Unveiling the accelerated spontaneous photodecomposition of perfluorooctanoic acid in frozen solution.

Per- and polyfluoroalkyl substances (PFAS) are highly persistent in the environment, posing a significant challenge to their effective decomposition. Current degradation techniques relying on hydrated electrons (eaq-) typically necessitate anaerobic and alkaline conditions, which are too stringent for practical applications. Herein, we demonstrate for the first time that perfluorooctanoic acid (PFOA) can be spontaneously degraded during freezing under an oxygen-containing atmosphere. The degradation efficiency reaches 85.6% within 12 h at -20°C, without any foreign additives. The corresponding defluorination efficiency is approximately 9.9 times higher than that in unfrozen water. Mechanistic analyses reveal that the photodegradation of PFOA in frozen solutions is primarily driven by the generation of eaq-. Confocal Raman spectroscopy further confirmed that PFOA is concentrated and confined within the micron-scale unfrozen liquid layer (ULL, 2.57-8.33 μm) during freezing, triggering the freeze-concentration and freeze-confinement effects. These combined effects further lead to enrichment of PFOA and eaq- in the ULL, thereby enhancing the reactivity between them and thus accelerating the photodegradation process of PFOA. The general applicability of this mechanism was further verified for other PFAS. These findings demonstrate a novel strategy for the effective degradation of PFAS, unveiling an undiscovered transformation path of PFAS in natural environments.