Calcium Metal Mechanochemically Defluorinates PFAS to Environmentally Benign CaF2

Abstract

Fluoropolymers and other per- and polyfluoroalkyl substances (PFAS) are among the most persistent anthropogenic materials. Although bulk fluoropolymers such as polytetrafluoroethylene (PTFE) are often regarded as chemically inert, weathering and abrasion generate long-lived fluorinated microplastics, while molecular PFAS accumulate in the environment as mobile contaminants. Existing destruction strategies can cleave C–F bonds, but typically require incineration or aggressive reagents and frequently generate water-soluble fluoride products that remain environmentally problematic. Here we show that calcium metal enables solvent-free mechanochemical defluorination of PTFE and perfluorooctanoic acid (PFOA) at ambient starting temperature without external heating, while irreversibly sequestering fluorine as calcium fluoride, a nontoxic, insoluble, and environmentally benign solid. Quantitative solid-state 19F NMR, with detection limits explicitly assessed, shows no resolvable PTFE after 30 min of milling with Ca and Mg (<1.0% residual PTFE); Sr gives near-complete conversion (<10% residual PTFE), whereas Ba gives only partial (∼64% residual PTFE) conversion after 30 min of milling. TEM and SEM-EDX reveal CaF2-rich nanocrystalline domains intimately mixed with an oxygen-containing disordered carbonaceous material. By coupling efficient defluorination with fluoride sequestration, this work establishes a mechanochemical strategy for mineralization of persistent fluorinated materials without generating soluble alkali-metal fluoride waste as a secondary environmental hazard, while defining the analytical and process-safety constraints relevant to further development.