Z-Scheme heterojunction Ag/NH2-MIL-125(Ti)/CdS with enhanced photocatalytic activity for ketoprofen degradation: Mechanism and intermediates

In this study, the Z-scheme heterojunction photocatalysts Ag/NH2-MIL-125(Ti)/CdS (AMC-5, AMC-10 and AMC-20) were first successfully obtained by stepwise deposition of Ag and CdS. The morphology, crystallinity and photochemical properties of the materials were investigated. The N2 adsorption–desorption isotherm showed that AMC-10 had a large specific surface area (363.41 m2/g). Cyclic tests proved that the heterojunction exhibited good stability. In comparison, AMC-10 exhibited a significantly higher photocatalytic activity. Results suggested that 94.2% ketoprofen (10 mg/L) can be photocatalytically degraded to intermediates and small molecules after 180 min of simulated sunlight illumination. AMC-10 had a large degradation rate constant 0.0168 min−1, which is 3.15 and 2.50 times higher than that of CdS and NH2-MIL-125(Ti), respectively. The total organic carbon removal percentage of ketoprofen was 57.5%. Free radical trapping experiments indicated that ·O2− and h+ were the main active species, which can also be verified by the detection of electron paramagnetic resonance. Possible degradation pathways and reaction mechanisms were proposed by band potential calculations and liquid chromatography-mass spectrometry. The results of Vibrio qinghaiensis sp.-Q67 culture demonstrated that the biotoxicity of ketoprofen increased first and then decreased with time. It can be concluded that the Z-scheme heterojunction of Ag/NH2-MIL-125(Ti)/CdS was beneficial for the separation of photo-generated electrons and holes and the noble metal Ag nanoparticles play the important role in the photocatalytic degradation with the surface plasmonic resonance effect and Schottky junction.