Efficient removal of Sr2+ and Cs+ from aqueous solutions using a sulfonic acid-functionalized Zr-based metal–organic framework

"Ion-imprinting" strategy towards metal sulfide scavenger enables the highly selective capture of radiocesium

Highly selective capture of radiocesium is an urgent need for environmental radioactive contamination remediation and spent fuel disposal. Herein, a strategy is proposed for construction of "inorganic ion-imprinted adsorbents" with ion recognition-separation capabilities, and a metal sulfide Cs2.33Ga2.33Sn1.67S8·H2O (FJSM-CGTS) with "imprinting effect" on Cs+ is prepared. We show that the K+ activation product of FJSM-CGTS, Cs0.51K1.82Ga2.33Sn1.67S8·H2O (FJMS-KCGTS), can reach adsorption equilibrium for Cs+ within 5 min, with a maximum adsorption capacity of 246.65 mg·g-1. FJMS-KCGTS overcomes the hindrance of Cs+ adsorption by competing ions and realizes highly selective capture of Cs+ in complex environments. It shows successful cleanup for actual 137Cs-liquid-wastes generated during industrial production with removal rates of over 99%. Ion-exchange column filled with FJMS-KCGTS can efficiently treat 540 mL Cs+-containing solutions (31.995 mg·L-1) and generates only 0.12 mL of solid waste, which enables waste solution volume reduction. Single-crystal structural analysis and density functional theory calculations are used to visualize the "ion-imprinting" process and confirm that the "imprinting effect" originates from the spatially confined effect of the framework. This work clearly reveals radiocesium capture mechanism and structure-function relationships that could inspire the development of efficient inorganic adsorbents for selective recognition and separation of key radionuclides.

Decorating Channel Walls in Metal-Organic Frameworks with Crown Ethers for Efficient and Selective Separation of Radioactive Strontium(II)

Nuclear accidents and the improper disposal of nuclear wastes have led to serious environmental radioactive pollutions. The rational design of adsorbents for the highly efficient separation of strontium(II) is essential in treating nuclear waste and recovering radioactive strontium resources. Metal-organic frameworks (MOFs) are potential materials for the separation of aqueous metal ions owing to their designable structure and tunable functionality. Herein, a novel 3D MOF material MOF-18Cr6, in which 1D channels are formed using 18-crown-6-ether-containing ligands as channel walls, is fabricated for strontium(II) separation. In contrast to traditional MOFs designed by grafting functional groups in the framework pores, MOF-18Cr6 possesses regular 18-crown-6-ether cavities on the channel walls, which not only can transport and intake strontium(II) via the channels, but also prevent blockage of the channels after the binding of strontium(II). Consequently, the functional sites are fully utilized to achieve a high strontium(II) removal rate of 99.73 % in simulated nuclear wastewater. This study fabricates a highly promising adsorbent for the separation of aqueous radioactive strontium(II), and more importantly, can provide a new strategy for the rational design of high-performance MOF adsorbents for separating target substances from complex aqueous environments.

Multiscale structural control of thiostannate chalcogels with two-dimensional crystalline constituents

Chalcogenide aerogels (chalcogels) are amorphous structures widely known for their lack of localized structural control. This study, however, demonstrates a precise multiscale structural control through a thiostannate motif ([Sn2S6]4-)-transformation-induced self-assembly, yielding Na-Mn-Sn-S, Na-Mg-Sn-S, and Na-Sn(II)-Sn(IV)-S aerogels. The aerogels exhibited [Sn2S6]4-:Mn2+ stoichiometric-variation-induced-control of average specific surface areas (95-226 m2 g-1), thiostannate coordination networks (octahedral to tetrahedral), phase crystallinity (crystalline to amorphous), and hierarchical porous structures (micropore-intensive to mixed-pore state). In addition, these chalcogels successfully adopted the structural motifs and ion-exchange principles of two-dimensional layered metal sulfides (K2xMnxSn3-xS6, KMS-1), featuring a layer-by-layer stacking structure and effective radionuclide (Cs+, Sr2+)-control functionality. The thiostannate cluster-based gelation principle can be extended to afford Na-Mg-Sn-S and Na-Sn(II)-Sn(IV)-S chalcogels with the same structural features as the Na-Mn-Sn-S chalcogels (NMSCs). The study of NMSCs and their chalcogel family proves that the self-assembly principle of two-dimensional chalcogenide clusters can be used to design unique chalcogels with unprecedented structural hierarchy.

Selective removal of Sr(II) from saliferous radioactive wastewater by capacitive deionization

⁹⁰Sr-containing radioactive wastewater during Fukushima nuclear accident (FNA) aroused extensive consideration for its disposal. Massive coexisted Na⁺ ions seriously inhibited Sr²⁺ removal, aggravating the expenditure of radioactive wastewater treatment. Herein, a chestnut shell derived porous carbon material modified with aryl diazonium salt (ADS) of sodium 4-aminoazobenzene-4'-sulfonate (SPAC) was developed as capacitive deionization electrode for selective removal of Sr²⁺ from saliferous radioactive wastewater. Based on ADS modification, the Sr²⁺ electrosorption capacity of SPAC electrode was improved to 33.11 mg g^-1 with fast ion removal rate of 2.89 mg g^-1 min^-1, comparing with only 16.10 mg g^-1 before modification. The isothermal adsorption and kinetics by SPAC electrode fitted well with Langmuir and pseudo-second-order model, achieving a maximum Sr²⁺ electrosorption capacity of 58.21 mg g^-1, superior cycling stability, and excellent charge efficiency (77.63%). Fascinatingly, the SPAC electrode exhibited superhigh Sr²⁺ selectivity of 70.65 against Na⁺ in Na⁺-Sr²⁺ mixed solution with molar ratio of Na⁺:Sr²⁺ as 20:1. Density functional theory (DFT) simulation, combining with electrochemical and spectral analyses, revealed that the high overlap of electron cloud between Sr²⁺ ion and anionic sulfonic group (-SO₃^-) provided SPAC with remarkable selectivity of Sr²⁺ ion, and illustrated the ion-swapping mechanism of Sr²⁺ selectivity.

Highly Efficient Uptake of Cs+ by Robust Layered Metal-Organic Frameworks with a Distinctive Ion Exchange Mechanism

137Cs with strong radioactivity and a long half-life is highly hazardous to human health and the environment. The efficient removal of 137Cs from complex solutions is still challenging because of its high solubility and easy mobility and the influence of interfering ions. It is highly desirable to develop effective scavengers for radiocesium remediation. Here, the highly efficient uptake of Cs+ has been realized by two robust layered metal-organic frameworks (MOFs), namely [(CH3)2NH2]In(L)2·DMF·H2O (DMF = N,N'-dimethylformamide, H2L= H2aip (5-aminoisophthalic acid) for 1 and H2hip (5-hydroxyisophthalic acid) for 2). Remarkably, 1 and 2 hold excellent acid and alkali resistance and radiation stabilities. They exhibit fast kinetics, high capacities (q m Cs = 270.86 and 297.67 mg/g for 1 and 2, respectively), excellent selectivity for Cs+ uptake, and facile elution for the regeneration of materials. Particularly, 1 and 2 can achieve efficient Cs+/Sr2+ separation in a wide range of Sr/Cs molar ratios. For example, the separation factor (SF Cs/Sr) is up to ∼320 for 1. Moreover, the Cs+ uptake and elution mechanisms have been directly elucidated at the molecular level by an unprecedented single-crystal to single-crystal (SC-SC) structural transformation, which is attributed to the strong interactions between COO- functional groups and Cs+ ions, easily exchangeable [(CH3)2NH2]+, and flexible and robust anionic layer frameworks with open windows as "pockets". This work highlights layered MOFs for the highly efficient uptake of Cs+ ions in the field of radionuclide remediation.