Diffusion in granular zeolites

According to the Langmuir model (Eq.2) the adsorption capacity qm for Cd is 2.5 times grater than for Zn and adsorption capacity qm for Pb is 2 times grater than Zn when granular activated carbon is used. When natural zeolite is used as adsorbent, the adsorption capacity qm for Zn is 5 times lower than Cd and Pb. So, qm varied in the order Cd (II)> Pb(II) >Zn(II) for GAC, and Pb(II) = Cd(II)>Zn(II) for the natural zeolite as adsorbent. Ricordel et al (2001) and Tsoi and Zhao (2004) reported a similar relationship when different adsorbents were used. This can be explained on the basis of their ionic radii, hydration energy, ionic mobility and diffusion coefficient. The explanations of different authors were given on the basis of the surface covered by the adsorbed metal ions or on the basis of metal surface complexation constants and thermodynamic parameters values. 

---