Limits...
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.

Zhang N, Brugger J, Etschmann B, Ngothai Y, Zeng D - PLoS ONE (2015)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

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.

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.

Show MeSH

Related in: MedlinePlus

Comparison experimental and predicted water activity for NiCl2 solution at room temperature.The line is predicted one in this work and the dot line represents the prediction one without considering effect of complexes in system, corresponding to the dot line in Fig 9; the circles are literature values [63].
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4401718&req=5

pone.0119805.g011: Comparison experimental and predicted water activity for NiCl2 solution at room temperature.The line is predicted one in this work and the dot line represents the prediction one without considering effect of complexes in system, corresponding to the dot line in Fig 9; the circles are literature values [63].

Mentions: The solubility products from this work are mostly in agreement with the literature values [46–48]. The solubility isotherms calculated using the parameters from Tables 3 and 4 are displayed in Fig 10, and are consistent with the experimental values [64]. If ion association is neglected for the NiCl2-MgCl2-H2O ternary system, the results are unsatisfactory as shown in the dashed line of Fig 10 using the Pitzer completely dissociated model (see [47,67] for details of calculation procedure), although the solubility products of the solid phases are comparable in both models as tabulated in Table 3. Furthermore, the predicted water activity in the NiCl2-H2O system in this work is in excellent agreement with the literature value [63] (Fig 11). In the absence of Ni(II)-chloride complexing, the calculated water activity for pure NiCl2 solutions deviated from the experimental values at high salt concentration (dotted line in Fig 11). The deviation occurs at higher concentration for NiCl2 solutions than that for ZnCl2 solutions (~2.5 mol∙kg-1 vs ~0.05 mol∙kg-1, respectively [65]); this is consistent with the smaller extent of chloro complex formation in Ni(II) chloride solution relative to Zn(II) chloride solutions [68–70]. We also attempted to fit the experimental mean activity coefficient of NiCl2 solutions without considering the NiCl+ complex (dotted line in inset of Fig 9), with no significant decrease in fit quality.


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.

Zhang N, Brugger J, Etschmann B, Ngothai Y, Zeng D - PLoS ONE (2015)

Comparison experimental and predicted water activity for NiCl2 solution at room temperature.The line is predicted one in this work and the dot line represents the prediction one without considering effect of complexes in system, corresponding to the dot line in Fig 9; the circles are literature values [63].
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4401718&req=5

pone.0119805.g011: Comparison experimental and predicted water activity for NiCl2 solution at room temperature.The line is predicted one in this work and the dot line represents the prediction one without considering effect of complexes in system, corresponding to the dot line in Fig 9; the circles are literature values [63].
Mentions: The solubility products from this work are mostly in agreement with the literature values [46–48]. The solubility isotherms calculated using the parameters from Tables 3 and 4 are displayed in Fig 10, and are consistent with the experimental values [64]. If ion association is neglected for the NiCl2-MgCl2-H2O ternary system, the results are unsatisfactory as shown in the dashed line of Fig 10 using the Pitzer completely dissociated model (see [47,67] for details of calculation procedure), although the solubility products of the solid phases are comparable in both models as tabulated in Table 3. Furthermore, the predicted water activity in the NiCl2-H2O system in this work is in excellent agreement with the literature value [63] (Fig 11). In the absence of Ni(II)-chloride complexing, the calculated water activity for pure NiCl2 solutions deviated from the experimental values at high salt concentration (dotted line in Fig 11). The deviation occurs at higher concentration for NiCl2 solutions than that for ZnCl2 solutions (~2.5 mol∙kg-1 vs ~0.05 mol∙kg-1, respectively [65]); this is consistent with the smaller extent of chloro complex formation in Ni(II) chloride solution relative to Zn(II) chloride solutions [68–70]. We also attempted to fit the experimental mean activity coefficient of NiCl2 solutions without considering the NiCl+ complex (dotted line in inset of Fig 9), with no significant decrease in fit quality.

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

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.

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.

Show MeSH
Related in: MedlinePlus