NH₂-MIL-125(Ti)/TiO₂ heterojunction with non-disturbed dual reactive centers for synchronous photocatalytic removal of Cr(VI) and organic dyes

Chromium (VI) (Cr(VI)) generally coexists with organic dyes in industrial effluents, posing a formidable challenge in water purification. Herein, NH₂-MIL-125(Ti)/TiO₂ Z-scheme heterojunction with intimate interfacial contact was synthesized for synchronous removal of pollutant in coexisting Cr(VI)/dyes system under simulated solar irradiation. Structural and optical investigations indicated that a well-defined interface was formed by establishing a Ti-N-C bond, facilitating the spatial separation of the photoexcited carriers of the Z-scheme heterojunction. The optimum NH₂-MIL-125(Ti)/TiO₂ nanocomposites show superior performance in photocatalytic removal of the pollutants in the Cr(VI) (5 mg/L, 97.2%)/MB (40 mg/L, 100%) coexistence systems within 120 min, which is comparable to that in the single system. The electron spin resonance (ESR) tests, radicals scavenging experiments, and density functional theory (DFT) cannulations unveiled that TiO₂ could serve as oxidation centers to generate hydroxyl radicals (•OH) for MB oxidation, while the NH₂-MIL-125(Ti) with exposed Ti nodes could act as reduction centers to effectively adsorb Cr₂O₇^2- and inject photo-generated electrons (e^-) to accomplish the in-site photoreduction of Cr(VI) into Cr(III) under illumination. Particularly, owing to the spatial separation and non-disturbed dual reactive centers, the reduction and oxidation processes could be well accommodated, which could allow MB and Cr(VI) to be removed synchronously. This work demonstrated the great potential of applying duel reactive centers to eliminate multipollutant simultaneously in the actual scenarios for wastewater treatment.