Rational Construction of Porous Metal-Organic Frameworks for Uranium(VI) Extraction: The Strong Periodic Tendency with a Metal Node

Significance of MOF adsorbents in uranium remediation from water

Uranium is considered as one of the most perilous radioactive contaminants in the aqueous environment. It has shown detrimental effects on both flora and fauna and because of its toxicities on human beings, therefore its exclusion from the aqueous environment is very essential. The utilization of metal-organic frameworks (MOFs) as an adsorbent for the removal of uranium from the aqueous environment could be a good approach. MOFs possess unique properties like high surface area, high porosity, adjustable pore size, etc. This makes them promising adsorbents for the removal of uranium from contaminated water. In this paper, sources of uranium in the water environment, human health disorders, and application of the different types of MOFs as well as the mechanisms of uranium removal have been discussed meticulously.

Light-driven uranyl-organic frameworks used as signal-enhanced photoelectrochemical sensors for monitoring anthrax

The semiconductor-like characteristics and light absorption ability of metal-organic frameworks (MOFs) make it have the potential for photoelectrochemical sensing. Compared with composite and modified materials, the specific recognition of harmful substances directly using MOFs with suitable structures can undoubtedly simplify the fabrication of sensors. Herein, two photosensitive uranyl-organic frameworks (UOFs) named HNU-70 and HNU-71 were synthesized and explored as the novel "turn-on" photoelectrochemical sensors, which can be directly applied to monitor the biomarker of anthrax (dipicolinic acid). Both sensors have good selectivity and stability towards dipicolinic acid with the low detection limits of 1.062 and 1.035 nM, respectively, which are far lower than the human infection concentration. Moreover, they exhibit good applicability in the real physiological environment of human serum, demonstrating a good application prospect. Spectroscopic and electrochemical studies show that the mechanism of photocurrent enhancement results from the interaction between dipicolinic acid and UOFs, which facilitates the photogenerated electron transport.

Efficient uranium(VI) adsorbing bioinspired nano-sized hydroxyapatite composites: synthesis, tuning, and adsorption mechanism

The production of large amounts of uranium-containing wastewater and its potential hazards has stimulated green and efficient material removal of uranium (VI). Inspired by the natural mineralization of bone, a facile and eco-friendly biomimetic synthesis of nano-hydroxyapatite (HAP) was carried out using chitosan (CS) as a template. It was found that the reaction temperature and the amount of precursors influence the particle size, crystallinity and specific surface area of the CS/HAP nanorods, and consequently their U(VI) adsorption efficiency. Moreover, the synthesized CS/HAP-40 with smaller particle size, lower crystallinity, and larger specific surface area show a more efficient U(VI) removal compared with CS/HAP-55 and CS/HAP-55-AT. It has a maximum adsorption capacity of 294.12 mg·g^-1 of the CS/HAP-40. Interestingly, the U(VI) removal mechanism of CS/HAP-40 in acidic (pH = 3) and alkaline (pH = 8) aqueous solutions was found to be different. As one of the main results, the U(VI) adsorption mechanisms at pH 8 could be surface complexation and ion exchange. On the contrary, three different mechanisms could be observed at pH 3: dissolution-precipitation to form chernikovite, surface complexation, and ion exchange.

Ionic metal-organic frameworks (iMOFs): progress and prospects as ionic functional materials

Metal-organic frameworks (MOFs) have been a research hotspot for the last two decades, witnessing an extraordinary upsurge across various domains in materials chemistry. Ionic MOFs (both anionic and cationic MOFs) have emerged as next-generation ionic functional materials and are an important subclass of MOFs owing to their ability to generate strong electrostatic interactions between their charged framework and guest molecules. Furthermore, the presence of extra-framework counter-ions in their confined nanospaces can serve as additional functionality in these materials, which endows them a significant advantage in specific host-guest interactions and ion-exchange-based applications. In the present review, we summarize the progress and future prospects of iMOFs both in terms of fundamental developments and potential applications. Furthermore, the design principles of ionic MOFs and their state-of-the-art ion exchange performances are discussed in detail and the future perspectives of these promising ionic materials are proposed.

Highly efficient adsorptive extraction of uranium from wastewater by novel kaolin aerogel

An environment-friendly, low-cost and efficient kaolin aerogel adsorbent (named as KLA) was synthesized via a freeze-drying-calcination method to solve the defect of low uranium removal rate for kaolin (KL). The removal rate of uranium on KLA reached 90.6 %, which was much higher than that of KL (69.2 %) (C₀ = 10 mg L^-1, t = 24 h, pH = 5.0, T = 298 K and m/V = 1.0 g L^-1). The uranium removal behavior on KLA was satisfied with Pseudo-second-order and Langmuir model, which meant that the uranium ions were immobilized on the surface of KLA via chemical reaction. Meanwhile, high temperature was in favor of the removal of uranium on KLA, indicating that the removal process was a spontaneous endothermic reaction. Compared with KL, KLA also presented better cycle ability and its removal rate of uranium was up to 80.5 % after three cycles, which was still higher than that of KL at the first cycle (74.5 %). On basis of the results of SEM, XRD, FT-IR and XPS, it could be concluded that uranium ions were adsorbed by KLA via complexation. Hence, KLA could be regarded as a feasible candidate for the removal of uranium from aqueous solution.

Efficient Photocatalytic Oxidative Coupling of Benzylamine over Uranyl-Organic Frameworks

Visible-light-driven organic transformation photocatalyzed by metal-organic frameworks (MOFs) under mild conditions is considered a feasible route to conserve energy and simplify synthesis. Herein, a light-sensitized, three-dimensional uranyl-organic framework (HNU-64) with twofold interpenetration and its derivatives HNU-64-CH₃ and HNU-64-Cl with functionalized ligands of -CH₃ and -Cl groups were obtained. These MOFs have broad optical absorption bands and suitable band energy levels in photooxidation, which makes them exhibit high activity and selectivity for the photooxidation of benzylamine to N-benzylbenzoimide under mild conditions. This work provides an efficient and simple synthetic option for oxidative coupling of amines to directly produce imines.

Metal-Organic Framework-Based Materials for Adsorption and Detection of Uranium(VI) from Aqueous Solution

The steady supply of uranium resources and the reduction or elimination of the ecological and human health hazards of wastewater containing uranium make the recovery and detection of uranium in water greatly important. Thus, the development of effective adsorbents and sensors has received growing attention. Metal-organic frameworks (MOFs) possessing fascinating characteristics such as high surface area, high porosity, adjustable pore size, and luminescence have been widely used for either uranium adsorption or sensing. Now pertinent research has transited slowly into simultaneous uranium adsorption and detection. In this review, the progress on the research of MOF-based materials used for both adsorption and detection of uranium in water is first summarized. The adsorption mechanisms between uranium species in aqueous solution and MOF-based materials are elaborated by macroscopic batch experiments combined with microscopic spectral technology. Moreover, the application of MOF-based materials as uranium sensors is focused on their typical structures, sensing mechanisms, and the representative examples. Furthermore, the bifunctional MOF-based materials used for simultaneous detection and adsorption of U(VI) from aqueous solution are introduced. Finally, we also discuss the challenges and perspectives of MOF-based materials for uranium adsorption and detection to provide a useful inspiration and significant reference for further developing better adsorbents and sensors for uranium containment and detection.

Waste feather fiber based high extraction capacity bio-adsorbent for sustainable uranium extraction from seawater

Recycling uranium from seawater is of great significance to the development of nuclear industry. However, due to high salinity and low uranium concentration in seawater, there are still many challenges in current seawater uranium extraction technology. In this study, waste feather fibers (FF) were used as raw materials to develop a phosphonate-functionalized feather fiber (FF-PT). The study on the adsorption performance shows that FF-PT have good adsorption and recycling performance for uranium. The study on the adsorption performance shows that FF-PT have good adsorption and recycling performance for uranium. The adsorption capacity reaches up to 342.5 mg·g^-1 in the 8 ppm uranium solution, and service life of at least 10 cycles were obtained. In addition, in the environment with high salinity and the coexistence of metal competitive ions, FF-PT also shows excellent selectivity, and it can reach 3.22 mg·g^-1 adsorption capacity after immersed in natural seawater for 30 days. Combined with the results of economic analysis, we believe that the FF-PT has broad application prospects in the industrialized uranium extraction from seawater.

Rational synthesis of isomorphic rare earth metal–organic framework materials for simultaneous adsorption and photocatalytic degradation of organic dyes in water

Two isomorphic rare earth metal–organic frameworks (MOFs) were synthesized by a solvothermal method. These MOFs have good removal effects on cationic and neutral dyes through simultaneous adsorption and photocatalysis.

Metal-organic frameworks as superior porous adsorbents for radionuclide sequestration: Current status and perspectives

Efficient separation of hazardous radionuclides from radioactive waste remains a challenge to the global acceptance of nuclear power due to complex nature of the waste, high radiotoxicities and presence of large number of interfering elements. Sorption of radioactive elements from liquid phase, gas phase or their solid particulates on various synthetic organic, inorganic or biological sorbents is looked as one of the options for their remediation. In this context, highly porous materials, termed as metal-organic frameworks (MOFs), have shown promise for efficient capturing of various types of radioactive elements. Major advantages that have been advocated for the application of MOFs in radionuclide sorption are their excellent chemical stability, and their large surface area due to abundant functional groups, and porosity. In this review, recent developments on the application of MOFs for radionuclide sequestration are briefly discussed. Focus has been devoted to address the separation of few crucial radioactive elements such as Th, U, Tc, Re, Se, Sr and Cs from aqueous solutions, which are important for liquid radioactive waste management. Apart from these radioactive metal ions, removal of radionuclide bearing gases such as I₂, Xe, and Kr are also discussed. Aspects related to the interaction of MOFs with the radionuclides are also discussed. Finally, a perspective for comprehensive investigation of MOFs for their applications in radioactive waste management has been outlined.

Metal-Organic-Framework Based Functional Materials for Uranium Recovery: Performance Optimization and Structure/Functionality-Activity Relationships

Uranium recovery has profound significance in both uranium resource acquisition and pollution treatment. In recent years, metal-organic frameworks (MOFs) have attracted much attention as potential uranium adsorbents owing to their tunable structural topology and designable functionalities. This review explores the research progress in representative classic MOFs (MIL-101, UiO-66, ZIF-8/ZIF-67) and other advanced MOF-based materials for efficient uranium extraction in aqueous or seawater environments. The uranium uptake mechanism of the MOF-based materials is refined, and the structure/functionality-property relationship is further systematically elucidated. By summarizing the typical functionalization and structure design methods, the performance improvement strategies for MOF-based adsorbents are emphasized. Finally, the present challenges and potential opportunities are proposed for the breakthrough of high-performance MOF-based materials in uranium extraction.

Polymer Ligand-Sensitized Lanthanide Metal-Organic Frameworks for an On-Site Analysis of a Radionuclide

Herein, a new strategy to increase the sensitivity of a lanthanide metal-organic framework (Ln-MOF) to UO₂²⁺ was proposed by using polymeric ligands. By utilizing [Tb(1,3,5-benzenetrisbenzoate)]n (Tb-TBT) MOF as the host, preloaded 2-vinyl terephthalic acid (VTP) was polymerized in situ, which produced a novel fluorescent composite denoted as PVTP⊂Tb-TBT. Benefiting from the coordination of PVTP to the Tb nodes, the polymeric chains performed both as molecular scaffolds that improved the water stability of the framework and as additional antennae that sensitized the photoluminescence of the Tb nodes. More importantly, the detection sensitivity and selectivity of PVTP⊂Tb-TBT to UO₂²⁺ were much improved compared to those of Tb-TBT. Detailed characterizations indicated that the incorporation of PVTP efficiently enriched UO₂²⁺ in the probe, which promoted the energy dissipation to UO₂²⁺. Besides, UO₂²⁺ was also supposed to release PVTP from PVTP⊂Tb-TBT and, thus, exposed the open metal sites to water molecules, which interrupted the sensitization effect of PVTP and induced a nonradiative energy dissipation. A limit of detection (LOD) as low as 0.75 nm was recorded by suspending the PVTP⊂Tb-TBT probe in a water sample, far below the limit in drinking water set by the United States Environmental Protection Agency (130 nm). Furthermore, a remotely controlled sampling and an on-site analysis of real water samples were realized by facilely loading PVTP⊂Tb-TBT on thin films (TFs). The LOD for UO₂²⁺ was 2.5 nm by using the TFs. This study reports a new strategy for boosting the sensitivity and selectivity of Ln-MOF to monitor UO₂²⁺ and expands the application of the strategy to an on-site analysis.

Origins of Clustering of Metalate-Extractant Complexes in Liquid-Liquid Extraction

Effective and energy-efficient separation of precious and rare metals is very important for a variety of advanced technologies. Liquid-liquid extraction (LLE) is a relatively less energy intensive separation technique, widely used in separation of lanthanides, actinides, and platinum group metals (PGMs). In LLE, the distribution of an ion between an aqueous phase and an organic phase is determined by enthalpic (coordination interactions) and entropic (fluid reorganization) contributions. The molecular scale details of these contributions are not well understood. Preferential extraction of an ion from the aqueous phase is usually correlated with the resulting fluid organization in the organic phase, as the longer-range organization increases with metal loading. However, it is difficult to determine the extent to which organic phase fluid organization causes, or is caused by, metal loading. In this study, we demonstrate that two systems with the same metal loading may impart very different organic phase organizations and investigate the underlying molecular scale mechanism. Small-angle X-ray scattering shows that the structure of a quaternary ammonium extractant solution in toluene is affected differently by the extraction of two metalates (octahedral PtCl₆^2- and square-planar PdCl₄^2-), although both are completely transferred into the organic phase. The aggregates formed by the metalate-extractant complexes (approximated as reverse micelles) exhibit a more long-range order (clustering) with PtCl₆^2- compared to that with PdCl₄^2-. Vibrational sum frequency generation spectroscopy and complementary atomistic molecular dynamics simulations on model Langmuir monolayers indicate that the two metalates affect the interfacial hydration structures differently. Furthermore, the interfacial hydration is correlated with water extraction into the organic phase. These results support a strong relationship between the organic phase organizational structure and the different local hydration present within the aggregates of metalate-extractant complexes, which is independent of metalate concentration.

Controlling the secondary assembly of porous anionic uranyl-organic polyhedra through organic cationic templates

Herein, we report a new uranyl-organic polyhedron U₄L₄ (L = BTPCA) assembled from uranyl and a semirigid tritopic ligand. By adjusting the carbon chain length of organic templates, two complexes can be obtained based on the diverse secondary assembly of U₄L₄ cages. The mechanism of different arrangements of U₄L₄ cages induced by organic templates was explored in detail.

Efficient Photocatalytic Extraction of Uranium over Ethylenediamine Capped Cadmium Sulfide Telluride Nanobelts

The photocatalysts for hexavalent uranium (U(VI)) reduction suffered from the low uranium uptake capacity and weak long-wavelength light absorption. Herein, we synthesized the CdS_xTe_1-x nanobelts capped by ethylenediamine (EDA), which provided amino groups as the adsorption sites. With the increase of the Te content, the amino groups on the CdS_xTe_1-x nanobelts decreased because of the variation of the electron density of Cd²⁺, whereas the light adsorption was enhanced due to the narrowed bandgap. In photocatalytic reduction of U(VI), the CdS_0.95Te_0.05-EDA nanobelts exhibited a considerable U(VI) removal ratio of 97.4% with a remarkable equilibrium U(VI) extraction amount on per weight unit of the adsorbent (q_e) of 836 mg/g. The bandgap structure and Fourier transform infrared spectroscopy (FT-IR) spectra analysis revealed that the optimum photocatalytic activity of CdS_xTe_1-x nanobelts was achieved at a 5% of Te^2- doping, which balanced the factors of amino groups and bandgap. This adsorption-photoreduction process offers an ultrahigh uranium extraction capacity over wide uranium concentrations.

In-situ synthesis of uranyl-imprinted nanocage for selective uranium recovery from seawater

Adaptive coordination structure is vital for selective uranium extraction from seawater. By strategy of molecular imprinting, uranyl is introduced into the  m ultivariate metal-organic framework (MOF) during the synthesis process to guide the  in-situ  construction of proper nanocage structure for targeting uranyl binding. Except for  the  coordination between uranium with four oxygen from the materials, the axial oxygen of uranyl also forms hydrogen bonds with hydrogen from the phenolic hydroxyl group, which enhances the binding affinity of the material to uranyl. Attributing to the high binding affinity, the adsorbent shows high uranium binding selectivity to uranyl against not only the interfering metal ions, but also the carbonate group that coordinates with uranyl to form [UO 2 (CO) 3 ] 4 -  in seawater. In natural seawater, the adsorbent realizes a high uranium adsorption capacity of 7.35 mg g -1 , t ogether with an 18.38 times higher selectivity to vanadium.  Integrated into account the high reusability, this adsorbent is a promising alternative for uranium recovery from seawater.

Tuning of the Network Dimensionality and Photoluminescent Properties in Homo- and Heteroleptic Lanthanide Coordination Polymers

Targeted synthesis, through a heteroleptic methodology, has resulted in three types of lanthanide (Ln) coordination polymers (CPs) with tailored dimensionality, tunable photoluminescent colors, and distinct luminescence quenching upon UV and X-ray irradiation. The homoleptic Ln(tpbz)(NO₃)₂ [CP-1; tpbz = 4-(2,2':6',2″-terpyridin-4'-yl)benzoate] is assembled from Ln cations and bridging tpbz ligands, accompanied by the decoration of NO₃^- anions, forming a one-dimensional (1D) chain structure. The presence of ancillary dicarboxylate linkers, 1,4-benzenedicarboxylate (bdc) and 2,5-thiophenedicarboxylate (tdc), promotes additional bridging between 1D chains to form a two-dimensional layer and a three-dimensional framework for Ln(tpbz)(bdc) (CP-2) and Ln(tpbz)(tdc) (CP-3), respectively. The multicolor and luminescence properties of the obtained CPs were investigated, displaying typical red Eu^III-based and green Tb^III-based emissions. The Sm^III-bearing CP-1-CP-3, however, exhibit diverse ratiometric Ln^III- and ligand-based emissions, with the photoluminescent colors varying from pink to orange to cyan. Notably, the Tb^III-containing CP-1-CP-3 display distinct luminescence quenching upon continuous exposure to UV and X-ray irradiation. To our best knowledge, CP-2-Tb represents one of the most sensitive UV dosage probes (3.2 × 10^-7 J) among all CPs.

Unexpected ultrafast and highly efficient removal of uranium from aqueous solutions by a phosphonic acid and amine functionalized polymer adsorbent

P(DMAA–B2MP) was prepared by solvothermal polymerization and exhibits fast and efficient sorption of uranium(vi) from aqueous solutions.

Insights into the structure-performance relationships of extraction materials in sample preparation for chromatography

Sample preparation is one of the most crucial steps in analytical processes. Commonly used methods, including solid-phase extraction, dispersive solid-phase extraction, dispersive magnetic solid-phase extraction, and solid-phase microextraction, greatly depend on the extraction materials. In recent decades, a vast number of materials have been studied and used in sample preparation for chromatography. Due to the unique structural properties, extraction materials significantly improve the performance of extraction devices. Endowing extraction materials with suitable structural properties can shorten the pretreatment process and improve the extraction efficiency and selectivity. To understand the structure-performance relationships of extraction materials, this review systematically summarizes the structural properties, including the pore size, pore shape, pore volume, accessibility of active sites, specific surface area, functional groups and physicochemical properties. The mechanisms by which the structural properties influence the extraction performance are also elucidated in detail. Finally, three principles for the design and synthesis of extraction materials are summarized. This review can provide systematic guidelines for synthesizing extraction materials and preparing extraction devices.