Comparative Removal of Cr(VI) and F− Ions from Water by Freezing Technology

Abstract

Trace element ions, such as Cr(VI) and F−, are of particular interest due to their environmental impact. Both ions exhibit an anionic nature in water that can show similar removal tendencies except for their significant differences in ionic radius, speciation forms, and kosmotropic-chaotropic behaviors. Accordingly, partial freezing was performed to examine the comparative freeze separation efficiencies of Cr(VI) and F– from aqueous solutions. Freeze desalination influencing parameters such as initial ion concentration, salt addition, and freeze duration were explored. Under optimal operating conditions, freeze separation efficiencies of 90 ± 0.12 to 95 ± 0.54% and 58 ± 0.23% to 60 ± 0.34% from 5 mg/L of Cr(VI) and F–, respectively, were demonstrated. The salt addition into the F–-containing solutions revealed more F– ion intercalation into the ice, initiating the decrement of freeze separation efficiency. The influences of structuring-destructuring (kosmotropicity-chaotropicity) and the size-exclusion nature of ice crystals were used to explain the plausible mechanism for the difference in freeze separation efficiency between Cr(VI) and F– ions.