| Issue |
Natl Sci Open
Volume 4, Number 1, 2025
Special Topic: Nuclear Environment Advances
|
|
|---|---|---|
| Article Number | 20240007 | |
| Number of page(s) | 4 | |
| Section | Earth and Environmental Sciences | |
| DOI | https://doi.org/10.1360/nso/20240007 | |
| Published online | 03 June 2024 | |
PERSPECTIVE
Promising porous materials for 99TcO4− removal from nuclear wastes
1
School of Environmental Science and Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, China
2
College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, China
* Corresponding authors (emails: h.yang@ncepu.edu.cn (Hui Yang); xkwang@ncepu.edu.cn (Xiangke Wang))
Received:
23
February
2024
Revised:
16
March
2024
Accepted:
8
April
2024
As a kind of green and clean energy resource, nuclear energy has attracted extensive attention and utilization [1]. Technetium-99 (99Tc) is a byproduct of 235U, which exists extensively in the spent fuel and radioactive waste generated during nuclear energy production (about 6%) [2]. The volatile 99Tc can escape during the vitrification process of nuclear waste, posing the risk of leakage [3]. 99Tc usually occurs in the form of pertechnetate anion (99TcO4−), under neutral, oxidative or even slightly reducing conditions. Due to characteristics of non-complexing and high water-solubility, 99TcO4− can diffuse into ecological systems easily, resulting in difficult separation during the treatment of contaminants. The 99TcO4− removal is complex from nuclear waste due to the high alkaline environment at Harford and locations like Savannah River, the strong acidic and various coexisting ions of highly radioactive liquid wastes [4]. Hence, the development of materials and techniques for 99TcO4− removal under practical conditions remains a significant challenge.
So far, a variety of materials have been developed for 99TcO4− removal, including layered double hydroxides (LDHs), anion exchange resins, inorganic composites (Sn(II)-based materials, Fe(II)-based materials, and so on), and others [5]. Among them, resins and LDHs are used for ion exchange, and some inorganic composites could convert Tc(VII) into insoluble Tc(IV). However, these inorganic materials have drawbacks such as poor acid or alkali resistance, low removal rate, and poor regeneration. Recently, porous materials have been considered as potential candidates for 99TcO4− removal and ReO4− (as a nonradioactive surrogate for 99TcO4−) by ion exchange, such as covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and organic polymers [5]. It is promising that these porous materials can overcome the limitations mentioned earlier, which also present some challenges during ion exchange, including (1) reducing costs and improving product yields in the preparation process of materials; (2) maintaining high stability and radiation resistance of materials under practical applications; (3) improving reuse and economic efficiency.
Ionic polymers have moved to the forefront of materials research because of their predictable structure type, tunable pore diameter, and flexible functionalization. Li et al. reported that SCU-CPN-1 (Figure 1A) has excellent radiation resistance and hydrolytic stability, which can reach the adsorption capacity of 876 mg/g for ReO4−. Meanwhile, SCU-CPN-1 exhibited cost-effective and practical application potential due to its decent radiation resistance and reusability [6]. Then, Li et al. prepared SCU-CPN-2 [7] and SCU-CPN-4 [8] for the uptake of 99TcO4− in nuclear waste. SCU-CPN-2 exhibited a high uptake capacity of 1467 mg/g for ReO4−, and SCU-CPN-4 showed fast removal kinetics and excellent uptake selectivity from the simulated SRS (Savannah River Site) solutions. Yang et al. utilized imidazolium-N+ nanotraps by tuning the surrounding environments via a halogenation strategy, which realized a high 99TcO4− adsorption capacity of 1434.1±24.6 mg/g, fast kinetics and excellent selectivity. The XPS, X-ray absorption fine structure (XAFS), and molecular dynamics (MD) simulations demonstrated the mechanism of ion exchange and the relationship between the functional structure (imidazolium-N+ sites assisted by halogen) and adsorption performance (Figure 1B) [9]. Overall, due to their simplicity, low cost, and highly efficient properties, the ionic polymer materials show promising application potential for 99TcO4− removal from nuclear wastes.
 |
Figure 1 Schematic showing (A) synthesis route of SCU-CPN-1 and its anion-exchange applications [6]; (B) the 99TcO4− removal by iCOPs under different conditions [9]; (C) the synthesis of Ag-TPPE through directly mixing raw materials [12]; (D) the application of NCU-3-X (X = Cl, Br) on ReO4−/TcO4− removal [13]; (E) the reaction mechanisms for the Ru@HNCC-R catalyzed extraction of ReO4− during the adsorption-electrocatalysis [14]. |
COFs have increasingly attracted attention for 99TcO4−/ReO4− and target radionuclide removal due to their high crystallinity, porosity, multi-functionality, pore space adjusting and stability properties [10]. For instance, Hao et al. reported a cationic pyridinium salt-based COF (PS-COF-1) with a specific surface area of around 2703 m2/g, exhibiting fast kinetics and high removal capacity towards ReO4−(1262 mg/g)/99TcO4−. Mechanism study showed that the saturation adsorption was attributed to ion exchange between ReO4−/ 99TcO4− and Cl− on the pyridine N(+) sites [11]. This work demonstrated the function of the pyridinium salt group and the possibility of COFs for environment remediation by virtue of radiation resistance, ordered pore structure and chemical stability.
Benefitting from the advantages of high specific surface area, high porosity, tunable shapes, and facile functionalization, MOFs have also received much attention for ReO4−/99TcO4− removal from aqueous solutions. Kang et al. synthesized a fourfold interpenetrated cationic MOF (Ag-TPPE) (Figure 1C) with structural stability under extreme conditions (8M NaOH). Ag-TPPE could selectively remove 99TcO4− in the presence of excess SO42− or NO3−. The removal percentages remained above 99% when the molar ratio of SO42−:ReO4− was 1000:1. In addition, the simulated potential of mean force (PMF) results revealed the molecular mechanism of ion exchange [12]. Furthermore, the introduction of halogen atoms (such as Br and Cl) into MOF skeletons can facilitate the adsorption process. Hu et al. regulated different halogen atoms to construct two isostructural MOFs, NCU-3-X (X=Br, Cl), in which NCU-3-Br exhibited a maximum capacity of 483 mg/g towards ReO4− (Figure 1D). The excellent performance could be attributed to the synergy of the halogen bond (between ReO4− and bromine atoms) and the interaction (between imidazole groups and ReO4−) [13].
In addition to these materials, carbon-based materials have also been studied extensively. Liu et al. reported hollow porous N-doped carbon capsules loaded with ruthenium cluster that was modified with the cationic polymeric networks (Ru@HNCC-R), which reached the removal capacity of 449 mg/g in 3M HNO3 for ReO4− via the adsorption-electrocatalytic method (Figure 1E) [14]. The generation of the insoluble Re(VI)O3 through electrocatalytic redox promoted the removal efficiency, which offered a new idea for ReO4−/99TcO4− removal from aqueous solutions. The developed Ru@HNCC-R represented good durability.
In recent years, significant progress has been made for 99TcO4− removal by porous materials both in adsorption capacity and selectivity, especially in the face of extreme conditions of nuclear waste environment. Although many effective adsorbents and promising adsorption/catalytic pathways have been reported, most investigations were conducted under laboratory conditions. The practicality, effectiveness, and stability of the material should be further approved through practical applications under nuclear waste environments. In addition, given the economic feasibility, the cost of the materials and the conditions for synthesis should also be taken a reasonable consideration. Improving the reuse of materials also dramatically reduces costs, especially for powdered materials. Therefore, it is particularly vital to develop materials with scalable, low-cost, efficient, and recyclable properties, which will benefit the disposal of nuclear waste and the sustainable development of the future nuclear energy industry. At the same time, compliance with environmental safety standards and government policies is the basis for ensuring the smooth conduct of all experiments.
Data availability
The original data are available from corresponding authors upon reasonable request.
Funding
This work was supported by the National Natural Science Foundation of China (22322603, U2167218 and 22276054) and the Beijing Outstanding Young Scientist Program.
Author contributions
Z.C., S.W., H.Y. and X.W. supervised the project. X.Z., M.H. and X.Y. wrote the manuscript. All authors reviewed and edited the manuscript.
Conflict of interest
The authors declare no conflict of interest.
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© The Author(s) 2024. Published by Science Press and EDP Sciences.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
All Figures
|
Figure 1 Schematic showing (A) synthesis route of SCU-CPN-1 and its anion-exchange applications [6]; (B) the 99TcO4− removal by iCOPs under different conditions [9]; (C) the synthesis of Ag-TPPE through directly mixing raw materials [12]; (D) the application of NCU-3-X (X = Cl, Br) on ReO4−/TcO4− removal [13]; (E) the reaction mechanisms for the Ru@HNCC-R catalyzed extraction of ReO4− during the adsorption-electrocatalysis [14]. |
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