Biochar supported Fe–TiO2 composite for wastewater treatment: Solid-state synthesis and mechanistic insights

A low-cost, solvent-free solid-state method for synthesizing an iron-doped TiO2–biochar composite using licorice root waste as the carbon source was proposed. Licorice was selected after a comprehensive adsorption study using methylene blue (MB) and paracetamol (PC), which revealed its superior performance among four tested biochars. The adsorption followed pseudo-second-order kinetics and was best described by the Langmuir model, indicating monolayer uptake on homogeneous active sites, with Qmax values of 949 mg/g for MB and 923 mg/g for PC. When coupled with Fe-doped TiO2, the licorice-derived biochar enhanced the photocatalytic properties by increasing the surface area, reducing the band gap to 2.06 eV, and improving charge separation. Scavenger tests indicated a shift in the primary degradation pathway from hydroxyl to superoxide radicals upon biochar incorporation. The composite showed excellent removal efficiency under visible light and strong stability across cycles, making it a promising, sustainable material for advanced wastewater treatment applications.