BiOI-MIL Binary Composite for Synergistic Azo Dye AR14 Discoloration

Acid red 14 (AR14) is a widely used azo dye that belongs to a major family of commercial dyes employed extensively in the textile industry. The present study aimed to investigate the photocatalytic discoloration of AR14 using a visible-light-responsive catalyst. The composite catalyst was synthesized by integrating thermally modified MIL-101 (M-MIL) integrated into bismuth oxide. Thermal modification of MIL-101 produced octahedral α-Fe2O3 particles with a size of 1–2 μm, which were incorporated into bismuth oxyiodide (BiOI) featuring a nanosheet structure. BiOI@M-MIL composite exhibited an enhanced photocatalytic activity. The bandgap energy, Eg, of BiOI was reduced from 1.95 eV to 1.73 eV in the composite. Photocatalytic reactions were performed under visible-light irradiation using a 5 W cold LED lamp. The AR14 discoloration tests demonstrated that BiOI@M-MIL was 1.81-fold more efficient compared to pristine BiOI. Key parameters affecting AR14 discoloration —such as catalyst dosage, pH, dye concentration, and contact time, were investigated. The composite achieved almost a complete dye removal efficiency of 94.26% under experimental conditions. Radical trapping tests highlighted the crucial role of superoxide radicals, O2.−, in the dye discoloration process. BiOI@M-MIL takes advantage of simultaneous adsorption and photocatalysis with the highest adsorption capacity of 45.32 mg g−1 and 32.2 mg g−1, based on Sips and Langmuir models, respectively. The catalyst also showed good reusability and ~14% loss in removal efficiency after five consecutive cycles. In conclusion, the BiOI@M-MIL composite demonstrates excellent photocatalytic performance, combining low energy consumption with material stability, making it a promising candidate for AR14 discoloration.