Improving selective separation of Cu(II) from acidic polymetallic media with 2-ethylhexyl 4-pyridinecarboxylate ester: Extraction behaviors, coordination structure and microscopic mechanism

Abstract

In this work, the extraction selectivity on Cu(II) against other impurity metal ions is greatly improved with 2-ethylhexyl 4-pyridinecarboxylate ester (LII) as a synergist where dinonylnaphthalene sulfonic acid (HDNNS) was used as an extractant. To illuminate the synergistic extraction mechanism, a model complex of Cu(II) with nahpthalene-2-sulfonic acid (HNS) and n-hexyl 4-pyridinecarboxylate ester (LI) was synthesized and characterized by single crystal X-ray diffraction, elemental analysis, thermo-gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR) and electrospray ionization mass spectrometry (ESI-MS), respectively. The crystal structure analysis indicates that the molecular structure of the Cu(II) model complex is determined as [Cu(NS)2(LI)2(H2O)4], where Cu(II) is directly coordinated with two deprotonated HNS, two monodentate LI and two water molecules to form a twisted octahedral configuration. Furthermore, in order to provide parallels to actual solvent extraction, the extracted Cu(II) complex in the non-polar organic phase was also prepared and studied using FT-IR and ESI-MS. Compared with the spectra of both the Cu(II) model complex and the extracted Cu(II) complex, it could be inferred that the extracted Cu(II) complex probably has an identical coordination configuration with the Cu(II) model complex, and the inner coordination sphere of Cu(II) might be occupied with N atoms from pyridine ring in two LII molecules, O atoms from two water molecules and O atoms of two deprotonated HDNNS molecules, respectively. The microscopic mechanism on the selective extraction of Cu(II) was proposed.