Hydrothermal pretreatment and pyrolytic conversion of biogas residue into biochar for efficient adsorption of tetracycline

Biochar preparation is a promising strategy for the application of biogas residue. Modification of biochar to enhance its adsorption performance is worthy of consideration. In this work, the biogas residue was hydrothermally pretreated first, and then directly pyrolyzed to biochar (HBRBC). Pristine biochar and hydrochar were also prepared as controls. The adsorption characteristic of biochars toward tetracycline (TC) were explored. A predominantly mesoporous structure was observed in HBRBC. Furthermore, hydrothermal pretreatment enhanced the carbon defects and the polarity of biochar. Moreover, hydrothermal pretreatment caused the change of composition and functional groups in biochar. Sorption kinetics for HBRBC could be well described with both two-compartment and pseudo-second-order models. Meanwhile, the Freundlich and Temkin models were suitable for the isotherm data of TC sorption onto HBRBC. The maximum adsorption capacity of HBRBC toward TC reached 289.1 mg⋅g−1, which was ascribed to the joint contribution of pore filling, π–π interaction, hydrogen bonding, and electrostatic interaction. Hydrothermal pretreatment mainly intensified the pore filling, π–π interaction, and hydrogen bonding due to the developed mesoporous structure, more functional groups, and high polarity. HBRBC exhibited a high adsorption performance toward TC with a wide pH range and coexisting ion interference. Thus, hydrothermal pretreatment combined with pyrolytic conversion was a feasible pathway for biogas residue utilization.