title: 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.
abstract: 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 RNi-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.
authors.affiliation: College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P. R. China; School of Chemical Engineering, The University of Adelaide, Adelaide 5000, South Australia, Australia; School of Geosciences, Monash University, Clayton 3800, Victoria, Australia.
authors.forename: Ning
authors.initials: N
authors.lastname: Zhang
authors.affiliation: Division of Mineralogy, South Australian Museum, Adelaide 5000, South Australia, Australia; School of Geosciences, Monash University, Clayton 3800, Victoria, Australia.
authors.forename: Joël
authors.initials: J
authors.lastname: Brugger
authors.affiliation: School of Chemical Engineering, The University of Adelaide, Adelaide 5000, South Australia, Australia; Division of Mineralogy, South Australian Museum, Adelaide 5000, South Australia, Australia; School of Geosciences, Monash University, Clayton 3800, Victoria, Australia.
authors.forename: Barbara
authors.initials: B
authors.lastname: Etschmann
authors.affiliation: School of Chemical Engineering, The University of Adelaide, Adelaide 5000, South Australia, Australia.
authors.forename: Yung
authors.initials: Y
authors.lastname: Ngothai
authors.affiliation: College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P. R. China.
authors.forename: Dewen
authors.initials: D
authors.lastname: Zeng
mesh_terms: D056831:Coordination Complexes / Q000737:chemistry*; D004573:Electrolytes / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D015636:Magnesium Chloride / Q000737:chemistry*; D008956:Models, Chemical; D009532:Nickel / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D056831:Coordination Complexes; D004573:Electrolytes; D007477:Ions; D012996:Solutions; D015636:Magnesium Chloride; D014867:Water; C022838:nickel chloride; D009532:Nickel