Mechanochemical degradation of perfluorooctane sulfonate using Fe and α-Al₂O₃: achieving complete defluorination without sulfonate group overreduction

Abstract

Perfluorooctane sulfonate (PFOS) resists complete defluorination under ambient conditions. We developed a mechanochemical (MC) method using α-Al₂O₃ and Fe that achieved complete defluorination of PFOS. Among the tested metal oxides and zero-valent metals, the combination of α-Al₂O₃ and Fe enabled complete defluorination of PFOS while preventing overreduction of the sulfonate group to generate toxic H₂S. Instead, the reaction produced SO₃^2–, F^–, and amorphous carbon as the main products. The defluorination rate in this MC system was more than three times faster than that reported for other MC systems. This method could not only be applied to PFOS alternatives but also enabled the defluorination of PFOS in soil. During ball milling, the in situ generated coordinative unsaturated Fe³⁺ and Al³⁺ on the surface of the milling reagents bound the sulfonate group of PFOS to promote desulfonation. Fe has a moderate reducing ability that facilitated reductive desulfonation and subsequent defluorination while simultaneously inhibiting the overreduction of the sulfonate group. α-Al₂O₃ suppressed Fe aggregation and critically inhibited the protonation of the desulfonation fragment (i.e., C₈F₁₇^–) owing to the small amount of surface OH^–/H₂O. This ability enhanced the electron transfer from Fe to C₈F₁₇^–, leading to complete defluorination without the generation of hydrofluorocarbons.

Graphical abstract

Keywords

Perfluorooctane sulfonate / Defluorination / Desulfonation / Mechanochemistry

Highlight

	● A combination of α -Al2O3 and Fe enabled PFOS full defluorination under milling.
	● In-situ formed Fe3+ and Al3+-VO bounded SO3– to enhance reductive desulfonation.
	● Fe as reducing reagents inhibited the sulfonate group overreduction.
	● α -Al2O3 inhibited Fe aggregation and C8F17– pronation, facilitating full defluorination.
	● This method could be applied to PFOS alternatives and PFOS in soil.

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