A simple melamine-assisted cellulose pyrolysis synthesis of magnetic and mesoporous N-doped carbon composites with excellent adsorption of Congo red

[Display omitted] •Metal/N doped carbon composite is prepared by a melamine-assisted cellulose pyrolysis.•Metal/N doped carbon composite is magnetic and quickly separated from solution.•Magnetic composite has big mesopore volume and electron deficient conjugated surface.•Dye adsorption on magnetic composite is reached via electrostatic and π-π interaction.•Magnetic composite has a high adsorption capacity of Congo red, especially at high pH. A magnetic and mesoporous metal/N-doped carbon composite is prepared using a melamine-assisted pyrolysis method of hydroxypropyl methylcellulose with Fe(NO3)3. The magnetic composite has a 3D hierarchical structure composed of 1D nanostructure interlaced with each other and a large pore volume distributed mainly in mesopores. Meanwhile, its N-doped carbon surface with sp2 hybridized structure is positively charged due to the promotion action of its mixed Fe phase, Fe3C and Fe3N, inducing the production of electron deficient conjugated structures as electron acceptors. Its characteristic microstructure determines that the adsorption of anionic Congo red on its surface is achieved mainly through multiple adsorption mechanisms of electrostatic and π-π interaction. Thus, the magnetic composite has an excellent dye adsorption capacity, even improved dramatically at high pH conditions, and simultaneously, the spent composite is quickly separated from wastewater solution by using its magnetic properties. In contrast, these undoped or N-doped carbon composites prepared in the absence of melamine and Fe salts have very poor or partially improved adsorption efficiencies to the Congo red dyes due to their inactive characteristic microstructures, such as lower pore volume, more microporous structures and negatively charged mesoporous surface.