Affinity Distribution Description of Competitive Ion Binding to Heterogeneous Materials

Abstract

Semiempirical equilibrium models of competitive, two-component binding of simple ions to heterogeneous materials are based on a combination of independent binding isotherms, such as two-component Langmuir isotherms or exchange isotherms. Such models are usually sufficiently flexible to rationalize various experimental data sets quantitatively. Since such models are thermodynamically consistent, they have substantial predictive capabilities. Furthermore, such models are rather simple, may depend on a relatively small number of parameters, and provide an intuitive framework for comparison of binding properties of different materials. We solve the ill-posed data inversion problem with constrained, regularized least-squares techniques. This approach to the interpretation of two-component competitive ion binding to heterogeneous materials is illustrated with two data sets; the binding of Sr2+ and H+ to ferrihydrite (amorphous FeOOH) and that of Cu2+ and H+ to a humic acid.

Sulfate Adsorption on Goethite

“…8 that only a certain value of the charge distribution (Ϫ0.35, Ϫ1.65), as found earlier, is able to predict the correct pH dependence of the adsorption between pH 5 and 8. The calculated proton adsorption in 0.033 M Na 2 SO 4 for different values of charge distribution also shows that this kind of data can be used to calculate the charge distribution of adsorbed sulfate.…”

“…A correct description of pH dependence is most critically tested with proton-ion adsorption stoichiometry data. Based on the principle of thermodynamic consistency both are related (4,20). Recently it has been shown by Rietra et al (19) that in case of the presence of only one adsorbed species, as is the case for sulfate (16), the modeling of proton-ion adsorption stoichiometry is independent of the intrinsic affinity.…”

“…Electrostatic surface complexation models (1)(2)(3) are in principle able to make predictions in more complex systems. The advantage of these mechanistically oriented binding models according to Č ernic et al (4) is that they can use the same formalism in all systems of interest, and enable in principle the prediction of the characteristics of the individual binding sites and complexes. Today the application of electrostatic surface complexation models is restricted by the uncertainty of the choice of the electrostatic profile at the mineral-water interface and the location of the ions in it.…”

Fluoride Adsorption on Goethite in Relation to Different Types of Surface Sites

“…1b), which is equivalent with OH release. This coadsorption is related to the pH dependency of the adsorption process according to the principle of thermodynamic consistency (34,35). The mean H/F ratio, which follows from the slope of the line, is about 0.65 mol/mol.…”

“…As mentioned above, the proton/ion stoichiometry is coupled to the pH dependency of the adsorption via the principle of thermodynamic consistency (34,35). It implies that with a proper description of the experimental proton/fluoride ratio (Fig.…”