Enhancement of Borosilicate Glass Dissolution by Silica Sorption and Diffusion in Compacted Bentonite: A Model Study.

ABSTRACTExperimental evidence indicates that glass corrosion rates decrease proportionally with the increase of silicic acid concentration in the solution contacting the glass surface. A minimum corrosion rate (Rsat) is reached when the solution becomes saturated with respect to an unidentified amorphous siliceous compound. In a repository where the vitrified waste form is surrounded by compacted bentonite, the silica dissolved from the glass will diffuse into the pore solution and concentration gradients will be established throughout the backfill material. The silicic acid concentration at the glass-bentonite interface, and thus the glass corrosion rate, will then be diffusion controlled. Moreover, experimental work suggests that significant sorption of silica by clay minerals in bentonite may accelerate glass corrosion.A model describing glass corrosion coupled with diffusive transport and sorption of silica in bentonite has been developed and incorporated in a FORTRAN computer code (GLADIS). The model assumes: (a) a linear isotherm for the sorption of silica (KD), (b) time and space invariant pH, temperature and ionic strength, (c) proportionality between the quantity of silica precipitated and the amount of glass dissolved and (d) cylindrical geometry. Preliminary calculations with a particular parameter set at 90°C, assuming no silica sorption on the bentonite, indicate for an unfractured glass block that a stationary state is rapidly reached in which the silica concentration at the glass-bentonite interface is lower than the saturation concentration. This implies that the glass corrodes at a more rapid rate than Rsat (RsS ∼ 8 Rsat) If moderate silica sorption is assumed (KD = 0.5 m3 kg−1), the attainment of stationary conditions is delayed by the removal of silicic acid from solution, and the average corrosion rate is further increased by a factor ∼ 2.