Borosilicate glass is an important material used in various industries due to its chemical durability, such as for the immobilization of high-level nuclear waste. However, it is susceptible to ...aqueous corrosion, recognizable by the formation of surface alteration layers (SALs). Here, we report in situ fluid-cell Raman spectroscopic experiments providing real-time insights into reaction and transport processes during the aqueous corrosion of a borosilicate glass. The formation of a several-micrometre-thick water-rich zone between the SAL and the glass, interpreted as an interface solution, is detected, as well as pH gradients at the glass surface and within the SAL. By replacing the solution with a deuterated solution, it is observed that water transport through the SAL is not rate-limiting. The data support an interface-coupled dissolution-reprecipitation process for SAL formation. Fluid-cell Raman spectroscopic experiments open up new avenues for studying solid-water reactions, with the ability to in situ trace specific sub-processes in real time by using stable isotopes.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Currently accepted mechanistic models describing aqueous corrosion of borosilicate glasses are based on diffusion-controlled hydrolysis, hydration, ion exchange reactions, and subsequent ...re-condensation of the hydrolyzed glass network, leaving behind a residual hydrated glass or gel layer. Here, we report results of novel oxygen and silicon isotope tracer experiments with ternary Na borosilicate glasses that can be better explained by a process that involves the congruent dissolution of the glass, which is spatially and temporally coupled to the precipitation and growth of an amorphous silica layer at an inwardly moving reaction interface. Such a process is thermodynamically driven by the solubility difference between the glass and amorphous silica, and kinetically controlled by glass dissolution reactions at the reaction front, which, in turn, are controlled by the transport of water and solute elements through the growing corrosion zone. Understanding the coupling of these reactions is the key to understand the formation of laminar or more complex structural and chemical patterns observed in natural corrosion zones of ancient glasses. We suggest that these coupled processes also have to be considered to realistically model the long-term performance of silicate glasses in aqueous environments.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The replacement of celestine (SrSO4) by strontianite (SrCO3) has extensively been studied over the past few decades. It also represents an ideal reaction to in situ study the kinetic and fundamental ...mechanistic details of mineral replacement reactions in aqueous solutions via fluid-cell Raman spectroscopy. This technique allows us to study the reaction process in real time by continuously analysing the solution or by imaging the solid–liquid interface where the reaction takes place and while the replacement process is ongoing. Two sets of experiments were conducted, the first of which was carried out with celestine powder and an equimolar Na2CO3 solution in a heated fluid cell between 30 and 60 °C. The progress of the reaction was followed by Raman spectroscopic in situ measurements of the solution. Another experiment was performed with a polished cuboid cut from a single celestine crystal and a 1 M Na2CO3 solution in a fluid cell at room temperature (21 °C). In this experiment, the reaction was studied in situ, spatially resolved, and in real time. The results of both types of experiments revealed that the replacement occurs via a coupled dissolution–precipitation mechanism and that the evolution of the solution composition cannot fully be explained by a single rate law derived from a shrinking core model. By applying the model-independent time-to-a-given-fraction method, three kinetic regimes could be identified and the associated activation energies quantified.
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CEKLJ, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Abstract
The co-precipitation of sulphate minerals such as celestine and barite is widely studied because their formation is ubiquitous in natural and anthropogenic systems. Co-precipitation in ...porous media results in crystallization of solid solutions yielding characteristics such as oscillatory zoning that are rarely observed in bulk solution or in batch experiments. In the past, the precipitation of compositionally-zoned (Ba,Sr)SO
4
crystals was observed post-mortem in macroscopic silica gel counter-diffusion experiments. Their formation was originally explained by the difference in the solubility products of the end-members combined with diffusion-limited transport of solutes to the mineral-fluid interface, while a later study favored the idea of kinetically controlled reactions. With recent advances combining in-operando microfluidic experiments and reactive transport modelling, it is now possible to verify hypotheses on the driving forces of transport-coupled geochemical processes. We developed a “lab on a chip” experiment that enabled the systematic study of the nucleation and growth of oscillatory-zoned (Ba,Sr)SO
4
crystals in a microfluidic reactor. The compositions of the solid solutions were determined by in-situ Raman spectroscopy. Our investigation shows (1) that the composition of the nucleating phases can be approximated using classical nucleation theory, (2) that the oscillatory zoning is not solely controlled by the limited diffusional transport of solutes, and (3) that nucleation kinetics plays a major role in the switch between different stoichiometric compositions. The zoning phenomena is governed by the complex interplay between the diffusion of reactants and the crystallization kinetics as well as other factors, e.g. surface tension and lattice mismatch.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
One important application of borosilicate glass is its use as a nuclear waste form to immobilize high-level nuclear waste. Understanding the corrosion mechanism of borosilicate glasses in aqueous ...solutions is essential to reliably predict their long-term behavior in the worst-case scenario of glass-groundwater contact in a geologic repository. Traditional models evaluate the long-term corrosion process on the basis of diffusion-controlled hydration and ion exchange reactions that are followed by solid-state reconstruction of the hydrolyzed glass network. Here we report textural, chemical, and
18O and
26Mg isotope tracer results from corrosion experiments with a borosilicate glass in an acidic aqueous solution (initial pH ≈
0,
T
=
150
°C, 6 to 336
h) that contradict such a paradigm. We propose a new mechanistic model for glass corrosion under acidic conditions that is based on congruent (stoichiometric) dissolution of the glass that is spatially and temporally coupled to the precipitation of amorphous silica at an inward moving reaction front. The model potentially provides a novel framework to understand apparently contradictory observations made under more moderate conditions and to evaluate the long-term aqueous durability of silicate glasses.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Abstract
Fluid-cell Raman spectroscopy is a space and time-resolving application allowing in operando studies of dynamic processes during solution–solid interactions. A currently heavily debated ...example is the corrosion mechanism of borosilicate glasses, which are the favoured material for the immobilization of high-level nuclear waste. With an upgraded fluid-cell lid design made entirely from the glass sample itself, we present the polymerization of the surface alteration layer over time in an initially acidic environment, including the differentiation between pore and surface-adsorbed water within it. Our results support an interface-coupled dissolution-precipitation model, which opposes traditional ion-exchange models for the corrosion mechanism. A sound description of the corrosion mechanism is essential for reliable numerical models to predict the corrosion rate of nuclear waste glasses during long-term storage in a geological repository.
The pseudomorphic replacement of Carrara marble by calcium phosphates was used as a model system in order to study the influence of different fluid pathways for reaction front propagation induced by ...fluid–rock interaction. In this model, grain boundaries present in the rock as well as the transient porosity structures developing throughout the replacement reaction enable the reaction front to progress further into the rock as well as to the center of each single grain until transformation is complete. Hydrothermal treatment of the marble using phosphate bearing solutions led to the formation of hydroxylapatite and β-TCP; the formation of the latter phase was probably promoted by the presence of ∼0.6 wt.% Mg in the parent carbonate phase. Completely transformed single grains show a distinctive zoning, both in composition and texture. Whereas areas next to the grain boundary consist of nearly pure hydroxylapatite and show a coarse porosity, areas close to the center of the single grains show a high amount of β-TCP and a very fine porous microstructure. If fluorine was added as an additional solution component, up to 3 wt.% of F were incorporated into the product apatite and the formation of β-TCP was avoided. The use of the isotope 18O as a chronometer for the replacement reaction makes it possible to reconstruct the chronological development of the calcium phosphate reaction front. Raman analysis revealed that the incorporation of 18O in the PO4 tetrahedron of hydroxylapatite results in the development of distinct profiles in the calcium phosphate reaction front perpendicular to the grain boundaries of the marble. Through the use of the 18O chronometer, it is possible to estimate and compare the time effectiveness of the different fluid pathways in this model system. The results demonstrate that the grain boundaries are an effective pathway enabling the fluid to penetrate the rock more than one order of magnitude faster compared to the newly developing channel-like porosity structures, which act as pathways towards the center of single mineral grains. Thus, after only short reaction durations, it may be possible for the fluid to progress relatively large distances along the grain boundaries without developing broad reaction fronts along the path.
•Hydrothermal replacement of marble by hydroxylapatite and β-TCP.•Formation of β-TCP is avoided if fluorine is present in solution.•18O used as a chronometer to study time effectiveness of different fluid pathways.•Grain boundaries represent more effective fluid pathways than porosity structures.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Serpentinization-fueled systems in the cool, hydrated forearc mantle of subduction zones may provide an environment that supports deep chemolithoautotrophic life. Here, we examine serpentinite clasts ...expelled from mud volcanoes above the Izu–Bonin–Mariana subduction zone forearc (Pacific Ocean) that contain complex organic matter and nanosized Ni–Fe alloys. Using time-of-flight secondary ion mass spectrometry and Raman spectroscopy, we determined that the organic matter consists of a mixture of aliphatic and aromatic compounds and functional groups such as amides. Although an abiotic or subduction slab-derived fluid origin cannot be excluded, the similarities between the molecular signatures identified in the clasts and those of bacteria-derived biopolymers from other serpentinizing systems hint at the possibility of deep microbial life within the forearc. To test this hypothesis, we coupled the currently known temperature limit for life, 122 °C, with a heat conduction model that predicts a potential depth limit for life within the forearc at ∼10,000 m below the seafloor. This is deeper than the 122 °C isotherm in known oceanic serpentinizing regions and an order of magnitude deeper than the downhole temperature at the serpentinized Atlantis Massif oceanic core complex, Mid-Atlantic Ridge. We suggest that the organic-rich serpentinites may be indicators for microbial life deep within or below the mud volcano. Thus, the hydrated forearc mantle may represent one of Earth’s largest hidden microbial ecosystems. These types of protected ecosystems may have allowed the deep biosphere to thrive, despite violent phases during Earth’s history such as the late heavy bombardment and global mass extinctions.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Vertebrate fossils embedded in amber represent a particularly valuable paleobiological record as amber is supposed to be a barrier to the environment, precluding significant alteration of the ...animals' body over geological time. The mode and processes of amber preservation are still under debate, and it is questionable to what extent original material may be preserved. Due to their high value, vertebrates in amber have never been examined with analytical methods, which means that the composition of bone tissue in amber is unknown. Here, we report our results of a study on a left forelimb from a fossil Anolis sp. indet. (Squamata) that was fully embedded in Miocene Dominican amber. Our results show a transformation of the bioapatite to fluorapatite associated with a severe alteration of the collagen phase and the formation of an unidentified carbonate. These findings argue for a poor survival potential of macromolecules in Dominican amber fossils.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Fossilization processes and especially the role of bacterial activity during the preservation of organic material has not yet been well understood. Here, we report the results of controlled ...taphonomic experiments with crayfish in freshwater and sediment. 16S rRNA amplicon analyzes showed that the development of the bacterial community composition over time was correlated with different stages of decay and preservation. Three dominating genera, Aeromonas, Clostridium and Acetobacteroides were identified as the main drivers in the decomposition of crayfish in freshwater. Using micro-computed tomography (µ-CT), scanning electron microscopy (SEM) and confocal Raman spectroscopy (CRS), calcite clusters were detected after 3-4 days inside crayfish carcasses during their decomposition in freshwater at 24 °C. The precipitation of calcite clusters during the decomposition process was increased in the presence of the bacterial genus Proteocatella. Consequently, Proteocatella might be one of the bacterial genera responsible for fossilization.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
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