Yuezhou Wei

A study of the Ce(III)/Ce(IV) redox couple for redox flow battery application

Abstract In this study, the electrochemical behavior of the Ce(III)/Ce(IV) redox couple in sulfuric acid medium with various concentrations and the influence of the operating temperature were investigated. A change of the concentration of sulfuric acid mainly produced the following two results. (1) With an increase of the concentration of sulfuric acid the redox peak currents decreased. (2) The peak potential separation for the redox reactions increased with rising concentration of sulfuric acid from 0.1 to 2 M and then decreased with further increase of the concentration. Elevated temperature was electrochemically favorable for Ce(III)/Ce(IV) couple, which caused an increase of the peak currents for the redox reactions and a decrease of the peak potentials separation. Constant-current electrolysis shows that the current efficiency was 73% for the oxidation process of Ce(III) and 78% for the reduction process at 298 K, and could be improved by elevating the temperature. The open-circuit voltage of the Ce–V cell, after full charging, remained constant at 1.870±0.005 V for more than 48 h, and is about 29% higher than that of the all-vanadium batteries. The coulombic efficiency was approximately 87%, showing that self-discharge of the Ce–V battery was small. The preliminary exploration shows that the Ce(III)/Ce(IV) couple is electrochemically promising for redox flow battery (RFB) application.

Studies on the separation of minor actinides from high-level wastes by extraction chromatography using novel silica-based extraction resins

To develop an advanced partitioning process by extraction chromatography using a minimal organic solvent and compact equipment to separate minor actinides such as Am and Cm from nitrate acidic high-level waste (HLW) solution, several novel silica-based extraction resins have been prepared by impregnating organic extractants into the styrene-divinylbenzene copolymer, which is immobilized in porous silica particles (SiO2-P). The extractants include octyl(phenyl)-N, N-diisobutylcarbamoylmethylphosphine oxide (CMPO), di(2-ethylhexyl)-phosphoric acid (HDEHP), and bis(2,4,4-trimethylpentyl)dithiophosphinic acid (Cyanex 301). Compared to conventional polymer-matrix resins, these new types of extraction resin are characterized by rapid kinetics and significantly low pressure loss in a packed column. The results of separation experiments revealed that trivalent actinides and lanthanides can be separated from other fission products, such as Cs, Sr, and Ru in simulated HLW solution containing concentrated nitric acid by extraction chromatography using a CMPO/SiO2-P resin-packed column. Satisfactory separation between Am(III) and a macro amount of lanthanides from simulated HLW solution with pH 4 was achieved by using a newly purified Cyanex 301/SiO2-P resin. However, the Am(III) separation was very sensitive to the purity of Cyanex 301, and the improvement of its stability is an important task for practical utilization.

Development of the MAREC process for HLLW partitioning using a novel silica-based CMPO extraction resin

A new partitioning technology named “MAREC” process has been proposed for the separation of minor actinides (MA=Am, Cm) from high level liquid waste (HLLW) by extraction chromatography using a novel porous silica-based CMPO extraction resin. Separation experiments for simulated HLLW solutions containing typical fission product (FP) elements were carried out by a column packed with the CMPO/SiO2-P extraction resin. The experimental results showed that MA as a mixture with some heavy rare earths can be effectively separated from other FP elements by using two chromatographic columns. Furthermore, some specific FP elements such as Pd, Zr and Mo were also efficiently separated from the simulated HLLW. The separation behavior of the elements are considered to result from the difference of their adsorption and elution selectivity based on the complex formation with CMPO and the eluents such as DTPA and H2C2O4. These results revealed that the proposed MAREC process for HLLW partitioning is essentially feasible.

Understanding the bonding nature of uranyl ion and functionalized graphene: a theoretical study.

Studying the bonding nature of uranyl ion and graphene oxide (GO) is very important for understanding the mechanism of the removal of uranium from radioactive wastewater with GO-based materials. We have optimized 22 complexes between uranyl ion and GO applying density functional theory (DFT) combined with quasi-relativistic small-core pseudopotentials. The studied oxygen-containing functional groups include hydroxyl, carboxyl, amido, and dimethylformamide. It is observed that the distances between uranium atoms and oxygen atoms of GO (U-OG) are shorter in the anionic GO complexes (uranyl/GO(-/2-)) compared to the neutral GO ones (uranyl/GO). The formation of hydrogen bonds in the uranyl/GO(-/2-) complexes can enhance the binding ability of anionic GO toward uranyl ions. Furthermore, the thermodynamic calculations show that the changes of the Gibbs free energies in solution are relatively more negative for complexation reactions concerning the hydroxyl and carboxyl functionalized anionic GO complexes. Therefore, both the geometries and thermodynamic energies indicate that the binding abilities of uranyl ions toward GO modified by hydroxyl and carboxyl groups are much stronger compared to those by amido and dimethylformamide groups. This study can provide insights for designing new nanomaterials that can efficiently remove radionuclides from radioactive wastewater.

Density Functional Theory Studies of UO22+ and NpO2+ Complexes with Carbamoylmethylphosphine Oxide Ligands

The UO(2)(2+) and NpO(2)(+) extraction complexes with n-octyl(phenyl)-N,N-diisobutylmethylcarbamoyl phosphine oxide (CMPO) and diphenyl-N,N-diisobutylcarbamoyl phosphine oxide (Ph(2)CMPO) have been investigated by density functional theory (DFT) in conjunction with relativistic small-core pseudopotentials. For these extraction complexes, especially the complexes of 2:1 (ligand/metal) stoichiometry, UO(2)(2+) and NpO(2)(+) predominantly coordinate with the phosphoric oxygen atoms. The CMPO and Ph(2)CMPO ligands have higher selectivity for UO(2)(2+) over NpO(2)(+), and for all of the extraction complexes, the metal-ligand interactions are mainly ionic. In most cases, the complexes with CMPO and Ph(2)CMPO ligands have comparable metal-ligand binding energies, that is, the substitution of a phenyl ring for the n-octyl group at the phosphoryl group of CMPO has no obvious influence on the extraction of UO(2)(2+) and NpO(2)(+). Moreover, hydration energies might play an important role in the extractability of CMPO and Ph(2)CMPO for these actinyl ions.

Adsorption of Ce（ IV ） Anionic Nitrato Complexes onto Anion Exchangers and Its Application for Ce （IV） Separation from Rare Earths （ III ）

Abstract Ce(IV) nitrato complexes were adsorbed on two anion exchangers based on polyvinyl pyridine (PVP) and quaternized PVP incorporated into porous silica matrix. The effect of nitric acid concentration (0.5∼6 mol·L−1) and temperature (278∼318 K) on Ce(IV) sorption efficiency was investigated. Sorption increased with increasing nitric acid concentration, indicating that [Ce(NO3)6]2− complex is the main adsorbed Ce(IV) species. Oxidation of sorbents by adsorbed Ce(IV) species resulting in Ce(III) release to the solution was observed. Pyridine based anion exchangers exhibited higher oxidation stability compared to the commercial strong base anion exchanger. Ce(IV) reduction was temperature dependent and obeyed pseudo-first-order reaction kinetics. Column separation of Ce(IV) from La(III) and Y(III) was carried out from 6 mol·L−1 nitric acid with PVP based anion exchanger. Reasonable Ce(IV) breakthrough capacity (0.7 mol·kg−1 PVP) was achieved. No remarkable decrease of capacity was observed within 3 consequent runs. In contrast, Ce(III) leakage due to reduction decreased and breakthrough capacity slightly increased. This effect was more pronounced with increasing temperature. Regeneration with 0.1 mol·L−1 nitric acid was successful (recovery 100%±4%) and Ce solution of high purity (>99.97%) with respect to La and Y content was gained.

Properties and mechanism of molybdenum and zirconium adsorption by a macroporous silica-based extraction resin in the MAREC process

Abstract To achieve effective separation of molybdenum and zirconium in the MAREC process, the adsorption properties and mechanism of Mo(VI) and Zr(IV) with a macroporous CMPO/SiO2‐P (CMPO: octyl(phenyl)‐N,N‐diisobutylcarbamoylmethylphosphine oxide) extraction resin have been studied. By investigating the influence of the aqueous concentrations of H+ and NO3 − on the adsorption of Mo(VI), the composition of complex of Mo(VI) and CMPO/SiO2–P is determined as H2MoO4 · 2CMPO/SiO2–P for dilute aqueous HNO3 and H2MoO3 (NO3)2 · 2CMPO/SiO2–P for concentrated aqueous HNO3, respectively. Similarly, the composition of Zr(IV) and CMPO/SiO2–P is determined as ZrO(NO3)2 · 2CMPO/SiO2–P or Zr(NO3)4 · 2CMPO/SiO2–P in 0.3–4.0 M HNO3, while ZrO2 · 2H2O · 2CMPO/SiO2–P is assumed for lower HNO3 concentration. Based on the compositions of Mo(VI) and Zr(IV) with CMPO/SiO2–P and the elution behavior of Mo(VI) and Zr(IV) by using 0.05 M diethylenetriaminepentaacetic acid (DTPA) at 0.01–1.0 M HNO3, a dynamic interconversion equilibrium between the complexes of Mo(VI) or Zr(IV) and CMPO/SiO2–P is demonstrated to take place in the elution process. To verify the adsorption mechanism, the adsorption and elution behavior of Mo(VI) and Zr(IV) with 0.05 M DTPA‐pH 2.0 was performed from a simulated high level radioactive liquid waste (HLLW) containing Pd(II), Gd(III), Y(III), Eu(III), Sm(III), Mo(VI), and Zr(IV). The results indicate that Mo(VI) and Zr(IV) not only can be efficiently eluted with 0.05 M DTPA‐pH 2.0, but also the elution efficiency is much better than that of 0.5 M H2C2O4 previously used in the MAREC process. The reverse equilibrium of complexes between Mo(VI) or Zr(IV) and CMPO/SiO2–P in high and low acidity was demonstrated, respectively.

Chromatographic Separation of Strontium (II) from a Nitric Acid Solution Containing some Typically Simulated Elements by a Novel Silica‐Based TODGA Impregnated Polymeric Composite in the MAREC Process

Abstract A new kind of macroporous silica‐based N,N,N′,N′‐tetraoctyl‐3‐oxapentane‐1,5‐diamide (TODGA) chelating polymeric adsorption material (TODGA/SiO2‐P) was developed and synthesized by impregnating TODGA molecules into the pores of ∼50 µm SiO2‐P particles to separate strontium effectively from high‐level liquid waste (HLLW). The adsorption of some typically simulated elements Na(I), K(I), Cs(I), Rb(I), Sr(II), Ba(II), and Ru(III) towards TODGA/SiO2‐P adsorbent was investigated by examining the influence of contact time and HNO3 concentration. It was found that with an increase in the HNO3 concentration, the adsorption of Sr(II) onto TODGA/SiO2‐P adsorbent increased quickly from 0.5 M to 2.0 HNO3 and then decreased. TODGA/SiO2‐P exhibited excellent adsorption ability and selectivity for Sr(II) over all of the tested elements, which showed almost no adsorption. Based on the batch experiments, the separation of Sr(II) from a 2.0 M HNO3 solution containing ∼5×10−3 M of the simulated elements was performed by TODGA/SiO2‐P packed column at 25°C and 50°C, respectively. Cs(I), K(I), Ru(III), Ba(III), Rb(I), and Na(I) were found to facilely elute out column and flow into effluent along with feed solution and 2.0 M HNO3 because of no adsorption. Sr(II) adsorbed towards TODGA/SiO2‐P was desorbed sufficiently by distilled water and separated completely from the simulated elements. Its recovery percentage was 100.2% at 25°C and 99.7% at 50°C. Furthermore, the leakage behavior of TODGA from its silica‐based adsorbent was investigated. It was found that the quantity of TODGA leaked was basically equivalent to its solubility in the corresponding HNO3 solution.

Complexation behavior of Eu(III) and Am(III) with CMPO and Ph2CMPO ligands: insights from density functional theory.

A series of extraction complexes of Eu(III) and Am(III) with CMPO (n-octyl(phenyl)-N,N-diisobutyl-methylcarbamoyl phosphine oxide) and its derivative Ph2CMPO (diphenyl-N,N-diisobutyl carbamoyl phosphine oxide) have been studied using density functional theory (DFT). It has been found that for the neutral complexes of 2:1 and 3:1 (ligand/metal) stoichiometry, CMPO and Ph2CMPO predominantly coordinate with metal cations through the phosphoric oxygen atoms. Eu(III) and Am(III) prefer to form the neutral 2:1 and 3:1 type complexes in nitrate-rich acid solutions, and in the extraction process the reactions of [M(NO3)(H2O)7](2+) + 2NO3(-) + nL → ML(n)(NO3)3 + 7H2O (M = Eu, Am; n = 2, 3) are the dominant complexation reactions. In addition, CMPO and Ph2CMPO show similar extractability properties. Taking into account the solvation effects, the metal-ligand binding energies are obviously decreased, i.e., the presence of solvent may have an significant effect on the extraction behavior of Eu(III) and Am(III) with CMPOs. Moreover, these CMPOs reagents have comparable extractability for Eu(III) and Am(III), confirming that these extractants have little lanthanide/actinide selectivity in acidic media.

Separation of actinides from simulated spent fuel solutions by an advanced ion-exchange process

Abstract In order to develop an advanced ion-exchange process to recover U, Pu and other actinide elements from the spent fuels of light-water reactors (LWR), we have manufactured a new type of anion-exchanger characterized by a rapid adsorption–elution rate, high mechanical strength and relatively excellent radiation-resistance. The separation behavior of some actinide elements from simulated spent fuel solutions containing concentrated nitric acid was examined experimentally by ion-exchange chromatography using dilute nitric acid, uranous and thiourea as eluents. U(VI), Np(V), Np(IV) and Np(VI) showed similar adsorption–elution behavior and could be separated from most fission product elements (FPs) such as Cs(I), Sr(II), Mo(VI), Rh(III), Pd(II) and Tc(VII) and trivalent rare earths. Pu(IV) was strongly adsorbed by this anion-exchanger and was effectively eluted as Pu(III) using U(IV)–N 2 H 4 as reductive eluent. Am(III) was not adsorbed and mixed with the non-adsorptive FPs.