Estimation of Possible Supersaturation of Solutions during Ion Exchange in Porous Media

Abstract

The phenomenon of isothermal supersaturation of solutions in a porous medium during ion exchange is studied using mathematical modelling. The phenomenon consists in the formation of a solution with a concentration significantly exceeding the maximum solubility of the substance under ordinary conditions, and no precipitation occurs within the pores of the ion exchanger. In the works of several authors, the general nature of the phenomenon has been noted, and an explanation has been provided for the stability of the supersaturated solution in micropores. The question of why precipitation does not occur in larger pores remains open. In the present work, a model for the variation in the number of particles of the condensed phase in pores is proposed, and estimations are obtained that make it possible to determine the possible supersaturation depending on the process parameters. It is shown that the phenomenon under consideration can be explained not by a change in the thermodynamic conditions in the porous medium relative to ordinary conditions, but by the effect of dynamic equilibrium between the aggregation of condensed phase particles in the interior of the pores, their diffusion, and their decomposition near the surface of the ion exchanger grains. The proposed hypothesis is supported by quantitative analysis and comparison with available experimental data.