(538b) Visible-Light-Driven and Chemical-Free Photocatalytic Decomposition of per- and Polyfluoroalkyl Substances Using Commercially Available Cerium Dioxide

The threat posed by “forever chemicals”—per- and polyfluoroalkyl substances (PFASs)—has emerged as a global challenge for water treatment scientists. The exceptional strength of carbon–fluorine (C–F) bonds, among the strongest in chemistry, renders PFASs highly resistant to conventional wastewater treatment techniques. Therefore, the development of efficient and sustainable technologies for PFAS removal from water is urgently needed to ensure a safe and reliable living environment.

Photocatalysis has attracted increasing attention as a promising PFAS treatment method due to its high efficiency and reliance on clean energy. In this study, we developed a chemical-free, solar-light-driven photocatalytic system for the decomposition of perfluorocarboxylic acids (PFCAs) using a commercially available semiconductor—cerium dioxide (CeO₂). Three types of CeO₂ with different particle sizes (5 nm, 25 nm, and 300 nm) were evaluated for their performance in decomposing perfluorooctanoic acid (PFOA), a representative PFAS. Among them, 300 nm CeO₂ exhibited the highest photocatalytic efficiency, achieving 95% degradation of an initial 100 mg/L PFOA in 4 hours, with 57% defluorination after 12 hours. Material characterization using low- and high-resolution transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) revealed that the inferior performance of the 5 nm and 25 nm CeO₂ was attributed to lower purity, limited colloidal stability, and a deficiency in oxygen vacancies.

Reactive species involved in PFOA degradation were investigated via chemical quenching experiments using ethylenediaminetetraacetic acid (EDTA) for photogenerated holes (h⁺), methanol and tert-butanol for hydroxyl radicals (^•OH), p-benzoquinone for superoxide species (O₂^•―/HO₂^•), and furfuryl alcohol (FFA) for singlet oxygen (¹O₂). Only EDTA and p-benzoquinone significantly inhibited PFOA degradation, indicating a co-contribution of h⁺ and O₂^•―/HO₂^•. The formation of O₂^•―/HO₂^• was further confirmed by electron paramagnetic resonance (EPR) spin-trapping. Notably, PFOA degradation was most effective at pH 3.0, with higher pH leading to a sharp decrease in degradation efficiency—suggesting that HO₂^• plays an actual role in PFOA degradation instead of O₂^•―. The detection of short-chain PFCAs as degradation products supports a mechanism involving carbon-chain cleavage. Additionally, photocatalytic tests on shorter-chain PFCAs showed a gradual decrease in degradation efficiency with decreasing chain length. Optimal experimental conditions, including CeO₂ and PFOA concentrations, pH, and light intensity, were systematically determined. The system also demonstrated effective PFOA degradation in groundwater, underscoring its potential for practical applications. Finally, the 300 nm CeO₂ showed good reusability over four cycles.

This study not only introduces an efficient and environmentally friendly system for PFCA decomposition but also advances the understanding of the photocatalytic degradation mechanisms of PFASs, offering valuable insights for controlling PFAS contamination in aquatic environments.