Adsorption of Methylene Blue Onto Novel Biochars Prepared from Magnolia Grandiflora Linn Fallen Leaves at Three Pyrolysis Temperatures

The adsorption properties and mechanisms of methylene blue (MB) onto novel biochars produced by the fallen leaves of Magnolia grandiflora Linn (MGL), at different pyrolysis temperatures (450 °C, 500 °C, 550 °C) were explored. Results of the adsorption experiments revealed that the fallen leaf-biochar of MGL (MGLB) pyrolyzed at 450 °C (MGLB450) had the highest adsorption capacity of MB (114.15 mg g−1) and MGLB pyrolyzed at 500 °C (MGLB500) was lowest (88.13 mg g−1). The characterization results showed that the BET surface area (41.784 m2 g−1) and total pore volume (0.043 cm3 g−1) of MGLB450 were low, but the contents of oxygen-containing functional groups were highest. Oxygen-containing functional group might have a greater impact on the adsorption of MB than its physical characteristics. The adsorption capacity increased with reaction temperature, indicating that the MG adsorption onto biochars was endothermic. The higher initial concentrations of MB and pH were beneficial to adsorption. The adsorption kinetics showed that the adsorption followed pseudo-second-order kinetics model. The obtained equilibrium data were fitted better by Langmuir model rather than Freundlich model.