SON68 glass alteration under Si-rich solutions at low temperature (35-90 °C): kinetics, secondary phases and isotopic exchange studies

Pristine and Si-29-doped SON68 glass was leached in dynamic mode under silica-rich synthetic CallovoOxfordian (COx) groundwater at pH 8, a glass surface-to-liquid volume (S/ V) ratio (900-1800 m(-1)) and at 35, 50 and 90 degrees C. The solutions were analysed by ICP-MS and ion chromatography, the alteration products were studied by electron microscopy and Raman spectroscopy, and the Si-isotopes profiles were obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS). The glass alteration seems to be governed by both diffusion and surface reaction processes. After 653 days of alteration the normalized leaching rates were 2.6 (+/- 0.2) x 10(-5), 9.9 (+/- 0.8) x 10(-5) and 2.4 (+/- 0.2) x 10(-3) g m(-2) d(-1) at 35, 50 and 90 degrees C, respectively. The major alteration secondary phase is clay-type Mg-silicates at all temperatures in addition to powellite and apatite at 90 degrees C. SIMS studies clearly showed uptake of Si-29 by the surface gel via condensation and in the Mg-rich phyllosilicates via precipitation from solution. The precipitation of phyllosilicates at all temperatures constitutes the main process which destabilises the gel layer, thus maintaining a long-term glass dissolution rate in COx groundwater, higher than in pure water.