Influence of silica release on clay characteristic: Scientific report for WP1. Experiment 1 of RP.WD.008 (experimental programme 2000-2003 on the long-termbehaviour of nuclear waste glass for NIRAS/ONDRAF)

Well identified Boom Clay was exposed to clay water with a high silica concentration in a flow-through cell. To enrich the clay water with Si, it was equilibrated with amorphous silica or SON68 glass frit. The Si-rich water was then pumped through the cell with Boom clay at an optimal flow rate. The solution at the inlet and the outlet of the cell was analysed repeatedly, to follow the concentration of dissolved species, especially Si. The difference in concentration allows to determine the amount of Si immobilized by the clay. The mineralogy of the clay was examined before and after the test to look for mineralogical changes. The tests have shown that short term immobilization of Si on Boom Clay can be considered as a reversible sorption of silicate ions, for Si concentrations up to at least 108 mg/l. This is the Si concentration range that we can expect in situ. The resulting Kd values (0.011–0.012 m³/kg) probably underestimate the actual Kd, due to experimental artefacts. Therefore, it is better to rely on Kd values derived from percolation experiments. The experiments have given no indications for the existence of a long-term Si fixation process or related changes in mineralogy.