Room temperature ionic liquid as a novel medium for liquid/liquid extraction of metal ions

Abstract

Room temperature ionic liquids (RTILs) have been used as novel solvents to replace traditional volatile organic solvents in organic synthesis, solvent extraction, and electrochemistry. The hydrophobic character and water immiscibility of certain ionic liquids allow their use in solvent extraction of hydrophobic compounds. In this work, a typical room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate [C₄mim][PF₆], was used as an alternative solvent to study liquid/liquid extraction of heavy metal ions. Dithizone was employed as a metal chelator to form neutral metal–dithizone complexes with heavy metal ions to extract metal ions from aqueous solution into [C₄mim][PF₆]. This extraction is possible due to the high distribution ratios of the metal complexes between [C₄mim][PF₆] and aqueous phase. Since the distribution ratios of metal dithiozonates between [C₄mim][PF₆] and aqueous phase are strongly pH dependent, the extraction efficiencies of metal complexes can be manipulated by tailoring the pH value of the extraction system. Hence, the extraction, separation, and preconcentraction of heavy metal ions with the biphasic system of [C₄mim][PF₆] and aqueous phase can be achieved by controlling the pH value of the extraction system. Preliminary results indicate that the use of [C₄mim][PF₆] as an alternate solvent to replace traditional organic solvents in liquid/liquid extraction of heavy metal ions is very promising.

Introduction

Room temperature ionic liquids (RTILs) have aroused increasing interest for their promising role as alternative media in synthesis [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], separation [12], [13], [14], and electrochemistry [15], [16] as a result of their unique chemical and physical properties [17], [18], [19], [20], [21]. RTILs can dissolve a wide spectrum of organic, organometallic, and inorganic compounds [18], [19]. Also, they have no detectable vapor pressure and are relatively thermal stable. So, there is no loss of solvent through evaporation with ionic RTILs. This will avoid environmental and safety problems due to volatilization, as is the case in traditional organic solvents. Therefore, they are proposed as novel solvent systems to replace traditional solvents that are generally toxic, flammable, and volatile. RTILs are regarded to have the potential to be alternative reaction media for “Green Chemistry” [22], [23].

Applications of RTILs in analytical chemistry have also started to receive attention recently [12], [14], [24], [25], [26], [27], [28]. For the pioneer works of using RTILs in the extraction of metal ions, several groups presented the use of crown ethers for the extraction of group 1 and 2 ions from aqueous phase [29], [30], [31]. RTILs were further extended to the extraction of heavy metal ions. Rogers and coworkers incorporated thiourea, thioeter, and urea into derivatized imidazolium cations and used these functionized ILs as the extractant in liquid/liquid extraction of Hg²⁺ and Cd²⁺[32]. In that report, low distribution ratios were obtained with less expensive ILs, such as 1-butyl-3-methylimidazolium hexafluorophosphate [C₄mim][PF₆] as extractant for Hg²⁺ and Cd²⁺. The distribution ratios were highly improved when functionized ILs were mixed with [C₄mim][PF₆]. Therefore, functionized ILs played the role of extractant and hydrophobic phase. Rogers and coworkers also utilized [C₄mim][PF₆] with organic and inorganic extractants for the liquid/liquid extraction of heavy metal ions with radiotracer technique [33]. Hence, further analytical applications should be explored for the extraction of heavy metal ions with these novel media. In this work, dithizone, a popular organic metal chelator for analytical applications, was utilized as an organic extractant for liquid/liquid extraction of heavy metal ions with ionic liquid-[C₄mim][PF₆]. UV-Vis spectrometer was employed to examine the formation of metal–chelator complex by observing the spectra of dithizone and metal–dithizone complexes. Atomic absorption (AA) spectrometry, instead of isotope tracer [33], was then used to determine the liquid/liquid extraction efficiencies of metal ions at various conditions. To evaluate the potential use of RTILs to replace traditional volatile organic compounds (VOCs) in liquid/liquid extraction of heavy metal ions, the extractions of metal ions with VOCs and ionic liquid are compared.