Silicate Binding and Precipitation on Iron Oxyhydroxides

Area of Science:

• Geochemistry
• Surface Chemistry
• Mineralogy

Background:

• Silica-bearing waters interact with mineral surfaces, altering reactivity through Si complex and solid deposition.
• Understanding these interactions is crucial for predicting the fate of silica in natural environments.

Purpose of the Study:

• To identify specific surface hydroxo groups on goethite and lepidocrocite involved in ligand exchange with silicate species.
• To elucidate the initial attachment sites and mechanisms of silicate oligomerization and polymerization on these mineral surfaces.

Main Methods:

• Fourier transform infrared (FTIR) spectroscopy was employed to analyze hydroxo groups on goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) surfaces.
• Samples were reacted with monomeric silicate species in aqueous solutions at varying pH and time, followed by drying under N₂(g) for spectral analysis.
• Analysis focused on O-H stretching bands corresponding to singly (-OH), doubly (μ-OH), and triply coordinated (μ₃-OH) groups.

Main Results:

• Silicate species preferentially exchanged singly coordinated (-OH) groups on goethite and lepidocrocite surfaces.
• Doubly (μ-OH) and triply coordinated (μ₃-OH) groups were not involved in the initial ligand exchange.
• Silicate attachment predominantly occurs in rows of mononuclear monodentate species, initiating oligomerization at loadings >∼1 Si/nm².

Conclusions:

• The preferential exchange of -OH groups dictates the initial sites for silicate adsorption and subsequent polymerization.
• These findings clarify the mechanism of silica deposition on iron oxyhydroxides, influencing water-mineral interactions.
• Understanding these surface processes is vital for predicting silica behavior in geological and environmental systems.