Synthesis and characterization of phosphate-based glass-ceramics for nuclear waste immobilization: Structure, thermal behavior, and chemical stability

Accepted Manuscript Synthesis and characterization of phosphate-based glass-ceramics for nuclear waste immobilization: Structure, thermal behavior, and chemical stability Jinfeng Liu, Fu Wang, Qilong Liao, Hanzhen Zhu, Dongsheng Liu, Yongchang Zhu PII:

S0022-3115(18)31064-X

DOI:

https://doi.org/10.1016/j.jnucmat.2018.11.002

Reference:

NUMA 51292

To appear in:

Journal of Nuclear Materials

Received Date: 6 August 2018 Revised Date:

31 October 2018

Accepted Date: 1 November 2018

Please cite this article as: J. Liu, F. Wang, Q. Liao, H. Zhu, D. Liu, Y. Zhu, Synthesis and characterization of phosphate-based glass-ceramics for nuclear waste immobilization: Structure, thermal behavior, and chemical stability, Journal of Nuclear Materials (2018), doi: https://doi.org/10.1016/ j.jnucmat.2018.11.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Synthesis and characterization of phosphate-based glass-ceramics for nuclear waste immobilization: Structure, thermal behavior, and

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chemical stability

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Jinfeng Liu1, Fu Wang1, Qilong Liao*,1, Hanzhen Zhu1, Dongsheng Liu1, Yongchang Zhu2

State Key Laboratory of Environment-friendly Energy Materials, School of Materials Science and

China Building Materials Academy, Beijing 100024, PR China

Corresponding author.

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∗

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Engineering, Southwest University of Science and Technology, Mianyang 621010, PR China

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ACCEPTED MANUSCRIPT Abstract: Phosphate-based glass-ceramics were prepared by a traditional glass melt-quenching method in a high-temperature furnace. The effects of the ZrO2 substituted for Na2O on the crystal phase, thermal behavior, structure, and chemical

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stability of the prepared glass-ceramics were investigated in detail. X-ray diffraction (XRD) analysis showed that NaZr2(PO4)3 is the main crystalline phase of all the studied

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samples. ZrP2O7 and FePO4 respectively appeared when the ZrO2 substituted for Na2O reached or exceeded 8 mol% and 12 mol%. FTIR and Raman spectra showed that the

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network structure of the studied glass-ceramics consisted mainly of orthophosphate units, pyrophosphate units, and a small amount of metaphosphate units and [BO4] units. For the samples containing 10 mol% ZrO2 or less, the normalized leaching rates of Zr (LRZr), Fe (LRFe), Na (LRNa), and P (LRP) remained low (2.5 × 10−6 g·m−2·d−1, 3.3 × 10−6

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g·m−2·d−1, 2.5 × 10−3 g·m−2·d−1, and 6.2 × 10−4 g·m−2·d−1, respectively) after immersion in deionized water at 90 °C for 28 days. The obtained results suggest that the traditional

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glass making process may potentially be applicable to the synthesis of some phosphate-based glass-ceramics for immobilizing nuclear waste.

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Keywords: Glass-ceramics; Synthesis; Structural features; Chemical durability

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ACCEPTED MANUSCRIPT 1. Introduction Vitrification is one of the most important methods of dealing with high-level radioactive waste (HLW) generated from the reprocessing of spent fuel [1-4].

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Borosilicate glass is the first-generation HLW host of choice; however, the low solubility of certain elements such as actinides in the glass matrix and the formation of

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cracks due to decay heat coming from the radioactive elements are major limitations [5]. Therefore, it is necessary to seek alternate matrices for the disposal of nuclear waste

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containing these elements. Matrices, including glasses, ceramics, and glass-ceramics, have been studied as potential host materials for these nuclear wastes [6-8]. Specifically, the thermal stability, structure, irradiation stability, and polaronic mobility of Fe2O3-B2O3-P2O5 glasses have been extensively studied with the aim of loading HLW

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with a high phosphate, sulfate, and chloride content [9-11]. B2O3 gave the most acceptable performance compared with Al2O3 and SiO2, considering the improvement

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of glass forming ability and thermal stability [9]. Previous reports [12, 13] proved that B2O3 is useful in the immobilization of radioactive waste and could improve the thermal

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stability and chemical durability when incorporated into some phosphate glasses. Further, the mass absorption coefficients and thermal neutron absorption of B2O3 are two orders of magnitude higher than those of P2O5 [10]. Karabulut [14] demonstrated that 36Fe2O3-10B2O3-54P2O5 glass had superior irradiation stability, thermal stability, and similar chemical durability compared with Fe2O3-P2O5 glasses, where the latter have been proven to be a suitable matrix for the immobilization of certain nuclear

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ACCEPTED MANUSCRIPT wastes. Moreover, Fe2O3-B2O3-P2O5 glasses containing certain amounts of Na2O/K2O, CeO2, ZrO2, La2O3, Cr2O3, Gd2O3, or simulated nuclear wastes exhibit good properties and structures [11, 15-20]. The O/P ratio of glass composition effects the glass structure.

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The ultraphosphate glass with an O/P ratio of 2.5 consists of only Q3 units, which is hygroscopic and volatile. With increasing O/P ratio, Q3 units gradually convert to Q2

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units (O/P = 3.0) to Q1 units (O/P = 3.5), and to Q0 units (O/P = 4.0). These structural units correspond to metaphosphate glass, pyrophosphate glass, and orthophosphate glass

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respectively [21-23]. The introduced metal cations usually break the P-O-P bond in phosphate glasses and form M-O-P (M: metal) bond which is more hydro-resistant than P-O-P bond [24].

It has been proven that ceramic phases are formed during the traditional glass

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melt-quenching process when some elements exceed the solubility limit of the Fe2O3-B2O3-P2O5 glass matrix, where precipitation of the durable ceramic phase from

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the base glasses insignificantly affected the chemical durability of the base glasses [18, 19]. The formed glass-ceramic matrices, which contain both stable crystalline phases

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and the glass phase, still exhibited higher chemical durability, thermal stability, and chemical durability than borosilicate or phosphate glass [25-29]. For instance, glass-ceramics containing monazite crystals and a small amount of the ZrP2O7 crystalline phase exhibited the requisite performance [30, 31], and the formation of both monazite and FePO4 crystalline phases in the structure of cerium phosphate glass-ceramics did not affect the stability of these ceramics [32].

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ACCEPTED MANUSCRIPT Zirconium is one of the high-concentration elements in HLW originating from the zircaloy cladding of used nuclear fuel rods [33]. The presence of ZrO2 raises an interesting question due to its relatively low solubility in phosphate glass and its strong

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ability to induce the crystallization of phosphate glasses [34]. However, Zr is an effective nucleation agent to the formation of glass-ceramic material, and the

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replacement of Na+ (0.961) by high cation field strength of Zr4+ (7.716) improves the structure and compactability of the IBP glass [11]. Inspired by this and the summarization,

in

this

study,

iron

boron

phosphate-based

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above-mentioned

glass-ceramics are prepared by a traditional glass melt-quenching method using a high-temperature furnace. The effects of substitution of ZrO2 for Na2O on the crystal phase, structure, thermal properties, and chemical stability of the prepared

2. Experimental

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glass-ceramics were investigated.

2.1 Preparation of glass-ceramics

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Glass-ceramics were prepared by heating the raw materials in a high-temperature

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furnace. The raw materials are comprised of analytical grade ZrO2 (99.0%), NaH2PO4·2H2O (99.0%), Fe2O3 (99.0%), (NH4)H2PO4 (99.0%), and H3BO3 (99.5%) in the ratios of the desired composition. The composition of the glass-ceramics is listed in Table 1. The actual chemical compositions were measured by ICP-OES. An extra 5% of boric acid and ammonium dihydrogen phosphate was added to the raw materials to compensate for the volatility, respectively. The blending raw materials were placed into a corundum crucible and heated to 450 °C for 2 h to remove NH3 and H2O; the 5

ACCEPTED MANUSCRIPT temperature was then raised to 1200 °C at a heating rate of 5 °C·min−1 and held at 1200 °C for about 3 h in air atmosphere. Subsequently, the high-temperature liquids were poured onto a preheated graphite plate, then annealed at 450 °C for 1 h to

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eliminate the thermal stress of the glass phase, followed by naturally cooling to room temperature. The sample notation is based on the amount of ZrO2 substituted for Na2O;

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for instance, Zr4 represents the 4ZrO2-16Na2O-80(36Fe2O3-10B2O3-54P2O5) sample. 2.2 Characterization

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The crystal phases were confirmed by using an X-ray diffractometer (DMAX1400, Rigaku Company, Japan), using Cu-Kα radiation, operating at 40 kV and 70 mA in the 2θ range of 3−80°, with a scanning speed of 8°⋅min-1 and an acquisition step of 0.02°. Comprehensive precision of standard SI powder is less than 0.02°. The microstructure

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and the chemical composition of the glass-ceramics were observed by scanning electron microscopy (SEM) with an energy dispersive X-ray spectroscopy (EDXS) attachment

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(Ultra 55 FESEM, Carl Zeiss Instrument Co., Germany). The error is about 5 at. %. All samples were corroded with 2 mol% hydrofluoric acid for 60 s, rinsed, and dried, then

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sprayed with gold before the SEM analysis. Differential scanning calorimetry (DSC) curves were obtained by using a synchronous thermal analyzer (STA449F5, NETZSCH). The error in the temperature measurements is ± 2 °C, and powdered samples (~8 mg; particle size: <45 µm) were placed into the Al2O3 sample cups for the analysis. For the thermal analysis, the temperature was ramped from room temperature to 1000 °C at a heating rate of 20 °C·min−1 under a stream of N2 gas (20 mL·min-1). An automatic

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ACCEPTED MANUSCRIPT electronic density meter (MH-300A, Xiamen, China), the estimated error is ± 0.001 g·cm−3, was used to measure the density of samples. The principle of instrument is Archimedes method and distilled water as the immersion liquid. Raman spectra were

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acquired with powder samples in the range of 200−2000 cm−1 using a Renishaw InVia Raman spectrophotometer with an argon ion laser (k = 514.5 nm) as the excitation

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source; the resolution ratio of the Raman is 1~2 cm−1 and light power is 1.7 mW. FTIR spectra were collected on a ThermoNicolet Smart-380 Fourier transform infrared

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spectrometer in the range of 400−2000 cm−1 using KBr pellets. The pellets were prepared by grinding and pressing a mixture containing about 1−2 mg powder samples and 100−200 mg spectroscopic grade dry KBr powder. The accuracy of FTIR is estimated to be ± 4 cm−1.

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2.3 Static leaching experiments

The Product Consistency Test (PCT) is a standard for evaluating chemical

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durability of glasses and glass-ceramics that mix nuclear waste and hazardous waste [35]. Typically, the studied glass-ceramic powder (100−200 mesh) was washed with

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acetone, dried, and weighed (± 0.0001 g). A 1.5 g sample of the powder and 15 ml deionized water (pH = 7) were put into a polytetrafluoroethylene vessel (100 mL) and the vessel was sealed. The vessel was then placed in a constant temperature device at 90 ± 1.0 °C. After 1, 3, 7, 14, 28, and 53 days, the leaching solution was removed and centrifuged at a speed of 2000 rpm in a centrifuge. Then 15 ml fresh deionized water was added to the polytetrafluoroethylene reactors. The concentrations of P, Fe, and Zr in

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ACCEPTED MANUSCRIPT the solution were analyzed by using inductively coupled plasma optical emission spectroscopy (ICP-OES, iCAP6500) and the concentration of Na was analyzed by atomic absorption spectrometry (AAS, AA700). The relative standard deviation of the

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ICP-OES and the AAS is less than 1% and 0.2%, separately. The normalized leaching rate LRi (g·m−2·d−1) of element i was calculated using the formula [36]:

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LRi = Ci ⋅ V /( f i ⋅ S ⋅ ∆t ) , where Ci is the concentration of element i (g/L), V is the volume of the leaching solution (L), fi is the mass fraction of the element in the samples,

value of S/V is 2000 m−1. 3. Results and discussion

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S is the surface area of the samples (m2), and ∆t is the experimental period (days). The

3.1 Analysis of crystalline phase and microstructure

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Figure 1 shows the XRD pattern of the as-prepared glass-ceramics. The XRD data show that the main crystalline phase of the Zr4 and Zr6 samples was NaZr2(PO4)3

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(JCPDS-PDF No. 33-1312). Moreover, a small amount of crystalline ZrP2O7 (JCPDS-PDF No. 71-2286) was detected. The Zr8 and Zr10 samples mainly contained

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NaZr2(PO4)3 and ZrP2O7 as the crystalline phases. When the content of Zr exceeded 10 mol%, both NaZr2(PO4)3 and ZrP2O7 were the main crystalline phases, and crystalline iron phosphate FePO4 (JCPDS-PDF No. 29-0715) became detectable. Trace amounts of crystalline Fe7(PO4)6 (JCPDS-PDF No. 49-1088) were also detected in the Zr14 and Zr16 samples. Figure 1 also shows that the intensity of the peaks corresponding to NaZr2(PO4)3 increased with an increase in the ZrO2 content from 4 mol% to 10 mol%.

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ACCEPTED MANUSCRIPT The formation of NaZr2(PO4)3 was curtailed when the substitution of ZrO2 for Na2O exceeded 12 mol%, which is attributed to the significant decrease in the available sodium in the system. Conversely, the amount of ZrP2O7 formed increased with

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increasing ZrO2 content.

Figure 2(a)−(g) present the SEM images of the glass-ceramics. Rectangular

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crystals (in the N-site) were observed for all the samples. The EDXS (Figure 2(h)) data show that the rectangular crystals contained Zr, P, O, and Na, and the molar ratio of

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Na/Zr/P/O was about 1:2:3:12. Taken together with the XRD data, the rectangular crystals were determined to be NaZr2(PO4)3. For the Zr10, Zr12, Zr14, and Zr16 glass-ceramics, some spheroidal crystals were observed (in the Z-site existed). The EDXS data (Figure 2(i)) show that the constituent elements of the spheroidal crystals

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were Zr, P, and O, and the molar ratio of Zr/P/O was about 1:2:7, which confirms that the crystalline phase corresponds to ZrP2O7. Many strip crystals were observed in the

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Zr14 and Zr16 samples. Figure 2(j) the shows EDXS profiles of the strip crystals (F-site). The data indicate that the strip crystalline phase is comprised of Fe, P, and O in

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a molar ratio of 62.83:17.09: 0.08 (about 1:1:4), which indicates that the strip crystals were FePO4. As shown in Figure 2, crystalline phases were apparent for all samples, but the fracture surface was still extremely compact in the samples with ZrO2 contents of 10 mol% or less. The compactability of the fracture surface became less (i.e., the surface became loose) when the ZrO2 content increased to 12 mol%. The fracture surfaces of the samples also showed some cracks, and the number of cracks increased with a further

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ACCEPTED MANUSCRIPT increase in the ZrO2 content, as shown in Figure 2(f) and (g), because excessive microcrystal increased high-temperature viscosity. 3.2 Density and DSC analysis

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The density of the glass-ceramics, as shown in Table 2, was related to the compactability and the crystalline phases. It increased initially on account of the higher atomic weight of Zr element than Na and the formation of crystalline phase. The

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tendency began to reduce when ZrO2 content further exceeded 12 mol% because the

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cracks in the SEM images. Figure 3 shows the DSC curves of the studied glass-ceramics, recorded from room temperature to 1000 °C. The profiles were characterized by the glass transition temperature (Tg), crystallization onset temperature (Tr), and the onset temperature (TL) of the exothermic effect connected with the first appearance of the

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liquid phase. The Tg was determined by the intersection that the extension line of baseline before conversion and the tangent line at the feature point. The obtained Tg, Tr,

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TL, (Tr − Tg) and (TL − Tr) values are listed in Table 2. From the Zr4 to Zr8 samples, the Tg shifted to lower values with increasing NaZr2(PO4)3 content. In general, the

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formation of Zr-O-P bonds decreases the barrier height between two adjacent areas in glass transition region [37]. The glass transition activation energies decreased and glass transition temperature increased. However, the NaZr2(PO4)3 and ZrP2O7 crystallite increased the barrier height [38]. Accordingly, the glass transition activation energies increased and the glass transition temperature decreased. The Tg values of the Zr8, Zr10, and Zr12 samples fluctuated within a narrow range, indicating no significant effect of ZrO2 substitution on the Tg value of these samples. The endothermic peaks in the DSC 10

ACCEPTED MANUSCRIPT curves disappeared when the substitution of ZrO2 exceeded 12 mol% because only a small amount of glass was present. The Zr-O-P bond is stronger than P-O-P bond, so Zr-O-P bond enhances phosphate chains and cohesion of glass structure. Although the

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addition of ZrO2 depolymerized the glass, the Zr-O-P bond contributed to the increase in Tr from Zr4 to Zr6 sample [19]. When the substitution exceeded 6 mol%, Tr decreased

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because the impact of crystalline phase was more significant than the formation of Zr-O-P bonds. This can be explained that large amounts of the preexisting NaZr2(PO4)3

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and ZrP2O7 crystallite provided the surfaces and interfaces, simultaneously, the structure of crystallite was similar to the short-range ordered structure of the glass [38]. No exothermic peaks were discernible in the DSC curves of the Zr14 and Zr16 samples as crystal formation almost depleted the residual glass. According to Hruby’s method [39],

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the forming tendency of glass is proportional to the thermal stability and the Kgl is a numerical measure for the glass-forming tendency, K gl = (Tr − Tg ) (TL − Tr ) . The Zr4

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and Zr8 sample was 0.56, indicating that the two samples are easily prepared and have good thermal stability. Zr6 sample had the Kgl value of about 0.77 and so the best

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thermal stability. For Zr10 sample, the Kgl value significantly decreased, suggesting that addition of excess Zr element significantly decreased the thermal stability of residual glass in the sample. 3.3 FTIR analysis

The FTIR spectra in Figure 4 provides some information about the structural variation with substitution of ZrO2 for Na2O. The features at 549 cm−1 and 1113 cm−1 are attributed to the bending modes of O-P-O in (P2O7)4− (Q1 groups) and the 11

ACCEPTED MANUSCRIPT (PO3)2−asym stretch in the Q1 structure [21], respectively. The shoulder at 643 cm−1 is attributed to the stretching vibrations of the Fe-O-P bonds [40]. The band at 748 cm−1 is assigned to the bending vibrations of the B-O-B bonds in the [BO4] groups [41]. The

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absorption bands of (PO4)3−asym appears 1039 cm−1 which corresponds to Q0 group [21]. The bands at 1384 cm−1 and 1465 cm−1 are characteristic of [BO3] and BO2O− [42]. The

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more intense band at 1626 cm−1 represents the bending vibrations of P-OH and B-OH vibrations, or those of atmospheric water that may have been absorbed by the

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hygroscopic KBr pellets [42]. For the O/P ratio molar of ≥4.0 (Table 1) and the formed various crystalline phases, Q0 and Q1 units would appear in the glass-ceramics. However, a small fraction of Q2 units existed in the phosphate network and the possible reason was the existence of disproportionation reaction: 2Q1 ↔ Q2 + Q0 [22]. The

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absorption band assignments for studied samples are listed in Table 3. It can be concluded that the Q0 units, Q1 units, and a small number of Q2 units and [BO4] units

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made up the network structure of the samples. Figure 4 shows that the intensity of some of the absorption peaks changed,

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although there were no significant differences in the peak position. The intensity of the bands at 549 cm−1 and 1113 cm−1 have been ascribed to the Q1 units decreased initially, then increased with increasing ZrO2 content. The intensity of the 1039 cm−1 band, which is ascribed to the Q0 units, followed the opposite trend compared with that of the Q1 units. As O/P increases, the glass phase could be depolymerized by ZrO2 based on the modifier function and the Q0 units increased. But the numerous crystals strongly

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ACCEPTED MANUSCRIPT absorbed free oxygen, which leads to a decrease in the O/P molar ratio in the glass phase and the Q0 units decreased. Related research [11] showed that doping ZrO2 into the ferroboron phosphate glass before crystallization, free oxygen introduced by ZrO2,

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which lead to [BO3] group transforming to [BO4] group. However, the appearance of crystallization caused that [BO4] group begin to change into [BO3] group. In this study,

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the change of [BO4] units and [BO3] units were not obvious due to the two effects cancelled out.

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3.4 Raman spectra

Figure 5(a) shows the Raman spectra of the glass-ceramics in the range of 200−1400 cm−1; the Raman data were used to investigate the effects of ZrO2 substitution. The shape of the absorption peaks in the Raman spectra differed notably with variation

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of the composition. With the increase in the substitution of ZrO2, the shape of the peaks became sharper due to serious crystallization of the glass. Figure 5(b) shows the

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deconvoluted Raman spectrum of the Zr4 sample. The band assignments of the Zr4 sample are characteristic of the Q0 structure of orthophosphate, with the most prominent

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band at 1027 cm−1 [43]. The shoulder at 1048 cm−1 corresponds to the symmetric stretching mode of non-bridging (PO3)−1, Q1 [43, 44]. Moreover, the low-intensity band at 622 cm−1 corresponds to bridging oxygen atoms (P-O-P)sym in the Q2 phosphate tetrahedra [43, 44]. The low frequency bands <600 cm−1 are assigned to the bending mode of the phosphate polyhedra with iron as a modifier ion [43]. The Zr6 and Zr4 samples had similar spectral features. For the Zr8 sample, sharp Raman bands were

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ACCEPTED MANUSCRIPT observed at 1022 and 1057 cm−1, corresponding to the NaZr2(PO4)3 and ZrP2O7 crystalline phases, respectively [45], as previously confirmed by XRD measurement. Zr12, Zr10, and Zr8 gave rise to similar spectral features; however, with increasing

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substitution of ZrO2, the bands became sharper and more intense. Zr14 and Zr16 also had similar spectral profiles. Sharp bands were observed at 250, 276, 390, 413, and 432

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cm−1 for the Zr14 and Zr16 samples. These peaks correspond to the FePO4 crystalline phase [43]. Simultaneously, a sharp peak appeared at 1169 cm−1. Previous studies of

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borophosphate glasses [19, 46] indicated that the Raman scattering efficiency of borate groups is much lower than that of phosphate groups. Therefore, there was no obvious borate vibration in the obtained Raman spectra of the studied glass-ceramics. The absorption band assignments for the studied samples are listed in Table 4.

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Notably, the most prominent band of the studied samples (from Zr4 to Zr10) tended to shift towards low wavenumber. Although the Raman frequencies of glasses

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and their corresponding crystalline compounds fall into similar ranges, the position of the peak of the corresponding structural unit of the crystalline compound is slightly

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lower than that of the glass [43]. With increasing substitution of ZrO2, the intensity of the peaks in the range of 1007−1022 cm−1, which are assigned to the PO4 symmetric stretch (Q0), increased because of the presence of the NaZr2(PO4)3, FePO4, and Fe7(PO4)6 crystalline phases. Simultaneously, the bands at 1057−1095 cm−1 related to the PO3-symmetric stretch (Q1) became sharper with increasing ZrO2 substitution (from Zr4 to Zr16) due to growth of the ZrP2O7 crystals. The peak at 622−655 cm−1

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ACCEPTED MANUSCRIPT corresponding to Q2 decreased in intensity with increasing substitution of ZrO2 in the compositions of the studied glasses. The assignment of the band at about 1169−1189 cm−1 is ambiguous [43]. The peaks at 1169−1189 cm−1 for the samples with low ZrO2

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substitution may have overlapping contributions from symmetric PO2-stretching modes and asymmetric PO3-stretching modes [43]. However, it is established that the Raman

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peaks of crystalline Fe4(P2O7)3 and Fe3(P2O7)2 associated with the P2O74− anions are located at 1150−1200 cm−1 [43]. It can be inferred that the peak at 1169 cm−1 in the

crystals in this study. 3.5 Chemical durability

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profiles of the Zr14 and Zr16 samples is assigned to Q1 units and corresponds to ZrP2O7

Figure 6 shows the normalized rates of leaching of Zr (LRZr), Fe (LRFe), Na (LRNa)

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and P (LRP) from the samples immersed in 90 °C deionized water for 1, 3, 7, 14, 28, and 53 days. In general, the LR of all samples decreased with the immersion time, which may be caused by a protective amorphous gel layer formed at the reaction interface [47,

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48]. Figure 6(a) shows that the LRZr of all the samples decreased quickly in the initial 7

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days, then decreased slowly from 7 days to 28 days. The LRZr remained constant at about 2.5 × 10−7 to 2.5 × 10−6 g·m−2·d−1 after 28 days. The LRZr varied little with the immersion time and with ZrO2 substitution for the samples placed in the leaching liquids, and the amount of the elements leached was found to be near the detection limit of ICP-OES (0.001 mg·L−1) after 28 days. Moreover, the LRZr decreased slightly with an increase in the zirconium-containing crystal phases because zirconium has low glass-forming ability and zirconium mainly forms ceramics, which indicates that 15

ACCEPTED MANUSCRIPT NaZr2(PO4)3 and ZrP2O7 have high chemical durability. As shown in Figure 6(b), the LRFe of the Zr4, Zr6, Zr8, and Zr10 samples decreased initially with time and did not

vary much thereafter, remaining at a low value of about 3.3 × 10−6 g·m−2·d−1 after 28

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days. Figure 6(d) shows that the LRNa of the Zr4, Zr6, Zr8, and Zr10 samples was about 2.5 × 10−3 g·m−2·d−1 after 28 days. The LRP of the Zr4 and Zr6 samples changed slightly

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from 28 to 53 days (Figure 6(f)), with a value of about 6.8 × 10−4 g·m−2·d−1 at 53 days. The LRP of the Zr8 and Zr10 samples varied insignificantly and remained at a low value

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of about 6.2 × 10−4 g·m−2·d−1 after 28 days.

It can be concluded that the LRFe, LRNa, and LRP decreased slightly with increasing substitution of ZrO2 for Na2O (the substitution of ZrO2 for Na2O was 4−10 mol%). Thus, in this range, the presence of the NaZr2(PO4)3 and ZrP2O7 crystalline phases does not

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degrade the chemical durability in aqueous medium. The excellent chemical stability is attributed to the relatively stable zirconium-containing crystal phases and glass phase.

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Moreover, the LRFe, LRNa, and LRP values of the Zr12 sample, which contains the FePO4 crystalline phase, were an order of magnitude higher than those of the Zr4, Zr6, Zr8,

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and Zr10 samples. This is because FePO4 is also a chemically stable crystalline phase and it does not reduce the chemical stability of glass-ceramics [32]. The raw materials formed a homogeneous mixture of microcrystal and glass melt at 1200 °C. Excessive microcrystal would consume large amounts of oxygen, leading to the decrease of O/P and the increase of high-temperature viscosity, which weakened forming ability of glass [2]. Therefore, the excess NaZr2(PO4)3, FePO4, and ZrP2O7 crystalline phases of Zr12,

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ACCEPTED MANUSCRIPT Zr14 and Zr16 samples weaken the forming ability of glass, as described in the thermal analysis. A weakened structure of the glass phase in the samples seriously affected the chemical stability of glass-ceramics. Therefore, Figure 6(c), (e), and (g) show that the

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LRFe, LRNa, and LRP were significantly higher when the substitution of ZrO2 for Na2O

was more than 12 mol%.

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4. Conclusions

Phosphate-based glass-ceramics were synthesized by the simple traditional glass

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melt-quenching method. The effects of substitution of ZrO2 for Na2O on the crystal phase, thermal behavior, structure, and chemical stability were investigated in detail. The results show that NaZr2(PO4)3 is the main crystalline phase for all the studied samples. ZrP2O7 and FePO4 crystalline phases were detected in the studied glasses

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containing 8 mol% ZrO2 or more and in those with 12 mol% ZrO2 or more, respectively. The substitution of ZrO2 promotes the crystallization of NaZr2(PO4)3, ZrP2O7, and

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FePO4. The network structure of the studied glass-ceramics mainly consists of Q0 units, Q1 units, [BO4] units, and a small amount of Q2 units. Under the combined action of O/P

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and crystalline phases, the changes were caused by ZrO2 substituted for Na2O, Q0 units increased initially, then decreased with increasing ZrO2 content. Formation of the glass phase decreased dramatically when the substitution of ZrO2 for Na2O exceeded 12 mol%, and the addition of excess Zr decreased the thermal stability of the glass and chemical durability of glass-ceramics. The normalized leaching rates (LRZr, LRFe, LRNa, and LRP) of the samples containing 10 mol% ZrO2 or less were almost the same and

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ACCEPTED MANUSCRIPT remained at low values of about 2.5 × 10−6 g·m−2·d−1, 3.3 × 10−6 g·m−2·d−1, 2.5 × 10−3 g·m−2·d−1, and 6.2 × 10−4 g·m−2·d−1, respectively, after immersing the samples in deionized water at 90 °C for 28 days. The obtained results suggest that the traditional

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glass making process may potentially be applicable to the synthesis of some

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phosphate-based glass-ceramics for immobilizing nuclear waste.

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Acknowledgments

This work was supported by the National Natural Science Foundation of China (51702268), by the China Industrial Technology Development Program (2017−1407),

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18LZXT01).

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and by the Southwest University of Science and Technology (18LZX302 and

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ACCEPTED MANUSCRIPT Data availability The raw data required to reproduce these findings are available for download from [https://data.mendeley.com/datasets/dr6ytpbvgk/draft?a=aec978e2-9284-4406-82f9-71c

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2a355ec31].

The processed data required to reproduce these findings are available for download

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from

[https://data.mendeley.com/datasets/dr6ytpbvgk/draft?a=aec978e2-9284-4406-82f9-71c

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2a355ec31].

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ACCEPTED MANUSCRIPT References [1] M.G. Mesko, D.E. Day, Immobilization of spent nuclear fuel in iron phosphate glass, J. Nucl. Mater. 273 (1999) 27−36.

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Composition-Structure Relationships in Simplified Nuclear Waste Glasses: 1. Mixed Alkali Borosilicate Glasses, J. Am. Ceram. Soc. 94 (2011) 92−100.

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ACCEPTED MANUSCRIPT Figure captions Figure 1. XRD pattern of the studied glass-ceramic samples. Figure 2. SEM images of (a) Zr4, (b) Zr6, (c) Zr8, (d) Zr10, (e) Zr12, (f) Zr14, and (g)

Figure 3. DSC curves of the prepared glass-ceramics. Figure 4. FTIR spectra of the studied glass-ceramics.

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Figure 5. Raman spectra of the studied samples.

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F-site: FePO4 crystal.

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Zr16 samples and EDXS data for (h) N-site: NaZr2(PO4)3, (i) Z-site: ZrP2O7, (j)

Figure 6. Normalized leaching rates of (a) Zr, (c) Fe, (e) Na, and (g) P for all studied glass-ceramics and (b) Fe, (d) Na, and (f) P for Zr4, Zr6, Zr8, Zr10, Zr12 samples during PCT test at 90 °C. A magnified view of the curves for longer times is also

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shown in the figures.

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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ACCEPTED MANUSCRIPT Table 1 Chemical compositions, molar ratio of O/P and density of the as-prepared glass-ceramics. Samples Batch composition (mol%)

Measured composition (mol%)

Molar ratio

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ZrO2 Na2O Fe2O3 B2O3 P2O5 ZrO2 Na2O Fe2O3 B2O3 P2O5 Al2O3 of O/P 4

16

28.8

8

43.2

4.1

14.0

29.4

7.8

44.3

0.3

4.05

Zr6

6

14

28.8

8

43.2

6.0

13.2

28.7

8.1

43.4

0.3

4.07

Zr8

8

12

28.8

8

43.2

7.6

11.7

27.9

8.2

44.1

0.3

4.10

Zr10

10

10

28.8

8

43.2

9.9

9.5

28.6

8.0

43.5

0.2

4.12

Zr12

12

8

28.8

8

43.2

12.4

7.5

27.3

7.9

44.0

0.6

4.14

Zr14

14

6

28.8

8

43.2

13.6

5.5

28.1

7.6

44.4

0.4

4.17

Zr16

16

4

28.8

8

43.2

14.9

4.7

28.9

7.5

43.6

0.1

4.19

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Zr4

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ACCEPTED MANUSCRIPT Table 2 Density and thermal parameters Tg, Tr, TL, (Tr − Tg), (TL − Tr) and Kgl of the glasses waste forms containing different amounts of ZrO2.

Zr4

Zr6

Z8

Zr10

Zr12

Tg ± 2/°C

509

502

500

505

504

−

−

Tr ± 2/°C

615

628

613

587

−

−

−

TL ± 2/°C

803

790

813

824

827

829

841

Tr−Tg ± 2/°C

106

126

113

82

−

−

−

TL−Tr ± 2/°C

188

162

Kgl

0.56

0.77

ρ± 0.001/g⋅cm−3

3.105

3.128

3.076

3.061

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0.56

0.34

3.139

3.147

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Zr14

Zr16

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Samples

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ACCEPTED MANUSCRIPT Table 3 Assignments of FTIR spectra for the studied samples. Band assignments

Ref.

549

bending modes of O-P-O in (P2O7)4− (Q1 groups)

[21]

643

Fe-O-P bonds

748

bending vibrations of B-O-B bonds in [BO4] groups

[41]

975

Symmetric stretching mode of Q0 type of linkage

[21]

1039

(PO4)3−asym stretch in Q0 group

[21]

1113

(PO3) −asym stretch in Q1 group

[21]

1198

(PO2) −asym stretch in Q2 structure

[21]

1465

the modes of boron–oxygen triangular units ([BO3] and [42]

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bending vibrations of P-OH and B-OH vibrations

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Wavenumber (cm-1)

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[42]

ACCEPTED MANUSCRIPT Table 4 Assignments of Raman spectra for the studied samples. Band assignments

Ref.

< 600

bending mode of the phosphate polyhedra

[43]

622

bridging oxygen atoms (P-O-P)sym in Q2 phosphate [43, 44] tetrahedra

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Wavenumber (cm-1)

1007−1022

PO4 symmetric stretch (Q0)

1048−1095

PO3-symmetric stretch (Q1)

1169−1189

PO2-stretching modes and asymmetric PO3-stretching [43]

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P2O74− anions

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[43]

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[43, 44]

ACCEPTED MANUSCRIPT Highlights • Phosphate based glass-ceramics for nuclear waste immobilization were synthesized. • NaZr2(PO4)3 is the main crystalline phase of all studied glass-ceramics.

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• Main structural units of the glass-ceramics are Q0, Q1, and a small amount of Q2 and [BO4] units.

• LRZr and LRFe of the samples containing ZrO2≤10 mol% are 2.5 × 10−6 and 3.3 ×

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10−6 g·m−2·d−1 respectively.