Sorption mechanism of organic dyes on a novel self-nitrogen-doped porous graphite biochar: Coupling DFT calculations with experiments

A novel N-doped porous graphitized biochar (NPGBC) with a large specific surface area (950.52 m²/g), partly graphitized structure, and high nitrogen (N) doping (3.61%) was prepared by co-carbonization of alfalfa, ferric chloride (FeCl₃), and potassium hydroxide (KOH). This biochar was used as an adsorbent with excellent methylene blue (MB) (326.90 mg/g) and methyl orange (MO) (906.52 mg/g) sorption capacities from wastewater. The sorption of MB and MO by NPGBC followed pseudo-second-order kinetics and Sips models. Density functional theory (DFT) calculations showed that the pyrrole N and pyridine N played decisive roles in the elimination of MO. When the content of pyridine N was more than 6.67%, or the content of pyrrole N was more than 3.00% in the calculation model, it was conducive to the sorption of MO. The interactions between NPGBC and organic dyes included π–π stacking and electrostatic and hydrogen-bonding interactions. The above results provide important reference values for the preparation and application of high-efficiency organic dye adsorbents.