Figure 1. Highly selective capture of Cs+ ions by a highly stable layered metal sulfide (KInSnS4) under strongly acidic conditions.
Cesium (137Cs) is one of the main sources of radioactivity in spent fuel. It possesses long half-life and emits high-energy γ rays, and thus has a significant impact on the storage and disposal of nuclear waste. Moreover, it is highly soluble and easy to migrate in the environment and biosphere, posing serious threats to human health and environmental safety. Therefore, the rapid and selective capture of radioactive Cs+ ions from complex solutions, especially the high-level liquid wastes (HLLWs), is of great importance to environmental protection, human health and the sustainable development of nuclear energy. However, HLLWs are generally acidic and extremely complex, containing not only Cs+, but also Sr2+, Ln3+, Na+ and so on, which poses a great difficulty for the selective sequestration of Cs+ ions from HLLWs. The ion-exchange has received much attention as an effective method for the removal of radioactive cations owing to its advantages of convenience, high efficiency, low cost and low secondary contamination. However, its application in the capture of Cs+ ions from acidic solutions are scarcely reported as most current ion exchange materials don’t well perform under the acidic conditions due to the stability issue of materials and the strong competition of protons. On the other hand, the Cs+ capture mechanism under strongly acidic conditions is unclear.
To solve the above problem, we have constructed a robust layered metal sulfide ion exchange material KInSnS4 (InSnS-1) inspired by the relatively stable sulfides KMS-1 and KMS-5. InSnS-1 can effectively capture Cs+ under both neutral and acidic conditions by the ion exchange method (Figure 1). InSnS-1 has excellent acid and irradiation resistances. It can maintain its layered network under strongly acidic (1 - 4 mol/L HNO3) and irradiation (200 kGy γ irradiation) conditions. It possesses fast kinetics (teCs ≤ 5 min, RCs ≥ 90%) and high adsorption capacity (qmCs = 316 mg/g) for Cs+ ions in neutral solutions (Figure 2), which is higher than that of most Cs+ ion-exchangers[1, 3-5].
Figure 2. Adsorption kinetics, adsorption isotherms, pH-dependent studies and irradiation stability studies of InSnS-1 for the removal of Cs+ ions under the neutral and acidic conditions (1 M = 1 mol/L HNO3, 2 M = 2 mol/L HNO3, 3 M = 3 mol/L HNO3).
Materials that can remove Cs+ under strong acid conditions are currently very limited, which mainly focus on ammonium phosphomolybdate and cupric aromatic crown ethers and their composites[6-9]. Here, InSnS-1 can still effectively capture Cs+ ions in 1 mol/L HNO3 solution (qmCs = 98.6 mg/g, Figure 2). InSnS-1 exhibits the excellent selectivity for Cs+ ions (Figure 3) even in the presence of high concentrations of competing Mn+ ions (Mn+ = Na+, Sr2+, Ca2+ and La3+) and has high separation factors SFCs/M in strongly acidic solutions (1 and 3 mol/L HNO3).
Given the good acid stability of InSnS-1, we systematically investigated the effect of H3O+ on its selective capture of Cs+. In this process, we were pleasantly surprised to discovered that the presence of H3O+ is very favorable for enhancing the selective trapping ability of InSnS-1 for Cs+. This may be attributed to the repulsive effect of H3O+ on Na+, Sr2+ and La3+. H3O+ has a small radius and positive charge. Sr2+ and La3+ also have relatively small radii and highly positive charges compared to Cs+. Na+ has a smaller radius than Cs+, although it has the same positive charge as Cs+. Therefore, H3O+ has a relatively stronger repulsive effect on Na+, Sr2+ and La3+, which prevents them from entering the structure. Overall, the excellent selective capture of Cs+ by InSnS-1 can be attributed to the stronger repulsion of H3O+ ions from competing ions and the stronger interaction between soft basic S2- and relatively soft acidic Cs+. This firstly and systematically reveals the effect of H3O+ ions on the selective capture of Cs+ by sulfide ion exchange materials.
Additionally, the material can be used as a stationary phase in ion exchange columns to effectively remove Cs+ from neutral and acidic solutions containing high concentrations of Cs+, with treatment capacities of 1300 (C0Cs = 190.13 mg/L, neutral) and 650 (C0Cs = 84.25 mg/L, 1 mol/L HNO3) bed volumes, respectively (Figure 4). These excellent properties indicate that InSnS-1 has potential applications in the field of radiocaesium remediation.
Figure 3. Selective capture of Cs+ by InSnS-1 in the presence of Sr2+ and/or La3+ under the neutral and acid conditions (Sr/La/Cs Molar ratio: A = 1.38:1.27:1, B = 13.9:8.58:1, C = 123:73.9:1, D = 1.47:0.925:1, E = 13.6:8.45:1, F = 122:73.1:1, G = 1.45:0.880:1, H = 13.5:8.36:1, I = 123:73.7:1).
Figure 4. Dynamic capture of Cs+ by InSnS-1 as stationary phase in ion exchange columns under the neutral and acid conditions (neutral solution: C0Cs = 190.13 mg/L; 1 mol/L HNO3 solution: C0Cs = 84.25 mg/L).
Importantly, the ion exchange of Cs+ or/and H3O+ ions by InSnS-1 has been directly visualized by single-crystal structural analysis, and thus the underlying mechanism of selective Cs+ capture from acidic solutions has been illuminated at the molecular level (Figure 5). The selective Cs+ capture of InSnS-1 originates from the extremely strong interactions between soft S2- of sulfide layers and relatively soft Cs+, the adjustable interlayer spacing, and structural flexibility of InSnS-1 under acidic conditions. In our research, the important influence of the shortest K-S distance of K+-directed metal sulfides on the ion exchange performance has also been revealed for the first time through a comparative structure-performance study.
Figure 5. Crystal structures diagrams of InSnS-1 before and after Cs+ and/or H3O+ ion exchange.
This work represents a breakthrough in the study of the Cs+ capture under highly acidic conditions through developing an effective sulfide material and providing a deep insight into the mechanism of selective Cs+ capture from the view of microstructural illumination. In addition to focusing on the performance of the materials, our research has also conducted a regularity search through a large number of comparative studies and systematic experiments. This is a pioneering work in the systematic study of the selective capture of Cs+ by metal sulfides under strongly acidic conditions, which sheds light on the design of novel acid-tolerant sulfide-based ion exchange materials for radiocesium decontamination with practical applications.
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