Task-Specific Ionic Liquids Incorporating Novel Cations for the Coordination and Extraction of Hg²⁺ and Cd²⁺:  Synthesis, Characterization, and Extraction Studies

Abstract

A series of hydrophobic task-specific ionic liquids designed to extract Hg2+ and Cd2+ from water were prepared by appending urea-, thiourea-, and thioether-substituted alkyl groups to imidazoles and combining the resulting cationic species with PF6-. The new ionic liquids were characterized and investigated for their metal ion extraction capabilities. When used in liquid/liquid extraction of Hg2+ and Cd2+ from aqueous solutions, the metal ion distribution ratios increased several orders of magnitude, regardless of whether the ionic liquids were used as the sole extracting phase or doped into a series of [1-alkyl-3-methylimidazolium][PF6] (alkyl = n-C4-C8) ionic liquids to form a 1:1 solution. In the 1:1 mixtures, as the length of the alkyl chain increased from butyl to hexyl to octyl, the metal ion distribution ratios increased. Increasing the ratio TSIL/[C4mim][PF6] resulted in higher distribution ratios for both Hg2+ and Cd2+. Overall, the thiourea- and urea-derivatized cations yielded the highest distribution ratios, and those for Hg2+ were higher than those for Cd2+; however, a change in aqueous-phase pH does not promote the stripping of metal ions from the extracting phase. The combination of these imidazolium cations and PF6- produced ionic liquids with decreased thermal stability in comparison to [C(n)mim]-[PF6]. Gaussian98 restricted Hartree-Fock geometry optimizations for one of the thiourea-appended cations shows the charge delocalization around the ring and suggests that the thiourea group may aid in deprotonating the imidazolium ring and may be responsible for the lowered thermal stability of these cations.

Ionic Liquids in Analytical Chemistry

“…Most water-immiscible RTILs contain either PF 6 -or bis[(trifluoromethyl)sulfonyl]imide anion ( Table 1). The functionality of the cations was manipulated to coordinate Hg 2+ and Cd 2+ and extract them from water (28). These task-specific ILs incorporate urea-, thiourea-, and thioether-substituted alkyl groups into the imidazolium parent compound, and then this cation is combined with PF 6 -to form the water-immiscible RTIL (Table 1).…”

Selective Extraction of Copper, Mercury, Silver, and Palladium Ions from Water Using Hydrophobic Ionic Liquids

“…[22][23][24][25] Task-specific ionic liquids (TSIL), can extract cations without ion-exchange. [26][27][28] TSILs are ionic liquids with functional groups appended to the cation. Some TSILs appear to be efficient for extraction with high selectivity for some metal ions such as mercury, cadmium, 27,28 or actinides.…”

“…[26][27][28] TSILs are ionic liquids with functional groups appended to the cation. Some TSILs appear to be efficient for extraction with high selectivity for some metal ions such as mercury, cadmium, 27,28 or actinides. 26 In this work, we investigate the ability of selected ionic liquids to extract offensive or valuable cations from water.…”

Cyclodextrin-ionic liquid polyurethanes for application in drinking water treatment

“…However, the most important property of ionic liquids that has been exploited in this study is their ability to extract metals from aqueous media (Cruz, 2000;Liu et al, 2005). In this regard, Visser et al (2002) have reported the successful extraction of cadmium (Cd 2+ ) and mercury (Hg 2+ ) from an aqueous medium using ionic liquids via the formation of an IL-metal complex.…”