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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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Principal component analysis results for NiCl2-H2O and Ni-MgCl2-H2O system in 350–550 nm and 580–850 nm absorbance bands, respectively.The circle is calculated by 100 ×, where xcalc and xmeas are the model and observed value, respectively. The gray area stands for the level of experimental error of 0.2%-0.5%.
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pone.0119805.g006: Principal component analysis results for NiCl2-H2O and Ni-MgCl2-H2O system in 350–550 nm and 580–850 nm absorbance bands, respectively.The circle is calculated by 100 ×, where xcalc and xmeas are the model and observed value, respectively. The gray area stands for the level of experimental error of 0.2%-0.5%.

Mentions: Based on the analysis of XAS data above, two main Ni(II) species are present in NiCl2 solutions: the fully hydrated aqua ion and the monochloro complex. Principal component analysis (PCA) [44] confirms that two species are required to explain the UV-Vis data for the pure NiCl2 solutions within experimental error (Fig 6). In contrast, three species are required to explain the NiCl2-MgCl2-H2O system dataset (Fig 6), suggesting that [NiCl2]0 may be present in the solutions with the highest ionic strength. This is also consistent with the XAS results of Tian et al. [5]; consequently, the analysis was conducted using the Ni2+, NiCl+ and [NiCl2]0 species. The minor species H+ and [HCl]0 were also included in the speciation model, with the formation constant of the neutral ion pair, [HCl]0, taken from the result of Sverjensky et al. [45] (Table 3).


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)

Principal component analysis results for NiCl2-H2O and Ni-MgCl2-H2O system in 350–550 nm and 580–850 nm absorbance bands, respectively.The circle is calculated by 100 ×, where xcalc and xmeas are the model and observed value, respectively. The gray area stands for the level of experimental error of 0.2%-0.5%.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0119805.g006: Principal component analysis results for NiCl2-H2O and Ni-MgCl2-H2O system in 350–550 nm and 580–850 nm absorbance bands, respectively.The circle is calculated by 100 ×, where xcalc and xmeas are the model and observed value, respectively. The gray area stands for the level of experimental error of 0.2%-0.5%.
Mentions: Based on the analysis of XAS data above, two main Ni(II) species are present in NiCl2 solutions: the fully hydrated aqua ion and the monochloro complex. Principal component analysis (PCA) [44] confirms that two species are required to explain the UV-Vis data for the pure NiCl2 solutions within experimental error (Fig 6). In contrast, three species are required to explain the NiCl2-MgCl2-H2O system dataset (Fig 6), suggesting that [NiCl2]0 may be present in the solutions with the highest ionic strength. This is also consistent with the XAS results of Tian et al. [5]; consequently, the analysis was conducted using the Ni2+, NiCl+ and [NiCl2]0 species. The minor species H+ and [HCl]0 were also included in the speciation model, with the formation constant of the neutral ion pair, [HCl]0, taken from the result of Sverjensky et al. [45] (Table 3).

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