Development and Application of Space- and Time-Resolved Hyperspectral Raman Imaging to in situ Study Mineral- and Glass-Water Reactions
Final Report Abstract
Information on the mechanism of fluid-driven phase transformations is usually obtained by ex-situ analyses of the reaction products after quenching the experiments. To overcome problems associated with this experimental approach, we have applied in-situ and real-time hyperspectral Raman spectroscopic imaging to investigate the corrosion of borosilicate glasses and the replacement of celestine by strontianite. For these investigations, we designed and built special fluid cells that allow measuring the formation of the reaction product(s) and the solution with the direction of the laser beam parallel to the direction of the reaction front plane with a lateral resolution in the order of 5 to 20 µm. With this setup, the dynamics of the various reaction and transport processes in both systems could be studied. For example, in situ, space-resolved, confocal Raman imaging of the reaction between a ternary Na borosilicate glass and a bicarbonate solution at 85°C revealed an interfacial solution between a silica-based surface alteration layer (SAL) and the underlying glass, and pH gradients towards the glass surface and within the SAL. A sudden drop of the reaction rate was observed after the SAL thickness had reached ~30 µm. By exchanging the solution with a deuterated bicarbonate solution, it was possible to quantify the diffusion of molecular water through the SAL which revealed that water transport through the SAL was not rate-limiting. In addition, during aqueous corrosion of the same borosilicate glass at 70°C in 0.1M HCl we observed the fast formation of amorphous silica that replaced the glass and initially acted as a protective barrier, slowing down the corrosion process. With increasing reaction time, however, the sudden formation of a crack/gap at the glass-silica interface could be observed that caused an increase of the glass-silica replacement rate. Moreover, the polymerization of the surface alteration layer over time in an initially acidic environment could be quantified. Furthermore, two fluid-cell Raman spectroscopic experiments were conducted with a Ba-bearing soda-lime boroaluminosilicate glass at ~ 90 °C for 180 and 260 h that otherwise solely differed in the solution volume (SV) while keeping the surface area constant. After initial congruent dissolution of the glass, the formation of a crystalline, saponite-based SAL, as well as subsequent zeolite precipitation, witherite coating, and carbonate precipitation within pore spaces of the saponite layer were observed. 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 that could be quantified from the in-situ data. 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. To assess the long-term stability of nuclear waste glasses, it is crucial to understand how self-irradiation affects the structural state of the glass and influences the glass dissolution behavior. In this study, we therefore studied the effect of heavy ion irradiation on the forward dissolution rate of a non-radioactive ternary borosilicate glass. To create extended radiation defects, the glass was subjected to heavy ion irradiation using 197Au ions that penetrated ~50 µm deep into the glass. The structural damage was characterized by Raman spectroscopy, revealing a significant depolymerization of the silicate and borate network in the irradiated glass and a reduction of the average boron coordination number. Real time, in situ fluid-cell Raman spectroscopic corrosion experiments were performed with the irradiated glass in a silica-undersaturated, 0.5 M NaHCO3 solution at temperatures between 80 and 85 °C (initial pH ≈ 7.1). The time- and space-resolved in situ Raman data revealed a 3.7 ± 0.5 times increased forward dissolution rate for the irradiated glass compared to the non-irradiated glass, demonstrating a significant impact of irradiation-induced structural damage on the dissolution kinetics. In general, all in situ data unambiguously support an interface-coupled dissolution–precipitation (ICDP) model for the formation of the SAL and casts doubts on the validity of published kinetic glass corrosion models. Considering the replacement of celestine by strontianite, the most intriguing observations are that (1) the strontianite that replaced the celestine in a non-equilibrated Na carbonate solution made with H218O became enriched in 18O at about the same time when the strontianite rim was observed to shrink and (2) that already crystallized strontianite also became richer in 18O, generating an 18O front that moved in the opposite direction than the ICDP front. The most striking feature is that the strontianite in direct contact with the equilibrated 18O-rich aqueous carbonate solution had the lowest 18O content. The oxygen isotopic composition of the strontianite product obviously re-equilibrated along a front that moved in the opposite direction to the ICDP front, indicating that the strontianite rim re-equilibrated, likely by an Ostwald ripening process, which is in line with the observation that the strontianite rim started to shrink. The results of this project unambiguously demonstrate the power of fluid-cell Raman spectroscopic experiments for studying solid-water interactions, with the ability to trace specific sub-processes inoperando and spatially resolved by using stable isotopes.
Publications
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Real-time in situ observations of reaction and transport phenomena during silicate glass corrosion by fluid-cell Raman spectroscopy. Nature Materials, 18(4), 342-348.
Geisler, Thorsten; Dohmen, Lars; Lenting, Christoph & Fritzsche, Moritz B. K.
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The Effect of Heavy Ion Irradiation on the Forward Dissolution Rate of Borosilicate Glasses Studied In Situ and Real Time by Fluid-Cell Raman Spectroscopy. Materials, 12(9), 1480.
Lönartz, Mara Iris; Dohmen, Lars; Lenting, Christoph; Trautmann, Christina; Lang, Maik & Geisler, Thorsten
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Corrosion of ternary borosilicate glass in acidic solution studied in operando by fluid-cell Raman spectroscopy. npj Materials Degradation, 5(1).
Lenting, Christoph & Geisler, Thorsten
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Feedbacks and non-linearity of silicate glass alteration in hyperalkaline solution studied by in operando fluid-cell Raman spectroscopy. Geochimica et Cosmochimica Acta, 329, 1-21.
Müller, Gerrit; Fritzsche, Moritz B.K.; Dohmen, Lars & Geisler, Thorsten
