Validation of glass dissolution and Si diffusion parameters with a combined glass dissolution-diffusion experiment in Bo
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Validation of glass dissolution and Si diffusion parameters with a combined glass dissolution-diffusion experiment in Boom Clay Karel Lemmens, Marc Aertsens Waste and Disposal Department SCKCCEN (Belgian Nuclear Research Centre), Boeretang 200, B-2400 Mol, Belgium [email protected], [email protected] ABSTRACT Existing knowledge on glass dissolution and silica diffusion in Boom Clay is validated by experiments where both phenomena could be studied simultaneously. SON68 glass coupons, doped with radioactive 32Si, were sandwiched between two cores of fresh humid Boom Clay and heated to 30°C. At the end of the experiment, the system was dismantled, the mass loss of the glass coupon was measured, and the clay core was sliced to determine the diffusion profile of the 32 Si dissolved from the glass. These data were completed with analyses of the clay water and surface analyses for analogous tests with undoped glass. The results are interpreted by assuming congruent glass dissolution at a constant rate, with a glass silica saturation concentration between 14 and 20 mg/l, a forward glass dissolution rate (at zero silica concentration) of 0.028 g.m-2day-1, an apparent silica diffusion coefficient in the clay of 1.4 10-12 m2sec-1, and a distribution coefficient for silica on Boom Clay between 0.010 and 0.075 m³kg-1. These parameter values are close to the range found in literature. It was not necessary to consider diffusion through the gel, precipitation or detailed geochemical reactions. The modeling exercise shows that the existing knowledge about the subsystems glass and clay can succesfully be integrated to describe the coupled processes in the whole system.
INTRODUCTION The dissolution of waste glass in contact with near field materials is often described as a series of coupled processes. The most frequently considered processes are matrix dissolution, ion exchange, diffusion through the gel, saturation of the solution and the removal of dissolved glass species by diffusion, sorption and/or precipitation in the near field [1]. Sorption and precipitation are difficult to distinguish. Previous experiments with FoCa7 bentonite have suggested that the silica immobilization consists of a first stage, interpreted as sorption, and a second stage, interpreted as precipitation [2]. In Boom Clay (see [3] for the mineralogical composition), there is evidence for silica sorption, but only indirect evidence for silica precipitation. Sorption of silica in Boom Clay has been demonstrated directly in conditions where the clay water solution was oversaturated in silica with regard to the clay [1], and in conditions where the clay water was close to silica saturation w.r.t. the clay [4]. For the precipitation of silica in Boom Clay, no direct evidence exists. Still, many experiments in Boom Clay have revealed high mass losses combined with relatively thin alteration layers. This suggests that either, the alteration layers were lost before the surface was analysed, or the alteration layers were partly dissolved and its components taken u
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