Cation Effect of Chloride Salting Agents on Transition Metal Ion Hydration and Solvent Extraction by the Basic Extractant Methyltrioctylammonium Chloride

Abstract

The addition of a nonextractable salt has an important influence on the solvent extraction of metal ions, but the underlying principles are not completely understood yet. However, relating solute hydration mechanisms to solvent extraction equilibria is key to understanding the mechanism of solvent extraction of metal ions as a whole. We have studied the speciation of Co(II), Zn(II), and Cu(II) in aqueous solutions containing different chloride salts to understand their extraction to the basic extractant methyltrioctylammonium chloride (TOMAC). This includes the first speciation profile of Zn(II) in chloride media with the three Zn(II) species [Zn(H 2 O) 6 ] 2+ , [ZnCl 3 H 2 O] − , and [ZnCl 4 ] 2– . The observed differences in extraction efficiency for a given transition metal ion can be explained by transition metal ion hydration due to ion–solvent interactions, rather than by ion–solute interactions or by differences in speciation. Chloride salting agents bearing a cation with a larger hydration Gibbs free energy reduce the free water content more, resulting in a lower hydration for the transition metal ion. This destabilizes the transition metal chloro complex in the aqueous phase and increases the extraction efficiency. Salting agents with di- and trivalent cations reduce the transition metal chloro complex hydration less than expected, resulting in a lower extraction efficiency. The cations of these salting agents have a very large hydration Gibbs free energy, but the overall hydration of these salts is reduced due to significant salt ion pair formation. The general order of salting-out strength for the extraction of metal ions from chloride salt solutions is Cs + < Rb + < NH 4 + ≈ K + < Al 3+ ≈ Mg 2+ ≈ Ca 2+ ≈ Na + < Li + . These findings can help in predicting the optimal conditions for metal separation by solvent extraction and also contribute to a broader understanding of the effects of dissolved salts on solutes.

Thermodynamic Modeling of Salting Effects in Solvent Extraction of Cobalt(II) from Chloride Media by the Basic Extractant Methyltrioctylammonium Chloride

“…The free water content of the aqueous phase can be lowered by increasing the salt concentration and by increasing the Gibbs free energy of hydration of the salt cation, while taking self-association of the cation and the anion of the salt into account. 26 …”

“…The full lines represent the model calculations, while the points are experimental data from our previous papers. 25 , 26 …”

“… 51 The shift in formation of CoCl + and CoCl 4 2– to higher chloride concentrations in CaCl 2 , MgCl 2 , and AlCl 3 media can be explained by a lower free chloride concentration due to ion-pair formation between those salt cations and chloride. 26 …”

“…The Co(II) speciation in aqueous and organic phases and the Co(II) extraction data from different salt solutions by 0.2 mol L –1 TOMAC in toluene were taken from our previous papers and further literature analysis. 25 , 26 , 51 Other experimental data were determined within this work. Standard-state thermodynamic data were taken from the literature.…”

Modulation of High-Spin Co(II) in Li/Co-MOFs as Efficient Fenton-like Catalysts

“…In the initial Li­(I)/Co­(II)/carboxylate ligand/DMF/H 2 O system, the Li­(I) ion would be significantly hydrated to form Li + (H 2 O) 4 for its high charge density, thus decreasing the amount of free water molecules available for the hydration of the Co­(II) ion . We speculate that the Co­(II) ion exists in the form of Co 2+ (H 2 O) n (DMF) 6– n ( n ≤ 6).…”