Predictions of Silica Adsorption and Surface Speciation at the Mineral-Water Interface

The mineral-water interface governs an extensive range of geochemical processes on the Earth's surface such as weathering, contaminant migration, and the adsorption and concentration of organic acids important in biochemistry. Aqueous silica is a prevalent constituent of soilwater, groundwater and riverwater, and experiments indicate that it strongly adsorbs on oxide surfaces. Here we use the Extended Triple Layer Model (ETLM) to predict the surface complexation of aqueous silica on the minerals goethite, hydrous ferric-oxide (HFO) and magnetite. We have predicted three species of silica that adsorb at the mineral-water interface: a mononuclear inner-sphere complex, a binuclear inner-sphere complex, and an outer-sphere complex. Under soilwater conditions, the binuclear complex is predicted to dominate on the surface of goethite, whereas the outer-sphere complex is the principle species on HFO. Previous Si isotope studies indicate that the goethite surface is enriched in lighter Si isotopes relative to the HFO surface suggesting that surface speciation and environmental factors, such as pH and ionic strength, might play a role in Si fractionation.