Abstract
Sorption, fixation and desorption kinetics of Cd (super 2+) on calcium-exchanged zeolite-X were studied using an isotopic dilution technique utilizing (super 109) Cd. The technique provided reliable measurements of time-dependent fixation of Cd and was validated using chabazite, which demonstrated wholly reversible Cd (super 2+) ion exchange. A first-order kinetic model was developed to describe the progressive transfer of Cd (super 2+) to a less reactive form in X-zeolite, following initial sorption, and subsequent desorption of Cd subject to different initial contact times. The kinetic model differentiates between two "pools" of sorbed Cd (super 2+) on zeolite-X, designated labile and non-labile sorbed Cd in which the labile sorbed Cd is in immediate equilibrium with the free Cd (super 2+) ion activity in solution. Additionally, an intra-particle diffusion model was developed and compared with the kinetic model to determine whether time-dependent Cd sorption is controlled by reaction kinetics or diffusion within zeolite particles. The kinetic model provided a much better fit to the experimental data (R (super 2) = 0.987) than the diffusion model. The rate constants describing Cd dynamics in CaX zeolite gave a half-time for Cd desorption of approximately 77 d, for release to a "zero-sink". Abstract Copyright (2006) Elsevier, B.V.