Design of a magnetic Fenton-like catalyst by decorating diamond-shaped MIL-88A with Chenopodium-derived biochar for nitrophenol degradation: optimization and mechanistic insights

Abstract

An effective Fenton-like Fe₃O₄/MIL-88A/BC catalyst was fabricated by combining magnetite nanoparticles (Fe₃O₄) with diamond-shaped MIL-88A and Chenopodium-derived biochar for the degradation of 2-NP. The elemental composition, morphology, functional groups, surface net charge, and crystallographic phase of the Fe₃O₄/MIL-88A/BC catalyst were examined using various characterization techniques, including XPS, SEM, FTIR, ZP, and XRD. Optimization experiments were conducted to determine the optimal Fenton-like degradation conditions for 2-NP using H₂O₂/Fe₃O₄/MIL-88A/BC. Laboratory experiments showed that the 2-NP degradation efficiency by H₂O₂/Fe₃O₄/MIL-88A/BC reached 91.04% within 120 min at pH = 5, Fe₃O₄/MIL-88A/BC = 10 mg, and H₂O₂ concentration = 500 mg L^-1. Kinetic studies indicated that the Fenton-like degradation of 2-NP followed a second-order model, while H₂O₂ decomposition was best described by a first-order model. Quenching tests indicated that the Fenton-like reaction of 2-NP proceeded via a radical mechanism and confirmed that the ˙OH radicals are the controlling reactive O-species. The degradation mechanism of 2-NP was proposed based on the XPS spectra of the neat and used Fe₃O₄/MIL-88A/BC catalysts. The intermediates obtained from the Fenton-like degradation of 2-NP by the Fe₃O₄/MIL-88A/BC catalyst were predicted from the GC-MS spectrum.

Fig. 1. Schematic representation of the fabrication process of the Fe₃O₄/MIL-88A/BC catalyst.

Fig. 2. (A) FTIR spectra, (B) XRD of MIL-88A, Fe₃O₄, BC, and Fe₃O₄/MIL-88A/BC composites, and (C) zeta potentials of the Fe₃O₄/MIL-88A/BC composites.

Fig. 3. XPS spectra of the Fe₃O₄/MIL-88A/BC composite: (A) O 1s, (B) C 1s, and (C) Fe 2p.

Fig. 4. SEM images of (A) BC, (B) Fe₃O₄, (C) MIL-88A, and (D) Fe₃O₄/MIL-88A/BC composite.

Fig. 5. (A) Comparison test (dose = 10 mg, temperature = 25 °C, [2-NP] = 50 mg L⁻¹, [H₂O₂] = 500 mg L⁻¹, and pH = 5); (B) effect of pH on the catalytic system (dose = 10 mg, [H₂O₂] = 500 mg L⁻¹, temperature = 25 °C, [2-NP] = 50 mg L⁻¹, and pH = 3–11); (C) effect of catalyst dose ([H₂O₂] = 500 mg L⁻¹, temperature = 25 °C, [2-NP] = 50 mg L⁻¹, pH = 5, and dose = 5–20 mg); and (D) effect of temperature on the Fenton-like degradation of 2-NP (temperature = 25–55 °C, [2-NP] = 50 mg L⁻¹, pH = 5, [H₂O₂] = 500 mg L⁻¹, and dose = 10 mg).

Fig. 6. (A) Effect of oxidant concentrations (pH = 5, [2-NP] = 50 mg L⁻¹, [H₂O₂] = 100–1000 mg L⁻¹, catalyst dose = 10 mg, and temperature = 25 °C), (B) decomposition of H₂O₂ ([2-NP] = 50 mg L⁻¹, pH = 5, [H₂O₂] = 500 mg L⁻¹, catalyst dose = 10 mg, and temperature = 25 °C); and (C) effect of 2-NP concentrations on the Fenton-like degradation efficiency ([2-NP] = 50–300 mg L⁻¹, pH = 5, [H₂O₂] = 500 mg L⁻¹, catalyst dose = 10 mg, and temperature = 25 °C).

Fig. 7. Kinetic study of the Fenton-like degradation of 2-NP by the H₂O₂/Fe₃O₄/MIL-88A/BC system: (A) first-order and (B) second-order, and kinetic study of the H₂O₂ decomposition: (C) first-order and (D) second-order.

Fig. 8. (A) Quenching test of the Fe₃O₄/MIL-88A/BC, (B) XPS survey of the used Fe₃O₄/MIL-88A/BC catalyst, (C) high resolution of Fe 2p, and (D) high resolution of O 1s.

Fig. 9. GC-MS of 2-NP after Fenton-like degradation by the Fe₃O₄/MIL-88A/BC composite.

Fig. 10. Schematic representation of the degradation of 2-NP by the Fe₃O₄/MIL-88A/BC composite.

Fig. 11. (A) Cycling test of the Fe₃O₄/MIL-88A/BC catalyst for five 2-NP degradation cycles and (B) leaching test of Fe species from the Fe₃O₄/MIL-88A/BC composite during the five degradation cycles.