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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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Background-subtracted UV-Vis spectra of (a) NiCl2-H2O system with salt concentration range from 0.1 to 4.5 mol∙kg-1 at room temperature; background subtracted molar absorbance of Ni(ClO4)2 solutions with 0.05 and 4.46 mol∙kg-1 salt concentration are shown in inset; and (b) NiCl2-MgCl2-H2O system, for a constant NiCl2 concentration of 0.05 m and MgCl2 concentrations from 0 to 5.7 m.The inset show the location of the band at ~400 nm as a function of total Cl concentration.
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pone.0119805.g005: Background-subtracted UV-Vis spectra of (a) NiCl2-H2O system with salt concentration range from 0.1 to 4.5 mol∙kg-1 at room temperature; background subtracted molar absorbance of Ni(ClO4)2 solutions with 0.05 and 4.46 mol∙kg-1 salt concentration are shown in inset; and (b) NiCl2-MgCl2-H2O system, for a constant NiCl2 concentration of 0.05 m and MgCl2 concentrations from 0 to 5.7 m.The inset show the location of the band at ~400 nm as a function of total Cl concentration.

Mentions: The baseline-corrected room-temperature spectra (Raw spectral data was collected in S9 and S10 Tables) for both sets of solutions, one with systematically increasing NiCl2 concentration and the other containing constant Ni(II) (0.05 mol∙kg‒1) but systematically increasing the MgCl2 concentration, are presented in Fig 5a and 5b, respectively. The spectra show two characteristic absorption features at 350–500 and 600–800 nm. With increasing NiCl2 concentration, the absorbance increases and the spectra display a systematic red-shift: the band in the range of 350–500 nm shifts from ~393 to ~405 nm (arrow A in Fig 5a), and the bands in the 600–800 nm range (arrows B and C in Fig 5a) shifts from ~720 to ~737 nm and ~656 to ~670 nm, respectively. These features become more pronounced for the Ni-Mg-Cl system at higher MgCl2 concentration, with the ~393 nm band shifting up to ~415 nm (inset in Fig 5b) and the ~720 band shifting to ~770 nm. The shoulder at ~654 nm becomes weaker (arrow B in Fig 5b, almost vanishing) at the higher chloride concentration. To check the effect of ionic strength on the UV-Vis spectra, solutions of 0.05 and 4.46 mol∙kg-1 Ni(ClO4)2 were measured (inset in Fig 5a). No shift of the peak position was observed, and the molar absorbance coefficients remained nearly constant. Deviations from the Beer-Lambert law at high salinity [75,40] can be due to the influence of the dielectric constant of the medium on the absorption of light by a particular complex [41], or to changes in the electronic structure of the complex induced by changes in the structure of the solvent and in the outer coordination shells of the complex. These effects can be difficult to quantify. The Ni(II) perchlorate data in Fig 5a, however, clearly indicate that the structure of the Ni(II) aqua ion is not sensitive to changes in solvent structure as the salt concentration changes from 0.05 to 4.5 mol∙kg-1; this stable structure was also confirmed by the neutron diffraction study of Newsome et al. [42]. Consequently, the Beer-Lambert law appears to be obeyed up to high salinity in this system.


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)

Background-subtracted UV-Vis spectra of (a) NiCl2-H2O system with salt concentration range from 0.1 to 4.5 mol∙kg-1 at room temperature; background subtracted molar absorbance of Ni(ClO4)2 solutions with 0.05 and 4.46 mol∙kg-1 salt concentration are shown in inset; and (b) NiCl2-MgCl2-H2O system, for a constant NiCl2 concentration of 0.05 m and MgCl2 concentrations from 0 to 5.7 m.The inset show the location of the band at ~400 nm as a function of total Cl concentration.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4401718&req=5

pone.0119805.g005: Background-subtracted UV-Vis spectra of (a) NiCl2-H2O system with salt concentration range from 0.1 to 4.5 mol∙kg-1 at room temperature; background subtracted molar absorbance of Ni(ClO4)2 solutions with 0.05 and 4.46 mol∙kg-1 salt concentration are shown in inset; and (b) NiCl2-MgCl2-H2O system, for a constant NiCl2 concentration of 0.05 m and MgCl2 concentrations from 0 to 5.7 m.The inset show the location of the band at ~400 nm as a function of total Cl concentration.
Mentions: The baseline-corrected room-temperature spectra (Raw spectral data was collected in S9 and S10 Tables) for both sets of solutions, one with systematically increasing NiCl2 concentration and the other containing constant Ni(II) (0.05 mol∙kg‒1) but systematically increasing the MgCl2 concentration, are presented in Fig 5a and 5b, respectively. The spectra show two characteristic absorption features at 350–500 and 600–800 nm. With increasing NiCl2 concentration, the absorbance increases and the spectra display a systematic red-shift: the band in the range of 350–500 nm shifts from ~393 to ~405 nm (arrow A in Fig 5a), and the bands in the 600–800 nm range (arrows B and C in Fig 5a) shifts from ~720 to ~737 nm and ~656 to ~670 nm, respectively. These features become more pronounced for the Ni-Mg-Cl system at higher MgCl2 concentration, with the ~393 nm band shifting up to ~415 nm (inset in Fig 5b) and the ~720 band shifting to ~770 nm. The shoulder at ~654 nm becomes weaker (arrow B in Fig 5b, almost vanishing) at the higher chloride concentration. To check the effect of ionic strength on the UV-Vis spectra, solutions of 0.05 and 4.46 mol∙kg-1 Ni(ClO4)2 were measured (inset in Fig 5a). No shift of the peak position was observed, and the molar absorbance coefficients remained nearly constant. Deviations from the Beer-Lambert law at high salinity [75,40] can be due to the influence of the dielectric constant of the medium on the absorption of light by a particular complex [41], or to changes in the electronic structure of the complex induced by changes in the structure of the solvent and in the outer coordination shells of the complex. These effects can be difficult to quantify. The Ni(II) perchlorate data in Fig 5a, however, clearly indicate that the structure of the Ni(II) aqua ion is not sensitive to changes in solvent structure as the salt concentration changes from 0.05 to 4.5 mol∙kg-1; this stable structure was also confirmed by the neutron diffraction study of Newsome et al. [42]. Consequently, the Beer-Lambert law appears to be obeyed up to high salinity in this system.

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