A comparative study of ion exchange kinetics in zinc/lead-modified zeolite-clinoptilolite systems.

The kinetics of zinc and lead ions removal by modified zeolite-clinoptilolite has been investigated. The rate of the ion exchange process for lead ions is faster than for zinc ions, as well as the time needed to reach the equilibrium. The ion exchange capacity of zeolite of lead ions is doubly higher than that of zinc ions. Diffusion models according to the Vermeulen's approximation, the parabolic diffusion model and the homogeneous diffusion model have been tested with the experimental data of ion exchange for zinc and lead. For both systems examined, the best fit of the models proposed with the experimental data was shown by the Vermeulen's approximation and the homogeneous diffusion model with t-->t(infinity). The diffusion coefficients are calculated from kinetic models of lead ions they are of the order of 10(-6)cm(2)/min, constant for all examined initial concentrations and not dependent on time. The diffusion coefficients in the system of zinc ions is of the order of 10(-8)cm(2)/min, also independent of initial concentrations, but decreasing with time from the beginning of ion exchange to the equilibrium.

Investigations of different kinetic models for zinc ions uptake by a natural zeolitic tuff.

The kinetics of uptake of zinc ions from aqueous solutions by natural zeolitic tuff has been investigated. Batch experiments at constant temperature and hydrodynamic conditions have been performed. A decrease in the initial zinc concentration in aqueous solutions prolongs the time needed for equilibrium. Various kinetic models including the film-diffusion model, the surface diffusion model and the heterogeneous diffusion model have been tested for the description of the experimental results of zinc concentration in the solution over time. Diffusion through the film and diffusion through the surface of the particle could be the rate limiting steps at the initial reaction time. However, the heterogeneous diffusion model seems to be the best model providing a satisfactory fitting of the experimental results from the beginning of the process to the point of equilibrium, particularly at lower initial zinc concentrations.