Experimental study of zeolitic diffusion by use of a concentration-dependent surface diffusion model

Abstract

Surface diffusivity in adsorption and ion exchange processes is probably the most important property studied expensively in the literature but some aspects, especially its dependence on solid phase concentration, is still an open subject to discussion. In this study a new concentration-dependent surface diffusion model, equipped with a flexible double selectivity equilibrium relationship is applied on the removal of Pb 2+ , Cr 3+ , Fe 3+ and Cu 2+ from aqueous solutions using a natural zeolite. The model incorporates the Chen-Yang surface diffusivity correlation able to deal with positive and negative dependence with surface coverage. The double selectivity equilibrium relationship successfully represents the experimental equilibrium data, which follow Langmurian isotherm type for Pb 2+ , sigmoidal for Cr 3+ and Fe 3+ and linear for Cu 2+ . The concentration-dependent surface diffusion model was compared with the constant diffusivity surface diffusion model and found to be moderately more accurate but considerably more useful as it provides more insights into the diffusion mechanism. The application of the model resulted in an average deviation of 8.56 ± 6.74% from the experimental data and an average solid phase diffusion coefficients between 10 −9 and 10 −10 cm 2 /s. The results showed that the diffusion of metal ions in the zeolite structure is unhindered following the surface diffusion mass transfer mechanism.

Experimental and Modeling Studies of Sr2+ and Cs+ Sorption on Cryogels and Comparison to Commercial Adsorbents

“…Some studies apply simplified diffusionbased models not mirroring the complex mechanisms taking place during surface diffusion, for instance, the occurrence of nonmonotonic patterns of the surface diffusivity in the course of the sorption process. 36,37 In the case of adsorption onto cryogels, a limited number of papers apply rigorous dynamic models; for example, a rate constants distribution model also accounting for diffusion limitations was recently proposed to interpret Cu(II) adsorption on a polyethylenimine cryogel. 38 In the present paper, a detailed understanding of intraparticle transport mechanisms is gained by applying a diffusion-based model accounting for fluid film, pore, and surface transport of the adsorbate.…”