Photocatalytic/Ultrasonic Catalytic Degradation of Antibiotic Contaminants and Hydrogen Production with Ni-Doped ZnFe₂O₄/g-C₃N₄ Nanocomposites

In recent years, the accumulation of doxycycline hydrochloride (DOXH) in the environment has emerged as a significant ecological concern, particularly for aquatic and terrestrial organisms. This has prompted researchers to explore advanced oxidation technologies such as photocatalysis and Fenton oxidation for effective water treatment. This study evaluates the efficacy of doped and heterojunction-constructed catalysts in the Fenton oxidation treatment of DOXH. Initially, spherical Ni2+-doped ZnFe2O4 (ZF2–xNxO) was synthesized using a straightforward hydrothermal method. The incorporation of Ni2+ alters the band gap of the spherical ZFO, enhancing light absorption and facilitating the separation and transfer of electrons and holes. Subsequently, ZF1.94N0.06O was combined with block g-C3N4 (CN) for the ultrasonically/photocatalytically driven Fenton degradation of DOXH. The synergistic effect of doping and the heterojunction structure, coupled with the addition of trace H2O2, resulted in a more stable supply of active radicals within the composite system. This optimization led to the 15%-ZF1.94N0.06O4/CN nanocomposite demonstrating highly efficient ultrasonic Fenton activity, achieving a removal efficiency of 53% in 35 min, which is 26% and 41% higher than those of ZF1.94N0.06O and CN, respectively. Furthermore, the 15%-ZF1.94N0.06O/CN composite exhibited the highest photo-Fenton degradation efficiency of 94% in 120 min surpassing ZF1.94N0.06O and CN by 30% and 42%, respectively. Free radical trapping experiments and EPR tests indicated the involvement of e–, h+, •O2–, and •OH in the degradation process, with high-performance liquid chromatography–mass spectrometry analysis revealing reduced ecotoxicity of the degradation intermediates. Additionally, the 15%-ZF1.94N0.06O/CN composite exhibited exceptional H2 production performance, with a rate of 88.25 μmol·g–1·h–1, which is 72 and 63 times higher than that of ZF1.94N0.06O and CN, respectively. This study provides a crucial reference for investigating the degradation of pollutants using CN-based nanomaterials and offers insights into achieving efficient and rapid degradation of antibiotic pollutants.