Elution Phenomena of Acidic Ions from Granular Snow and Spherical Ice

Abstract

In the field of freeze separation, there have been few studies regarding the separation of solutes by forming ice spheres from the solution and melting them. In order to enhance the elution of sulfate ions from ice, spheres formed of sulfate-containing water were examined, and melting experiments were performed.
First, ice spheres containing sulfate ions were packed in a column and melted, the concentration of sulfate ions in the drained effluent was then measured. The ice spheres were formed using two different methods. In the first method, a plastic mold having spherical holes of 18.0 mm diameter was used. The mold was refrigerated for 12 h in order to form spherical ice crystals. The other method involved the formation of falling droplets of the solution through a refrigerant controlled at about –30°C and rapidly freezing them. The ice spheres formed in this way had an average diameter of 3.5 mm. Second, the ice spheres including sulfate were melted from the surface to the center in a glass vessel filled with an organic solvent and the change in the concentration of sulfate due to the melting was measured and the sulfate distribution in the sphere was estimated. In this experiment, the ice spheres having a diameter of 18.0 mm were melted.
The results of the melting experiment using the column showed higher concentrations of sulfate ions in the effluent drained first and lower concentrations in the later effluent. When using solvent for the melting experiment, the distribution of sulfate in an ice sphere was found to be biased; the amount of sulfate was apparently larger near the surface than that around the center of the sphere. It was also found that the solution containing a relatively high amount of sulfate ions for forming the ice sphere tended to establish a uniform sulfate distribution. The maximum concentration near the surface was lower than that observed in the first effluent using the column. Consequently, it was suggested that when the ice spheres containing sulfate ions were packed in a column and melted, a higher concentration of sulfate ions preferentially appeared in the first drained effluent according to the repeated melting and refreezing cycle.