Two-phase homogeneous diffusion model for the fixed bed sorption of heavy metals on natural zeolites

In this work, the fixed bed removal kinetics of Pb²⁺, Zn²⁺, Mn²⁺, Cr³⁺, Fe³⁺ and Cu²⁺ from aqueous solutions on natural zeolites was studied. For this aim, a non-dimensional two-phase homogeneous solid diffusion model including axial dispersion and equipped with a universal double-selectivity equilibrium model is developed and applied. In total 9 isotherms, representing 128 experimental points and 25 breakthrough curves, representing 764 experimental points are used in modeling. The application of the model is satisfactory resulted in an average deviation from the experimental data of 11.19 ± 5.53%. The solid phase diffusion coefficients are between 10⁻⁷ and 10⁻⁹ cm²/s depending on the metal, flow rate and particle size in the decreasing order of Cu > Fe, Cr > Zn, Pb > Mn. The study is supplemented by an extended literature review on fixed bed models and experimentally derived solid phase diffusion coefficients in zeolites.