Hierarchical porous biochar from plant-based biomass through selectively removing lignin carbon from biochar for enhanced removal of toluene

This study proposed a simple and green air oxidation (AO) method to prepare hierarchical porous biochar by selectively removing lignin carbon from biochar after the pyrolysis of plant-based biomass, based on the fact that the thermal decomposition temperature in air between lignin carbon and cellulose/hemicellulose carbon was different. Three kinds of biomass with different lignocellulose contents were used, including walnut shell, cypress sawdust and rice straw. The results found that AO treatment could effectively improve the pore structure of the three biochar. The specific surface area of WCO-4, CCO-4 and RCO-4 was 555.0 m2/g, 418.7 m2/g and 291.9 m2/g, respectively, which was significantly higher than those of WC (319.5 m2/g), CC (381.7 m2/g) and RC (69.6 m2/g), respectively. Among these, walnut shell biochar with air oxidation (WCO) had higher surface area of 555.0 m2/g and mesopore volume of 0.116 cm3/g, this was related to its high content of lignin, which could facilitate the formation of mesopores by AO treatment with high selectivity. The toluene adsorption capacity of WCO reached 132.9 mg/g, which increased by 223.4% from that without AO treatment. The kinetics study indicated that the diffusion rates of toluene molecule were improved due to the increased mesopores volume of biochar and micropores also play an important role in the adsorption of toluene. The results demonstrate that AO treatment is a promising method to develop hierarchical porous structure for lignocellulose-rich plant-based biomass with low cost and environmental-friendly, which greatly enhanced the toluene adsorption capacity.