Lead (Pb) and cadmium (Cd) are considered to be two of the major heavy metal pollutants that co-exist in substantial amounts in industrial discharges, mining waste, and fertilizer impurities. They are nonbiodegradable and mobile in the soil where they poison underground water leading to detrimental human health effects. The similarities in the chemical (ionic charge and hydrated radius) and electrochemical (reduction potential) properties of Pb²⁺ and Cd²⁺ make them challenging ions to be efficiently separated from feed streams. In this presentation, we illustrate the electric field-driven formation of orientated ionic liquid [e.g., 1-ethyl-3-methylimidazolium chloride, (EMIMCl)] layers at aqueous electrode-electrolyte interfaces (EEIs) and their functionality toward selective complexation and electrodeposition of divalent Pb²⁺ and Cd²⁺ cations at the electrified interface. We characterized the desolvation, diffusion, and charge transfer properties of EMIMCl-coordinated heavy metal complexes at EEIs and how these properties influence the efficiency, selectivity and nucleation kinetics of targeted metal electrodeposits as a function of chemical stoichiometry (Pb²⁺, Cd²⁺, and EMIMCl) and applied potential employing a combination of experimental (i.e., electrochemical impedance, Raman, and nuclear magnetic resonance spectroscopies as well as mass spectrometry/ion soft landing) and theoretical (density functional theory and molecular dynamics) techniques. We found that the presence of EMIMCl interphase layers improved the Pb²⁺extraction efficiency form a mixture containing both PbCl₂ and CdCl₂ and enabled a superior selectivity factor of 10 after electrochemical cycling. Our results demonstrate the novel functionality of ionic liquid-based aqueous EEIs for the electrochemical separation of challenging critical metal cations with similar chemical and electrochemical properties.

Sandra Pratt

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