Topical report on glass dissolution experiments at very high surface area to solution volume

Glass dissolution experiments were conducted at very high SA/V ratio in absence of cement in order to reach a very high reaction progress, allowing to follow pH evolution and secondary phases formed in these conditions, accelerating the processes that may occur on the long term. In this study, two inactive glasses, i.e. SON68 glass and ISG, were used and tests were carried out either in a KOH solution or in young cement water (abbreviated as YCWCa), both leaching solutions having a pH(25 °C) of 13.5. Both temperatures 30 and 70 °C were investigated, this latter temperature favouring the precipitation of secondary phases, more particularly zeolites, which can be responsible for glass alteration resumption. Due to the very high SA/V ratio, a pH decrease was measured in all experiments, with a maximum decrease of 2 pH units at 30 °C and 3 pH units at 70 °C. Such a pH decrease can be attributed to both glass dissolution and precipitation of secondary phases. Regardless the leaching solution and the temperature, low long-term rates in the range of 6 x 10-6 – 2 x 10-4 g/m2d were determined. Such low glass dissolution rates can be explained by the combined effect of the very high SA/V ratio and the important pH decrease. The very thin alteration layer in contact with the pristine glass observed by SEM might also be protective, but further investigations would be needed to get more information about its morphology and composition. The presence of zeolite phases with their typical rod-like shape was observed by SEM. The formation of these phases probably triggered the glass dissolution, thereby postponing the formation of a protective alteration layer, but as a result of the decreasing pH, their precipitation rate decreased, resulting in a gradual decrease of the dissolution rate. Hence, an alteration resumption was not observed in these experiments. Geochemical modelling was done with the PHREEQC code using the Thermochimie and the high salinity database PITZER in order to relate the pH evolution to the amount of dissolved glass. In general, there is a good agreement between the modelled and measured pH as a function of the amount of dissolved glass, even though simplications or assumptions have to be made when applying the PITZER database due to the lack of parameters for certain elements and phases such as Al and zeolites. On the other hand, the results also indicate that the pH change due to dissolution of certain elements from the glass are somehow compensated by precipitation of those elements. For both glasses, the lower the reaction progress, the better the agreement between the model calculation and the measurements, because at low reaction progress the ionic strength effects and precipitation are minimal. The effects of the precipitation of secondary phases on the pH seem to become more important at high reaction progress. However, because the secondary phases formed in the experiments were mostly amorphous and thermodynamic data for such phases are missing, it is difficult to identify the pH effect of precipitation at high reaction progress. In the context of the Belgian deep geological disposal for the vitrified high-level waste, the high pH due to the use of concrete potentially has a large effect on the glass dissolution rate. The expected pH evolution (decrease) at the glass surface will be favorable and may lead to low long-term glass dissolution rates, but a better understanding of the dissolution and transport mechanisms is necessary to quantify the impact and the timescale. The dissolution experiments in the absence of cement at very high reaction progress showed that an important pH drop can occur, leading to residual glass dissolution rates as in neutral pH conditions. This pH decrease depends much on the type of phases that are formed.