Thermodynamic and kinetic study of CaS in aqueous systems

One of the main sources of sulfide in fuel combustion ashes is the hazardous compound calcium sulfide (CaS), which decomposes in water and releases sulfide compounds into the environment. In order to safely use hydrotransport and open air deposition of wet ash, as well as to treat alkaline ash transportation waters, it is important to know the dissolution behavior of CaS, in this regard, several values of both CaS solubility and equilibrium constants of dissolution reactions are illustrated in the scientific literature. To reconcile these differences, both kinetics and equilibrium of CaS dissolution reactions under inert and air conditions at 25°C were studied. The water solubility of CaS under an inert atmosphere (with N2 treated water) is lower (2.29·10−3mol/L) than under air (2.74·10−3mol/L). A more complex reaction mechanism for open air dissolution, elucidated using HsC Chemistry® 7.1 simulations, was also presented. Various reaction mechanisms as well as equilibrium and rate constants were evaluated to model CaS dissolution data. The relatively small deviations between calculated values and experimental data confirm that the suggested models could be employed to describe CaS dissolution under both atmospheric and inert conditions.