Silicic acid (H4SiO4) adsorbs at the ferrihydrite-water interface as monomeric or oligomeric surface silicate complexes. ATR-IR spectra were used to determine the proportions of monomeric and oligomeric surface silicate as a function of pH and Si surface concentrations (Gamma(Si)) for H4SiO4 in ferrihydrite suspensions. At each pH the proportion of adsorbed silicate present as monomers decreased as Gamma(Si) increased while at a given Gamma(Si) the proportion of adsorbed silicate present as monomers was higher at higher pH. ATR-IR spectra for ferrihydrite suspensions in combination with the adsorption isotherm data were used to calibrate the Diffuse Layer Model (DLM) to describe H4SiO4 adsorption as monomers and oligomers on ferrihydrite surface sites ( FeOH). Using a set of reactions that were consistent with the ATR-IR spectra the DLM could accurately describe the H4SiO4 adsorption isotherms, the distribution of surface monomeric and oligomeric silicates, and the decrease in surface potential with Gamma(Si). The reactions included the formation of monomeric complexes ( FeH(3-n)SiO4(-n)) and trimeric silicate complexes formed between two surface sites( Fe2H(6-n)Si3O10(-n)) This oligomer stoichiometry is consistent with previous studies suggesting the surface silicate oligomer is formed by a solution H4SiO4 bridging two adjacent adsorbed monomers to form a linear trimer. This study also showed that carbonate can enhance H4SiO4 adsorption between pH 9 and 11. The data were consistent with formation of an outer-sphere complex between a solution H3SiO4- and a protonated adsorbed carbonate species which is analogous to the mechanism by which carbonate enhances the goethite adsorption of sulfate. (C) 2010 Elsevier Inc. All rights reserved.