The application of oxidized carbon derived from Luffa cylindrica for caffeine removal. Equilibrium, thermodynamic, kinetic and mechanistic analysis

The present work investigates the adsorption of caffeine (CF) by oxidized carbon derived from Luffa cylindrica (OLC). Batch equilibrium experiments were carried out and the effect of adsorption parameters such as pH, contact time, temperature and ionic strength, have been investigated and discussed in detail. Maximum adsorption efficiency was observed at pH 4.0 and the adsorption of CF onto OLC reached equilibrium after 80 min. The kinetic experimental data followed the pseudo-second-order kinetic model and the equilibrium adsorption data fitted well to both Langmuir and Freundlich isotherm models. The maximum adsorption capacity was estimated to be 59.9 mg/g at pH 4 and 298 K. Increasing ionic strength from 0 to 1 M NaClO4 led to a significant decline of the maximum adsorption capacity up to 50%. Thermodynamic studies revealed that the adsorption of CF onto OLC was spontaneous, exothermic with increasing randomness in the solid/solution system interface. The adsorption of CF on OLC at pH values between 4 and 10.4 seems to occur mainly via electrostatic attraction between the negative OLC surface and cationic form of CF. At pH values lower from 3 and higher from 10.4 other mechanisms such as pi-cation and dipol-dipol interactions may determine the CF adsorption on oxidized carbon surfaces. (C) 2019 Elsevier B.V. All rights reserved.