Rapidly immobilization of simulated An3+ radioactive contaminated soil as glass-ceramics by microwave sintering

The development of nuclear technology leads to the generation of radionuclide contaminated soils, which require e�cient and rapid disposal methods. In this study, with microwave sintering method, the simulated radionuclides contaminated soils were successfully immobilized as glass-ceramics at 1200 °C with 30 min. The temperature effect, phase evolution, morphology, element distribution of simulated radioactive contaminated soil were investigated. The XRD results showed that Nd 3+ were immobilized into glass-ceramic phases with Nd 2 O 3 content increased up to 25 wt %. In addition, molecular dynamics simulation (MD) results showed the simulated radioactive contaminated soils were composed of [SiO 4 ] tetrahedron, and Si-O-Si bonds had an important effects on the glass structure forming.


Introduction
With the development and application of nuclear technology, nuclear energy has played an important role in energy eld [1].In the process of utilizing nuclear energy, the public health is threatened by the release of harmful radionuclides [2].Especially after Fukushima Daiichi Accident, the effective treatment and management of harmful radionuclides become a topic of global concern [3].Radionuclide contaminated soils are originated from the uranium mining, nuclear facility operation, nuclear decommissioning and nuclear tests, which pose a great threat to human health [4].Therefore, it is a challenging task to safely disposal of radionuclide contaminated soils.
In situ vitri cation (ISV) technology is a commonly effective method of remediation contaminated soils [5].Early in the 1980s, ISV technology by joule heating has been developed by the Paci c Northwest Laboratory of the US, and then applied to the disposal of contaminated soils in Maralinga nuclear test site of the UK [6].In the process of joule heating, the materials are heated to melting through high power currents, and then cooled with rapid speed.The traditional joule heating method requires the long heating time, and the uniformity of the solidi cation is di cult to obtain [7].Due to the advantages of homogeneous heating and remediation time reducing, microwave sintering technique has been applied in the eld of remediation contaminated soils [8,9].The microwave sintering technique can convert absorb microwave energy into heat energy, and the materials are heated at the same time [10].
To explore the effective methods of remediation contaminated soils, many scientists have devoted to microwave sintering technique [11][12][13][14][15][16][17][18].Jou [15] have reported that the Pb polluted soils are successfully immobilized by microwave sintering technique, with a low Pb leaching concentration.The hydrocarbon contaminated soils have been remediated through the microwave sintering technique, and the impact factors of microwave power and moisture content have been investigated [16].In our previous work, Cedoped simulated radionuclides contaminated soils [17] and uranium-contaminated soils [18] have been vitri ed with microwave sintering.Therefore, it can be inferred that the microwave sintering technique is an effective method to treat the contaminated soils.However, few works have been reported on the immobilization mechanism through the combined with experimental and theoretical methods.
In this work, simulated radionuclides contaminated soils were investigated by the experimental and theoretical methods.A series of simulated radionuclides contaminated soils were immobilized by microwave sintering at different temperatures.Nd 2 O 3 was considered to be a simulated radionuclides of An 3+ (such as Am 3+ , Pu 3+ , Cm 3+ ), owing to the same valence and similar ionic radius (R Nd3+ = 1.11Å, R Am3+ = 1.01 Å, R Pu3+ = 1.00 Å and R Cm3+ = 0.97 Å) [19].The phase evolution, morphology and element distribution of sintered samples are collected by using the experimental method.Molecular dynamics simulations were employed to exploring the structure information of simulated radionuclides contaminated soils.

Fabrication
The pristine soil composition was listed in Table 1, which was measured by XRF (Axios, Netherlands).To remove adsorptive water, the soil (200 mesh) and Nd 2 O 3 powder (Tianjin Kermel Co. Ltd., purity ≥ 99.99%) were preheated at 100 °C for 12 h.Four kinds of Nd 2 O 3 addition contents were selected as 10 wt %, 15 wt %, 20 wt % and 25 wt %, respectively.The mixed soil samples were weighed at 5.0 g.Then, the pristine soil and the Nd 2 O 3 powder were su ciently mixed in a mortar by grinding with ethyl alcohol (AR grade) until the mixed powder dried absolutely.All mixed soil samples were sintered by microwave sintering furnace (HAMiLab-M1500) at 1000 °C 1100 °C and 1200 °C for 30 min.The samples were heated at the rate of 30 °C/min, and then cooled down to room temperature.

Characterization
To con rm the phase structure of samples, X-ray diffraction (XRD, X'Per PRO, Netherlands) was used with Cu Kα radiation (λ = 1.5406Å).The scanning ranged from 10 o to 80 o with the scanning rate of 8 o /min.In addition, Fourier Transform Infrared Spectroscopy (FT-IR, PE, Spectrum One) was employed to obtain the detailed structure information.The micro-morphology and element distribution of samples were analyzed by using Scanning Electron Microscope (SEM, TM-4000, Hitachi) and energy dispersive spectrometry (EDS), respectively.

Computational methods
In MD simulations, the General Utility Lattice Program (GULP) code [20] was chosen to analyze the microstructure of system.The 25 wt % Nd 2 O 3 doped soil was selected.The Buckingham potential was employed to describe the inter-atom interaction, which can be expressed as follow [21] where r ij is the distance between atoms i and j, Z i and Z j are the effective charges of atoms i and j, A ij , ρ ij and C ij are the parameters of atom i-j interaction.The related parameters [21,22] of Buckingham potential were shown in Table 2. Around 2100 atoms were distributed randomly in simulations box, which were used as the initial modes of MD simulations.The 11 Å cutoff distance and 1 fs time step were applied in the whole simulations process.At rst, the canonical (NVT) ensemble was employed to heat the randomly distributed atoms to 5000 K for 100 ps.Then the systems were cooled to 300 K with the 5 K/ps cooling speed, and the NVT ensemble was applied in the cooling process.Lastly, at 300 K, the systems were reduced the inner stress with the microcanonical (NVE) ensemble for 100 ps, and the related trajectory data were collected by the nal 800 con gurations for structural analysis.

XRD analysis
In order to investigate the structure information of samples, the XRD patterns of the simulated radioactive waste samples are sintered at 1000 °C, 1100 °C and 1200 °C, which are shown in Fig. 1. Figure 1 (

Morphology analysis
Figure 2 shows the macro-morphology of simulated radioactive contaminated samples before and after sintering.The results show that the volumes of samples reduce after sintering, which are bene t to the transportation process and geological disposal [23,24].The sample of No.8 has a rough surface, and the samples of No. 12 and 16 have smooth and lustrous surfaces.
The SEM and corresponding element mapping images of sintered samples are investigated to observe the micro morphology of samples, which are presented in Fig. 3.The SEM results reveal that the holes exists in the sample surface at 1000 °C, while smooth surfaces are observed in the samples at 1100 °C and 1200 °C.Furthermore, Fig. 3 (d) -(f) show that the Nd elements present uniform distribution without enrichment phenomenon with the doped amount of 25 wt %.

FT-IR analysis
The detailed structure information of sintered samples has been measured by FT-IR for con rming the existing form of neodymium.Figure 4 shows the FT-IR spectra of sintered samples with 10-25 wt % doped content of Nd 2 O 3 at different temperatures.The range of main absorption peaks are located between 400 cm − 1 and 2000 cm − 1 .From Fig. 4 (a) to 4 (c), it can be seen that the absorption bands are located at around 1627 cm − 1 and 1633 cm − 1 , which are assigned to H-O-H bonds [25].In addition, weak exural vibration peaks of 1384 cm − 1 are attributed to the -CH 3 bonds, which may be attributed to the residual alcohol in the process of preparing samples [26].The strong and wide absorption peaks appear in the range of 850 cm − 1 to 1300 cm − 1 , which are related to the anti-symmetric stretching vibration peaks of Si-O-Si bonds [27].The characteristic bending vibrations ranging from 760 cm − 1 to 850 cm − 1 and at 467 cm − 1 are caused by Si-O bonds [28].Figure 4 (d) shows that the intensity of Si-O-Si bonds increase with the increase of temperature.The temperature has little effect on the intensity of H-O-H and -CH 3 bonds.Moreover, the FT-IR results show that no related Nd bonds are observed in all samples.Combining with the results of XRD and SEM, it can be inferred that the Nd has been solidi ed in the soil matrix.
According to the above discussed, the sintered samples mainly consist of the Si-O-Si bonds.To investigate the network structure of sintered samples, the numbers of bridging oxygen bonds (BO) and non-bridging oxygen bonds (NBO) are studied, which can be represented by the Q n (n is the number of bridging oxygen bonds) species of Si-O tetrahedron.Generally, the FT-IR bands at about 870 cm − 1 , 920 cm − 1 , 990 cm − 1 , 1090 cm − 1 and 1200 cm − 1 are assigned to the Q 0 , Q 1 , Q 2 , Q 3 and Q 4 , respectively [29]. Figure 5 shows the Q n species of 25 wt % Nd 2 O 3 doped soils at different temperature.It is shown that the network connectivity of sintered samples is affected by the temperature, and the non-bridging oxygen bonds translate into the bridging oxygen bonds.At 1000 °C, the Q 2 , Q 3 and Q 4 are 29.31%,44.07%and 26.62%, respectively.As the temperature increases, the numbers of Q 2 decrease, indicating that the numbers of NBO decrease in sintered samples.The numbers of Q 3 + Q 4 increase with the increase of temperature, which can explain that the temperature improves the intensity of Si-O-Si bonds.The sintered sample at 1200 °C has more than 75% of Si-O tetrahedrons with three and four BO, indicating that the glass network structure becomes stability.The structure information will be discussed by the following MD simulation results.

Structure analysis with MD simulations
MD simulations are applied to the system with 2100 atoms, and the structure corresponds to 25 wt % Nd 2 O 3 doped sintered soils, which is performed with the following molar ratio: 74. 53   Generally, the bond length can be deduced by the rst peak position of RDF [30].The bond length of Si-O is 1.62 Å in simulated soils, which is close to the experimental values of 1.61 Å [31].The bond length of Al-O is 1.75 Å, which is consistent with the experimental results of Extended X-ray absorption ne structure (EXAFS) [32].Our simulated results agree well with the experimental results, indicating that the simulated method is reasonable.The order of bond length is Si

Conclusion
In summary, the simulated radionuclides contaminated soils were rapidly immobilized as glass-ceramics material by microwave sintering at 1200 °C with 30 min.With the increasing of temperature, the crystal diffraction peaks of decreased, and the diffraction peaks of amorphous phase increased, indicating that the glass forming ability increased at elevated temperature.Nd 3+ were immobilization into glassceramics structure with the Nd 2 O 3 doped content increases up to 25 wt%.The SEM results showed that Nd element distributed uniform in the soil matrix.Experimental and theoretical results showed that the sintered soil samples were mainly composed of [SiO 4 ] tetrahedron, which will affect the stability of glass network structure.The microwave sintering has been posed to be a potential method for rapidly and effectively disposing of radionuclides contaminated soils.
a) shows that the main peaks of sintered samples are SiO 2 and Ca 2 SiO 4 with the 10 wt % Nd 2 O 3 content.When the amounts of Nd 2 O 3 range from 20 wt % to 25 wt %, the peaks of Ca 2 SiO 4 disappear, and the peaks of Mg 2 SiO 4 and Ca 3 Mg(SiO 4 ) 2 appear.This suggested that Ca 2 SiO 4 is related to the forming of Mg 2 SiO 4 and Ca 3 Mg(SiO 4 ) 2 .With the Nd 2 O 3 content increasing, the intensity of SiO 2 peaks increases.As shown in Fig. 1 (b), it can be seen that the peaks of NaCaAlSi 2 O 7 disappear and the peaks of Mg 2 SiO 4 , Ca 3 Mg(SiO 4 ) 2 and amorphous phase appear until the Nd 2 O 3 content increases up to 15 wt %.The intensity of SiO 2 peak increases with the increasing of Nd 2 O 3 content.The XRD results of samples with 10-25 wt.% Nd 2 O 3 concentrations at 1200 °C are shown in Fig. 1 (c).It is shown that the crystal peak of SiO 2 and the amorphous characteristic are detected when the amounts of Nd 2 O 3 are in the range of 10 wt % to 25 wt %.As the Nd 2 O 3 content increase, the main peaks of SiO 2 increase, and the peaks of Mg 2 SiO 4 and Ca 3 Mg(SiO 4 ) 2 appear.

Figure 1 (
Figure 1 (d) shows that the XRD results of samples with the doped contents of 25 wt.% Nd 2 O 3 at different temperatures.When temperature varies from 1000 °C to 1200 °C, the main peaks of SiO 2 , Mg 2 SiO 4 and Ca 3 Mg(SiO 4 ) 2 are detected, and the peaks of amorphous phase at 2θ = 20°-30° are observed, indicating that the glass-ceramic phases appear in sintered soil samples.With the increase of temperature, the crystal diffraction peaks of SiO 2 , Mg 2 SiO 4 and Ca 3 Mg(SiO 4 ) 2 decrease, and peaks of amorphous phase become distinct.It can be deduced that the glass forming ability increases at elevated temperature.In addition, the XRD results show no obvious crystal peaks of the related Nd compounds are observed with the doped content of 25 wt % Nd 2 O 3 , indicating that Nd 3+ are immobilized into glassceramic phases.
2 and 0.07 mol % Nd 2 O 3 , as shown in Fig.6.It is shown that Nd distributes uniform in our simulated sintered soils, which agrees with the experimental results.

Figure 7
Figure 7 shows the simulated radial distribution function (RDF) for 25 wt % Nd 2 O 3 doped sintered soils.
which is presented in Fig.7.It can be assumed that Al and Si atoms are close to O atoms.As shown in Fig.7, the rst peaks of Si-O are narrow and sharp, whereas the rst peaks of Nd-O are board and low intensity.It is implied that the bonding ability of Si-O is strongest in all bonds.From above Q n species analysis, it is shown that the numbers of [SiO 4 ] increase as the temperature increases.It can be concluded that the Si-O bonds play an important role in forming glass ability.Simulated bond angle distributions for 25 wt % Nd 2 O 3 doped sintered soils are shown in Fig. 8.It is shown that the bond angles of O-Si-O and Si-O-Si are around 105° and 136°, which indicates [SiO 4 ]tetrahedron exists in our simulated sintered soils.It is consistent with the Q n results that more than 75% of Si-O tetrahedrons with three and four BO.With high energy X-ray diffraction, Poulsen et al[32] investigated that the bond angles of O-Si-O and Si-O-Si are around 109° and 147°, respectively.The main peak of O-Al-O bond angle distributions is around 104°, which is lower than that of O-Si-O.The O-Al-O bond angle distributions have two apparent peaks: one main peak at around 117° and the other small peak at around 91°.

Figures
Figures

Table 1
Composition of the pristine soil.

Table 2
Parameters of the inter-atomic interaction potential.