The migration law of magnesium ions during freezing and melting processes.

To explore the migration law of magnesium ions (Mg) during freezing and melting processes, laboratory simulation experiments involving freezing and melting were carried out to investigate the influence of ice thickness, freezing temperature, initial concentration, and initial pH on the distribution of Mg in the ice-water system. The distribution coefficient "K" (the ratio of the Mg concentration in the ice layer to the Mg concentration in the water layer under ice) was used to characterize the migration ability of Mg. The results showed that during the freezing process, the concentration distribution of Mg in the ice and water two-phase system was as follows: ice layer < water before freezing < water layer under ice; in other words, it migrated from ice layer to the water layer under ice. "K" decreased with increasing ice thickness, freezing temperature, initial concentration, and initial pH; the higher the ice thickness, freezing temperature, initial concentration, and initial pH were, the higher the migration efficiency of Mg into the water layer under ice was. During the melting process, Mg was released in large amounts (50-60%) at the initial stage (0-25%) and in small amounts (25-100%) uniformly in the middle and later periods. According to the change of Mg concentration in ice melt water, an exponential model was established to predict Mg concentration in ice melt period. The migration law of Mgduring the freezing and melting process was explained by using first principles. Graphical abstract.