Abstract:

The kinetics of extraction of tetravalent cerium from nitric acid solution by ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (C₄mimNTf₂) in 1,2-dichloroethane was studied by the constant interface cell method. The molecular dynamics (MD) simulations were performed to reveal the effect of the interface on the extraction kinetics. In the experimental section, the extraction rate regime was analyzed according to the dependences of extraction rates on the stirring speed, interfacial area, and temperature. The effect of solution compositions on the extraction rate was also investigated. The experimental results show that a mixed diffusional-kinetic regime occurs, where the diffusion resistance is in the aqueous phase, and chemical reactions occur at the interphase. The aqueous nitric acid concentration significantly influences the extraction rate, because it affects the coordination behaviors of tetravalent ceric ions. The apparent rate constant under the experimental condition was determined as 0.094 L·cm·mol^-1·min^-1 by fitting the experimental data. It also indicates that the additional tri-n-butyl phosphate (TBP) in the organic phase exerts the antagonistic effect on the kinetic with C₄mimNTf₂. The MD simulations employed the general AMBER force field (GAFF) combined with the restrained electrostatic potential (RESP) atom charge. The scaled charge scheme with a scaled factor of 0.8 was adopted for ionic liquids to avoid overestimating the interaction energy between cations and anions. A mixed dissociation model of nitric acid was employed to consider the state of nitric acid molecules in the aqueous phase. In simulations, the number and charge density distribution along the axial, the radial distribution functions (RDF), and diffusion coefficients were calculated. The simulation results show that C₄mimNTf₂ in the organic bulk tends to adsorb at the interface with the polar groups toward the aqueous phase. The center-of-mass RDF between C₄mim⁺ and \mathrm{N}\mathrm{T}{\mathrm{f}}_{\mathrm{2}}^{-} depicts that C₄mimNTf₂ exists in the form of ion pairs or clusters. With the addition of TBP in the organic phase, C₄mimNTf₂ can bond with TBP by the hydrogen bonds between the imidazolium ring and the terminal oxygen atom of the P\stackrel{\text{????}}{?}O group in TBP. Meanwhile, the simulated diffusion coefficients of C₄mimNTf₂ and TBP decrease with the increased organic phase TBP concentration. The hydrogen bond interactions and the variation of diffusion coefficients lead to the antagonistic effect. This study demonstrates that the interface behavior significantly affects the extraction of Ce(Ⅳ) from nitric acid solutions, which provides some reference for the extraction process and mechanism of the ionic liquid-based extraction system

Key words: solvent extraction, kinetics, ionic liquid, molecular simulation, antagonistic effect, interface

摘要：

通过恒界面池法研究离子液体1-丁基-3-甲基咪唑双三氟甲磺酰亚胺盐（C₄mimNTf₂）萃取硝酸溶液中四价铈的动力学，并结合分子动力学模拟分析C₄mimNTf₂在萃取相界面附近的分布及结构特征。萃取动力学实验通过考察搅拌速率、相界面面积、温度以及两相不同组分浓度对Ce(Ⅳ) 萃取速率的影响，表明萃取过程为水相扩散-界面反应控制模式，且磷酸三丁酯（TBP）和C₄mimNTf₂间存在动力学的反协同萃取效应。分子动力学模拟分析不同组分在两相中的分布规律，结果验证了C₄mimNT₂在相界面处存在明显吸附现象。研究表明界面过程对离子液体萃取体系的动力学行为有重要影响，可以为离子液体萃取体系的萃取过程和机理的研究提供一定参考。

关键词: 溶剂萃取, 动力学, 离子液体, 分子模拟, 反协同效应, 界面