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Results of irradiation on the mechanisms controlling the residual charge of an alumino-borosilicate glass

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Desk 2 teams the info obtained for the gel thicknesses, from SEM statement, ToF-SIMS analyses and ICP analyses. A world consistency of the outcomes can’t be achieved for 2 causes:

  • Sure small sides of the glass coupons haven’t been polished previous to leaching experiment. Unexpectedly, these sides underwent extreme and uneven corrosion, possible as a result of surfaces contained small cracks that favoured alteration and triggered floor enhance25. As small sides characterize 31–35% of the full floor of the samples, the impression of the phenomenon is just not negligible however not possible to quantify.

  • B is partly retained within the gel. This was not taken into consideration for normalised mass loss calculation however it may be quantified for samples A–D from ToF-SIMS profiles. B retention in gel is mentioned additional.

Regardless of these limitations, to evaluate the impression of irradiation on glass alteration, direct and dependable comparability could be comprised of SEM observations carried out on giant faces of the coupons, together with ToF-SIMS analyses, which had been additionally carried out on giant faces. These comparisons present that regardless of the pH, pre-irradiated samples had been extra altered than non-irradiated samples. This consequence was already noticed at pH 911, however it was by no means reported in impartial and acid pHs. The distinction in gel thickness between irradiated and non-irradiated samples will increase with reducing pH (from an element 2.8 to an element 4.0 for pH 9 to pH 3, respectively). This could be associated to the capability of the gel to reorganise, which is favoured in primary pH, as urged by a earlier work on ISG15.

The residual charge, rr, was calculated by linear regression on the normalised mass losses for B between 55 and 83 days, i.e., past the inflexion level for the pre-irradiated samples. The worth of rr was not calculated for pattern F, as alteration went past the irradiated space. It ought to be borne in thoughts that the worth of the residual charges is indicative for the reason that calculation relies on the geometric floor, and due to this fact ignore the roughness of the small sides. Nonetheless, the residual charges could be in contrast two by two assuming that the burden of the small sides over the entire of the alteration is analogous for a given pH. The outcomes are given in Fig. 10.

Fig. 10: Residual charges calculated from the discharge of B into the answer between 55 and 83 days for five of the 6 experiments.
figure 10

An uncertainty of 30% for the worth of rr is taken. A, C, E correspond to non-irradiated samples, whereas B and D are associated to pre-irradiated samples. Outcome for experiment F is just not supplied on condition that alteration went deeper than the irradiated depth.

For experiment A, rr was near that decided after 1 yr of alteration for CJ2 glass powder altered at pH 9 (3.2 × 10−3 g m−2 d−1 after normalisation on the particular floor of the powder). This provides ~8 × 10−3 g m−2 d−1 after normalisation on the geometric floor, a worth, which could be in comparison with that of our examine19. On the idea of the powder experiment, which lasted greater than 20 years, it may be anticipated that the residual charge in experiment A will proceed to lower with time, and that the alteration will stick with it uniformly over all the floor with out the formation of secondary minerals. Curiously, the rise in rr for the irradiated pattern (experiment B vs. A) comparable to noticed in our experiments is aligned with residual charge measurements performed on 238/239Pu or 244Cm-doped glasses that underwent ballistic damages on account of recoil nuclei26. Right here, the ratio between rr (irradiated)/rr(non-irradiated) is 2.5 and 1.5 at pH 9 and pH 7, respectively. These values are decrease than the rise in gel thickness on the finish after 83 days of alteration. This helps the thought proposed in a current paper that the gels shaped on irradiated samples reorganised sooner than on reference samples11.

The retention of B within the alteration layer came about within the pristine glass/alteration layer interface zone. It was notably excessive when the glass was drastically altered (positions of the arrows in Fig. 11). As above defined, for the experiments E and F, this quantification was not potential utilizing the ToF-SIMS mappings. When evaluating the B releases into resolution for the experiments performed at pH 9 (A and B), pH 7 (C and D), and at pH 3 (E and F), it was famous above that the latter adopted a sq. root of time evolution for the non-irradiated samples (A, C, and E) whereas the pre-irradiated samples confirmed an equal development for 15 to 25 days earlier than the curve bent (Fig. 5). The glass alteration charges due to this fact decreased extra notably for the pre-irradiated glasses than for the non-irradiated samples. These observations counsel firstly that the damaging impact of pre-irradiation on glass behaviour may very well be transitory, and secondly that the damaging impact of the answer acidity on the residual charge may very well be counter-balanced by an accumulation of B within the alteration layer, whose impact could be to decelerate diffusion of water molecules and thus the progress of alteration (Fig. 10). These coupled phenomena, linking B and water diffusion within the gel, was demonstrated in a earlier ISG glass examine6. This mechanism might be mentioned in additional element beneath.

Fig. 11: ToF-SIMS B profiles for experiments A–D.
figure 11

The dotted vertical strains point out the place of the pristine glass/alteration layer interface, and the arrows present the B content material at that interface. The retention of B within the alteration layer elevated with the thickness of the altered glass.

For ISG-type glasses, the residual charge resulted from the power of B to proceed dissolving after the silicate community not (or extraordinarily slowly) dissolved, as a result of Si reached the answer saturation degree. In glass compositions like that studied right here, there are few B‒O‒B bonds, and due to this fact the bonds, which do exist can’t type a percolating sub-network. Nevertheless, there are as many Si‒O‒B bonds as there are for Si‒O‒Si27. This implies {that a} sub-network constructed with Si‒O‒B linkages, a priori extra fragile than the purely silicate community5 percolates throughout the construction and may result in dissolution of the B atoms with out essentially breaking the Si‒O‒Si bonds close by. Moreover, it was calculated that B is chargeable for round 90% of the pore formation in gels28. Aside from the formation of a cavity during which water can readily diffuse, the hydrolysis of a Si‒O‒B bond and the dissolution of B results in the formation of a silanol group (Si‒OH). If two silanols are shut sufficient to one another, they will re-form a siloxane bond (Si‒O‒Si) by expelling a water molecule29. The departures of B and Na along with the condensation of silanols create the porous construction of the gel. The pH might impression water mobility in gels by way of variations in hydrogen bonding interactions30. One other key course of that must be thought of, is that the gel construction evolves through the response, because the hydrolysis and condensation reactions proceed throughout the gel, regardless that the composition of each the fluid and the gel stay fixed28,30,31,32. The reactivity of the silicate community ensuing from the departure of cell parts results in its reorganisation and to the closing of some pores31,33,34. Understanding that water diffusion coefficient by way of closed pores is round 10 orders of magnitude decrease than in open channels31, it seems that gel maturation/reorganisation can impression the residual charge. As famous above, a earlier examine confirmed that pre-irradiated ISG samples altered in circumstances just like samples A and B (90 °C, pH 9) corroded round 4 occasions sooner than non-irradiated reference samples and shaped gels that reorganised sooner11. The final consequence was obtained utilizing TEM in cryogenic mode to protect the gel construction throughout statement.

The comparability of the non-irradiated and irradiated glasses highlights vital variations among the many three pH ranges studied. Whether or not the glass was irradiated or not, the chemical parts making up the fabric remained the identical, and it has been proven from Si isotopes that the silicate networks (Si‒O‒Si bonds) had been solely barely affected by alteration within the Si saturation circumstances of the answer. For a given pH, the distinction in alteration charges might thus be as a result of affect of the structural damages brought on by irradiation (structural modifications at brief and medium distance and variations within the inner vitality state of the system) on the hydrolysis of the Si‒O‒B bonds and/or on diffusion coefficients of aqueous species throughout the alteration layers. In a diluted medium, it has been proven that at pH 9 there is a rise (issue of 4.7) within the preliminary dissolution charge of the pre-irradiated CJ2 glass in comparison with a non-irradiated pattern35. This worth is near that decided by Lonartz et al. below comparable circumstances for a Na-borosilicate glass36. Keep in mind that the preliminary dissolution charge is decided based mostly on the discharge of Si in resolution in a extremely diluted medium. These circumstances allow the intrinsic reactivity of the glass to be examined within the absence of an alteration layer (resistance to hydrolysis of the silicate community). Within the irradiation circumstances carried out in our examine, the rise within the Si‒O‒M (M = Si, B, Al) bond hydrolysis charge of the glass was established, however in a residual regime the place the glass alteration was managed by a number of mechanisms (hydrolysis of the Si‒O‒B bonds, transport of water and of B within the gel), it isn’t identified precisely which of those mechanisms was impacted.

To research whether or not the transport properties of the aqueous species within the alteration layer had been affected by the structural modifications throughout the glass, a further “tracing” experiment was carried out on samples A and B at pH 9. This concerned submerging the 2 samples in an answer enriched in H218O and methylene blue for twenty-four h, after which utilizing ToF-SIMS to analyse the penetration profiles of the isotopically labelled water and the methylene blue, a chemically inert molecule with a hydrodynamic diameter of 1 nm (in comparison with 0.27 nm for the water molecule). The experiment was carried out at ambient temperature to keep away from producing additional glass alteration, and the ToF-SIMS analyses had been carried out after freezing the samples in liquid nitrogen. A cooled pattern holder was used to be able to protect the pore water as a lot as potential through the evaluation. It was a tough evaluation scenario, and work needed to be carried out rapidly, because the excessive vacuum within the evaluation chamber may cause water sublimation regardless of the low temperatures. The outcomes had been as follows (Fig. 12):

  • S, the methylene blue label, penetrated greater than 10 nm into the alteration layer of the non-irradiated glass, and 20 nm into that of the pre-irradiated glass. Because the abrasion was quick (6 nm steps for experiment A and 25 nm steps for experiment B), it isn’t potential to provide extra exact penetration depths. The profiles are due to this fact not introduced within the determine.

  • 18O, the water molecule label for the answer, penetrated so far as the alteration border, marked by the departure of B atoms.

  • 18O exchanged rather more with the 16O within the alteration layer of the irradiated glass than in that of the non-irradiated pattern.

Fig. 12: ToF-SIMS profiles for samples A and B after tracing for twenty-four h in an answer enriched in H218O.
figure 12

The alteration fronts are proven by the vertical dotted strains. The isotopic ratios 18O/16O are quantitative, and given on the axis on the fitting. The alteration layer shaped on the irradiated glass included ~3 occasions extra H218O than the non-irradiated glass.

Based mostly on our outcomes, the next reasoning could be adopted. On the speculation that 18O primarily exchanged with O from water molecules current in pores of the gel31, this tracing experiment revealed the existence of open channels throughout the gel, which solely enabled small molecules (water, however not methylene blue) to diffuse quickly between the answer and the glass floor. An essential level to notice is that the amount of exchangeable water, and due to this fact the proportion of channels enabling a swift circulation of water molecules, is ~3 occasions bigger in irradiated than in non-irradiated glass (comparability between the plateau ranges). This consequence strongly means that the construction of the 2 gels are completely different. The upper quantity of open channels within the pre-irradiated pattern might consequence from damages created by irradiation within the stable (fission tracks, enhance in giant sized-rings, and enhance in NBO). One other implication of this result’s that the gel former atoms (Si and Al) weren’t cell through the glass alteration. Certainly, if all of the Si and Al atoms had been hydrolysed earlier than precipitating throughout the gel layer, it’s possible that the gel would have misplaced its glass structural traits. This aligns with previous outcomes displaying that glasses irradiated with 252Cf past the formation of traces of fission maintain these traces of their morphology throughout the alteration movie37. The distinction when it comes to O trade charge is also on account of a maturation course of occurring at a sooner charge on the pre-irradiated samples, as urged by Tribet et al.11, and main to larger and extra interconnected pores. Certainly, as above defined, gel reorganises over time. Due to that, the gels characterised on the finish of experiments A and B have advanced after they’ve shaped. If one considers than Si and Al are low cell, the place alongside the profile could be associated to time. It signifies that the portion of the gel close to the interface with the pristine glass (inside gel) is youthful than that positioned nearer to the answer. The latter is thus extra mature and reorganised than the previous. Evaluating younger and mature gels shaped on the 2 supplies (Fig. 12), it seems than the variations are most for the younger gels, suggesting that reorganisations throughout the gel performs a serious function on alteration in residual charge regime.

Whether it is thought of that the alteration came about at a relentless quantity, which appears to have been the case right here at pH 7 and pH 9, following the strategy utilized to ISG28, the free quantity created by B and Na leaching would result in 43% porosity. Based mostly on this, an isotopic depend for the O atoms exhibits that round 4.3% of the O atoms current within the alteration layer pore water had been exchanged through the tracing experiment on non-irradiated glass, in comparison with 13% for the irradiated samples. These fractions stay fairly low, displaying both that the porosity within the layers is especially closed, or that exterior layers of the gel (near the interface with the answer) inhibit water diffusion, or just that the trade charge is sluggish. The fractions for the O exchanged on this examine had been, nevertheless, increased than these decided for ISG glass after 1 yr of alteration at pH 7, 90 °C. For the latter, the fraction of pore water exchanged was 2.5%, as discovered by tracing equivalent to that utilized in our examine31. This weaker trade must be thought of in relation to a sooner charge lower (issue of 1000 for ISG glass, in comparison with about 250 for the CJ2 glass). At this level within the investigations, no conclusion could be drawn as to the placement within the gel the place the diffusion is the slowest.

The experiments carried out throughout this examine didn’t reveal whether or not the gels will have a tendency in direction of having the identical diffusion properties due to the maturation course of (pore closures and coalescence). Additional work might be wanted to make clear this level.

This examine results in the next concluding remarks. The CJ2 glass pre-irradiated with heavy ions altered extra rapidly than the non-irradiated glass below residual charge circumstances in acidic, impartial, or primary media. For the three pH ranges studied, the alteration charge decreased with time, due to the formation of a passivating alteration layer. This layer retained a part of the hydrolysed B, which might impression water diffusion and due to this fact reinforce its passivating properties, as has been proven in a earlier examine6. In a primary medium, this work has demonstrated that the alteration layer shaped on irradiated glass is much less passivating, at the very least within the early phases, than on non-irradiated glass. We now have thus established that the construction of the alteration layer initially inherited from the glass construction, after which matured, possible altering the diffusion coefficients of water and aqueous species. In line with a earlier examine on ISG, evidently maturation happens at a sooner charge on pre-irradiated samples11, which might clarify why the residual charge, even increased initially of the experiment decreased sooner than for the non-irradiated materials. As issues the controversy concerning the alteration layer formation mechanisms, this work has confirmed that the gel shaped on this kind of glass is just not shaped by congruent dissolution and re-precipitation, however moderately by a preferential dissolution mechanism for the B atoms, adopted by silanol re-condensation involving very low Si atom mobility. From these varied observations, it may be concluded that the consequences of pre-irradiation on glass dissolution within the residual charge regime are transient.

Following this work, you will need to clearly establish the alteration layer zone or zones contributing to diffusion inhibition. To be able to clarify the variations between irradiated and non-irradiated samples, the authors of the examine above cited11, urged that the driving pressure for glass alteration is exerted solely on the interface zone between the gel and the glass. One other examine carried out with SON68 glass in residual charge regime proposed that water diffusion within the uncorroded glass controls the discharge of Na by way of ion trade and B dissolution38. On the idea of our outcomes, it will seem that the query should stay open. General, coherency must be established amongst all the outcomes accessible within the literature. Though the impact of pre-irradiation with 1 GeV Xe on rr is near that noticed on Pu and Cm-doped glass26, the situations of irradiation had been completely different (unidirectional irradiation and decoupling between irradiation and alteration in our case), in order that this comparability is just not enough to conclude about the perfect circumstances to simulate the behaviour of nuclear glass in geological repository.

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