7.2 Ionic Strength Effects

The ionic strength is a convenient mathematical expression for the concentration of ionic charges in solution and is used in any theory for activity coefficients. The ionic strength, I, is calculated as shown in Equation 8 where zi is the charge on an ion, i, of molality, mi.

[latex]\displaystyle I=\frac{1}{2}\sum z{_{i}}^{2}m_{i}[/latex] (8)

The presence of other charged species in solution affects both equilibrium solubility and the kinetics of reaction. Most minerals, indeed, most compounds, exhibit an increase in solubility when dissolved in a solution with increased amounts of some other non-reacting solute such as NaCl. If there is no other reaction taking place, this increase is ascribed to the ionic strength effect. If the mineral remains at equilibrium solubility and the ionic strength increases, the ion-activity product is constant, but the activity coefficients decrease (Butler, 1998) so the concentrations of the ions must increase.

Fluorite solubility in pure water at 25 °C is about 7.4 mg F/L. As the concentration of NaCl increases, the F concentration in solution increases, as shown in Figure 4a. These simulations were computed with the PhreeqcI code (Parkhurst and Appelo, 2013) and are presented for purposes of showing factors that affect F solubility. One of the obvious consequences for groundwaters is that they can increase in salt content through seawater intrusion in coastal areas (Chen et al., 2020a; Gao et al., 2007), through mixing with other saline waters, or through dissolution of an evaporite bed. If a soluble fluoride-bearing mineral is present, more F will dissolve from this mixing. However, if the saline fluid is enriched in calcium (Ca), then mixing will decrease the F concentration because of the common-ion effect. The same is true for fluorapatite dissolution in NaCl, but because of its lower solubility, the effect is much less (Figure 4b).

Graphs showing solubilities of fluorite and fluorapatite in NaCl solutions

Figure 4  Solubilities of fluorite a) and fluorapatite b) in NaCl solutions at 25 °C simulated using the PhreeqcI code and the wateq4f.dat database.


Fluoride in Groundwater Copyright © 2022 by Pauline L. Smedley. All Rights Reserved.