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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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The distribution maps of analysis error of XAS spectra for 2 mol∙kg-1 (a) and 5.05 mol∙kg-1 (b) NiCl2 solution as a function of number of Cl ligands.The gray area represents 90% confidence level.
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pone.0119805.g004: The distribution maps of analysis error of XAS spectra for 2 mol∙kg-1 (a) and 5.05 mol∙kg-1 (b) NiCl2 solution as a function of number of Cl ligands.The gray area represents 90% confidence level.

Mentions: The Ni K-edge EXAFS k- and R-space spectra for the two solid standards and the four solutions are shown in Fig 3. The small amount of replacement of water molecules by chloride ions upon increasing NiCl2 concentration is clearly shown by the phase shift in the imaginary part of the EXAFS Fourier transform (arrow in Fig 3c). In the final analysis, Ni(II) coordination was constrained to be 6, consistent with the result of octahedral configuration obtained from XANES data; the fitting results are listed in Table 2. For all solutions, the refined Ni-O bond distances are 2.07(2) Å, which is consistent with earlier studies [31,37]. For the 1 and 2 mol∙kg-1 NiCl2 solutions, the fully hydrated ([Ni(H2O)6]2+) model reproduced well the experimental k-space and R-space EXAFS spectra. The effect of replacing one of the water molecules with a chloride ligand on the fit quality (reduced chi-square, χ2 [38]) of the 2 mol∙kg-1 NiCl2 solution is illustrated in Fig 4a. The absolute minimum of χ2 is located at 0 chloride, and the residuals increase slowly with addition of Cl, resulting in a large uncertainty at the 90% confidence level. In contrast, χ2 shows a minimum at 0.44 Cl for the 5.05 mol∙kg-1 NiCl2 corresponding to the “deepest” depression with an uncertainty of +/– 0.23 Cl (Fig 4b). As expected, the Cl-ion resides at a longer distance than O (RNi-Cl 2.35(2) Å vs. RNi-O 2.07(2) Å) due to the larger hard-sphere radius. These results are close to those obtained using XRD by Waizumi et al. [25]; for a 5.06 mol∙kg-1 NiCl2 solution, these authors reported NNi-O = 5.6 ± 0.2 and RNi-O = 2.06(2) Å; and NNi-Cl = 0.4 ± 0.2 and RNi-Cl = 2.37(2) Å.


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)

The distribution maps of analysis error of XAS spectra for 2 mol∙kg-1 (a) and 5.05 mol∙kg-1 (b) NiCl2 solution as a function of number of Cl ligands.The gray area represents 90% confidence level.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0119805.g004: The distribution maps of analysis error of XAS spectra for 2 mol∙kg-1 (a) and 5.05 mol∙kg-1 (b) NiCl2 solution as a function of number of Cl ligands.The gray area represents 90% confidence level.
Mentions: The Ni K-edge EXAFS k- and R-space spectra for the two solid standards and the four solutions are shown in Fig 3. The small amount of replacement of water molecules by chloride ions upon increasing NiCl2 concentration is clearly shown by the phase shift in the imaginary part of the EXAFS Fourier transform (arrow in Fig 3c). In the final analysis, Ni(II) coordination was constrained to be 6, consistent with the result of octahedral configuration obtained from XANES data; the fitting results are listed in Table 2. For all solutions, the refined Ni-O bond distances are 2.07(2) Å, which is consistent with earlier studies [31,37]. For the 1 and 2 mol∙kg-1 NiCl2 solutions, the fully hydrated ([Ni(H2O)6]2+) model reproduced well the experimental k-space and R-space EXAFS spectra. The effect of replacing one of the water molecules with a chloride ligand on the fit quality (reduced chi-square, χ2 [38]) of the 2 mol∙kg-1 NiCl2 solution is illustrated in Fig 4a. The absolute minimum of χ2 is located at 0 chloride, and the residuals increase slowly with addition of Cl, resulting in a large uncertainty at the 90% confidence level. In contrast, χ2 shows a minimum at 0.44 Cl for the 5.05 mol∙kg-1 NiCl2 corresponding to the “deepest” depression with an uncertainty of +/– 0.23 Cl (Fig 4b). As expected, the Cl-ion resides at a longer distance than O (RNi-Cl 2.35(2) Å vs. RNi-O 2.07(2) Å) due to the larger hard-sphere radius. These results are close to those obtained using XRD by Waizumi et al. [25]; for a 5.06 mol∙kg-1 NiCl2 solution, these authors reported NNi-O = 5.6 ± 0.2 and RNi-O = 2.06(2) Å; and NNi-Cl = 0.4 ± 0.2 and RNi-Cl = 2.37(2) Å.

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