Thermodynamic modeling of poorly complexing metals in concentrated electrolyte solutions: an X-ray absorption and UV-Vis spectroscopic study of Ni(II) in the NiCl₂-MgCl₂-H₂O system

Knowledge of the structure and speciation of aqueous Ni(II)-chloride complexes is important for understanding Ni behavior in hydrometallurgical extraction. The effect of concentration on the first-shell structure of Ni(II) in aqueous NiCl₂ and NiCl₂-MgCl₂ solutions was investigated by Ni K edge X-ray absorption (XAS) and UV-Vis spectroscopy at ambient conditions. Both techniques show that no large structural change (e.g., transition from octahedral to tetrahedral-like configuration) occurs. Both methods confirm that the Ni(II) aqua ion (with six coordinated water molecules at R_Ni-O = 2.07(2) Å) is the dominant species over the whole NiCl₂ concentration range. However, XANES, EXAFS and UV-Vis data show subtle changes at high salinity (> 2 mol•kg^-1 NiCl₂), which are consistent with the formation of small amounts of the NiCl⁺ complex (up to 0.44(23) Cl at a Ni-Cl distance of 2.35(2) Å in 5.05 mol•kg^-1 NiCl₂) in the pure NiCl₂ solutions. At high Cl:Ni ratio in the NiCl₂-MgCl₂-H₂O solutions, small amounts of [NiCl₂]⁰ are also present. We developed a speciation-based mixed-solvent electrolyte (MSE) model to describe activity-composition relationships in NiCl₂-MgCl₂-H₂O solutions, and at the same time predict Ni(II) speciation that is consistent with our XAS and UV-Vis data and with existing literature data up to the solubility limit, resolving a long-standing uncertainty about the role of chloride complexing in this system.