Feedbacks and non-linearity of silicate glass alteration in hyperalkaline solution studied by in operando fluid-cell Raman spectroscopy

Abstract

Mechanisms, feedbacks and resulting non-linearity during silicate glass alteration in a hyperalkaline carbonate solution were studied through hyperspectral Raman imaging of heated fluid-cells. Our experimental setup enabled in operando visualization and rate measurements of glass dissolution and secondary phase precipitation, complemented by spectral characterization of the phases involved and semi-quantitative monitoring of the ionic strength of the solution close to the glass interface. After initial congruent dissolution of the Ba-bearing soda-lime boroaluminosilicate glass, the formation of a crystalline, saponite-based surface alteration layer (SAL), as well as subsequent zeolite precipitation, witherite coating, and carbonate precipitation within pore spaces of the saponite layer were observed. Two in operando experiments were conducted at ∼ 90 °C for 180 and 260 h that otherwise solely differed in the solution volume (SV) while keeping the surface area constant. The high SV experiment exhibited a transient upward excursion of initial dissolution rates, followed by continuously rapid glass dissolution along with slow SAL growth and sustained oscillations in ionic strength. Contrastingly, in the low SV experiment, glass dissolution monotonically decreased after the onset of rapid SAL growth and no sustained oscillations were observed. We find that growth conditions and resulting properties of the SAL exert dominant, non-linear effects on the evolution of glass dissolution rates. In turn, SAL formation depends on nucleation/growth kinetics and the accumulation of glass-derived solutes at the reaction front. Both, dissolution and precipitation, feedback with solution chemistry and transport processes, together controlling the evolution of the corrosion process. Additionally, fracturing, delamination, and the evolution of surface morphology may affect glass dissolution rates and transport pathways. Such interpretations of decelerating reaction rates in response to the growth of a protective layer are consistent in micro-scale experiments and in outcrop- to global-scale observations, as is the accelerating effect of surface area creation by physical disruption and morphology. Thus, these µm-scale mechanistic insights could help elucidating local to global environmental feedbacks (e.g., erosion or weathering patterns) as well as process dynamics in engineered environments (e.g., nuclear waste disposal) and may assist the improvement predictive models.