Ionic liquid extraction of heavy metal ions by 2-aminothiophenol in 1-butyl-3-methylimidazolium hexafluorophosphate and their association constants

The ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM]PF6, was synthesized and characterized by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. The optimum conditions for the extraction of Ni2+, Cu2+ and Pb2+ in water by liquid–liquid extraction method were investigated using the synthetic ionic liquid combined with 2-aminothiophenol ligand. The results showed that while the optimum pH for the extraction of Ni2+ and Pb2+ was 4–6 and 5, respectively; the extraction of Cu2+ was independent of the pH of solution. The extraction equilibria of Ni2+, Cu2+ and Pb2+ were reached within 120, 30 and 30 min, respectively. The existence of sodium, calcium, magnesium, sulfate and chloride ions did not significantly affect the extraction efficiency of all metal ions. The stoichiometries of complexes between Ni2+, Cu2+ and Pb2+ and the ligand are 1:3, 1:2 and 1:2, respectively. The extracted Ni2+ in ionic liquid phase could be back-transferred into 3% H2O2 in 0.5 M HNO3 at 20 min of contact time and the extracted Cu2+ and Pb2+ could be stripped with 1 M HNO3 at the contact time of 20 and 10 min, respectively, with the percentage stripping higher than 95%. The extraction efficiency of all metal ions with the ligand in the ionic liquid was higher than that obtained in chloroform using the same conditions. The association constants of complexes between Ni2+and Pb2+ and the ligand in ionic liquid are 2.26 × 107 and 2.97 × 102, respectively.