Ionic liquid assisted Fe<sub>3</sub>/ZnIn<sub>2</sub>S<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub> S-scheme heter

<p indent="0mm">Targeting the dominant role of ·O₂⁻ in the degradation process of tetracycline (TC), a visible light-driven continuous Extraction Coupled Photocatalytic Degradation for Tetracycline (EPDT) system was constructed. In this system, Fe₃/ZnIn₂S₄/g-C₃N₄ (FZCN) composite photocatalysts were prepared using a ZnIn₂S₄/g-C₃N₄ (ZCN) S-scheme heterojunction as a carrier, with trinuclear iron cluster (Fe₃) serving as an innovative co-catalyst. Although the photocatalytic degradation of TC has great application potential, the limited dispersion of the photocatalyst greatly inhibits its contact with the organic substrate and greatly reduces the reaction rate. Therefore, the system performance was optimized by utilizing [BMIM]PF₆ ionic liquid (IL) and acetone as extractant and dispersant, respectively. Firstly, the prepared layered S-Scheme heterojunction carrier ZCN enables the stable and uniform distribution of Fe₃ co-catalysts, thereby passivating the material surface, endowing FZCN with a larger specific surface area and abundant micropores. This facilitates the adsorption more TC molecules and intermediate products. Subsequently, the construction of the S-scheme heterostructure was confirmed through various characterizations and density functional theory (DFT) calculations, which significantly separate and retain strong photogenerated e⁻ and h⁺, and facilitate the rapid generation of ·O₂⁻ by further transferring e⁻ at the metal sites of Fe₃. Furthermore, the capture experiment and ESR analysis determined that ·O₂⁻ and h⁺ are the main active species controlling the primary reaction efficiency of TC degradation, while ·O₂⁻ is also converted into a minor amount of ·OH, which participates in secondary reactions. When the Fe₃ loading amount is <sc>30 mg,</sc> the optimal degradation rate constants of FZCN-3 are 5.2 times, 3.9 times, and 1.6 times those of g-C₃N₄, ZnIn₂S₄, and ZCN, respectively, and it maintains good structure and performance after four cycles. In addition, [BMIM]PF₆ imidazole-based extractant not only extracts TC through π-π stacking interactions, but also serves as the reaction medium and the dispersant stabilizer for catalyst. Therefore, establishing an “Ionic Liquid Bridge” significantly enhances the mass transfer efficiency between the aqueous phase and photocatalyst. Additionally, due to the photocatalytic degradation, the concentration of TC in the IL phase decreases, thereby disrupting the extraction equilibrium and inducing a continuous migration of TC into the IL phase, forming a continuous system. The EPDT system achieves deep removal of TC through repeated cycles of TC extraction and photocatalytic degradation, resulting in the extraction coupled photocatalytic degradation rate of EPDT reaching 83.8% within <sc>60 minutes,</sc> which is 2.1 times higher than that without extractant, and increasing to 90.5% at <sc>120 minutes.</sc> In addition, through single factor experiments, it was found that the optimal amount of IL added in the EPDT experiment is <sc>7 mL,</sc> the optimal amount of FZCN-3 is <sc>5 mg,</sc> and the optimal pH for the reaction is 3.5. However, the competition between SO₄²⁻ and TC for reactive sites and the elimination effect of HCO₃⁻ on ·OH and h⁺ are not conducive to the EPDT reaction. Lastly, an examination of the constituents involved in the degradation of TC was carried out, revealing that TC undergoes decomposition into various small molecular intermediates under the combined influence of h⁺, ·O₂⁻, and ·OH radicals. Subsequently, these intermediates are further mineralized into inorganic compounds like CO₂ and H₂O. This efficient EPDT system can be applied to various organic pollutants and actual water matrices, and is expected to provide a new perspective for the design of methods for organic pollutant degradation.