Static through-diffusion experiments were performed to study the diffusion
of alkali- and alkaline earth-metals in fine-grained granite and
medium-grained Äspö-diorite. Tritiated water was used as an inert reference
tracer. Radionuclides of the alkali- and alkaline earth-metals (mono- and
divalent elements which are not influenced by hydrolysis in the pH-range
studied) were used as tracers, i.e.
22Na+, 45Ca2+ and Sr The
effective diffusivity and the rock capacity factor were calculated by
fitting the breakthrough curve to the one-dimensional solution of the
diffusion equation. Sorption coefficients, Kd, that were derived
from the rock capacity factor (diffusion experiments) were compared with
Kd determined in batch experiments using crushed material of
different size fractions.
The results show that the tracers were retarded in the same order as was
expected from the measured batch Kd. Furthermore, the largest
size fraction was the most representative when comparing batch Kd
with Kd evaluated from the diffusion experiments. The observed effective
diffusivities tended to decrease with increasing cell lengths, indicating
that the “transport” porosity decreases with increasing sample lengths used
in the diffusion experiments.