Chiyao Bai

A catechol-like phenolic ligand-functionalized hydrothermal carbon: One-pot synthesis, characterization and sorption behavior toward uranium

We proposed a new approach for preparing an efficient uranium-selective solid phase extractant (HTC-btg) by choosing bayberry tannin as the main building block and especially glyoxal as crosslinking agent via a simple, economic, and green one-pot hydrothermal synthesis. The results of characterization and analysis show that after addition of glyoxal into only bayberry tannin-based hydrothermal reaction system, the as-synthesized HTC-btg displayed higher thermal stability, larger specific surface area and more than doubled surface phenolic hydroxyl groups. The sorption behavior of the sorbents toward uranium under various conditions was investigated in detail and the results indicated that the process is fast, endothermic, spontaneous, and pseudo-second-order chemisorption. The U(VI) sorption capacity reached up to 307.3 mg g(-1) under the current experimental conditions. The selective sorption in a specially designed multi-ion solution containing 12 co-existing cations over the range of pH 1.0-4.5 shown that the amount of uranium sorbed accounts for about 53% of the total sorption amount at pH 4.5 and distinctively about 85%, unreported so far to our knowledge, at pH 2.0. Finally, a possible mechanism involving interaction between uranyl ions and phenolic hydroxyl groups on HTC-btg was proposed.

An adaptive supramolecular organic framework for highly efficient separation of uranium via an in situ induced fit mechanism

On the basis of the unusual coordination structure of UO22+ combined with the adaptive nature of supramolecular organic frameworks (SOFs), here we have designed and prepared a novel SOF-based solid phase extraction adsorbent (MA–TMA) using N-donor-containing melamine (MA) and O-donor-containing trimesic acid (TMA) as bifunctional building blocks mutually linked via hydrogen bonds. The as-prepared MA–TMA, with a rich N-/N- and N-/O-heterocyclic structure throughout its framework, provides an accessible coordination geometry and/or ligand environment for the uranyl ion, which builds the crucial structural basis for the pre-organized adaptive frameworks closely related to the “induced-fit” and selective recognition of uranyl ions. The main results are as follows: (1) the highest selectivity of 92%, so far unreported, and a considerable capacity of 324 mg g−1 for uranium adsorption by MA–TMA are observed in weak acidic multi-cation solution (pH 2.5), accompanied by a distribution coefficient Kd value of 16 000 mL g−1, 100-fold or more over other 11 competitive cations; (2) MA–TMA could reach its limiting saturation capacity of 1028 mg g−1 at pH 4.5 in pure-U(VI) solution; (3) noteworthily, the morphology of MA–TMA changed from a ribbon-like structure with a nano-diameter before adsorption into aggregated granules with a size of tens of microns after adsorption, which would be much more favorable for subsequent solid–liquid separation. Furthermore, possible mechanisms for the selective recognition of uranyl ions and the morphological changes of MA–TMA after adsorption are explored based on experimental characterization and chemical rationale.

Three novel triazine-based materials with different O/S/N set of donor atoms: One-step preparation and comparison of their capability in selective separation of uranium

Cyanuric chloride was chosen as a core skeleton which reacted with desired linker molecules, urea, thiourea and thiosemicarbazide, to prepare three novel functional covalent triazine-based frameworks, CCU (O-donor set), CCTU (S-donor set) and CCTS (S, N-donor set) respectively, designed for selective adsorption of U(VI). The products have high nitrogen concentration (>30 wt%), regular structure, relatively high chemical and thermal stability. Adsorption behaviors of the products on U(VI) were examined by batch experiments. CCU and CCTU can extract U(VI) from simulated nuclear industrial effluent containing 12 co-existing cations with relatively high selectivity (54.4% and 54.2%, respectively). Especially, effects of donor atoms O/S on adsorption were investigated, and the outcomes indicate that the difference in coordinating ability between the donor atoms is weakened in large conjugated systems, and the related functional groups with originally very strong coordination abilities may not be the best choice for the application in selective adsorption of uranium and also other metals. The as-proposed approach can easily be expanded into design and preparation of new highly efficient adsorbents for selective separation and recovery of uranium through adjusting the structures, types and amounts of functional groups of adsorbents by choosing suitable linkers.

Ligand-exchange mechanism: new insight into solid-phase extraction of uranium based on a combined experimental and theoretical study

In numerous reports on selective solid-phase extraction (SPE) of uranium, the extraction of uranium is generally accepted as a direct coordination of the ligands on the solid matrix with the uranyl, in which the critical effect of the hydration shell on the uranyl is neglected. The related mechanism in the extraction process remains unclear. Herein, the detailed calculation of activation energy and the geometry of the identified transition states reveal that the uranium extraction by a newly-synthesized urea-functionalized graphite oxide (Urea-GO) is in essence an exchange process between the ligands on Urea-GO and the coordinated water molecules in the first hydration shell of the uranyl. Moreover, we demonstrate that it is the ketone oxygen in the urea ligand to displace the coordinated water molecule of uranyl due to its stronger bonding ability and lower steric-hindrance, whereas the nitrogen atom in the same ligand is proved to be an electron donor that enables the oxygen atom to have stronger affinity for uranium through electron delocalization effects evaluated on the basis of calculations of the second-order interaction energy between donor and acceptor orbitals. We therefore propose a new ligand-exchange mechanism for the SPE process. This study advances the fundamental understanding of uranium extraction, and provides theoretical and practical guidance on ligand design for selective complexation of uranium(VI) and other metal ions in aqueous solution. Finally, the effect of nitrate ions on the extraction of uranyl was successfully explained based on the experimental and theoretical study.

Chaos to order: an eco-friendly way to synthesize graphene quantum dots

We developed a rapid, simple and pollution-free method to synthesize highly ordered graphene quantum dots (GQDs), which adopts cheap and readily available activated carbon and environmentally friendly hydrogen peroxide as raw materials through simple microwave and hydrothermal treatment, and the fine products are obtained as uniformly sized particles. The proposed strategy enables the difficult transformation from amorphous carbon to highly ordered GQDs for the first time while completely avoiding the use of concentrated sulphuric acid, concentrated nitric acid and other caustic reagents, and the purification procedure is relatively simple. Furthermore, the as-prepared products possess low toxicity, high biological compatibility and good fluorescence properties, which are excellent properties for bio-labelling applications.

Modifiable diyne-based covalent organic framework: a versatile platform for in situ multipurpose functionalization

Recently, research into functional materials with various desired properties has been ceaselessly promoted. However, the development of convenient and practical strategies for building flexible material platforms to realize multi-functionalization and to prepare various functional derivatives is still a challenge. Herein, we propose for the first time a new strategy that introduces conjugated carbon–carbon triple bonds into the skeleton of covalent organic frameworks (COFs) that can act as both building blocks and active sites to create a versatile platform. Based on the platform, in situ multipurpose functionalization according to diverse real needs can be achieved more easily and efficiently with different addition reactions, compared with current approaches used for the preparation of functional materials. In this paper, a novel COF material (TCD), which contains diynes acting as a skeleton of the material platform, was directly prepared under microwave irradiation. Then, three new functional materials with a cyano group (TCD-CN), an amidoxime group (TCD-AO) and a hydroxyl group (TCD-OH) were obtained by further functionalization of the CC bonds in TCD. The capabilities of the prepared materials for the selective separation of uranium from a simulated nuclear industrial effluent were examined, and TCD-AO was found to exhibit the highest separation efficiency.

Synthesis of microporous covalent phosphazene-based frameworks for selective separation of uranium in highly acidic media based on size-matching effect

On the basis of high stability of phosphorus-oxygen linkage, we constructed two microporous covalent phosphazene-based frameworks (CPFs), for the first time, by choosing hexachlorocyclotriphosphazene as a core unit and polyhydroxy aromatic compounds (hydroquinone or phloroglucinol) as monomers, named CPF-D and CPF-T, respectively. Characterization studies by using Fourier transform infrared, nuclear magnetic resonance, thermal gravimetric analysis, 60Co γ-ray irradiation, and so forth, demonstrated that both of the CPF materials have excellent acid and radiation stability and relatively higher thermal stability. The results of batch adsorption experiments show that CPF-T is significantly more capable of sorbing uranium than CPF-D. In a pure uranium system with higher acidity (pH 1), the uranium sorption amount of CPF-T can reach up to 140 mg g-1. Distinctively, in mixed-metal solution with 12 coexisting cations, CPF-T shows relatively stable and excellent uranium adsorption capability over a wide range of acidity (pH 4 to 3 M HNO3), and the difference in uranium sorption amounts is less than 30% with the maximum of 0.26 mmol g-1 at pH 4 and the minimum of 0.20 mmol g-1 at 3 M HNO3, which is far superior to that of the conventional solid-phase extractant (SPE) materials previously reported. The research results suggested that the sorption model based on the speculated mechanism of size-matching plus hydrogen bond network has played a dominant role in the process of uranium adsorption. The proposed strategy for the one-pot fabrication of an acid-resistant microporous framework materials by bridging the aromatic monomers via P-O bonds provides an alternative approach for the design and synthesis of new SPE materials with size-matching function desired for effective separation of uranium or other valuable metals from highly acidic environments.