Objective Biochar demonstrates significant potential for dye wastewater treatment. To prepare adsorbent with high dye adsorption performance and enhance the comprehensive utilization of biomass resources, biochar was prepared by expansion pretreatment of cork.

Method Cork was subjected to expansion pretreatment using water-boiling and microwave methods, followed by pyrolysis in a tubular furnace at 550 ℃ for 60 min to produce cork biochar. The microstructure and chemical properties of cork biochar were characterized using SEM, FTIR, and BET analyses. Batch adsorption experiments were conducted to evaluate the adsorption performance of cork biochar for methylene blue (MB) and Congo red (CR), two cationic and anionic dyes. The adsorption mechanisms were elucidated through kinetic, isotherm, and thermodynamic model fitting, combined with structural characterization.

Result The cork biochar obtained through microwave expansion pretreatment (MBC-90) exhibited a rougher surface with abundant micropores and mesopores, alongside a reduction in —OH functional groups and an enhancement of aromatic structures. These characteristics made MBC-90 more suitable for adsorbing small dye molecules MB. In contrast, the cork biochar produced via water-boiling expansion pretreatment (WBC-90) displayed larger pores and irregular pore wall structures, while retaining more —OH functional groups, making it better suited for adsorbing larger dye molecules CR. The maximum adsorption capacity of MBC-90 for MB reached 193.63 mg/g and that of WBC-90 for CR reached 203.55 mg/g, which were better than the maximum adsorption capacity of unexpanded cork biochar for MB (129.18 mg/g) and CR (121.44 mg/g). The adsorption of MBC-90 on MB and WBC-90 on CR showed excellent pH adaptability, maintaining high adsorption capacity in pH range of 2−10. After five adsorption-desorption experiments, the adsorption stability and regeneration performance of MBC-90 on MB (95.43 mg/g) and WBC-90 on CR (138.17 mg/g) remained good. The adsorption process followed a pseudo-first-order kinetic model and a Langmuir isotherm model, indicating that the adsorption process was dominated by physical adsorption, with monolayer coverage forming on the biochar surface. Thermodynamic analysis further confirmed that the adsorption was a spontaneous, endothermic reaction, and the expansion pretreatment significantly enhanced the spontaneity of adsorption process. Overall, the comprehensive analysis revealed that the adsorption mechanism of cork biochar for dye molecules involved multiple interactions, including electrostatic attraction, hydrogen bonding, π-π stacking and pore filling.

Conclusion The dye adsorption performances of cork biochar are improved by microwave and boiled expansion pretreatment. In addition, the prepared cork biochar also exhibits good pH adaptability and recycling performance, which is expected to be applied to industrial wastewater treatment and will expand the comprehensive utilization of biomass resources.