Application of film-pore diffusion model for the adsorption of metal ions on coir in a fixed-bed column

The most important criterion in the design of fixed-bed adsorption systems is the prediction of column breakthrough or the shape of the adsorption wave front, which determines the operating life-span of the bed and the regeneration time. In this study, the investigation of column breakthrough was carried out using coir, a by-product from coconut processing industry, as adsorbent to remove Pb(II) and Cu(II) from aqueous solutions. A two resistance film-pore diffusion (FPD) model was applied to predict the concentration profiles for various system variables in this study, including metal solution concentration, flow rate and bed height. It was found that the FPD model could be applied to predict the breakthrough kinetics of the column. The systems under investigation (Pb/coir and Cu/coir) could be described by a single effective diffusivity (Deff) over the operating range of flow rates and initial metal concentrations. The Deff values were (1.31 ± 0.36) × 10−5 cm2/s and (8.24 ± 1.03) × 10−6 cm2/s for Pb/coir and Cu/coir systems, respectively. On the other hand, the values of external mass transfer coefficient (βc) were found to increase with decreasing flow rates and remained constant for the variation in initial adsorbate concentration. Biot number was used as an indicator for the intraparticle diffusion. The Biot number was found to increase with increasing flow rate and initial concentration, indicating an increase in intraparticle diffusion resistance.