A unified approach for modeling the variable reactivity of goethite : surface complexation modeling of proton, alkaline earth metal ion, and transition metal ion adsorption

Developing tools to predict the fate and transport of contaminants in the environment has been the focus of many environmental studies. Among these efforts, surface complexation models (SCM) have been suggested as a promising tool for simulating the adsorption behavior of hazardous metal cations and oxyanions onto metal oxide surfaces. However, the predictive capability of SCMs is often limited to system conditions over which the modeling parameters were estimated, and hence, a self-consistent model capable of making accurate predictions over a wide range of solution and surface conditions observed in natural environments is still lacking. The goal of this study was to develop a modeling approach that broadens the predictive capability and applicability of SCMs. To achieve this goal, two issues related to the complex interactions among the constituents of the mineral-water interfacial system were addressed and investigated: 1) variable reactivity of metal oxide surfaces, and 2) interactions between alkaline earth metal ions and the surface. The scope of this study is limited to goethite. The selection of goethite was based on the fact that it is one of the most important minerals that controls adsorption of ions in common natural soils and has been extensively investigated as a sorbent mineral in many previous SCM studies. In the current research, the variation of surface reactivities observed for different goethite morphologies was captured in a charge distribution – multisite complexation (CD-MUSIC) model by incorporating established relationships between crystal face contribution (CFC), inner-Helmholtz capacitance (C₁), and protonation constants (pK [subscript a] ’s) with the specific surface area (SSA) of goethites. By using the surface parameters estimated by these relationships, CD-MUSIC model was able to predict proton, Cd²⁺, and SeO₃²⁻ adsorption onto a wide range of different goethite morphologies (21 – 105 m²/g SSA) using data obtained in our laboratory and reported in the literature. The adsorption behavior of alkaline earth metal (AEM) ions was also investigated. Experimental results showed that adsorption of AEM cations of weaker affinity onto goethite was more highly impacted by the types and concentrations of background electrolytes. It is suggested that the electrolyte effect (i.e., reduction in extent of adsorption) is due to the combined effect of competitive adsorption and aqueous complexation with electrolyte cations and anions, respectively. This result indicates background electrolytes should not be indiscriminately assumed inert or indifferent, especially when assessing the adsorptive behavior of weakly sorbing solutes. Specifically, discrete effects of electrolytes must be considered and accounted for when determining equilibrium constants of other ions using experimental data. Finally, surface complex model species and equilibrium constants of AEM ions ( [superscript int] K subscript Me] ) were determined by calibrating adsorption binding constants using selected data sets and validating the model with a larger range of data. It was proven that with these modeling parameters, a CD-MUSIC model was capable of accurately predicting AEM adsorption onto three preparations of goethite (50 m²/g, 64.5 m2/g, and 73 m²/g SSA) over a range of solution conditions (i.e., pH 5-11, ionic strength 0.01M – 0.7M, various background electrolytes). In addition, the model was also able to predict the impact of high concentrations of Mg²⁺ on Cd²⁺ adsorption, and the impact of Cd²⁺ on Mg²⁺ adsorption. Despite the fact that both ions were considered to form inner-sphere complexes with goethite, the presence of both solutes showed no effect on adsorption of either ion. The results of this study demonstrate that the suggested modeling approach could greatly improve the predictability of a CD-MUSIC surface complexation model; the SSA values of pure synthetic goethites were able to capture the change of CFC, capacitance and pK [subscript a], and accurate predictions have been made for proton, AEM ions, Cd²⁺ and SeO₃²⁻ adsorption onto various preparations of goethite over a wide range of solution conditions.