Concentration dependence of diffusion-adsorption rate in activated carbon

A new method based on transfer rate measurements of a radioactive solute inside activated carbon grains is presented which allows to isolate the relative contributions of the chemical (reversible adsorption) and geometrical (tortuous diffusion paths) effects on the overall effective diffusion coefficient. Transfer rate measurements were performed with two different initial conditions: carbon grains either initially saturated with a solution of the same concentration as the bath (self-diffusion) or free of solute. In the self-diffusion case the adsorption equilibrium for the solute is undisturbed during the process and the adsorption dynamics for tracer particles becomes linear. These processes can be analyzed in terms of an effective self-diffusion coefficient (D-eff). The concentration dependence of the transfer rate is shown to be controlled by nonlinearities of the adsorption isotherm. We also show that the self-diffusion coefficient (D-s) is one order of magnitude lower than in free bulk diffusion. Finally, in the second case (grains initially free of solute) we predict satisfactorily sorption rate curves, as well as the dependence of the transfer rate on the initial concentration, using results obtained in the self-diffusion case. (C) 1999 Elsevier Science Ltd. All rights reserved.