Fate of Lu(III) sorbed on 2-line ferrihydrite at pH 5.7 and aged for 12 years at room temperature. II: insights from STEM-EDXS and DFT calculations

Environmental Science and Pollution Research - Transformation products of two-line ferrihydrite associated with Lu(III) were studied after 12 years of aging using aberration-corrected...     

Actinide geochemistry: From the molecular level to the real system

Geochemical processes leading to either mobilization or retention of radionuclides in an aquifer system are significantly influenced by their interaction with rock, sediment and colloid surfaces. Therefore, a sound safety assessment of nuclear waste disposal requires the elucidation and quantification of those processes. State-of-the-art analytical techniques as e.g. laser- and X-ray spectroscopy are increasingly applied to study solid–liquid interface reactions to obtain molecular level speciation insight.We have studied the sorption of trivalent lanthanides and actinides onto aluminium oxides, hydroxides and purified clay minerals by the time-resolved laser fluorescence spectroscopy and X-ray-absorption spectroscopy. Chemical constitution and structure of surface bound actinides are proposed based on spectroscopic information. Open questions still remain with regard to the exact nature of mineral surface ligands and the mineral/water interface. Similarities of spectroscopic data obtained for M(III) sorbed onto γ-alumina, and clay minerals suggest the formation of very comparable inner-sphere surface complexes such as S–O–An(III)(OH)_x^(2 − x)(H₂O)_5 − x at pH > 5. Those speciation data are found consistent with those predicted by surface complexation modelling. The applicability of data obtained for pure mineral phases to actinide sorption onto heterogeneously composed natural clay rock is examined by experiments and by geochemical modelling. Good agreement of experiment and model calculations is found for U(VI) and trivalent actinide/lanthanide sorption to natural clay rock. The agreement of spectroscopy, geochemical modelling and batch experiments with natural rock samples and purified minerals increases the reliability in model predictions.The assessment of colloid borne actinide migration observed in various laboratory and field studies calls for detailed information on actinide–colloid interaction. Kinetic stabilization of colloid bound actinides can be due to inclusion into inorganic colloid matrix or by macromolecular rearrangement in case of organic, humic/fulvic like colloids. Only a combination of spectroscopy, microscopy and classical batch sorption experiments can help to elucidate the actinide–colloid interaction mechanisms and thus contribute to the assessment of colloids for radionuclide migration.     

Interaction of latex colloids with mineral surfaces and Grimsel granodiorite

Bentonite clay is considered as possible backfill material for nuclear waste repositories in crystalline rock. The same material may also be a source of clay colloids, which may act as carriers for actinide ions possibly released from the repository. Depending on the geochemical parameters, these colloids may be retained by interaction with mineral surfaces of the host rock. In the present study interaction of carboxylated fluorescent latex colloids, used as a model for bentonite colloids, with natural Grimsel granodiorite and some of its component minerals is studied by fluorescence microscopy and SEM/EDX. The experiments are carried out by varying the pH from 2–10. Strong adsorption is observed at pH values close to or below the points of zero charge (pH_pzc) of the mineral surfaces. The influence of Eu(III), used as a chemical homologue for trivalent actinide ions, on colloid adsorption is investigated. Depending on mineral phase and pH, a significant increase of colloid adsorption is observed in the presence of Eu(III).     

The influence of colloid formation in a granite groundwater bentonite porewater mixing zone on radionuclide speciation

In the context of deep geological storage of high level nuclear waste the repository will be designed as multiple barrier system including bentonite as buffer/backfill material and the host rock formation as geological barrier. The engineered barrier (bentonite) will be in contact with the host rock formation and consequently it can be expected that bentonite porewater will mix with formation groundwater. We simulate in this study the mixing of Grimsel groundwater (glacial melt water) with synthetic Febex porewater (assuming already saturated state) in a batch-type study and investigate the formation of colloids by laser-induced breakdown detection (LIBD) and SEM-EDX as well as the changes in radionuclide (U, Th, Eu) speciation via ultrafiltration or via time-resolved laser fluorescence spectroscopy (TRLFS) analysis in the case of Cm(III). Based on PHREEQC saturation index (SI) calculations a precipitation of calcite might be expected at low Febex porewater (FPW) content (< 20%), fluorite precipitation at FPW contents < 60% and gibbsite precipitation at FPW contents above 10%. The colloids generated in the mixing zone aggregate when the synthetic FPW content exceeds 10%. LIBD analysis of the time-dependent colloid generation/aggregation revealed a low concentration of colloids to be stable with an estimated plateau value around 100–200 ppt and an average colloid diameter around 30 nm after 140 days reaction time at FPW admixture > 10%. SEM/EDX mostly identifies Al/Si containing colloidal phases and some sulfates could be found under certain admixture ratios. TRLFS studies show that the Cm speciation is strongly influenced by colloid formation in all solutions. In the Febex pore water/GGW mixing zone with high groundwater contents (> 80%) colloids are newly formed and Cm is almost quantitatively associated with most likely polysilicilic acid colloids.