Iron Sulfide Solubility Measurement and Modeling Over Wide Ranges of Temperatures, Ionic Strength, and pH

Abstract

Summary Iron sulfides constitute a diverse group of solid phases and aqueous complexes, many of which are critical in both natural and industrial processes, such as marine sedimentation, oil and gas productions, metallic equipment corrosion by hydrogen sulfide, and water treatment, to mention a few. As the fundamental basis of related research on iron sulfides, an accurate thermodynamic model for iron sulfide solubility prediction under wide ranges of conditions (e.g., water compositions, temperature, pressure, and pH) is required. In this study, a plug flow reactor was built to measure iron sulfide solubility at various temperatures (25–90°C), pH (5.9–6.9), and ionic strength (0.150–4.27 mol/kg). Using the mackinawite solubility data measured in this study and in literature, a new Pitzer theory-based thermodynamic model with the explicit presence of ion complexes was developed. In this model, a neutral aqueous ion complex, FeSaq, was included explicitly. The mackinawite solubility product constant was fitted with a temperature-dependent equation from 23 to 125°C: pKsp,m=−94.97+4,444TK+14.64×ln(TK), with TK as temperature in Kelvin. This model can accurately predict the mackinawite solubility with standard error of estimate (SEE) of the mean SI = ±0.015 SI units, under wide ranges of temperature (23–125°C), pH (3.16–9.66), ionic strength (0–5 mol/kg water), and Fe(II) to S(-II) molar ratio (from 1:1,000 to 10,000:1). The Gibbs free energy and enthalpy of formation for mackinawite were also derived. This new model could help improve FeS scaling and corrosion control in complicated industrial systems, as well as the sulfide-related heavy metal transport in natural aqueous systems.