Visible-light-responsive Fe-Based oxide nanomaterials for sustainable photo-Fenton degradation of hazardous phenols

Persistent pollutants like 2,4-dinitrophenol (2,4-DNP) pose serious risks to environmental and human health due to their toxicity and resistance to biodegradation. This study explores the degradation of 2,4-DNP using heterogeneous Fenton and photo-Fenton processes catalyzed by iron-based oxide nanomaterials synthesized hydrothermally. Four catalysts—Bi-Fe, Cu-Fe, Ag-Fe, and C/Ag-Fe, were prepared and characterized using XRD, SEM, and TEM to confirm their crystalline phases and nanostructures. Among them, Bi-Fe oxides exhibited the highest catalytic performance under visible light due to its porous morphology and mixed-phase composition. Under optimal photo-Fenton conditions (pH 3, 100 mg/L catalyst, 1000 mg/L H2O2), Bi-Fe oxides achieved over 99.9% degradation of 10 mg/L 2,4-DNP within 240 min, with a rate constant of 0.024 min-1. Cu-Fe demonstrated the best performance in conventional Fenton conditions, reaching 73.4% removal after 60 min. Ag-Fe and C/Ag-Fe showed moderate but consistent activity, with removal efficiencies up to 81.8% under photo-Fenton conditions. The impact of operational parameters—including catalyst dose, H2O2 concentration, solution pH, and common anions—was systematically investigated. Sulfate and phosphate ions notably enhanced the photo-Fenton process. Zeta potential governs adsorption and stability of Bi–Fe, Cu–Fe oxides, and Fe2O3 in Fenton systems, with all being strongly positive in acidic media but shifting negative at their respective isoelectric points (Cu–Fe ≈ 5.8, Bi–Fe ≈ 6.3, Fe2O3 ≈ 7–7.8). Bi–Fe catalysts show higher stability under photo-Fenton (~ 92% activity, 0.7 mg/L leaching) than in dark Fenton (~ 82%, 1.2 mg/L), highlighting the role of light in reducing dissolution and sustaining long-term efficiency. Degradation pathways were elucidated using HPLC and IC, confirming the formation and subsequent breakdown of trihydroxybenzene and oxalic acid as intermediates. The results demonstrate that Bi-Fe system, driven by visible light, offers a sustainable and highly effective catalytic platform for degrading persistent phenolic pollutants. This study highlights the practical potential of photo-Fenton systems using iron-based nanomaterials for real-world wastewater treatment applications under mild and energy-efficient conditions.