[en] Interaction of trivalent lanthanides and Cm(III) with humic acid (HA) (l0mg/L) is studied as a function of metal ion concentration (1 x 10⁸ to 5 x 10^-5 mol/L) and pH (2.5 to 7.0) in 0.01 mol/L NaClO₄. Time resolved laser fluorescence spectroscopy (TRLFS) reveals a slight blue-shift/broadening of the fluorescence peak position for the Cm(III)-HA complex combined with an enhanced contribution of a short-lived fluorescence component with increasing metal ion loading and decreasing pH. Fluorescence decay for Cm(III)-HA is non-monoexponential under all investigated conditions and lifetimes can be described by assuming a short lived (τ = 61 μs) and a longer lived contribution (τ = 142 μs). These findings are explained by the existence of a certain heterogeneity in binding sites inducing slightly variable ligand-field splitting in the Cm(III)-HA complex. Batch experiments with a Eu(III)-HA solution contacted with a chelating resin reveal a decrease of the solid/liquid distribution coefficient by decreasing the occupancy of HA ligand sites from 4% to 0.2%. This observation indicates that complexation constants for the Eu(III)-HA complex vary with metal loading. Differences in related constants are, however, smaller than one order of magnitude. Variations in fluorescence lifetimes are interpreted as a consequence of increasing HA agglomeration at increasing metal loading and decreasing pH, thus leading to an enhanced local density of chromophoric groups close to the Cm(III) ion favouring non-radiative energy transition. HA agglomeration at low pH and increasing metal ion loading is well known from previous studies using flow-field flow fractionation (FFF) coupled to ICP-mass spectrometry and UV-Vis spectrophotometry. Our experimental results demonstrate the dynamic properties of metal-HA complexes depending on chemical conditions. (orig.)

[en] For the long-term performance assessment of nuclear waste repositories, knowledge concerning the interactions of actinide ions with mineral surfaces is imperative. The mobility of released radionuclides is strongly dependent on the sorption/desorption processes at mineral surfaces. Therefore, it is necessary to characterise the surface species formed and to elucidate the reaction mechanisms involved. Insight into the sorption mechanisms and identification of surface species is of cardinal importance for a reliable predictive modeling of sorption reactions. Time resolved laser fluorescence spectroscopy (TRLFS) allows due to its high fluorescence yield speciation studies of Cm(III) in the nanomolar concentration range. Exemplary the results of TRLFS studies for the interaction of Cm(III) with three different minerals are presented. (orig.)

[en] The geochemical approach for the safety assessment of nuclear waste disposal is focused on actinide elements, which will dominate the radiotoxicity of the nuclear waste over long time periods. Discussed are chemical reactions, which may mobilize or immobilize the actinides under the geochemical conditions in the near and far field of a repository. Examples are given for the formation of ternary hydroxo carbonate complexes of tetravalent actinides and for interface reactions of trivalent actinides with mineral surfaces. This work will provide thermodynamic data as well as process understanding of reactions relevant for the long-term prediction of actinide migration in the geosphere. (orig.)

[en] The aim of the present work is to investigate the dissociation kinetics of actinide ions and their chemical homologues by the Chelex batch technique. A chelating cation exchanger is used to scavenge dissociated ions of (1) Eu(III) complexed with purified humic acid as a function of the equilibration time; and (2) actinides and actinide homologues which are constituents of natural non-purified humic colloids in groundwater. The groundwater samples originate from the Gorleben aquifer system. Dissociation rates are measured and used to compare the dissociation behaviour of 'laboratory' and 'real' systems. (orig.)

[en] Photodynamic processes of the Cm³⁺ ion complexed with four different fulvic acids are discussed. The four different fulvic acids of different origins have different optical properties, reflected in the fluorescence intensity of the complexed Cm(III) fulvate. The fluorescence intensity of fulvate complexed curium ion is enhanced relative to the non-complexed aqueous Cm³⁺ ion. This enhanced fluorescence intensity is shown to be related to the fluorescence intensities of the different fulvic acids. Contrary to this, the fluorescence intensity from direct excitation of the fulvate complexed Cm(III) show no significant trend with the fluorescence properties of the fulvic acids. The study is of somewhat preliminary nature because of the small number of substances used and the complexity of the involved photodynamic processes. In order to delineate individual process contributions, organic model ligands will need to be studied. (orig.)

[en] Mechanistic insight into radionuclide interaction with mineral surfaces is of great importance for performance assessment of nuclear waste repositories. For a fundamental understanding of sorption/incorporation processes and a reliable thermodynamic description, e.g. by surface complexation models, all relevant processes have to be identified and quantified. In the present study, the Cm(III) sorption onto different crystal planes (018), (104), (012), (110), (001) of sapphire single crystals (area: 1 cm²) is investigated at low Cm(III) concentrations and at pH 4.5 and 5.1 by time resolved laser fluorescence spectroscopy (TRLFS), XPS, α-spectrometry and autoradiography. (orig.)

[en] Powellite (CaMoO₄) is one of the various crystalline secondary alteration phases which form during the corrosion of high level waste (HLW) glasses. Due to their structural variability, powellite can accommodate considerable chemical substitutions including trivalent actinides. Batch adsorption and coprecipitation experiments in mixed flow reactors have been used to study quantitatively the uptake of Cm(III) and Eu(III) (as a nonradioactive chemical homologue for trivalent actinides) from aqueous solution under repository relevant conditions. A metal ion concentration-independent K_d of 1200 ml/g has been determined for adsorption above pH 4 up to 3 μmol/L Eu(III). The partition coefficient for coprecipitation varies between 10 and 800 depending on the precipitation rate. Time-resolved laser fluorescence spectroscopy has been used to study the aqueous complexation of Cm(III)/Eu(III) and MoO₄^2- as well as their incorporation into the powellite crystal lattice during coprecipitation. A red shift of the Cm(III) fluorescence emission of the transition indicates the formation of aqueous CmMoO₄⁺ (598 nm) and Cm(MoO₄)₂^2- (601 nm) complexes. The incorporation of Cm(III) is indicated by a significant red shift. The corresponding lifetime of 165 μs indicates quench effects. A similar behaviour has been observed for Eu(III). The life time of the ⁵D₀-⁷F₂ transition is 350 μs for an incorporated Eu(III) species, suggesting also intrinsic quench effects, due to the local crystal field or impurities. (orig.)

[en] Column experiments were carried out to investigate the influence of humic colloids on subsurface uranium migration. The columns were packed with well-characterized aeolian quartz sand and equilibrated with groundwater rich in humic colloids (dissolved organic carbon (DOC): 30 mg dm^-3). U migration was studied under an Ar/1% CO₂ gas atmosphere as a function of the migration time, which was controlled by the flow velocity or the column length. In addition, the contact time of U with groundwater prior to introduction into a column was varied. U(VI) was found to be the dominant oxidation state in the spiked groundwater. The breakthrough curves indicate that U was transported as a humic colloid-borne species with a velocity up to 5% faster than the mean groundwater flow. The fraction of humic colloid-borne species increases with increasing prior contact time and also with decreasing migration time. The migration behavior was attributed to a kinetically controlled association/dissociation of U onto and from humic colloids and also a subsequent sorption of U onto the sediment surface. The column experiments provide an insight into humic colloid-mediated U migration in subsurface aquifers

[en] The temperature upon which thermodynamic data used for performance assessment purposes is based is usually 25 C. However, in the near field environment of a spent fuel or vitrified high level waste repository the temperature will remain elevated for a long time period. Thermal calculations (Johnson et al., 2001) have shown that the temperature in the bentonite may attain 70-90 C under saturated conditions for 1000 years after repository closure. According to these calculations temperature then slightly decreases to 40-50 C after about 10000 years, which is the expected life span for the canisters. For the Swedish KBS-3 concept temperatures for the bentonite backfill are calculated to reach up to 80 C at the inner boundary (Ageskog and Jansson, 1999). After 1000 years temperatures are assumed to decrease to 40-55 C. From the above references it is apparent that there may be significant temperature differences for long time periods between thermodynamic standard conditions (i. e., T= 25 C) and the ones expected in the repositories. Therefore in this study the effect of temperature (20-80 C) on sorption of Eu(III) onto smectite was studied by wet chemistry, time resolved laser fluorescence spectroscopy (TRLFS), scanning transmission X-ray microscopy (STXM) and pyrolysis GC-MS. (orig.)

[en] The present paper describes the surface complexation behaviour of trivalent metal ions, Am(III) and Eu(III), on well characterised γ-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 γ-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. (orig.)