<p>Octahedral Fe₃O₄-modified coke Fenton catalyst (Fe₃O₄/PCWQ) was prepared via in-situ one-pot oxidation method inspired by grapefruit peel, and characterized by SEM, EDS, XRD, XPS, FTIR, BET, VSM, and Raman, respectively. Fe₃O₄ crystals was predominantly in octahedral morphology with an average particle size of 60 nm. Fe₃O₄/PCWQ exhibited graphene-like structure. The synergistic effect between oxygen functional group and Fe²⁺/Fe³⁺ cycle in Fe₃O₄/PCWQ enhances the degradation performance of p-nitrophenol (P-NP). Under the optimal conditions (1.0 g/L catalyst, 30 mmol/L H₂O₂, pH 3.0, 25 °C), Fe₃O₄/PCWQ exhibits high degradation efficiency of P-NP (91.25% in 30 min and 98.21% in 180 min) and stability (90.72% after 6 cycles) with low iron leaching (&lt;0.528 mg/L), following the quasi-first-order degradation kinetics. Fe₃O₄/PCWQ has better catalytic performance than pure Fe₃O₄ under the action of H₂O₂, and is an efficient, stable and repeatable green catalyst.</p>