Lei Mei

Efficient removal of uranium from aqueous solution by zero-valent iron nanoparticle and its graphene composite

Zero-valent iron nanoparticle (ZVI-np) and its graphene composites were prepared and applied in the removal of uranium under anoxic conditions. It was found that solutions containing 24 ppm U(VI) could be completely cleaned up by ZVI-nps, regardless of the presence of NaHCO3, humic acid, mimic groundwater constituents or the change of solution pH from 5 to 9, manifesting the promising potential of this reactive material in permeable reactive barrier (PRB) to remediate uranium-contaminated groundwater. In the measurement of maximum sorption capacity, removal efficiency of uranium kept at 100% until C0(U) = 643 ppm, and the saturation sorption of 8173 mg U/g ZVI-nps was achieved at C0(U) = 714 ppm. In addition, reaction mechanisms were clarified based on the results of SEM, XRD, XANES, and chemical leaching in (NH4)2CO3 solution. Partially reductive precipitation of U(VI) as U3O7 was prevalent when sufficient iron was available; nevertheless, hydrolysis precipitation of U(VI) on surface would be predominant as iron got insufficient, characterized by releases of Fe(2+) ions. The dissolution of Fe(0) cores was assigned to be the driving force of continuous formation of U(VI) (hydr)oxide. The incorporation of graphene supporting matrix was found to facilitate faster removal rate and higher U(VI) reduction ratio, thus benefitting the long-term immobilization of uranium in geochemical environment.

Exploring Actinide Materials Through Synchrotron Radiation Techniques

Synchrotron radiation (SR) based techniques have been utilized with increasing frequency in the past decade to explore the brilliant and challenging sciences of actinide-based materials. This trend is partially driven by the basic needs for multi-scale actinide speciation and bonding information and also the realistic needs for nuclear energy research. In this review, recent research progresses on actinide related materials by means of various SR techniques were selectively highlighted and summarized, with the emphasis on X-ray absorption spectroscopy, X-ray diffraction and scattering spectroscopy, which are powerful tools to characterize actinide materials. In addition, advanced SR techniques for exploring future advanced nuclear fuel cycles dealing with actinides are illustrated as well.

Silver Ion-Mediated Heterometallic Three-Fold Interpenetrating Uranyl–Organic Framework

A unique case of a uranyl-silver heterometallic 3-fold interpenetrating network (U-Ag-2,6-DCPCA) from a multifunctionalized organic ligand, 2,6-dichloroisonicotinic acid, in the presence of uranyl and silver ions is reported. It is the first report of a heterometallic uranyl-organic interpenetrating network or framework. Notably, a (4,4)-connected uranyl building unit in U-Ag-2,6-DCPCA, which is available through combined influences of structural halogenation and silver ion additive on uranyl coordination, plays a vital role in the formation of a 3-fold interpenetrating network. Halogen substitution effectively changes structural features and coordination behaviors of isonicotinate ligand and contributes to the control of uranyl coordination. Meanwhile, it exerts influence on the stabilization of 3-fold interpenetrating networks by halogen-halogen interactions. Theoretical calculation suggests that the silver ion should mainly serve as an inductive factor of uranyl species through strong Ag-N binding affinity, directly leading to the formation of a (4,4)-connected uranyl building unit and finally a heterometallic 3-fold interpenetrating network. Related experimental results, especially an interesting postsynthetic metalation, afford further evidence of this induction effect.

Tetranuclear Uranyl Polyrotaxanes: Preferred Selectivity toward Uranyl Tetramer for Stabilizing a Flexible Polyrotaxane Chain Exhibiting Weakened Supramolecular Inclusion

Introduction of mechanically interlocked components into actinide-based metal-organic materials such as polyrotaxanes will generate an entirely new type of inorganic-organic hybrid materials showing more supramolecular encapsulation-based dynamics. In this work, tetranuclear uranyl-directed polyrotaxanes (UO2 )4 O2 -C5A3-CB6 (1) and (UO2 )4 O2 -C6A3-CB6 (2), which are the first actinide pseudorotaxanes with high-nuclearity uranium centers, were obtained through systematic extension of the string spacer in pseudorotaxane ligands from 1,4-butylene (C4) to 1,5-pentylene (C5) and 1,6-hexylene (C6). Both of the as-synthesized tetranuclear uranyl polyrotaxanes were structurally characterized and analyzed. Considering the structure of UO2 -C4A3-CB6 and the 1,4-butylene string spacer, the preference for the uranyl tetramer may be related to the configurational inversion of the pseudorotaxane ligands from trans mode to cis mode on coordination to the uranyl center. Detailed structural analysis suggests that the length of the stretched string molecules for CB6-encapsulated pseudorotaxanes has remarkable effect on the supramolecular inclusion interactions and the configurations of pseudorotaxanes, and should be responsible for the configurational inversion of pseudorotaxane spacers and subsequent distinct changes of the uranyl building units and geometric structures.

Solvent‐Dependent Synthesis of Porous Anionic Uranyl–Organic Frameworks Featuring a Highly Symmetrical (3,4)‐Connected ctn or bor Topology for Selective Dye Adsorption

Two highly symmetrical (3,4)-connected uranyl-organic frameworks (UOFs) were synthesized by a judicious combination of D3h -symmetrical triangular [UO2 (COO)3 ]- and Td symmetrical tetrahedral tetrakis(4-carboxyphenyl)methane (H4 MTB). These two as-synthesized UOFs possess similar structural units and coordination modes but totally different topological structures, namely ctn net and bor net. Solvent-induced interpenetration and a morphology change are observed. The two compounds exhibit crystal transformation via a dissolution-crystallization process. Adsorption experiments in CH3 OH solution indicate that both of them can selectively remove positively charged dyes over negatively charged and neutral dyes. Moreover, the electronic structural and bonding properties of the two compounds were systematically explored by density functional theory (DFT) calculations.

Supramolecular inclusion-based molecular integral rigidity: a feasible strategy for controlling the structural connectivity of uranyl polyrotaxane networks

The assembly of two-dimensional (2D) large channel uranyl-organic polyrotaxane networks as well as structural regulation of uranyl-bearing units using jointed cucurbit[6]uril-based pseudorotaxanes with integral rigidity based on supramolecular inclusion is presented for the first time. This construction strategy concerning controlling molecular integral rigidity based on supramolecular inclusion may afford an entirely new methodology for coordination chemistry.

The first case of actinide triple helices: pH-dependent structural evolution and kinetically-controlled transformation of two supramolecular conformational isomers

The first actinide triple helices, including two supramolecular conformational isomers of uranium(VI), have been synthesized with the aid of a flexible V-shaped ligand and a rigid aromatic base. The isomers exhibit an intriguing pH-dependent structural evolution and a kinetically-controlled transformation via a novel conformational rearrangement of the organic base.

Size-tunable synthesis of monodisperse thorium dioxide nanoparticles and their performance on the adsorption of dye molecules

We report a facile method to synthesize monodisperse, uniform and size-controllable thorium dioxide nanoparticles via a hydrothermal precursor synthesis and a subsequent calcination treatment. Highly uniform spherical precursor nanoparticles with a tunable diameter of 38–274 nm and monodisperse irregular precursor nanoparticles with a size range of 20–45 nm can be successfully synthesized using thorium nitrate pentahydrate, urea and glycerol as the metal source, nucleation agent, and size-controlling agent, respectively. The chemical composition, growth process, and size control of the precursor nanoparticles have been systematically investigated, and a probable formation mechanism of these nanoparticles was also proposed. Calcination of precursor nanoparticles at 800 °C leads to the formation of uniform thorium dioxide nanoparticles without any morphological deformation. Furthermore, the size and shape effects of thorium dioxide nanoparticles on the uptake of dye molecules have been studied. The corresponding adsorption behaviors of as-prepared samples can be elucidated very well by the nitrogen adsorption–desorption analysis.

An Unprecedented Two‐Fold Nested Super‐Polyrotaxane: Sulfate‐Directed Hierarchical Polythreading Assembly of Uranyl Polyrotaxane Moieties

The hierarchical assembly of well-organized submoieties could lead to more complicated superstructures with intriguing properties. We describe herein an unprecedented polyrotaxane polythreading framework containing a two-fold nested super-polyrotaxane substructure, which was synthesized through a uranyl-directed hierarchical polythreading assembly of one-dimensional polyrotaxane chains and two-dimensional polyrotaxane networks. This special assembly mode actually affords a new way of supramolecular chemistry instead of covalently linked bulky stoppers to construct stable interlocked rotaxane moieties. An investigation of the synthesis condition shows that sulfate can assume a vital role in mediating the formation of different uranyl species, especially the unique trinuclear uranyl moiety [(UO2 )3 O(OH)2 ](2+) , involving a notable bent [O=U=O] bond with a bond angle of 172.0(9)°. Detailed analysis of the coordination features, the thermal stability as well as a fluorescence, and electrochemical characterization demonstrate that the uniqueness of this super-polyrotaxane structure is mainly closely related to the trinuclear uranyl moiety, which is confirmed by quantum chemical calculations.

Supramolecular Isomers of Coordination-Directed Side-Chain Polypseudorotaxanes Based on Trimeric Uranyl Oxalate Nodes.

Although the prosperity of rotaxane coordination polymers with rotaxane molecules serving as main-chain linkers is known, side-chain metal-organic polypseudorotaxanes incorporating macrocyclic host molecules have not been reported to date. Herein a new type of coordination-driven cucurbit[6]uril-bearing side-chain polypseudorotaxane, with two-dimensional trimeric uranyl-oxalate as main chains, has been synthesized. This was carried out through hydrothermal reactions of uranyl components with an in situ-formed carboxylated pseudorotaxane ligand in the presence of oxalate co-ligands. Varying the substitution site of coordination groups led to two different supramolecular isomers. Further mechanistic analysis indicated that condition-dependent hydrolysis of the cyano groups of the pseudorotaxane ligand, as well as the participation of oxalate groups into the coordination sphere of uranyl moieties, contributes to the formation of this new type of side-chain polypseudorotaxane.