The impact of ionic strength on the proton reactivity of clay minerals

Abstract

The reactivity of clay mineral surfaces exerts a fundamental control on elemental cycling in environmental systems. However, the effect of solution ionic strength (IS) on their reactivity is poorly understood. Here we quantified the effect of changes in IS on clay mineral reactivity through the application of surface complexation modeling (SCM) based on acid-base titration data for three different clay minerals (kaolinite, illite, and montmorillonite). The results demonstrate that the acidity constants (Ka values) for proton interactions on the clay surface vary linearly as a logarithmic function of solution IS, and that proton binding ability is inversely related to solution IS. Two possible explanations for the observed relationship are proposed: (1) solution electrolytes (e.g., Na+) competitively bind to clay surface sites relative to protons, and (2) that the surface electrostatic field is attenuated by solution electrolytes. We conclude that the impact of increasing solution IS on clay surface reactivity is the result of a weakening negative surface electric field that impacts the surface potential. This mechanism ultimately leads to different adsorption behavior for cations and anions under changing IS conditions. The empirical relationship between K values and IS provided here can be applied to predict clay surface reactivity under varied environmental IS conditions and has specific implications for trace metal behavior in environments, such as estuaries, where changes in IS can be significant.