Betel Leaf Extract-Mediated Engineering of Iron Oxide/Biochar Catalysts for Efficient Dye Degradation

The escalating discharge of synthetic dyes poses a serious threat to aquatic ecosystems, demanding catalytic water-treatment technologies with high efficiency, sustainability, and mechanistic tunability. This study presents a novel polyphenol-mediated interface engineering strategy for constructing a mixed-valence Fe x O y /biochar catalyst using betel leaf extract. Rather than serving solely as a green reducing reagent, the polyphenol-rich extract regulates the nucleation, dispersion, and stabilization of triple-phase iron oxides (α-Fe 2 O 3 , Fe 3 O 4 , and FeO) within a porous carbon matrix, enabling the formation of catalytically active Fe 2+ /Fe 3+ interfacial domains. The resulting catalyst exhibits densely distributed iron oxide nanoparticles (150–300 nm) across both the external surface and internal pore structure. Although the surface area decreases from 556.8 to 264.2 m 2 /g after modification, the engineered Fe x O y –carbon interface delivers highly efficient peroxymonosulfate (PMS) activation, achieving 90.87 ± 0.06% rhodamine B degradation efficiency within 20 min at an initial dye concentration of 50 ppm, using only 0.4 g/L catalyst and a low PMS dosage of 0.2 g/L. This performance surpasses most reported waste-derived catalysts under milder oxidant conditions, highlighting the enhanced oxidant utilization efficiency of our as-fabricated biochar. Mechanistic investigations reveal a synergistic oxidation pathway involving singlet oxygen, sulfate radicals, and hydroxyl radicals, arising from the cooperative redox cycling of mixed-valence iron species and the engineered interface. This work demonstrates that plant extract-mediated synthesis, particularly with betel leaf extract, can serve as a green fabrication route and a powerful strategy for tuning catalytic interfaces and oxidation pathways, offering broader design insights for sustainable advanced oxidation technologies in wastewater purification.