Nitrogen containing functional groups of biochar: An overview

[Display omitted] •Pyridinic-N, pyrrolic-N, and quaternary-N dominate biochar N-functional groups.•N-functional groups of biochar can be analyzed by FTIR, XPS, NMR and XANES.•N-functional groups are mainly results of biomass, pyrolysis temperature & N-doping.•Biochar N-functional groups favor pollutant removal, redox reaction & energy storage. Biochar is a carbonaceous material produced by thermal treatment, e.g., pyrolysis, of biomass in oxygen-deficient or oxygen-free environment. Nitrogen containing functional groups of biochar have a wide range of applications, such as adsorption of pollutants, catalysis, and energy storage. To date, many methods have been developed and used to strengthen the function of N-containing biochar to promote its application and commercialization. However, there is no review available specifically on the development of biochar technologies concerning nitrogen-containing functional groups. This paper aims to present a review on fractionation, analysis, formation, engineering, and application of N-functional groups of biochar. The effect of influencing factors on biochar N-functional groups, including biomass feedstock, pyrolysis parameters (e.g., temperature), and additional treatment (e.g., N-doping) were discussed in detail to reveal the formation mechanisms and performance of the N-functional groups. Future prospective investigation directions on the analysis and engineering of biochar N-functional groups were also proposed.