High adsorption of methyl orange by nitrogen-doped activated carbon derived from kraft lignin via self-activation

As adsorbents, nitrogen-doped activated carbons are attracting increasing interest in the field of environmental protection, pharmaceutical and food industries due to their tunable pore structures, abundant surface functional groups and variable charges. However, the main factor limiting the commercialization and mass production of the activated carbons is the cost of adsorbent preparation. A nitrogen-doped activated carbon derived from kraft lignin in black liquor was produced by self-activation via endogenous alkali, and was used as an adsorbent of anionic methyl orange for its very low cost. It was indicated that the activated carbon with a rich pore structure, having a specific surface area of 1124.78 m2 g−1 and a total pore volume of 0.60 cm3 g−1, was obtained under a mass ratio of 1:2 of organic matter in the black liquor to diethylenetriamine and an activation temperature of 850 °C. Furthermore, the adsorption capacity of the activated carbon for methyl orange in simulated wastewater was evaluated, and it exhibited excellent performance with a maximum adsorption capacity of 373.95 mg g−1 under the optimized conditions as the methyl orange solution is at an initial concentration of 175 mg L−1, pH 7.8 and an adsorbent dosage of 0.4 g L−1. Adsorption kinetics, adsorption isotherms and adsorption thermodynamics were also studied. And the adsorption process of the activated carbon was found to be in accordance with the Pseudo-second-order kinetic model and the Langmuir model. This work provides a novel strategy for producing lignin-derived activated carbon from dried black liquor by self-activation with a low cost and timesaving approach.