Accelerated Leach Testing of GLASS (ALTGLASS): I. Informatics approach to high level waste glass gel formation and aging

Abstract

Glass corrosion data from the ALTGLASS™ database were used to determine if gel compositions, which evolve as glass systems corrode, are correlated with the generation of zeolites and subsequent increase in the glass dissolution rate at long times. The gel compositions were estimated based on the difference between the elemental glass starting compositions and the measured elemental leachate concentrations from the long‐term product consistency tests (ASTM C1285) at various stages of dissolution, ie, reaction progress. A well‐characterized subset of high level waste glasses from the database was selected: these glasses had been leached for 15‐20 years at reaction progresses up to ~80%. The gel composition data, at various reaction progresses, were subjected to a step‐wise regression, which demonstrated that hydrogel compositions with Si*/Al* ratios of <1.0 did not generate zeolites and maintained low dissolution rates for the duration of the experiments. Glasses that formed hydrogel compositions with Si^*/Al^* ratios ≥1, generated zeolites accompanied by a resumption in the glass dissolution rate. The role of the gel Si/Al ratio, and the interactions with the leachate, provides the fundamental understanding needed to predict if and when the glass dissolution rate will increase due to zeolitization.

Accelerated Leach Testing of Glass (ALTGLASS): II. Mineralization of hydrogels by leachate strong bases

“…For the glasses that resume accelerated alteration, the strong bases continue to be leached out of the glasses as a function of reaction progress instead of the [WA]. For glasses whose [SB] decreases at long times, there is also a rise of alkali in the gel layer (Part I, Reference ‐ Table S2), which then provides the altered layer with all the components necessary for zeolitization. As the precipitation of zeolites occurs, it consumes two moles of OH − for every Al that converts to zeolite (Equation ) (see Section ).…”

“…Where [OH − ] is the total alkali content, [SiO 2 ] is the silica content of the solution phase in the batch, b is a fitting parameter, and the number one is implicitly the (Si/Al) zeolite equal to one, which becomes (Si/Al) zeolite greater than one as the [SiO 2 ] of the solution phase increases and induces zeolitization. An (Si * /Al * ) ratio of greater than one is exactly the ratio of the activated complexes defined for zeolitization to occur as defined in Part I of this study using the informatics approach . A detailed discussion of the formation of zeolites has been given by Jansen and by Feijen, et al, and more recently by Cundy and Cox .…”

“…The emerging mechanism for the resumption of accelerated dissolution is that of a process which starts with the formation of an aluminosilicate hydrogel on the surface of HLW glasses during dissolution. That hydrogel has activated complexes, which can be described by the ratio, Si*/Al*. If Si*/Al* is less than one, no resumption of dissolution occurs.…”

“…These Fe‐Si gels were studied by XRD and spectroscopic techniques including Nuclear Magnetic Resonance (NMR) and Extended X‐ray Absorption Fine Structure (EXAFS). The gels were composed of hisingerite (Si 2 O 5 (OH) 4 ·2H 2 O), which is a poorly‐crystallized precursor of ferric smectite that possess the same local structure as nontronites as noted in Part I of this study …”

“…In Stage III glass corrosion, this consumption of hydroxide associated with the newly forming zeolitic phases may lead to a decrease in the measured pH of the leachate solution. In the next section, the (OH − ) ex , determined for the ALTGLASS HLW glasses discussed in Part I, will be discussed in which (OH − ) ex is given in terms of excess strong base ([SB] ex ) after a methodology first applied to HLW leachates by Jantzen, et al in 1995, which is equivalent to Equation .…”