Photo-Fenton-Based Degradation of Methylene Blue Dye Using Hydroxyapatite Nanoparticles Doped with Fe₃O₄/γ-Fe₂O₃

In this study, we evaluated the catalytic properties of hydroxyapatite (HA) nanoparticles (NPs) doped with Fe₃O₄/γ-Fe₂O₃ for the degradation of methylene blue (MB) dye using a photo-Fenton process at pH 2.5, 7.0, and 9.0. The HA NPs were characterized using X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectrometry, Raman spectroscopy, thermogravimetric analysis, fluorescence spectrometry, and point of zero charge experiments. Upon using unmodified HA NPs, 1.33 mL of H₂O₂ 30% (v/v), and ultraviolet radiation (125 W Hg lamp), the discoloration values achieved at pH 2.5, 7.0, and 9.0 were 88.4, 44.8, and 58.1%, respectively. For the HA NPs modified with Fe₃O₄/γ-Fe₂O₃, the composition Ca_2.5Fe^II_2.5Fe^III₅(PO₄)₆(OH)₂ yielded the best results during the photo-Fenton-based process, and discolorations of 100, 100, and 95.2% were achieved at pH 2.5, 7.0, and 9.0, respectively. To confirm the effectiveness of the Ca_2.5Fe^II_2.5Fe^III₅(PO₄)₆(OH)₂ NPs, the total organic carbon (TOC) and toxicity effects on Lactuca sativa L. were evaluated, and reproducibility tests were performed. The TOC removal of the Ca_2.5Fe^II_2.5Fe^III₅(PO₄)₆(OH)₂ NPs was ca. 90% and the toxicity of the MB dye was eliminated after 120 min of reaction; moreover, the NPs retained their physical stability and activity and were reused for 11 consecutive degradation experiments.

Keywords:
magnetite; maghemite; heterogeneous Fenton process; structural modification