Sorption of Am(III) and Eu(III) onto gamma-alumina: experiment and modelling

The present paper describes the surface complexation behaviour of trivalent metal ions, Am(III) and Eu(III), on well characterised gamma -alumina. Experiments are conducted at different pH (4-8) and ionic strength (0.001-0.1 M NaClO4) either in the presence or absence of CO2. By varying the metal ion concentration from 10(-9) to 10(-4) mol/L, the sorption isotherm is established under each given experimental condition. Different surface complexation models are applied to the experimental results to interpret and appraise the:surface sorption processes under different experimental conditions. A comparison of the present results is made with the Eu(III) sorption onto the hematite mineral surface investigated previously. It has been shown that sorption properties of hematite and gamma -alumina seem to be quite similar. For both systems, a good agreement is found between experimental data and modelling, once using the two site surface complexation model and the same complexation constants for the respective monodentate surface complex. The Eu(III) sorption reaction is additionally studied in-situ by time resolved laser fluorescence spectroscopy (TRLFS). Formation of inner-sphere complexes can be deduced from the emission spectra. The continuous increase of the fluorescence life time with increasing pH starting from pH = 5.0 indicates that surface complexation is accompanied by a decreasing number of hydration water in the first coordination sphere.