Synthesis and characterization of trioctylammonium dicarboxylate ionic liquids for heavy metal extraction from aqueous solutions.
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Five hydrophobic trioctylammonium-based ionic liquids (ILs) functionalized with phthalate, succinate, adipate, suberate, and sebacate anions were synthesized for heavy metal extraction from aqueous solutions. The ILs were characterized using 1H NMR, 13C NMR, and FT-IR spectroscopy, with FT-IR spectroscopy confirming the synthesis of the ILs. The thermal stability of the ILs was investigated using thermogravimetric analysis, and the synthesized ILs had decomposition temperatures between 205 and 222 °C. Of the five synthesized ILs, [HTOA][phthalate], [HTOA][adipate], and [HTOA][sebacate] were liquid at room temperature (18 °C) with [HTOA][succinate] and [HTOA][suberate] having melting points of 20 and 26 °C, respectively. The temperature dependences of density, viscosity, and electrical conductivity for the liquid ILs were investigated in the temperature range of 293.15–333.15 K and modelled using a linear model, the Vogel–Fulcher–Tammann model, and the Arrhenius model, respectively. [HTOA][phthalate], [HTOA][adipate], and [HTOA][sebacate] were highly viscous, having viscosities of 734.3, 3369.7, and 2471.7 mPa s, respectively, at 25 °C. The addition of a molecular solvent was shown to be effective in reducing the viscosity as IL mixtures of [HTOA][phthalate], [HTOA][adipate], and [HTOA][sebacate] with 10 wt.% methanol had viscosities of 82.8, 250.7, and 193.5 mPa s, respectively, at 25 °C. Walden plots were prepared to investigate the extent of proton transfer in the liquid IL systems. Analysis of these plots indicated that these ILs had low ionicities. The efficacy of [HTOA][phthalate] and [HTOA][adipate] as extraction solvents in liquid-liquid extraction of Cd(II), Cu(II), and Zn(II) from aqueous solutions was then determined by investigating the effect of extraction time on the extraction process, with four extraction times being used: 5, 15, 30, and 60 min. [HTOA][suberate] was not water stable and was not investigated further. [HTOA][adipate] performed better than [HTOA][phthalate], having extraction efficiencies of 85, 67 and 69% for Cd(II), Cu(II), and Zn(II), respectively, after 60 min. In comparison, [HTOA][phthalate] had extraction efficiencies of 85, 53, and 37% for Cd(II), Cu(II), and Zn(II), respectively, after 60 min. The lower performance of [HTOA][phthalate] may be due to the steric hindrance exerted by the benzene ring present in the phthalate ion. Both ILs showed a higher affinity for Cd(II), with extraction efficiencies for Cd(II) being significantly higher than those for Cu(II) and Zn(II). The recyclability of the [HTOA][adipate] was investigated by using 0.1 M EDTA solution as the regenerating agent. The EDTA solution was effective in extracting Cd(II) and Cu(II), with back-extraction efficiencies of 89 and 82%, respectively. Its extraction of Zn(II) was less effective with a back-extraction efficiency of 41% being achieved.