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Colloid Concentration (ppm)
160
140
120
100
80
60
40
20
0
BACKppm
1.2DES
1.4DES
1.6DES
2.21
0.75
0.24
1.6 g/cm 3
Background
1.4 g/cm 3
0 100 200 300 400 500
Time (days)
1.2 g/cm 3
334
Colloid concentration (ppm)
160
140
120
100
80
60
40
20
0
Deionised water
Grismel water
NaCl 10 -2 M
1.2 1.4 1.6
Compaction density (g/cm 3 )
Figure 3: Left: Generation of bentonite colloids from bentonite plugs at different initial densities in
deionised water. Right: Dependence on the colloid concentration with the compaction density in
three different waters (deionised, Grimsel groundwater and NaCl 0.01 M. As received FEBEX clay.
Ca-homoionised clay did not form colloid in appreciable concentration, but the presence of Na in
the exchange complex (20 %) completely changes the generation behaviour (for example, asreceived
FEBEX bentonite). Finally, the surface exposed to hydration (and the consequent existence
extrusion paths) also affects colloid generation. Generated bentonite colloids are stable over
month in low mineralised and alkaline pH and their stability may increase in the presence of humic
acids.
Neretnieks and Liu [34] suggested a ‘zero order model’ to describe the colloid generation from
compacted bentonite for PA purposes. The criterion of colloid release is the critical coagulation
concentration corresponding to 1 mmol/L Ca 2+ . The studies performed within FUNMIG clearly
show that taking a certain Ca concentration as a colloid generation criterion does not describe the
real behaviour. Calculated colloid release rates according to the ‘zero order model’ in general are
by far higher than measured in experiments. The new experimental data can be used to develop an
improved colloid generation model.
In most of the transport studies performed in fractured cores, the breakthrough curves of colloids
always presented an elution peak in a position very similar to that of conservative tracers but that
their recovery critically depended on the colloid concentration and on the water flow rate (Figure
4).
In the case of bentonite colloids, significant colloid filtration was observed in spite of the existence
of “unfavourable” electrostatic conditions for colloid-rock attachment. In the Grimsel case, the geochemical
conditions favour high colloidal stability, nonetheless, the filtration of bentonite colloid
increased significantly when the hydrodynamic conditions approached the ones expected in a repository
(low water flow rates) and when the roughness of the fracture surface increased [35].