Finite bath and boundary layer effects on solute release kinetics from solid matrices

Abstract

The effects of deviations from the theoretical boundary conditions, and, in particular, the infinite bath and perfectly agitated bath conditions, on the transient release kinetics of a solute from a solid matrix are considered. The efficiency in this respect of two experimental designs, referred to as the “agitated specimen” and “continuous elution” techniques, was evaluated using a cellulose acetate-4-aminoazobenzene model system. The dependence of release rate and kinetics on various experimental parameters was shown to be in conformity with the predictions of simple theory and to lead to consistent estimates of stagnant boundary solution layer thickness. The agitated specimen technique was found to compare favorably with the more usual alternative agitation methods in efficiency, with the potential advantage of wider applicability. The continuous elution technique was designed primarily for the laboratory evaluation of the maximum water leachability of “solidified” medium-to-low-level radioactive waste products, which is an important criterion relating to the safe disposal of these products in the environment. Release kinetic data of Cs + and Sr 2+ (which are among the main radionuclides present in radioactive wastes) from cement matrices (loaded with Cs 2SO 4 or SrSO 4, respectively) obtained by the usual “stagnant eluant” and by the new continuous elution method, are reported. It is shown that (i) finite bath and imperfect agitation effects can seriously reduce the release rate measured by the former technique, in the case of the salt of low aqueous solubility, and (ii) these effects are efficiently eliminated by the latter method (which, incidentally, also prevents the even more serious reduction in Sr 2+ release rate caused by exposure of the alkaline leachate to atmospheric CO 2).