A sustainable preparation strategy for the nitrogen-doped hierarchical biochar with high surface area for the enhanced removal of organic dye

Abstract Biochar is a potential porous carbon to remove the contaminants from aquatic environments. Herein, N-doped hierarchical biochar was produced by the combined approach of ammonia torrefaction pretreatment (ATP) and alkali activation. ATP could not only incorporate N element into poplar wood, but obtain the loose structure of poplar wood. The highest surface area of N-doped hierarchical biochar was 2324.61 m 2 g −1 after ammonia wet torrefaction pretreatment, which was higher than that of activation carbon (1401.82 m 2 g −1 ) without torrefaction pretreatment, the hierarchical biochar (2111.03 m 2 g −1 ) without ammonia atmosphere. The N-doped hierarchical biochar presented the highest adsorption capacity (564.7 mg g −1 ) of methyl orange (MO), which was 14.64-fold of that on biochar without N doping. In addition, the pseudo-second-order and Langmuir model fitted well with the adsorption kinetics and isotherms of the N-doped hierarchical biochar. The incorporation of nitrogen element could not only tune the distribution of surface electrons on biochar, but optimize the ambient condition of adsorption active sites as well. The adsorption of MO might occur on the N-/O-containing functional groups through the electrostatic interaction, the π-π dispersion interaction, and the hydrogen bonding. The density functional theory showed that the graphitic-N and pyridinic-N were the dominant adsorption active sites. Graphical Abstract