Wool-Derived Biochar via Composite Modification: Preparation and Enhanced Adsorption Performance for Methylene Blue

Amid the continuous escalation of wastewater discharge from the global dye manufacturing industry, the development of sustainable and high-performance adsorbent materials has become a critical research priority. By utilizing waste wool fibers as a precursor and employing a hybrid modification strategy that combines molten salt activation with nitrogen doping, we successfully synthesized a novel wool-based biochar (WBC). A comprehensive investigation into its adsorption mechanisms for methylene blue (MB) was conducted along with an assessment of its regeneration capability and effectiveness in removing a wide range of dye pollutants. Characterization results demonstrated that WBC possesses a high specific surface area (954.7 m²/g) and a large total pore volume (0.579 cm³/g), a hierarchical porous structure dominated by 3-4 nm mesopores. The maximum adsorption capacity was up to 887.4 mg/g under the optimum adsorption parameters with 0.01 g of adsorbent dose, 60 mg/L of initial MB concentration, pH of 9-10 and 60 °C of the adsorption temperature. In the regeneration property test, the adsorption capacity of WBC after five cycles remained at 72.26 mg/g, which was approximately 60.2% of the initial adsorption capacity. The adsorption process followed the pseudo-second-order kinetic model (R² > 0.99) and Langmuir isotherm model, respectively, indicating that the MB adsorption on WBC was monolayer chemisorption through chemical interactions predominates. MB adsorption by WBC predominantly occurs via hydrogen bonding, π-π conjugation, and electrostatic attraction. Furthermore, WBC demonstrated broad-spectrum applicability to diverse dyes. This study valorizes waste wool into a sustainable adsorbent, advancing resource recycling and environmental management.