Contact isotopic- and contact ion exchange between two adsorbents-II Kinetics of film diffusion controlled processes.

The kinetics of contact ion exchange between two adjacent ion exchanger beads in water was investigated. Since in this case diffusion of the counter ions in the intermingling electric double layers between the beads was the rate determining step, the rates of these processes were described theoretically with help of the Nernst-Planck equations. Model calculations revealed that the half times of contact ion exchange are the longer the stronger the ion exchanger particles absorb one of the two counter ions (separation factor α2 1 ≫ 1 or α2 1 ≪ 1). Contact isotopic exchange between labelled and unlabelled ions 1 will be faster than contact ion exchange between ions 1 and ions 2 if D1/D2 > 1 (Di = diffusion coefficients), regardless of the value of the separation factor. Experimentally, the rates of the contact exchange processes Cs+ -Cs+, Cs+ -Na+ and Cs+ -Ag+ were determined, using 134Cs as a radiotracer. The half times of these reactions were found to be between 20 and 30 hr, if the upper of the two 0.8 mm Φ beads rested with its own weight on the other one. If the upper bead was weighted with a mass of 2g, the half times were reduced by a factor of about 20. The experiments are in quantitative agreement with the theory