Degradation of Perfluorooctanoic Acid (PFOA) on Aluminum Oxide Surfaces: New Mechanisms from Ab Initio Molecular Dynamics Simulations

Perfluorooctanoic acid (PFOA) is a part of a large group of anthropogenic, persistent, and bio-accumulative contaminants known as per- and polyfluoroalkyl substances (PFAS) that can be harmful to human health. In this work, we present the first ab initio molecular dynamics (AIMD) study for systematically investigating a wide range of temperature-dependent degradation dynamics of PFOA on (100) and (110) surfaces of gamma-Al2O3. Our results show that PFOA degradation does not occur on the pristine (100) surface, even when carried out at high temperatures. However, introducing an oxygen vacancy on the (100) surface facilitates an ultra-fast (< 100 fs) defluorination of C-F bonds in PFOA. We also examined degradation dynamics on the (110) surface and found that PFOA interacts strongly with Al(III) centers on the surface of gamma-Al2O3, resulting in a stepwise breaking of C-F, C-C, and C-COO bonds. Most importantly, at the end of the degradation process, strong Al-F bonds are formed on the mineralized gamma-Al2O3 surface, which prevents further dissociation of fluorine into the surrounding environment. Taken together, our AIMD simulations provide critical reaction mechanisms at a quantum level of detail and highlight the importance of temperature effects, defects, and surface facets for PFOA degradation on reactive surfaces, which have not been systematically explored or analyzed.