MOLECULAR STATICS CALCULATIONS OF PROTON BINDING TO GOETHITE SURFACES: THERMODYNAMIC MODELING OF THE SURFACE CHARGING AND PROTONATION OF GOETHITE IN AQUEOUS SOLUTION - ADSORPTION MODELS

Abstract

Molecular statics calculations of proton binding at the hydroxylated faces of goethite are used to guide the development of a thermodynamic model which describes the surface charging properties of goethite in electrolyte solutions. The molecular statics calculations combined with a linear free energy relation between the energies of the hydroxylated surface and the aqueous solvated surface predict that the acidity constants for most singly (aqua or hydroxo), doubly (μ-hydroxo), and triply (μ3-hydroxo or μ3-oxo) coordinated surface sites all have similar values. This model which binds protons to the goethite 110 and 021 faces satisfactorily describes the surface charging behavior of goethite, if pair formation between bulk electrolyte species, i.e., Na+, Cl-, and NO3-, is included in the model. Inclusion of minor species of quite different charging behavior (designed to describe the possible presence of defect species) did not improve our predictions of surface charge since the protonation of the major surface sites changed when these minor species were introduced into the calculations thereby negating the effect of small amounts of defect species on the overall charging behavior. The final thermodynamic model is shown to be consistent with the surface charging properties of goethite over a range of pH values, NaNO3, and NaCl concentrations.