Orange peel-derived iron-doped biochar for efficient and rapid Fenton-like degradation of Rhodamine B: Role of temperature-induced iron species evolution

Abstract

This study developed an efficient Fenton-like catalyst by pyrolyzing a mixture of orange peel and ferric nitrate in an inert atmosphere to produce iron-doped biochar (Fe/BC) for Rhodamine B (RhB) degradation. The materials were characterized by XRD, Raman, SEM, TEM, XPS, and low-temperature N₂ adsorption-desorption measurements. The results showed that pyrolysis temperature significantly influenced the iron species in Fe/BC. Fe₃O₄ was dominant at 600 °C, while metallic iron (Fe⁰) and iron carbide (Fe₃C) prevailed above 650 °C. These structural changes directly affected catalytic activity. The density functional theory (DFT) calculations based on adsorption energies and charge transfer analysis indicated that the superior activity of Fe₃C-Fe⁰/BC over Fe₃O₄/BC was associated with its stronger RhB adsorption and enhanced interfacial electron transfer. The Fenton-like system composed of Fe/BC-800 (pyrolyzed at 800 °C) demonstrated optimal performance, achieving 97.3% RhB degradation under specific conditions (0.3 mL H₂O₂, 0.04 g Fe/BC-800, 50 mL 20 mg·L⁻¹ RhB, pH 4.82, 25 °C, and 25 min). Hydroxyl radicals (·OH) were confirmed as the primary oxidants based on radical trapping tests and EPR spin trapping evidence to drive RhB degradation through sequential reactions including stepwise N-deethylation and cleavage of the chromophore structure, thereby achieving its thorough degradation. This work not only presented a high-performance, biomass-derived catalyst for advanced oxidation processes but also provided fundamental insight into the critical role of iron speciation in governing Fenton-like catalytic activity. These findings offered a valuable strategy for sustainable water treatment.