Synthesis and characterization of iron phosphate based glass-ceramics containing sodium zirconium phosphate phase for nuclear waste immobilization

Journal Pre-proof Synthesis and characterization of iron phosphate based glass-ceramics containing sodium zirconium phosphate phase for nuclear waste immobilization Fu Wang, Jinfeng Liu, Yuanlin Wang, Qilong Liao, Hanzhen Zhu, Li Li, Yongchang Zhu PII:

S0022-3115(19)31396-0

DOI:

https://doi.org/10.1016/j.jnucmat.2020.151988

Reference:

NUMA 151988

To appear in:

Journal of Nuclear Materials

Received Date: 31 October 2019 Revised Date:

15 December 2019

Accepted Date: 6 January 2020

Please cite this article as: F. Wang, J. Liu, Y. Wang, Q. Liao, H. Zhu, L. Li, Y. Zhu, Synthesis and characterization of iron phosphate based glass-ceramics containing sodium zirconium phosphate phase for nuclear waste immobilization, Journal of Nuclear Materials (2020), doi: https://doi.org/10.1016/ j.jnucmat.2020.151988. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.

Fu Wang: Conceptualization; Writing – original draft; Writing – review & editing; Project administration; Funding acquisition; Methodology; Supervision Jinfeng Liu: Investigation; Writing – original draft; Validation Yuanlin Wang: Investigation; Resources; Writing – review & editing Qilong Liao: Project administration; Funding acquisition; Writing – review & editing; Supervision Hanzhen Zhu: Resources; Software; Writing – review & editing Li Li: Resources; Software Yongchang Zhu: Project administration, Data curation

Graphical Abstract

1

1 2

Synthesis and characterization of iron phosphate based

3

glass-ceramics containing sodium zirconium phosphate phase

4

for nuclear waste immobilization

5 6

Fu Wang a, 1,*, Jinfeng Liu a, 1, Yuanlin Wang a, Qilong Liao a ,*, Hanzhen Zhu a, Li Li a,

7

Yongchang Zhu b

8 a

9

State Key Laboratory of Environment-friendly Energy Materials, School of Material

10

Science and Engineering, Southwest University of Science and Technology, Mianyang

11

621010, PR China b

12

China Building Materials Academy, Beijing 100024, PR China

13 14

∗

Corresponding author.

15

1

16

E-mail address: [email protected] (F. Wang); [email protected] (Q.L.

17

Liao)

Wang and Liu contributed to this work equally.

18 19

1

1

Abstract: Stable sodium zirconium phosphate (NZP)-iron phosphate glass-ceramics for

2

nuclear waste immobilization were synthesized by a melt-quenching process and the

3

effect of B2O3 on their phases, structure and properties were investigated in detail. The

4

results show that the NZP-iron phosphate glass-ceramics with good thermal stability

5

and chemical durability are obtained by the melt-quenching process. It is found that

6

B2O3 addition is the key point to achieve the glass-ceramics with NZP as crystalline

7

phase and the reasons on the formation of NZP phase due to B2O3 addition were also

8

analyzed and explained. The obtained conclusions indicate that the iron phosphate based

9

glass-ceramics containing NZP phase are potential matrix for immobilizing specific

10

nuclear waste and suggest that melt-quenching process is a possibly applicable

11

technology to prepare certain glass-ceramics containing nuclear waste.

12 13

Keywords: Glass-ceramics; Sodium zirconium phosphate; Phase analysis, Structural

14

features; Chemical durability

15

2

1

1 Introduction

2

To avoid environmental problems, large amounts of high-level radioactive waste

3

(HLW) generated from the reprocessing of nuclear fuel need to safely deal with.

4

Immobilization using glass matrix is one of the best methods to deal with HLW in

5

industrial scale [1, 2]. Borosilicate glasses are the first matrices for HLW

6

immobilization [3, 4]. However, certain HLW coming from nuclear power plants are

7

rich in sodium, phosphates, iron oxide, zirconium oxide and heavy metals which are

8

poorly soluble in widely used borosilicate glasses [1, 2, 4, 5]. Therefore, immobilization

9

of the HLW is still challenging until now. In recent decades, iron phosphate based

10

glasses, specifically, the 40Fe2O3–60P2O5 glass, have been received much attention due

11

to their high loading capacity of the HLW and good chemical durability [6-8]. The iron

12

and phosphate in the HLW can be the raw materials. Moreover, the radiation stability of

13

iron phosphate glasses doped with B2O3 is comparable to widely used borosilicate

14

glasses due to the high neutron absorption coefficient and mass absorption coefficient of

15

B element [8, 9]. Most actinides have ~10 wt% solubility in glass waste forms [5,

16

10-13]. For example, the solubility of cerium, usually as surrogate for An (III, IV),

17

where An represents actinide elements, is less than 9 wt% in iron phosphate based

18

glasses [10].

19

As potential alternatives to conventional glass waste forms, glass-ceramics

20

consisting of stable crystalline phases and glassy phase are of interest. Ideally, stable

3

1

crystalline phases preferentially incorporate long-lived nuclides such as actinides, and

2

glassy phase can confine other nuclides and elements in HLW and is also the second

3

barrier for the nuclides incorporated in the crystalline phases [14, 15]. Thus

4

glass-ceramics containing stable crystalline phases usually show high chemical

5

durability and waste loading capacity [14, 16, 17]. For instance, barium borosilicate

6

glass-ceramics containing zirconolite (CaZrTi2O7), one of the most durable mineral

7

phases on the Earth, as main crystalline phase show good properties [17]. Phosphate

8

based glass-ceramics containing monazite phase exhibit the requisite performances and

9

the formation of monazite phase improves the stability of the final waste forms [18, 19].

10

Moreover, comparing with their glassy counterparts, the waste loadings could be further

11

increased by designing glass-ceramic waste forms [17, 19].

12

Sodium zirconium phosphate NaZr2(PO4)3 (NZP), as a potential host for storage

13

and disposal of radioactive waste, shows high stability, excellent chemical durability

14

and radiation resistance [20, 21]. Moreover, NZP structure allows incorporation of more

15

than 40 nuclides and highly flexible isomorphic substitutions [22, 23]. In addition, the

16

dissolution time, which is related to chemical durability, of NZP takes 20,000 times

17

longer than quartz [22]. Therefore, crystals having NZP structure is one of most suitable

18

crystalline phases in glass-ceramic waste forms, especially in phosphate based

19

glass-ceramic waste forms. Besides heavy metals including actinide elements, a HLW

20

coming from a nuclear power plant in China contains large amounts of sodium (Na) and

4

1

zirconium (Zr). Based on its compositional features, we propose that NZP-phosphate

2

glass-ceramics potentially are good hosts for this kind of HLW. Zr and Na can be the

3

raw materials. However, how to synthesize the glass-ceramic waste forms is the key

4

point. As we all know that vitrification by traditional melt-quenching process is the only

5

way to immobilize HLW in industrial scale until now [14]. If the glass-ceramics can be

6

prepared by the melt-quenching process, we can take advantage of the facilities used for

7

vitrification. According to previous studies, one-step synthesis methods without

8

reheat-treatment processes at high temperature, such as slow-cooling method and

9

melt-quenching process, are simple processes to potentially prepare certain

10

glass-ceramics for HLW immobilization [12, 24-26]. Previously, the effect of

11

substitution of ZrO2 for Na2O on the thermal behavior, structure and chemical stability

12

of a phosphate based glass-ceramic were studied [24]. It is difficult to achieve target

13

phases when melt-quenching process was adopted, although practicable properties of

14

the final glass-ceramics are obtained [24]. It is known that crystalline phases would be

15

controlled by varying composition and synthesis parameters [27]. Based on the

16

above-mentioned summaries, in this paper, we further try to synthesize NZP-iron

17

phosphate glass-ceramics for nuclear waste immobilization by using melt-quenching

18

process. The variations of phases, structure and normalized leaching rates of the

19

prepared glass-ceramics due to composition modification were investigated in detail.

20

2 Experimental

5

1

2.1 Sample preparation

2

Based on the previous investigation [24] and trial studies (Fig. S1), baseline

3

glass-ceramic having composition of 10ZrO2–10Na2O–48P2O5–32Fe2O3 (mol%) was

4

determined. Moreover, it was found that B2O3 significantly affects the crystalline phases

5

of the glass-ceramics (Fig. S1). Therefore, glass-ceramics with general stoichiometric

6

composition xB2O3–10ZrO2–10Na2O–(80-x) (60P2O5–40Fe2O3)，where x=0, 4, 8, 10, 12

7

and 16 mol%, were prepared by a melt-quenching process in a high temperature furnace

8

(KBF1400, Nanjing Nanda, China) to investigate the variations of properties due to

9

B2O3. The analytic reagents, purchased from KeLong Chemical Reagent Co., Ltd.,

10

China, of ZrO2 (≥99.0%), NaH2PO4·2H2O (≥99.0%), Fe2O3 (≥99.0%), (NH4)H2PO4

11

(≥99.0%) and H3BO3 (≥99.5%) were used as raw materials. The specifications of the

12

raw materials are listed in Tables S1 and S2. The batches were made by thoroughly

13

mixing with designed proportions of the corresponding raw materials and placed in

14

corundum crucibles. Subsequently, the batches were melted at 1200 °C in air for about 3

15

h based on the previous trial study (Fig. S2). Then, the obtained melts were poured onto

16

a graphite plate before transferring into an annealing furnace with temperature of

17

450 °C for 1 h to eliminate the potential inner stress. Finally, glass-ceramic samples

18

were

19

xB2O3–yZrO2–(20-y)Na2O–(80-x)(60P2O5–40Fe2O3)

20

glass-ceramic, where x=0 and 4, y=4, 6, 8 and 10 mol%, were prepared by using the

obtained.

For

comparison,

glass-ceramics

6

with

derived

composition

from

the

of

baseline

1

same method. The brief schematic of the melt-quenching process is shown in Fig. 1.

2

The obtained glass-ceramic samples are designated as Bx-Zry, where x is the molar ratio

3

of B2O3, and y is the molar concentration of ZrO2. The composition, melt temperature

4

and melting time adopted are listed in Table S3.

5

2.2 Characterization

6

X-Ray diffraction (XRD) data of samples were collected from a X-ray

7

diffractometer (DMAX1400, Rigaku Company, Japan), with Cu Kα radiation (λ = 1.54

8

Å), operating at 40 kV and 70 mA, 2θ from 5 to 80°, step width of 0.03° and a time per

9

step of 10.16 s. The collected XRD data were analyzed by using Jade 6.5 software. The

10

elementary distributions and microstructures of the polished surface, after etching with

11

2 wt.% HF solution for 60 s, of glass-ceramics were investigated by using scanning

12

electron microscope (SEM) (Ultra 55, Zeiss) equipped with energy dispersive X-ray

13

detector (EDS) (Oxford IE450X-Max80). A simultaneous thermal analyzer (STA449C,

14

NETZSCH) was used to obtain the differential scanning calorimetric (DSC) curves of

15

samples. The measurement of DSC was from 100 to 1000 °C at a heating rate of 20 °C

16

min−1 using nitrogen as protective atmosphere, and about 30 mg of each sample with

17

particle size less than 45 µm was used. The characteristic temperatures were determined

18

with precision ±1 °C using a microprocessor of thermal analyzer.

19

2.3 Structural features

20

The structural features of samples were characterized by Fourier transform infrared

7

1

(FTIR) spectra and Raman spectra. FTIR spectra were measured from 400 to 2000 cm−1

2

by standard KBr pellet technique using a Spectrum One Infrared Spectrometer

3

(PerkinElmer Instrument Co. USA). The pellets were prepared by grinding and pressing

4

about 1–2 mg of powder samples with 100–200 mg of anhydrous KBr. Raman spectra

5

in the range of 200 to 1400 cm-1 with spectral resolution of 1–2 cm-1 were acquired on

6

powder samples by using a Renishaw InVia Raman spectrophotometer. The spectra

7

were recorded in back-scattering geometry under excitation with Ar ion laser radiation

8

(k=514.5 nm) at a light power of 1.7 mW.

9

2.4 Static leaching experiment

10

Chemical durability of samples was evaluated with the standard Product

11

Consistency Test (PCT) via static leaching experiment [28]. Typically, glass-ceramic

12

powders (100−200 mesh) were washed with acetone, dried until getting weight constant

13

(± 0.0001 g). A 1.5 g dried powder of each sample was put into a tightly sealed

14

polytetrafluoroethylene vessel filled with 15 mL deionized water at 90 ± 1 °C. After 1, 7,

15

14, 28 and 56 days, the leaching solution was removed for measurement and 15 mL

16

fresh deionized water was added to the polytetrafluoroethylene vessels. The

17

concentrations of P and Fe in the leaching solution were measured by inductively

18

coupled plasma optical emission spectroscopy (ICP-OES, iCAP6500, Thermo Fisher

19

Corporation, USA). The concentration of Zr was analyzed by inductively coupled

20

plasma emission spectrometer–mass spectrometer (ICP-MS, Aiglent1200/7700x). The

8

1

normalized leaching rates of element i (LRi, g m−2 d−1) were calculated using the

2

equation (1) [24, 29]:

3

LRi =

Ci ⋅ V f i ⋅ S ⋅ ∆t

(1)

4

where Ci is the concentration of element i (g L-1), fi is the mass fraction of the element

5

in the samples, V is the volume of leaching solution (L), ∆t is the leaching time (days)

6

and S is the surface area of the samples (m2). The S/V is 2000 m−1 based on the standard.

7

3 Results

8

3.1 Phase analysis

9

Fig. 2 shows effect of B2O3 on XRD patterns of the glass-ceramics prepared at

10

1200 °C for about 3 h. Based on XRD in Fig. 2, the crystalline phases of the

11

glass-ceramic without B2O3 include main ZrP2O7 phase (PDF# 71-2286, space group:

12

Cubic, Pa-3(205)) and a small amount of NZP phase (PDF# 33-1312, space group:

13

Hexagonal, R-3c(167)) (Table S4). In contrast, the B2O3-doped glass-ceramics show the

14

main crystalline phase of NZP, only one XRD peak located at 2θ of about 21.5°

15

representing to ZrP2O7 phase is detected (Fig. 2 and Fig. S1). Moreover, with the

16

content of B2O3 increasing from 4 mol% up to 10 mol%, the peak corresponding to

17

ZrP2O7 phase continuously diminishes and the content of ZrP2O7 phase decreases from

18

about 1.7 wt% to 0.2 wt% (Table S4). When the content of B2O3 is 12 mol%, no any

19

XRD peaks representing to ZrP2O7 phase are detected. That is to say, iron phosphate

20

based glass-ceramics containing single NZP crystalline phase are synthesized by the 9

1

melt-quenching process. Further increasing B2O3 to 16 mol%, insignificantly variations

2

of XRD pattern are observed. Since XRD data and FTIR spectra of B8-Zr10 and

3

B10-Zr10 insignificantly change (Fig. S3), and the sample containing 10 mol% B2O3

4

(B10-Zr10) is the turning point to obtain the glass-ceramics with single NZP crystalline

5

phase

6

xB2O3–10ZrO2–10Na2O–(80-x) (60P2O5–40Fe2O3), where x=0, 4, 10, 12 and 16 mol%,

7

for further measurement and analysis to investigate the effect of B2O3 on their

8

properties.

(Fig.

2),

we

chosen

the

glass-ceramics

with

composition

of

9

Fig. 3 shows SEM images of the typical glass-ceramics. From the electronic photos

10

(Fig. S4), the prepared glass-ceramics after casting and annealing look like glass and

11

show glassy luster, and have compact fracture section. It can be seen from Fig. 3 that all

12

samples show very compact microstructure with lots of micro-crystals embedded in

13

glassy phase. Based on EDS results, the crystalline phases at N-sites in Fig. 3 are

14

comprised of Zr, P and O in molar ratio of about 1:1:4 (Fig. S5a), which reconfirms that

15

the main crystalline phase of the glass-ceramic without B2O3 is ZrP2O7. In addition, the

16

main elements in the crystalline phases at F-sites in Fig. 3 include Na, Zr, P and O in

17

molar ratio of about 1 : 2 : 3 : 12 (Fig. S5b). Together with XRD analysis, the main

18

crystalline phase of the glass-ceramics with different amounts of B2O3 is further

19

determined to be NZP. From EDS maps of B0-Zr10 and B12-Zr10 samples (Fig. 4), it

20

can be observed that Fe, O and B are rich in the glassy phase. In contrast, P and Zr are

10

1

rich in the crystalline phases formed. Na is mainly concentrated in the glassy phase for

2

B0-Zr10 sample and almost homogeneously distributed for B12-Zr10 sample. This

3

result is consistent with XRD analysis.

4

3.2 FTIR and Raman results

5

Fig. 5 shows normalized FTIR spectra of the typical glass-ceramic samples.

6

Generally, the FTIR spectra are mainly characterized by absorption bands around 551

7

cm–1, 643 cm–1, 881 cm–1, 1047 cm–1, 1120 cm–1, 1199 cm–1, 1400 cm–1 with a shoulder

8

at 1384 cm–1, 1458 cm–1, 1593 cm–1 and 1630 cm–1. Based on the published literatures

9

and reasonable analysis, main assignments of the FTIR bands have been done and are

10

summarized in Table 1. The broadest absorption bands located at 1047 cm–1 and 1120

11

cm–1 are attributed to asymmetric stretching vibrations of (PO4)3– units (Q0 units) [30,

12

31] and asymmetric stretching vibrations of (PO3) – in (P2O7)4– units (Q1 units) [32]

13

respectively. The absorption band at 881 cm–1 is assigned to stretching vibrations of

14

B-O bonds in [BO4] units [33, 34] and P–O–B bonds [10, 35]. The weak band at 1199

15

cm–1 is ascribed to asymmetric stretching vibrations of (PO3)– in Q1 units and (PO2)+ in

16

metaphosphate (Q2) units [30-32, 36]. The band at 551 cm–1 corresponds to bending

17

mode of O–P–O in Q1 groups [30]. The band at about 643 cm–1 is characteristic of

18

Fe–O–P bonds [10]. In addition, the bands between 1593 and 1630 cm–1 can be ascribed

19

to bending vibration modes of H–O–H, B–O–H and P–O–H [33, 37]. The bands at 1400

20

with a shoulder at 1384 cm–1 and 1458 cm–1 are related to symmetric stretching

11

1

vibrations of P=O bonds [31] and vibration modes of [BO3] and BO2O– [38]. The band

2

at 981 cm-1 in IR spectrum of B0-Zr10 sample has been assigned to the P−O– groups

3

(chain terminator) [6, 39] and the band at about 740 cm-1 is related to ZrP2O7 phase [40,

4

41]. Some discernible modifications in FTIR spectra were caused due to B2O3 addition,

5

although no significant differences of band locations are observed. The intensity of the

6

bands at about 551 cm–1 and 1120 cm–1 which have been related to Q1 units significantly

7

decreases with B2O3 addition. The intensity of the bands concentrated on 1047 cm–1

8

which has been assigned to Q0 units and 643 cm–1 which has been attributed to Fe–O–P

9

bonds increases with the content of B2O3. Moreover, the bands at 981 cm-1 and 740 cm-1

10

disappear for the samples containing B2O3. The absorption band at 881 cm–1 which has

11

been related to [BO4] units and P–O–B bonds appears and increases in intensity with the

12

content of B2O3. It also should be noted that the intensity of the bands between

13

1384–1458 cm–1 obviously increases due to the formation of [BO3] or/and BO2O− [31,

14

38] when the content of B2O3 is higher than 10 mol% .

15

Fig. 6 shows normalized Raman spectra of the typical glass-ceramics. Usually, no

16

obvious peaks related to borate units in Raman spectra are observed [13, 42]. Based on

17

literatures, the Raman peak at 1021 cm–1 is induced by symmetric stretching vibrations

18

of Q0 units [43]. The peaks at 1053 cm–1 and 1089 cm–1 are assigned to symmetric

19

stretching vibrations of (PO3)– in Q1 units [43, 44]. The weak peak at 630 cm–1 is

20

assigned to symmetric stretching vibrations of P–O–P bonds in Q2 units [43, 44]. The

12

1

peaks below 600 cm–1 can be assigned to complicated internal vibrations such as

2

skeletal deformation vibrations of phosphate chains and pyrophosphate segments [43].

3

Q2 units should come from a disproportion reaction between Q2 and Q0 units [45] or are

4

induced by adding B2O3 in high concentrations because B16-Zr10 show high intensity

5

of the peak at about 630 cm–1. This also attributes the variation of the band at 1199 cm–1

6

in FTIR spectra. Main peaks and assignments of Raman spectra are listed in Table 2. It

7

can be seen from Fig. 6 that the shape of the Raman spectrum of B0-Zr10 sample is

8

obviously different from the glass-ceramics containing B2O3. The maximum peak of

9

B0-10Zr sample is located at 1089 cm–1, and the maximum peak of the glass-ceramics

10

containing B2O3 is located at about 1021 cm–1. Moreover, the intensity of the peak at

11

1089 cm–1 decreases with B2O3 and disappears when the concentration of B2O3 is higher

12

than 10 mol%. Since Raman spectra of glassy material usually show broad scattering

13

peaks [13] and the sharp peaks in Raman spectra usually are induced by crystalline

14

phases [13, 46], it can be concluded that this results are consistent with XRD analysis.

15

3.3 DSC result

16

Fig. 7 shows DSC curves of the typical glass-ceramic samples. The curves were

17

mainly characterized by a glass transition temperature (Tg), onset crystallization

18

temperature (Tr) and onset liquid temperature (TL). The inset in Fig. 7 shows how these

19

thermal parameters were determined and the obtained Tg, Tr, TL, (Tr–Tg) and (TL–Tr)

20

values are listed in Table 3. As shown in Fig. 7 and Table 3, Tg and Tr slightly increase

13

1

with content of B2O3, indicating enhanced structure of glassy phase obtained by B2O3

2

addition. And TL tends to decrease due to the low melting temperature of B2O3 [47]. The

3

density (Table 3) of the glass-ceramics gradually decreases because of the low atomic

4

weight of B. According to Hrubý’s method [24, 48], glass forming ability is

5

proportional to thermal stability of a glass and K (K = (Tr–Tg)/(TL–Tr)) is a quantitative

6

measurement for glass forming ability. As listed in Table 3, all of the investigated

7

glass-ceramics except for B16-Zr10 sample show good thermal stability, which are

8

comparable with conventional silicate glasses having K values usually varied from

9

about 0.14 to 1.3 [49, 50]. The addition of B2O3 leads to the increase of the K value,

10

which means higher thermal stability of residual glass in samples due to the formation

11

of stronger B–O bonds in the glassy phase [47]. B10-Zr10 and B12-Zr10 samples have

12

the K value of about 0.42 and show the best thermal stability. For B16-Zr10 sample, the

13

slight decreased K value (0.367), as listed in Table 3, may be caused by the additional

14

formation of [BO3] (Fig. 5) [47].

15

3.4 Normalized leaching rate

16

Fig. 8 shows normalized leaching rates of P (LRP) and Fe (LRFe) of typical

17

samples immersing in deionized water at 90 °C for 1, 7, 14, 28 and 56 days. Fig. 8(a)

18

shows that LRP of all samples decrease quickly after immersion within the initial 14

19

days, then slowly decrease from 14 to 28 days and almost remain constant between 1.83

20

and 2.87 × 10−4 g m−2 d−1 depending on B2O3 content after 28 days. Fig. 8(b) shows that

14

1

LRFe also decrease quickly during the first 14 days, and remain almost unchanged

2

after 21 days. There are no regular changes of LRFe but the values of LRFe are from

3

about 10−6 to 10−7 g m−2 d−1 for all glass-ceramic samples. The first quickly decrease of

4

LR may be due to ion exchanges occurring between the glass composition and the

5

leaching solution during the initial days [41, 51] and the following lower LR are

6

ascribed to the formation of protective passivation layer [13, 51].

7

Low LR means high chemical durability of glass-ceramics [24, 28, 29]. All of the

8

studied glass-ceramics except for B16-Zr10 show similar LR values which are one or

9

two order of magnitude lower than the glass-ceramics containing various phosphate

10

crystalline phases [24, 41] and certain iron phosphate glass for nuclear waste

11

immobilization [6]. It can be seen that B2O3 addition (not higher than 12 mol%) does

12

not degrade the chemical durability of the glass-ceramics containing NZP phase. The

13

concentrations of Zr element in leaching solution for the samples immersing in 90 °C

14

deionized water within 28 days are hardly detected, and Zr concentration in the leaching

15

solution for 56 days is close to the detected limit (about 10-7 g L-1) of ICP (Table S5).

16

That is to say, the LRZr of glass-ceramics are believed to be less than 10−7 g m−2 d−1.

17

4 Discussion

18

4.1 Role of B2O3 in the formation of NZP phase

19

Based on phase analysis, adding B2O3 into the batches is the key point to achieve

20

the glass-ceramics containing NZP phase by melt-quenching process. According to

15

1

previous investigation, liquids firstly form and NH4FeP2O7 phase coming from the

2

reaction between NH4H2PO4 and Fe2O3 appears at about 300 °C in the melt-quenching

3

process [26]. Therefore, nucleation and crystallization usually occur in the process of

4

phase formation [27] and crystalline phases having pyrophosphate group such as ZrP2O7

5

tends to form (B0- Zr10 sample). Moreover, ZrO2 in batches is an effective nucleating

6

agent for crystalline phase having monoclinic, tetragonal or cubic structure [27], which

7

facilitates the formation of ZrP2O7 phase having cubic space group. Therefore, the

8

obtained glass-ceramic without B2O3 contains the main crystalline phase of ZrP2O7 and

9

only about 2.3 wt% NZP phase (Table S4).

10

When B2O3, as the other glass forming oxide, exists in a composition together with

11

P2O5, various phosphate units try their best to do not connect each other in glass liquids

12

[47]. This result is also proved by FTIR and Raman analysis which indicate the increase

13

in Q0 phosphate units by B2O3 incorporation (Figs. 4 and 5). Q0 units facilitates the

14

formation of NZP phase having orthophosphate group, thus the glass-ceramics

15

containing B2O3 show the main crystalline phase of NZP (>93%, Table S4). This result

16

is also consistent with the conclusion reported by S. Liu, et al. who proved that

17

phosphate glass without B2O3 only showed the crystallization of phase containing Q1

18

units and the glass containing B2O3 showed the increasing crystallization of phase

19

containing Q0 units at the expense of Q1 containing phase [52]. It should be noted that

20

single NZP phase is difficult to obtain in the glass-ceramics. That is because: (1) single

16

1

NZP ceramic itself is difficult to be synthesized and the second ZrP2O7 phase always

2

appears easily during the preparation, even when traditional solid-state reaction method

3

used [46]; (2) as mentioned before, nucleating agent ZrO2 in batches is in favor of

4

ZrP2O7 formation [27]. In this work, when the content of B2O3 is higher than 10 mol%,

5

glass-ceramics containing single NZP crystalline phase are obtained. This indicates that

6

high content of B2O3 significantly separates various phosphate units. Meanwhile, the

7

formation of NZP phase consumes Zr in composition, which also decreases the effective

8

content of nucleating agent ZrO2.

9

4.2 Preparation

10

At present, glass-ceramic waste forms were mostly synthesized by the process of

11

pressing and sintering the mixture of glass powders and HLW [14, 18, 23, 41], or the

12

synthesis process by which the raw materials firstly form glasses using melt-quenching

13

method and then the glasses are reheated to separate the target phases [15, 17]. The two

14

preparing processes both are complex and difficult to realize industrial preparation

15

comparing with vitrification [14, 16]. That is also probably why vitrification through

16

traditional melt-quenching process is the only way to immobilize HLW in industrial

17

scale until now [14]. This report further confirms the feasibility to prepare certain

18

phosphate glass-ceramic waste forms by melt-quenching process. This is an important

19

finding. If this method can be used to prepare glass-ceramic waste forms with required

20

properties, it can take advantage of nuclear facilities used for vitrification. Moreover,

17

1

comparing with corresponding glass waste forms, the waste loading capacity could be

2

further increased. For example, based on previous investigation, the solubility of Zr in

3

iron phosphate glasses is less than 6 mol% [53]. In this work, up to 10 mol% instead of

4

6 mol% Zr could be incorporated in the corresponding glass-ceramics, the loading

5

capacity increases by about 50%. This will enable to decrease the number of glass

6

canisters and reduce the disposal cost [14].

7

4.3 Chemical durability

8

Chemical durability is one of the most important properties for nuclear waste

9

forms [2, 13, 14, 41, 51]. Based on the literature reported by Horlait et al. [54], the

10

surface area determined by geometrical method probably leads to higher leaching rates,

11

but it can still briefly evaluate the chemical durability of samples according to PCT

12

standard. As presented in section 3.4, the results of static leaching experiment

13

demonstrate that good chemical durability (LRFe, ∼10−7 g m−2 d−1; LRP, ∼10−4 g m−2 d−1)

14

of the glass-ceramics is achieved [14]. For glass-ceramics, if the glassy phase and

15

crystalline phases both show high aqueous resistance, good chemical durability would

16

be obtained [17, 24, 27, 41]. In this work, iron phosphate glass is one of potential

17

matrixes for immobilizing specific nuclear waste and possesses high aqueous resistance

18

(LR, ∼10−7 g m−2 d−1) [2, 6, 7, 14], and adding ∼10 mol% B2O3 does not degrade its

19

durability [8-10, 13] and thermal parameters (DSC analysis in Fig. 7). Moreover,

20

ZrP2O7 and NZP phase have high aqueous resistance (LR, ∼10−8 g m−2 d−1) and

18

1

structural stability [24, 40]. Therefore, the good chemical durability is believed to be

2

attributed to the high aqueous resistance and structural stability of B2O3-doped iron

3

phosphate glassy phase, ZrP2O7 and NZP phase. Since water resistance of NZP is higher

4

than ZrP2O7 [20, 22, 23], so slightly higher chemical durability (lower LR values) of the

5

glass-ceramics containing NZP phase is obtained. However, excessive B2O3 addition

6

(more than 12 mol% in this work) would decreases the stability of glassy phase based

7

on literatures [8, 9] and DSC analysis (Fig. 7 and Table 3), so B16-Zr10 sample shows

8

one or two order of magnitude higher LR values than other samples.

9

4.4 Implications on nuclear waste immobilization

10

It is known that material having NZP structure is one of most suitable hosts for the

11

immobilization of radioactive waste [22, 23, 46, 55, 56]. NZP structure allows

12

incorporation of various elements in HLWs and highly flexible isomorphic substitutions

13

[22, 55, 56]. For example, alkali and di-valent cations can be incorporated into NZP

14

structure by entering Na sites, rare-earth ions can enter Zr sites [23, 57]. Even highly

15

charged species such as hexavalent molybdenum can directly substitute P with suitable

16

charge compensation on the other sites [22, 23]. Moreover, the structure of NZP loaded

17

with waste elements insignificantly changes [23]. Another advantage is that Na, Zr, P

18

and Fe in HLWs can be the raw materials for the preparation of the glass-ceramics and

19

stable iron phosphate glassy phase provides the second barrier to confine nuclides. Only

20

one endothermic peak on the DSC curves confirms a homogeneous glassy phase in the

19

1

glass-ceramics [25, 47], and about 495 °C (>450 °C) of Tg and good thermal stability

2

which is comparable with conventional silicate glasses also meet the requirements of

3

deep geological disposal of nuclear waste forms [14]. More importantly, vitrification in

4

industrial scale can be performed efficiently at the temperature of 1200 °C [14].

5

Therefore, the NZP-iron phosphate based glass-ceramics synthesized by melt-quenching

6

process are potential hosts for the immobilization of specific HLW.

7

5 Conclusions

8

Glass-ceramics with NZP microcrystal were synthesized by melt-quenching

9

process and effect of B2O3 on their phases, structural features and properties were

10

discussed in detail. The results show that the main crystalline phase of the iron

11

phosphate based glass-ceramics without B2O3 is ZrP2O7, and that of the glass-ceramics

12

with B2O3 is NZP phase. When an appropriate content (12 mol%) of B2O3 is

13

incorporated, iron phosphate glass-ceramic with single NZP crystalline phase is

14

obtained. FTIR and Raman spectra show that adding B2O3 separates various phosphate

15

units connecting each other, resulting in formation of NZP phase and increasing content

16

of Q0 phosphate units. Moreover, the obtained phosphate glass-ceramics with NZP as

17

main crystalline phase possess good thermal stability and chemical durability based on

18

DSC analysis and Static leaching experiment. The prepared glass-ceramics have LRp

19

and LRFe of ∼2.0 × 10−4 g m−2 d−1 and ∼10−6 g m−2 d−1 respectively after 28 days.

20

Almost no Zr in the leaching solutions within 28 days is detected.

20

1

Acknowledgements

2

This work was supported by the National Natural Science Foundation of China

3

[grant number 51702268] and the China Industrial Technology Development Program

4

[grant number 2017-1407].

5 6

21

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[57] A. Bohre, O.P. Shrivastava, Crystal chemistry of immobilization of divalent Sr in

7

ceramic matrix of sodium zirconium phosphates, J. Nucl. Mater. 433 (2013) 486–493.

8

https://doi.org/10.1016/j.jnucmat.2012.10.012.

Ceram.

Int.

43

9

31

(2017)

9522–9530.

1 2

Table 1 Main absorption bands and assignments of FTIR spectra for the glass-ceramic

3

samples. Wavenumber Band assignments

References

551

Bending mode of O–P–O in Q1 units

[30]

643

Symmetric stretching vibrations of Fe–O–P bonds

[10]

740

Related to ZrP2O7 phase

[40, 41]

881

stretching vibrations of B-O bonds in [BO4] units

[33, 34]

P–O–B bonds

[10, 35]

981

P–O– units

[6, 39]

1047

Asymmetric stretching vibrations of (PO4)3– in Q0 units

[30, 31]

1120

Asymmetric stretching vibrations of of (PO3) – in Q1 units

[32]

1199

Asymmetric stretching vibrations of (PO3) – in Q1 units and [30,

–1

(cm )

(PO2)+ in Q2 units

36]

1384–1458

[BO3], BO2O− and P=O bonds

[31, 38]

1593–1630

Bending vibration modes of H–O–H, B–O–H and P–O–H [33, 37] bonds

4 5 6

32

31,

1 2

Table 2 Main peaks and assignments of Raman spectra of the glass-ceramics. Wavenumber (cm–1)

Band assignments

References

＜600

Internal vibrations related to phosphate polyhedral,

[43]

chains and segments 630

Symmetric stretching vibrations of P–O–P bonds in

[43, 44]

Q2 units 1021

Symmetric stretching vibrations of Q0 units

[43]

1053 and 1089

Symmetric stretching vibrations of (PO3)– in Q1

[43, 44]

units 3 4

33

1 2

Table 3 Density (ρ) and related DSC thermal parameters of the typical glass-ceramic

3

samples. Samples

B0-Zr10

B4-Zr10

B10-Zr10

B12-Zr10

B16-Zr10

Tg ± 1 (°C)

489

492

496

494

496

Tr ± 1 (°C)

551

588

597

591

583

TL ± 1 (°C)

906

889

839

823

819

Tr–Tg (°C)

62

96

101

97

87

TL–Tr (°C)

355

301

242

232

236

K=(Tr-Tg)/(TL-Tr)

0.175

0.319

0.417

0.418

0.367

ρ±0.001(g cm–3)

3.170

3.154

3.113

3.118

3.079

4 5

34

1

Figure captions

2

Fig. 1. Brief schematic of the synthesis of the iron phosphate based glass-ceramics by

3

melt-quenching process.

4

Fig. 2. XRD pattern of the glass-ceramic samples containing different amounts of B2O3

5

prepared at 1200 °C for about 3 h.

6

Fig. 3. SEM images of polished sections of the typical glass-ceramics.

7

Fig. 4. EDS maps of (a) B0-Zr10 and (b) B12-Zr10 glass-ceramic samples.

8

Fig. 5. Normalized FTIR spectra of the typical glass-ceramic samples.

9

Fig. 6. Normalized Raman spectra of the typical glass-ceramic samples.

10

Fig.7. DSC curves of the prepared glass-ceramic samples.

11

Fig. 8. Normalized leaching rates of (a) P and (b) Fe for the glass-ceramic samples

12

immersing in deionized water at 90 °C for different days.

13

35

1

2 3

Fig. 1

4 5

36

1

2 3

Fig. 2

4 5 6

37

1 2

Fig. 3

3

38

1

2 3

Fig. 4

4 5

39

1 2

3 4

Fig. 5

5 6

40

1 2

3 4

Fig. 6

5 6

41

1

2 3

Fig. 7

4 5 6

42

1

2

3 4

Fig. 8

43

Highlights - NZP-iron phosphate glass-ceramics were synthesized by a melt-quenching process. - B2O3 addition is the key point to achieve the NZP-iron phosphate glass-ceramics. - Reasons on the formation of NZP due to B2O3 addition were explained. - Melt-quenching process is a possible technology to prepare certain glass-ceramics. - NZP-phosphate glass-ceramics are potential matrix for disposal of specific HLW.

Declaration of interests × The authors declare that they have no known competing financial interests or personal relationships □ that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: