Degradation of indometacin by simulated sunlight activated CDs-loaded BiPO₄ photocatalyst: Roles of oxidative species

In this study, novel carbon dots/BiPO₄ (CDBP) photocatalytic complexes were successfully synthesized via a facile hydrothermal-calcination synthesis strategy. The physicochemical properties of the synthesized samples were studied by X-ray diffraction (XRD), UV–vis diffuse reflectance spectra (DRS), Fourier infrared spectrometer (FT-IR), Raman spectrometer, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), photoluminescence26spectra (PL), electrochemical workstation, etc. The activities of the CDBP were evaluated through the photocatalytic degradation of Indometacin(IDM) in an aqueous solution under simulated sunlight irradiation. With increasing concentrations of carbon dots (CDs), the photocatalytic activity of the CDBP initially increased, and then decreased. A CDs content of 3.0 wt% shows 12 times higher photocatalytic activity than that of pristine BiPO₄. Reactive oxidative species, particularly O₂[rad]⁻ and h⁺, were the two critical reactive oxidative species to mediator immediate the photocatalytic degradation of IDM. A notable sign of 5, 5-dimethyl-1-pyrrolidone-N-oxyl(DMPOX) was observed through electron spin resonance spectroscopy(EPR) with CDBP as the photocatalyst, which indicated higher oxidability than pristine BiPO₄ under simulated sunlight irradiation. This enhanced photocatalytic activity might due to high-efficiency charge separation, unique up-converted PL properties, as well as the bandgap narrowing of the CDs. Moreover, the byproducts of IDM were detected by HPLC–MS/MS and GC–MS, and the probable pathways were deduced. The acute toxicity at three trophic levels initially increased slowly and then decreased rapidly as the IDM dechlorination and total organic carbon(TOC) decreased during photocatalytic degradation.