Abstract
To explore the migration law of magnesium ions (Mg2+) 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 Mg2+ in the ice-water system. The distribution coefficient “K” (the ratio of the Mg2+ concentration in the ice layer to the Mg2+ concentration in the water layer under ice) was used to characterize the migration ability of Mg2+. The results showed that during the freezing process, the concentration distribution of Mg2+ 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 Mg2+ into the water layer under ice was. During the melting process, Mg2+ 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 Mg2+ concentration in ice melt water, an exponential model was established to predict Mg2+ concentration in ice melt period. The migration law of Mg2+during the freezing and melting process was explained by using first principles.Graphical abstract
Highlights
Icebound is an important hydrological feature of surface water at high latitudes
(3) To study the effects of initial concentration on the migration law of Mg2+ in the freezing process, referring to Guidelines for Drinking-water Quality set by the WHO (2011), the maximum allowable level of total hardness of drinking water shall not exceed 500 mg·L−1
The distributions of Mg2+ in the ice and water two-phase under different ice thickness conditions are shown in Fig. 2: As shown in Fig. 2, when the ice thicknesses were 4 cm, 8 cm, 12 cm, 16 cm, and 20 cm, the concentrations of Mg2+ in ice layer were significantly lower than that of raw water by 500 mg·L−1, while the concentrations of Mg2+ in the water layer under ice were significantly higher than that of raw water by 500 mg·L−1
Summary
More than 50 million lakes regularly freeze every year in the world, and the annual icebound time is more than 150 days (Verpoorter et al 2014). The penetration rate of light in water is weakened (Catalan 1992; Welch et al 1987), which affects the photosynthesis of aquatic plants under the ice (Jewson et al 2009). Liu et al (2017, 2019) found that during the growth period of sea ice in Lake Ulansuhai, due to the difference in equilibrium gradient, Hg, Zn, and Pb concentrations in water first increased during the freezing process; the dynamic balance of ions between water and sediments was disturbed; and a portion of the Hg, Zn, and Pb concentrations migrated to sediments. Among the published literature on fresh water, only 2% involves the freezing process of water bodies (Hampton et al 2015), and these studies mainly focus on heavy metals and nutrients. Pieters and Lawrence (2009) found in winter that in the Tailings Lake in northwest Canada, when the ice thickness reached 60–80 cm, approximately 99% of the salt in the ice was discharged into the water layer under ice. Liu et al (2017, 2019) found that during the growth period of sea ice in Lake Ulansuhai, due to the difference in equilibrium gradient, Hg, Zn, and Pb concentrations in water first increased during the freezing process; the dynamic balance of ions between water and sediments was disturbed; and a portion of the Hg, Zn, and Pb concentrations migrated to sediments. Melak et al (2016) found that in the Rift Valley
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