Modelling equilibrium distribution of ions and molecules in a heterogeneous system of CaCO3– water: gas phase under both equilibrial and non-equilibrial conditions

Abstract

In many places, including Northern Estonia, the soil bedrock is limestone, consisting mainly of CaCO₃. Equilibrium processes in aqueous medium involving dissolved CO₂ and solid CaCO₃ play a vital role in many biological and technological systems. In this study, a model for equilibrium distribution of ions and molecules in the ternary heterogeneous system solid CaCO₃–water–gas phase containing CO₂ under both equilibrial and non-equilibrial conditions was developed. The model can be used for the determination of concentrations of all components in the water phase over a wide range of concentrations of CO₂ in water, allowing the assessment of the impact of anthropogenic processes on the natural environment and could be usefully applied in water and wastewater technology. It can also be useful for developing innovative methods for the measurement of aqueous CO₂. As algae in water utilize dissolved CO₂ in photosynthesis, it leads to an increase in pH and bacteria will produce CO₂, which, in turn, leads to a decrease in pH. Therefore the concentration of CO₂ will vary in a large scale in the aqueous environment. The equilibrium concentrations of dissolved CO₂ in the liquid phase at a given partial pressure of CO₂ in the gaseous phase was calculated in the range of p(CO₂)_G ¨=(8.08x´10^-7÷2.37´x10⁴) ppm at 25 °C. Quantitative evaluation of the equilibrium distribution of ions and molecules in the system CaCO₃–CO₃ ^2––HCO₃ ––H₂CO₃–CO₂ at an equilibrium with gas phase identified a relevant minimum solubility of s[CaCO₃]_min ¨= 0.1 mmol/L. The model has been experimentally validated.