One-step pyrolysis of pulp and paper mill wastes for Fe0/Fe3O4/Cu0@biochar: A pH - responsive Fenton-like catalyst with switchable ROS pathways and enhanced electron transfer for efficient organic dye degradation

This study pioneers a sustainable “waste-to-wealth” strategy for fabricating a high-performance, pH-universal Fenton-like catalyst (FeCu 2 @BC T4 ) through the one-step co-pyrolysis of acid-precipitated black liquor (APBL) and Fenton sludge with a copper precursor. The innovation lies in the precise engineering of a Fe 0 /Fe 3 O 4 heterojunction synergistically coupled with Cu 0 nanoparticles on a functional APBL-derived biochar. By tuning the Cu/Fe mass ratio and pyrolysis time, the phase composition and electron-transfer properties were optimized. The resulting catalyst exhibits exceptional activity and stability across a broad pH range (3–9), achieving >92.3% methylene blue degradation within 60 min, and demonstrates superior removal efficiency toward various cationic dyes (rhodamine B, crystal violet, malachite green) and an anionic dye (methyl orange). Mechanistic investigations reveal a clear pH-governed transition of reactive oxygen species (ROS) pathways: under acidic conditions (pH 3), •OH-dominated homogeneous–heterogeneous catalysis prevails; under neutral conditions, the system shifts to a heterogeneous process driven by synergistic •OH/ 1 O 2 oxidation. This unique ROS switch is sustained by efficient electron mediation from the biochar matrix, coupled with robust electron supply and transfer enabled by the Fe 0 /Fe 3 O 4 heterojunction and Fe Cu galvanic coupling. This work provides a novel paradigm for converting industrial solid wastes into advanced catalytic materials with a well-elucidated mechanism, effectively overcoming the critical pH limitation of conventional Fenton technologies and showing broad application prospects for organic wastewater remediation.