Enhanced degradation of tetracycline hydrochloride by recyclable 3D N-TiO2/Fe3O4/rGO magnetic heterojunction driving peroxydisulfate and H2O2 activation under visible light irradiation: Performance and mechanism

In this work, a simple one-step hydrothermal method was employed to synthesize recyclable magnetic heterojunction N-TiO₂/Fe₃O₄/rGO (NTFG) nanocomposites for the activation of peroxydisulfate (PDS) and H₂O₂ in the removal of the emerging contaminant tetracycline hydrochloride (TCH). Under visible irradiation, the TCH removal rate in the NTFG/PDS/Vis system was nearly complete at 99.9% (the kinetic rates of 0.1531 min^-1) compared to 98.8% in the NTFG/H₂O₂/Vis system (the kinetic rates of 0.1133 min^-1). The excellent catalytic activities of both systems were attributed to the special 3D structure of magnetic heterojunction N-TiO₂/Fe₃O₄ (NTF) nanoparticles uniformly dispersed on wrinkled rGO, which promoted interfacial contact between different components. The incorporation of rGO and the formation of a direct Z-scheme heterojunction between the N-TiO₂ and Fe₃O₄ broadened the visible-light response range and effectively prevented the recombination of photogenerated electrons and holes. Encouragingly, rGO functions as an electron transfer medium to expedite electron transfer, while Fe(III)/Fe(II) cycling on the surface of NTFG further enhances the activation of PDS in the NTFG/PDS/Vis system. For the NTFG/H₂O₂/Vis system, rGO not only decomposed H₂O₂ through an electron transfer mechanism but also effectively promoted Fe(III)/Fe(II) cycling in the Fenton process. Both systems maintained high TCH removal rates after five cycles (the NTFG/PDS/Vis system for 98.2% while the NTFG/H₂O₂/Vis system for 91.9%), demonstrating NTFG reusability and magnetic recoverability. Additionally, the NTFG/PDS/Vis system demonstrates a broader pH application range and greater ability to interfere with higher concentrations of humic acid (HA). In both systems, ·OH, ${\text{O}}_2^{\mathbf{·}-}$${\text{O}}_2^{\mathbf{·}-}$, and h⁺ are key active species during TCH degradation, while ${\text{SO}}_4^{\bullet -}$${\text{SO}}_4^{\bullet -}$ being particularly significant in the NTFG/PDS/Vis system. Furthermore, possible degradation pathways for TCH in both systems have been proposed, and importantly, the biotoxicity of its intermediates has significantly decreased. The synthesis of NTFG provides new insight into the design of new magnetic heterojunctions with high performance and recyclability.