Ionic liquids are widely used as green and functional solvents in separation science including critical metals extraction and recovery via ion adsorption and/or exchange. Selective coordination of ionic liquid with the targeted metal ions in the aqueous solutions may be attributed to the emergence of higher specificity of the ionic liquid-based separation process. However, it is still unclear how the structure and stoichiometry of ionic liquid clusters that exist in the solution phase lead to the origin of specific adsorption properties and that control the formation of ionic liquid-metal complexes during targeted metal-ion removal. Hence, in this talk, we report a joint experimental and theoretical investigation, using electrospray ionization mass spectrometry (ESI-MS), nuclear magnetic resonance (NMR), and ab initio molecular dynamics simulations to reveal the formation and their adsorption properties of the 1-Ethyl-3-methylimidazolium chloride (EMIMCl) in solution phase. Sequentially, we employed EMIMCl in the electrode-electrolyte interface (EEI) to selectively adsorb Pb2+ ions in a solution containing both Pb and Cd through the formation of IL-Pb complex during redox process. We found that the redox properties of Pb2+ ions in the EMIMCl-Pb complex changed significantly compared to EEI with only Pb2+ ions without EMIMCl. Further analysis of cyclic voltammograms (CV) of redox cycle of Pb2+ ions with and without EMIMCl reveals that the diffusion of Pb ions and heterogeneous electron transfer was improved during reduction scan of Pb2+ ions to Pb0 on the EMIMCl sites, however, the Pb0 was hardly oxidized back to Pb2+ during oxidation scan, therefore leaves the precipitate of Pb on the electrode. Contrastingly, Cd could not form such complex with EMIMCl owing to Cd as soft Lewis metal ion compared to Pb. It is well known that Pb and Cd are usually co-existed in the wastes produced from metal-finishing, mining, and chemical industries and hard to be removed separately. Our findings demonstrate the selective removal of Pb from Cd (Pb/Cd adsorption ratio = 10) using electrified interfaces with EMIMCl clusters as binding sites. Therefore, we illustrate that understanding the physical and electronic properties of ionic liquids will enable its widespread application of other critical metal removals in separation science.

Understanding the role of ionic liquid clusters in redox-mediated selective metal removal

ACS Spring 2022

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