Rapid immobilization of simulated radioactive soil waste by microwave sintering

Journal of Hazardous Materials 337 (2017) 20–26

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Rapid immobilization of simulated radioactive soil waste by microwave sintering Shuai Zhang a , Xiaoyan Shu a , Shunzhang Chen a , Huimin Yang a , Chenxi Hou a , Xueli Mao a , Fangting Chi a , Mianxin Song a , Xirui Lu a,b,∗ a Key Subject Laboratory of National Defense for Radioactive Waste and Environmental Security, Southwest University of Science and Technology, Mianyang 621010, PR China b China Academy of Engineering Physics, Mianyang 621900, PR China

h i g h l i g h t s • • • •

The radioactive contaminated soil is fast disposed by microwave technology. Simulated nuclides are immobilized and uniformly distributed in glass matrix. The density of vitrification matrix ranges from 2.813 g/cm3 to 3.215 g/cm3 . The normalized leach rate of Nd is less than ∼10−4 g/(m2 day) after 42 days.

a r t i c l e

i n f o

Article history: Received 2 February 2017 Received in revised form 2 May 2017 Accepted 3 May 2017 Available online 3 May 2017 Keywords: Radioactive contaminated soil Microwave Vitrified forms Chemical durability FT-IR

a b s t r a c t A rapid and efficient method is particularly necessary in the timely disposal of seriously radioactive contaminated soil. In this paper, a series of simulated radioactive soil waste containing different contents of neodymium oxide (3–25 wt.%) has been successfully vitrified by microwave sintering at 1300 ◦ C for 30 min. The microstructures, morphology, element distribution, density and chemical durability of as obtained vitrified forms have been analyzed. The results show that the amorphous structure, homogeneous element distribution, and regular density improvement are well kept, except slight cracks emerge on the magnified surface for the 25 wt.% Nd2 O3 -containing sample. Moreover, all the vitrified forms exhibit excellent chemical durability, and the leaching rates of Nd are kept as ∼10−4 –10−6 g/(m2 day) within 42 days. This demonstrates a potential application of microwave sintering in radioactive contaminated soil disposal. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Soil in certain regions might be heavily contaminated by radionuclides, owing to nuclear accident, mining tailing and nuclear testing, etc. [1–5]. Radionuclides in the contaminated soil may travel around the world with air stream, heavy rains or groundwater, and pose a long-term threat to living organisms [6]. Furthermore, the composition of both radionuclides and soil are normally different among varied regions. This makes some disposal means to be invalid, especially those which require precise ratio and strong selectivity or that are time-consuming [5]. Therefore, it

∗ Corresponding author at: Key Subject Laboratory of National Defense for Radioactive Waste and Environmental Security, Southwest University of Science and Technology, Mianyang 621010, PR China. E-mail address: [email protected] (X. Lu). http://dx.doi.org/10.1016/j.jhazmat.2017.05.003 0304-3894/© 2017 Elsevier B.V. All rights reserved.

is driving a search for a simple, fast and flexible method in radioactive contaminated soil disposal under such serious and complex situations. Vitrification route seems to be attractive for the mentioned purpose, since it can immobilize a range of nuclides at one time, as well as other contaminants in soil, e.g. heavy metal ions, complicated organics and mixed wastes [7]. Moreover, its resulting waste form is easy to be stored safely in a geological repository [8], due to the reduced volume and high chemical durability of glassy product (Normalized leaching rate ranges from 10−4 to 10−1 g/(m2 day)) [9]. Regrettably, vitrification method at present is mainly applied to the treatment of low- and intermediate-level radioactive waste (LILW) [10], or occasionally industrial soil waste [11], while seldom researches have focused it on the disposal of radioactive contaminated soil. Moreover, the traditional Joule heating technology and complex pretreatment process (e.g. raw material ratios) have weaken the advantage of this potential method more or less.

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Fig. 1. Schematic diagram of microwave system.

Table 1 Composition of the pristine soil used in the present work.

Content (wt.%)

Table 2 Mixture ratio of simulated radioactive contaminated soil.

SiO2

Al2 O3

Fe2 O3

CaO

K2 O

MgO

Na2 O

TiO2

66.32

16.57

5.87

4.67

2.86

1.66

0.81

0.74

As a new sintering method, microwave technology has attracted much attention in recent years [12–14]. It is referred as dielectric heating, and heat is generated within the material instead of external sources [15]. Also, it is well established as a fast sintering method, since it can achieve more than 1000 ◦ C within 20 min [16]. Therefore, as compared with other treatment methods, microwave sintering technology shows its superiority in the aspects of rapid, efficient and environmental friendliness [17,18]. Moreover, microwave technology has already shown excellent performance in the remediation of soils containing heavy metals [19–23] and toxic organic compounds [24,25], which indicates a wider application of microwave in soil treatment is rather possible. Normally, the main actinides in contaminated soils are Am, Pu, Cm, et al. [26,27], and Nd3+ is generally taken as a surrogate for An3+ due to their equal valence and similar ionic radius: RNd 3+ = 1.11 Å and An3+ (RAm 3+ = 1.01 Å; RPu 3+ = 1.00 Å and RCm 3+ = 0.97 Å) [28]. In this paper, simulated radioactive contaminated soil containing neodymium has been systematically vitrified by microwave sintering in a direct way, without complex pretreatment. In order to understand the solidifying mechanism of nuclides and to acquire the aqueous durability of as obtained vitrified forms, a series of characterizations were carried out. XRD was taken to study the phase evolution as a function of neodymium oxide content, FTIR and Raman spectra were collected to get the detailed local information, FESEM-EDX was performed to distinguish the micromorphology and element distribution evolution. The chemical durability of vitrified forms was examined using the standard PCT [29] static leach test. 2. Experimental 2.1. Fabrication The pristine soil (200 mesh, the composition is shown in Table 1) and Nd2 O3 powder (Tianjin Kermel Co. Ltd., purity ≥ 99.99%) were chosen as the reactants. All the raw materials were heated at 100 ◦ C for 12 h to remove adsorptive water. Then, simulated radioactive contaminated soil was prepared according to Table 2, and the total mass of each sample is 10 g. To obtain homogenous soil sample, the raw materials in appropriate ratio were sufficiently mixed in a mortar by grinding with ethyl alcohol (AR grade). After drying, the soil sample held by a corundum crucible (vol, 5 ml; 1700 ◦ C ≤ T ≤ 1800 ◦ C) was put into the high temperature

Content (wt.%) Pristine Soil Nd2 O3

97 3

95 5

90 10

85 15

80 20

75 25

Fig. 2. Temperature-microwave power-time (T-P-t) profile of soil.

microwave muffle furnace (HAMiLab-M1500) by Syno Therm for sintering. Before sintering, the temperature calibration was carried out via inserting a standard platinum rhodium thermocouple into the soil sample to measure the temperature inside the crucible, and taking the infrared thermometer to test the outer wall of the crucible. The temperature difference was less than 5 ∼ 10 ◦ C above 900 ◦ C (about 3 ∼ 5 ◦ C at 1300 ◦ C). Surrounding the crucible, a mullite sagger (containing silicon carbide) was used to improve the sintering ability of different kinds of soil, especially at high temperatures [14,16,30]. The schematic diagram of the microwave system is shown in Fig. 1 and the temperature-microwave powertime (T-P-t) profile of soil is shown in Fig. 2. For comparison, the conventional sintering method in ambient air was also adopted at 1300 ◦ C for 2 h, with a heating rate of 5 ◦ C/min. 2.2. Characterization To determine the glass transition temperature, simultaneous TG-DTG measurements were carried out on a thermal analysis apparatus (SDT Q 600) in air atmosphere by heating from room temperature to 1300 ◦ C at a heating rate of 20 ◦ C/ min. The phase structure of solidified soil body was confirmed by X-ray diffraction (XRD, X’Per PRO, Netherlands) with Cu K␣ radiation (␭ = 1.5406 Å),

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Fig. 3. Schematics of static leach test.

operating at 2.2 kW. The 2 range of collected data was 10◦ –80◦ , and the scanning rate was 2◦ /min. Besides, the Fourier transform infrared spectra (FT-IR, PerkinElmer Co., USA) were recorded with the KBr pellet standard method. All Raman spectra were collected by a Raman spectrometer (Renishow inVia, U.K.) equipped with Argon ion laser (514 nm). The microstructure of the sintered sample was observed by scanning electron microscope (SEM, Ultra 55. Germany). The element distribution was analyzed using energy dispersive X-ray spectrometer (EDX) attached to the SEM equipment. The density of each sample was measured at room temperature with the Archimedes method, using water as an immersing liquid. 2.3. Leaching experiments The chemical durability of the solidified samples was assessed by static leaching experiments (Product Consistency Test, PCT). After crushing and sieving (75–150 ␮m), the specific surface area of the glass powders were measured with N2 adsorption method by a specific surface area tester (Gold APP Co. F-Sorb X400 China). About 100 mg of glass powder were immersed in 10 ml pure water (pH = 6.6) contained by a hydrothermal reaction vessel, which is composed of Teflon inner vessel and stainless steel outer vessel (as shown in Fig. 3). After being sealed, the vessel was heated to a constant temperature (90 ◦ C ± 1 ◦ C), and the solution sample was taken at regular intervals (3, 7, 14, 28, 35, 42 days). The as obtained aqueous solution was then analyzed by an inductively coupled plasma mass spectroscopy (ICP-MS, Agilent 7700x, USA). The normalized elemental (i) leaching rates (NLi , g m−2 d−1 ) were calculated using the following formula [29]: NLi =

Ci · V SA · fi

Where Ci is the concentration of the element in the solution, V is the volume of the leachate (m3 ), SA is the surface area of powders (m2 ), fi is the mass fraction of element i in the solidified soil (wt.%). The SA/V ratio is about 2000 m−1 . 3. Results and discussion 3.1. Thermal analysis by TG-DTG The TG-DTG curves of the pristine soil in the temperature range of 30–1300 ◦ C are shown in Fig. 4. Among 30–750 ◦ C, the weight loss is normally attributed to the removing of adsorption water or crystal water and a small amount of volatile organic substances [31]. This is notable in this work, since obvious weight loss (about 5%) is observed in soil sample at about 300 ◦ C to 700 ◦ C. At about 895 ◦ C and 1150 ◦ C, the TG curve is slightly higher than the back-

Fig. 4. TG-DTG curves of the simulated radioactive contaminated soil.

ground level of 1200 ◦ C. The weak peak at 895 ◦ C is related to the decomposition of a small amount of calcite (CaCO3 ) in the pristine soil [32]. The peak at about 1150 ◦ C would result from the decomposition reaction of feldspar ((K, Na)AlSi3 O8 ), which tends to reach equilibrium after 1200 ◦ C according to the XRD results (Fig. 5(a)). Therefore, 1100 ◦ C, 1200 ◦ C and 1300 ◦ C were chosen as the sintering temperatures to explore the best solidifying process. 3.2. Structural analysis through XRD, FT-IR and Raman spectroscopies In order to explore the phase formation process of soil and to understand the sintering difference between microwave and conventional technologies, the samples were sintered by both methods. The microwave sintering experiments were carried out at 1100 ◦ C, 1200 ◦ C and 1300 ◦ C for 30 min, while the conventional sintering was performed at 1300 ◦ C for 2 h. The XRD patterns obtained on these samples are shown in Fig. 5(a). As can be seen, typical steamed broad peaks appear in all the XRD patterns, indicating the formation of amorphous structures. Furthermore, the crystal peaks disappear at 1300 ◦ C after microwave sintering for 30 min, whereas the crystal peaks of silicon oxide are still sharp after conventional sintering at 1300 ◦ C for 2 h. Combined with the TG-DTG analysis in 3.1, it seems that holding at 1300 ◦ C for 30 min is a reasonable choice for microwave sintering in the following work. Fig. 5(b) shows the XRD patterns of sintered samples with different Nd content. All of them exhibit typical amorphous characteristics. More significantly, no obvious crystal peaks of Nd2 O3 or neodymium containing compounds are detected in the XRD patterns. Therefore, neodymium has been successfully incorporated into the solidified soil body, though the existing form of Nd could not be determined by XRD. With the attempt to find out the existing form of introduced Nd, the structure of solidified soil samples have been further tested by FT-IR and Raman spectroscopies. Fig. 6 shows the FT-IR spectra of samples with different contents of Nd2 O3 . The absorption bands at 3461 cm−1 and 1626 cm−1 are assigned to the hydroxyl groups [33]. The bands at 2923 cm−1 and 2852 cm−1 are attributed to the asymmetric aliphatic C H stretching vibration and symmetric aliphatic C H stretching vibration in methylene [34,35]. Their stable positions and shapes suggest that a small amount of impurities containing hydroxyl and aliphatic C H may exist in all the samples, which might originate from the preparing process just before characterization. Moreover, the main absorption peaks in the samples are concentrated in the range of 400 cm−1 to 1800 cm−1 . The sensitive bands at 474 cm−1 and 789 cm−1 are characteristic bending vibrations of sil-

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Fig. 5. XRD patterns of samples obtained at varied conditions (a) and samples with different Nd concentration (b) (C, Conventional sintering; M, Microwave sintering).

Fig. 7. Raman spectra of samples with different contents of Nd2 O3 .

3.3. Microstructure characterization and density evolution Fig. 6. FT-IR spectra of samples with different contents of Nd2 O3.

icon oxides, which are related to Si–O–Si bonds and O–Si–O bonds, respectively. Moreover, the asymmetric shape of the wide band, ranging from 800 cm−1 to 1200 cm−1 , indicates that it’s a composite absorption band, which contains a first band at 950 cm−1 and a second band ranging from 1020 cm−1 to 1060 cm−1 [36]. Among them, the band at 950 cm−1 is due to the asymmetric Si–O stretching vibration, while the band in the range of 1020 cm−1 to 1060 cm−1 is attributed to the asymmetric Si–O–Si stretching vibration. Furthermore, the position of the main absorption bands are consistent with each other. In our previous work, the Raman active band for Nd (sample containing 3 wt.% of Nd2 O3 ) totally disappeared after sintering at 1300 ◦ C [37]. In addition, as shown in Fig. 7, no substantial changes were found in the Raman region related to Nd with the increase of Nd2 O3 content (from 5 wt.% to 25 wt.%). That is to say, no bonds related to Nd have been found in the FT-IR or Raman curves, and no obvious crystal peaks of Nd2 O3 or neodymium containing compounds have been detected by XRD. Therefore, authors assume that Nd3+ might be incorporated in the vitrified material at an atomic scale without participating in any chemical reactions [38].

In order to understand the morphology, neodymium distribution and the density variation with enhanced Nd content, the solidified soil samples were further characterized. Fig. 8 shows the photo of samples before (a) and after (b)–(e) microwave sintering. Before sintering, soil samples have the same macro-profile as that shown in Fig. 8(a). After microwave treatment, samples have bright and smooth surfaces, and patterns similar to that of the blue and white porcelain appear on the sample surfaces as the Nd content increases gradually (Fig. 8(b)–(e)). In addition, the sintering trace on the wall of crucible indicates obvious shrinkage (about 42%–50%) has resulted from microwave treatment (shown in Fig. 8(b)). This is in a good agreement with the volume reduction principle which prevails in radioactive waste disposal. Since special technology and room are required in manipulating growing amounts of radioactive waste, the volume reduction could bring much convenience in the transportation and the final deep geological storage [8–10,39]. Fig. 9 shows the SEM and corresponding Nd element mapping images of the vitrified forms. SEM pictures of samples doped with 3 wt.%–25 wt.% of Nd2 O3 are presented in Fig. 9(a)–(d). It reveals that the smooth and homogeneous surface is essentially consistent with the macro-profile. However, for a Nd2 O3 content of 25%, the sample surface turns to be rough, and some minor cracks appear. On the other hand, Nd in all samples is homogeneously distributed

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Fig. 8. Photos of soil before and after microwave sintering (a: morphology of samples before sintering; b, c, d, e: morphology of the sintered samples with 3 wt.%, 5 wt.%, 15 wt.%, 25 wt.% of Nd2 O3 , respectively. All the solidified bulk show obvious shrinkage ranges.).

Fig. 9. SEM image (a-d) and corresponding element mapping image of Nd (e-h).

Fig. 11. NLNd Variation of vitrified forms bearing different Nd2 O3 contents (3 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%, 25 wt.%). Fig. 10. Bulk density of glass waste forms.

3.4. Chemical durability measurement by aqueous leaching test in the corresponding region, as shown in Fig. 9(e)–(h), and element aggregation has not been observed even for a Nd2 O3 content of 25%. High density is usually considered to be helpful in improving the immobilization behavior of target radioactive elements. Fig. 10 shows the bulk density of vitrified forms. It indicates that all the samples have density values ranging from 2.813 g/cm3 to 3.215 g/cm3 . Furthermore, the density of the samples keeps a rising trend with enhanced Nd2 O3 content, except a slight decrease from 3.215 g/cm3 to 3.175 g/cm3 as the content of Nd2 O3 increased to 25 wt.%. This may be explained by the minor cracks in this sample in Fig. 7(d), which lead to a decline in structural compactness.

For glasses bearing radionuclides, the thermal, mechanical, chemical and irradiation stability are mainly required, and the chemical durability is preliminarily studied in this work [40,41]. The solidified bulks were studied according to PCT procedure to evaluate the leaching behavior of simulated nuclides in aqueous solution. Fig. 11 shows the normalized mass losses of Nd (NLNd ) in solidified soil body with different Nd2 O3 content, and their variation with time. Some attractive information can be drawn from Fig. 11. First of all, NLNd of all samples decrease rapidly from 3 to 14 days, and then gradually slow down. After 21 days,

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the NLNd of all samples remains almost unchanged (between 0.5 and 2 × 10−5 g/(m2 day)) with increasing time. This phenomenon may be due to the formation of a silica-rich layer at the interface with solution. Such a layer is known to slowdown the leaching rate [42,43]. Secondly, the NLNd of sample “25 wt.%” is significantly higher than other groups, especially in the first 21 days. This might result from the enhancement of neodymium content, and this phenomenon can also be deduced from other sets of data. However, the highest NLNd of all samples is still less than 1.6 × 10−4 g/(m2 day) within the discussed time, and it turns to below 6.37 × 10−6 g/(m2 day) after 42 days. This is comparable to the reported leaching rate of An and other simulated nuclides in glassy products (10−4 to 10−1 g/(m2 ·day)) [9]. It demonstrates the considerable chemical durability of vitrified forms obtained from the simulated radioactive contaminated soil. 4. Conclusions In summary, a series of simulated radioactive contaminated soil containing neodymium have been successfully vitrified by microwave sintering. Based on the phase, microstructure, and chemical durability analysis, the preliminary possibility of rapidly dealing with radioactive soil waste by microwave technology has been revealed. However, the treatment of radioactive contaminated soil is a huge and complex problem, and many further challenges must be considered in the follow-up works (e.g. the volatilization issue of cesium). In this paper, some basic conclusions could be summarized in the following. (1) A series of simulated radioactive contaminated soil containing 3–25 wt.% of Nd2 O3 has been successfully solidified by microwave sintering in 30 min. (2) The solidified bulks are of amorphous structure, and no bands related to Nd have been detected by XRD, FT-IR and Raman spectroscopies. (3) Homogeneous Nd distribution were well kept in all vitrified forms, and the solidified bodies have a density values ranging from 2.813 g/cm3 to 3.215 g/cm3 . (4) The soil solidified body exhibits excellent chemical durability, and the normalized elemental leaching rate of Nd was below 6.37 × 10−6 g/(m2 day) after 42 days. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21677118), the Pre-Research Foundation of Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory (No. 15yyhk01), the Postgraduate Innovation Fund Project by Southwest University of Science and Technology (16ycx033). Thanks for the technical supported from Engineer Lei Yuan in HUNAN CHANGE MICROWAVE TECHNOLOGY CO., LTD. References [1] A.O. Aidarkhanov, S.N. Lukashenko, O.N. Lyakhova, et al., Mechanisms for surface contamination of soils and bottom sediments in the Shagan River zone within former Semipalatinsk nuclear test site, J. Environ. Radioact. 124 (2013) 163–170. [2] Y.E. Dubrova, R.I. Bersimbaev, L.B. Djansugurova, et al., Nuclear weapons tests and human germline mutation rate, Science 295 (2002), 1037–1037. [3] D.E. Crean, F.R. Livens, M. Sajih, et al., Remediation of soils contaminated with particulate depleted uranium by multi stage chemical extraction, J. Hazard. Mater. 263 (2013) 382–390. [4] T.J. Yasunari, A. Stohl, R.S. Hayano, et al., Cesium-137 deposition and contamination of Japanese soils due to the Fukushima nuclear accident, Proc. Natl. Acad. Sci. U. S. A. 108 (2011) 19530–19534. [5] Y.G. Zhu, G. Shaw, Soil contamination with radionuclides and potential remediation, Chemosphere 41 (2000) 121–128.

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