Abstract
Molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) have been combined to study the coordination of the Co2+ and Ni2+ ions in ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl)imide ([Tf2N]−) anion and having different organic cations, namely, 1-butyl-3-methylimidazolium ([C4mim]+), 1,8-bis(3-methylimidazolium-1-yl)octane ([C8(mim)2]2+), N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium ([choline]+), and butyltrimethylammonium ([BTMA]+). Co and Ni K-edge XAS data have been collected on 0.1 mol L–1 Co(Tf2N)2 and Ni(Tf2N)2 solutions and on the metallic salts. MD simulations have been carried out to obtain structural information on the metal ion coordination. The analysis of the extended X-ray absorption fine structure (EXAFS) spectra of the solutions has been carried out based on the atomistic description provided by MD, and the studied ILs have been found to be able to dissolve both the Co(Tf2N)2 and Ni(Tf2N)2 salts giving rise to a different structural arrangement around the metal ions as compared to the solid state. The combined EXAFS and MD results showed that the Co2+ and Ni2+ ions are surrounded by a first solvation shell formed by six [Tf2N]− anions, each coordinating in a monodentate fashion by means of the oxygen atoms. The nature of the IL organic cation has little or no influence on the overall spatial arrangement of the [Tf2N]− anions, so that stable octahedral complexes of the type [M(Tf2N)6]4– (M = Co, Ni) have been observed in all the investigated ILs.
Highlights
The extraction and separation of the Co2+ and Ni2+ ions from complex aqueous matrixes have recently received much attention for the recovering of these metals from acidic aqueous solutions originating from mining activity[1,2] and for their recycling from devices that are at their end-of-life.[3]
ionic liquids (ILs) emerged as new media for a variety of applications, such as electroplating, batteries, solar cells, corrosion protection, catalysis, food and pharmaceutical applications.[8−14] ILs have been found to be suitable as the hydrophobic receiving phase for the separation of target metal ions from aqueous solutions.[15−20] thanks to the wide electrochemical windows accessible, they can serve as suitable media for metal electrodepositions.[11,21−25]
M−N g(r)s obtained from the Molecular dynamics (MD) simulations (Figure 3) were calculated to characterize the local structure around the metal ions
Summary
The extraction and separation of the Co2+ and Ni2+ ions from complex aqueous matrixes have recently received much attention for the recovering of these metals from acidic aqueous solutions originating from mining activity[1,2] and for their recycling from devices that are at their end-of-life.[3]. The use of ILs in the simultaneous recovery of cobalt and nickel from aqueous solutions has been investigated in several recent studies aimed at developing more sustainable separation processes either using the “neat” ILs or in the presence of auxiliary extracting ligands.[6,26−30]. In this framework, the knowledge of the nature of the interactions between the dissolved metal ions and the IL anions is key information to understand the solvation/desolvation phenomena[31−33] and the thermodynamics of complex for-. The thermodynamic parameters obtained in some studies on the formation of Ni2+
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