S-Scheme charge transfer in topotactically transformed LTO/g-C3N4 for photo-Fenton mineralization in complex water matrices.

Abstract

The development of visible-light-driven photo-Fenton catalysts for the mineralization of recalcitrant organic pollutants remains a challenge in wastewater treatment. In this study, an S-scheme-type ZnFe₂O₄/ZnO/CuO@g-C₃N₄ (GLTO) heterojunction was prepared via topotactic transformation of a Zn-Cu-Fe layered triple hydroxide (LTH) precursor, followed by ultrasonic assembly with exfoliated g-C₃N₄. Structural and optical characterizations revealed intimate interfacial contact, a mesoporous architecture (mean pore diameter, 31.0 nm; specific surface area, 49.6 m² g^-1), and an optical band gap of 2.4 eV. The GLTO composite demonstrated visible-light photo-Fenton activity for the degradation of rhodamine B dye (RhB), achieving 93.5% dye removal and 65% total organic carbon (TOC) reduction within 28 min. Liquid chromatography-mass spectrometry revealed progressive conversion to low-molecular-weight products, with the detected intermediates tentatively assigned. The apparent rate constant (0.0877 min^-1) was ∼30-fold higher than that of bare LTO. The catalyst maintained >90% degradation efficiency at 10 mM NaCl and 72% degradation at 10 mg L^-1 humic acid under the tested model water conditions, and retained >95% of its initial activity after five cycles. Electrochemical impedance spectroscopy and electron spin resonance spectroscopy supported enhanced charge separation and the involvement of reactive oxygen species, consistent with the proposed S-scheme mechanism. In this model, electrons are retained on g-C₃N₄, while holes are retained on the LTO component. ECOSAR modeling predicted a progressive reduction in the potential ecotoxicity of the detected intermediates. These findings demonstrate the potential of LTH-derived multimetal oxides coupled with g-C₃N₄ for the photo-Fenton treatment of organic pollutants under the tested model water conditions.