Insights into the Composite Scale Formation and Coprecipitation Behavior of CaSO₄ and SrSO₄ at different salinity level

The dependency of the calcium sulfate (CaSO₄) deposit formation on the presence and concentration of co-precipitating ions, is a fundamental question to the advancement of crystallization science. In this paper, the thermodynamics and kinetics of the static precipitation of 47.5 mM calcium sulfate, in isolation and presence of strontium sulfate (SrSO₄) as common co-crystallizing salt, are examined at 30°C. The deposition of pure and composite sulfate scales is studied by varying the salinity of the medium from 0.35 to 1.50 M NaCl. To evaluate the effect of the co-precipitating, strontium ion concentration was altered from 0 to 20.0 mM. In order to elucidate the crystallization mechanism in terms of reaction rate and extent, the concentration of the free scaling ions, including that of Ca²⁺, SO₄²⁻ and Sr²⁺, is systematically monitored at different intervals until equilibrium is achieved. Moreover, crystal deposits collected at regular intervals from the isothermal incubation experiments are characterized for changes in their microstructure, morphology, and elemental distribution, to establish correlations with the data obtained on the evolution of the solution composition. The analysis of the crystallographic properties of the deposits substantiates the dependency of the thermodynamics and kinetics of the CaSO₄ precipitation process on the salinity and co-existence of supersaturated SrSO₄ in the mixture. Particularly, the morphology of calcium sulfate deposits is altered in the presence of SrSO₄, which corroborates the assumption of co-crystal interactions. These results indicate that heterogeneous crystal nucleation and growth of the mixed salts can be prompted at temperatures and salinity conditions as low as 30°C and 0.35 M, when the concentration of Sr²⁺ is increased up to 20.0 mM. Using the Pitzer model derived ion activity coefficients, discrepant thermodynamic solubility product constant (K_sp) values are also determined for the pure salts when these co-precipitate in mixed systems. Reaction kinetics for calcium sulfate crystallization is also highly sensitive to the ionic strength effect as well as to the presence and concentration of strontium ions. Accordingly, these observations confirm the need to account for both contributing and non-contributing ions in assessing the thermodynamics and kinetics of crystal formation.