Study on the behavior and mechanism of NiFe-LDHs used for the degradation of tetracycline in the photo-Fenton process

Abstract

An environment-friendly 3D NiFe-LDHs photocatalyst was fabricated via a simple hydrothermal method and characterized by means of SEM, XRD, BET, XPS and FT-IR. It is a highly efficient heterogeneous photo-Fenton catalyst for the degradation of TC-HCl under visible light irradiation. After exploring the effects of catalyst dosage, initial concentration of TC-HCl, solution pH and H₂O₂ concentrations, the optimal reaction conditions were determined. The experiment results showed that the degradation efficiency can reach 99.11% through adding H₂O₂ to constitute a photo-Fenton system after adsorption for 30 min and visible light for 60 min. After four cycles, the degradation rate decay is controlled within 21.2%, indicating that NiFe-LDHs have excellent reusable performance. The experimental results of environmental factors indicate that Fe²⁺ and Ca²⁺ promoted the degradation of TC-HCl, both Cl^- and CO₃^2- inhibited the degradation of TC-HCl. Two other antibiotics (OTC and FT) were selected for research and found to be effectively removed in this system, achieving effective degradation of a variety of typical new pollutants. The radical trapping tests and ESR detection showed that ·OH and ·O₂^- were the main active substances for TC degradation in the photo-Fenton system. By further measuring the intermediate products of photodegradation, the degradation pathway of TC-HCl was inferred. The toxicity analysis demonstrated that the overall toxicity of the identified intermediates was reduced in this system. This study provides a theoretical and practical basis for the removal of TC in aquatic environments.

Fig. 1. Structural characterizations of NiFe-LDHs. (a) SEM image, (b) XRD pattern, (c) FT-IR and (d) BET and aperture distribution.

Fig. 2. (a) XPS survey spectra of 3D NiFe-LDHs; (b) XPS O 1s spectrum; (c) XPS C 1s spectrum; (d) XPS Fe 2p spectrum; (e) XPS Ni 2p spectrum.

Fig. 3. (a) Effect of NiFe-LDHs dosage on TC degradation in the photo-Fenton process. [TC] = 30 mg L⁻¹, pH = 6.01 and H₂O₂ = 1 mmol L⁻¹, (b) effect of effect of TC concentrations on TC degradation in the photo-Fenton process. NiFe-LDHs dosage = 30 mg, pH = 6.01 and H₂O₂ = 1 mmol L⁻¹, (c) effect of pH on TC degradation in the photo-Fenton process. NiFe-LDHs dosage = 30 mg, [TC] = 30 mg L⁻¹ and H₂O₂ = 1 mmol L⁻¹, (d) effect of H₂O₂ concentrations on TC degradation in the photo-Fenton process. NiFe-LDHs dosage = 30 mg, [TC] = 30 mg L⁻¹ and pH = 8.01.

Fig. 4. (a) Degradation of TC in different systems, (b) reaction kinetics fitting curves.

Fig. 5. Effect of anions, cations and HA on TC degradation in the photo-Fenton process.

Fig. 6. Recycling experiments of NiFe-LDHs in the photo-Fenton process.

Fig. 7. The effects of different scavengers on TC degradation in the photo-Fenton process and EPR spectra of (b) DMPO-·OH, (c) DMPO-·O₂⁻.

Fig. 8. Proposed mechanisms of the degradation of TC in the photo-Fenton process.

Fig. 9. Potential degradation pathways of TC in the photo-Fenton process.

Fig. 10. The fathead minnow LC 50–96 h (a), daphnia magna LC50-48 h (b), developmental toxicity (c) and mutagenicity (d) of tetracycline and its possible degradation intermediates.