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

The effect of ionic strength on reactions at aqueous interfaces can provide insights into the nature of the chemistry involved. The adsorption of H4SiO4 on iron oxides at low surface silicate concentration (Gamma(Si)) forms monomeric silicate complexes with Fe-O-Si linkages, but as Gamma(Si) increases silicate oligomers with Si-O-Si linkages become increasingly prevalent. In this paper, the effect of ionic strength (I) on both Gamma(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 Gamma Si, to decrease at a given H4SiO4 solution concentration, while the proportion of oligomers on the surface at a given Gamma(Si) increases. At pH 10, the opposite is observed; Gamma(Si) increases as Increases, while the proportion of surface oligomers at a given Gamma(Si) decreases. Ionic strength has only a small effect on the surface chemistry of H4SiO4 at pH 7, but at low Gamma(Si) this effect is in the direction observed at pH 4 while at high Gamma(Si) the effect is in the direction observed at pH 10. The pH where the surface has zero charge decreases from approximate to 8 to 6 as Gamma(Si) increases so that the surface potential (Psi) is positive at pH 4 for all Gamma(Si) and at pH 7 with low Gamma(Si). Likewise, Psi < 0 at pH 10 for all Gamma(Si) and at pH 7 with high Gamma(Si). The diffuse layer model is used to unravel the complex and subtle interactions between surface potential (F) and chemical parameters that influence interfacial silicate chemistry. This analysis reveals that the decrease in the absolute value of Psi as I increases causes Gamma(Si) to decrease or increase where Psi is, respectively, positive or negative. Therefore, at a given Gamma(Si), the solution H4SiO4 concentration changes with I, and because oligomerization has a higher H4SiO4 stoichiometry coefficient than monomer adsorption, this results in the observed dependence of the extent of silicate oligomerization on I.