Fabrication of N-doped micro-mesoporous carbons from industrial alkali lignin with urea assistance for high-efficiency adsorption of methylene blue

N-doping is a prospective strategy to improve the adsorption efficiency of adsorbents. Herein, N-doped porous carbons (PCs) were synthesized from industrial alkali lignin (IAL) with urea assistance for adsorbing methylene blue (MB). The nitrogen content and pore structure could be adjusted by changing urea dosage. The PCs were characterized via scanning electron microscopy (SEM), N2 adsorption-desorption experiments, Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and Zeta potential. The optimized weight ratio of urea to IAL was 1:1. NLC-1 synthesized under this condition possessed high micropore volumes (Vmicro) and mesopore volumes (Vmeso), namely 0.3531 and 0.3480 cm3/g, respectively, and the specific surface area (SSA) of NLC-1 reached 1131 m2/g. The adsorption capacities of PCs for MB were compared and discussed. Notably, the as-obtained NLC-1 exhibited the highest adsorption capacity of 785.8 mg/g. This was attributed to the pore filling and the appropriate N content (3.780%). The adsorbing process of MB onto NLC-1 was well reproduced by the Langmuir model. Furthermore, the kinetic data conformed to the pseudo-second-order (PSO) model, and the MB adsorption process was spontaneous according to the thermodynamic parameters. Overall, this study indicates that PCs from IAL have a great potential for MB adsorption.