离子液体萃取硝酸中Ce(Ⅳ)的动力学研究

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 NTf^-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==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.