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[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(NO 3) 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, Fe 3 C and Fe 3 N, 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ni doped Ce 2 O 3 @CeO 2 /CN composite is prepared in pyrolysis by tuning nickel doping. • CeO 2 surface is reduced in air to form Ce 2 O 3 @CeO 2 structure by carbon nitrides. • Carbon nitride structure is activated and positively charged by Ce 2 O 3 @CeO 2 structure. • Ni doped Ce 2 O 3 @CeO 2 /CN composite has highest adsorption capability of Congo red. Sheet-like Ni doped Ce 2 O 3 @CeO 2 /CN composite is synthesized by tuning the introduction of nickel in a facile and low-cost pyrolysis process. Due to the activation of the shell-core Ce 2 O 3 @CeO 2 structure, the nitrogen chemical state in the carbon nitrides is mainly in the form of an N-(C 3) structure, inducing the production of excellent electronic conductivity and positive charges on carbon nitrides. For the pyrolytic synthesis of the composite, the amount of nickel doping is a crucial factor. Under no nickel condition, only CeO 2 is synthesized and positively charged. With increasing nickel content, Ni doped CeO 2 /CN composite is prepared and negatively charged due to its main component of CeO 2 and the change of its carbon nitride structure. Ni doped CeO 2 prepared with high enough nickel content presents individual NiO particles and its negatively charged surface. The removal of these designed composites for anionic Congo red is utilized to reveal the significant influence of their surface structure. Ni doped Ce 2 O 3 @CeO 2 /CN composite depicts the highest removal performance. Ni doped CeO 2 /CN composite, despite the great change in its carbon nitride structure, still has much higher removal efficiency than pure CeO 2. However, Ni doped CeO 2 is found to have lower adsorption performance than pure CeO 2. [ABSTRACT FROM AUTHOR]