Adsorption of Phosphate and Ammonium Ions on Ice Surface in the Aqueous Solution

SYNOPSIS The distribution of impurities, such as phosphate or ammonium ions, in an ice sphere was examined by melting an ice sphere frozen using a spherical plastic mold containing a solution of phosphate or ammonium ions. The concentrations of phosphate or ammonium ions near the surface of the ice sphere were lower, and those around the center of the ice sphere were higher. This concentration distribution curve with a maximum near the center of the ice sphere was attributed to the direction of the ice formation from the outer surface to the center. In order to interpret this biased distribution, the adsorption phenomena of phosphate or ammonium ions on the surface of ice crystals were also experimentally examined. The amounts adsorbed immediately after the establishment of a quasi-equilibrium state obeyed a Langmuir adsorption isotherm. Therefore, it was suggested that the adsorption phenomena of the impurities on the ice surface caused the biased distribution and should probably be a significant factor in freeze separations.