Thermodynamic properties and revised Helgeson-Kirkham-Flowers equations of state parameters of the hydrated and dehydrated monomeric silica species at t = 0.01–600oC, P = 1–3000 bars, ρH2O = 0.35–1.1 g cm−3, and Im = 0 m

Abstract

The thermodynamic properties and revised Helgeson-Kirkham-Flowers equations of state (r-H-K-F EoS) parameters of the hydrated (Si(OH)₄(aq), SiO(OH)₃^– and SiO₂(OH)₂²⁻) and corresponding dehydrated (SiO₂(aq), HSiO₃⁻ and SiO₃²⁻) monomeric silica species are important to describe the pH, composition, temperature, and pressure dependence of formation/breakdown reactions of all silicon-bearing compounds globally. Experimental log₁₀ equilbrium constant, K values describing the formation reactions of these hydrated and dehydrated monomeric silica species were therefore compiled from the literature, extrapolated to zero ionic strength by specific ion interaction theory as required and used to derive their thermodynamic properties and r-H-K-F EoS parameters.

Consideration of all formation reactions in the same study provides a collective, internally consistent update to the thermodynamic properties and r-H-K-F EoS parameters of the monomeric silica species that are able to provide satisfactory matches to the available experimental log₁₀ K values at t = 0.01–600^oC, P = 1–3000 bars, ρ_H2O = 0.35–1.1 g cm⁻³, and zero ionic strength. These temperature and pressure limits comfortably bracket t = 0.01–100^oC and P = 1–270 bars relevant to the geological disposal of radioactive wastes at depths of up to 1 km.

Updates to the thermodynamic properties of silicon-bearing compounds in all of the available geochemical thermodynamic databases are necessary, especially if reaction properties are used or given. Internal consistency between the hydrated and dehydrated species means that the hydrated species alone can be used as entries in geochemical thermodynamic databases.