Assessing the role of silicate polymerization on metal oxyhydroxide surfaces using X-ray photoelectron spectroscopy

The iron oxide phases that precipitate in weathered environments can contain large amounts of adsorbed silicic acid (H4SiO4). This adsorbed silicate can be present as monomeric or oligomeric silicate species and affects many properties of iron oxides such as morphology, phase stability and surface charge. Therefore understanding the chemistry of H4SiO4 at iron oxide interfaces is important to describe the geochemistry of iron oxides and the elements that are associated with iron oxides. In this study we have measured the Si 2s and Si 2p binding energies (BE) in the XPS spectra of silicate adsorbed onto preformed ferrihydrite under conditions where the silicate surface chemistry is well defined. The positions of both the Si 2s and Si 2p peaks are indicative of the degree of polymerization of adsorbed silicate, but the Si 2p peaks overlap with an Fe 3p multiplet splitting peak. For ferrihydrite-adsorbed H4SiO4 the Si 2s peak BE increased from 153.1 to 153.9 eV as the H4SiO4 surface concentrations (ΓSi in mol Si (mol Fe)− 1) increased from 0.03 to 0.2. The low ΓSi Si 2s BE value is slightly lower than the Si 2s BE of the orthosilicate almandine (153.3 eV), while the high ΓSi value lies midway between almandine and the tectosilicate quartz (154.6 eV). The positions of the Si 2s peaks correlate with the Si–O stretching bands (ν(Si–O)) in the FTIR spectra and are consistent with a model in which monomeric silicate species are present at low ΓSi and one dimensional oligomers are formed at higher ΓSi. The Si 2s BE and ν(Si–O) for synthetic and natural ferrihydrites co-precipitated with silicates are substantially lower than those for silicate adsorbed onto preformed ferrihydrite, clearly indicating a lower degree of silicate polymerization for silicate adsorbed onto the internal surfaces between ferrihydrite domains. The Si 2s and ν(Si–O) for co-precipitated ferrihydrites appear to be indicative of the age of the ferrihydrite and the rate of ferric precipitation.