Although the Ca2+-F− association is of great importance for aqueous environments and industrial systems containing F−, as well as for defluorination processes, many details of the association solvation structures and...

Solubility data for the reciprocal system Ca2+, Mn2+//F–, SO4 2– + H2O is very important for the removal of fluoride or calcium ions from the MnSO4 aqueous solution by CaF2(s) precipitation; however, these data have not been reported in literature to date. In this work, the solubility data for the ternary systems CaF2 + MnSO4 + H2O, CaF2 + MnF2 + H2O, and MnF2 + MnSO4 + H2O and the quaternary systems CaF2 + CaSO4 + MnSO4 + H2O and CaF2 + MnF2 + MnSO4 + H2O were measured in detail at 298.15 K. The results showed that CaF2(s) solubility increases monotonically with increasing MnSO4 concentration in the CaF2 + MnSO4 + H2O system, and MnSO4 produces a stronger salting-in effect on CaF2(s) than ZnSO4. CaF2(s) solubility decreases with the increase in the MnF2 concentration and increases again after reaching the minimum value. In the MnF2 + MnSO4 + H2O system, the addition of MnSO4 results first in a decrease and then in an increase of the MnF2(s) solubility, similar to the MgF2(s) solubility behavior in the MgF2 + MgSO4 + H2O system. In the CaSO4·2H2O(s)-saturated solution, CaF2(s) solubility increases with increasing MnSO4 concentration but is generally lower than that in the pure MnSO4 aqueous solution. In the MnF2(s)-saturated solution, CaF2(s) solubility increases with increasing MnSO4 concentration but is remarkably lower than that in the pure MnSO4 aqueous solution. The reciprocal system Ca2+, Mn2+//F–, SO4 2– + H2O exhibits cosaturated phase fields of CaSO4·2H2O(s)-CaF2(s)-MnSO4·H2O(s), CaF2(s)-MnSO4·H2O(s), and CaF2(s)-MnSO4·H2O(s)-MnF2(s).

Solubility of sparingly soluble salts, i.e., CaF2(s), MgF2(s), and ZnF2(s), in ZnSO4 aqueous solution is necessary for the process design to remove Ca2+, Mg2+, and F– ions from zinc hydrometallurgical systems. However, these data have been unavailable in any literature up to now. In this work, the solubilities of MF2(s) (M = Ca, Mg, Zn) in ZnSO4 and MF2(s) (M = Ca, Mg) in ZnF2 aqueous solutions have been elaborately measured at 298.15 K. The results showed that the CaF2(s) and MgF2(s) solubilities increase with the increasing ZnSO4 concentration; for example, their solubility in 2 mol·kg–1 ZnSO4 aqueous solution increases to about 6 times of that in pure water. The ZnSO4 has a slightly stronger salt-in effect on CaF2(s) than on MgF2(s). However, the ZnF2(s) solubility in ZnSO4 aqueous solution decreases at first and then increases with increasing ZnSO4 concentration. As expected, ZnF2 possesses strong salting-out effect on the solubility of CaF2(s) and MgF2(s). A remarkable character is that the CaF2(s) solubility goes through a minimum point and then increases again with the ZnF2 addition; inversely, the MgF2(s) solubility decreases with the ZnF2 addition with time. Their different solubility characters have been interpreted from the aspect of ion association interaction. An ion association model has been applied to predict the CaF2(s) and MgF2(s) solubility in ZnF2 aqueous solution. The predicted results agree with the experimental ones quite well, which indicates that the ion association dominates the excess properties of these concerned systems.

LaF 3(s) with extremely low solubility in pure water is the most promising compound to remove F − from heavy-metal sulfate aqueous solutions. However, LaF 3(s) solubility data in sulfate aqueous solutions of heavy metals have never been reported in any literature up to now. First, the LaF 3(s) solubility in pure water was measured in this work. The obtained results show that the LaF 3(s) solubility in pure water at 298.15 and 348.15 K is determined to be lower than 1.7 × 10 −6 mol•kg −1 by the fluoride ion-selective electrode (FISE) and lower than 1.4 × 10 −7 mol•kg −1 by inductively coupled plasma optical emission spectroscopy (ICP-OES), which are the lower detection limits of each analysis method. Second, the LaF 3(s) solubility in MSO 4 (M = Zn, Mn, Mg) aqueous solutions at 298.15 and 348.15 K was measured by ICP-OES. The experimental results show that the LaF 3(s) solubility increases with an increasing MSO 4 (M = Zn, Mn, Mg) concentration at both temperatures. At each temperature, the salting-in effect of LaF 3(s) is continuously enhanced in the order ZnSO 4 < MnSO 4 < MgSO 4 at a certain salt concentration. For example, the LaF 3(s) solubility in a 3.583 mol•kg −1 MgSO 4 aqueous solution at 348.15 K is 1.59 × 10 −3 mol•kg −1 , tens of thousands of times higher than that in pure water. Furthermore, the LaF 3(s) solubility at 348.15 K is about 2.1−6.2 times that at 298.15 K in MSO 4 (M = Zn, Mn, Mg) aqueous solutions of the same concentration, indicating that a lower temperature is more favorable to remove fluoride ions from MSO 4 (M = Zn, Mn, Mg) aqueous solutions by LaF 3(s) .

The thermodynamic and kinetic behaviors of Mg2+-F– ion pairing in aqueous solution are investigated theoretically and experimentally and are contrasted to those of Ca2+-F–. Thermodynamically, similar to CaF x (H2O)14 2–x (x = 1 and 2), MgF­(H2O) y + (y = 14–20) contact ion pairs (CIPs) are more stable than their solvent-shared ion pairs (SSIPs), whereas the CIPs and SSIPs of MF2(H2O) y are almost isoenergetic. However, in kinetics, the conversion of SSIPs to CIPs for M2+-F– (M = Mg2+ and Ca2+) ion pairing must overcome a high energy barrier due to the strong hydration of Mg2+ and F–. The kinetics dominate after the thermodynamics and kinetics are balanced, which hinders the formation of M2+-F– CIPs in practical MF2 aqueous solutions (less than or equal to saturated concentrations). This result is also supported by the 19F nuclear magnetic resonance spectra of saturated MF2 solutions. Although the interaction between Mg2+ and F– is slightly stronger than that between Ca2+ and F– due to the smaller radius of Mg2+, the formation of Mg2+-F– CIPs needs to go through two rate-limiting steps, the dehydration and entrance of F– (i.e., via exchange mode) with a higher energy barrier, due to the ability of strongly bound water molecules and rigorous octahedral coordinated configuration of Mg2+, while the formation of Ca2+-F– CIPs only goes through a single rate-limiting step, the entrance of F– (i.e., via swinging mode) with a lower energy barrier, due to the flexible coordination configuration of Ca2+. This is responsible for precipitation in MgF2 aqueous solution requiring a larger supersaturation degree and a lower precipitation rate than in CaF2. These kinetic factors lead to the association constants previously reported for MF+ determined by a fluoride ion-selective electrode (ISE) combined with the titration method, where the MF2 solutions were always unsaturated at the titration end point, which actually corresponds to those of the ligand process going from completely free M2+ and F– to their SSIPs. A possible strategy to accurately determine the association constants of MF+ and MF2(aq) CIPs by fluoride ISEs is proposed. The present results suggest that judging the formation of M2+-F– CIPs in practical solutions from a theoretical calculation perspective requires significant consideration of the kinetic factors, except for the thermodynamic factors.