One-step construction of magnetic biochar with a hierarchical porous structure and its adsorption properties

Abstract

Biochar materials are rich in sources and environmentally friendly and play a valuable role in sewage treatment. To improve the adsorption properties of biochar, it must be modified and magnetized. In this paper, sunflower straw, Fe(NO₃)₃·9H₂O and 1,3,5-benzoic acid were used as precursors to prepare a hierarchical porous magnetic biochar composite (MBC) by one-step solid-phase anoxic low-temperature pyrolysis. The MBC samples were characterized by SEM–EDS, TEM, XRD, N₂ adsorption–desorption, FT-IR and VSM. The results showed that the hierarchical porous magnetic biochar was successfully prepared. The introduction of the surfactant 1,3,5-benzoic acid inhibited the decomposition of Fe(NO₃)₃·9H₂O in the magnetic γ-Fe₂O₃ crystal state. Additionally, carbon fiber formed by the decomposition of 1,3,5-benzoic acid under anoxic conditions covered the γ-Fe₂O₃ nanoparticles. These γ-Fe₂O₃ nanoparticles coated with carbon fiber were firmly loaded on the surface of the biochar, magnetizing the porous biochar system and enriching its surface structure. The inorganic small-molecule gas generated by the decomposition of 1,3,5-benzoic acid opened up the original pore structure of the sunflower-based biochar, enriched the pore structure of the system and increased the specific surface area of the system. MBC-20 prepared with 20% γ-Fe₂O₃ (mass percentage) showed strong adsorption performance for rhodamine B. The adsorption rate of MBC-20 reached more than 99.45% in 30 min. The adsorption behavior was consistent with quasi-second-order kinetics and the Langmuir isotherm model. The maximum adsorption capacity was 48.03 mg g⁻¹; the material was suitable for magnetic recovery and maintained high adsorption performance after 5 cycles. The prepared magnetic biochar MBC-20 not only has good adsorption properties for rhodamine B but also has good adsorption properties for other cationic dyes, anionic dyes and actual pollutants.