Silicic acid adsorption and oligomerization at the ferrihydrite-water interface: interpretation of ATR-IR spectra based on a model surface structure.

Oxide surfaces can promote specific lateral interactions between adsorbed species that become concentrated in specific orientations at an interface. In this article, in situ attenuated total reflectance (ATR) IR spectra were collected over time (from 0 to approximately 100 h) as the iron oxide ferrihydrite reacted with H(4)SiO(4) (between 0.007 and 1.65 mM) and at a pH of 4, 7, or 10. Under all conditions, the first product formed was a monomeric surface species with distinct bands at 945 and 880 cm(-1), and a bidentate (2)C complex with SiO(4) sharing corners with two edge-linked Fe octahedra was proposed. Once a certain surface concentration (Gamma(Si)) of monomers was reached, a condensed oligomeric surface species with Si-O-Si linkages was observed on the surface with bands at 1005, 917, and 827 cm(-1) and one or more bands at >1050 cm(-1). This species was observed as a minor surface component at Gamma(Si) that was up to 10 times lower than the calculated density of (2)C adsorption sites on ferrihydrite and became the dominant surface species at higher Gamma(Si). This formation of a specific oligomer is rationalized on the basis of a recent model for the ferrihydrite surface, with the arrangement of (2)C adsorption sites on the (021) ferrihydrite face causing adjacent Si monomers to be held in an orientation that is conducive to the formation of a condensed Si species upon insertion of a solution H(4)SiO(4). Therefore, this model predicts that the ferrihydrite surface may act as a template for oligomerization in one dimension forming segments of pyroxene-like structures. The ATR-IR spectra and changes in the surface species' composition with time are consistent with such a model.