9. PROGRAMACIÓN DIDÁCTICA
9.2 Unidades de trabajo
A number of different aspects concerning the colloid/nanoparticle formation and fate have been tackled within the KOLLORADO-1 project and the investigation has now been contin- ued within its follow-on project KOLLORADO-2.
Radionuclide speciation and interaction with bentonite/montmorillonite. Experiments carried out with fracture filling materials from Äspö and Grimsel under Grimsel groundwater conditions (glacial melt water reference) demonstrate different interaction behavior of long- lived radionuclides with colloids and mineral surfaces. Within Kollorado-2 batch experiments on RN sorption reversibility kinetics with Zn– and Ni-labeled montmorillonite colloids in the presence of granite fracture filling material and Grimsel groundwater have been conducted identical to previous experiments using natural FEBEX bentonite colloids ([74]). The new experiments generally confirmed the previous results and show comparable behavior of the synthetic montmorillonite colloids. The tri-and tetravalent radionuclides 232Th(VI), 242Pu(IV) and 243Am(III) are almost quantitatively associated with clay colloids and show sorption re- versibility. The reversibility kinetics are slower in case of the synthetic Zn- and Ni- montmorillonite colloids compared to the FEBEX colloids. The difference in surface area ratio (colloid/FFM) due to the higher specific surface area (~ a factor of 2) for the synthetic Zn-/Ni- clay colloids is hypothesized as explanation. As observed in earlier studies 99Tc(VII), 233U(VI) and 237Np(V) are not initially associated to the synthetic Zn- and Ni-montmorillonite colloids. However, regarding these RN, within the duration of the experiments a distinct drop in redox potential is observed. In consequence, the redox sensitive elements 99Tc(VII), 233U(VI) and 237Np(V) are prone to reduction processes in this study. The similar behavior of 99Tc(VII) and
over the whole experimental duration may be either due to a slow 233U(VI) sorption and/or reduction to 233U(IV).
Colloid generation at the compacted bentonite fracture interface. A pH and ionic strength dependent extent of colloid generation from a compacted bentonite source was ob- served very much driven by the respective CCC. For conditions below the CCC, colloid gen- eration is confirmed, whereas in experiments with an ionic strength lying above the CCC, colloid generation within the observation period is negligible. The experiments substantiate the important influence of the CCC for the potential release of clay colloids from bentonite. Furthermore, mock-up tests to optimize the final design of the CFM long-term in situ study and show the general feasibility are performed. The results will be used to optimize the ana- lytical long-term monitoring program for the field experiments. Both, mechanical erosion due to the swelling into the artificial 1 mm fracture as well as chemical erosion evidenced by chemical long-term changes in contact water composition (chloride, sulfate) have been ob- served. The swelling pressure is sufficient to break the glass ampules filled with labeled syn- thetic Ni-montmorillonite, Na fluorescein and the sorbed tri- and tetravalent elements. Eroded bentonite concentrations determined under the adjusted flow velocity for the CFM experiment of 1.8 - 2.3∙10-5 m/s reach 6 – 15 mg/L with a bimodal size distribution of < 45 nm colloids and colloids in the size range of 150-500 nm, respectively. Concerning the labeling with glass ampules the contact area and released proportion of total colloid mass has to be taken into account concerning the detection limits of the analytics. Radioisotopes and pre-concentration methods (exchange resins in collaboration with Helsinki University) are currently tested in the ongoing laboratory program for the CFM long-term in situ study.
The implementation of this data in currently available erosion models is an ongoing field of research activity within CFM and the EU project CP BELBaR.
Colloid – Mineral surface interactions. The results of the KOLLORADO-1 project showed
that the interaction of colloids with mineral surfaces is mainly controlled by electrostatic inter- actions. Strongest attractive forces are observed close to or below the individual point of zero charge (pHpzc) of the minerals. According to the results, colloid adsorption (adhesion forces) in alkaline regime (e.g., Grimsel groundwater conditions) is weak. Investigation using latex microspheres revealed only attractive forces for the mineral phase apatite, which occurs as accessory mineral in the Grimsel Granodiorite. Separate measurements show that Ca(II) concentrations as low as 10-4 M as present in Grimsel groundwater reduce colloid-mineral surface repulsive forces by a factor of about three and adhesion forces become detectable even at high pH. Therefore, the probability of colloid attachment to mineral surfaces increas- es also in low mineralized groundwater at high pH. Experimental observations of weak col- loid retention in such systems can be qualitatively also explained by such kinetically con- trolled processes. A further microscopic investigation in Kollorado-2 project emphasized the importance of surface heterogeneity and its role to retain colloidal particles onto surfaces under unfavorable conditions (surfaces with same charge) using vertical scanning interfer- ometry (VSI) and atomic force microscopy (AFM) techniques. The results presented suggest
fluid dynamics are involved in the complex behavior of colloid retention increase at the rough rock surface under unfavorable conditions.
Colloid mobility. Based on the results of the laboratory migration studies Febex bentonite
colloid mobility is relatively high and only small colloid fractions are found to be retained in the fracture under the Grimsel groundwater conditions. As mentioned above this finding is in qualitative agreement with colloid-mineral interaction studies. Compared to the colloid recov- ery the initially colloid associated metal recovery is significantly lower and decreases with increasing residence time clearly indicating the relevance of kinetically controlled desorption processes under these conditions. This confirms earlier findings obtained by other authors [23, 41, 76] at considerably shorter migration times. Tetravalent actinide ions such as Th(IV) show slower dissociation rates as compared to those of the trivalent metal ions Eu(III) and Tb(III). The time dependent decrease of metal ion recovery observed in column experiments is again in qualitative agreement with the outcome of batch sorption tests. Both studies con- firm the importance of considering radionuclide desorption kinetics for the assessment of colloid mediated radionuclide transport for nuclear waste disposal (see discussion below). Without this consideration, results of short-term laboratory experiments cannot be applied to make long-term predictions.