Ion adsorption at the goethite/water interface: A new face distribution—site density model

Goethite shows an anomalous adsorption behavior when its particle size increases from nanorods to microrods, in which the adsorption capacity also increases progressively when normalized by surface area. This precludes the use of a unified thermodynamic theoretical approach to describe the adsorption behavior of (non-ideal) goethites. A new surface structural model is proposed for the latter where higher proportions of face (010) are assumed responsible for this increase in surface reactivity. The model explains quantitatively the higher adsorption capacities and pH adsorption edges of a considerable number of ions on these goethites, including protons, and is coupled to a surface complexation model for a thermodynamic description of its adsorption behavior on a per-site basis. As such, the model has highly predictive capabilities for all goethites. Additionally, the model developed for highly reactive goethites may be further extrapolated to adequately describe proton charge behavior of ferrihydrite.