X‐ray Photoelectron Spectroscopy as a Tool to Differentiate Silicon‐Bonding State in Amorphous Iron Oxides

Abstract

An x‐ray photoelectron spectroscopy (XPS) study was conducted to: (i) compare the Si (2p) and O (1s) peaks of selected Si‐containing minerals, and (ii) evaluate the application of XPS in identifying probable Si‐bonding environments in Si‐containing ferrihydrites. For silica gel, the Si (2p) and O (1s) XPS peaks occurred at 104.0 and 534.2 eV, respectively; with biotite, these peaks occurred at 102.8 and 532.4 eV. The difference in the Si (2p) binding energies between silica gel and biotite was attributed to differences in the Si‐bonding environment and the number of ‐Si‐O‐Si‐ linkages at corners of the SiO4 tetrahedron, and the charge deficit in the tetrahedral layer of biotite. The Si (2p) XPS peak for silicate adsorbed on ferrihydrite occurred at 100.9 eV, which indicated the probable presence of ligand‐bound silicate or small units of polymerized silica at the ferrihydrite surface. For ferrihydrite samples treated with an excess of Si (≥75 g kg−1), peaks attributable to a separate Si‐rich phase were identified on decomposition of the Si (2p) and O (1s) XPS peaks. In the case of ferrihydrite coprecipitated with silicate at Si/Fe molar ratios ≥0.10, the Si (2p) peak occurred at approximately 102.8 eV, which is a position similar to that observed for layer silicates, indicating the possible presence of Si, i.e., the presence of ‐Fe‐O‐Sl‐ bonds, within the ferrihydrite structure.

Inhibition of Sintering by Si During the Conversion of Si-Rich Ferrihydrite to Hematite

“…The peak at 530 eV was assigned to O(ls) bound in ferric oxides (Mclntyre and Zetaruk, 1977). The shoulder at higher energy was attributed to O(1 s) coordinated in hydroxyl groups associated with Fe (Vempati et al, 1990). The Si(2s) peak occurred at 153.3 eV (Figure 4c).…”

“…Hence the particle is primarily composed of surface area and near-surface area, and this suggests that the bulk structure is of lesser significance. Vempati et al (1990) argued that >90% of associated Si is structure bound in a ferrihydrite precipitated at a Si/ Fe molar ratio of 1.0 (see Vempati et al, 1990, Figure 3). This conclusion is based on the XPS Si(2p) signal, which occurs in an energy range where Fe(3s) has a satellite peak.…”

“…Another possibility is that Si substitution causes increased stability. Based on Xray photoelectron spectroscopy (XPS) data, Vempati et al (1990) postulated that Si is structure bound in Present address: Department of Microbiology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada, ferrihydrite. On the other hand, Yoshinaga and Kanasaki (1993) precipitated ferrihydrite in the presence of Si and Ge, which have significantly different ionic radii, and concluded from the similarity in d-values that these elements did not substitute in the ferrihydrite structure.…”

Biotransformation of two-line silica-ferrihydrite by a dissimilatory Fe(III)-reducing bacterium: formation of carbonate green rust in the presence of phosphate

“…3) indicated that Si content (at least in the range studied here) had a small effect on the biotransformation of ferrihydrite in terms of rate, extent, and biomineralization. Sorbed Si exists as a surface complex [-Fe-O-Si(OH) 3 ] on ferrihydrite at lower Si/Fe ratios (Ͻ0.05) and as a structural constituent (-Fe-O-Si-bonds) when Si/Fe Ͼ0.10 (Vempati et al, 1990). Ferrihydrite exhibits a higher affinity for phosphate than silicate [as -Fe-O-Si(OH) 3 ]; and PO 4 3Ϫ displaces H 3 SiO 4 Ϫ easily from ferrihydrite surfaces (Parfitt, 1978;Vempati and Loeppert, 1989;Parfitt et al, 1992).…”

Characterization of solids in produced water from wells fractured with recycled and fresh water

“…A possible explanation for this lower binding energy shift could be that silica or silicate was bonded with metal ions. For example, Vempati et al (1990) discovered that, when ferrihydrite co-precipitated with silica, the binding energy of silicon was shifted lower to 100.8 eV. Moreover, a study done by Gonzalez-Elipe et al (1988) showed that, in the phyllosilicate minerals framework, substitution of Al or Mg for Si conferred net negative charge to the tetrahedral sheet, thus lowered the photoelectron binding energies of silicon and oxygen.…”