Biomass-derived N-doped carbon(BNC)is an important environmental material and widely used in the fields of water purification and soil remediation.However,the toxicant in the commonly used synthesis process of BNC materials have been largely ignored.Herein,we firstly report the presence of a highly toxic by-product(KCN)in the activation process of BNC materials consequential of the carbothermal reduction reaction.Because this carbothermal reduction reaction also regulates the N-doping and pore development of BNC materials,the KCN content directly relates with the properties of BNC material properties.Accordingly,a high KCN content(～611 mg)can occur in the production process of per g BNC material with high specific surface area(～3600 m2/g).Because the application performance of BNC material is determined by the surface area and available N doping,therefore,production of a BNC material with high performance entails high risk.Undoubtedly,this study proves a completely new risk recognition on a familiar synthesis process of biomass-based mate-rial.And,strict protective device should be taken in fabrication process of biomass-derived carbon material.