3.1 Microstructures and phase determination of the simulated slag
Fig. 4 shows the microstructures of the simulated slag sintered in air at different temperatures. The individual particles can be detected when sintered at 1000 ℃, which indicates that the particles have not been melted. When the sintering temperature rises up to 1100 ℃, the particles are partially melted and evident sintering necking can be seen. Besides, the size of particle increases obviously. When the temperature reaches up to 1200 ℃ and 1300 ℃, the microstructure shows the solidification structure of molten liquid, suggesting that severe reaction may occur during the sintering process. Besides, the locally globular protrusions can be clearly seen on the surface of the simulated slag even sintered at 1300 ℃.
Fig. 5 shows the microstructure of simulated slag sintered in argon at different temperature. The slag shows two specific morphologies: (I) When the sintering temperature is lower than 1100 ℃, the particles are relatively independent with lots of pores, which means that the reaction is insufficient. (II) The particles are melted and then form densified products with smooth and flat morphology when sintered at 1200 and 1300 ℃. Compared with the simulated slag sintered in air, it can be inferred that the reaction is severer in argon, because of the disappearance of locally globular protrusions.
Fig. 6 shows the XRD spectrum of simulated slag sintered at different temperature in air. It obviously shows that Fe2O3 and Fe3O4 peaks occur when sintered at 1100 and 1200 ℃ rather than fayalite phase, and also remains the grainy structure. The high content of oxygen inhibits the formation of fayalite and/or promotes the oxidization of fayalite. Afterwards, the Fe-Si oxide phase ((Fe0.914Si0.086)(Fe0.998Si0.002)2O4, PDF#89-6227, referred to as “Fe3-xSixO4” hereafter) is formed owing to the further reaction among silica, Fe2O3 and Fe3O4 particles. However, some silica particles can also be detected in the simulated slag when sintered at 1300 ℃ because of the high melting point and relatively low diffusion rate of SiO2.
When sintered in argon, fayalite phase can be formed even at 1000 ℃, as the XRD spectrum shown in Fig. 7. However, silica particles can not be reacted completely, and therefore inhibit the diffusion of atoms in the simulated slag. Thus, the simulated slag presents porous morphology when sintered below 1100 ℃. The Fe3O4 and silica phases disappear when the sintering temperature reaches up to 1200 ℃, which can also be confirmed by the smooth and flat morphology of the simulated slag. In addition, the diffraction peaks of arsenic-bearing phases can not be found because of the relatively low content of arsenic and its compound.
3.2 TCLP tests
TCLP tests have been utilized to evaluate the stabilization of As elements inside the simulated slag sintered at different temperature and atmosphere, as shown in Fig. 8. It can be seen that the leached arsenic concentration of the simulated slag sintered in air at 1000 ℃ and 1100 ℃ reaches up to 188.7 and 243.3 mg L-1, respectively, much higher than the limitation of arsenic concentration (5.0 mg L-1) stipulated by the identification standard for hazardous substances (GB 5085.3-2007). The leaching concentration of As increases when the sintering temperature improves from 1000 ℃ to 1100 ℃ in air, which can be attributed to the improved mobility of arsenic atoms. When sintered at 1200 ℃ and 1300 ℃ in air, the leaching concentration of arsenic is as low as 2.916 and 0.339 mg L-1, respectively. It can be inferred that As elements are stabilized in the simulated slag. As for the simulated slag sintered in argon, the leached arsenic concentration satisfies the identification standard for hazardous substances except sintered at 1000 ℃. According to the XRD spectrum, we can confirm that fayalite phase possesses better As stabilization ability than that of Fe-Si oxide phase.
3.2 XPS analysis of the simulated slag
In order to study the valence state of As and Si elements inside the simulated slag, XPS analysis has been applied. As shown in Fig. 9, the peak of Si2p shifts towards to the low binding energy with increasing temperature, indicating the increasing of the valence electron density of Si atoms (Dalby et al. 2007). The research (Zhao et al. 2016) certifies that the Si-O-Si bindings can be translated into Si-O-Fe and Si-O-As bindings when arsenic atoms are introduced into glass. As can be seen in Fig. 9(a), there exists two types of Si (Zhao et al. 2016) when sintered at 1000℃ in air: One is the bond for bridging oxygen which occurs at high binding energy (103.59 eV) and the other is the bond for non-bridging oxygen at low binding energy (102.47 eV). The relative percentages of these two types of Si are 81.4% and 18.6%, respectively. Thus, it can be deduced that silicon atoms existed as Si-O-Si bond do not link to other atoms. When sintering temperature reaches to 1200 ℃, as show in Fig. 9(b), Si atoms exist as Si-O-Si, Si-O-Fe and Si-O-As bindings, and the corresponding percentages are 65.34%, 27.5% and 7.16%, respectively. The existence of Si-O-Fe bond is due to the formation of Fe-Si oxides during sintering process. When sintered at 1100 ℃ and 1200 ℃ in argon, as show in Fig. 9(c) and (d), the proportion of Si-O-As are 13.75% and 15.76%, respectively. It is evidently that with the increase of temperature, Si-O-Si bonds can be replaced by Si-O-As bonds. The Si-O-As bond is rather stable and thereby reduces the leaching concentration of As in solution.
The spectrum of As3d under different conditions is shown in Fig. 10. According to the previous research (Bang et al. 2005), the binding energy of AsO43- and HAsO42- was at 44.9 eV and 45.5eV, respectively. In this work ,only As(V) peak (AsO43-) can be detected in As3d spectrum when sintered in air. The peak is shift towards to the low binding energy with the increase of temperature. It is due to the electron density of As is higher than Si in Si-O-As, causing the electron density around As atom increases and the binding energy of arsenic shift to lower energy. It can be deduced that the arsenic have participated in the glass structure in the form of Si-O-As. In argon, XPS spectra of As in Fig. 10 shows that the binding energy at 43.8 eV is regarded as As (III), while the binding energy at 44.9eV is presented as AsO43-. The relative percentages of As (III) at these two temperatures are 29.3% and 20.7%, respectively. As (III) can be formed through the reduction of As (Ⅴ) by Fe. The As(V) is more stable and less toxic than As(III) (Wang et al. 2019; Xiu et al. 2016), and thus it is favorable to immobilize arsenic as As(V). With the increase of temperature, the percentage of As (III) decreases. Therefore, it is favorable for arsenic compound to replace SiO4 and form As-O-Si covalent bond, resulting in the enhanced properties of arsenic solidification and stabilization of the simulated slag.
3.5 Theoretical calculation
In order to identify the promotional effect of diffusion rate by formed fayalite phase, the FPMD calculation is applied. Here, the (Ca0.5Fe0.5)H(As0.5Si0.5)O4 structure is used to simulate the mixture of calcium hydrogen arsenate (CaHAsO4) and fayalite. The diffusion rate and the mean square displacements of different atoms can be calculated according to the following equation:
Where ri is the displacement of the atom and t represents the time.
Fig.11 (a) and (b) show the mean square displacement of different atoms in CaHAsO4 and (Ca0.5Fe0.5)H(As0.5Si0.5)O4 structures, respectively, which can directly reflect the atomic diffusion behaviors (Wu 2004). The results show that the atom diffusion in (Ca0.5Fe0.5)H(As0.5Si0.5)O4 structure is evidently higher than that in CaHAsO4, which can be inferred that the fayalite phase can promote the movement of atoms and therefore enhance the diffusion rate. As discussed above, the simulated slag shows smooth and flat morphology sintered in argon while that possesses locally globular protrusions sintered in air, which can be attributed to the formation of fayalite phase and thus accelerate the diffusion rate in argon. However, the promotional effect of diffusion rate by Fe2O3 and Fe3O4 phases are evidently lower than that of fayalite phase, and thus, the melting temperature of simulated slag is rather high when sintered in air.
Fig. 12 shows the simulated structure of (Ca0.5Fe0.5) H(As0.5Si0.5)O4 by sintering of CaHASO4 and Fe2SiO4 powders. The AsO4 and SiO4 tetrahedrons can be seen in the simulated structure, reflecting the occurrence of As-O-Si covalent bond. As mentioned above, As-O-Si covalent bond is relatively stable. Therefore, As atoms can not escape easily from the crystal, and therefore inhibit the leaching of As atoms.
In summary, we can confirm that the stabilizing ability of As elements is improved when the content of oxygen is relatively low, which can be ascribed as the following reasons: (I) The fayalite phase can be preserved at high temperature with low content of oxygen; (II) The fayalite phase inside the slag can reduce the melting temperature and enhance the diffusion rate; (III) As atom can take place of Si atom through diffusion process and form AsO4 tetrahedron; (IV) The stability of Si-O-As covalent bond is rather high, which can inhibit the extraction of As from the slag.