Determination of Distribution and Structures of Manganese Aqua and Chloro Complexes in Hydrochloric Acid Solutions at 298 K

Abstract

As part of the research on metal aqua and chloro complexes in hydrochloric acid solutions, which is one of the fundamentals in the development of marine resource and the hydrometallurgical process, we have investigated distributions and structures of metal aqua and chloro complexes have been analyzed. The adsorption behavior of manganese species on anion exchange resin was investigated to understand the approximate distribution of manganese aqua and chloro complexes and found that anionic manganese complexes are formed in the concentrated hydrochloric acid solutions. The UV-vis absorption spectra of the manganese species in hydrochloric acid solutions at 298 K were collected for analysis of complex formation behavior. It was determined that the number of complex species to be three using the first derivative test. A series of UV-vis absorption spectra was analyzed using the thermodynamic model fitting analysis. As the results, the distribution of manganese aqua and chloro complexes and their stability constants were determined. The XANES and EXAFS spectra of individual species were obtained dividing a series of the spectra by the distribution of complexes in matrix form. The individual EXAFS spectra were examined using FEFF fitting analysis. The three manganese aqua and chloro complexes are [Mn^II(H₂O)_x]²⁺ (x = 4, 5, or 6), [Mn^IICl₂(H₂O)₄]⁰, and [Mn^IICl₄]²⁻. Only [Mn^IICl₂(H₂O)₄]⁰ was confirmed to have an octahedral structure. The structures of the rest of complexes were not clearly determined.