Complete mechanochemical defluorination of perfluorooctanoic acid using Al2O3 and Al powders through matching electron-mediated reduction with decarboxylation

This work reports a mechanochemical (MC) method for complete defluorination of perfluorooctanoic acid (PFOA) by using Al and Al 2 O 3 as milling agents. Both the Al/Al 2 O 3 molar ratio ( n Al / n Al 2 O 3 ) and the pre-thermal treatment of Al 2 O 3 strongly influenced the defluorination of PFOA. When commercial γ -Al 2 O 3 was pre-treated at 1200 °C, the use of Al and heat-treated γ -Al 2 O 3 with n Al / n Al 2 O 3 of 1: 1 led to PFOA defluorination of 100% after ball milling for 26 min at 350 rpm, being much higher than those (8.3%–58.1%) for using singlet milling agents or binary milling agents containing γ -Al 2 O 3 pre-heated at temperatures lower than 700 °C. It was clarified that the carboxylate-mediated adsorption of PFOA on Al 2 O 3 was essential for the MC decarboxylation as a degradation initiation step, and the in-situ generated electron on milled Al consequently caused the reductive dissociation of C–F bonds in the decarboxylation intermediate. A larger n Al / n Al 2 O 3 increased the in-situ electron generation rate ( r e ), and a higher heat-treatment temperature decreased OH − /H 2 O adsorbed on Al 2 O 3 to low the PFOA decarboxylation rate ( r dec ). The r e / r dec ratio determined defluorination pathways, and the percentage of the defluorination of PFOA in its total degradation including the generation of any degradation intermediates increased linearly with increasing r e / r dec .