UK HLW glass powder dissolution in water
The average pH of the solution in contact with the simulant UK HLW glass powder was pH 9.8 ± 0.1, compared with the blank pH of ~6.1. These values were little changed over the duration of the experiment, indicating that the pH was quickly buffered to a stable value (Fig. 1a). The normalized mass loss of boron (NLB) is shown in Fig. 1b. Boron is a well-recognized soluble tracer element for network dissolution and is thought not to become incorporated into alteration products during glass alteration; hence, it may be used as an indication of forward glass dissolution rate. NL(B) data for all experimental conditions are shown in Table 2. NL(B) occurred under two main dissolution regimes: an initial rapid rate was observed between 0 and 28 days, followed by a slower rate from 28 days until the end of the experiment at 168 days. Similar dissolution trends were also observed for NL(Li) (Fig. 1c) and NL(Na) (Fig. 1d), although Na dissolution was greater than that for B and Li (Table 2); however, this may be expected as a result of the alkali-rich nature of the glass (Table 1). Dissolution rates are shown in Table 3. In the course of the experiment, the curves did not reach a plateau value, indicating that equilibrium or, in the case of glass, “quasi-equilibrium” was not achieved.
Table 2. Normalized mass loss (g/m2) of B, Li, Na, and Si during the three main phases of dissolution in water and saturated Ca(OH)2 solution for all experimental conditions, described by the end time point of each regime
|28 days||(3.81 ± 0.3) × 10−3||(6.87 ± 0.7) × 10−3||(1.48 ± 0.1) × 10−2||(1.05 ± 0.1) × 10−3|
|168 days||(6.45 ± 0.6) × 10−3||(9.22 ± 0.9) × 10−3||(2.11 ± 0.2) × 10−2||(9.94 ± 0.9) × 10−4|
| Powder |
|28 days||(1.67 ± 0.2) × 10−3||(2.83 ± 0.2) × 10−3||(5.88 ± 0.6) × 10−3||(3.14 ± 0.3) × 10−5|
|84 days||(3.30 ± 0.3) × 10−3||(5.35 ± 0.5) × 10−3||(1.30 ± 0.1) × 10−2||(4.41 ± 0.4) × 10−4|
|168 days||(4.45 ± 0.5) × 10−3||(6.78 ± 0.6) × 10−3||(1.65 ± 0.2) × 10−2||(8.16 ± 0.8) × 10−4|
| Monolith |
|70 days||(1.96 ± 0.2) × 10−2||(2.67 ± 0.3) × 10−2||(3.39 ± 0.4) × 10−2||(0.0 ± 0.0)|
|168 days||(3.02 ± 0.3) × 10−2||(4.40 ± 0.4) × 10−2||(6.89 ± 0.2) × 10−2||(0.0 ± 0.0)|
Table 3. Normalized dissolution rates (g/m2/day) of B, Li, Na, and Si during the three main regime of dissolution in water and saturated Ca(OH)2 solution for all experimental conditions
|Initial (0–28 days)||(1.29 ± 0.1) × 10−4||(2.16 ± 0.2) × 10−4||(5.24 ± 0.5) × 10−4||(3.32 ± 0.3) × 10−5|
|Residual (28–168 days)||(1.88 ± 0.2) × 10−5||(1.68 ± 0.2) × 10−5||(4.49 ± 0.5) × 10−5||(4.21 ± 0.4) × 10−7|
| Powder |
|Initial (0–28 days)||(5.92 ± 0.6) × 10−5||(9.32 ± 0.9) × 10−5||(2.02 ± 0.4) × 10−4||(3.32 ± 0.3) × 10−7|
|Intermediate (28–84 days)||(2.91 ± 0.3) × 10−5||(4.50 ± 0.5) × 10−5||(1.27 ± 0.1) × 10−4||(7.31 ± 0.7) × 10−6|
|Residual (84–168 days)||(1.37 ± 0.1) × 10−5||(1.71 ± 0.2) × 10−5||(4.19 ± 0.4) × 10−5||(4.47 ± 0.5) × 10−6|
| Monolith |
|Initial (0–70 days)||(1.68 ± 0.2) × 10−4||(3.53 ± 0.4) × 10−4||(6.70 ± 0.7) × 10−4||(0.0 ± 0.0)|
|Intermediate (70–168 days)||(5.44 ± 0.7) × 10−5||(1.77 ± 0.4) × 10−4||(3.57 ± 0.4) × 10−4||(0.0 ± 0.0)|
Figure 1. (a) Measured solution pH for UK HLW glass powders and monoliths dissolved in water and/or saturated Ca(OH)2 solution. Normalized mass loss (g/m2) of: (b) B, (c) Li, (d) Na, and (e) Si from glass powder dissolved in water or Ca(OH)2; (f) solution concentration (mg/L) of dissolved Ca in blank (i.e. no glass) saturated Ca(OH)2 solution and for UK HLW glass powder; and normalised mass loss (g/m2) of (g) Mg and (h) Al from glass powder dissolved in water or Ca(OH)2. Error bars are derived from standard deviation of triplicate samples.
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The normalized mass loss of silicon, NL(Si), between 0 and 28 days was rapid (Fig. 1e), at a rate of (3.32 ± 0.3) × 10−5 g/m2/day (Table 3). This dissolution behavior was followed by a decrease in the dissolution rate to (4.21 ± 0.4) × 10−7 g/m2/day until a steady state was achieved at NL(Si) of (9.94 ± 0.9) × 10−4 g/m2. This is indicative of silicon saturation in solution, likely a result of the high surface-area-to-volume ratio of the glass powder employed in this study. The normalized mass loss of Mg and Al was significantly less than those observed for B, Li, Na, and Si (Figs1g and h, respectively). The NL(Mg) was measured at relatively high concentrations between 0 and 28 days with a maximum NL(Mg) value of (5.5 ± 0.4) × 10−5 g/m2. Subsequently, the NL(Mg) rapidly decreased to a constant value of (5.9 ± 1.1) × 10−6 g/m2 until the end of the experiment. The NL(Al) was initially high, decreasing from 0 to 7 days to an approximately constant value of (8.8 ± 0.2) × 10−7 g/m2. The dissolution behavior of both Mg and Al is indicative of incorporation into a gel layer on the surface of the glass powders.
UK HLW glass powder dissolution in saturated Ca(OH)2
The pH of the saturated Ca(OH)2 solution containing UK HLW glass powder decreased rapidly from an initial value of pH 12.7 ± 0.1 to pH 10.5 ± 0.2 at 84 days, where it remained constant, as shown in Fig. 1a. This is in comparison with the blank saturated Ca(OH)2 solution, which had a constant pH of 12.5 ± 0.3 for the duration of the experiment (Fig. 1a). The pH of the solution was consistently higher than pH 11.5, indicating that Ca(OH)2 precipitation from solution (due to carbonation) was minimized.
The normalized mass loss of B, Li, and Na is shown in Figs 1b–d. It is clear that the dissolution behavior of these elements in saturated Ca(OH)2 was different to that observed in water. Firstly, lower NL(i) values and dissolution rates (Tables 2 and 3) were observed, and secondly, the dissolution appeared to occur over three main dissolution regimes: from 0 to 28 days, 28 to 84 days, and 84 to 168 days. Tables 2 and 3 detail the NL(i) and RL(i) for each period, respectively. For B, the NL(i) was initially fast, with RL(B) of (5.92 ± 0.6) × 10−5 g/m2/day between 0 and 28 days. From 28 to 84 days, the dissolution rate decreased to (2.91 ± 0.3) × 10−5 g/m2/day and decreased further from 84 to 168 days (RL(B) = (1.37 ± 0.1) × 10−5 g/m2/day). The data suggest that “quasi-equilibrium” was achieved, but further data are required to confirm this.
The NL(Si) is shown in Fig. 1e. Initially, Si was not detectable in solution. At 28 days, it was possible to measure Si, giving NL(Si) of (3.14 ± 0.3) × 10−5 g/m2 (Table 2). After this initial “incubation” regime, dissolution of silicon proceeded rapidly between 28 and 84 days. Between 84 and 168 days, the dissolution rate decreased to (4.47 ± 0.5) × 10−6 g/m2/day, where an apparently constant NL(Si) of (8.16 ± 0.8) × 10−4 g/m2 was attained. This dissolution behavior is in contrast to that observed for glass powder dissolved in water, which demonstrated a rapid increase between 0 and 28 days followed by a constant NL(Si) until the end of the experiment (Fig. 1e). These data suggest that the presence of Ca(OH)2 in solution directly affected the NL(Si).
The concentration of Ca remained approximately constant in the blank Ca(OH)2 solution at 770 ± 68 mg/L, as shown in Fig. 1f. Conversely, for the Ca(OH)2 solution in contact with UK HLW glass powder, the Ca concentration rapidly decreased from a starting value of 770 mg/L at 0 days to 200 mg/L at 28 days. Concentrations continued to decrease between 28 and 84 days at a much slower rate (to ~80 mg/L), and finally, after 84 days, a constant Ca concentration of 65 ± 10 mg/L was observed. These changes in Ca removal from solution are concurrent with the decrease in pH (Fig. 1a), which was also constant at 84 days and beyond. Figs 1g and h show the evolution of NL(Mg) and NL(Al), respectively. In contrast to the NL(Mg) observed for UK HLW dissolved in water, it was not always possible to detect Mg in solution. The only detectable concentrations occurred at 28 and 42 days, giving NL(Mg) of ~10−7 m2/g. Interestingly, the NL(Al) was observed to rapidly increase in solution, reaching a maximum value of (4.36 ± 0.2) × 10−5 g/m2 between 14 and 28 days. After this time, the NL(Al) slowly decreased to a value of (1.55 ± 0.1) × 10−6 g/m2 at 168 days. This is in contrast to the behavior observed in water only, where the NL(Al) was constantly at low values.
Geochemical modeling of the experimental solution resulting from UK HLW glass powder dissolution in saturated Ca(OH)2 was conducted using PHREEQC. Saturation indices of phases shown to be typically relevant to glass dissolution at high pH[4, 17] and those containing Ca are shown in Fig. 2a, as a function of the three apparent regimes of dissolution. Also included were Si phases containing Al and Mg, as a result of the distinctive normalized mass loss of these elements (Figs. 1g-h). The phases investigated were the following: analcime (Na0.96Al0.96Si2.04O6·H2O), which is known to precipitate during the dissolution of silicate glasses; phillipsite-Ca (CaAl2Si5O14·5H2O); sepiolite (Mg4Si6O15OH·6H2O); and tobermorite (Ca5Si6(O,OH)18·5H2O). These phases are commonly referred to as silicate hydrates, which can incorporate Ca (C), Al (A), Na (N), and/or Mg (M) in a variety of combinations and wide range of compositions, for example Ca-Al-silicate hydrate or C-A-S-H. Ca(OH)2 saturation was also modeled.
Figure 2. (a) Saturation indices of selected species from the UK HLW glass powder dissolution in saturated Ca(OH)2 solution during the three main phases of dissolution. Phases are: analcime (Na0.96Al0.96Si2.04O6·H2O), phillipsite-Ca (CaAl2Si5O14·5H2O), tobermorite (Ca5Si6(O, OH)18·5H2O), sepiolite (Mg4Si6O15OH·6H2O), and Ca(OH)2, and (b) relative saturation of Ca-silicate species in each main dissolution phase as a function of Ca:Si ratio, derived from the following PHREEQC species: 1.5 = jennite (Ca9Si6O32·H2O), 1.33 = foshagite (Ca4Si3O9(OH)2.0·5H2O), 1 = xonotolite (Ca6Si6O17(OH)2), 0.83 = tobermorite (as above), and 0.5 = okenite (CaSi2O4(OH)2.H2O).
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Between 0 and 28 days, Ca(OH)2 was saturated in solution. The Mg-bearing silicate phase sepiolite was also saturated, indicating that it was thermodynamically possible for M-S-H phases to precipitate from solution during this time. All other phases were undersaturated, suggesting that C-(N)-A-S-H precipitation was not a major process governing the dissolution behavior during this phase. Between 28 and 84 days, Ca(OH)2 became undersaturated, concurrent with the Ca concentration decrease (Fig. 1f). In addition to sepiolite saturation, tobermorite became saturated, suggesting the likely precipitation of C-S-H phases during this dissolution regime. Between 84 and 168 days, C-(N)-A-S-H phases became saturated in solution (e.g. phillipsite-Ca and analcime). This behavior is indicative of Si saturation in solution. The changing composition of C-S-H phases with dissolution time can be expressed by the Ca-to-Si ratio in solution. Figure 2b shows that during the first phase of dissolution, only C-S-H phases with a Ca:Si ratio of >1 were likely to precipitate from solution (e.g. xonotolite, foshagite, and jennite). In the second and third phases of dissolution, C-S-H phases with a Ca:Si ratio of <1 were favored to precipitate (okenite, tobermorite, xonotolite, and foshagite), with the lower Ca:Si C-S-H species showing a greater propensity for saturation in the third phase of dissolution.
This change in C-S-H composition as a function of dissolution time is confirmed when the Ca and Si concentration data are plotted on a CaO-SiO2-H2O phase diagram, as shown in Fig. 3. At 21 and 28 days (at the beginning of the second regime of dissolution), the data plot on the C-S-H(I) metastable solubility curve, where C-S-H(I) corresponds to noncrystalline C-S-H phases with a Ca:Si ratio >1.5. Between days 42 and 84, the data gradually shift towards the C-S-H(II) solubility curve, where C-S-H(II) corresponds to crystalline C-S-H phases with a Ca:Si ratio between 0.5 and 1. It is likely that solution compositions that plot between these fields would give rise to precipitates of C-S-H-(I). Data from day 140 and 168 plot near the junction between the C-S-H + H2O and SiO2(H2O)x + H2O stability fields (data not shown), suggesting that the solution was close to Si saturation. It should be noted that this diagram is relevant to the CaO-SiO2-H2O system at 25°C, not at 50°C as in the current study; therefore, Si saturation cannot be confirmed without further data.
Figure 3. Plot of Ca and Si solution concentrations from the dissolution of UK HLW glass powder in saturated Ca(OH)2 experimental solution at 50 °C on the CaO-SiO2-H2O phase diagram at 25°C, adapted from Jennings (1986). Triangles show data points from the current study, from the initial detection of Si at day 21 to day 112 (further data points crowded the top of the chart). Coloured line shows trajectory of changing C-S-H composition in Regime 1 (red), Regime 2 (blue), and Regime 3 (green).
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UK HLW glass monolith dissolution in saturated Ca(OH)2
To investigate the dissolution of UK HLW glass at surface-area-to-volume ratios more relevant to the conditions likely to be encountered in a GDF, monolith samples with a surface-area-to-volume ratio of 10/m were used. The pH closely resembled that of the Ca(OH)2 blank (Fig. 1a), which was approximately constant at pH 12.5 ± 0.3 for the 168-day duration of the experiment. Only the soluble glass-forming elements, B, Li, and Na, were detectable in solution. The normalized mass loss of these elements was higher than for the powdered glass experiments (Fig. 4a, Table 2), due to solution saturation effects in the powder experiments, resulting from the high surface-area-to-volume ratio. In contrast to the glass powder experiments, the dissolution of these elements was observed to proceed via a two-step process. Between 0 and 70 days, the dissolution rate was relatively rapid, as shown in Fig. 4a and Table 3. The dissolution rates decreased and began to plateau after 70 days. It was not possible to detect Si in solution for the duration of the experiment. The concentration of Ca in the solution used in the monolith experiment was observed to decrease between 0 and 70 days from 830 mg/L to ~680 mg/L, as shown in Fig. 4b. After 70 days, the Ca concentration was approximately constant at 690 ± 5 mg/L. The dissolution rate change of the soluble glass elements (B, Li and Na) appeared to correlate with the rate change in Ca removal from solution.
Figure 4. (a) Normalized mass loss (g/m2) of B, Li, Na, and Si as a function of time for UK HLW glass monoliths dissolved in saturated Ca(OH)2 at 50 °C and (b) solution concentration (mg/L) of dissolved Ca in saturated Ca(OH)2.
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Analysis of the monolith sample by powder XRD revealed the presence of an M-S-H phase (sepiolite Mg4Si6O15(OH)2·6H2O, card 13–595) on the surface of the glass, which appeared after 3 days of dissolution (Fig. 5a). Also present at the surface were small (~10 μm) acicular crystallites and rounded particles ~1 μm in size (SEM inset, Fig. 5a). XRD and SEM/EDS analysis indicated that the rounded particles may be un-dissolved or precipitated Ca(OH)2 ; the acicular RuO2 crystallites were present in the as-prepared glass. In addition, large areas (>250 μm) comprised of a phase rich in Ca and Si, with no distinct crystalline texture, were observed after 3 days (Fig. 5b, EDS analysis not shown). After 168 days, the distribution of these phases was increased, and they had increased in size (up to ~400 μm, Fig. 5c).
Figure 5. (a) X-ray diffraction spectrum of surface precipitates observed on the surface of UK HLW glass monoliths after 3 days of dissolution in saturated Ca(OH)2 solution at 50°C. Spectrum was taken after 168 days of dissolution. Insert shows SEM images of large, amorphous surface precipitates after (b) 3 days and (c) 168 days of monolith glass dissolution.
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Figure 6 shows a back scatter electron (BSE) cross-section image of the glass monolith reacted for 168 days embedded in epoxy resin. Between the light (glass) and black (epoxy) areas, there is a two-component zone of material of intermediate BSE intensity. The first is ~3 μm in thickness immediately adjacent to the glass, rich in Si, which appears separated due to dehydration during drying of the sample. Immediately adjacent to the Si-rich layer, there is an area ~60 μm thick containing large precipitated phases. An EDS line scan taken across the second region of the altered zone indicated that where high intensities of Ca occurred, there were also increased intensities of Si, Mg, and Al, providing an indication that C-(A)-S-H and M-S-H phases may be forming a layer on the surface of the glass as a result of dissolution.
Figure 6. (a) Backscatter electron image and (b) EDS line scans taken along the cross section of the UK HLW glass monolith surface after 168 days of dissolution in saturated Ca(OH)2 solution at 50°C showing the morphology and chemical composition of the Ca-, Si-, Al-, and Mg-rich altered layer.
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