Design and mechanistic study of mixed-ligands Fe-CP/TiO2 composites for organic pollutant removal via enhanced visible-light photocatalysis

Coordination polymers (CPs) are promising materials for wastewater treatment owing to their multifunctionality. In this study, a mixed-ligands Fe-based CPs, designated as REL-4, was successfully synthesized via a solvothermal method using 1,4-benzenedicarboxylic acid (H2BDC) and 2,2′-bipyridine (2,2′-bpy). To enhance its photocatalytic activity, REL-4 was integrated with TiO2 to form a REL-4/TiO2 heterojunction composite. Structural and physicochemical characterisations confirmed the successful dispersion of TiO2 on the hexagonal spindle-like REL-4 framework without disrupting its crystallinity (93.68% for REL-4 and 87.81% for the composite). The composite (1:1 ratio) exhibited a specific surface area of 29.7 m2/g and a narrowed optical bandgap of 2.9 eV, enabling significant visible-light absorption compared to pristine TiO2. The optimized 1:1 REL-4/TiO2 composite achieved 99% Rhodamine B (RhB) degradation within 120 min under visible-light irradiation. Radical scavenging experiments identified ∙O2- as the dominant reactive species, with additional contributions from h+ and •OH. DFT calculations predicted the susceptible reactive sites of RhB, while LC-MS analysis revealed intermediates associated with N-deethylation, chromophore cleavage, and ring-opening reactions. Based on these findings, a plausible degradation pathway prior to complete mineralization was proposed, demonstrating the potential of REL-4/TiO2 as an efficient visible-light-driven photocatalyst for dye-contaminated wastewater treatment.

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