Ion migration at the ice–water interface during the freezing process of salt solution – Experimental investigation

Abstract

Within the scope of this work, systematic investigations of the ion migration process at the ice–water interface during the freezing of salt solutions are performed via experimental investigations (part I) and numerical investigations (part II). In this study, the ion migration trend under different freezing temperatures was revealed, and the formation process of the brine pockets during phase transition and the mechanism of ion release in ice were explored. The results show that the migration capabilities of Na+ and K+ are greater than those of Ca2+ and Mg2+, whereas the migration capability of Cl− is greater than those of HCO3− and SO42−. At −2 °C, the ion concentration in the surface ice decreased by 47.64 %, indicating the occurrence of ion release within the ice. When the moving speed of the phase transition interface is greater than the ion diffusion ability, the brine pockets form in the ice. Under a temperature gradient, the brine pockets in ice become thermodynamically unstable, resulting in the inevitable ion release toward the high-temperature side. The ion release depends on the temperature gradient and the brine pocket concentration. Furthermore, the unsynchronous migration of ions is related to the freezing temperature, diffusion coefficient, hydration radius, and hydration free energy.