Migration of Inorganic Salt Ions During Salt Water Freezing and Melting

Abstract

In this study, we performed frozen saltwater desalination experiments on salt water from southern Xinjiang, China, to investigate ion migration during freezing and melting. We also analyzed the key factors that are responsible for differences in ion migration.

Our results show significant differences in the ion migration rate during freezing, related to the initial ion concentration, ionic radius, and freezing temperature (P<0.05). Consequently, freezing selectively removes ions from saltwater, with monatomic ions with a higher concentration and a larger radius being more likely to be 'trapped' by ice crystals. Cl^-, Na⁺, Ca²⁺, CO₃^2-+HCO₃^- (P<0.05) ions were also more easily 'trapped' by ice crystals when the freezing temperatures were low; SO₄^2- was relatively difficult to remove during freezing regardless of freezing temperature. During gradual melting, Ca²⁺, Mg²⁺, and K⁺ saw the most significant removal during the first of the five melting stage, while Cl^-, SO₄^2-, CO₃^2-+HCO₃^-, and Na⁺ were significantly removed during the first two melting stages. Water quality also changed with progressive melting, from Cl-Na to Cl•SO₄-Na type between the early and middle stages. the ion concentration data of ice meltwater was fitted better by the ion deviation coefficient equation (R₂>0.92).

Keywords: frozen desalination, salt water, ion migration, gravity-induced brine drainage, fitting