Glass Dissolution Kinetics Research Articles

The initial dissolution rates of simulated UK Magnox–ThORP blend nuclear waste glass as a function of pH, temperature and waste loading

AbstractThe first comprehensive assessment of the dissolution kinetics of simulant Magnox–ThORP blended UK high-level waste glass, obtained by performing a range of single-pass flow-through experiments, is reported here. Inherent forward rates of glass dissolution were determined over a temperature range of 23 to 70°C and an alkaline pH range of 8.0 to 12.0. Linear regression techniques were applied to the TST kinetic rate law to obtain fundamental parameters necessary to model the dissolution kinetics of UK high-level waste glass (the activation energy (Ea), pH power law coefficient (η) and the intrinsic rate constant (k0)), which is of importance to the post-closure safety case for the geological disposal of vitreous products. The activation energies based on B release ranged from 55 ± 3 to 83 ± 9 kJ mol–1, indicating that Magnox–THORP blend glass dissolution has a surface-controlled mechanism, similar to that of other high-level waste simulant glass compositions such as the French SON68 and LAW in the US. Forward dissolution rates, based on Si, B and Na release, suggested that the dissolution mechanism under dilute conditions, and pH and temperature ranges of this study, was not sensitive to composition as defined by HLW-incorporation rate.

In-situ characterization of borate glass dissolution kinetics by μ-Raman spectroscopy

The dissolution behavior of binary and ternary Na–Ca-borate glasses in phosphate-containing aqueous solutions was studied by micro-Raman spectroscopy. Raman spectra collected from the solutions provided information about the types and concentrations of borate anions released from the glass as a function of time. Boric acid molecules (H3BO3) and tetrahydroxyl borate anions (B(OH)4−) can be detected in solution and their relative concentrations depend on the solution pH. By monitoring the areas under peaks associated with these two species, the overall borate release rates and variations in solution pH were determined. Binary Na-borate glasses exhibit linear dissolution kinetics and the dissolution rates decrease with Na2O-contents up to 30mol%. The ternary Na–Ca-borate glasses exhibit parabolic dissolution kinetics because of the formation of a surface Ca-phosphate phase, and these reaction rates also decrease with increasing [Na2O+CaO] content.

Micro-channel as a New Tool to Investigate Glass Dissolution Kinetics

A reliable modeling of the long-term dissolution of HLW glass requires sufficient evaluation of the glass dissolution kinetics including a sound understandings of reaction mechanism. For the evaluation of glass dissolution kinetics, we need much more data on the glass dissolution rate measured precisely, consistently and systematically under various well-constrained test conditions. The current standard test methods, unfortunately, cannot provide enough data for the kinetic evaluation. Therefore, we should improve or develop the test methods to provide precise and consitent data for the kinetic evaluation. In the present paper, therefore, major standard test methods currently applied to the measurement of glass dissolution are summarized with their advantages and disadvantages, and some newly developed test methods are introduced. In addition, problems to be solved are discussed to advance the kinetic evaluation.

Experimentally determined dissolution kinetics of SON68 glass at 90 °C over a silica saturation interval: Evidence against a linear rate law

Flow-through dissolution experiments were carried out on the SON68 glass—the inactive analog of the French high-level waste glass—at 90°C and pH=9 over a silica saturation interval [0–150ppm Si(aq)]. Two types of specimens were subjected to dissolution studies: powders (249–150, 149–75 or 74–45μm diameter size fractions) or monoliths (∼1cm×1cm×4mm). At each Si concentration interval individual coupon and glass powder experiments were run at the same flow-to-surface area (q/S) conditions. Under dilute solution conditions, the dissolution rate becomes independent of q/S and define the forward rate of reaction of 0.30g/(m2d). Dissolution rates of powders normalized on either a geometric or a B.E.T. surface area basis were compared to rates determined on monoliths. In every case, the geometric surface area normalized rates matched those of the monoliths, conclusively showing that B.E.T. normalized rates are too low by a factor of ∼3×. In Si-free solutions changes in flow rates resulted in differences in Al released to solution (∼50 to 3140ppb), correlating inversely with dissolution rates. Si was added to input solutions and kept constant (“direct”), but in select experiments Si concentrations were above target concentrations, and then dropped to their target values (“indirect”). Both “direct” and “indirect” Si addition experiments yielded identical dissolution rates within experimental uncertainty. Progressive addition of Si to the input solution caused a decrease in rates from 0.30 to 0.0020g/(m2d), or a factor of ∼150×, but the decrease is strongly non-linear with respect to Si. These data are inconsistent with simple models relating glass dissolution to the chemical affinity of reaction.

Thermodynamic Approach for Predicting Actinide and Rare Earth Concentrations in Leachates from Radioactive Waste Glasses

Studies aimed primarily at determining leach rates of different elements from doped glasses have resulted in computerized models for predicting leachate concentrations. However, leach rate related data should be limited to predicting the stability behavior of the glass matrix; the radionuclide release data based on these studies are empirical and are highly dependant on many variables and processes which have not been systematically evaluated and thus do not provide a reliable method of predicting leachate concentrations. A better approach is available for those elements that can readily form relatively insoluble solids during preparation of glass or glass/water interactions. This alternate approach relies on the experimental solubilities of pulverized doped glasses, in a wide range of well-controlled important variables such as pH and pe, and their comparisons at the given aqueous composition to predicted solubilities of known solid phases from the thermodynamic data. These comparisons are used to indirectly identify specific solubility-controlling solids in doped glass/water systems to determine scientifically defensible aqueous concentrations of different elements for any given groundwater composition, independent of glass dissolution kinetics and independent of time. This paper summarizes data available for the application of this alternate approach to reliably predict concentrations of thorium, uranium, neptunium, plutonium, and trivalent actinides and rare earth elements leachable from the doped glasses. The thermodynamic data, in addition to that reported in recent critical reviews, includes new data that were developed for the solubility products of Th3(PO4)4(s) and the solid solutions of trivalent actinides and rare earth hydroxides. Thermodynamic interpretations of the doped glass solubility data show specifically that tetravalent actinide hydrous/crystalline oxides and solid solutions of trivalent actinides and rare earths hydroxides in non-phosphate glasses and Th3(PO4)4(s) and MPO4(s), where M denotes trivalent actinides or rare earths, in phosphate-containing glasses are the dominant solubility-controlling solids. Needed future research in this area is briefly outlined.

Composition effects on synthetic glass alteration mechanisms: Part 1. Experiments

Alteration of nuclear waste glasses and silicate minerals is governed by complex processes regulated by several coupled mechanisms. Among these processes is reactive mass transfer through the amorphous gel layer (known as the passivating reactive interphase (PRI) in case of a rate-limiting effect) located between the pristine glass and the bulk solution. In order to assess the influence of the glass composition and the pH on the properties of the PRI, and thus on the nuclear glass durability, an experimental leaching study was performed on borosilicate glass samples with or without Ca, Al, and Zr. Experiments were conducted to understand the influence of the pH and glass composition on the solvated cation diffusion coefficient within the PRI and to generate data for calibration of a PRI solubility model (not presented here). All the experiments were carried out at high S/ V ratios so that silicon rapidly reached apparent saturated conditions and the PRI could form: in such conditions glass alteration is controlled only by diffusion of water and dissolved species through the PRI and by precipitation of crystallized secondary phases. The constituents in the PRI and the crystallized secondary phases depend to a large extent on the glass composition and pH. Alkali metal (Na) or preferentially alkaline earth (Ca) elements are retained in the PRI for charge compensation of Al and Zr. The apparent diffusion coefficient calculated from the release of boron, a good tracer, varies with the pH from less than 4 × 10 −22 to 9 × 10 -18 m 2 s −1 in the studied glasses. These very low diffusion coefficients decrease as the pH increases. Concerning the PRI composition we show that Si, Al, Ca and Zr have strong interactions and thus major consequences on the glass durability. Our findings indicate that the SiO 2aq activity is relatively constant and independent of the pH below pH 9, followed by a drop at pH 10. In addition, the activity of SiO 2aq is affected by the glass composition, and especially by aluminum and zirconium. As soon as dissolved silicon reaches steady state in solution the aluminum and zirconium concentrations start to decrease, probably due to silicon, aluminum and zirconium interactions with retention in the PRI. The formation of crystallized secondary phases is observed at pH 10 for aluminum-free glasses, which diminishes the saturation state of amorphous silica in solution. In these glasses the saturation index indicates that the solution is oversaturated with respect to calcium silicate hydrates (ex: tobermorite, gyrolite). Moreover, the formation of crystallized secondary phases causes dissolution of the PRI and the glass, which sustains renewed alteration. This study leads to the conclusion that modeling nuclear glass dissolution kinetics over a wide pH range (typically from pH 7 to pH 10) must take into account (1) PRI composition variations and relations between the PRI composition and properties (solubility, diffusion coefficient); and (2) crystallized secondary phases that can consume elements from the PRI. Applying PRI modeling concepts to other kinds of natural glasses or even multi-oxide minerals might prove useful for enhancing our understanding of alteration mechanisms.

A fractured roman glass block altered for 1800 years in seawater: Analogy with nuclear waste glass in a deep geological repository

Fractured archaeological glass blocks altered 1800 years in seawater are investigated because of their morphological analogy with vitrified nuclear waste. They provide an opportunity for understanding glass alteration in variable confined media (cracks), by studying an actual ancient system in a known stable natural environment. Characterization of the crack network from two-dimensional trace maps (length, alteration thickness, orientation) allows us to determine the three-dimensional geometric parameters (crack density, fracture ratio) and the percentage of alteration, using stereological relations. This methodology could be applied to nuclear glass. From a representative archaeological glass block, we showed that the surface developed by the cracks is 86 ± 27 times greater than the geometric surface but the volumetric alteration is 12.2 ± 4.1%, which is only 12 times greater than the volumetric alteration of the block periphery (about 1 vol%). This unexpected low value is explained by the large variation of the alteration thicknesses in the different types of cracks in relation with their location in the block. The alteration thickness is usually smaller in the internal zone than in the border zone. The alteration layers resulted from three main mechanisms (interdiffusion, glass dissolution, and secondary phase precipitation) leading to two different alteration products (a sodium-depleted layer and mainly a Mg-smectite). Geometric parameters such as the glass surface area/solution volume ratio and transport parameters (renewal of the alteration solution) strongly affected the glass dissolution kinetics. The confined conditions and the diffusive transport of reactive species favor low alteration kinetics. The precipitation of secondary phases also results in sealing of the cracks. Consequently, although it is not known if subcritical crack growth occurred, internal cracks account for only a minor contribution to the overall alteration. These results improve our understanding of alteration in cracks for assessing the predominant physical and chemical parameters that must be considered in long-term nuclear glass modeling.

SON68 nuclear glass dissolution kinetics: Current state of knowledge and basis of the new GRAAL model

This article summarizes the present state of knowledge concerning aqueous alteration of R7T7-type nuclear containment glasses, represented mainly by the inactive reference glass designated SON68. Based on this review, we propose to describe the glass alteration kinetics up to and including the final residual rate regime by means of a new mechanistic model known as GRAAL ( glass reactivity with allowance for the alteration layer). Phenomenological analysis findings are reviewed for the various glass alteration regimes: interdiffusion, initial rate, rate drop, residual rate and, under very particular circumstances, resumption of alteration. These alteration regimes are associated with predominant mechanisms. Published work interpreting and modeling these mechanisms was examined in detail. There is a broad consensus on the general mechanisms of the initial rate and even the interdiffusion regime, whereas the mechanisms controlling the rate drop remain a subject of dispute not only with regard to nuclear glasses but also for the dissolution of silicate minerals. The reaction affinity responsible for the rate drop is expressed differently by different authors and depending on the underlying theories. The disagreement concerns the nature of the phase (glass or gel) or the activated complex controlling the rate drop, which in turn determines the elements that must be taken into account in the overall affinity term. Progress in recent years, especially in identifying the mechanisms responsible for the residual rate, has shed new light on these issues, allowing us to propose new theoretical foundations for modeling the different kinetic regimes of SON68 nuclear glass dissolution. The GRAAL model considers that water diffusion in the passivating reaction zone (the gel formed under saturation conditions) is a rate-limiting step in the overall glass dissolution kinetics. Moreover, this passivation zone is a soluble phase whose stability is directly dependent on the nature of the secondary phases likely to precipitate and on the solution renewal conditions.

Experimentally determined dissolution kinetics of Na-rich borosilicate glass at far from equilibrium conditions: Implications for Transition State Theory

The dissolution kinetics of five chemically complex and five chemically simple sodium silicate glass compositions (Na–Si±Al±B) were determined over a range of solution saturation values by varying the flow-through rates (1–100mL/d) in a dynamic single-pass flow-through (SPFT) apparatus. The chemically complex borosilicate glasses are representative of prospective hosts for radioactive waste disposal and are characterized by relatively high molar Si/(Si+Al) and Na/(Al+B) ratios (>0.7 and >1.0, respectively). Analysis by X-ray absorption spectroscopy (XAS) indicates that the fraction of ivB to iiiB (N4) varies from 0.66 to 0.70. Despite large differences in bulk chemistry, values of δ29Si peak shift determined by MAS-NMR varies only by about 7ppm (δ29Si=−94 to −87ppm), indicating small differences in polymerization state for the glasses. Forward rates of reaction measured in dynamic experiments converge (average log10 rate [40°C, pH 9]=−1.87±0.79 [g/(m2d)]) at high values of flow-rate (q) to sample surface area (S). Dissolution rates are independent of total Free Energy of Hydration (FEH) and this model appears to overestimate the impact of excess Na on chemical durability. For borosilicate glass compositions in which molar Na>Al+B, further addition of Na appears to stabilize the glass structure with respect to hydrolysis and dissolution. Compared to other borosilicate and aluminosilicate glasses, the glass specimens from this study dissolve at nearly the same rate (0–∼56×) as the more polymerized glasses, such as vitreous reedmergnerite (NaBSi3O8), albite, and silica. Dissolution of glass follows the order: boroaluminosilicate glass>vitreous reedmergnerite>vitreous albite>silica glass, which is roughly the same order of increasingly negative 29Si chemical shifts. The chemical shift of 29Si is a measure of the extent of bond overlap between Si and O and correlates with the forward rate of reaction. Thus, dissolution appears to be rate-limited by rupture of the Si–O bond, which is consistent with the tenants of Transition State Theory (TST). Therefore, dissolution at far from equilibrium conditions is dependent upon the speed of the rate-controlling elementary reaction and not on the sum of the free energies of hydration of the constituents of boroaluminosilicate glass.

Long-term corrosion of two nuclear waste reference glasses (MW and SON68): A kinetic and mineral alteration study

Powder samples of two inactive borosilicate glasses (MW and SON68), used as references for vitrified nuclear waste in Switzerland, were leached statically in pure water over more than 12a at 90 °C. Solution aliquots were taken at regular intervals in order to investigate the glass dissolution kinetics and the retention of elements representing radionuclides. At 5.7 and 12.2a, single tests were interrupted to investigate the corroded samples. Boron and Li concentration data indicate that the glass corrosion kinetics of the MW glass is about 10 times faster than for the SON68 glass, both in the transient and asymptotic phase of the leaching process. The alteration products were studied by X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM-STEM) coupled with energy dispersive spectrometry (EDS) analyses. Alteration of the MW glass produced abundant magnesian clay minerals, as well as nanoparticles of lanthanide and Zr phosphates, whereas only small amounts Ca–Zn–Fe–Ni clay minerals were formed as alteration product of SON68. Retention factors were above 99% for most trace elements, indicating almost quantitative fixation of many radionuclides in the secondary phases. Solution concentration data were used to calculate aqueous speciations and saturation indices of potential secondary solids. The solutions are close to saturation with respect to simple lanthanide phosphates (in agreement with the TEM data) and quartz. The presence of quartz in the altered SON68 samples is corroborated by XRD data. In conclusion, the corrosion and radionuclide retention properties of SON68 seem to be more favorable than those of the MW glass. A major finding of this investigation was that glass degradation may strongly depend on minor compositional changes in the glass composition. The presence of Mg in the MW glass triggers the nucleation of secondary clay minerals, thereby promoting glass corrosion via silica removal. In the Mg-free SON68 glass the formation of clays, and hence the glass degradation, were considerably slower.

Son68 Glass Dissolution Kinetics at High Reaction Progress: Mechanisms Accounting for The Residual Alteration Rate

ABSTRACTIn a highly confined medium corresponding to geological repository conditions, the alteration rate of the French SON68 inactive nuclear reference glass drops by about four orders of magnitude below the initial rate. However, extended experiments lasting months or years provide evidence of a virtually constant or slowly decreasing residual alteration rate. Although very low, this rate could account for most of the altered glass thickness after 10 000 years. Experiments at high temperatures and especially high glass-surface-area-to-solution-volume ratios were performed to reveal and quantify the predominant mechanisms underlying the residual rate. The authors describe the characterization of the solution chemistry, the crystallized secondary phases, and the amorphous gel observed after alteration of the French SON68 inactive reference glass, and discuss their implications in terms of long-term behavior modeling. A slow diffusion mechanism is identified in the solid, involving alkalis in particular but also boron. This mechanism results in higher concentrations in solution that affect the system chemistry, not only by slightly modifying the pH and element speciation in solution (e.g. silicon), but also by inducing the precipitation of new crystallized secondary phases that can consume glass constituent elements in the same way as simple solution renewal. Diffusion and the precipitation of secondary phases are two mechanisms to be considered to account for the residual rate.

SON68 Glass Dissolution Kinetics at High Reaction Progress: Experimental Evidence of the Residual Rate

ABSTRACTSON68 borosilicate glass dissolution involves several parallel mechanisms: interdiffusion, hydrolysis of the silicate network, recondensation of dissolved species leading to the formation of an amorphous gel layer and precipitation of secondary crystalline phases. In a highly confined medium corresponding to geological disposal conditions, the gel formed during SON68 glass alteration quickly becomes protective and the hydrolysis rate drops by about 4 orders of magnitude compared with the initial dissolution rate. This study focuses on the state of reaction progress corresponding to apparent saturation of the solution. At 90°C in static mode, new experimental data show that the decreasing rate due to the protective gel formation is followed by a quasi-constant residual rate. Several hypotheses are discussed. We first demonstrate that silica dissociation related to a slowly rising pH is not a valid explanation. This hypothesis proposed 15 years ago was consistent with a rate controlled by affinity (equilibrium between the glass and solution). The hypothetical precipitation of secondary crystalline phases that consume elements from the protective gel layer is then considered in the light of data obtained with a simplified 8-oxide glass, but this mechanism is not sufficient to account for residual rate of the SON68 glass. Other hypotheses based on the gel evolution (dissolution, evolution of the porous texture) are also proposed. The proposed mechanisms imply the persistence of a protective gel with a constant thickness. Finally, this paper shows that the residual rate could be a decisive phenomenon affecting the long term behavior of glass in disposal conditions.

Dissolution kinetics of basaltic glasses: control by solution chemistry and protective effect of the alteration film

Basaltic glasses are considered as natural analogs for industrial nuclear aluminoborosilicate glasses. Alteration experiments were conducted in closed and open systems at 90°C with a synthetic basalt glass doped with 1% lithium (dissolution tracer). The evolution of the alteration kinetics over time was assessed by comparison of reaction progress at different degrees in closed system experiments. The maximum dissolution rate (initial rate, r 0) was comparable to the value observed for an SON68-type nuclear glass; the basaltic glass alteration rate subsequently dropped by four orders of magnitude. The kinetic models currently proposed in the literature to account for the alteration kinetics of basaltic glasses, nuclear glasses or aluminosilicate minerals are based on the concept of chemical affinity: the chemical affinity alone is assumed to control the dissolution kinetics. When applied to the experimental data for the closed system tests with basaltic glass, these models failed to account for the low measured rates. An inhibiting effect of dissolved silica was then investigated through open system basaltic glass alteration experiments with silicon-enriched solutions. The basaltic glass dissolution rate dropped by a factor not exceeding 200 (or about two orders of magnitude) compared with r 0 at the high imposed silicon concentrations (130 ppm). A protective effect of the alteration film was advanced to account for the four-orders-of-magnitude rate drop observed in closed system experiments, based notably on an examination of natural basaltic glasses ranging in age from several thousand to a few million years. The mean alteration rates can be estimated from the measured palagonite thicknesses, taking into account the age of the natural glasses; the rates were comparable to those measured in the laboratory for high reaction progress.

Use of orthophosphate complexing agents to investigate mechanisms limiting the alteration kinetics of French SON 68 nuclear glass

This investigation was carried out to assess the protective properties of the alteration film that develops during aqueous alteration of the French SON 68 (R7T7-type) nuclear glass, notably by examining the behavior of some network-forming cations in the presence of complexing anions. Glass alteration was studied here in the presence of orthophosphate ions. Comparisons were established between two series of tests performed with a solution containing orthophosphate ions and control tests performed under the same conditions but without phosphates. The first series of experiments was performed under initial rate conditions (i.e. in dilute media) to assess the effect of pH and phosphate concentration on the initial glass dissolution rate. Under these conditions, which ensure maximum chemical affinity of the glass dissolution reaction, phosphate adsorption occurs at the reaction interface only with acid pH values, at which the glass dissolution reaction is strongly inhibited. The elements that form complexes with the phosphates (Al, Fe, etc.) partially control glass dissolution in acidic media. Additional experiments carried out under saturated conditions — notably with respect to Si — in a solution enriched with phosphates showed that rare earth and Ca phosphates precipitated in the outer region of the alteration film, maintaining a glass dissolution rate significantly higher than in the control experiment. These observations have several implications. (1) Comparing the results obtained in the presence of phosphates and in the reference medium, the authors demonstrate deductively that glass dissolution is limited by the inner portion of the alteration film, i.e. the amorphous gel. (2) A kinetic law of SON 68 glass dissolution cannot be based on silica alone; the results of these experiments contradict Grambow’s model. (3) With regard to control of the glass dissolution kinetics by the protective properties of the gel, this type of experiment shows that the relation between the chemical composition and the microstructure of the gel is an important aspect in modeling the glass alteration kinetics, but that it is still poorly understood.

Kinetics of leaching near the percolation threshold

Numerical simulation of random binary solid leaching kinetics in which one component is selectively soluble were examined. The kinetics are supposed to be controlled by the dissolution rate of this component. Time power laws were observed near percolation threshold for the advance of the leaching front ( x m∼ t a with a=0.73 in three dimension (3D)) and for the amount of dissolved atoms ( n d∼ t b with b=0.43 in 3D). The exponents a and b are shown to be related to the fractal dimension d f and to the minimum path dimension d m of the percolation clusters: a=1/ d m and b=( d f + 1 − d)/ d m. Application to glass dissolution kinetics is discussed.

Measurement of kinetic rate law parameters on a NaCaAl borosilicate glass for low-activity waste

The dissolution kinetics of a NaCaAl borosilicate glass, being studied for immobilization of low-activity waste, were measured between 20 and 90°C and solution pH between 6 and 12 using the single-pass flow-through method. Dissolution kinetics measurements are needed to parameterize a mechanistic model that is being used to compute the corrosion rate of the glass waste form as a function of temperature, pH, and the concentrations of the other glass components in water percolating through a proposed shallow-land disposal facility. The key factors that were found to influence the test results include test duration and background subtraction of the raw data. Background subtraction is shown to be important to prevent a non-physical increase in the computed rate with increasing flow rate, particularly in tests run at higher flow rates. Experimental factors that were found to have no detectable influence on the test results included the glass particle size and buffer type. We also illustrate how flow rate variations can be used to obtain information about the reaction order and equilibrium constant parameters in a conventional transition-state theory rate law.

Attenuation of glass dissolution in the presence of natural additives

The dissolution kinetics of silicate glasses in aqueous environments in systems which included a variety of natural crystalline solids in addition to the glass itself and the aqueous phase are reported. The results demonstrate the possibility of a dramatic decrease in the rate of dissolution of silicate glass in the presence of certain varieties of olivine-based materials. This decrease in dissolution rate was shown to be due to the fact that these additives consist mostly of Mg-based material but also contain minor amounts of Al and Ca. The combined presence of Mg with these minor species affected the corrosion rate of the glass as a whole, including its most soluble components such as boron. This study has potentially important implications to the durability of glasses exposed to natural environments. The results may be relevant to the use of active backfill materials in burial sites for nuclear waste glasses, as well as to better understanding of the environmental degradation of natural and ancient glasses.

Experimental investigation of aqueous corrosion of R7T7 nuclear glass at 90°C in the presence of organic species

The initial dissolution kinetics of the French “R7T7” inactive LWR reference glass was investigated in organic media at 90°C by static experiments at imposed pH values. Tests were conducted with various organic acids or acid salts (formic acid, citric acid, humic acids, sodium citrate and sodium formate) at different concentrations and at pH values ranging from 3 to 9. The leachates were submitted to ICP-AES analysis, and the alteration films on the glass specimens were examined by SEM (equipped with an energy-dispersive chemical analysis systm) and XRD. Results are discussed for the following areas. Effect of pH: different mechanisms are involved at pH 2.5 and pH 9, but in both cases, the R7T7 glass dissolution kinetics are always controlled by a surface reaction mechanism. The most significant effects of the organic acids on the glass dissolution kinetics were observed at pH 3, which was not unexpected. Nature of the acid (or acid salt): this is an essential parameter. Formic acid (monoacid) and sodium formate had no effect on the initial glass dissolution rate nor on the nature of the alteration film; conversely, citric acid (triacid) and sodium citrate significantly affected glass dissolution. The role of citric acid in the surface reaction mechanism has been demonstrated experimentally at pH 2.5; it is also known to favor that extraction of Si (directly or indirectly) and Zr and probably of Fe. The humic acids are strong catalysts of the R7T7 glass dissolution reaction. The conclusions drawn from these observations corroborate other published findings, suggesting that the catalytic effect is appreciable only for species with at least two carboxylic acid functions. Effect of concentration: the effect of the concentration was observed at basic pH values in the presence of sodium citrate and humic acids. This parameter may thus be quantified and incorporated in the glass dissolution kinetic law.

Dissolution kinetics of silica glass as a function of pH between 40 and 85°C

Anhydrous vitreous silica glass was reacted in solutions buffered from pH 1.1 to pH 10.9 between 40 and 85°C for up to 1500 h. The initial release of silicon to solution is proportional to the square root of time, consistent with a surface diffusion process. At later times, the release becames linear with time, indicating surface reaction control. The reproducibility of square root of time release rates with previously used glass can be explained within the context of current models of vitreous silica structure. At 65°C, above pH 6.5 the log diffusional rate constants increase with pH such that δ log k/δpH = 0.4, while below pH 6.5 the diffusional rate constants are nearly pH-independent. The surface reaction release rate constants show a similar pH dependence with δ log k/δpH = 0.5 above pH 7, while below pH 7 the rate constants are nearly pH-independent. The activation energy of surface reaction is 22.7 ± 3.7 kcal mol−1 at pH 4 for vitreous silica. This value is consistent, but slightly higher than that of Rimstidt and Barnes [Geochim. Cosmochim. Acta 44 (1980) 1683] for amorphous silica.

Modeling of Solution Renewal with the Kindis Code: Example of R7T7 Glass Dissolution at 90°C

AbstractThe deep underground environment that would correspond to a geological repository is a system open to fluid flow. It is therefore necessary to investigate the effects of solution renewal on the long-term behavior of glass in contact with water. These effects can now be simulated using the new version of the geochemical Kindis model (thermodynamic and kinetic model).We tested the model at 90°C with an SAIV ratio of 400 m−1 at twelve renewal rates of pure water ranging from 200 to 0 vol% per day. With renewal rates between 200 and 0.065 vol% per day, steady-state conditions were obtained in the reaction system: i.e. the glass corrosion rate remained constant as did the concentrations of the dissolved species in solution (although at different values depending on the renewal rate). The ionic strength never exceeded 1 (the validity limit for the Debye-Huckel law) and long term predictions of the dissolved glass mass, the solution composition and the potential secondary mineral sequence are possible. For simulated renewal rates of less than 0.065 vol% per day (27 vol% per year), the ionic strength rose above 1 (as in a closed system) before steady-state conditions were reached, making it critical to calculate long-term rates; A constant and empirical long-term rate, derived from laboratory measurement, have to be extrapolated. These calculations were based on a first order equation to describe the glass dissolution kinetics. The results obtained with the KINDIS code show discrepancies with some major experimental kinetic data (the long term rate must decrease with the « glass-water » reaction progress, under silica saturation conditions). This clearly indicates that a more refine kinetic relation is needed for the glass matrix.