Trinh Dinh Dinhab,
Dongxiang Zhang*a and
Vu Ngoc Tuanc
aSchool of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 102488, China. E-mail: boris@bit.edu.cn; Tel: +86 13366112230
bVilas 849, Quality Testing Lab, Center for Research and Development Science Technology Tien Nong, Thanh Hoa, 442410, Vietnam
cFaculty of Electric-Electronic Engineering, Nam Dinh University of Technology Education, Nam Dinh, 420000, Vietnam
First published on 7th April 2020
The effective adsorption of radioactive iodine is greatly desirable, but is still a significant challenge. In this manuscript, we report the synthesis of a bismuth-modified zinc aluminium layered double hydroxide (BiZnAl-LDH) via a co-precipitation method for the highly efficient absorption of iodine. Based on the robust chemical attraction between Bi and I2, BiZnAl-LDH exhibited highly effective iodine capture. Furthermore, to evaluate BiZnAl-LDH as an effective sorbent, it was characterized via X-ray powder diffraction (XRD), scanning electron microscopy-energy dispersion spectroscopy (SEM-EDS), and Fourier-transform infrared spectroscopy (FITR). In addition, to determine the morphology and iodine adsorption properties of BiZnAl-LDH, several studies were conducted. Through experiments, its elemental composition and vibration before and after iodine adsorption were analyzed via EDS and X-ray photoelectron spectroscopy (XPS). During the capture process, I2 is reduced to I− by the intercalated Bi3+ via chemical adsorption, and the maximum adsorption capacity of BiZnAl-LDH for iodine reached up to 433 mg g−1, which had a surface area, average pore diameter, and pore volume of 36.259 m2 g−1, 2.374 nm, and 0.128 m3 g−1, respectively. Compared with several previous sorbents for iodine adsorption, BiZnAl-LDH exhibited an iodine adsorption of approximately two times that of the commercial Ag-exchange zeolite X, and furthermore BiZnAl-LDH is cost-effective. Thus, the substantial iodine capture by BiZnAl-LDH indicates that it is a capable sorbent for the effective elimination of radioactive iodine from reprocessing plant emissions.
Among the existing isotopes of iodine, 127I is the most common and stable one. Also, 127I can be commonly found in foodstuff, where it is used as an additional nutritional supplement (e.g. iodized salt). Furthermore, considering its application potential as an antimicrobial agent, it is also used for immediate water sterilization in emergency or critical cases.5 In addition, to reduce the chances of the physical uptake of rarer radioactive isotopes from the environment, 127I can be used in large doses. Nevertheless, studies reveal6,7 that this approach is dangerous in the long term.
As a result of its application in nuclear industrial energy, the 131I isotope has received considerable attention in recent years. 131I has found successful applications in the medical industry, where it has become a widely used product for medical imaging in nuclear medicine and radiation therapy. As a byproduct of the nuclear energy industry, where it is produced as a result of the fission of uranium, its acquisition, and application in countries that do not use atomic energy is limited. Based on the comparatively short radioactive half-life of 131I, which is 8 days, it is considered a major health hazard. In some cases, it has been found where a nuclear accident occurred within the first week.8 On the other hand, the 129I isotope has a long-term radioactive half-life of over a few million years and is substantially less dense than other isotopes. However, 129I is of significant interest in several fields and disciplines such as geological and environmental sciences. To quantitatively measure the human impact on the atmosphere, 129I can be used as a significant unit measure, in particular the effect of the nuclear blast during nuclear bomb testing on the new atmosphere.9
There is a vast body of research primarily focused on the radioactive waste products from the exhaust stream. Indeed, there are well-known wet processes designed to trap radioactive iodine. For example, mercury and alkaline scrubbing have proven to have highly effective removal capacities, anion exchange resins,10,11 zeolite-based materials,12,13 Ag-based material,14,15 porous metal–organic frameworks (MOFs),16–19 Cu-based materials,20,21 and in the case of adsorbents for the adsorption of radioactive I−, activated carbon.22 In addition, several research groups have studied the depletion potential of different iodine species containing I− and IO3− from aqueous solutions of layered double hydroxides (LDHs).8,9
LDHs are known as anionic clays materials or hydrotalcite-like compounds, which are coating-stratified clay metals based on a similar structure to the brucite Mg(OH)2. LDHs also present a layered structure that carries a net positive charge. Their exchangeable anions are balanced by the positive charge constituents of the alternating cation layers in the alternating regions of LDHs. The common formula of LDHs is characterized by M2+1−yM3+y(OH)y+2(An−)y/n·mH2O where the divalent metal M2+ is Ni2+, Cu2+, Mg2+ and Zn2+ cation, the trivalent metal M3+ is Ti3+, Al3+, Bi3+ and Fe3+ cation, usually chloride, nitrate or carbonate is the exchangeable anion and y is usually 0.2 < y < 0.33.23,24 The structure and composition of LDH compounds have been the focus of considerable research. In addition, the recent work by Mills et al. based on hydrotalcite LDHs exhibited an interesting property in terms of composition and nomenclature called the reform effect (or remembrance).25 Basic metal adsorbents have been effectively applied in iodine adsorption. Moreover, Nenoff and colleagues conducted substantial work aimed at advancing layered hydrotalcite-like oxides-I-Bi as waste forms for iodine precipitation. As a result, it was found that the iodine phase composition in oxides-I-Bi was determined according to the Bi:
I ratio,26,27 and the compounds containing bismuth showed several important advantages such as the ability to react with iodine and iodide capture capacity, and Bi2O3 was also comprehensively investigated for iodine immobilization adsorption.28–30
It was reported that the surface modification of layered double hydroxide electrodes with an additive, such as Bi, can enhance the chemical properties of the active materials.31–33 Also, layered double hydroxide materials have been investigated as additives due their effect on the environment. The main additives to Al include bismuth and other non-metallic elements, including other bismuth compounds and Bi2O3. These bismuth compounds can prove the linkage efficiency between Bi and the zinc metal matrix.34 Moreover, an Al–Bi mixed system can allow Bi-phase stabilization and overcome some of the limitations of Al.
We investigated bismuth for the capture of iodine gas. The reactions between iodine and bismuth demonstrate the characteristic properties of the gas–solid reaction in the iodine adsorption process. Table 1 clearly shows the highly exothermic nature of this reaction and that the iodine adsorption capacity of Bi is better than that of silver and Bi2O3.
Reaction | ΔG (kcal) | Spontaneity |
---|---|---|
Ag + 1/2I2(g) = AgI | −16.059 | Spontaneous |
Bi + 1/2O2(g) + 1/2I2(g) = BiOI | −52.428 | Spontaneous |
Bi + 3/2I2(g) = BiI3 | −33.376 | Spontaneous |
5Bi + 7/2O2(g) + 1/2I2(g) = Bi5O7I | −274.993 | Spontaneous |
5/2Bi2O3 + 1/2I2(g) = Bi5O7I + 1/4O2(g) | −5.867 | Spontaneous |
2Bi2O3 + 2I2(g) = 4BiOI + O2(g) | 5.588 | Nonspontaneous |
Bi2O3 + 3I2(g) = 2BiI3 + 3/2O2(g) | 40.898 | Nonspontaneous |
High iodine adsorption efficiency is highly desirable, but there are still major challenges in long-term iodine storage requirements for adsorbent materials. Recently, many LDH-type materials (such as NiTi-LDH and MgAl-LDH) have shown high iodine adsorption efficiency based on chemical adsorption. However, the chemical adsorption of iodine is still limited in an aqueous environment.4,9 Chemical adsorption combined with physical adsorption on NiTi-Sx-LDH in a humid air environment was considered to be an effective method for iodine adsorption for short-term storage.36
Herein, various LDHs compounds were synthesized and characterized, where different amounts of Bi were used to modify the chemical activate characteristics of the LDHs. The experiments used the strong chemical attractions between iodine and bismuth in ZnAl LDH modified by Bi absorbents. Accordingly, BiZnAl-LDH was applied for iodine adsorption and storage over long periods.
![]() | (1) |
![]() | ||
Fig. 2 (a) FTIR spectra of ZnAl-LDH (B0), Bi0.2ZnAl0.8-LDH (B2), Bi0.3ZnAl0.7-LDH (B3) and Bi0.4ZnAl0.6-LDH (B4). (b) FTIR spectra of BiZnAl-LDH (Bi/Zn/Al=0.4![]() ![]() ![]() ![]() |
The FTIR spectrum in Fig. 2b after the sorption experiment shows that the I− ion was present in BiZnAl-LDH-I. The strong bands in the range of 400–700 cm−1 are due to the LDH-iodine crystal lattice vibrations (BiI3 and I2–ZnAl2O4).
The characteristic SEM images of BiZnAl-LDH with different Bi/Zn/Al mole ratios and ZnAl-LDH are depicted in Fig. 3. It is obvious that all three BiZnAl-LDH samples exhibit a layered hexagonal structure similar to the typical structure of ZnAl-LDH with the layered hexagonal structure of ZnAl2O4.39 The particle size and thickness of BiZnAl-LDH is about 200–300 nm and 40 nm, respectively. These results show that the lattice structure of the material was not destroyed after the addition of the appropriate amount of Bi.40 Thus, based on the analysis above, BiZnAl-LDH was successfully synthesized through the co-precipitation method.39
Furthermore, the layered hexagonal structure of BiZnAl-LDH was studied via EDS, HRTEM, and TEM, as showed in Fig. 4. The TEM image in Fig. 4a and HRTEM image in Fig. 4b demonstrate that BiZnAl-LDH is mostly composed of different diameters of the hexagonal spherical particles. The typical HRTEM image, as presented in Fig. 4b, shows the layer structure of BiZnAl-LDH with d-spacings of 0.23 and 0.24 nm, corresponding to the different lattice planes of Bi and ZnAl2O4, respectively.41–43 Fig. 4d shows the EDS mapping of a spherical particle, which was used to determine the composition of the hexagonal spherical particles of BiZnAl-LDH, confirming the results from the XPS analysis.
![]() | ||
Fig. 4 (a) TEM image, (b) HRTEM image, (c) EDS results and (d) element mapping image of BiZnAl-LDH (Bi/Zn/Al=0.4![]() ![]() ![]() ![]() |
The XRD patterns of the samples are compared in Fig. 5. According to JCPDS: 48–1023, ZnAl-LDH shows extreme diffraction peaks located at 2θ = 11.98°, 23.84°, 35.06° and 60.78°, corresponding to the (003), (006), (009), and (110) diffraction planes of ZnAl-LDH, respectively. BiZnAl-LDH exhibited similar diffraction peaks corresponding to the planes of ZnAl-LDH, and the total volume from the calculated LDH was greater than 80% of the reaction product. A broader peak indexed to Bi was discovered in BiZnAl-LDH due to the low percentage of Bi in BiZnAl-LDH at 2θ = 30.46°, which corresponds to the diffraction pattern of Bi (ICDD PDF No. 44-1246). As the molar ratio of Bi increased, the typical of XRD peaks of Bi become more intense, especially with the B4 sample, which showed the maximum intensity. Thus, based on the BiZnAl-LDH hexagonal structure reported in the literature (JCPDS No. 05-0669), the diffraction peak locations for ZnAl-LDH exhibited no change. All samples exhibited the characteristic diffraction peaks of the BiZnAl-LDH material, indicating that the layer structure of the LDH was not destroyed by the bismuth atoms.39
![]() | ||
Fig. 5 XRD patterns of ZnAl-LDH, Bi0.2ZnAl0.8-LDH (B2), Bi0.3ZnAl0.7-LDH (B3) and Bi0.4ZnAl0.6-LDH (B4). |
TG and DTG measurements were performed on the BiZnAl-LDH sample, and the results are shown in Fig. 6. According to the TG and DTG results for BiZnAl-LDH, three weight loss regions were observed. Specifically, the region between 40 °C to 200 °C (determined at 169.4 °C in DTG) is attributed to the loss of physically adsorbed water, between 200 °C to 600 °C (determined at 284.6 °C in DTG) is due to the loss of interlayer water dihydroxylation of the layered structure, and over 500 °C is due to the decomposition of carbonate, which is consistent with that for the previously reported ZnAlCO3 LDH.44 Similarly with LDH, the intrinsic form of dehydration at 40–200 °C (at 60.6 °C and 158.8 °C in DTG) and LDH layered structure of dihydroxylation distributed between 200 and 600 °C (especially the volume dropped sharply at 224.3 °C in DTG) can also be identified in the curves of BiZnAl-LDH.
![]() | ||
Fig. 6 TG-TDG curves of BiZnAl-LDH (Bi/Zn/Al=0.4![]() ![]() ![]() ![]() |
![]() | ||
Fig. 7 (a) Optical image of BiZnAl-LDH, (b) optical image of BiZnAl-LDH-I, (c) EDS result and (d) element mapping image of BiZnAl-LDH-I. |
The functional groups and components of the material would have changed due to iodine adsorption. Therefore, we used XPS, XRD, and further characterization methods to further describe the reaction between BiZnAl-LDH and I2. In Fig. 8, the XRD patterns of BiZnAl-LDH and BiZnAl-LDH-I show that the sorbents retained their typical structure after the iodine adsorption of hydrotalcite, and the characteristic absorption peaks of LDH still existed. The location of the peaks shifted and their intensity changed. In Fig. 8, the XRD pattern of BiZnAl-LDH-I after iodine adsorption demonstrates a change in the intensity of the peaks because of the nature of the hydrotalcite layers of the adsorbed iodine molecules.45,46
In Fig. 8, the XRD pattern of BiZnAl-LDH-I shows similar diffraction peaks to that of BiZnAl-LDH before iodine adsorption. Also, the characteristic adsorption peaks of LDH such as (003), (006), (009), and (110) still existed. However, the position and intensity of the peaks changed clearly after iodine adsorption. This shows that after iodine adsorption, the basal (d003) spacing of 0.24 nm in BiZnAl-LDH increased to 0.32 nm in BiZnAl-LDH-I. This indicates that the iodine exists in BiZnAl-LDH-I in the form of BiI3, and also clearly shows that the atoms that compose the ZnAl-LDH crystal such as ZnAl2O4 participate in the iodine adsorption process. Also, the significant deviations in the intensity, and simultaneous shift in the position of the peaks in the XRD pattern of BiZnAl-LDH-I are clearly demonstrated in the charge displacement produced in the XPS result in Fig. 9b for Bi 4f, Fig. 9c for Zn 2p and Fig. 9d for I 3d.
![]() | ||
Fig. 9 (a) XPS survey spectra, (b) Bi 4f, (c) Zn 2p and (d) I 3d of BiZnAl-LDH (1) and BiZnAl-LDH-I (1-iodine). |
The survey XPS spectra (Fig. 9a) of before BiZnAl-LDH and after BiZnAl-LDH-I iodine adsorption show the presence of peaks for Zn 2p, Al 2p, Bi 4f, S 2p, and O 1s. After iodine adsorption, as shown in Fig. 9a, a new peak appeared at a binding energy of 618.6 eV for the BiZnAl-LDH-I sample, indicating that iodine was adsorbed. According to the XPS spectrum of I 3d (Fig. 9d), two peaks of I3− 3d3/2 (at 630.1 eV) and I3− 3d5/2 (at 618.6 eV) appeared.47,48 Fig. 9b shows that the peaks of Bi 4f7/2 and Bi 4f5/2 in BiZnAl-LDH at 158.3 eV 163.6 eV transformed to the peaks of Bi 4f7/2 and Bi 4f5/2 in BiZnAl-LDH-I at 158.7 eV and 163.9 eV, respectively. However, the peaks of Zn 2p3/2 and Zn 2p1/2 also shifted from 1021.1 eV and 1044.2 eV in BiZnAl-LDH and to 1021.3 eV and 1044.4 eV for BiZnAl-LDH-I, respectively. In addition, a small volume of Al was also involved in iodine absorption process, when there is also small electronic shift from 73.7 eV to 73.9 eV. This can be explained by the electron adsorption of iodine atoms by the Bi atoms, Zn atoms and Al atoms. In the I 3d spectrum of BiZnAl-LDH-I, the peaks of I2 at 618.6 eV and 630.1 eV shifted to 619.4 eV and 630.9 eV, respectively, for the molecule of BiI3 containing iodine atoms, and 619.6 eV for iodine atom in ZnI2.30,49,50
In Fig. 6c, the curve (TGA) of BiZnAl-LDH-I shows three mass loss region. The first mass loss in the range of 40 °C to 200 °C is mainly due to the elimination of some hydration water and physically adsorbed iodine.36 The second mass loss (200–350 °C) is attributed to the further removal of dehydroxylated hydrotalcite. The third obvious mass loss (350–600 °C) is assigned to the removal of I− in the metal iodide. Thus, according to the comparison of the TGA curves of BiZnAl-LDH and BiZnAl-LDH-I, it can be concluded that the adsorption process is mainly based on chemical adsorption.4
3/2 I2 + Bi = BiI3 | (2) |
Fig. 10 shows the result of the physical adsorption during the second step of the aforementioned mechanism for the iodine adsorption, where a relatively small quantity of iodine exists in the form of I2 in the BiZnAl-LDH compound. Finally, in the third step of the mechanism, a considerable quantity of iodine is captured due to ZnAl2O4, which is an expected result considering the chemical doping nature of the reaction involved in this third and final of the adsorption mechanism.
This journal is © The Royal Society of Chemistry 2020 |