Diffusion and Sorption of ³²Si-labelled Silica in the Boom Clay

The long term corrosion rate of nuclear waste glass in a repository might be controlled by the steady-state diffusion of dissolved silica and would be enhanced by the sorption of silica onto clay minerals. Irreversible sorption and moderate retardation have been observed for dissolved silica in Boom Clay. Values of nR between 10 and 20 have been determined by means of four Flow-Through migration experiments, while K_d in the range 20 to 100 cm³ ·g^-1 have been measured by batch sorption tests with ³²Si on fresh and slightly oxidized Boom Clay. As non specific interactions cannot explain the sorption of neutral Si(OH)₄, or of negative silicate species (Donnan exclusion), onto negatively charged clay minerals other mechanisms must be invoked: i.a., the chemisorption of dissolved silica Si(OH)₄ onto specific Lewis acid sites (Al³⁺ Fe³⁺) present at the clay surface. The suggested mechanism could be similar to this explaining the irreversible chemisorption of oxy-anions of weak acids (as phosphate, or borate) onto aluminum and iron hydroxides in soils. Ligand exchange of aqueous silica with a hydroxyl group of Al(OH)₃ may form a hydroxy-aluminosilicate (HAS) surface complex. Gibbsite layers accessible on the basal plane of kaolinite and on the edges of illite and smectite are possible sorption sites for the dissolved silica in Boom Clay. Moreover, hydrous ferric oxide produced by pyrite oxidation significantly increases the extent of silica sorption.