![]() The successful determination of the single-crystal structures of InSnS-1 and its three ion-exchange products help visualize the ion exchange between Cs + or hydrated protons (H 3O +) and interlayered K + ions of InSnS-1. Moreover, ion-exchange columns loaded with InSnS-1 can effectively treat neutral and acidic solutions containing high concentrations of Cs + (190.13 and 84.25 mg/L, respectively) with treatment efficiencies up to 1300 and 650 bed volumes, respectively. Under the coexistence of high-concentration competing ions such as Na +, Sr 2+, and La 3+, InSnS-1 exhibits high selectivity for Cs + in acidic solutions, which endows InSnS-1 with excellent Cs-Sr or Cs-La separation in the acidic environment. ![]() InSnS-1 has high Cs + ion-exchange capacity ( q m Cs = 316.0 mg/g in neutral solutions q m Cs = 98.6 mg/g in 1 mol/L HNO 3 solutions). Specifically, it can maintain its n n− layers even in 1–4 mol/L HNO 3 solutions. Such an approach leads to a robust K +-directed layered metal sulfide KInSnS 4 (denoted as InSnS-1) with excellent acid and irradiation resistances. Herein, the strategy to improve stability of materials by introducing Sn 4+ and In 3+ with high valency and large radii into sulfides has been applied. Therefore, it is of vital significance to develop acid-tolerant ion exchangers that can selectively capture Cs + from strongly acidic solutions for radioactive liquid waste treatment and to clarify the mechanism of Cs + removal for revealing the structure–function relationship. However, the capture of Cs + by metal sulfides under strongly acidic conditions (nitric acid concentrations >1 mol/L) is challenging, and in particular, the adsorption mechanism has been not clearly identified. In recent years, metal sulfides have been a very promising class of radioactive ion exchangers 12, 17, 18, 19, 20 which display excellent removal performance for Cs +. In contrast, materials that can effectively remove Cs + ions under extremely acidic condition are still very limited, exemplified mainly by ammonium phosphomolybdate and its complexes or cupric aromatic crown ether-modified silyl compounds 13, 14, 15, 16. Although ion exchange is considered an ideal method for controlling radioactive contamination due to its simplicity of operation, high efficiency, and lack of secondary contamination 11, the development of stable and highly selective ion exchangers for the efficient capture of Cs + in acidic solutions still remains a great challenge.ĭue to the instability or poor selectivity of materials for Cs + capture under acidic conditions, most of the current studies are restricted to that under neutral or weak acidic conditions 9, 11, 12. These methods, however, have some disadvantages in terms of operation, cost, or selectivity, such as the expensive and toxic extractants used in solvent extraction and the great volume of radioactive sludge produced by precipitation 10. Methods such as solvent extraction, chemical precipitation, and ion exchange/sorption have been used for the enrichment and separation of radiocesium 9. Such complex components would pose a huge challenge for the selective separation of Cs + from HLLWs. However, HLLWs are extremely complex, containing not only Cs +, but also Sr 2+, Ln 3+, Na + and so on 6, 8. The separated and purified 137Cs can also be prepared as isotope sources and reused in agriculture and medical fields 3. The key for the disposal of HLLWs is the removal of strongly radioactive ions (such as 137Cs) to reduce their radioactivity level 6. The strongly acidic high-level-liquid-wastes (HLLWs) resulting from the recovery of uranium and plutonium from spent fuel by the PUREX process contain 137Cs in ionic form 1, 5, 6, 7. ![]() Therefore, the disposal of 137Cs has received much attention due to its potential threat to the environment. For example, the Fukushima nuclear accident released a large number of radioactive Cs + ions into the environment, causing total radioactivity levels in some fish to remain above the limit (100 Bq/kg) until now 4. Once radioactive cesium is released into the environment, it will be extremely polluting and cause harm to the entire ecosystem 3. It is highly migratory in the environment, causing cell damage, cancer, and even death in humans 2. 137Cs with a long half-life ( t 1/2 ~ 30.17 years) can emit γ-rays 1. 137Cs, as the fission product of 235U, is one of the main sources of radioactivity in spent fuel (1230 g/ton) 1. With the rapid development of nuclear energy, growing concerns about its safety have been raised.
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