (1990), see Electronic annex. The concentration of colloidal ferrihydrite was calculated by allowing precipitation of ferrihydrite with a log K s of 2.69 at 25°C in the speciation simulations (a reaction enthalpy of À100.4 kJ/mol was used for recalculations to other temperatures).…”

Sorption of Eu(III)/Cm(III) on Ca-montmorillonite and Na-illite. Part 1: Batch sorption and time-resolved laser fluorescence spectroscopy experiments

“…The reported M-O-distance of 2.47 to 2.50 Å lies in the range of those measured for the aquo complex (2.494 Å). The fact that a second coordination shell of Si/Al atoms at a larger distance than the oxygen shell is not resolved in the EXAFS has been interpreted as indication for monodentate surface complex formation (Dardenne et al, 2001;Stumpf et al, 2004a). Am-Al/Si distances are assumed to be shorter for bidentate than for monodentate complexes and therefore easier to detect by EXAFS.…”