Insights into the degradation mechanism of perfluorooctanoic acid under visible-light irradiation through fabricating flower-shaped Bi5O7I/ZnO n-n heterojunction microspheres

Perfluorooctanoic acid (PFOA), which is an emerging contaminant, has received extensive attention in recent times due to its high toxicity and environmental risk. In this study, BiOI supported on Zn-Al hydrotalcite (BOI0.04-BHZA) was calcined at 400 ℃ to obtain flower-shaped Bi5O7I/ZnO n-n heterojunction microspheres for the photocatalytic degradation of PFOA under visible light irradiation. The samples were characterized by X-ray diffraction, Fourier transform infrared, UV–vis diffuse reflectance spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. Compared with BOI (k1 = 0.0044 h−1), BHZA (k2 = 0.0054 h−1), and BOI0.04-BHZA (k3 = 0.0073 h−1), the degradation rate constant of Bi5O7I/ZnO n-n heterojunction microspheres (k4 = 0.013 h−1) increased by 2.9, 2.4, and 1.8 times, respectively. Approximately 91% of PFOA was degraded after 6 h of irradiation. The excellent photocatalytic performance was ascribed to the Bi5O7I/ZnO n-n heterojunction formed by calcination, which enlarged the photoresponse to the visible light region and increased the separation efficiency of electron-hole pairs. Moreover, the degradation pathway of PFOA was investigated using high-performance liquid chromatography-mass spectrometry and ion chromatography assisted by density functional theory calculations. The results demonstrated that the carboxylic groups of PFOA was vulnerable to attack by the photoproduction hole. The formed unstable perfluoroheptyl radicals transformed into shorter chain perfluorocarboxylic acids by the elimination of CF2 units. It was expected that this Bi5O7I/ZnO n-n heterojunction photocatalyst would be a promising candidate for PFOA treatment.