Elucidation of adsorption mechanisms and mass transfer controlling resistances during single and binary adsorption of caffeic and chlorogenic acids

In this work, the potential of activated carbon to remove caffeic and chlorogenic acids in aqueous solution was investigated. The study focused on evaluating the single and binary adsorption equilibrium, as well as investigating the mass transfer resistances present during the process by applying diffusional models for a future scale-up of the process. For both compounds, the single adsorption equilibrium was studied at pH values of 3, 5, and 7. The experimental adsorption isotherms were interpreted using the Langmuir and Freundlich models, obtaining maximum adsorption capacities of 1.33 and 1.62 mmol/g for caffeic and chlorogenic acid, respectively. It was found that the adsorption mechanisms for both compounds were derived from π-π, electrostatic, and H-bonding interactions. Also, the binary adsorption equilibrium was performed, and the experimental data were interpreted using the extended multicomponent Langmuir model. The results evidenced that the binary adsorption of caffeic acid and chlorogenic acid is antagonistic in nature. Finally, the experimental adsorption rate data were interpreted by an external mass transport model and a diffusional model, finding that the overall adsorption rate is governed by intraparticle diffusion. Moreover, the surface and pore volume diffusion mechanisms were meaningful.