Adsorption of methylene blue and methyl violet by camellia seed powder: kinetic and thermodynamic studies

Adsorption of methylene blue (MB) and methyl violet (MV) onto camellia seed powder (CSP) was conducted, and the equilibrium data were fitted with Langmuir, Freundlich and Dubinin-Radushkevich (D-R) models to describe the equilibrium isotherms. The kinetics rates were modeled using pseudo-first-order, pseudo-second-order kinetic equations, and intra-particle diffusion model. The results revealed that adsorption of MB and MV onto CSP was affected slightly by the pH value, and the maximum adsorption amount was achieved at the pH of 5.5, the unadjusted value. For MB, the adsorption process could be depicted primly by Langmuir function, while for MV, the equilibrium data agree well with all of the Langmuir, Freundlich, and D-R models. The Gibbs free energy (Delta G(0)), enthalpy change (Delta H-0), and entropy change (Delta S-0) for MB were all below zero, which indicated the adsorption was spontaneous and exothermic process; while for MV, the negative Delta G(0) value and positive Delta H-0 value imply that its adsorption process is spontaneous and endothermic. For both MB and MV, the kinetics data were better fitted with the pseudo-second-order kinetic model and the intra-particle diffusion controls the adsorption rate.