The separation of Ni(II) over base metal ions in acidic polymetallic medium: Synergistic extraction and structural evidence

The selective extraction of Ni(II) from a polymetallic solution was achieved with an extractant mixture containing dinonylnaphthalene disulfonic acid (H2DNNDS) and 2-ethylhexyl 4-pyridinecarboxylate ester (4PC, L-II). The equilibrium results indicated that Ni(II) extraction was sensitive to the molar fraction of 4PC in the mixture containing H2DNNDS and 4PC. It was found that individual H2DNNDS or 4PC resulted in low extraction efficiency of Ni(II). Interestingly, when the mole fraction of 4PC in the mixture surpassed 50%, significant synergistic effect was observed for Ni(II) separation over Fe(III), Al(III), Ca(II), Mg(II) and Mn(II), thus highlighting the synergistic role of 4PC. In order to provide structural evidence for synergistic mechanism, two nickel model complexes were synthesized with 1,5-naphthalene disulfonic acid (H2NDS) in the absence/presence of methyl isonicotinate (L-I), which were the corresponding short-chain analogues of H2DNNDS and 4PC, respectively. Combined with single crystal X-ray diffraction of nickel model complexes, the coordination chemistry of nickel complex in the loaded organic phases was disclosed with the Fourier Transform infrared spectroscopy (FT-IR) and electrospray ionization mass spectrometry (ESI-MS). The results showed that Ni(II) was directly coordinated with two monodentate ligands L-I and four water molecules, while no direct interaction existed between Ni(II) and the O atoms of the deprotonated H2NDS in both two nickel model complexes. Hydrogen-bonding interaction between the H atom of coordinated water and the O atom of the deprotonated H2NDS played important role in stabilization of coordination configuration. The selective extraction of Ni(II) might be attributed to the incorporation of the pyridine based nitrogen donor into the inner coordination sphere of Ni(II) to supplant coordinated water.