Lijian Ma

Simple small molecule carbon source strategy for synthesis of functional hydrothermal carbon: preparation of highly efficient uranium selective solid phase extractant

In this paper, simple small molecules, glyoxal and acrylonitrile, are chosen as starting materials to prepare an amidoxime-functionalized hydrothermal carbon-based solid phase extractant (HTC-AO) via a one-step hydrothermal process following a simple oximation. The resulting HTC-AO exhibits the anticipated properties, i.e., low porosity (0.01 cm3 g−1) and intraparticle diffusion coefficient (kint = 0.042 mmol g−1 min−0.5), high content of amidoxime groups (1.66 mmol g−1) and minimal undesired functional groups (typically carboxylic group: 0.07 mmol g−1; phenolic group: 0.38 mmol g−1; lactonic group: 0.01 mmol g−1). Moreover, the results of irradiation experiments under γ-ray dosages between 1 and 100 kGy indicate that HTC-AO has good radiation stability. The sorption behavior of U(VI) onto HTC-AO is investigated in detail using batch sorption experiments. A saturation U(VI) sorption capacity over that of all the uranium sorbents reported previously is found to be 1021.6 mg g−1 at pH 4.5 in single uranium solution, and a so far unreported highest uranium selectivity of 81.6% with a sorption capacity of 268.9 mg g−1 is observed at pH 2.5 in multi-ion solution. The significant outcomes in this work confirms that the “simple small molecule carbon source” strategy is practical and efficient, and may have the potential for the preparation of other types of functional materials such as highly specific catalysts, drug targeting carriers and others.

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.

Pore-free matrix with cooperative chelating of hyperbranched ligands for high-performance separation of uranium.

A new strategy combining a pore-free matrix and cooperative chelating was proposed in the present paper in order to effectively avoid undesired nonselective physical adsorption and intraparticle diffusion caused by pores and voids in porous sorbents, and to greatly enhance uranium-chelating capability based on hyperbranched amidoxime ligands on the surface of nanodiamond particles. Thus, a pore-free, amidoxime-terminated hyperbranched nanodiamond (ND-AO) was designed and synthesized. The experimental results demonstrate that the strategy endows the as-synthesized ND-AO with the following expected features: (1) distinctively high uranium selectivity (SU = qe-U/qe-tol × 100%) from over 80% to nearly 100% over the whole weak acidity range (pH < 4.5); especially, the SU can reach up to unprecedented >91% at pH 4.5, more than 20% of selectivity increment over any analogous sorbent materials reported so far, with a uranium sorption capacity of 121 mg/g in simulated nuclear industry effluent samples containing 12 coexistent nuclide ions; (2) superfast equilibrium sorption time of <30 s; and (3) one of the highest distribution coefficients (Kd) of ∼3 × 106 mL/g for U(VI) as well as a fairly high sorption capacity of 212 mg/g at pH 4.5 in pure uranium solution. The strategy could also provide an optional approach for the design and fabrication of other new high-performance sorbing materials with prospective applications in selective separation of other interested metal ions.

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.

The influence of different hydroponic conditions on thorium uptake by Brassica juncea var. foliosa

The effects of different hydroponic conditions (such as concentration of thorium (Th), pH, carbonate, phosphate, organic acids, and cations) on thorium uptake by Brassica juncea var. foliosa were evaluated. The results showed that acidic cultivation solutions enhanced thorium accumulation in the plants. Phosphate and carbonate inhibited thorium accumulation in plants, possibly due to the formation of Th(HPO4)2+, Th(HPO4)2, or Th(OH)3CO3− with Th4+, which was disadvantageous for thorium uptake in the plants. Organic aids (citric acid, oxalic acid, lactic acid) inhibited thorium accumulation in roots and increased thorium content in the shoots, which suggested that the thorium-organic complexes did not remain in the roots and were beneficial for thorium transfer from the roots to the shoots. Among three cations (such as calcium ion (Ca2+), ferrous ion (Fe2+), and zinc ion (Zn2+)) in hydroponic media, Zn2+ had no significant influence on thorium accumulation in the roots, Fe2+ inhibited thorium accumulation in the roots, and Ca2+ was found to facilitate thorium accumulation in the roots to a certain extent. This research will help to further understand the mechanism of thorium uptake in plants.

Growth of high-quality covalent organic framework nanosheets at the interface of two miscible organic solvents

Stronger covalent bonds between monomers, relatively more complex growth processes (polymerization, crystallization, assembly, etc.) and π-π stacking interactions between adjacent layers make it extremely difficult to obtain highly ordered crystalline 2D covalent organic framework (COF) nanosheets. So more effective solutions have to be developed to push the methods reported so far beyond their inherent limitations. Herein, we report the first example of growing high-quality 2D COF nanosheets (NS-COF) at the interface of two miscible organic solvents. The novel approach, which is named as a buffering interlayer interface (BII) method, can be achieved by simply adding a low-density solvent interlayer, as a buffer layer, between the two miscible main solvents based on the self-propelled directed motion of the interface driven by the density differences among the solvents involved. The as-synthesized NS-COF exhibits a super-large size and a relatively regular shape with a smooth surface, which have not been observed before. The proposed strategy offers a facile and effective approach for growing well-structured 2D COF nanosheets and also other kinds of nanosheets.

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.

Effective charge-discriminated group separation of metal ions under highly acidic conditions using nanodiamond-pillared graphene oxide membrane

It has been a major challenge for separation science to separate selectively valuable metal ions from multi-cation aqueous solutions, especially in highly acidic media. In this study, a novel nanodiamond-pillared graphene oxide (NPG) membrane was, for the first time, prepared via intercalation of nanodiamond (ND) into graphene oxide (GO) by using a simple vacuum-assisted self-assembly method. Comprehensive characterization and comparison of GO and its derivative membranes show that after intercalation of ND into GO, the interlayer space and intrastratal nanochannels (similar to “the number of theoretical plates” in chromatography) are markedly increased; the channel-size distribution is profitably narrowed, and the thermal stability and hydrophilicity are enhanced; distinctively, the surface microroughness is significantly improved. The results of ion-sieving experiments demonstrate that the NPG nanofiltration membrane possesses a precise charge-discriminated group separation ability in a strong acidic solution containing 10 coexisting cations. Compared with the unmodified GO membrane, the average filtration rates of the NPG membrane were improved by 76.8% (for monovalent ions), 63.5% (for divalent ions), 118.0% (for trivalent lanthanide ions), 71.0% (for UO22+) and 105.1% (for Th4+) respectively, under the same conditions, meanwhile, the permeation rate of the multivalent cations with same charges shows much better consistency. It’s worth mentioning that UO22+ has the largest hydrated diameter among the cations used in this work, but the filtration rate of UO22+ is actually similar to that of the trivalent lanthanide cations, which may be attributed to the effective charge near +3 of UO22+, as reported in the literature. Finally, the reusability of the as-prepared nanofiltration membrane was investigated in 4 M nitric acid solution. After 5 runs of recycling tests, the group separation ability of the NPG membrane was basically maintained, and the filtration rate was reduced by up to 15.2% (for K+). These findings suggest that the as-prepared carbonaceous nanofiltration membrane could be among the most attractive membrane materials for the separation of valuable metal ions and have great potential applications in not only the nuclear power industry, but also the metallurgical, chemical manufacturing, electroplating and metal finishing industries.

Conversion of supramolecular organic framework to uranyl-organic coordination complex: a new “matrix-free” strategy for highly efficient capture of uranium

Herein, an innovative “matrix-free” strategy has been proposed for highly efficient uranium capture via uranyl-induced disassembly and reassembly of the two functional building blocks of the as-prepared hydrogen-bonded supramolecular organic framework (HSOF), which is composed of N-donor-containing melamine and O-donor-containing trimesic acid self-assembled through hydrogen bonding. The batch experimental results demonstrated that HSOF possesses excellent extraction capacity (qm = 444 mg g−1), >99% removal efficiency in the range of tested U(VI) concentration (20–130 ppm) with a considerably large KUd value of 1.3 × 107 mL g−1 at 130 ppm, and very fast extraction rate (<10 min) for UO22+. Especially, the uranium selectivity (SU = qe-U/qe-tol) of HSOF stays above 80% over the pH range tested in a uranium-containing solution with 11 competing cations, and distinctively, reaches the so far unreported 99% with a great capacity of 309 mg g−1 at pH 2.5. It is worth noting that a clear morphology transformation of HSOF nanowires to nanosheets of the uranyl-organic coordination complex (UOCC) after extraction has been observed only in the presence of uranyl ions. Moreover, according to experimental characterization and DFT studies, a possible mechanism for the efficient capture of uranium is proposed: the stronger coordination interaction among uranyl, TMA and MA could replace the weaker hydrogen-bond interaction originally linking the two building blocks in HSOF in the extraction process.