Rachid Bouakkaz*a,
Abdesselam Abdelouasa,
Yassine El Mendilia,
Bernd Grambowa and
Stéphane Ginb
aSUBATECH – Ecole des Mines de Nantes-CNRS/IN2P3-Université de Nantes, 4, rue Alfred Kastler, B.P. 20722, 44307 Nantes, France. E-mail: rachidbouakkaz@yahoo.fr; Rachid.Bouakkaz@subatech.in2p3.fr
bCEA Marcoule DTCD SECM LCLT, 30207 Bagnols-Sur-Cèze, France
First published on 14th July 2016
Pristine and 29Si-doped SON68 glass was leached in dynamic mode under silica-rich synthetic Callovo-Oxfordian (COx) groundwater at pH 8, a glass surface-to-liquid volume (S/V) ratio (900–1800 m−1) and at 35, 50 and 90 °C. The solutions were analysed by ICP-MS and ion chromatography, the alteration products were studied by electron microscopy and Raman spectroscopy, and the Si-isotopes profiles were obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS). The glass alteration seems to be governed by both diffusion and surface reaction processes. After 653 days of alteration the normalized leaching rates were 2.6 (±0.2) × 10−5, 9.9 (±0.8) × 10−5 and 2.4 (±0.2) × 10−3 g m−2 d−1 at 35, 50 and 90 °C, respectively. The major alteration secondary phase is clay-type Mg-silicates at all temperatures in addition to powellite and apatite at 90 °C. SIMS studies clearly showed uptake of 29Si by the surface gel via condensation and in the Mg-rich phyllosilicates via precipitation from solution. The precipitation of phyllosilicates at all temperatures constitutes the main process which destabilises the gel layer, thus maintaining a long-term glass dissolution rate in COx groundwater, higher than in pure water.
Many different studies under water saturated conditions have demonstrated that glass corrosion is a result of various processes. The first one is the hydration of the glass and the diffusion of water into the glass network followed by ion exchange between protons in solution and alkali metals located at network terminal sites. This process leads to the formation of a hydrated glass surface region.21 The release of glass elements follows a slope of −1/2 in double logarithmic diagrams, indicating a diffusion process.1 A recent study by Neeway et al.22 using Atom Probe Tomography (APT) clearly developed an interdiffusion model based on Fick’s second law to derive diffusion coefficients of alkalis. In silica saturated conditions, the mass loss of glass tracers follows also an inverse square root of time dependence.23 Many studies showed that the hydration and interdiffusion processes also involves the hydrolysis of the bonds of boron in the hydrated glass, its dissolution and release of B species although it is a glass former.24,25 As a consequence of these reactions, the remaining silicate network undergoes deep reorganizations to balance the charges and minimize the free energy of the system.25 The second key process is the hydrolysis, which implies the attack of the Si–O–M bonds (M = Si, Al, Zr,…) by OH−, H2O or H3O+, leading to a further disturbance of the silica network connectivity.26 This process leads to dissolution of silica. A recent study by Hellmann et al.27 proposed another mechanism for glass corrosion using a variety of characterization tools including APT. The mechanism, denoted as ‘interfacial dissolution–precipitation’, describes the glass dissolution as an interaction between water molecules and nanometre-scale level glass surface leaving the soluble species in solution (Na, Li, B,…) while the less soluble species (Si, Fe, Al,…) precipitate.
In systems with limited water volume, accumulation of dissolved Si will occur until a pH dependent certain concentration limit, which is often termed “saturation concentration”. For Si concentrations far below saturation, the glass dissolution rate is the fastest and depends essentially on the temperature, pH and glass composition28–31 and to a lesser extent to the presence of some inorganic32 or organic species.33 Saturation occurs due to the existence of a backward reaction of hydrolysis, i.e. the condensation reaction of silica. The backward reaction occurs in a wide range of pH and involves partly or totally detached species from the glass surface.34 This process leads to the formation of a gel at the glass surface. This step is tied to a slowing of the dissolution rate. The drop of the rate is due to (1) an affinity process (i.e. saturation of the solution with dissolved species;6), (2) the growth of the gel layer that forms a diffusion barrier between the pristine glass and the leachate.7,35 After this step, the dissolution rate of the glass continues to drop to reach the residual rate. For the SON68 glass a residual rate of 10−4 g m−2 d−1 is assumed in deionized water at 90 °C, a value about four orders of magnitude smaller than the initial rate.36,37 This regime corresponds to the so called residual rate. It has been observed under specific conditions (pH > 10 and or T > 100 °C) that a resumption of alteration may occur after some time of glass corrosion with the residual rate. This effect was attributed to the precipitation of zeolite minerals such as analcime or philipsite at the gel surface2,38,39 changing the solution chemistry and the transport properties of the gel. Fig. 1 reminds the relationships between the basic mechanisms and kinetic regimes of glass dissolution. It appears that the study of the formation of the gel layer is crucial to better understand and model the mechanisms of glass alteration. The gel is amorphous, porous and rich in silica, in very sparingly soluble elements (Al, Fe, Zr,…), in alkaline-earth elements and in actinides.40 However, it is depleted in soluble glass constituents (Na, Li, B, Mo,…) and alkali elements.41 The gel structure (e.g. mechanical cohesion, porosity,…) are greatly influenced by several parameters such as pH, temperature, length of experience, solution composition and the glass composition.42 At present, the mechanism of gel formation is controversial. Two major hypotheses have been proposed. For basaltic glasses, many authors consider a mechanism of dissolution/precipitation.12,43,44 This process is often thought of requiring a thermodynamic equilibrium with the entire solution (case of static test conditions). The layer formed on the basalt glass surface, denoted palagonite, has a different nature from that of the pristine glass. This phenomenon has been observed in the alteration of basaltic glass in seawater.45 Concerning nuclear waste glasses, some authors46,47 studied the structure of the gels by Raman spectroscopy and nuclear magnetic resonance (NMR) for 29Si and 17O isotopes, and highlighted the hypothesis of in situ silica condensation. Unlike the mechanism of dissolution/precipitation, this process involves a thermodynamic equilibrium with local chemical environments, which can be different from that of the entire solution. Both proposed mechanisms are not incompatible and could coexist. In diluted solutions they may be linked by the rate of transport of dissolved silica in the aqueous solution, while in saturated solutions both processes appear to be identical. To study the mechanism of dissolution/precipitation versus Si condensation, previous experiments have been done using 29Si-rich tracing solutions: (1) in static mode at a high S/V ratio,48 and (2) in dynamic mode at low S/V ratio.49,50 The isotopic analysis of altered glass layers by ionic microprobe (SIMS) revealed that the gel was formed by hydrolysis/condensation mechanism whereas phyllosilicates were formed by dissolution/precipitation mechanism.49 A recent study performed with the international simple glass (ISG), a 6 oxide glass with the same molar ratio as the R7T7 reference glass,51 showed that under silica saturation conditions, the gel is only formed by in situ condensation of Si–O–M bonds (M = Si, Al, Zr) following the release of Na, Ca and B atoms.25
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Fig. 1 A schematic representation of the main kinetic regimes of the glass corrosion process (adapted from Poinssot and Gin53). |
The present study was carried out with two main goals: (a) to investigate the dissolution rate of SON68 glass in flowing solutions through experiments at S/V ratio between 900 and 1800 m−1 and at 35, 50 and 90 °C. The choice of these temperature values was motivated by the wish to cover several situations: in the French design 90 °C is the expected temperature shortly after the closing of the disposal galleries; 50 °C is the expected temperature after the breach of the overpack containers and 35 °C is close to the temperature of Callovo-Oxfordian formation at 500 m depth.52 Many experiments have been conducted at 90 °C but only few data are available at lower temperature. (b) To better understand the mechanisms responsible for the formation of the alteration layer (gel + secondary crystalline phases) by tracing isotopic exchanges between the solution and the glass. To do so, we performed two additional experiments using 29Si doped SON68 and 29Si enriched COx water. Despite the advantages of using 29Si as a tracer, we are not aware of literature work on 29Si doped SON68 glass. Here, insight into glass corrosion mechanisms relies on the use of isotope sensitive analytical techniques in the solid and liquid phase.
Sgeo = (3/ρ × r) |
Oxide | wt% | Oxide | wt% | Oxide | wt% |
---|---|---|---|---|---|
SiO2 | 45.85 | ZnO | 2.53 | Nd2O3 | 2.04 |
B2O3 | 14.14 | P2O5 | 0.29 | Pr2O3 | 0.46 |
Na2O | 10.22 | SrO | 0.35 | Ag2O | 0.03 |
Al2O3 | 5.00 | ZrO2 | 2.75 | CdO | 0.03 |
CaO | 4.07 | MoO3 | 1.78 | SnO2 | 0.02 |
Li2O | 1.99 | Cs2O | 1.12 | TeO2 | 0.23 |
Fe2O3 | 3.03 | BaO | 0.62 | Ce2O3 | 0.97 |
NiO | 0.43 | Y2O3 | 0.20 | Others | 0.39 |
Cr2O3 | 0.53 | La2O3 | 0.93 |
Recent publications suggested the use of geometric surface area rather than BET data because of high uncertainties for glass powder.55,56 Fournier et al.55 conducted dissolution experiments with a soda lime glass in the form of monoliths with a known geometric surface area and glass powder. They found that the rates measured on powders using geometric surface area (Sgeo) instead of BET surface area (SBET) are closest to those found for monoliths. They suggested that features (e.g. defects) at the atomic scale could contribute to overestimation of the SBET. Thereby, we chose to use the geometric surface area for glass dissolution kinetics calculations.
Component | Concentration (mmol L−1) | ||
---|---|---|---|
35 °C | 50 °C | 90 °C | |
Cl | 41 | 41 | 41 |
S(VI) | 15.6 | 14 | 10 |
Na | 45.6 | 42 | 39 |
K | 1 | 1 | 0.96 |
Ca | 7.36 | 9.9 | 10 |
Mg | 6.67 | 4.1 | 2.5 |
Sr | 0.17 | 0.2 | 0.17 |
Si | 0.18 | 0.35 | 0.84 |
TIC | 2.62 | 3.05 | 3.3 |
Parameter | Leaching experiments | ||
---|---|---|---|
Non-doped SON68 + non doped COx water | 29Si-doped SON68 + non-doped COx water | Non-doped SON68 + 29Si-doped COx water | |
Temperature | 35, 50 and 90 °C | 90 °C | 90 °C |
Flow rate | 4 ± 1 mL per day | 4 ± 1 mL per day | 4 ± 1 mL per day |
Mass of the glass | 0.5773 g | 1.0598 g | 0.5575 g |
Surface | 0.027134 m2 | (0.0515 + 0.00022) m2 | (0.0262 + 0.00022) m2 |
Volume | 30 mL | 29 mL | 29 mL |
S/V | ∼900 m−1 | ∼1800 m−1 | ∼900 m−1 |
Experiment period | 653 days | 485 days | 456 days |
The morphological and chemical analyses of corroded monoliths and powders were achieved using a JSM 5800 LV scanning electron microscope (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX). The samples were coated with a thin carbon layer in order to get a better resolution of the K-lines for lighter glass components (Si, Al and Ca). Analyses were done under an accelerating voltage of 15 kV. The final compositions of analysed areas were calculated assuming oxide stoichiometry and normalisation to 100%. Altered samples were also analysed by X-ray diffractometry (XRD). For all samples, XRD patterns were characteristic of amorphous materials with the absence of diffraction peaks.
Corrosion products were identified by micro-Raman spectroscopy. Measurements were performed at room temperature using a T64000 Jobin-Yvon/LABRAM spectrometer equipped with a 600 lines mm−1 diffraction grating. The instrument is equipped with an Olympus microscope (×100 objectives) and Ar+–Kr+ laser (514 nm exciting line). The power of illumination was about 5 mW at the sample in order to prevent any deterioration of the material. Single spectra were obtained in the 100–2200 cm−1 range with an integration time of 600 s.
TOF-SIMS technique was used to obtain isotopic Si ratios and elemental depth profiles in the corrosion layers. Measurements were performed at the University of Lyon using an IONTOF GmbH TOF SIMS instrument. Profiles were achieved by alternating abrasion and analyses cycles. Abrasions were run with a 2 keV primary O2+ ion beam on an area measuring 300 × 300 μm2. Analyses were carried out using a focused Bi+ primary beam at 25 keV on an area measuring 100 × 100 μm2.
![]() | (1) |
Normalized dissolution rate NLRi+1 was calculated in units of grams of glass dissolved per square metre per day (g m−2 d−1) from the following relation:
![]() | (2) |
Experimental uncertainty of the glass dissolution rate was determined by calculating the standard deviation of the function f(x1, x2,⋯, xn) from the following equation:
![]() | (3) |
![]() | (4) |
Resulting errors of dissolution rates were near 17% and were dependent essentially on the errors associated with the tracer concentrations in the solution.
The concentrations of the leached glass elements in the outlet solution at 90 °C are plotted versus the alteration time in Fig. 3(b), and the same trends are observed at 35 and 50 °C. During the first 14 days, concentrations increase rapidly reaching maximum values, followed by a decrease before reaching a steady state. This phenomenon may be due to the transient reactor operation. The behaviours of B, Li, Cs and Mo are similar. These elements are released from the glass until 653 days. The increase in the concentration of B towards 260 days is an experimental artefact likely due to solution filtration (filter failure) or apparatus malfunctioning. The concentration of Si represents only a calculated concentration value corresponding to the release by the glass, calculated by subtracting Si in the input solution from Si in the leachate. It must be kept in mind that: (1) a fraction of the leached Si is retained in both alteration gel and secondary precipitates, and (2) a part of Si from the input solution can also form secondary phases on the glass surface. This calculation leads to negative concentrations after 163 days, suggesting that some Si is consumed during the reaction. The 29Si isotopic tracing allows us to accurately perform mass balance calculation and specifically determine the fraction of Si released by the glass (see below).
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Fig. 4 Evolution of elements dissolution rates (NLR, g m−2 d−1) as a function of corrosion time at 35 °C (a), 50 °C (b) and 90 °C (c and d). Errors in NLR values are near 17%. |
Element | NLR (g m−2 d−1) | ||
---|---|---|---|
35 °C | 50 °C | 90 °C | |
B | 2.6 (±0.2) × 10−5 | 9.9 (±0.8) × 10−5 | 2.4 (±0.2) × 10−3 |
Li | 2.3 (±0.3) × 10−5 | 6.6 (±0.9) × 10−5 | 1.7 (±0.3) × 10−3 |
Cs | 1.9 (±0.3) × 10−5 | 4.6 (±0.6) × 10−5 | 1.3 (±0.2) × 10−3 |
Mo | 8.7 (±1.4) × 10−6 | 1.9 (±0.2) × 10−5 | 4.7 (±0.7) × 10−4 |
The normalized leaching rate of Si is lower than that of B. This is much more remarkable at 90 °C where NLR B is about 1.5 orders of magnitude higher than NLR Si. This may be explained by the incorporation of silica in secondary phases at this temperature. Other sparingly soluble glass constituents are probably as well incorporated in this phase.
As shown in Fig. 4(a) and (b), the release of tracers at 35 and 50 °C seems to follow a −0.8 gradient. This value represents neither a diffusion-controlled process (−0.5 gradient) or stop of reaction and dilution (−1 gradient),1 and a detailed interpretation is not available. For the experiment at 90 °C, the first 30 days of glass alteration show rather a −0.5 gradient (Fig. 4(c)), which indicates a diffusion process. Whereas for longer durations, a gradient of −1 suggests strong slow down of reaction rates or even a stop of reaction (the gradient of −1 is an indicator of a simple dilution process of initially leached glass constituents in the continuously flowing water, Fig. 4(d)). The results in Table 4 show that after 653 days of alteration, the residual rate of SON68 glass at 90 °C is reached (2.4 × 10−3 g m−2 d−1). This value is about 10 times higher than that obtained for glass dissolution in pure water in static mode.36,37 It should be noted that the rate of the glass altered at 35 °C is about two orders of magnitude smaller than the glass altered at 90 °C.
Fig. 5 shows the normalized dissolution rate for boron as a function of time at 90 °C obtained in this work and compared to previous results realised by Neeway.58 The experiments by Neeway58 were run under the same conditions as this work (pH, temperature and flow rate) by varying the solution composition (pure water), the silica concentration in inlet solutions (25 and 75 ppm) and the glass powder sizes (Ø = l μm). These experiments were stopped after 170 and 300 days for solutions with initial silica concentration of 75 and 25 ppm, respectively. This is due to the complete dissolution of the glass because the use of very fine powder. In our work the experiments were conducted up to 650 days. Also the initial glass dissolution rate obtained in this work is higher than that obtained by Neeway58 even for experiments using lower initial silica concentration (25 ppm). This is due to the presence of Mg in COx water used in this work, which leads to silica consumption via Mg-silicates formation and thus increases glass dissolution rate. Also, the high S/V used in Neeway58 work is expected to lead to a rapid saturation with regard to silica for instance and also a lower dissolution rate.
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Fig. 5 Normalized leaching rates of boron (NLR, g m−2 d−1) versus time at 90 °C, pH 8, and by using silica saturated COx water, results obtained in this work are compared to those obtained by Neeway58 under the same conditions by using silica saturated pure water. |
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Fig. 6 Normalized dissolution rates of B (a), Li (b), Cs (c) and Mo (d) (NLR, g m−2 d−1) as a function of time at different temperatures. |
Fig. 7(a) shows a plot of the logarithm of release rates for B, Li, Cs and Mo versus the inverse of the absolute temperature. The rates data are the normalized release rates calculated at 482 days leaching. The apparent activation energy (Ea, J mol−1) may be obtained from these plots using the Arrhenius equation:
![]() | (5) |
Elements | Activation energy (kJ mol−1) | |
---|---|---|
Time (days) | ||
3 | 482 | |
B | 75 (±10) | 70 (±11) |
Li | 73 (±11) | 70 (±12) |
Cs | 71 (±10) | 69 (±12) |
Mo | 71 (±12) | 69 (±13) |
The Ea values calculated in the current work are in good agreement with results published in the literature. Ferrand et al.1 calculated Ea from the water diffusion coefficients in the SON68 glass powder (Ø = 20 μm). Experiments were conducted in dynamic systems at 50 and 90 °C with silica rich solution at pH 4.8, 7.2 and 9.8, giving Ea values of 49, 52 and 85 kJ mol−1, respectively. Many other works11,12,54 conducted with different glass compositions, including basaltic glasses, give similar activation energy values (72–77 kJ mol−1).
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Fig. 8 SEM micrographs of glass powders altered at 35 (a), 50 (b) and 90 °C (c) during 653 days. SEM micrograph of the SON68 glass altered for 653 days at 90 °C with the corresponding EDX (d) and Raman spectra compatible with hydrated magnesium silicate (talc)60 (e). |
At 90 °C, the precipitation of Mg-silicate was important enough to drive the pH to a lower value than at lower temperatures. According to Debure et al.61 the reactions of Mg-silicate formation can be written as:
(![]() ![]() | (a) |
3Mg2+ + 4H4SiO4(aq) + 2H2O ↔ Mg3Si4O10(OH)2 + 6H3O+ | (b) |
According to this reaction, the release of magnesium into the pore water by Mg bearing minerals of the clay stone could have a negative effect on the long term behaviour of the glass through consumption of silica in solution, thus sustaining glass dissolution. Conversely, the long-term behaviour may be somewhat stabilized by the release of protons in this reaction, lowering the pH. Our results suggest that the corresponding pH is around 7.7 at 90 °C (Fig. 3(a)).
Jollivet et al.62 have also demonstrated, by modelling of the SON68 glass alteration in COx ground water with GRAAL model,2 that the precipitation of secondary Mg phyllosilicates lowers the pH of the leachate and enhances the dissolution of the passivating layer, increasing consequently the diffusivity of this layer. In static conditions, when all the magnesium in solution has precipitated, the simulations showed that the pH returns slowly to the values usually measured for the alteration of glass in pure water.
Fig. 9(a)–(c) shows the evolution of Si and Mg concentration as a function of time at different temperatures. We can notice that the initial measured concentration of Mg, and to a lesser extent of Si, are lower than the nominal concentrations indicated in Table 2. This can be explained by the partial precipitation of these elements during water storage. The first instants of glass dissolution are characterized by a significant increase in the Si concentration combined with a decrease in Mg concentration. Thereby, the concentration of Mg reaches a minimum value for a maximum Si concentration after 14 days of alteration. Gradually, the glass releases less Si and Mg concentration increases to reach a steady state. In the first step of alteration where the glass releases much Si, the pH increases and Mg reacts to form magnesium silicates. For the experiment carried out at 90 °C, the concentrations of Si and Mg in the input solution are about 38.5 and 55.9 ppm, respectively. At steady state, their concentrations are about 30.1 and 50.3 ppm, respectively (Fig. 9(c)). The difference in concentration of each element between the input solution and the leachate represents the precipitated concentration of element in the reactor. Therefore, the Si/Mg mass ratio of precipitated elements is equal to 1.37, which is not so far from that of talc stoichiometry (1.53). The difference could be attributed to Si consumption by the gel. For experiments carried out at 35 and 50 °C the Si/Mg mass ratios were 1.44 and 1.22, respectively.
The evolution of Si concentration as a function of time at different temperatures is presented in Fig. 9(d). The decrease of Si concentration is high at 90 °C, this is probably due to the fact that Mg silicate precipitation is favoured at high temperature. Thus the pH is controlled by both the formation this phase and the release of the glass alkali elements in solution. Indeed, during the first two months of alteration at 90 °C the pH remains relatively high (Fig. 3(a)) despite a precipitation of a large amount of Mg silicates which coincides with a sharp decrease in Mg concentration (Fig. 9(c)). During this period the glass releases the maximum of its components including the Si ([Si] > 42 ppm). A competition seems to occur between the two processes. The precipitation of Mg silicates becomes dominant and controls the pH of the solution when the release of the glass alkali elements drops. It should nevertheless be emphasized that the pH values in the leachate do not reflect the local pH values of water in the altered layer pores that can be higher.63
SEM/EDX analyses revealed the formation of a rich Mo and Ba phase on the glass sample altered at 90 °C, suggesting the precipitation of powellite. The presence of this phase is confirmed with micro-Raman analyses (Fig. 10). The retention of Mo in alteration products was also observed by Neeway et al.3 It is known that (Ba,Ca)MoO4 powellite phase is the primary Mo containing crystalline phase that forms in the alteration of nuclear waste glass.64 The powellite mineral is capable of incorporating a wide range of trivalent actinides (Am, Cm and Pu) and lanthanides (La, Nd).10,65,66 Powellite phase was observed only for experiments conducted at 90 °C. The drop in Mo release rate after 200 and 260 days at 50 and 35 °C, respectively, indicates that a significant fraction of Mo is retained in the alteration products, although the Mo bearing phase has not been identified. Two other Ca-containing phases were observed at 90 °C by SEM/EDX analyses. Micro-Raman analyses confirm the precipitation of apatite and calcite. These minerals can also incorporate trivalent actinides and lanthanides.10,65 Only calcite was observed at 35 and 50 °C.
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Fig. 10 Micro-Raman spectra obtained on the surface of the SON68 glass altered during 653 days at 90 °C, showing the various Raman bonds of calcite, apatite and powellite.67–70 |
The total concentrations of silicon released from the glass (Sireleased) were calculated from the elemental analyses of Si and isotopic analyses of 29Si/28Si ratios. The 30Si/28Si ratios were not measured and assumed to be constant 0.0335 (natural isotopic ratio), because the low enrichment of 29Si-doped SON68 glass in 30Si compared to 29Si.
The total concentration of silicon (Sitotal) = (28Si + 29Si + 30Si).
28Si = Sitotal/[1 + (30Si/28Si) + (29Si/28Si)] |
28Siinlet = Sitotal,inlet/[1 + 0.0335 + (29Si/28Si)inlet] |
29Siinlet = [(28Siinlet) × (29Si/28Si)inlet]. |
The concentration of 29Si in the outlet solutions (29Sioutlet) is obtained using the same method.
29Sioutlet = [(28Sioutlet) × (29Si/28Si)outlet], the (29Si/28Si)outlet ratio is presented by the points on the curve in Fig. 11(a).
Therefore, the concentration of 29Sireleased (released from the glass) is equal to (29Sioutlet − 29Sinlet) by assuming that 29Si and 28Si are released together with the initial isotopic ratio in the glass (29Si/28Si = 0.16).
The total silicon released from the glass Sitotal,released = [(29Sireleased × 100)/14]. The weight percentage of 29Si in total silicon of the glass is 14.
The evolution with time of the total Si released (Sitotal,released) for the (29Si doped SON68 + non-doped COx water) experiment is compared to that from the experiment with non-doped glass (SON68 + COx, 90 °C) (Fig. 11(b)). Both curves follow the same shape. The isotopic tracing of the glass allows us to monitor the evolution of Si released from the glass until the end of the experiment. For the experiment (non-doped SON68 + 29Si enriched COx water) the evolution of the Si concentration with time could not be monitored because the use of a very rich 29Si solution (99.9%). In fact, the results of measurement of the isotopic 29Si/28Si ratios give values equal to the background.
Fig. 12 shows the normalized leaching rates of glass elements for: (a) 29Si doped SON68 + non-doped COx water experiment, and (b) non-doped SON68 + 29Si enriched COx water experiments. The curves follow the same trend as that in Fig. 4(d). The dissolution rates tend towards a value of about 10−3 g m−2 d−1. Fig. 12(a) also shows the evolution of Si dissolution rate based on isotopic 29Si/28Si ratio calculations described above. Compared to the other glass tracers, the Si is strongly retained in the alteration layer. Fig. 12(c) shows the evolution with time of silicon retention factor fSi (expressed in %) in the gel. The value of fSi is calculated from the normalized mass loss of silicon NLSi (g m−2) and boron NLB (g m−2): fSi = (1 − (NLSi/NLB)) × 100. The silicon retention factor remains high during the experiment, it is around 96% at the beginning of the experiment and then decreases to 94% after 14 days (transient reactor operation) and after it increases to 100% after 458 days. The very high fSi value is due to the use of silica saturated solutions and particularly higher S/V ratio (1800 m−1). Indeed, S/V ratio is the determining parameter in the increase of silica retention factor.71 Even though low silicon diffusion coefficients are associated with high silicon retention factors, the authors showed that a variation in the S/V ratio between 1 and 2000 cm−1 results in a decrease of the apparent silicon diffusion coefficient by eight orders of magnitude. The very high fSi value confirms the very low silicon diffusion coefficient (see Section 3.2.2.) and indicates either that the gel formed is dense and protective or that the dissolution affinity, due to high silica concentrations is very low. The normalized dissolution rates of boron for different leaching experiments carried out at 90 °C, pH 8 and [Si]init = 42 ppm can be seen in Fig. 12(d). Whatever the combination, a decrease in the alteration rate is observed to achieve a value of about 10−3 g m−2 d−1 after 653 days of alteration. The slight difference in the dissolution rate between the non-doped and 29Si-doped SON68 glass is likely due to the difference in S/V ratio. The silicon retention in the gel is known to play a major role in the porosity decrease.72,73
In agreement with what is described above, the EDX mapping shows that the Mg (initially absent in the glass) is almost completely absent in the gel, though it is highly concentrated in the phyllosilicates. This suggests a super-saturation of solution towards the magnesium silicates which are massively precipitated on the glass surface, impeding the migration of Si through the gel. This phenomenon suggests that the precipitation of phyllosilicates is not kinetically limited but it is rather limited by Si-availability as shown by Jollivet et al.62 and Fleury et al.74
The Ca is highly enriched in the gel and in spherical structures in the glass/phyllosilicates interface. This enrichment occurs simultaneously with enrichment in phosphate, which suggests the formation of a calcium phosphate phase (apatite). The apatite mineral may be beneficial to storage because of its capacity to retain the actinides present in the glass. Valle et al.49 have also observed by TOF-SIMS analysis a high concentration of Ca at the glass/phyllosilicates interface after 6 months of SON68 glass alteration, as previously observed by Gin et al.72 who indicated the presence of a thin alteration layer rich in calcium and phosphates.
Phosphates have the tendency to incorporate the rare earth elements REE,75 this may explain the presence of La in the spherical structures. It has been shown that the protective properties of the amorphous gel are directly linked to the distribution of REE and Ca within the alteration layer,72 the authors have shown that the precipitation of rare-earth phosphates and Ca tend to maintain alteration rates substantially higher than when the same elements are uniformly distributed in the gel.
There is an apparent enrichment of Al in the gel layer in comparison to the pristine glass and a high concentration in the phyllosilicates. This trivalent element conserves its tetrahedral coordination during the alteration76 which requires the presence of charge compensators such us Na, Cs and Ca in order to maintain the charge balance.77 In this case, it is essentially the Na which provides this role because it is slightly present in the gel and highly concentrated in the phyllosilicates where Al is also concentrated. Caurel78 has shown that under extreme conditions of R7T7 glass alteration (pH > 10.5 and T ≥ 150 °C), Al precipitates as zeolite phases. The precipitation of this phase tends to consume Si present within the gel, the latter is thus poorly connected and loses its protective properties, leading to the resumption of alteration.79 This phenomenon has not been observed in laboratory experiments under moderated conditions of glass alteration (pH < 10 and T < 90 °C),38 as is the case in our study. Fe shows a slight enrichment in the gel compared to the glass and a strong presence in the phyllosilicates, which explains its very low concentration (sometimes below the detection limit) in the leachate. Fe may be present in the phyllosilicates as iron hydroxide form.
In the work of Valle et al.49 the isotopic signature of phyllosilicates found by SIMS method corresponds to that of the solution when the glass monolith is removed from the reactor. The authors suggest that the phyllosilicates are formed by dissolution/precipitation mechanisms while the gel is formed by hydrolysis/condensation reactions because it keeps an intermediate isotopic signature. However, the precipitation of secondary phases in the gel cannot be excluded (i.e. phosphates, molybdates, precursors of magnesium phyllosilicates,…).
It must also be noted the difference between the two works: (1) in our work we have added the glass powder in addition to the monolith, (2) we did not use the same parameters (flow rate = 12 mL per day and low S/V (34 m−1) as in the work of Valle et al.49), and (3) we particularly cleaned with ultrapure water the altered glass monoliths before analyzes. In fact, the cleaning performed on altered glass monoliths was able to eliminate or dissolve a portion of the external layer which was in contact with the solution. Therefore, the phyllosilicates keep a different isotopic signature from that of the alteration solution when the monolith is removed from the reactor. This hypothesis is further supported by hydration experiments of SON68 glass monoliths in the presence of vapor D218O.80 Unlike this work, the hydrated glass monoliths have not been cleaned with ultrapure water before analyzes. It is seen that the phyllosilicates keep an isotopic signature 18O/16O of the water vapor. In any case, all analysis results highlight a mechanism of dissolution/precipitation responsible for the phyllosilicates formation.
In the second experiment (Fig. 14(b)) four different zones are also noticed with the same behaviour of the elements studied. The depth profile of 29Si/28Si shows the penetration of 29Si from the solution into the non-doped SON68 altered glass. The value of 29Si/28Si ratio in the glass measured by TOF-SIMS technique is equal to 0.0509 and it is very close to the theoretical value (natural abundance = 0.05). The 29Si/28Si ratio reaches a maximum value in the phyllosilicates (29Si/28Simax phyllosilicates = 5.2). As in the first experiment, this value is lower than that measured in the leachate when the sample is removed from the reactor (29Si/28Sileaching solution > 40).
It is therefore clear that the phyllosilicates keep an intermediate isotopic signature between the pristine glass and the alteration solution. The fact that the gel keeps a relatively constant isotopic ratio along the profile would lead to the assumption that the condensation phenomena is almost instantaneous and follows the thermodynamic laws of local dissolution/precipitation.
TOF-SIMS profiles of all glass components for 29Si doped SON68 glass monolith altered in non-doped COx water after 485 days can be seen in Fig. 14(c)–(e). The boron profiles show a significant depletion in the entirety of the alteration layer. Li is also depleted in the alteration layer. Cs behaves in the same manner as B and Li in the gel, however a small amount of this element is retained in the phyllosilicates. The retention of Cs is due to its ability to be incorporated into solid phases, in particular the clays.81 The concentration of Mo drops sharply in the interfacial gradient and linearly in the gel, this behaviour indicates that an amount of Mo is retained in the gel. In agreement with solution analyses and SEM observations, Mo is also detected in the phyllosilicates. Unlike B and Cs, the gel is not completely depleted in Na. The presence of Al in the gel may cause the retention of Na which assumes the role of a charge compensator. Na is largely concentrated in the external layer of altered glass.
The profiles of Al and Fe are identical, they are especially enriched in the phyllosilicates. Ca behaves in the same manner as Fe in the gel, it is very concentrated at the interface between the gel and the phyllosilicates but its concentration is very low in the external zone. The relative enrichment in Ca compared to the pristine glass indicates its high incorporation into the alteration layer and secondary phases. The incorporation of Ca in the gel has been observed by Chave et al.82 during the alteration of the French reference nuclear glass in calcium-rich solution, this phenomenon slows the glass corrosion rate. Mg is very concentrated in the phyllosilicates as magnesium silicates. It should be noted that Mg penetrates into the gel from the solution and thus contributes to the destabilization of the alteration layer via its magnesium silicates transformation.34,61,83
For the alteration experiment of non-doped SON68 glass in 29Si enriched COx water after 456 days, TOF-SIMS profiles show that all elements behave in the same manner.
The diffusion of Si in the gel was determined according to the method described by Abdelouas et al.84 Thus the TOF-SIMS profile of 29Si/28Si isotopic ratio in the interfacial gradient was fitted using Origin 8.6 software. The experimental and fitting results are given in Fig. 14(f). The equation is based on Fick’s second law, which predicts how diffusion causes the concentration to change with time. It takes into account de diffusion of 28Si from the alteration solution toward the gel which is considered a semi-infinite medium.
Fick’s second law is given by:
![]() | (6) |
The solution of this equation is:
![]() | (7) |
This gives the equation:
![]() | (8) |
The obtained value of Si apparent diffusion coefficient is about 1.7 × 10−22 m2 s−1. This value is consistent with the modelled value obtained by Valle85 for alteration experiments of SON68 glass in silica concentration solutions at 90 °C and pH 9.2. The modelled value DSi, based on the Fick’s second law, was seen to range between 3 × 10−20 after 1 h and 7 × 10−23 m2 s−1 after 20 days. For the same experiments, but under silica unsaturated conditions, the author obtained a value of DSi between 5 × 10−14 and 4 × 10−13 m2 s−1. The low diffusion of silica through the gels formed under saturation conditions demonstrates the effect of the affinity term.
It should be noted that this method is a simplified approach from the TOF-SIMS measurements. In fact, silicon can be sorbed and/or precipitated in the gel. Therefore, the process of Si transport in the gel layer may be described in the reactive transport way and not the solute transport in the porous media. Thus, from the TOF-SIMS profiles we can provide apparent diffusion coefficients, the very low value of which indicates an important part of chemical reactivity.
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