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5.1 Sorption
As an example of the type of work carried out, one component of the sorption studies focused on
the influence of carbonate on the sorption of radionuclides. Carbonate is the main inorganic ligand
present in bentonite pore water and is known to form soluble complexes. Experiments were performed
to determine the sorption of Ni(II), Eu(III), and U(VI) in montmorillonite (SWy-1) and bentonite
(MX-80) as a function of carbonate concentration and pH ([9]; [10]) in a pH range from 7 to
9.5. The experiments were performed by contacting a NaHCO3 solution with a suspension of purified
Na-montmorillonite, the main mineral present in bentonite. The experimental data indicate that
Ni(II) sorption onto montmorillonite is rather insensitive to the presence of inorganic carbon at levels
up to 20 mM and pH values below 9, within the experimental uncertainties associated with the
measurements. Only at very high inorganic carbon concentrations (0.1 M) was a more pronounced
effect on sorption observable. On the other hand, a clear effect of inorganic carbon on Eu(III) and
an even more pronounced effect on U(VI) sorption on montmorillonite was obtained. Model predictions
were carried out using the 2SPNE SC/CE model [11] with the assumption that metal carbonate
complexes do not sorb. In the case of Ni(II), the data and the model predictions agree within
the uncertainty of the data. For Eu(III) and U(VI), the model prediction underestimate the measured
data.
The sorption of cations on dispersed and compacted bentonite in synthetic pore water was studied.
The sorption of 22 Na + and 85 Sr 2+ on dispersed and compacted material was found to be very similar,
indicating that compacting the material does not result in a decreased accessibility of the ion exchange
sorption sites.
5.2 Diffusion
A set of through-diffusion and out-diffusion studies with 36 Cl - [12]; 22 Na + and 85 Sr 2+ [13] and 134 Cs +
have been performed with Volclay KWK bentonite. In the case of 36 Cl - both the dry density (�) of
the bentonite (� = 1300–1900 kg m -3 ) and the composition of the background electrolyte (I = 0.01-
1M NaCl) were varied. In addition, mass balance calculations for 36 Cl - and stable Cl - were made.
The results show that anions are excluded from the interlamellar space of the montmorillonite, so
that diffusion takes place in the interparticle pore space of the bentonite. Due to external negative
charges, anions are also partially excluded from this interparticle pore space. The extent of exclusion
depends on the ionic strength of the pore water. Furthermore, the interparticle pore space also
depends on the bulk density of the bentonite. There is a relationship between the effective diffusion
coefficient of anions, De A [m 2 s -1 ] and the diffusion accessible porosity, � [-], given by:
D A A m
e Dw
where Dw A is the diffusion coefficient of anion A in bulk water and m is a constant, depending on
the porous medium (Fig. 5, right). This relationship covers results obtained by work performed in
NF-PRO [14], earlier work on MX-80 and FEBEX (Ca) bentonite. Because the effective porosity
depends on the ionic strength of the pore water and the bulk dry density of the bentonite, also the
effective diffusion coefficient depends on the pore water composition and bulk dry density. At high
ionic strength, diffusion becomes faster. Dilution of the pore water results in a decrease of diffusion
of anions in the bentonite. At high bulk dry density, diffusion is slow whereas at low bulk dry
density, diffusion is faster. In the case of the cations, only the bulk dry density was varied. The
solution used was a synthetic pore water [12]. In the case of cations, the situation is different.
Studies on the diffusion of cations that adsorb via an ion exchange process on montmorillonite
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