CO2-spherical activated carbon as a new adsorbent for Methylene Blue removal: Kinetic, equilibrium and thermodynamic studies

In this study, the efficient adsorption of methylene blue (MB) onto spherical activated carbon (SAC) prepared from the sucrose hydrothermally carbonized and physically activated with CO2 is reported for the first time. The obtained SAC surface exhibited acidic features, according to Boehm titration (1.00 mmol g−1 of acid groups) and the pHPZC (3.18). The optimum pH for MB adsorption onto SAC was of 7.0. Adsorption studies showed that kinetic data followed the pseudo-second-order and Elovich models, whereas the intraparticle diffusion and film diffusion models were tested and the results revealed that the rate-limiting step of adsorption process was not only the intraparticle diffusion. Equilibrium adsorption studies were best described by Dubinin-Radushkevich, Sips and Langmuir isotherm models, with a maximum monolayer adsorption of 211.3 mg g−1. Thermodynamic results showed negative values of Gibbs free energy change (ΔG°) and positive values of enthalpy (ΔH° = 3.13 kJ mol−1) and entropy change (ΔS° = 66.20 J mol−1 K−1), indicating that MB adsorption onto SAC is spontaneous and endothermic.