Coordination and extraction of mercury(ii) with an ionic liquid-based thione extractant

Abstract

A neutral thione extractant, 1,3-diethylimidazole-2-thione (C₂C₂ImT), was prepared from an ionic liquid (IL), 1,3-diethylimidazolium acetate, and used within a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethane)sulfonamide ([C₂mim][NTf₂]), to extract Hg(II) from aqueous solutions of HgCl₂ or Hg(OAc)₂. Investigations of the extraction mechanism, spectroscopic analyses of the extracted species, and crystallographic studies of the interactions of C₂C₂ImT with Hg(II) are reported, including the first structurally characterized Hg-NTf₂ coordination compound, Hg(C₂C₂ImT)₂(NTf₂)₂. Coordination complexes of the thione ligand with Hg(II) show variability in coordination numbers and geometries with stoichiometry, suggesting that the extraction mechanism is dependent on the speciation of mercury in aqueous solution. HgCl₂ can form neutral, extractable complexes with the thione in aqueous solution. Hg(OAc)₂ dissociates on dissolution in water and Hg(II) is extracted through a cation exchange mechanism involving [Hg(C₂C₂ImT)₂]²⁺ ions. The precipitation of neutral mercury complexes from the IL following the extraction of excess mercury suggests a simple and unusual way to recycle the IL.