Study of thermodynamic models and crystallization kinetics for gypsum precipitation in hypersaline solutions with electrochemical impedance spectroscopy (EIS)

Predicting solid precipitation is critical for many systems (e.g., heat exchangers, desalination systems) because of the associated impact on performance efficiency. Among the mineral salts most often associated with scaling issues in water treatment systems, Gypsum (CaSO 4 ⋅ 2H 2 O) is one of the major foulants that adversely impact performance (i.e., fouling, wetting). Commonly used Debye-Hückel limiting law for predicting precipitation reactions with multiple salts is only valid for dilute solutions where the electrolyte ions are assumed to be mainly affected by the long-range coulombic interactions and several models are widely adopted as Debye-Hückel limiting law extension in predicting precipitation reactions at high salinities. These models have been shown to work well for predicting ion activities and saturation indices in high salinity solutions, but their validity in kinetically limited conditions is still in doubt. Electrochemical Impedance Spectroscopy (EIS) is a surface-sensitive measurement that allows the detection of minimal surface changes and has been used in a variety of applications to study crystal formation. It uses the impedance potential of a system determined from a response to an alternating current (AC) of varying frequency to detect salt crystallization. In this study, we used synthetic solutions at supersaturated conditions as predicted by multiple models to study scale formation at the range of ionic strengths from 0.5 to 5 M. The objective of this study is to provide insight into the precipitation of salts in high-salinity aqueous solutions and investigate the impact of the kinetic constraints on the performance of high water recovery desalination processes. EIS was proven to be an efficient and precise method for detecting and tracking the kinetics of gypsum precipitation with sensitivity to nanometer-level crystals. The utility of the EIS technique was also verified by jar tests and reaction kinetic models. • Predicted SI of gypsum in hypersaline solution with 4 different thermodynamic models • Evaluated the accuracy of EIS for detecting gypsum precipitation in hypersaline solution • Validated EIS capacitance measurement with theoretical reaction models • EIS showed superiority for detecting gypsum crystallization over conventional methods.