Horst Geckeis

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

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.

The colloid and radionuclide retardation experiment at the Grimsel Test Site: influence of bentonite colloids on radionuclide migration in a fractured rock

Abstract The colloid and radionuclide retardation (CRR) experiment is dedicated to the study of the in situ migration behaviour of selected actinides and fission products in the absence and presence of bentonite colloids in a water-conducting feature (shear zone) in the Grimsel Test Site (GTS). The technical scenario considers the bentonite backfill/host rock interface as a potential source for colloids. The experiment investigates the migration behaviour of U, Th, Pu, Am, Np, Sr, Cs, I and Tc and the influence of smectitic bentonite colloids by two in situ tracer injections in a well-characterised dipole. The field experiments are supported by an extended laboratory and modelling programme. Colloid breakthrough is determined on-line by a mobile, laser-induced breakdown detection apparatus (LIBD), a mobile photon correlation spectrometer (PCS) and, afterwards in the laboratory, by a single particle counting method (SPC) using a laser light scattering technique. Bentonite colloids generated from bentonite backfill material were found to be stable in the experimental groundwater and the influence of pH and salinity on colloid stability was investigated. The in situ monitored breakthrough of the tri- and tetravalent actinides Am and Pu and of Cs followed the colloid breakthrough indicating some degree of colloid-mediated migration of these radionuclides in the experimental shear zone. But even when no colloids had been added to the tracer cocktail, part of Am(III) and Pu(IV) appears to migrate as colloids. The different colloid detection techniques revealed a colloid recovery between 80 and 90% of the injected bentonite colloids. The CRR experimental results are considered from the perspective of understanding the likely long-term behaviour of a deep geological repository for radioactive waste and as an indicator of the way forward to the next generation of in situ experiments.

Effect of humic acid on the sorption of Cm(III) onto γ-Al2O3 studied by the time-resolved laser fluorescence spectroscopy

Summary Sorption of Cm(III) onto γ-alumina coated with humic acid (HA) is studied by the Time Resolved Laser Fluorescence Spectroscopy (TRLFS). The experiments are performed at 0.1 M NaClO4, 0.44 g/L γ-Al2O3, 10 mg/L HA and at a metal ion concentration of 2×10-7 mol/L. At the investigated pH range (4 to 10) HA is completely sorbed to γ-Al2O3. The excitation spectrum of Cm(III) bound to HA/γ-Al2O3 in the wavelength range 370-400 nm exhibits broad flat bands very different from those obtained for the Cm(III) aquo ion and the Cm(III)-γ-Al2O3 surface complex, respectively. The spectrum, lacking distinctive structure due to intramolecular energy transfer processes, points to the predominant binding of the Cm(III) to surface-bound HA. TRLFS experiments performed at two different excitation wavelengths (λex=355 and 396.6 nm) allow for a differentiation of humic-bound and non-humic-bound Cm(III). Differences in fluorescence spectra obtained at the different excitation wavelengths are found at pH<6.9. They are due to the presence of the non-complexed Cm(III) aquo ion which is not detected in the indirect excitation mode (λex=355 nm). At pH≥7, the fluorescence spectra obtained by indirect and direct excitation become congruent and again point to the existence of only humic-bound Cm(III) species. Comparison of peak maxima and fluorescence lifetimes for Cm(III)-HA and Cm(III)-HA/γ-Al2O3, however, reveal differences. The results clearly indicate a contribution of the γ-Al2O3 surface to the Cm(III) binding and, thus, suggest the formation of ternary complexes such as >Al-O-Cm(III)(HA).

Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland: impact of reaction kinetics and speciation on radionuclide migration

Summary The influence of smectite colloids on the migration behaviour of U(VI), Th(IV), Pu(IV), Am(III), Np(V), Sr(II) and Cs(I) is investigated within the Colloid and Radionuclide Retardation experiment (CRR). Two in situ experiments in a well-characterized granitic fracture zone are carried out in presence and absence of bentonite colloids. Radionuclide retardation observed in the field studies increases in the sequence Np(V)∼U(VI)<Sr(II)<Cs(I), where a small fraction of colloid borne breakthrough is only stated for Cs(I) in presence of bentonite colloids. Am(III) and Th/Pu(IV) mainly migrate as colloids without retardation in the presence and absence of smectitic colloids. The radionuclide migration behaviour is discussed on the basis of results obtained in laboratory batch sorption experiments and spectroscopic studies. Consistent with the field observation, laboratory derived Kd values increase in the order Np(V)∼U(VI)<Sr(II)<Cs(I). Significant kinetic hindrance for the sorption to fault gauge minerals is observed for Sr(II) and Cs(I), but notably for Am(III) and Pu(IV). The slow sorption reaction of tri- and tetravalent actinide ions is explained by their kinetically hindered dissociation from colloidal species. In order to explain the colloidal behaviour of tri- and tetravalent actinides even in absence of bentonite colloids, ultracentrifugation and spectroscopic experiments are performed. It is found that up to 60% of Pu(IV) and Am(III) species can be centrifuged off. Adding Cm(III) (5×10-8 mol L-1) into both injection solutions instead of Am(III) allows for a spectroscopic study by using the time resolved laser fluorescence spectroscopy (TRLFS). Peak position and fluorescence lifetimes (λ=604 nm, τ=110-114 μs) together with the fact that Cm(III) can be widely separated by ultracentrifugation, suggest the existence of inner-sphere surface complexes on groundwater and bentonite colloids. Carbon K-edge XANES analysis of the bentonite colloids reveal the presence of natural organic constituents. They are mainly of aliphatic nature containing high fractions of carboxylate groups. A contribution of these organic species towards the bentonite colloid stability and sorption of actinides is assumed to be likely.

Aquatic colloids relevant to radionuclide migration: characterization by size fractionation and ICP-mass spectrometric detection

Abstract The application of the flow-field flow fractionation (FFFF) combined with on-line ICP-mass spectrometry (ICP-MS) to the characterization of aquatic colloids is described. The capabilities and drawbacks of the technique are discussed with the aid of two examples. (1) The size distribution of smectitic colloids dispersed from a natural bentonite is determined by FFFF and laser light scattering (LLS) and compared with the size information obtained by ICP-MS detection. Due to the pronounced size dependency of the scatter light intensity, the LLS detection tends to overestimate the larger sized particles. Therefore, the FFFF-ICP-MS fractogram delivers more reliable size information. (2) Groundwater humic/fulvic colloids and the humic matter extracted from the corresponding sediment, both taken from the Gorleben aquifer (Lower Saxony, Northern Germany), are analysed by FFFF-ICP-MS. The location of REE, U, Th and Ca in different colloid size fractions appears to be very similar in both samples. The element specific fractograms suggest in agreement with earlier studies the location of Th and the REE mainly in inorganic colloids>17 nm containing also Fe and/or Al. U and Ca appear to be distributed between larger colloids and the fulvic/humic acid fraction with a size

A mobile laser-induced breakdown detection system and its application for the in situ-monitoring of colloid migration

Abstract A mobile Laser-Induced Breakdown Detection (LIBD) system is developed for the field study of the aquatic colloid migration. The principle of LIBD is based on the plasma generation at breakdown of colloids in the laser focus region, which is then observed by either photo-acoustic detection or optical monitoring. The method provides information on the number density and average size of colloids. The present instrumentation built up particularly for the field study is applied for the aquatic colloid migration in a natural granite fracture. The study is performed within the Colloid and Radionuclide Retention project (CRR) in order to investigate the role of colloids on the radionuclide migration. For this purpose, bentonite colloids dispersed in granite groundwater are injected into a rock fracture zone with a dipole distance of 5 m. After passing through the fracture, the groundwater is guided into a flow-through detection cell of LIBD for the quantification of colloids. Another experiment is performed with the same colloids traced by tetra- and tri-valent metal ions, Th(IV), Hf(IV) and Tb(III). The recovery of colloids as found by LIDB appears to be about 55±5% of the injected colloid mass concentration, which is corroborated by ICP-MS analysis of Al present in bentonite colloids. An average size of recovered colloids is slightly decreased from the value of before injection and hence the relative number density increased in the course of the rock fracture migration. Recovered fractions of colloid-borne trace elements in the extracted groundwater are 78±8% for both tetra-valent elements and 33±3% for trivalent element of the originally traced concentrations. Plausible explanations are given for the different recoveries of heavy metal ions.

The INE-Beamline for actinide science at ANKA

Since its inauguration in 2005, the INE-Beamline for actinide research at the synchrotron source ANKA (KIT North Campus) provides dedicated instrumentation for x-ray spectroscopic characterization of actinide samples and other radioactive materials. R&D work at the beamline focuses on various aspects of nuclear waste disposal within INEs mission to provide the scientific basis for assessing long-term safety of a final nuclear waste repository. The INE-Beamline is accessible for the actinide and radiochemistry community through the ANKA proposal system and the European Union Integrated Infrastructure Initiative ACTINET-I3. Experiments with activities up to 1 × 10(+6) times the European exemption limit are feasible within a safe but flexible containment concept. Measurements with monochromatic radiation are performed at photon energies varying between ~2.1 keV (P K-edge) and ~25 keV (Pd K-edge), including the lanthanide L-edges and the actinide M- and L3-edges up to Cf. The close proximity of the INE-Beamline to INE controlled area labs offers infrastructure unique in Europe for the spectroscopic and microscopic characterization of actinide samples. The modular beamline design enables sufficient flexibility to adapt sample environments and detection systems to many scientific questions. The well-established bulk techniques x-ray absorption fine structure (XAFS) spectroscopy in transmission and fluorescence mode have been augmented by advanced methods using a microfocused beam, including (confocal) XAFS/x-ray fluorescence detection and a combination of (micro-)XAFS and (micro-)x-ray diffraction. Additional instrumentation for high energy-resolution x-ray emission spectroscopy has been successfully developed and tested.

Sorption of Cm(III) and Gd(III) onto gibbsite, α-Al(OH)3: a batch and TRLFS study.

Gd(III) and Cm(III) sorption onto a pure aluminum hydroxide, gibbsite (alpha-Al(OH)(3)), is studied by batch experiments and time-resolved laser fluorescence spectroscopy (TRLFS). The experiments are conducted under argon atmosphere to exclude the influence of atmospheric CO(2) on solution and surface speciation. Batch experiments are done in two different electrolytes 0.1 M NaClO(4) and 0.1/0.01 M NaCl at a constant gibbsite concentration of 2.2 g/L. Gadolinium concentrations are varied from 6.4x10(-9) to 6.4x10(-5) M. pH-dependent sorption is found to be congruent at Gd(III) concentrations up to 6.4x10(-7) M and a shift of the pH edge to higher pH values is observed for higher metal ion concentrations. Type of background electrolyte anion and ionic strength do not affect the metal ion sorption. The spectroscopic investigations are performed with Cm(III) and gibbsite concentrations of 2x10(-7) M and 0.5 g/L, respectively. From the strongly red-shifted emission spectra two different inner-sphere surface complexes can be identified. A third species appearing at pH 6-11 is assigned to a coprecipitated or incorporated Cm(III) species. This incorporated species is most likely formed as a consequence of the applied experimental procedure. By continuously increasing the pH from 4 we move from high to low gibbsite solubility domains. As a result, aluminum hydroxide precipitates from oversaturated solutions, either covering already adsorbed curium or forming a Al/Cm(OH)(3) coprecipitate. Fluorescence lifetimes for the surface-bound Cm(III) complexes and the incorporated species are at 140-150 and 180-200 micros, respectively. Emission bands of the Cm(III) gibbsite surface complexes appear at comparable wavelengths as reported for Cm(III) species bound to aluminum oxides, e.g., gamma-Al(2)O(3); however, lifetimes are longer. This could presumably arise from either shorter binding distances of the Cm to Al-O sites or a coordination to more surface sites.

U, Th, Eu and colloid mobility in a granite fracture under near-natural flow conditions

Summary Laboratory core migration experiments were performed in a granite fracture from the Grimsel Test Site (GTS, central Swiss Alps). The flow velocity was varied (46 m yr-1, 94 m yr-1, 187 m yr-1) and solutions with 10-6 mol L-1 U(VI), 10-8 mol L-1 Th(IV) and 10-8 mol L-1 Eu(III) without (cocktail I) and with addition of 2 mgL-1 bentonite colloids (cocktail II) have been injected. Results are compared with those obtained in a field study at the GTS. Flow field-flow fractionation and ultrafiltration measurements show that U(VI) is not colloid borne, different from Th(IV) and Eu(III) which are associated with colloids in both spiked natural groundwaters with and without bentonite colloid addition. The partly unretarded U(VI) migration observed at short contact times (1-2 h) diminished under near-natural groundwater velocities (∼46 m yr-1) and only a weak retardation (Rf=19.5) could be observed. Furthermore, the U(VI) mass recovery decreased with increasing contact time and was as expected unaffected by bentonite colloid addition. A groundwater colloid mediated Th(IV)/Eu(III) migration without bentonite colloid addition could be confirmed in the core experiments but only at fast groundwater flow rates. Experiments at a water flow rate of 94 m yr-1 with cocktail II demonstrated a partially bentonite colloid facilitated transport of Th(IV) and Eu(III) with 27% and 37% mass recovery, respectively. At long tracer residence times in the flow field, no breakthrough of colloidal Th(IV)/Eu(III) could be observed in both runs, indicating the strong dependence of reaction kinetics on their mobility. Reference colloid experiments using fluorescence dye labeled carboxylated polystyrene microspheres (25 nm, 50 nm and 100 nm) revealed in general a higher recovery of smaller colloid size classes increasing with groundwater velocity. Additionally, an earlier peak arrival time of colloids and colloid-associated Th(IV) and Eu(III) was observed with respect to the conservative tracer 3H in all experiments due to pore-size or charge exclusion effects. In general, the colloid recoveries found in the laboratory core experiments are lower than those obtained in the field studies. The decrease of colloid mobility with increasing residence time suggests the occurrence of colloid attachment to the rock surfaces even under the colloid stabilizing groundwater conditions.

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

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.