An improved film-pore-surface diffusion model in the fixed-bed column adsorption for heavy metal ions: Single and multi-component systems

The present work was investigated the adsorption of heavy metal ions including lead (Pb2+), Cadmium (Cd2+) and Nickel (Ni2+) onto a commercial activated carbon (AC) in single and multi-component aqueous fixed bed column. The model, named film pore-concentration dependant-surface diffusion (FPCDSD), was able adequately to consider the details of inter- and intra-pellet transport mechanisms in column mode and predict film, pore and surface diffusivities of the adsorption process considering a nonlinear isotherm model. A diffusional kinetic model was developed, to investigate the effects of several parameters like initial concentration, flow rate, and bed height, and also to explain the adsorption kinetics and evaluation of the mass transfer coefficients. The detailed calculation methods to determine the contribution of each resistance against mass transfer were addressed in the paper. The model with mass transfer parameters (with the order of magnitude of Dp ≈ 10−10 m2 s−1, Ds ≈ 10−15 m2 s−1, Dz ≈ 10−5 m s−2, and kf ≈ 10−5 m s−1) was found successful in correlating adsorption data in single, and multi-component system under different operating conditions. Sensitivity analysis revealed that the value of Dp and kf dictate the model performance. The Biot number was found to increase with increasing flow rate and initial concentration, indicating an increase in intraparticle diffusion resistance.