Enhanced photocatalytic degradation of perfluorooctanoic acid using carbon-modified bismuth phosphate composite: Effectiveness, material synergy and roles of carbon

Abstract

Per- and polyfluoroalkyl substances (PFAS) such as perfluorooctanoic acid (PFOA) are recalcitrant to conventional wastewater treatment processes. In search for a more effective technology, we prepared a carbon sphere (CS) modified bismuth phosphate composite photocatalyst (BiOHP/CS) through a hydrothermal process. BiOHP/CS was able to adsorb > 99% of PFOA in 2 h (dosage = 1.0 g/L, initial PFOA = 200 µg/L, pH = 7.0), and subsequently, in situ decompose adsorbed PFOA nearly completely in 4 h of UV irradiation. The PFOA degradation rate was ~18 times greater than that by neat BiOHP. Advanced spectroscopic analyses and density functional theory calculations revealed that the carbon modification not only enhances the PFOA adsorption capacity, but also facilitates a side-on adsorption configuration of PFOA on the photoactive sites. While only the head carboxylate group was involved in PFOA adsorption on neat BiOHP, both the CF2/CF3 entities and the head carboxylate group participated in adsorption of PFOA on BiOHP/CS, which facilitates the subsequent photocatalytic cleavage of the CF bonds. Photoluminescence emission analysis and density of states calculations confirmed the CS-facilitated side-on adsorption mode and CS-enhanced separation of electrons and holes. In addition, the CS modification facilitates activation of PFOA by changing the electron distribution of the C-F bond, and enhances the stability and reusability of BiOHP by preventing photo-corrosion of the composite material. Overall, the findings provide important insights into the roles of carbonaceous modification and adsorption configurations in enhanced photocatalytic destruction of PFOA, which may guide future design and fabrication of environment-friendly adsorptive photocatalysts.