Intraparticle diffusion of Cr(VI) through biomass and magnetite coated biomass: A comparative kinetic and diffusion study

Abstract

In this study, NaOH treated pine (NTP) and magnetite-pine composite (NTP-NC) were prepared and applied for Cr(VI) adsorption at varying temperatures and the impact of the inorganic-organic nanocomposite combination on the kinetics, diffusion, equilibrium and Cr(VI) uptake mechanism was determined. FE-SEM-EDX analysis of NTP-NC showed that the composite was homogeneous containing Fe, O and C and the iron oxide was well dispersed over the composite. XPS analysis of NTP-NC revealed a shift in the 1s O to higher binding energy suggesting the incorporation of Fe3O4 on oxygenated groups of lignin. Kinetic modelling showed pseudo-second and pseudo nth order model best fitted the experimental data for both adsorbents. Intraparticle diffusion modelling revealed that initial adsorption behaviour was classified as strong initial adsorption for NTP and approaching complete initial adsorption for NTP-NC. Intraparticle diffusion coefficient, Di, where in the range of 10−5 to 10−13 cm2/s indicating that intraparticle diffusion occurred in a chemisorption controlled process. This was confirmed by the good fit of the chemisorption-diffusion model with the experimental data. Film diffusion was found to be active in the mechanism of Cr(VI) uptake by NTP but pore diffusion was active for NTP-NC. Activation energies, Ea, entropies, ΔS# and enthalpies, ΔH# of activation for the kinetic and diffusion processes where calculated from the various rate constants. Pseudo-first order Ea and ΔH# had the highest values for NTP while pseudo-second order had the highest values for NTP-NC for adsorption kinetics while intraparticle diffusion and chemisorption-diffusion gave the highest Ea and ΔH# for NTP and NTP-NC for the diffusion processes.