Insights of the adsorption mechanism of methylene blue on biochar from phytoextraction residues of Citrus aurantium L.: Adsorption model and DFT calculations

Biochars prepared from agricultural and forestry biomass resources have been widely used to remove wastewater pollutants. However, the theoretical analysis of the role of biochar functional groups in pollutant removal and the visualization of the adsorption mechanism is still in its initial stage. In this study, a kind of biochar with satisfied adsorption potential was prepared from Fructus Aurantii Immaturus residues (FAIR). A Fructus Aurantii Immaturus residue biochar (FRB) structural model was constructed based on its physicochemical characterization, and the adsorption mechanism of FRB for methylene blue (MB) was clarified at the molecular level through the adsorption experiments, adsorption models and DFT calculations. The results indicated that the FRB prepared by NaOH activation has abundant functional groups, higher porosity and specific surface area, and its saturated adsorption capacity for MB reached 298.90 mg/g. The adsorption process of the FRB for MB can be suitably described by the pseudo-second-order kinetic model and the Freundlich isothermal adsorption model, and it is an endothermic entropy-driven non-spontaneous process. The adsorption mechanism depended on the electrostatic attraction, hydrogen bond, π–π interaction and van der Waals force formed by the interaction between the -COOH, -OH and -NH-CO- in the FRB structure and the amino groups in the MB structure. Notably, the results of the DFT calculations were highly consistent with the theoretical analysis of the adsorption experiments and adsorption models.