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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.

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Molar absorptivity spectra of individual Ni(II)-chloride species obtained from the analysis of two absorptive band (left, 350–550 nm and right 580–850 nm) spectroscopic data for NiCl2-H2O and NiCl2-MgCl2-H2O systems at room temperature.The molar spectrum for Ni(ClO4)2 solution at room temperature is plotted as solid line for comparison.
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pone.0119805.g007: Molar absorptivity spectra of individual Ni(II)-chloride species obtained from the analysis of two absorptive band (left, 350–550 nm and right 580–850 nm) spectroscopic data for NiCl2-H2O and NiCl2-MgCl2-H2O systems at room temperature.The molar spectrum for Ni(ClO4)2 solution at room temperature is plotted as solid line for comparison.

Mentions: The analysis provides a good agreement between experimental and calculated spectra; for the 350–550 nm range, the largest differences (in absorbance unit) were 0.05 (NiCl2-H2O system) and 0.07 (NiCl2-MgCl2-H2O system); and for the 580–850 nm range, 0.012 and 0.006, respectively. The regressed MSE ion interaction parameters are presented in Table 4, and the optimized formation constants of Ni(II)-chloride complexes and the solubility products of solids obtained from solid-liquid equilibrium data and spectral data are reported in Table 3 together with the comparison of log10Ksp/log10K values at 25°C from this work and the literature [7,10,12,46,47,48,], respectively. The uncertainties of formation constants for NiCl+ and [NiCl2]0 at the 90% confidence level are also listed; they are derived from residual maps as described in the literature [39]. The molar absorptivity spectra of individual Ni(II)-chloride complexes are shown in Fig 7. The molar absorptivity spectra of individual Ni(II)-chloride complexes show a shift to lower energy (red-shift) with increasing substitution of chloride ions. We also conducted a fit in which we refined the molar absorptivity coefficients for the Ni(II) species; the refined the molar absorptivity spectrum is close to the spectrum fixed from measurement of the Ni(ClO4)2 solution, indicating that the analysis method is reliable. There is a large increase in molar absorptivity from Ni2+ to NiCl+; the change in intensity between NiCl+ and [NiCl2]0 was model-sensitive, which is attributed to the low concentration of this complex in the studied solutions. For the 580–850 nm region (Fig 7), the analysis only reveals a red-shift for the molar absorbance spectrum of [NiCl2]0, but failed to provide a realistic molar absorptivity spectrum for this complex. The changes of the calculated spectra of each individual Ni(II) species are analogous with the results obtained for minor amounts of Ni(II) in NaCl solutions at elevated temperatures [10].


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)

Molar absorptivity spectra of individual Ni(II)-chloride species obtained from the analysis of two absorptive band (left, 350–550 nm and right 580–850 nm) spectroscopic data for NiCl2-H2O and NiCl2-MgCl2-H2O systems at room temperature.The molar spectrum for Ni(ClO4)2 solution at room temperature is plotted as solid line for comparison.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0119805.g007: Molar absorptivity spectra of individual Ni(II)-chloride species obtained from the analysis of two absorptive band (left, 350–550 nm and right 580–850 nm) spectroscopic data for NiCl2-H2O and NiCl2-MgCl2-H2O systems at room temperature.The molar spectrum for Ni(ClO4)2 solution at room temperature is plotted as solid line for comparison.
Mentions: The analysis provides a good agreement between experimental and calculated spectra; for the 350–550 nm range, the largest differences (in absorbance unit) were 0.05 (NiCl2-H2O system) and 0.07 (NiCl2-MgCl2-H2O system); and for the 580–850 nm range, 0.012 and 0.006, respectively. The regressed MSE ion interaction parameters are presented in Table 4, and the optimized formation constants of Ni(II)-chloride complexes and the solubility products of solids obtained from solid-liquid equilibrium data and spectral data are reported in Table 3 together with the comparison of log10Ksp/log10K values at 25°C from this work and the literature [7,10,12,46,47,48,], respectively. The uncertainties of formation constants for NiCl+ and [NiCl2]0 at the 90% confidence level are also listed; they are derived from residual maps as described in the literature [39]. The molar absorptivity spectra of individual Ni(II)-chloride complexes are shown in Fig 7. The molar absorptivity spectra of individual Ni(II)-chloride complexes show a shift to lower energy (red-shift) with increasing substitution of chloride ions. We also conducted a fit in which we refined the molar absorptivity coefficients for the Ni(II) species; the refined the molar absorptivity spectrum is close to the spectrum fixed from measurement of the Ni(ClO4)2 solution, indicating that the analysis method is reliable. There is a large increase in molar absorptivity from Ni2+ to NiCl+; the change in intensity between NiCl+ and [NiCl2]0 was model-sensitive, which is attributed to the low concentration of this complex in the studied solutions. For the 580–850 nm region (Fig 7), the analysis only reveals a red-shift for the molar absorbance spectrum of [NiCl2]0, but failed to provide a realistic molar absorptivity spectrum for this complex. The changes of the calculated spectra of each individual Ni(II) species are analogous with the results obtained for minor amounts of Ni(II) in NaCl solutions at elevated temperatures [10].

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