Microbes, Minerals and Material Heterogeneity—The Limitations of Laboratory Glass Dissolution Tests

Glass dissolution mechanisms have been well described as far as they relate to nuclear waste type silicate or borosilicate glass exposed to simple solutions. Successive stages of glass alteration are understood to progress from initial ion exchange, inter-diffusion and network hydrolysis, to the formation of secondary alteration phases leading to a rate drop and either a long-term reduction in glass alteration or eventual rate resumption. However, with countries now engaged in site selection for near surface or deep geological disposal facilities for vitrified waste, there is increased interest in understanding glass alteration in more complex systems. This talk will focus on the extent to which complex environments can be replicated in laboratory experiments and the limitations of standardised dissolution tests.<br/><br/>Recent studies of glass alteration in dynamic, open, natural environments highlight the role that external elements, sourced from groundwater and surrounding mineralogy, play in secondary phase formation. In particular, multiple studies show that externally sourced elements are sequestered into alteration layers as they form and have significant effect on alteration layer chemistry. Glasses exposed to aqueous environments for between 20 and 220 years offer insight into the processes by which alteration layers grow and evolve over time, the phenomenon of chemically distinct lamellae/banding and vermiform features. Similarly, studies performed in more complex systems provide evidenced that microorganisms can affect glass alteration indirectly by controlling the local chemical environment, particularly the speciation of Fe and the availability of P. Finally, where glasses produced under laboratory conditions are usually relatively homogeneous whereas those produced at scale may contain crystalline phases associated with cooling, phase separation, or incomplete vitrification. Recent work has shed light on how mineral phases that are either more or less durable than the bulk glass matrix can affect the measured test response in closed system laboratory tests and how these challenges can be overcome.