Taoxiang Sun

Performance and Mechanism of Uranium Adsorption from Seawater to Poly(dopamine)-Inspired Sorbents

Developing facile and robust technologies for effective enrichment of uranium from seawater is of great significance for resource sustainability and environmental safety. By exploiting mussel-inspired polydopamine (PDA) chemistry, diverse types of PDA-functionalized sorbents including magnetic nanoparticle (MNP), ordered mesoporous carbon (OMC), and glass fiber carpet (GFC) were synthesized. The PDA functional layers with abundant catechol and amine/imine groups provided an excellent platform for binding to uranium. Due to the distinctive structure of PDA, the sorbents exhibited multistage kinetics which was simultaneously controlled by chemisorption and intralayer diffusion. Applying the diverse PDA-modified sorbents for enrichment of low concentration (parts per billion) uranium in laboratory-prepared solutions and unpurified seawater was fully evaluated under different scenarios: that is, by batch adsorption for MNP and OMC and by selective filtration for GFC. Moreover, high-resolution X-ray photoelectron spectroscopic and extended X-ray absorption fine structure studies were performed for probing the underlying coordination mechanism between PDA and U(VI). The catechol hydroxyls of PDA were identified as the main bidentate ligands to coordinate U(VI) at the equatorial plane. This study assessed the potential of versatile PDA chemistry for development of efficient uranium sorbents and provided new insights into the interaction mechanism between PDA and uranium.

Visualization of Adsorption: Luminescent Mesoporous Silica-Carbon Dots Composite for Rapid and Selective Removal of U(VI) and in Situ Monitoring the Adsorption Behavior

The removal and separation of uranium from aqueous solutions are quite important for resource reclamation and environmental protection. Being one of the most effective techniques for metal separation, adsorption of uranium by a variety of adsorbent materials has been a subject of study with high interest in recent years. However, current methods for monitoring the adsorption process require complicated procedures and tedious measurements, which hinders the development of processes for efficient separation of uranium. In this work, we prepared a type of luminescent mesoporous silica-carbon dots composite material that has high efficiency for the adsorption of uranium and allows simultaneous in situ monitoring of the adsorption process. Carbon dots (CDs) were prepared in situ and introduced onto amino-functionalized ordered mesoporous silica (SBA-NH2) by a facile microplasma-assisted method. The prepared CDs/SBA-NH2 nanocomposites preserved the high specific surface area of the mesoporous silica, as well as the fluorescent properties of the CDs. Compared with bare SBA-NH2, the CDs/SBA-NH2 nanocomposites showed much improved adsorption ability and excellent selectivity for uranyl ions. Moreover, the fluorescence intensity of the composites decreased along with the increase of uranium uptake, indicating that the CDs/SBA-NH2 nanocomposites could be used for on-site monitoring of the adsorption behavior. More interestingly, the adsorption selectivity of the composites for metal ions was in good agreement with the selective fluorescence response of the original CDs, which means that the adsorption selectivity of CDs-based composite materials can be predicted by evaluating the fluorescence selectivity of the CDs for metal ions. As the first study of CDs-based nanocomposites for the adsorption of actinide elements, this work opens a new avenue for the in situ monitoring of adsorption behavior of CDs-based nanocomposites while extending their application areas.

Microplasma-assisted rapid, chemical oxidant-free and controllable polymerization of dopamine for surface modification

The development of green, controllable and simple pathways for the rapid polymerization of dopamine is of great importance in the applications of polydopamine surface chemistry. Herein, we developed a green strategy to accelerate and control the polymerization of dopamine by using microplasma electrochemistry. It was found that the microplasma cathode could trigger and dramatically accelerate the polymerization process of dopamine. The PDA coating on a silicon wafer could reach a very high deposition rate of about 53 nm h−1, which is comparable to the fastest methods. The on/off mode and the rate of the polymerization reaction could also be regulated easily by the input current. This method could also be applied for creating two-dimensional (2D) surface coating patterns on various substrates.

Comparative study on the extraction of trivalent americium and europium by CMPO in imidazolium-based ionic liquids and dodecane

ABSTRACT The extraction of Am3+ and Eu3+ by octyl(phenyl)-N,N-diisobutyl carbamoylmethylphosphine oxide (CMPO) in eight imidazolium-based ionic liquids and in n-dodecane was studied and compared. Slope analysis indicates that the ligand/metal ratio in the extracted complexes in ionic liquids is much larger than that in n-dodecane. Nitric acid has different impacts on the extraction, depending greatly on the nature of the ionic liquids. A mixed-extraction mechanism, namely cation exchange plus the formation of neutral complexes (solvation), was proposed for the extraction in the ionic liquid systems. In addition, temperature was found to have a remarkable influence on the extraction. Highly exothermic enthalpy changes were obtained for the extraction in ionic liquid systems. This work provides further insight into the particular role played by the unique properties of ionic liquids in the extraction of metal ions from nitric acid media.

Denitration of simulated high-level liquid waste by formic acid for the connection of PUREX process with TRPO process

Denitration of simulated high level liquid waste (HLLW) by formic acid was performed for the connection of Plutonium Uranium Recovery by EXtraction (PUREX) process with trialkyl phosphine oxide (TRPO) process. The concentration of acid, anions and metal ions in solution were monitored under different mole ratios of formic acid to nitric acid and denitration time. Precipitates formed were characterized by Scanning electron microscope, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and X-ray absorption near edge structure. This work presents a promising strategy for the connection of PUREX process with TRPO process and also provides some new information on the precipitation behavior in denitration of HLLW.

Extraction of U(VI) by a novel polyazamacrocycle extractant

The extraction of U(VI) from nitrate solution was investigated using a polyazamacrocycle-type C8-cyclen ligand as extractant. The effect of aqueous phase acidity, temperature, ligand concentration, nitrate concentration and irradiation dose on the distribution ratio of U(VI) was studied. The extraction of U(VI) was significantly affected by pH due to the protonation of C8-cyclen and the hydrolysis of U(VI). Uranyl nitrate was extracted via neutral complex reaction mechanism, forming a 1:1 complex with C8-cyclen. The extraction is an exothermic reaction with entropy reduction. U(VI) could be easily stripped from the loaded organic phase by sodium carbonate solution. C8-cyclen showed good stability toward γ-irradiation. No obvious decomposition was observed under the dose of 104 Gy. A small amount of C8-cyclen decomposed at the dose of 105 Gy and the distribution ratio of U(VI) increased to some extent.

Efficient co-extraction of strontium and cesium from nitric acid medium by mixtures of di-tert-butylcyclohexano-18-crown-6 and 1,3-di(2-propoxy)calix[4]arene-crown-6 in n-octanol

ABSTRACT The co-extraction of strontium and cesium from nitric acid medium by di-tert-butylcyclohexano-18-crown-6 (DtBuCH18C6) and 1,3-di(2-propoxy)calix[4]arene-crown-6 (iPr-C[4]C-6) in n-octanol was studied. The effects of contact time, nitric acid concentration, extractant concentration and temperature on the co-extraction behavior were systematically investigated. Effective extraction of the two metals was achieved under a variety of conditions. The co-extraction from a simulated high-level liquid waste (HLLW) was also conducted, and strontium and cesium could be selectively extracted in the presence of a large number of other metals. Results in this work illustrate the feasibility of partitioning radioactive strontium and cesium simultaneously from HLLW by a mixture of DtBuCH18C6 and iPr-C[4]C-6 in n-octanol.

Thermodynamic and Spectroscopic Study on the Solvation and Complexation behavior of Ln(III) in Ionic liquids: Binding of Ln(III) with CMPO in C4mimNTf2

Fundamental coordination chemistry of metal ions in ionic liquids (ILs) is of great importance to extend the application of ILs in the area of metal separation. In this work, the solvation of representative trivalent lanthanides (Nd, Eu and La) and their complexation with a functional ligand, octylphenyl-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO, denoted as L), in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C4mimNTf2) have been probed by spectroscopic, calorimetric and theoretical techniques. Absorption spectrophotometric titrations suggest that four successive Nd/CMPO complexes, NdLj3+ (j = 1–4), form both in “dry” (water content <250 ppm) and “wet” (water-saturated) ILs. However, the thermodynamic parameters vary distinctly in the two ILs. In “dry” IL, the complexation is stronger and overwhelmingly driven by exothermic enthalpies. In contrast, the complexation in “wet” IL is relatively weak and mainly driven by highly positive entropies. Comparisons between the fitted absorption spectra of Nd/CMPO complexes in “wet” IL and that of extractive samples from biphasic solvent extraction have clearly identified the extracted species as NdL43+ during the extraction. The formation of a 1 : 4 Ln/CMPO complex was further supported by DFT calculations and 31P-NMR results (La/CMPO). Additionally, luminescence emission spectra and lifetime of Eu(III) provide further evidence to illustrate the solvation and complexation behavior of Ln(III) in ILs. The results from this work shed light on how solvation affects the complexation of metal ions in ILs and how fundamental thermodynamic findings could help reveal the mechanism of biphasic extraction in real applications.

Complexation of U(VI) with diphenyldithiophosphinic acid: spectroscopy, structure and DFT calculations

The complexation of U(VI) with diphenyldithiophosphinic acid (denoted as HL) in acetonitrile was studied by UV–Vis, FT-IR, crystallography and DFT calculations. UV–Vis absorption spectrophotometry implies that three successive complexes, UO2L+, UO2L2, UO2L3−, form in the solution. Significant ligand to metal charge transfer occurs from soft atom S to U(VI) in all the three complexes. A crystal of UO2L2 complex was successfully synthesized from the solution. In the crystal both the two ligands coordinate to U(VI) in bidentate form. DFT calculations confirm the formation of UO2L3− complex and help illustrate the structures of all the U(VI) species in the solution.

Quantum Chemistry Study on the Extraction of Trivalent Lanthanide Series by Cyanex301: Insights from Formation of Inner- and Outer-Sphere Complexes

The extraction of lanthanide series by Cyanex301, i.e., bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HC301), has been modeled by density functional theory calculation, taking into account the formation of both inner- and outer-sphere complexes. The inner-sphere complex Ln(C301)3 and the outer-sphere complex Ln(H2O)9(C301)3 are optimized, followed by the analysis of interaction energy, bond length, Laplacian bond orders, and Mulliken populations. The covalency degree increases in Ln–S and Ln–O bonds in the inner- and outer-sphere complexes, respectively, as the lanthanide series is traversed. Mulliken population analysis indicates the important role of the 5d-orbital participation in bonding in the formation of inner- and outer-sphere complexes. Two thermodynamic cycles regarding the formation of inner- and outer-sphere complexes are established to calculate the extraction Gibbs free energies (ΔGextr), and relaxed potential energy surface scan is utilized to model the kinetic complexation of C301 anion with hydrated metal ions. Light lanthanides can form both inner- and outer-sphere complexes, whereas heavy lanthanides only form outer-sphere complexes in biphasic extraction. After adopting the data of forming inner-sphere complex for light Ln(III) and that of forming outer-sphere complexes for heavy Ln(III), the trend of the calculated −ΔGextr agrees very well with that of the experimental distribution ratios on crossing the Ln(III) series. Results from this work help to theoretically understand the extraction behavior of Cyanex301 with respect to different Ln(III).