RD&D-Programme 2004 - SKB

The results of the laboratory, Chemlab and Lot tests provide a clear picture of the diffusion
behaviour of the studied ions. The data obtained, i.e. diffusivities and distribution coefficients,
agree well in the three different studies.
Programme
SKB intends to evaluate an alternative description of the diffusion process in bentonite
based on ion equilibrium. The overlapping electrical double layers entail that there are
charge-compensating counterions in the entire space between the mineral layers. Since only
the counterions can diffuse freely, the ion balance with surrounding groundwater will be
controlled by the counterion concentration, section 17.2.15. This leads to lower concentration
of groundwater ions in the bentonite than in the surrounding groundwater. The cations from the
groundwater can, however, replace the original charge-compensating cations by means of ion
exchange. The anions, on the other hand, cannot replace the negatively charged mineral layers,
so that the concentration of introduced anions is lower than the concentration of introduced
cations (anion exclusion). The higher concentration of introduced cations leads to a higher
concentration gradient and thereby more rapid diffusive transport (surface diffusion). The
advantage of this description of diffusion in bentonite, if it turns out to be correct, is that it
permits a theoretical quantification and a better conceptual understanding.
Transport resistance for radionuclides in the interface between bentonite and water-conducting
fracture /17-25, 17-26/ will be investigated experimentally. The plans include both laboratory
and in situ experiments. The latter will be performed in the Chemlab probe.
17.2.26 Radionuclide transport – sorption
The surface of smectitic clays has a permanent negative charge. The charge imbalance is
neutralized by an exchange of cations between the layers. When the clay is water-saturated, the
exchangeable cations are hydrated and an electrical double layer is formed in the water/clay
interface. The charge-compensating cations can easily be exchanged for other cations from the
solution that is in contact with the clay surface. Sorption of cations in smectite minerals can be
described as ion exchange reactions and modelled with thermodynamic equilibrium constants or
selectivity coefficients. Ion exchange is the typical sorption mechanism for alkali and alkalineearth
metals. Many transition metals are also sorbed via ion exchange.
Radionuclides can also be sorbed via reactions with the surface and form surface complexes.
Most actinides and lanthanides form surface complexes. Nuclides sorbed as surface complexes
cannot be transported by surface diffusion.
Conclusions in RD&D 2001 and its review
Kasam believes that it should be possible to develop a more sophisticated sorption model (ion
exchange, adsorption and surface precipitation of radionuclides as a function of concentration
and pH).
SKI questions the way SKB dismisses the sorption process as it does in RD&D 2001.
Newfound knowledge since RD&D 2001
The results of in situ tests with the Chemlab probe have been described in section 17.2.25.
As a basis for SR-Can, SKB has published an updated report describing diffusion and sorption
of radionuclides in bentonite /17-27/. The report contains the data that will be used in the assessment
and a detailed description of uncertainties in data. Most diffusivities have been chosen
on the basis of diffusion tests with HTO, which are insensitive to the extreme parameters.
Thermodynamic sorption models have been used for the radionuclides where the data are good
enough.
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