Adsorption mechanism of a cationic dye on a biomass-derived micro- and mesoporous carbon: structural, kinetic, and equilibrium insight

In this research, equilibrium, kinetics, and adsorption mechanism of methylene blue on a relatively large surface area (1437 m2 g−1) Azolla-derived micro- and mesoporous carbon has been investigated. The porous activated carbon was characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman and also X-ray photoelectron spectroscopy (XPS), N2 sorption, and high-resolution transmission electron microscopy. Remaining methylene blue solution revealed that the adsorption energy and adsorption capacity of the porous carbon are 21.76 kJ mol−1 and 1930 mg g−1, respectively. Based on the kinetics results, pseudo-second-order kinetic model is dominant during the adsorption process. Diffusion of methylene blue in the porosities and in the space between the graphitic planes of carbons is the rate-limiting parameters. Adsorption of methylene blue into the graphitic planes and the micro-porosities leads to some compressive stresses on the graphitic planes. The relative graphitization value of the prepared carbons will decrease during adsorption process. FTIR and XPS results demonstrate the effect of surface nitrogen groups on the adsorption of methylene blue. During the adsorption process, a relative percent of pyridinic nitrogen will decrease, pyrrolic and graphitic nitrogen of graphitic layers will eliminate and N-(C)3 and H-N-(C)2 nitrogen appear.