Ionic Strength Effects on Silicic Acid (H₄SiO₄) Sorption and Oligomerization on an Iron Oxide Surface: An Interesting Interplay between Electrostatic and Chemical Forces

Abstract

The effect of ionic strength on reactions at aqueous interfaces can provide insights into the nature of the chemistry involved. The adsorption of H(4)SiO(4) on iron oxides at low surface silicate concentration (Γ(Si)) forms monomeric silicate complexes with Fe-O-Si linkages, but as Γ(Si) increases silicate oligomers with Si-O-Si linkages become increasingly prevalent. In this paper, the effect of ionic strength (I) on both Γ(Si) and the extent of silicate oligomerization on the ferrihydrite surface is determined at pH 4, 7, and 10, where the surface is, respectively, positive, nearly neutral, and negatively charged. At pH 4, an increase in ionic strength causes Γ(Si) to decrease at a given H(4)SiO(4) solution concentration, while the proportion of oligomers on the surface at a given Γ(Si) increases. At pH 10, the opposite is observed; Γ(Si) increases as I increases, while the proportion of surface oligomers at a given Γ(Si) decreases. Ionic strength has only a small effect on the surface chemistry of H(4)SiO(4) at pH 7, but at low Γ(Si) this effect is in the direction observed at pH 4 while at high Γ(Si) the effect is in the direction observed at pH 10. The pH where the surface has zero charge decreases from ≈8 to 6 as Γ(Si) increases so that the surface potential (Ψ) is positive at pH 4 for all Γ(Si) and at pH 7 with low Γ(Si). Likewise, Ψ < 0 at pH 10 for all Γ(Si) and at pH 7 with high Γ(Si). The diffuse layer model is used to unravel the complex and subtle interactions between surface potential (Ψ) and chemical parameters that influence interfacial silicate chemistry. This analysis reveals that the decrease in the absolute value of Ψ as I increases causes Γ(Si) to decrease or increase where Ψ is, respectively, positive or negative. Therefore, at a given Γ(Si), the solution H(4)SiO(4) concentration changes with I, and because oligomerization has a higher H(4)SiO(4) stoichiometry coefficient than monomer adsorption, this results in the observed dependence of the extent of silicate oligomerization on I.

Bacteria attenuation by iron electrocoagulation governed by interactions between bacterial phosphate groups and Fe(III) precipitates

“…3.1. In this electrolyte, EC precipitates were negatively-charged due to the sorption of P and, to a lesser extent, Si (P:Fe and Si:Fe molar solids ratios of 0.7 ± 0.1 and 0.06 ± 0.04, respectively) (Appenzeller et al, 2002;Hamid et al, 2011). In contrast to previous experiments in the absence of oxyanions, bivalent cations significantly interacted with the surface of EC precipitates in the groundwater-like electrolyte, as indicated by a substantially higher ζ-potential at larger Ca/Mg concentrations.…”

Microemulsion‐Assisted Preparation of a Mesoporous Ferrihydrite/SiO2 Composite for the Efficient Removal of Formaldehyde from Air

“…[43] Thus, there are some PEG molecules trapped inside the ferrihydrite. [48] Moreover, such a blue-shift indicates a less-open Si À O À Si network structure with higher internal stress in the newly formed silica NPs. [47] Based on the spectra of the Fe-SiO 2 -x samples, the TO mode of the Si À O À Si asymmetric stretching vibration band shows a blueshift of around 43 cm À1 with increasing aging time (from 1022 cm À1 for Fe-SiO 2 -1 to 1065 cm À1 for Fe-SiO 2 -5), owing to the higher surface concentration of SiO 2 in the samples that are prepared with prolonging aging times.…”

“…[47] Based on the spectra of the Fe-SiO 2 -x samples, the TO mode of the Si À O À Si asymmetric stretching vibration band shows a blueshift of around 43 cm À1 with increasing aging time (from 1022 cm À1 for Fe-SiO 2 -1 to 1065 cm À1 for Fe-SiO 2 -5), owing to the higher surface concentration of SiO 2 in the samples that are prepared with prolonging aging times. [48] Moreover, such a blue-shift indicates a less-open Si À O À Si network structure with higher internal stress in the newly formed silica NPs. [49,50] Figure 4 shows the thermogravimetric analysis (TGA) and differential thermal analysis (DTA) plots of the ferrihydrite and Fe-SiO 2 -x (x = 1, 3, and 5).…”

Polymerization of silicate on TiO2 and its influence on arsenate adsorption: An ATR-FTIR study

“…With this technique, the monomeric, oligomeric, and polymeric silicate surface species can be distinguished based on the position of the Si O stretching bands [16,24,25]. Moreover, the molecular As(V) adsorption structures on metal oxides were also extensively studied with ATR-FTIR spectroscopy [26,27].…”