# Bismuth-doped aluminoborosilicate glasses synthesized under high-pressure: Role of bismuth on iodine solubility

Sami Soudani, Michael Paris, Jonathan Hamon, Lionel Campayo, Yann Morizet

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Sami Soudani, Michael Paris, Jonathan Hamon, Lionel Campayo, Yann Morizet. Bismuth-doped aluminoborosilicate glasses synthesized under high-pressure: Role of bismuth on iodine solubility. Journal of Nuclear Materials, 2026, 618, pp.156218. 10.1016/j.jnucmat.2025.156218. hal-05458738

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1 Bismuth-doped aluminoborosilicate glasses synthesized under high-pressure: Role of

2 bismuth on iodine solubility

3 Sami SOUDANI<sup>a,b,c*</sup>, Michael PARIS<sup>b</sup>, Jonathan HAMON<sup>b</sup>, Lionel CAMPAYO<sup>d</sup>, Yann

4 MORIZET<sup>a</sup>

5

6 <sup>a</sup> Nantes Université, CNRS, UMR 6112, Laboratoire de Planétologie et Géosciences, F-44000

7 Nantes, France

8 <sup>b</sup> Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, F-44000

9 Nantes, France

10 <sup>c</sup> Institute of Materials Research, Washington State University, Pullman, WA 99164-2711,

11 USA

12 <sup>d</sup> CEA, DES, ISEC, DE2D, Université de Montpellier, Marcoule, F-30207, Bagnols sur Cèze,

13 France

14 *Corresponding author: Sami Soudani: sami.soudani@wsu.edu

15 Postal address:

16 Laboratoire de Planétologie et Géosciences (LPG), UMR 6112, Nantes Université.

17 2 rue de la Houssinière, 44322 Nantes Cedex (FRANCE)

18
20 Bismuth-aluminoborosilicate glasses have gained interest as a possible matrix for nuclear
21 waste immobilisation, as bismuth-based sorbents have enhanced affinity for volatile
22 radioisotopes removal. Conjointly, it has been recently demonstrated that using high-pressure
23 conditions is an interesting tool to improve volatile elements retention during glass synthesis.

24 This study tries to combine these two fields showing significant improvement in 129-iodine
25 incorporation into the structure of Bi-bearing glasses synthesised under high-pressure
26 conditions (1 GPa and 1250°C) by comparison with Bi-free glasses or Bi-containing glasses
27 obtained at ambient pressure.

28 Two series of glasses have been investigated in the SiO₂-Al₂O₃-B₂O₃-Na₂O-CaO-Bi₂O₃
29 system, either with high (~13 mol.% Al₂O₃) or low (~5mol.% Al₂O₃) Al₂O₃ and Bi₂O₃ content
30 ranging from 0.5 to 16 mol.%. We show that I incorporation increases with increasing Bi₂O₃
31 and can reach up to 2.8 mol.%, which is much higher than Bi-free comparable glasses
32 synthesised under the same conditions. The ¹¹B NMR analysis demonstrated a drastic
33 decrease in the proportion of tetrahedral BO₄ with increasing Bi₂O₃ content regardless of the
34 alumina content. ²⁷Al displayed a large majority of ⁴VAl, with a steady increase of CNₐ₁ for
35 the Al-rich series, and nearly constant CNₐ₁ for the Al-poor series as we increase Bi₂O₃
36 content. O 1s XPS spectra demonstrates a clear change towards a general depolymerisation of
37 the glass structure with increasing Bi₂O₃ content. It confirms the network-modifier role of
38 Bi³⁺ in such a glass system and explains the increase in I content. This work paves the way for
39 new glass composition to be synthesised using high-pressure to address the worldwide issue
40 of the volatile nuclear wastes.

41 1. Introduction

42 Nuclear power plants currently generate 10-20% of the world's total energy consumption.

43 Countries such as Slovakia, Ukraine, and France rely on nuclear power for over 50% of their
44 energy needs [1, 2]. In the context of global warming and CO2 production, the nuclear
45 industry plays an important role as an almost carbon-free source of energy. Nuclear waste
46 reprocessing is very efficient as 96% of the waste mass will be reprocessed uranium and
47 plutonium [3], leaving only a small fraction of ultimate wastes. This final waste is classically
48 incorporated in a glass matrix and stored in large metallic cans before disposal.

49 Indeed, a large majority of the countries with a developed nuclear industry made the choice of
50 vitrification for immobilising the final radioactive waste in aluminoborosilicate glasses [4],
51 known for their high mechanical and chemical stability over time and for being not
52 significantly affected by radiation or heating [5, 6, 7, 8]. However, the most volatile radio-
53 nuclides are lost during the high-temperature (>1000°C) treatment of the vitrification process
54 [9, 10, 11] and require an off-gas treatment. Some can be recovered in the vapour phase by
55 sorbent materials [12, 13, 14, 15]. Those nuclides are thus present in very low proportion in
56 the glass (e.g., typically <1 wt.%, for Se, Tc and I [16, 17, 18, 10]). Owing to their
57 volatilization trend, different strategies were utilized to manage them: off-gas recycling,
discharge by isotopic dilution, capture and removal from off-gas systems waiting for a
suitable solution to be found.

Many studies have proposed using high-pressure for investigating the volatile solubility in melts, drawing inspiration from petrological research on H₂O and CO₂ content in magmatic systems [19, 20, 21, 22, 23, 24, 25]. Recently, this approach has been shown to increase the solubility of iodine [26, 27, 28, 29] and chlorine [30, 31] by several orders of magnitude in nuclear waste aluminoborosilicate glasses as compared to the ambient pressure incorporation.

In the scope of enlarging the knowledge on iodine solubility in glasses under high-pressure conditions, there is a crucial need to study the effect of different family of component on iodine behaviour as it is known that glass composition plays an important role on iodine solubility. This has been made by changing the Na₂O content in [32], providing direct evidence of the link between iodine and network modifying species. Morizet [33] also explored the effects of changing the network modifier cation, both in the alkali and earth-alkaline systems, with or without Bi₂O₃ addition. They showed that sodium represents the best compensator for iodine in aluminoborosilicate glasses, and that with small addition of Bi₂O₃, iodine incorporation in alkali-containing glasses is promoted. Lere-Adams [34, 35] also synthesized bismuth-containing wasteform for radioiodine. They investigated a sodalite exchanged caustic scrubber with binding glass in the ZnO-Bi₂O₃-B₂O₃ system, that has been HIP-ed. They started their high-pressure experiments with pre-synthesized I-containing material, without adding any more I source (as also did Chong [37]), which thus did not
improve iodine retention in the material as compared to previous studies [36, 38]. However, they noticed a decrease in the material porosity, and a slight conversion from sodalite to nepheline, which would induce important change in the material physico-chemical properties.

Bismuth as Bi₂O₃ is gaining popularity in optical glasses for Pb replacement [39, 40, 41, 42], it has also proven useful for nuclear industry as a radiation shielding oxide [43, 44, 45, 46], and is an effective component of sorbent for radio-iodine gaseous effluents [14, 47, 48]. At a time when end-to-end processes for nuclear waste management are being investigated to limit the number of processing steps, the development of a specific matrix and method for immobilising volatile radioisotopes is a real prospect. However, there is still a large gap of understanding the range of modifications at the atomic level imposed by bismuth in aluminoborosilicate glasses. So far, only a few studies investigated the local environment for bismuth cations in borosilicate glasses and never in presence of iodine. Recent work by McKeown [49] based on X-ray Absorption Spectroscopy suggests that Bi³⁺ cations are surrounded by three oxygen atoms at 2.13 Å. Khanna [50] suggested that Bi³⁺ presence is associated with a large concentration in NBOs, which could explain the increase in iodine solubility with increasing Bi₂O₃ shown in Morizet [33] (also associated with a high NBO content). In borate glasses, Varsamis [51] also confirmed the association between bismuth and NBOs in within these glasses; however, they pointed out that Bi could adopt a mixed role between network former and network modifier with different clusters BiO₃ and BiO₆. In most of the previously mentioned studies, bismuth is always seen as Bi³⁺ cations, however, bismuth
98 has several redox states and can also be found as Bi⁵⁺ in glasses [52, 53]. Hence, considering
99 the versatile behaviour of Bi₂O₃, its various redox state and its positive effect on radiation
100 shielding, there is a need for further experimental investigations to constrain the relationship
101 between Bi atoms and I species.

102 In the present study, we present new results on the effect of Bi₂O₃ addition on the
103 incorporation of iodine, with a larger Bi₂O₃ compositional range than previous related studies.
104 Solid State Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) and X-ray
105 Photoelectron Spectroscopy (XPS) have been employed to determine changes in the glass
106 structure related to addition of Bi₂O₃ in I-free glasses. I-bearing equivalent glasses synthesised
107 under high-pressure conditions have been investigated for I retention using Scanning Electron
108 Microscope with Energy Dispersive Spectrometer (SEM EDS). For those glasses, the I and Bi
109 speciation and O environments have been determined using XPS to find evidence for possible
110 Bi-I direct or indirect correlations. We thus propose a discussion on the way I solubilises into
111 the glass structure and how it is related to the local environment of bismuth.

## 2. Material and methods

### 2.1. Synthesis of the parent glasses

112 Two series of glasses have been synthesized, either with low (~5 mol.%) or high (~15 mol.%)
113 Al₂O₃ content, with various Bi₂O₃ content (from 0.5 to 16 mol.%). All glasses have been
114 prepared from an initial aluminoborosilicate glass in the Al₂O₃-SiO₂-B₂O₃-Na₂O-CaO system,
115 which composition is a typical of a Low Activity Waste glass that corresponds to an average
116
117
glass composition reported in Piepel [54] and is shown in Table 1. The glass has been prepared from reagent grade oxides and sodium carbonate, melted in a Pt-Au crucible at 1100°C for 3h with intermediate steps at 400 and 800°C for dehydration and decarbonation. A heating ramp of 5°C/min was used between each step. The melted material was eventually poured into a graphite crucible pre-heated at 530°C and stayed 1h at this temperature for removing mechanical stresses.

The recovered glass is translucent homogeneous and does not show evidence of crystalline phase or unreacted materials. This glass is then crushed and mixed along with different quantity of Bi₂O₃ to form the Al-poor series and mixed with both Al₂O₃ and different quantity of Bi₂O₃ to form the Al-rich series. The Bi-bearing starting material powders were melted in a Pt crucible at 1250°C in a muffle furnace for 1h. The ambient pressure sample quench was done by dropping the bottom of the Pt crucibles in water (~150°C/s for the quench rate). Recovered glasses are light yellow to brown with increasing Bi₂O₃ content. They have been checked optically free of the presence of crystals and then analysed for major element concentrations (see Table 1).

## 2.2. High-pressure experimental synthesis

The high-pressure experiments were carried out at the Laboratoire de Planétologie et Géosciences (LPG) using an end-loaded piston-cylinder apparatus. Glass powders and iodine (as I₂ or I₂O₅) content corresponding to 5 mol.% elemental I were loaded into Pt capsules with dimensions of 2.9 mm outer diameter and 12 mm length, sealed at both ends through arc-
138 welding. Each experiment involved combining two capsules within a ¾ inch talc-Pyrex high-pressure assembly.
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140 The experiments were conducted at 1.0 GPa and 1250°C. The experimental procedure is detailed in previous works [26, 27]. The talc-Pyrex assembly was pressurized to 1.0 GPa and the temperature raised to 550°C. We maintained these conditions for 10 minutes to stabilize both temperature and allow the high-pressure assembly to relax. Subsequently, the temperature was increased to 1250°C. Pressure control was automated using a Stigma© needle pump to prevent pressure drop. Temperature was monitored using a Eurotherm© controller with a S-type (PtRh₁₀-Pt) thermocouple located at the top of the capsules. Pressure and temperature are accurate within 10% in relative and to 1°C, respectively. All experiments had a standardized run duration of 2h for reproducibility. At the end of the experiment, an isobaric quench is performed by cutting-off the power, and the cooling rate is estimated at ~100°C in the first 500°C.
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151 2.3. Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM
152 EDS) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA ICP
153 MS)

154 The major elements concentrations have been determined with a Scanning Electron
155 Microscope JSM 5800LV equipped with a SSD SAMx Energy Dispersive Spectrometer
156 (SEM EDS Institut des Matériaux de Nantes Jean Rouxel, IMN). Analytical conditions were
157 15 kV for voltage and 0.5 nA for current. Acquisitions were conducted on large zones at
magnification below 5k to avoid Na loss under the electron beam. At least five acquisitions of 60 s have been performed for each glass sample. The standards used for the different oxides are corundum for Al₂O₃, wollastonite for SiO₂ and CaO, NaCl for Na₂O, metallic bismuth and platinum for Bi and Pt, respectively, and RbI for iodine. The measured composition of the quenched glasses is shown in Table 1. The reported error bars correspond to the standard deviation obtained on the replicated measurement and witness the homogeneity of the element within the glass. For instance, for iodine, the expected error is on the order of ±0.2 mol.%.

The B₂O₃ content was determined using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA ICP MS, at LPG) for providing more reliable results than the difference for 100% that could be deduced from SEM EDS measurements. The spectrometer is an ArF excimer laser (193 nm, Analyte G2, Photon Machines) that is coupled to a quadrupole ICP-MS (Varian Bruker 820-MS). The ablations were performed in a HelEx II 2-Volume Cell with He as a carrier gas. We used a laser energy density of 4.54 J.cm⁻² with a repetition rate of 10 Hz. All the analyses were performed in a spot mode with a circular spot size of 110 µm in diameter. Five acquisitions were performed on each glass sample. Acquisition time was set to 30 s with points collected every ~0.25 s (total number of points per analysis = ~120), preceded and followed by a 30 s blank acquisition. We used the LJ6 glass, containing 20.5 mol.% B₂O₃, investigated in Jolivet [55] to calibrate the LA-ICP-MS signal.

2.4. ¹¹B and ²⁷Al Solid State Nuclear Magnetic Resonance spectroscopy
¹¹B, and ²⁷Al NMR data were acquired on a Bruker Avance III 500 MHz (11.7 T) spectrometer with a 2.5 mm Magic Angle Spinning (MAS) probe at the Larmor frequency of 160.46 and 130.32 MHz, respectively. The glass powder was loaded into ZrO₂ rotor with Teflon caps. ¹¹B and ²⁷Al MAS spectra were acquired at a MAS frequency of 20 kHz with a single π/12 excitation for a radio frequency field strength of 20 kHz. The recycle delays between consecutive scans were 5s for ¹¹B and 1s for ²⁷Al. Spectra are referenced against H₃BO₃ solution at 19.6 ppm for ¹¹B, and Al(NO₃)₃ solution at 0 ppm for ²⁷Al.

All spectra were fitted according to previously published methods [56, 57, 58] using the DMfit© software [59]. We started by fitting the low-Bi end-members and then propagated the fit to the other glasses. We fitted the spectra using two “amorphous” components accounting for the BO₃ region, and placed a Gaussian line around 0 ppm for BO₄[4Si]. We placed a second Gaussian line accounting for the presence of BO₄[3Si,1B] units at ~2 ppm [60, 61]. This additional component is consistent with our current knowledge of the boron species behaviour that mixes in the silicate network and observed using advanced NMR investigations [57, 62, 63]

## 2.5. X-ray Photoelectron Spectroscopy

We performed X-ray Photoelectron Spectroscopy (XPS) on a Kratos Axis Ultra with a X-rays monochromatic Al Kα source at 1486.6 eV and 300 W. The glass chips were scrubbed with ethanol and dried (air) before analysis to remove possible remaining iodine from the surface and loaded into the sample chamber operating under high vacuum (<8.10⁻⁸ mbar). The beam
size on the sample is 300x700 µm². We acquired high-resolution spectra with a 40 eV pass energy and a step of 0.1 eV. Under those conditions, the instrumental resolution is 0.54 eV. The spectral range taken for bismuth encompasses the whole Bi 4f signal and the Si 2s, from 141 to 169 eV. The I 3d spectrum ranges from 608 to 642 eV; and the O 1s goes from 526 to 543 eV. To follow a possible evolution of the signal under the X-ray beam, we performed 10 scans of 14s for Bi, of 24s for I and of 26s for O. This method allowed us to observe important modification of the bismuth signal, characterised by a general shift to lower binding energy. We also observed the same variation in some of the I 3d spectra, but conversely, we did not observe any change in the spectral line shape for O 1s. For a few samples, we could observe also surface charging for I 3d spectra that indicates a neutralization problem. Thus, we did not attempt to fit the Bi 4f and I 3d spectra and will focus on qualitative comparisons.
All spectra were processed using the CasaXPS© software and were first referenced against the C 1s transition of adventitious carbon at 284.8 eV before further analysis. The O 1s spectra were fitted with 4 Gaussian-Lorentzian lines, according to the protocol exposed in Morizet [32] and corresponding to a potential distribution between BO and NBO species.

## 3. Results

### 3.1. Iodine solubility in mixed bismuth-doped glasses

The I-bearing glass obtained from high-pressure experiments have been characterised for major element concentrations. Data are shown in Table 1. The measured PtO₂ is an impurity
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from the platinum capsules in the sample during the high-pressure experiments and possibly
also from the parental glass during the initial synthesis in the Pt crucible [64].

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The I content is reported as a function of Bi₂O₃ content in Figure 1. We observe that the measured I content in I₂O₅-loaded glasses is higher than I₂-loaded samples, for a given glass composition and identical experimental conditions. For instance, between I₂ and I₂O₅, the iodine solubility is almost multiplied by a factor two. For Bi₂O₃ = 2 mol.%, the I solubility changes from ~0.6 to ~1.3 mol.% and from ~1.5 to ~2.0 mol.% for Al-rich and Al-poor glass series, respectively. This result is consistent with recent previous works [28, 32, 65]. In between the two glass series (Al-rich and Al-poor), the increase in I solubility between I₂ and I₂O₅ is on the same order. Regardless of the glass structure, we demonstrate that there is a strong control of the fluid phase composition on I solubility. It must be emphasised that in the case of I₂O₅, there is a combined dissolution of iodine and oxygen coming from the fluid phase into the aluminoborosilicate liquid.

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The second important information is the large difference in I retention for similar Bi₂O₃ content between Al-poor and Al-rich glasses. Again, we observed a factor two on I solubility in between the two glass series as observed in Figure 1. As mentioned in previous studies, I in aluminoborosilicate glasses is either present as Γ or I⁵⁺ in IO₃⁻ clusters, both species require the negative charge compensation by up to 4 Na⁺ or 2 Ca²⁺ [27, 28, 65]. Previous studies [29, 32] suggested that I species scavenge the available Na⁺ cations from the NBO sites in glasses in the SiO₂-Al₂O₃-B₂O₃-Na₂O system; in the meantime, the local environment for the network

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forming species requiring charge compensation (i.e. AlO₄ and BO₄) is not affected. With a higher Al₂O₃ content, implying more AlO₄, there is a lower proportion of Na⁺ cations available to form NBO. The consequence is that there is less I able to enter the glass structure as demonstrated by recent work [32]. We observe that the difference in iodine retention is constant and does not decrease with increasing bismuth content, meaning that bismuth might not be as good as sodium for charge compensating iodine in this glass system. This was observed in the literature that the iodine retention is higher with sodium over calcium and lithium over other alkali [65, 66].

Finally, we observe a positive correlation between I solubility and Bi₂O₃ concentration for both glass series. Morizet [33] also showed that the addition of Bi₂O₃ increases I solubility for aluminoborosilicate glasses synthesised under high-pressure; although, they observed this correlation to a limited concentration in Bi₂O₃ (up to 4.5 mol.%). In the present work, we show that the increase in I solubility extends to higher Bi₂O₃ contents, up to 15 mol.%. The increase in I solubility appears non-monotonic and seems to be more important at the lower Bi₂O₃ content. The strong increase in I solubility with increasing Bi₂O₃ content in the glass could be ascribed to the Bi³⁺ cations acting as network modifying cations within the glass structure. In the case Bi act as a network modifier in this glass system, it is then possible that it has the same role than Na⁺ for iodine species: Γ and IO₃⁻; Bi³⁺ forming Bi-I bonds [67].

### 3.2. <sup>11</sup>B and <sup>27</sup>Al MAS NMR spectroscopy of the Bi-bearing I-free aluminoborosilicate glasses
In order to understand why I content increases with increasing Bi₂O₃, the characterization of the parental glass structure is required. Structural characterisation has been conducted using Solid State MAS NMR to determine the change in network structure as a function of Bi₂O₃ content. The obtained spectra for ¹¹B and ²⁷Al MAS NMR are presented in Figure 2 (A and C for the Al-poor samples, and B and D for the Al-rich ones), normalized with respect to the area so that observed changes with the Bi₂O₃ content directly reflect changes in the speciation.

The ¹¹B spectra are composed of a quasi-symmetric peak located around 0 ppm related to boron in tetrahedral coordination (BO₄) and a broad asymmetric band centred around 12 ppm related to boron trigonal plan coordination (BO₃). For the ¹¹B spectra (Figure 2A and 2B), structural changes are characterized by a clear decrease in the BO₄ component intensity with increasing Bi₂O₃ content. For the Al-poor series (Figure 2A), we also observe a slight shift of the peak maximum toward higher chemical shift values. It is preposterous to conclude that the same mechanisms are taking place for the Al-rich series (Figure 2B) given the very low initial BO₄ signal. However, it seems that as the BO₄ intensity decreases, the symmetry of the region is also perturbed by the addition of Bi₂O₃. For those samples, we observe a slight shift of the maximum of the BO₄ peak toward higher values. This part of the spectra is composed of different units, mainly BO₄[4Si], BO₄[3Si,1B] near -2 and 0 ppm, respectively, for similar aluminoborosilicate glasses [60, 61, 68, 69]. Thus, the observed shift might be related to an increase of the mixing of the B and Si network as less BO₄[4Si] might be present. As the BO₄ content decreases with increasing Bi₂O₃ content, the proportion of BO₃ increases. Considering
276 that the BO₃ unit is the only boron species able to bear NBO (as compared to BO₄), the BO₄
277 decrease might indicate that the glass is depolymerizing upon Bi₂O₃ incorporation. Looking at
278 the BO₃ region for both the Al-poor (Figure 2A) and Al-rich (Figure 2B) samples, we can
279 observe a clear increase in the high frequency part of the spectra near ~15 ppm, but there are
280 few changes of the whole low frequency domain between 5 and 10 ppm. As for BO₄, the BO₃
281 region is composed of multiple components associated to different topologies of the BO₃ unit,
282 either in “ring” or “non-ring” formations [57, 61, 70]. When comparing samples with the
283 same Bi₂O₃ content in Figure 2A and B (samples p-Bi2, 4 and 8), we observe a clear decrease
284 in the BO₄ region intensity with increasing Al₂O₃. Moreover, the shape of the BO₃ band
285 seems to be different between Al-poor and Al-rich samples. The BO₃ line shape for Al-rich
286 glasses exhibits more feature than the one for Al-poor glasses, especially in the high
287 frequency part at 15 ppm.

288 In order to evaluate the potential effect of Bi₂O₃ addition to the Al environment, ²⁷Al NMR
289 spectra have been acquired, and are presented in Figures 2C and 2D for the Al-poor and Al-
290 rich samples, respectively. The spectra show a very intense component centred at ~60 ppm
291 ascribed to four-fold coordinated Al in AlO₄ species, with a probable small feature located at
292 ~30 ppm attributed to five-fold coordinated Al in AlO₅ species. We did not find evidence for
293 the presence of signal at ~0 ppm corresponding to AlO₆ units [71, 72, 73, 74, 75, 76, 77].
294 Despite using the same analysis conditions, the p-Bi8-Alr sample spectrum has a poorer S/N
295 ratio compared to the others. We do not have explanation for that yet. If we ignore this
spectrum, we observe a near-perfect superposition of the <sup>27</sup>Al spectra for samples p-Bi2 and p-Bi4 of the Al-rich series. For the Al-poor series, we also observe a huge similarity between all spectra except for p-Bi16, seemingly slightly less intense, and shifted toward lower frequencies. Overall, it seems that Bi has no effect on the aluminium environment.

### 3.3. Bi 4f, O 1s and I 3d XPS spectroscopy of the Bi-bearing I-free and I-bearing aluminoborosilicate glasses

In order to determine the element environment complementary to the NMR spectroscopy presented above, we conducted XPS measurements on the elements of interest: Bi, I and O. In Figure 3, we show several spectra for p-Bi2-Alp and p-Bi4-Alr. The Bi speciation (i.e. Bi<sup>3+</sup> or Bi<sup>5+</sup>) was investigated in the region near 157 eV corresponding to the Bi 4f<sub>7/2</sub> binding energy.

Unfortunately, several spectra showed important asymmetry arising from poor surface neutralisation. Nonetheless, some of the spectra corresponding to the first acquisitions (over 10) are shown in Figure 3. On this figure we can observe two binding energy peaks attributed to the Bi 4f<sub>7/2</sub> and 4f<sub>5/2</sub> orbital splitting peaks (about 159.4 and 164.8 eV, respectively). The measured average spin-orbit split between the maxima of the 4f<sub>7/2</sub> and 4f<sub>5/2</sub> peak is about 5.4 eV, consistent with the measured gap in previous studies (i.e. 5.31 eV; [78, 79]). According to the literature [79, 80], Bi redox state in glasses is mainly 3+ (centred about 158.0 eV), with small proportion of 5+ (161.0 eV) and metallic Bi<sup>0</sup> (at 157.6 eV). Several other bismuth form can be found in the literature such as Bi<sup>1+</sup> and Bi<sup>2+</sup> but are difficult to observe on classical laboratory XPS. In Figure 3, we observe a main peak centred near 159.4 eV for both the Al-
poor and Al-rich. As this peak shows a slight asymmetry on the high binding-energy side that could be attributed to Bi⁵⁺, we can conclude that the majority of the Bi 4f<sub>7/2</sub> component corresponds to Bi³⁺. The difference between the position of our measured peak and the literature might arise from differences in the calibration method or surface effects (i.e. difference in the surface neutralization).

Those spectra also present a small proportion of metallic bismuth (Bi<sup>0</sup>) on the lower binding energy side of the main peak attributed to Bi<sup>3+</sup>. Even with having performed short scans to avoid reduction under the X-ray beam, the presence of Bi<sup>0</sup> cannot be ruled out from being caused by the analytical conditions. The main peak at 159.4 eV does not seem to be affected by the Bi content and thus the speciation stays constant.

Looking at the difference between the Al-rich and Al-poor series, we observe that the spectra for p-Bi4-Alr shown in Figure 3C display the same features than p-Bi2 sample in Figure 3F. However, we observe a very small shift toward higher binding energy between the Al-rich I-free (Figure 3C, top) and Al-poor I-free (Figure 3D, bottom) spectra. It must be said that this difference is rather small and can be in the order of the error on the data acquisition. However, it may also indicate a change in the electronegativity of the neighbouring atoms (i.e. increase in Bi<sup>3+</sup>-O-B, decrease of Bi<sup>3+</sup>-O-Si).

We also show in Figure 3 the spectra for I-bearing (i.e. I<sub>2</sub> and I<sub>2</sub>O<sub>5</sub>) glasses. If we account the presence of Bi<sup>0</sup> to be arising only from reduction under the X-ray beam, we observe no apparent difference in between the Bi 4f, suggesting that 1) pressure conditions and I
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incorporation do not dramatically affect the local environment for Bi atoms and 2) if there is an effect, it is too small to be scrutinized in the present work. Owing to the possible change in the spectrum line shape upon the X-ray beam, it is currently difficult to discuss the change in Bi speciation and further experiments are required to improve our understanding on the local environment in aluminoborosilicate glasses

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340

341

The spectra for O 1s region obtained by XPS are shown in Figure 3B and 3E for p-Bi4-Alr and p-Bi2-Alp, respectively. The O 1s spectra consist in an asymmetric peak with a maximum at ~531 eV and also the Na Auger peak at 536 eV. The O 1s spectrum shift toward lower binding energy as the proportion of NBO increases [81, 82, 83, 84]. In between the glass series, we observe a slight change in the peak asymmetry: the peak for p-Bi4-Alr (Figure 3B) being more asymmetric than the p-Bi2-Alp on the high binding energy side. The reason for that change is not clear, however, it has been observed for all glasses in between the series. Based on the work of Morizet [32], it suggests that the distribution in between the BO species (i.e. several BO species are expected) is changing depending on the Al₂O₃ content. The effects of I incorporation and the effect of Bi₂O₃ increase on the local environment for oxygen species are not visible here without proper deconvolution of the spectra.

342

343

344

345

346

347

348

349

350

351

352

It has been shown in the literature that the I speciation in recovered glass from high-pressure experiments is dependent on the I initial source: iodide species I⁻ with I₂ initial source, mainly iodate species I⁵⁺ accompanied by small proportion of I⁻ with I₂O₅ initial source [32, 65]. Invariably, I⁰ is also found as dissolved I₂ and not in nano-bubbles [32]. In the present work, it

353

354

355
356 appears that the story is more complex and the systematic evolution of the I speciation is not
357 clear in the presence of Bi₂O₃ in the glass structure. We show I 3d spectra in Figure 3A and D
358 for p-Bi4-Alr and p-Bi2-Alp, respectively. In the spectra, we observe two peaks located at 619
359 and 624 eV assigned to I⁻ and I⁵⁺ respectively. In those spectra, we do not observe the
360 presence of I⁰ at ~620 eV, however, it is an apparent feature for other spectra (see Suppl.
361 Mat.). For p-Bi4-Alr, the I speciation pattern is consistent with literature suggestion: I⁻ is the
362 predominant species in I₂-loaded glass experiment, I⁵⁺ is in relevant proportion in I₂O₅-loaded
363 glass experiment. Surprisingly, for p-Bi2-Alp spectra, regardless of the initial iodine source,
364 I⁵⁺ is observed systematically. On the contrary, for p-Bi8-Alp-I₂O₅, the expected I⁵⁺ species is
365 totally absent in the spectrum. It seems that there is a change in the I speciation that is not
366 correlated either with the I source or the Bi₂O₃ content.

367 4. Discussion

368 4.1. Aluminoborosilicate glasses structure from NMR spectroscopy as a function of Bi₂O₃
369 content for ambient pressure glasses

370 In order to obtain the N₄ value corresponding to the BO₄ proportion [BO₄]/[BO₃+BO₄]), the
371 ¹¹B spectra of the parent glasses have been fitted following the method described in Angeli
372 [56, 57]. Figure 4 illustrates the obtained fit, the quantitative data are shown in Table 2 and all
373 ¹¹B fits are given in the Suppl. Mat.. All fits have been performed using the DMfit software
374 [59]. With the proportion of each component, we were able to compute the N₄ value (see
375 Table 2). We plotted the N₄ values as a function of Bi₂O₃ content in Figure 5A. It is clear
from this figure that for both series (i.e. Al-rich and Al-poor), the proportion of BO₄ is decreasing with increasing Bi₂O₃ content. At first sight, it appears that the extent of the N₄ decrease is the same for both series; however, this point should be taken with caution considering that there are only three data points for the Al-rich glasses. A second striking feature is the difference in the N₄ values between Al-poor and Al-rich samples at the same Bi₂O₃ content. We measured N₄ at 0.42 and 0.13 for p-Bi2-Alp and p-Bi2-Alr, respectively, 0.39 and 0.12 for p-Bi4-Alp and p-Bi4-Alr, respectively, and 0.33 and 0.04 for p-Bi8-Alp and p-Bi8-Alr, respectively. Considering that only the proportion of Al₂O₃ is changing, from about 5 mol.% for the Al-poor samples, to more than 15 mol.% for the Al-rich samples, we suggest that this difference in N₄ value is ascribed to cation priority for compensation. Indeed, as both B³⁺ and Al³⁺ cations require charge compensation in order to form tetrahedral species (BO₄ and AlO₄, respectively), if the quantity of charge compensator is not enough for both, the Al³⁺ cations are prioritized for compensation over the B³⁺ cations (already observed in aluminoborosilicate glasses [63, 85, 86, 87]). Thus, the observed lower N₄ in Al-rich samples can be explained by the higher quantity of AlO₄ requiring compensation. With this information and as the difference between Al-poor and Al-rich samples seems to be constant, we can conclude that Bi cations are not acting here as charge compensators, as previously stated in similar glass system [42]. It is noteworthy to say that in specific systems, Bi₂O₃ can also act as a network forming oxide [79, 88]. Thus, in our glass system, bismuth is a network modifier cation and creates NBO atoms, as emphasised in previous works [50]. According to
396

Dell [89] in the Na₂O-B₂O₃-SiO₂ system, and Du and Stebbins [62, 63] for (Na₂O/K₂O/CaO)-

397

B₂O₃-SiO₂ glasses, increasing the proportion of non-network-forming cation (i.e. Na⁺)

398

increases the proportion of BO₄ until a certain limit at which BO₄ are not formed anymore,

399

and where the network-modifier cations are creating NBO on network-forming SiO₄ units.

400

After this threshold (corresponding to 0.5+K'/4, with K'=[(B₂O₃]+[Al₂O₃])/[SiO₂]), the

401

proportion of BO₄ decreases as the proportion of NBO-bearing BO₃ increases. This is

402

represented by different coloured regions in the Figure 5B. On this figure, the measured N₄

403

for our measured samples are plotted with red crosses, and the corresponding calculated N₄

404

with the model [63, 89] based on the chemical composition are plotted with black crosses.

405

Here, as Bi is not a network former species, we calculated R' as

406

([Na₂O]+[CaO]+[Bi₂O₃])/([Al₂O₃]+[B₂O₃]). The observed difference in Figure 5B between

407

the measured and calculated data points is explained by the limited ability for the used model

408

to account for the “charge-compensating capacity” of cations other than the ones involved in

409

this early model. Indeed, the Dell [89] model has been developed initially for simple alkali

410

glasses and extended after that to mixed alkali-earth-alkali glasses [63]. Some cations are

411

prone to act as charge compensator or network modifiers, establishing the main limitation of

412

the model, unable to account for these differences. However, on Figure 5B we observe that

413

both the calculated and measured data are present in the same region, indicating that the

414

model might still be used to predict the NBO regime. Here, we observe that our samples are

415

located in a region corresponding to the formation of NBO associated to BO₃ species and
416

NBO associated to silicon Qⁿ species due to the high R' value. We thus conclude that whether

417

Bi can act as a charge compensator for boron species, the total non-network forming elements

418

(thus including Bi) is too high to promote formation of BO₄.

419

To quantify the proportion of the different Al species, we simulated the ²⁷Al spectra using

420

Czjzek lines [90, 91]. We calculated the average Al coordination number as CN<sub>Al</sub> =

421

4.[AlO₄] + 5.[AlO₅] considering that AlO₆ species was not observed (see Figures 2 and 4B). Fit

422

results are presented in Table 2 along with the calculated CN<sub>Al</sub>. We plotted this CN<sub>Al</sub> as a

423

function of Bi₂O₃ content in Figure 6. The first key element on this figure is the constant

424

value at ~4.05 for the Al-poor series. As the Al₂O₃ content is very low (~5 mol.%), it was

425

expected that almost all Al³⁺ was present as AlO₄ as it is the priority species for charge

426

compensation over BO₄ [62, 63, 85, 87]. It is interesting to observe that Bi₂O₃ does not seem

427

to influence the aluminium environment for this series. On the other hand, the CN<sub>Al</sub> is higher

428

in the Al-rich glass series for the same Bi₂O₃ content. The CN<sub>Al</sub> for the p-Bi₂, 4 and 8

429

samples are 4.09, 4.1 and 4.17 for the Al-rich glass series, against ~4.05 for the Al-poor glass

430

series. The higher CN<sub>Al</sub> must be related to the larger Al₂O₃ content (i.e. almost equals to the

431

Na₂O content), which makes total compensation of all AlO₄ units less straightforward. This

432

glass series also undergoes modifications with the Bi₂O₃ increase. We observe a clear increase

433

in the CN<sub>Al</sub>, from 4.09 for the p-Bi₂-Alr sample to 4.17 for the p-Bi₈-Alr one, with 9 and 17%

434

AlO₅, respectively (see Table 2). From the literature, it has been demonstrated that some high

435

field strength non-network cations such as Bi can cause Al-coordination to increase. This is
436

the case with earth-alkali cations such as magnesium [92, 93, 94, 95], and with rare-earth elements [96, 97, 98]. Nevertheless, we do not have clear evidence for the proximity between AlO₅ species and Bi cations.

437

438

439

4.2. Degree of polymerization with varying Bi₂O₃ content as seen from O 1s XPS and link with iodine solubility

440

441

To follow the evolution of the oxygen environment with the change in Bi₂O₃ content, we fitted the O 1s XPS spectra. Typical fitted spectra are presented in Figure 7 for p-Bi2-Alr and p-Bi8-Alp. The fitting parameters are given Table 3 and in the Suppl. Mat., along with the calculated proportion of each oxygen species (see Morizet [32] for more details on the fitting procedure). We fitted the spectra with four Gaussian lines accounting for two BO species and two NBO species as described in Morizet [32]. We suspect that the two NBO species correspond to NBO related to 1) BO₃ at ~531 eV and 2) Qⁿ SiO₄ species at ~530 eV. In Figure 7, we show the spectra for ambient pressure, I₂ and I₂O₅ glasses. For Al-poor glasses (Figure 7B), we do not observe a clear change in the Gaussian peak intensity. On the contrary, for Al-rich glass (Figure 7A), there is a clear increase in the Gaussian peak intensity for NBO species. It suggests that there is a decrease in the degree of polymerization of the glass with the incorporation of iodine and the applied pressure, which goes against the expected effect of iodine and pressure that is an increase in polymerisation [29, 58]. Thus here, the dominant effect is the depolymerisation induced by the [Bi₂O₃] increase, which is consistent with the relative proportion of Bi and I.

442

443

444

445

446

447

448

449

450

451

452

453

454

455
The Figure 8 highlights the evolution of the different oxygen species with increasing Bi₂O₃ content. Consistently, we observe an increase of the proportion of NBO at the expense of the BO with increasing Bi₂O₃ content. This effect is visible both for the Al-poor and Al-rich series, indicating a similar role for Bi₂O₃ as a network-modifier oxide in this glass system. It should be emphasised that the proposed fit in Figure 7 is only a possible solution from many others. Thus, the attribution of the peak in Table 3 must be taken cautiously, and advanced ¹⁷O NMR spectroscopy and Bi X-ray Absorption Spectroscopy approach have to be conducted to obtain a better view on the glass structure. Nevertheless, the results presented in Figure 8 indicate that Bi incorporation into the glass induces a general decrease in the degree of polymerization. The fact that the trends are observed for the whole set of samples also indicate that Bi₂O₃ is predominant for imposing polymerization degree and I incorporation or pressure conditions do not play a significant role on the glass structure network connectivity. As mentioned in Morizet [32], oxygen availability is the main driver of iodine solubility in glass. Thus, the measured change in oxygen speciation gives clues to explain the increases in iodine solubility when increasing bismuth content. Indeed, by increasing the proportions of “available oxygens” (NBOs), bismuth enable more iodine to enter the glass structure.

Considering that Bi³⁺ is the main species of Bi for both glass series and because we thus suggest that Bi is acting as a network modifier species, we can establish simple reactions for the incorporation of Bi into the glass at ambient pressure:

$$ 3SiO_4^{BO} + \frac{1}{2}Bi_2O_3 \rightarrow 3(SiO_3^{BO}O^{NBO}) \cdots Bi^{3+} \quad (1) $$
$$ 476 \quad 3BO_3^{BO} + \frac{1}{2}Bi_2O_3 \rightarrow 3(BO_2^{BO}O^{NBO}) \cdots Bi^{3+} \quad (2) $$

In these equations, $O^{BO}$ represents Bridging Oxygens, thus shared between two tetrahedra, and $O^{NBO}$ represents Non-Bridging Oxygens, whose charge only rely on one tetrahedra and a network-modifier species. Thus, for a $SiO_4^{BO}$ having 4 shared oxygens, the tetrahedra is only bearing 4/2 oxygens, whereas a $Q^3$ ($SiO_3^{BO}O^{NBO}$) is bearing 3/2+1 oxygens.

Given the dominant presence of $AlO_4$ for both series and as it is very unlikely that Al species are carrying NBO, we cannot establish the same equations for Al species. Looking at both equations, we observe that a $Q^4$ species ($SiO_4^{BO}$) is converted into a $Q^3$ species with one additional NBO ($SiO_3^{BO}O^{NBO}$) and that a fully polymerised $BO_3$ is converted into a $BO_3$ bearing one NBO. However, this is only one example of depolymerisation induced by $Bi^{3+}$ cations. Indeed, it is possible that already present $Q^3$ are converted into $Q^2$, or $BO^3$ with one NBO to be converted into $BO^3$ with two NBOs. As compared to previous studies on alkali and earth-alkali borosilicate glasses [61, 63, 87, 99, 100], Bi appears to be a stronger network modifier. Indeed, where $Na^+$ cations can easily act as a charge compensator for network former species (i.e. $AlO_4$ or $BO_4$), $Ca^{2+}$ is not as good, and thus, we can conclude here that Bi might only be able to induce depolymerisation in our glass system.

The proposed equation can also be adapted to the dissolution of I content considering that I dissolves as $I^-$ or $IO_3^-$ species and considering $Bi^{3+}$ cations as a possible source of charge
compensators for iodine, in agreement with previous works [32]. For I⁻, we have the following:

$$496 \quad 3(\mathrm{SiO}_3^{\mathrm{BO}}\mathrm{O}^{\mathrm{NBO}}) \cdots \mathrm{Bi}^{3+} + \frac{1}{2}\mathrm{I}_2 \rightarrow \mathrm{SiO}_4^{\mathrm{BO}} + 2(\mathrm{SiO}_3^{\mathrm{BO}}\mathrm{O}^{\mathrm{NBO}}) \cdots \mathrm{Bi}^{3+} \cdots \mathrm{I}^{-} + \frac{1}{4}\mathrm{O}_2 \quad (3)$$

$$497 \quad 3(\mathrm{BO}_2^{\mathrm{BO}}\mathrm{O}^{\mathrm{NBO}}) \cdots \mathrm{Bi}^{3+} + \frac{1}{2}\mathrm{I}_2 \rightarrow \mathrm{BO}_3^{\mathrm{BO}} + 2(\mathrm{BO}_2^{\mathrm{BO}}\mathrm{O}^{\mathrm{NBO}}) \cdots \mathrm{Bi}^{3+} \cdots \mathrm{I}^{-} + \frac{1}{4}\mathrm{O}_2 \quad (4)$$

And for IO₃⁻, we can write the following:

$$499 \quad 3(\mathrm{SiO}_3^{\mathrm{BO}}\mathrm{O}^{\mathrm{NBO}}) \cdots \mathrm{Bi}^{3+} + \frac{1}{2}\mathrm{I}_2\mathrm{O}_5 \rightarrow \mathrm{SiO}_4^{\mathrm{BO}} + 2(\mathrm{SiO}_3^{\mathrm{BO}}\mathrm{O}^{\mathrm{NBO}}) \cdots \mathrm{Bi}^{3+} \cdots (\mathrm{IO}_3)^{-} \quad (5)$$

$$500 \quad 3(\mathrm{BO}_2^{\mathrm{BO}}\mathrm{O}^{\mathrm{NBO}}) \cdots \mathrm{Bi}^{3+} + \frac{1}{2}\mathrm{I}_2\mathrm{O}_5 \rightarrow \mathrm{BO}_3^{\mathrm{BO}} + 2(\mathrm{BO}_2^{\mathrm{BO}}\mathrm{O}^{\mathrm{NBO}}) \cdots \mathrm{Bi}^{3+} \cdots (\mathrm{IO}_3)^{-} \quad (6)$$

These proposed chemical equations are only one example of iodine entry in the glass, and the one believed to be the most probable in presence of bismuth in agreement with the literature [32, 58]. Here iodine present as I⁻ or IO₃⁻ ionically bond to a positive charge to rest in a neutral environment. This is usually done by charge compensation by up to 4 sodium or 2 calcium cations, sharing charges and bonding iodine to the glass network [65]. As mentioned previously, other structures might form, such as BiOI or BiI3, but we cannot conclude on their presence or absence only with the information presented in this manuscript. As emphasised in previous works [29, 32, 66], the I effect on the network structure is very small. Hence, even with relatively high I content (up to 2.8 mol.%), any effect on the structure might be too low to be detected in front of the huge depolymerizing effect of Bi. Nevertheless, in the optic of
511

improving I incorporation into a potential immobilisation matrix, the addition of Bi₂O₃ seems

512

to be beneficial, at least to a few mol.%.

513

## 5. Summary

514

Bi-bearing aluminoborosilicate glasses were investigated as a matrix for iodine radioisotope

515

immobilisation with a protocol involving high-pressure glass synthesis. Glasses with Bi₂O₃

516

content from 1 to 16 mol.% were first synthesized under normal pressure condition and

517

revealed a huge decrease of the BO₄ proportion with increasing Bi₂O₃ content, along with an

518

expected increase in the NBO proportion.

519

These glasses were used for high-pressure experiments along with iodine as I₂ or I₂O₅. The

520

obtained glasses revealed an I retention of the order of 1 to 2.8 mol.%, which is several orders

521

of magnitude higher than in similar glasses synthesized under normal pressure conditions. We

522

demonstrated that Bi₂O₃ promotes I incorporation by increasing I solubility from 1.7 to 2.8

523

mol.% for example between a glass containing 1 mol.% Bi₂O₃ and one containing 14 mol.%

524

Bi₂O₃.

525

O 1s XPS did not reveal significant effect of I on the glass structure, most probably hidden

526

by the depolymerizing effect of Bi. I 3d XPS has also been performed and showed major

527

disturbances in the I speciation. Indeed, the expected speciation is dictated mainly by the used

528

I source, Γ for loading I₂, and I⁵⁺ for loading I₂O₅. Here we found highly oxidized iodine in I₂-
529 loaded samples, and major proportion of reduced iodine in I₂O₅-loaded samples. These results

530 may indicate a more intricate relation between iodine and bismuth that remains to be assessed.

531 *Acknowledgments*

532 *The authors are grateful to the Agence Nationale de la Recherche, which financed the current*

533 work through the ANR project "Iodine-CLEAN-UP" (ANR-20-CE08-0018). The authors

534 thank the Région Pays de la Loire for funding the present research through the "Trajectoire

535 nationale de la recherche ligérienne" program. The authors thank the Laboratoire de

536 Planétologie et Géosciences. Solid State NMR and SEM/EDS measurements were performed

537 on IMN's equipment platform, PLASSMAT, Nantes, France.

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539

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