Speciation of Am(III)/Eu(III) sorbed on γ-alumina: effect of metal ion concentration

Abstract

Abstract The present paper describes the speciation of Am(III)/Eu(III) sorbed on γ-alumina, and its evolution with changing metal ion concentration, studied using batch sorption experiment, time resolved fluorescence spectroscopy (TRFS), extended X-ray absorption fine structure (EXAFS) and surface Complexation modeling (SCM). Though numerous studies exist in the literature on the speciation of trivalent actinides and lanthanides on alumina surface, the mechanism of sorption at high metal ion concentrations is not yet fully understood. Batch sorption experiments of Am(III) on γ-alumina under varying condition of pH (3–10), ionic strength (0.005–0.1 M NaClO4) and metal ion concentration (10−7–10−4 M) were performed. Higher metal ion concentration was achieved by the addition of Eu(III) considering it as an analogue of Am(III). Time resolved fluorescence spectroscopy (TRFS) study of Eu(III) sorbed on γ-alumina at the metal ion concentration of 5.0 × 10−5 M was carried out over pH 4 to 7. TRFS showed the presence of two surface species, with distinctly different fluorescence decay life times. The shorter lifetime value and its changing pattern with pH indicate the surface species corresponding to this component to be monodentate species >AlOAm2+ and its hydrolyzed forms. The sorbed Eu3+ species corresponding to the longer lifetime value has 2–3 water molecules in its first coordination sphere and is multidentate in its binding on alumina surface. Extended X-ray absorption fine structure (EXAFS) measurement of Eu:γ-Al2O3 sorption systems at pH 6.18 and 7.15 corroborate the existence of two surface complexes. Further it suggests the edge sharing bidentate binding of Eu on AlO6 octahedra as the co-ordination mode of the higher lifetime component. Surface Complexation Modeling (SCM) of Am(III)/Eu(III) sorption onto γ-alumina at pH ≤ 7 has been carried out using these two surface species. 2-pK surface complexation modeling coupled with constant capacitance model for surface charge, revealed monodentate surface speciation as the predominant species at 10−7–10−6 M, while at 10−5 M, above pH 5.5, bidentate surface species starts to compete with monodentate speciation, and becomes the predominant surface species of Am(III)/Eu(III) on alumina surface at 10−4 M.