Diffusion mechanisms and effect of adsorbent geometry on heavy metal adsorption

This work investigates the effect of adsorbent geometry on diffusion mechanisms assuming form factors as a new approach in the conception of mass transfer model. Experimental nickel adsorption on activated carbon and sugarcane bagasse were selected as case studies. According to SEM images, the activated carbon geometry can be approximated as a sphere with sphericity of 0.47, while the sugarcane bagasse geometry was considered an infinite cylinder. In both cases, the equilibrium data were suitable elucidated by Sips isotherm and the thermodynamic parameters revealed exothermic and spontaneous adsorption process. The numerical solution of the diffusional model was developed and presented in detail considering the geometry of each case study. Finite difference approximations and nonlinear least squares methods have been successfully used to estimate and predict the dynamic adsorption behavior in terms of mass transfer phenomena. It was observed that the adsorption rate was influenced by the geometry and textural characteristics of the adsorbents. Both surface diffusion (70%) and pore volume diffusion (30%) presented a significant contribution in the activated carbon adsorption. However, only surface diffusion (97%) was significant in the sugarcane bagasse adsorption, since its porosity is three times lower than activated carbon. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.