A combined glass dissolution/diffusion experiment in Boom Clay at 30°C

Silica diffusion in clay is possibly the rate determining process for the dissolution of vitrified waste disposed of in a clay layer. For testing this hypothesis, combined glass dissolution/ silica diffusion experiments are performed. Glass coupons doped with 32Si are sandwiched between two clay cores at 30°C. Due to glass dissolution, 32Si is released and diffuses into the clay. At the end of an experiment, the glass mass loss is measured and the diffusion profile of the 32Si in the clay is determined. Both mass loss and the 32Si diffusion profile in clay are described well by a model combining glass dissolution according to a linear rate law with silica diffusion in clay. Fitting leads to an apparent silica diffusion coefficient between 7×10-13 m2/s and 1.2×10-12 m2/s. The maximal glass dissolution rate for glass next to clay is around 1.6×10-7 gram glass/m2 s. In clay, the measured silica concentration is around 5 mg/liter. Combining these values with the previously measured product of accessible porosity and retardation factor, leads in two ways to a silica glass saturation concentration in clay between 8 and 10 mg Si/liter.