Removal of cadmium ions from aqueous solution using very small ionic liquids to water ratio without metal chelator and pH modifications

Solvent extraction is an energy-efficient technology which uses two immiscible phases. In this regard, solvents like hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate based ionic liquids have been used. The hydrophilicity of the metal ions is a challenge to use this method. Coordinating the metal ions by ligands, lowering the pH of the aqueous phase, modifying the ionic liquid itself in such a way that it can coordinate with the metal ions, employment of large ionic liquid to aqueous phase ratio (minimum of 1:1) were also the attempts made to improve the distribution coefficient of the ionic liquids. All these efforts are problematic in hindering the applications of ionic liquids in extraction. In this report, the extraction efficiencies of ionic liquids (C 4 mim][PF 6 ], [C 6 mim][PF 6 ] , [C 8 mim][PF 6 ] and [C 10 mim][PF 6 ]) from water samples containing Cd 2+ using very small ionic liquid to water ratio (1:6 and 1:12) from a solution of concentrations 0.005 mg/L with out using any coordinating agent as an extractant and the need of changing the pH are disclosed. ionic aqueous phase 1:12 demonstrated extraction efficiencies of 75%, 83.75%, 87.50% and 100%, respectively; the 1:6 ratio extracted 87.50%, 100%, 100% and 100%, respectively which shows suitability of the later ratio for better extraction. Moreover, the recyclabilities of [C 6 mim][PF 6 ] and [C 8 mim][PF 6 ] investigated and the result showed that they can be used at least for five cycles. A Linear calibration curve with good coefficient of determination was obtained during the analysis for determination of the metal in the extracts.


INTRODUCTION
The concentration of toxic heavy metals in natural water bodies has been increasing (Motsi et al., 2009;Hernandez et al., 2010;Pinto et al., 2011). Heavy metals such as cadmium, zinc, lead, chromium, nickel, copper, vanadium, platinum, silver, and titanium are generated in electroplating, electrolysis depositions, conversion coating, and anodizing-cleaning, milling, and etching industries.
Significant amount of heavy metals wastes like tin, lead, and nickel result from printed circuit board (PCB) manufacturing. Wood processing industries where a chromated copper-arsenate wood treatment produces arsenic containing wastes; inorganic pigment manufacturing producing pigments contain chromium compounds and cadmium sulfide; petroleum refining generates conversion catalysts contaminated with nickel, vanadium, and chromium; and photographic operations producing film with high concentrations of silver and ferrocyanide. All of these generators produce a large quantity of wastewaters, residues, and sludge that can be categorized as hazardous wastes requiring extensive waste treatment (Gunatilake, 2015). Consequently, they are accumilated in food chains that pose a threat to human health, animals and plants and ecological systems (Manahan, 2011; *Corresponding author: atakiltabebe1@gmail.com Gunatilake, 2015). Therefore, efficient treatment of water by removing toxic heavy metals has been one of the major concerns. A number of specialized processes have been developed for the removal of metals from waste discharges. These include chemical precipitation (Ku and Jung, 2001), coagulation/flocculation (Samrani et al., 2008), ion exchange (Kang et al., 2004), electrochemical operations (Wang et al., 2007), biological operations (Pavasant et al., 2006), adsorption (Fu and Wang, 2011), filtration, and membrane processes (Landaburu-Aguirre et al., 2009). The choice of method is based on the concentration of the metal ions in the solution and the cost of treatment (Richardson and Harker, 2002). Solvent extraction is an energy-efficient technology which uses two immiscible phases (conventionally an organic phase and an aqueous phase) (Alonso et al., 2006;de los Ríos et al., 2009). In this regard, different solvents have been used in the recovery and separation of metals from aqueous solutions. One way of removing metal ions from an aqueous phase is by dissolving extractants such as di(2-ethylhexyl) phosphoric acid, tris(2ethylhexyl)amine, liquid phosphine oxides in an organic solvent such as kerosene and toluene. One disadvantage of this method of extraction is the loss of organic solvents via volatilization, which has a detrimental impact on the environment and human health (Lancaster, 2010). Consequently, different attempts have been implemented to minimize these draw backs. Replacement of the volatile organic solvents by non-volatile ones such as ionic liquids (ILs) is among such efforts (Wei et al., 2003;Domańska and Zhao, 2005;Platzer et al., 2015).
Ionic liquids (ILs) are low temperature melting salts (below 100 0 C) (Villar-Garcia et al., 2012;Atakilt Abebe et al., 2013). They are fundamentally different from salt solutions and molecular solvents. They are well characterized by unique properties, such as negligible vapor pressure, good thermal stability, tunable viscosity and designed miscibility with water and organic solvents, good extractability for various organic compounds as well as possession of cavities in their three dimensional microstructure (Huang et al., 2005).
For ionic liquids to be effectively used as solvents in liquid-liquid extractions, the knowledge of the mutual solubilities between ILs and the second liquid phase is fundamental. The mutual solubilities of water and imidazolium-based ILs were extensively studied and reported elsewhere.
The results indicate that mutual solubilities are primarily defined by the anion followed by the  (Visser et al., 2001;Wei et al., 2003;Dietz, 2006 (Wei et al., 2003), grafting coordinating agents on the cations of the ILs themselves (Visser et al., 2002), modifying the pH condition using hydrochloric acid solutions (Wei et al., 2003;Hernandez et al., 2010). Large IL to water ratio quantity (1:1) was also employed (Earle and Seddon, 2000;Sereshti et al., 2014). Moreover, the extraction activities using ionic liquids were carried out from water samples containing far greater concentrations (Hernandez et al., 2010) than the metal ions found in real samples (Xu et al., 2010).
The addtional steps in modifying the ILs were found to incur additional synthetic cost and elongated time. Moreover, the use of large IL (which are expensive) (Abbott et al., 2004;Hayyan et al., 2013) to aqueous phase ratio makes the employment of these types of ILs impractical.
There is no literature report on the extraction of toxic heavy metals in general and Cd 2+ in particular minimizing the above problemes combined. Taking  reduce the concern about the high cost of the solvent jointly with the problems associated with their disposal. Thus, the scientific and industrial community may be encouraged to be benefited from the attractive numerous properties of ILs from the implementation of this technology in research and production.

Synthesis of the ionic liquids (ILs)
Three new ILs were synthesized and purified according to reported procedures (Bonhote et al., 1996). The ILs were synthesized in two steps.

The first step involved synthesis of ILs with
halide anions and the second step, exchange of the halide anions with hexafluorphosphate, PF 6 and characterized using 1 H and 13 C NMR.  Maximum integration times (sec.) Extraction using 1:12 IL to aqueous phase ratio Similar procedure as above was employed except that 0.5 mL of IL was used as extractant.

Calibration of the ICP-OES instrument
The ICP-OES was calibrated using standard solutions of cadmium ion concentrations indicated in Table 2. The results clearly show that the calibration curve with good coefficient of determination was obtained during the analysis.

Characterization of the ILs
Nuclear magnetic resonance (NMR) spectroscopy is one of the powerful techniques by which the synthesis and purity of ILs is investigated.
Therefore, proton( 1 H) and carbon ( (Table   3, Figures 1 and 2).   97, 22.48, 26.00, 28.77,28.89, 29.81, 31.59, 35.93, 49.93, 122.18, 123.60, 136.63  28.90, 29.29, 29.31, 29.45, 29.87, 31.81, 35.98, 49.95, 122.19, 123.83, 138.88   (Huang et al., 2005). It is also clear that the extraction ability of the ILs increases with the alkyl chain length on the cation (Table 4, Figure   3). This is speculated to be due to the increase in the three dimensional cavity size and density as the alkyl chain length increases. For a given IL, better extraction was achieved when the IL to aqueous phase ratio is 1:6 (Table 5) compared to 1:12 (Table   4). This may be due to the larger number of cavities in the IL in the former ratio.   Where (Ci) aq : concentration of the Cd +2 in the aqueous phase before extraction; (Cf) aq : concentration of the Cd +2 in the aqueous phase after extraction; and C IL : concentration of the Cd +2 in the ionic liquid phase. The re-usability of two ILs was tested by recycling five times each. The concentration of Cd 2+ before and after the extraction in the aqueous phase for each cycle is indicated in Table 6. The trend of the extraction ability of the ILs for each cycle is showed in Figure 4a-