Determination of structures of cobalt(II)-chloro complexes in hydrochloric acid solutions by X-ray absorption spectroscopy at 298 K
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The anion exchange reaction is fundamental to the adsorption and desorption of a specific species from a solution phase to an extracting phase, and it is widely used for separation and waste fluid treatment in industrial fields. However, the details of the anion exchange reaction are poorly understood. Quantitative thermodynamic analysis needs a precise solution condition before and after the exchange reaction. Identification of species adsorbed on the anion exchanger is also necessary because there are multiple species in the solution phase in general. Cobalt is a base metal that is widely used in modern society. One of the authors determined the distribution of cobalt-chloro complexes in hydrochloric acid solutions. It is necessary to know what species are adsorbed on anion exchangers for the thermodynamic analysis of the anion exchange reaction. The comparison in structures between the species in the solution phase and adsorbed on anion exchangers reveals what species are adsorbed. Therefore, the determination of the structures of cobalt-chloro complexes in the solution phase is the next step for quantitative analysis. X-ray absorption spectroscopy (XAS) was used for the structure analysis. Factor analysis can decompose extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra consisting of multiple species into individual spectra of single species using the distribution determined using UV-Vis absorption spectra. Fitting EXAFS theoretical models to the decomposed individual spectra determined the structures of three cobalt-chloro complexes: an octahedron of [CoII(H2O)6]2+, a distorted octahedron of [CoII(H2O)5Cl]+, and a tetrahedron of [CoIICl4]2−. The XANES spectra showed us that the Cl ligand in [CoII(H2O)5Cl]+ was attracted to the center atom of CoII by an electrostatic force, and the bonding system between Cl ligands and CoII in [CoIICl4]2− involved covalency.
KeywordsCobalt-chloro complex Structure X-ray absorption spectroscopy Extended X-ray absorption fine structure X-ray absorption near-edge structure Factor analysis
The authors wish to thank Mr. Yuji Baba and Mr. Kouji Nagahashi for their great devotion to this experimental work. The synchrotron radiation experiments of X-ray absorption spectroscopy were performed at BL14B2 of SPring-8 with the approval of the the Japan Synchrotron Radiation Research Institute (Proposal No. 2008A1941). This work was performed under the Research Program of “The Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” in “The Network Joint Research Center for Materials and Devices” and was one of the projects conducted in the MSTeC Research Center at the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University.
The manuscript was composed by contributions of all authors. All authors have given approval to the final version of the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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