ABSTRACT
In the present study, the sorption of europium and lutetium onto titanium dioxide from aqueous solutions is presented, as a function of pH, ionic strength and concentration. An acid base model for the titanium dioxide surface was determined from potentiometric titrations and zeta-potential measurements. The common intersection point of potentiometric titrations coincided with the isoelectric point from electrokinetic experiments, resulting in a pristine point of zero charge of about 6.1. The experimental data were in agreement with previously published results and a previously published MUSIC-type model was used as the basis to model the acid-base behavior. Comparison of europium and lutetium showed no difference in the adsorption behavior. Furthermore, no difference was observed both in uptake and spectroscopic studies whether carbonate was absent or present. The absence of a noticeable effect of the ionic strength on the adsorption behavior was indicative of strong binding. EXAFS revealed rough conservation of the coordination with 9-8 water and surface hydroxyl groups upon sorption. EXAFS results suggested the existence of different metal-oxygen distances, more varied than that observed for the respective aquo complex and thus indicative for inner-sphere surface complexation. A clear differentiation of surface complexation denticity was not possible based on spectroscopic data. A multisite surface complexation model approach was applied by assuming monodentate and multidentate binding to describe the trivalent metal uptake data. It is conceivable that mono- and multidentate species contribute to lanthanide sorption to titanium dioxide. In other words a distribution of states occurs in cation surface complexation reactions.
ABSTRACT
Three samples of calcite homogeneously doped with Eu(3+) were synthesized in a mixed-flow reactor. By means of selective excitation of the 5D0-->7F0 transition at low temperatures (T<20 K), three different Eu(3+) species (species A, B, and C, respectively) could be discriminated. For each one, the emission spectrum and lifetime were obtained after selective excitation of the single species. On the basis of these data, species C could be identified as Eu(3+) incorporated into the calcite lattice on the (nearly) octahedral Ca(2+) site. Species B was also identified as Eu(3+) incorporated into the calcite lattice, but the ligand field shows a much weaker symmetry. Species A, however, is not incorporated into the crystal's bulk, having 1-2 H(2)O ligands left in its first coordination sphere and showing very little symmetry, and is considered as Eu(3+) adsorbed onto the calcite surface. The emission spectra of species C for Eu:calcite grown in the presence of Na(+) were found to differ from those of Eu:calcite synthesized in the presence of K(+). The latter revealed a strong distortion in site symmetry, which was not observed in the samples grown in Na(+) solutions. This finding provides spectroscopic evidence in favor of an incorporation mechanism based on the charge-balanced coupled substitution of Na(+)+Eu(3+)<-->2Ca(2+).
ABSTRACT
The surface sorption of Cm(III) onto aqueous suspensions of alumina is investigated by time-resolved laser fluorescence spectroscopy (TRLFS). The experiment is performed under an Ar atmosphere at an ionic strength of 0.1 M NaClO(4). The pH is varied between 2 and 10 and the metal ion concentration between 2.7x10(-8) and 4.5x10(-5) mol/L. With increasing pH, two Cm(III)-alumina surface species are identified which are attributed to identical withAl-O-Cm(2+)(H(2)O)(5) and identical withAl-O-Cm(+)(OH)(H(2)O)(4). The two curium-alumina surface complexes are characterized by their emission spectra (peak maxima at 601.2 nm and 603.3 nm, respectively) and fluorescence emission lifetime (both 110&mgr;s). In the concentration range investigated, the surface complex formation is not dependent on the metal ion concentration but only on the pH. Additionally, the concentration ratio of the two surface species is found to be independent of the metal ion concentration. No spectroscopic evidence for the presence of "strong" and "weak" sites can be found at different surface coverages. Copyright 2001 Academic Press.
