N-doping effectively enhances the adsorption capacity of biochar for heavy metal ions from aqueous solution.

N-doping was successfully employed to improve the adsorption capacity of biochar (BC) for Cu and Cd by direct annealing of crop straws in NH. The surface N content of BC increased more than 20 times by N-doping; meanwhile the content of oxidized-N was gradually diminished but graphitic-N was formed and increased with increasing annealing temperature and duration time. After N-doping, a high graphitic-N percentage (46.4%) and S (418.7 m/g) can be achieved for BC. As a result, the N-doped BC exhibited an excellent adsorption capacity for Cu (1.63 mmol g) and Cd (1.76 mmol g), which was up to 4.0 times higher than that of the original BC. Furthermore, the adsorption performance of the N-doped BC remained stable even at acidic conditions. A positive correlation can be found between adsorption capacity with the graphitic N content on BC surface. The surface chemistry of N-doped BC before and after the heavy metal ions adsorption was carefully examined by XPS and FTIR techniques, which indicated that the adsorption mechanisms mainly included cation-π bonding and complexation with graphitic-N and hydroxyl groups of carbon surfaces.