Polymerization of silicate anions in solutions at low concentrations

Abstract

Adsorption of tetrahedral-forming ions, such as silicates and phosphates, is considered to be the main cause of retention of these anions on the surfaces of hydrated Al- and Fe-oxides. The importance of this retention in natural and man-caused phenomena has been the subject of many investigations. When adsorption studies are conducted, the role of the surface and the charges developed on it by the potentially determining ions tend to be emphasized, while the nature of the sorbate is rarely considered or investigated. Nevertheless, the excluded or equilibrium solution is the seminal source of information in the formulation of adsorption mechanisms or models. The ionic species present in the equilibrium solution are practically always taken for granted, that is, from data previously published. They are thought to be a simple function of solution pH, without further analysis of ionic speciation changes that occur during the sorption process. Here, we provide direct experimental evidence, using an advanced and sensitive spectroscopic technique, laser Raman spectroscopy (LRS), that aqueous equilibrium silicate solutions at low concentrations contain polymeric and/or other anionic complexes and not just monomeric species. These low concentrations are similar to those used in most previous sorption experiments on Fe- and Al-sesquioxides and on tropical acidic soils. Our findings clearly indicate that any mechanisms and models that are proposed to describe anion sorption on sesquioxides should consider that silicates, and probably some other tetrahedral-forming anions, are not present in solution solely as monomeric ionic species as most investigators have assumed^1–11. Irregularities in fitting experimental data to models previously reported prompted this research^2–4,6,12.