An attenuated total reflectance IR study of silicic acid adsorbed onto a ferric oxyhydroxide surface

Silicic acid (H4SiO4) can have significant effects on the properties of iron oxide surfaces in both natural and engineered aquatic systems. Understanding the reactions of H4SiO4 on these surfaces is therefore necessary to describe the aquatic chemistry of iron oxides and the elements that associate with them. This investigation uses attenuated total reflectance infrared spectroscopy (ATR-IR) to study silicic acid in aqueous solution and the products formed when silicic acid adsorbs onto the surface of a ferrihydrite film in 0.01 M NaCl at pH 4. A spectrum of 1.66 mM H4SiO4 at pH 4 (0.01 M NaCl) has an asymmetric Si–O stretch at 939 cm−1 and a weak Si–O–H deformation at 1090 cm−1. ATR-IR spectra were measured over time (for up to 7 days) for a ferrihydrite film (≈1 mg) approaching equilibrium with H4SiO4 at concentrations between 0.044 and 0.91 mM. Adsorbed H4SiO4 had a broad spectral feature between 750 and 1200 cm−1 but the shape of the spectra changed as the amount of H4SiO4 adsorbed on the ferrihydrite increased. When the solid phase Si/Fe mole ratio was less than ≈0.01 the ATR-IR spectra had a maximum intensity at 943 cm−1 and the spectral shape suggests that a monomeric silicate species was formed via a bidentate linkage. As the solid phase Si/Fe mole ratio increased to higher values a discrete oligomeric silicate species was formed which had maximum intensity in the ATR-IR spectra at 1001 cm−1. The spectrum of this species suggests that it is larger than a dimer and it was tentatively identified as a cyclic tetramer. A small amount of a polymeric silica phase with a broad spectral feature centered at ≈1110 cm−1 was also observed at high surface coverage. The surface composition was estimated from the relative contribution of each species to the area of the ATR-IR spectra using multivariate curve resolution with alternating least squares. For a ferrihydrite film approaching equilibrium with 0.044, 0.14, 0.40 and 0.91 mM H4SiO4 the area of the spectra accounted for by monomeric species were 92%, 49%, 23% and 6%, respectively. The remainder was oligomer apart from a small amount (<5%) of polymerized silica at the two higher H4SiO4 concentrations. The solid phase Si/Fe mole ratios for these samples were 0.020, 0.037, 0.071 and 0.138, respectively.