Predicting Chromate Adsorption on Iron Oxides: A Surface Complexation Modeling Study

Chromate is a common contaminant in the environment, and its mobility and reactivity is controlled by adsorption reactions on iron oxides and oxyhydroxides. Iron oxides are among the most reactive surfaces in the environment and various models have been developed to describe the reactivity of various phases such as goethite and ferrihydrite. Surface complexation models (SCMs) provide a mechanistic description of adsorption under various conditions but their application to mineral assemblages is still difficult, due to the high degree of complexity and parametrization. In this study, we propose a unified SCM to simulate and predict chromate adsorption on iron oxides, and specifically focusing on three iron oxides: ferrihydrite, hematite and goethite. The unified approach focused on employing a single set of electrolyte and specific adsorption equilibrium constants, while modeling surface charge using individual mineral structure and surface properties. Specifically, a 3–site model was used to describe surface protonation using site densities derived from the structure and morphology, and protonation constants derived from the literature or fitted to mineral-specific charging curves. For chromate adsorption, insights from spectroscopy and batch adsorption experiments were incorporated to build a model that is able to simulate adsorption using unified electrolyte and surface complexation constants. This approach showed very promising results, as it was able to describe chromate adsorption on the three iron oxides with minimal parametrization.