Diffusion in ion exchange resins

Abstract

AbstractObservations on the rate of ion exchange in Dowex 50W resins are reported. These measurements were made under conditions where solid diffusion was the governing phenomenon for six pairs of ions sodium‐zinc, sodium‐silver, silver‐aluminum, zinc‐copper, zinc‐aluminum, and aluminum‐cerium (trivalent). The effects of temperature and of resin cross‐linkage were studied with the first system. Exchange in both directions was studied for each pair except the last.Interpretation with a Fick's law model was accomplished, and diffusion coefficients were obtained for each pair. Values were greatly dependent on the direction of the exchange as well as on the particular pair.Interpretation with a Nernst‐Planck model was also accomplished and diffusivities for each ion were obtained. Values were greatly influenced by the nature of the second ion except in the case of sodium. The interpretation in this case required numerical solution of the flux equations. These solutions are presented for valence ratios of 1/3 and 2/3 with diffusivity ratios of 5, 10, and 20 and for valence ratios of 3 and 3/2 with diffusivity ratios of 1/5, 1/10, and 1/20.Activation energies were found to be 4 to 6 kcal/g. mole for either model. Increasing the resin cross‐linkage from 4 to 12% decreased the diffusion values by approximately 80%.The data were equally well represented with either model. In view of the simplicity of Fick's law, the use of this model is recommended for design purposes.