Effect of V2O5 on crystallization tendency and chemical durability of Mo-bearing aluminoborosilicate glass

The effect of V2O5 addition on molybdates crystallization tendency, glass structure and chemical durability of aluminoborosilicate glass belonging to SiO2-B2O3-CaO-Na2O-Al2O3-MoO3 system has been studied. The results confirm that V2O5 addition can effectively suppress the crystallization tendency of powellite and enhance the molybdenum solubility in the glass. The MoO3 solubility limit is found to be 2.8 mol% in the V2O5-containing aluminoborosilicate glass. Raman results reveal that V2O5 addition seems to modify the local structural environment of isolated MoO4 units and increase their chemical disorder in the glass, which is favored for molybdenum incorporation in the glass. The molar volume and glass transition temperature of samples are found to depend on V2O5 content. Product consistency test (PCT) results show that the normalized leaching rates of the V2O5-containing aluminoborosilicate glass maintain at a fairly low level compared with standard borosilicate glassy waste form.


Abstact
The effect of V 2 O 5 addition on molybdates crystallization tendency, glass structure and chemical durability of aluminoborosilicate glass belonging to SiO 2 -B 2 O 3 -CaO-Na 2 O-Al 2 O 3 -MoO 3 system has been studied. The results confirm that V 2 O 5 addition can effectively suppress the crystallization tendency of powellite and enhance the molybdenum solubility in the glass. The MoO 3 solubility limit is found to be 2.8 mol% in the V 2 O 5 -containing aluminoborosilicate glass. Raman results reveal that V 2 O 5 addition seems to modify the local structural environment of isolated MoO 4 units and increase their chemical disorder in the glass, which is favored for molybdenum incorporation in the glass. The molar volume and glass transition temperature of samples are found to depend on V 2 O 5 content. Product consistency test (PCT) results show that the normalized leaching rates of the V 2 O 5 -containing aluminoborosilicate glass maintain at a fairly low level compared with standard borosilicate glassy waste form.

Introduction
Highly radioactive liquid waste (HLW) resulting from spent nuclear fuel reprocessing is highly hazardous to human beings and the environment, due to its radioactivity and biological toxicity. The safe disposal of HLW has drawn a great deal of attention from the public and governments [1][2][3][4][5][6]. Currently, vitrification is a worldwide recognized method to immobilize HLW, and borosilicate glasses have been employed to immobilize HLW on an industrial scale in several countries [7][8][9].
Higher concentration of molybdenum is usually present in HLW streams resulting from the reprocessing of commercial spent nuclear fuel due to the high burn-up. Based on previous studies, molybdenum has a very low solubility in alkali borosilicate glass under usual elaboration conditions. Above the solubility limit, a molybdate salt phase ('yellow phase') is usually separated from glass melt during the melting stage. This yellow phase is dominated by crystals of alkali molybdates (Na 2 MoO 4 ), which would gather radioactive elements like 137 Cs, 90 Sr and minor actinides. On account of the solubility of Na 2 MoO 4 in water, the yellow phase can severely decrease the chemical durability of nuclear waste forms. In addition, the yellow phase would also corrode melter liners and impede the vitrification process [10][11][12].
Mo cations primarily exist as a hexavalent state Mo 6+ in borosilicate glasses prepared in oxidizing or neutral conditions and take the form of [MoO 4 ] 2− tetrahedra, which locate in the alkali and alkaline earth enriched depolymerized regions of the glass network [13][14][15][16]. In order to improve the solubility of MoO 3 via suppressing the formation of yellow phase in borosilicate glass, a number of studies have been conducted by developing new or modified glass formulation [17][18][19][20][21]. The strategies include adding proper amount of rare earth (e.g., Nd and La) [13,[17][18][19] or high field strength modifiers such as Li [20], Mg [21] to the components, adjusting the ratio of Ca to Na and Na to B of the components [22][23][24][25][26], and increasing the B 2 O 3 level [27] in borosilicate glass.
Recently, a potential glass waste form based on iron-phosphate (40Fe 2 O 3 -60P 2 O 5 ) has been developed by Hsu et al [28] to immobilize HLW with high content of MoO 3 . No phase separation was observed in this ironphosphate glass when the MoO 3 content reaches up to 30 wt%. Moreover, Mo cations can bond well with phosphate units in the glass structure, which indicates that the compatibility of molybdenum and phosphate tetrahedrons is better than that of molybdenum and silicate tetrahedrons. However, due to the poor thermal and chemical durability properties, phosphate glass is undesirable in nuclear industry. Therefore, to address these problems during nuclear waste immobilization, it is felt prudent to develop borosilico-vanadate glasses based on the similar 'crystallo-chemical' features of V 2 O 5 and P 2 O 5 . This glass matrix may not only present the ability to incorporate high content of MoO 3 but also have better chemical durability and thermal stability than phosphate glass. Vanadium ions in alkali borosilicate glass mainly occupy V(V)O 4 units and the introduction of V 2 O 5 can modify the structural around B-or Si-centred structural units induced by Mo ions [15,29]. Therefore, it is expect that vanadium addition can suppress the preference of alkali ions for Mo(VI)O 4 , owing to equivalent or even higher priority of alkali ions coordination with V(V)O 4 units, which would enhance the solubility of MoO 3 in borosilicate glass and suppress the formation of yellow phase precipitation. However, a detailed description the effect of V 2 O 5 addition on structure and physical properties (e.g., chemical durability, density, molar volume and glass transition temperature) of Mo-bearing borosilicate glass is still missing.
The aim of this work is to evaluate the influence of V 2 O 5 content on crystallization tendency and microstructure of a modified aluminoborosilicate, which contains simultaneously MoO 3 and V 2 O 5 . For the purpose, a Mo-enriched aluminoborosilicate glass composition has been chosen in the SiO 2 -B 2 O 3 -Na 2 O-CaO -Al 2 O 3 -MoO 3 system, which is derived from a more complex glass envisaged to immobilize Mo-enriched HLW. Accordingly, two series of aluminoborosilicate glasses have been synthesized and characterized. The structural re-adjustments influencement on the glass properties has also been studied. The raw materials of SiO 2 , CaCO 3 , Al 2 O 3 , H 3 BO 3 , Na 2 CO 3 , MoO 3 , and V 2 O 5 with purity higher than 99 wt% were mixed by ball-milling for 3 h. Compositions of batches were given in table 1. The obtained samples were dissolved in nitric acid and the solution was analyzed for Si, B, Ca, Al, Na, Mo and V by inductively coupled plasma-atomic emission spectroscopy [21,30]. The obtained results showed good agreement between experimental and nominal glass compositions. For each oxide, the relative molar loss during melting was always lower than 3% of the nominal composition values, which indicates that only weak loss of volatile oxides such as B 2 O 3 , Na 2 O and V 2 O 5 occurred during melting. About 50 g of the mixture was heated in alumina crucibles to dissociate the carbonates and borate and then melted isothermally for 3 h at 1300°C to form homogeneous melts, followed by quenching of the melts into water. The quenched samples were crushed and melted again at 1300°C for 2 h to ensure homogeneity, then poured onto a preheated steel plate.

Experimental
X-ray diffraction (XRD) data was collected on an X' Pert PRO diffractometer using Cu-K α radiation (λ=1.54187 Å) [31,32]. For XRD measurement, powdered samples with particle size80 μm were used. Raman spectra were measured using a micro Raman spectrometer. The microstructure and microtopography of the obtained samples were studied on a Zeiss Ultra 55 field emission scanning electron microscope in the backscattered electron mode. Glass transformation temperature of the samples was determined by differential temperature analysis (DTA, SDT Q600, TA Instruments Inc.) with heating at 20°C/min in air. Bulk densities of the obtained samples were determined using Archimedes method.
The chemical durability of the obtained samples was evaluated by PCT method according to ASTM C1285-14 [33]. The samples were crushed into granules, selected by sieving (75-150 μm), cleaned with deionized water and dried. Then 3 g sample was soaked in 80 mL deionized water (pH=7) in Teflon reactors and kept in an oven at 90±1°C. Leachates were taken from the reactors at the end of 1, 3, 7, 14, 28 days respectively. These powdered samples were cleaned with absolute ethanol, dried and then mixed with new deionized water after each removal of leachate. The ion concentrations in leachate were obtained by inductively coupled plasma (ICP) analysis using an iCPA 6500 spectrometer (Thermo Fisher Company, USA). The normalized leaching rate LR i was calculated according to the following equation: where C i is the concentration of the i-th element in the leachate (g l −1 ), f i is the weight fraction of the i-th element in the obtained samples, V is the volume of the solution (L), SA is the powered samples surface area (m 2 ) and Δt is the duration of the experiment days. SA was determined using a surface area analyzer in a BET nitrogen adsorption. The SA and SA/V ratio are about 0.07 m 2 g −1 and 2625 m −1 , respectively.

Results and discussion
The effect of V 2 O 5 content on the crystallization tendency of the glass has been qualitatively estimated on the basis of XRD analysis as shown in figure 1. It can be found that all samples exhibit a broad peak in the 2θ ranges of 15-35°, which is regarded as the characteristic amorphous nature pattern of the samples. For the samples with x=0, the diffraction peaks can be indexed to powellite phase CaMoO 4 . However, the intensity of these diffraction peaks progressively decreases with increasing V 2 O 5 content, and no more CaMoO 4 phase is detected for samples with more than 1.8 mol% V 2 O 5 , which indicates that the incorporation of V 2 O 5 in aluminoborosilicate glass can clearly increase the solubility of MoO 3 . Figure 2 shows BSE images of the as-prepared samples with different content of V 2 O 5 . For the samples with 0 mol % V 2 O 5 , it can be found that numerous Mo-rich white globules are homogenously dispersed throughout the matrix of glass with grain size of about 200 nm in diameter. This should be related to the fact that the crystallization of CaMoO 4 resulted from liquid-liquid phase separation during melt cooling. With increasing V 2 O 5 content, the size of the globules remains conformably but the number of the globules reduces in the samples from x=0 to x=1.2 mol% (as shown in figures 2(a)-(c), respectively). When the V 2 O 5 content reaches up to 1.8 mol%, no more globules are observed, which means that no phase separation and crystallization occurs during melt cooling and the obtained glass is homogeneous. These results are consistent with the obtained XRD data and indicate that V 2 O 5 addition in aluminoborosilicate glass can inhibit the crystallization tendency of molybdate salt phases.   [35].   tetrahedra in borovanadate and borosilicate glasses are partially present as isolated ion species and acts as a network modifier in low contents [29,34]. When V(V)O 4 tetrahedra are present in molybdenum-bearing borosilicate glass, the following priority is found for the alkali ion coordination: AlO 4 >VO 4 MoO 4 2− > SiO 4 (Q 2 or Q 3 ) or BO 4 [35]. V(V)O 4 tetrahedra can strongly modify the distribution of the network modifying cations in glassy network and suppress the preference of the alkali/alkaline-earth cations only around Mo(VI)O 4 tetrahedral. In addition, earlier Raman studies of vanadate-molybdate crystals revealed that (Mo, V)O 4 tetrahedra share oxygen atoms with (Li, Mg)O 6 octahedra and other six-coordinated Li, Mg-environments [37]. Therefore, the Vanadium ions added to the glass may locate in the nearness of the molybdate entities, which could disturb the clusterization tendency of Mo(VI)O 4 units. Consequently, these modifications of the environment around Mo(VI)O 4 tetrahedra in the glass structure would suppress the crystallization tendency of molybdate salt phase and improve MoO 3 solubility in aluminoborosilicate glass. In order to understand more precisely the relationship between molybdenum and vanadium in the glass structure, further structural characterization such as NMR ( 95 Mo, 11 B, 27 Al, 23 Na and 29 Si) will be performed in the following work.
To determine the MoO 3 solubility limit within the studied aluminoborosilicate glass, samples were prepared with incremental additions of MoO 3 up to 3 mol%. Figure 4 shows XRD and Raman scattering spectra of My-V3 (y: mol% MoO 3 ) series samples. It can be found that no crystalline phase is formed other than glass phase even MoO 3 addition up to 2.8 mol% in V 2 O 5 -containing aluminoborosilicate glass. When the content of MoO 3 is close to 3 mol%, the powellite phase CaMoO 4 is detected. These results indicate that V 2 O 5 addition can obviously enhance the solubility of MoO 3 in aluminoborosilicate glass and V-containing borosilicate glass samples can accommodate up to 2.8 mol% of MoO 3 .
The density of the Vx-M2 series samples was measured at room temperature by the Archimedes method. The molar volume (V M ) of each sample was evaluated using the following formula: Where x i is the molar fraction and M i is the molecular weight of the ith component, and ρ is the density. Figure5  (a) shows the density and molar volume of the Vx-M2 series samples as a function of V 2 O 5 content. It can be observed that density slightly decreases with increasing V 2 O 5 content, where as molar volume increases. Similar results has been observed in V 2 O 5 loaded soda-lime silicate glasses [38,39], which indicates that the network structure of the glass is more open and the structure becomes loosely packed. Previous reports revealed that incorporation of V 2 O 5 into silicate glasses results in the reconversion of BO 4 tetrahedra to BO 3 triangles by the breaking of B-O-B linkages and the formation of nonbridging oxygens (NBOs) [34,35]. The increase in molar volume may be regarded as an outcome of the regular evolution of more NBOs, which is characteristic of alteration of the volume concentration. To get better insight, by virtue of the approximated values of the density and molar volume, the molar volume of oxygen (V O ) (volume of glass in which 1 mol of oxygen is confined) and the oxygen packing density (OPD) were calculated using the following formula [40,41]:   where M is the molecular mass of the glass sample and n is the number of oxygen atoms per formula units. Figure 5(b) shows the molar volume of oxygen and OPD of the Vx-M2 series samples as a function of V 2 O 5 content. It can be observed that the molar volume of oxygen increases and the OPD decreases with a rise in the concentration of V 2 O 5 . It is generally believed that OPD is presentation of the compactness of glass structure and is dependent on the number of bridging and non-bridging oxygen atoms. The behavior of the molar volume of oxygen and OPD with increasing V 2 O 5 content indicates that the glass network is more open and the structure turns out to be less tightly packed because of the re-arrangement of NBOs [42]. The glass transition temperature (T g ) values of these samples determined by differential temperature analysis are 582.42°C, 577.94°C, 564.36°C, 561.52°C, and 560.26°C respectively. It can be found that T g of these samples decreases with increasing V 2 O 5 content, which is also attributed to the influence of V 2 O 5 addition on the glass network structure.
The leaching properties of the as-prepared samples were examined by PCT tests. The calculated normalized leaching rates of Si (LR Si ), Ca (LR Ca ), Mo (LR Mo ) and V (LR V ) for the samples are shown figure 6. It can be observed that the LR Si , LR Ca , LR Mo and LR V of all glass present a gradual downward trend with increasing leaching time in the first 14 days and remain basically unchanged after 14 days. This should be due to the passivation layer formed on the sample surface. In addition, the normalized leaching rates of V 2 O 5 -containing samples are slightly lower than that of V 2 O 5 freeing samples in the first 14 days. While, the normalized leaching rates at 28 days of all samples are almost equal. This indicates that V 2 O 5 addition has no obvious effect on the leaching properties of aluminoborosilicate glass. After 28 days, the LR Si , LR Ca , LR Mo and LR V of the sample with 3 mol% V 2 O 5 are about 1.21×10 −3 g·m −2 ·d −1 , 6.24×10 −4 g·m −2 ·d −1 , 1.35×10 −3 g·m −2 ·d −1 , and 9.54×10 −4 g·m −2 ·d −1 respectively, which are lower than those of the standard borosilicate glassy waste form [43].  aluminoborosilicate can accommodate 2.8 mol% of MoO 3 . The probable explanation for the enhancement of MoO 3 solubility in aluminoborosilicate glass by V 2 O 5 addition could be (1) the depletion of alkali/alkaline-earth ions in Mo(VI)O 4 surrounding (2) the increase of the dispersion of Mo(IV)O 4 units in the glass. The molar volume and glass transition temperature of samples are found to depend on V 2 O 5 content. After 28 days, the normalized leaching rates of all modified aluminoborosilicate glass maintain at a fairly low level compared with standard borosilicate glassy waste form. Results of this initial investigation indicate that V 2 O 5 -containing aluminoborosilicate glass is potential hosts for the immobilization of Mo-bearing high-level nuclear waste. This work will continue to be investigated by focusing on understanding more precisely the relationship between molybdenum and vanadium in the glass structure.