Synergistic separation of copper(II) in sulfate solutions through outer-sphere coordination of dinonylnaphthalene disulfonic acid

Abstract Synergistic extraction is a promising strategy to achieve the separation and concentration of metal ions from complex effluents. Herein, the selective extraction behaviors of Cu(II) with 2-ethylhexyl pyridine-4-carboxylate ester (L II ) and dinonylnaphthalene disulfonic acid (H 2 DNNDS) have been investigated. The extraction of impurity ions including Al(III), Fe(III), Mn(II), Mg(II) and Ca(II) can be effectively depressed at the high molar ratio of the ligand L II and H 2 DNNDS, and the separation factors of Cu/Al, Cu/Ca, Cu/Fe, Cu/Mg, and Cu/Mn can reach 840, 1707, 2205, 2241, and 1269, respectively. Moreover, the relationship between the coordination chemistry and the extraction behavior has been demonstrated by X-ray single crystal diffraction, Fourier transform infrared spectroscopy (FT-IR) and electrospray ionization mass spectrometry (ESI-MS). The spectra analyses indicate that the extracted Cu(II) complex would have a similar distorted octahedral coordination environment with the Cu(II) model complex with the corresponding short-chain analogues of the ligand L II and H 2 DNNDS. The inner coordination sphere of Cu(II) complex is occupied with three N atoms from pyridine ring of the ligand L II and three O atoms from water molecules, and the deprotonated H 2 DNNDS acts as a non-coordinating ion-pairing agent to form an outer-sphere coordination structure. Hence, the extraction efficiency enhancement of Cu(II) with the ligand L II after mixed with H 2 DNNDS, can be attributed to the dual contribution of inner- and outer-sphere coordination of synergistic ligands.