THE IMPORTANCE OF THE MINERAL/WATER INTERFACE FOR CONTROLLING THE MOBILITY OF IONS: MACRO- AND MICROSCOPIC INVESTIGATIONS UNIFYING THE ADSORPTION BEHAVIOR OF THE FE OXYHYDROXIDES GOETHITE AND FERRIHYDRITE

A large variety of environmental geochemical reactions occur at the mineral/water interface, and they are preceded by the physicochemical retention mechanism called “sorption”, which dictates global phenomena as important as the natural purification of water, the geochemical recycling of elements, and the formation of soil, among others. Sorption occurs at the molecular level and is manifested in a macroscopic fashion, but is only relevant on the smallest mineral particles because of their high specific surface areas (SSAs). In particular Fe (hydr)oxides are responsible for the adsorption of oxyanions, including important contaminants such as As(V), because of their high points of zero charge, and thus their positive surfaces in the normal environmental pH range.

Although highly sophisticated models have been developed for describing the interfacial behavior of Fe oxyhydroxides such as goethite, they are only satisfactory for ideal crystals, of SSAs above 90 m²/g. Larger particles, i.e., with SSAs below 80 m²/g, and down to 28 m²/g, have consistently shown increasing surface reactivities as the SSA decreases, when adsorption data are normalized by surface area. This has forced thermodynamic models of goethite to describe its adsorption reactions with affinity constants that are dependent on the SSA of the goethite preparation investigated. This is highly unsatisfying for the predictive capabilities desired of modeling exercises.

In this work we present the development of a model that unifies the behavior of all goethites by defining surface-site specific affinity constants through a variable crystal-face/site-density model, which adequately describes in a quantitative manner large sets of adsorption data. Use is made of theoretical crystallographic site-density data of the different goethite crystal faces exposed, and of adjustment of the face contribution proportions until obtaining adsorption data congruency when normalizing by site occupancy. This exercise is independent of the specific Surface Complexation Model used later to obtain the affinity constants, for which both the popular Triple Layer and MUSIC models were employed. Also, through this model, the points of zero charge of the individual surface groups were estimated, and the ferrihydrite adsorption behavior was described.