O. Vidal et al., PARTITIONING OF LA BETWEEN SOLID AND SOLUTION DURING THE AGING OF SI-AL-FE-LA-CA GELS UNDER SIMULATED NEAR-FIELD CONDITIONS OF NUCLEAR WASTE-DISPOSAL, Applied geochemistry, 10(3), 1995, pp. 269-284
Lanthanum-bearing gels of various stoichiometries (Si-Fe +/- Al +/- Ca
) and structures (hydrated Na-bearing and anhydrous Na-free materials)
, were aged in oxidizing aqueous solutions at 90 degrees C and under a
strong thermal gradient. The presence of La in the starting material
did not inhibit the crystallization of ferrous clays (nontronite) for
compositions with ratios Si/(Fe + Al) less than or equal to 2, whereas
crystallization failed for Si/(Fe + Al) 2.7. Clays are able to trap L
a, probably as an interlayer cation. In some experiments, a significan
t amount of La was also associated with newly formed silicated amorpho
us phases (APs), the precipitation of which from the solution was enha
nced by a decrease of temperature. Extrapolation of these results to n
uclear waste disposal conditions suggests that the radionuclide-rich h
ydrated amorphous layer (AL) which forms during the leaching of nuclea
r waste glasses is likely not to crystallize into clays. The hydrated
amorphous layer should thus be a stable secondary barrier against the
dispersion of radionuclides from leached nuclear waste glasses. Toward
s the far-field conditions, heavy elements are likely to be found in n
ewly formed clays and silicated amorphous phases.