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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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Distribution of Ni(II) species in NiCl2-H2O (a, b), and NiCl2-MgCl2-H2O (c) solutions and average number of ligand in the first shell of Ni2+ as a function of NiCl2 solution concentration.In (a) and (c), the distributions of species are calculated with the formation constant and MSE parameters determined in this study (all lines); in (b), the distributions of species was calculated for NiCl2 solutions using the model and parameters from literature [10]; In (a), the calculated average numbers of ligands in the first shell of Ni2+ are compared with the number of ligands extracted by EXAFS (Cl ligand, empty circles with error bar; O ligand, triangles with error bar) and XRD [21,25] (Cl ligand, filled circles) analysis are shown for comparison; and (b) the result using the model and parameters from literature [10].
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pone.0119805.g008: Distribution of Ni(II) species in NiCl2-H2O (a, b), and NiCl2-MgCl2-H2O (c) solutions and average number of ligand in the first shell of Ni2+ as a function of NiCl2 solution concentration.In (a) and (c), the distributions of species are calculated with the formation constant and MSE parameters determined in this study (all lines); in (b), the distributions of species was calculated for NiCl2 solutions using the model and parameters from literature [10]; In (a), the calculated average numbers of ligands in the first shell of Ni2+ are compared with the number of ligands extracted by EXAFS (Cl ligand, empty circles with error bar; O ligand, triangles with error bar) and XRD [21,25] (Cl ligand, filled circles) analysis are shown for comparison; and (b) the result using the model and parameters from literature [10].

Mentions: The formation constant of NiCl+ in this work is larger than that reported in the potentionmetric study of Libus and Tialowska [49] and in the spectrophotometric studies of Bjerrum [7], Liu et al. [10] and Paatero and Hummelstedt [12], whereas that of the neutral [NiCl2]0 complex is more negative than that reported by Liu et al. [10] (Table 3) but closer the values from Bjerrum [7] and Paatero and Hummelstedt [12]. The distributions of absorbing species in the NiCl2-H2O and NiCl2-MgCl2-H2O solutions calculated with the new model show a larger percentage of the NiCl+ complex in the NiCl2-MgCl2-H2O solutions (higher Cl/Ni ratio) compared to the NiCl2-H2O solutions, implying a stronger association in the former solutions (Fig 8a and 8c). In addition, the neutral complex [NiCl2]0 can account up to ~38% Ni(II) in the MgCl2 solutions. For comparison, we calculated the distribution of Ni(II) species in NiCl2-H2O solutions using Liu et al. [10]’s model, in which the activities of the species are calculated using the “b-dot” equation (Fig 8b). The agreement is good at low NiCl2 concentrations (< 1 mol∙kg-1 NiCl2). However, the predictions diverge with increasing electrolyte concentration, with Liu et al. [10]’s model showing the neutral complex, [NiCl2]0, playing an important role (up to ~30%) in pure NiCl2 solutions at high concentrations, while our model and UV-Vis data suggest that no significant [NiCl2]0 is present (Fig 8a). This apparent contradiction is the result of the poor description of activity-composition relationships in highly saline solutions in the HKF model. The experimental mean activity coefficient of NiCl2 solutions is compared to the values calculated using the MSE parameters and the model of Liu et al. [10] in Fig 9. The mean activity coefficient calculated in this work agrees well with the experimental data [63], while Liu et al. [10]’s model only reproduces the data at low NiCl2 concentrations (≤ 0.5 mol∙kg-1), with increasingly significant deviations occurring with increasing salt concentrations.


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)

Distribution of Ni(II) species in NiCl2-H2O (a, b), and NiCl2-MgCl2-H2O (c) solutions and average number of ligand in the first shell of Ni2+ as a function of NiCl2 solution concentration.In (a) and (c), the distributions of species are calculated with the formation constant and MSE parameters determined in this study (all lines); in (b), the distributions of species was calculated for NiCl2 solutions using the model and parameters from literature [10]; In (a), the calculated average numbers of ligands in the first shell of Ni2+ are compared with the number of ligands extracted by EXAFS (Cl ligand, empty circles with error bar; O ligand, triangles with error bar) and XRD [21,25] (Cl ligand, filled circles) analysis are shown for comparison; and (b) the result using the model and parameters from literature [10].
© Copyright Policy
Related In: Results  -  Collection

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

pone.0119805.g008: Distribution of Ni(II) species in NiCl2-H2O (a, b), and NiCl2-MgCl2-H2O (c) solutions and average number of ligand in the first shell of Ni2+ as a function of NiCl2 solution concentration.In (a) and (c), the distributions of species are calculated with the formation constant and MSE parameters determined in this study (all lines); in (b), the distributions of species was calculated for NiCl2 solutions using the model and parameters from literature [10]; In (a), the calculated average numbers of ligands in the first shell of Ni2+ are compared with the number of ligands extracted by EXAFS (Cl ligand, empty circles with error bar; O ligand, triangles with error bar) and XRD [21,25] (Cl ligand, filled circles) analysis are shown for comparison; and (b) the result using the model and parameters from literature [10].
Mentions: The formation constant of NiCl+ in this work is larger than that reported in the potentionmetric study of Libus and Tialowska [49] and in the spectrophotometric studies of Bjerrum [7], Liu et al. [10] and Paatero and Hummelstedt [12], whereas that of the neutral [NiCl2]0 complex is more negative than that reported by Liu et al. [10] (Table 3) but closer the values from Bjerrum [7] and Paatero and Hummelstedt [12]. The distributions of absorbing species in the NiCl2-H2O and NiCl2-MgCl2-H2O solutions calculated with the new model show a larger percentage of the NiCl+ complex in the NiCl2-MgCl2-H2O solutions (higher Cl/Ni ratio) compared to the NiCl2-H2O solutions, implying a stronger association in the former solutions (Fig 8a and 8c). In addition, the neutral complex [NiCl2]0 can account up to ~38% Ni(II) in the MgCl2 solutions. For comparison, we calculated the distribution of Ni(II) species in NiCl2-H2O solutions using Liu et al. [10]’s model, in which the activities of the species are calculated using the “b-dot” equation (Fig 8b). The agreement is good at low NiCl2 concentrations (< 1 mol∙kg-1 NiCl2). However, the predictions diverge with increasing electrolyte concentration, with Liu et al. [10]’s model showing the neutral complex, [NiCl2]0, playing an important role (up to ~30%) in pure NiCl2 solutions at high concentrations, while our model and UV-Vis data suggest that no significant [NiCl2]0 is present (Fig 8a). This apparent contradiction is the result of the poor description of activity-composition relationships in highly saline solutions in the HKF model. The experimental mean activity coefficient of NiCl2 solutions is compared to the values calculated using the MSE parameters and the model of Liu et al. [10] in Fig 9. The mean activity coefficient calculated in this work agrees well with the experimental data [63], while Liu et al. [10]’s model only reproduces the data at low NiCl2 concentrations (≤ 0.5 mol∙kg-1), with increasingly significant deviations occurring with increasing salt concentrations.

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