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 Cu²⁺ and Cd²⁺ by direct annealing of crop straws in NH₃. 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_BET (418.7 m²/g) can be achieved for BC. As a result, the N-doped BC exhibited an excellent adsorption capacity for Cu²⁺ (1.63 mmol g^-1) and Cd²⁺ (1.76 mmol g^-1), 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.