Ion Migration Patterns and Mechanisms of Melting Brackish Water Ice by Gravity-Induced Desalination

Abstract

The melting process of brackish water ice has more desalination effect than the formation process of brackish water ice. In this experiment, ice crystals produced by single-stage progressive freezing under different conditions of icing temperature and raw water ion concentration were utilized to carry out gravity-induced desalination (GD) and conventional ice melting tests. The migration pattern of Cl-, SO42-, CO32-+HCO3-, Na+, K+, Ca2+ and Mg2+ under the conditions of different ice crystal masses was explored. The results showed that the ion removal rate tended to decrease with the continuation of the GD ice melting process and the decrease of the average ice crystal ion concentration. When the icing temperature decreased, the ion removal rate showed a decreasing trend. The ion concentration of ice-melt water during the GD process showed an exponential decay, and the water type changed from Na·Mg-Cl to Na-Cl·SO4. There were differences in the migration rates of each ion during GD ice melt. Among them, the ion discharge rate showed a distribution pattern of Cl- > Na+ > SO42- > Ca2+, Mg2+, CO32-+HCO3- > K+. The ion removal rate showed a distribution pattern of Na+ > Cl- > K+, Ca2+, Mg2+ > CO32-+HCO3- > SO42-. The key factors affecting the ion discharge rate are ion relative mass, polarizability, ion radius, and average ion concentration. The key factors affecting the ion removal rate are ion relative mass, ion radius, icing temperature, and average ion concentration. And the ion discharge rate and removal rate are significantly and positively correlated with other key influencing factors except ion relative mass. The model can be used to accurately predict the ion concentration of mixed ice-melt water during GD ice melting process. Using the mixed ice-melt water ion concentration as the target ion concentration for user demand can significantly improve water production rates. Compared with using the instantaneous concentration as the ice-melt water collection point, the water production rate was improved by 27.40%. The application of the prediction model is presented by way of a case study.

Keywords: gravity-induced desalination, brackish water ice, ions, modeling, water yield