Understanding the chemistry of hydroxyaluminosilicates: from the mechanism of formation to the determination of an equilibrium constant

Schneider, Céline

Abstract

The proposition in recent years that silicic acid (Si(OH)4) acts as an environmental control of the biological availability of aluminium (Al) has presented inorganic chemistry with an intriguing scientific challenge. Si(OH)4 reacts with Al to form hydroxyaluminosilicates (HAS) and thereby ameliorates the toxic effects of Al. However, in spite of the recent progress made in the identification and characterisation of these materials, very little is known about the kinetics underlying the formation of HAS. In particular, the rate at which HAS are formed and achieve stability with respect to their environment will be critical to their role in defining the biological availability of aluminium.

This research project investigated different aspects of the chemistry of the formation and precipitation of HAS. The interaction between Al and methyl-substituted analogues of Si(OH)4 using solution and solid state NMR as well as SEM-EDX revealed that the substituents prevented any reaction with Al, suggesting a significant degree of specificity in the formation of HAS. Atomic force microscopy (AFM) was used in solution to follow the phenomenon of growth and agglomeration of HAS particles with time and provided new insight into the deposition of HAS on a silica substrate, in particular helping to discriminate two forms of HAS, HASa and HASB, by their charge and rate of agglomeration. X-ray photoemission spectroscopy (XPS) was used to characterise precipitates of HASa and HASb and provided new information on their structure from the observed shift in their binding energies. Fluorimetry was used to study the reaction of Si(OH)4 with Al. The formation of the aluminium-morin complex was used as an estimate of the free Al concentration. The establishment of a reliable experimental protocol enabled an indirect measurement of the formation of HAS and how both the concentrations of Al and Si(OH)4 influenced the reaction.

Most importantly, the quantification of the fluorimetrie results when coupled with speciation calculations using the SolGasWater Al speciation model resulted in the first significant attempt to determine an equilibrium constant for the formation of HAS. This constant (Log Khasb = -10.94 at 20°C) has enabled significant advances in bringing together experimental and theoretical data concerning HAS in the environment and the control of the bioavaibility of Al.

Citation

Schneider, C. Understanding the chemistry of hydroxyaluminosilicates: from the mechanism of formation to the determination of an equilibrium constant. (Thesis). Keele University. https://keele-repository.worktribe.com/output/1080246

• Thesis Type: Thesis
• Deposit Date: Feb 28, 2025
• Publicly Available Date: Feb 28, 2025
• Public URL: https://keele-repository.worktribe.com/output/1080246
• Award Date: 2003-01