Perfluorooctanoic acid (C₇F₁₅COOH, PFOA) has attracted attention due to its chemical durability, wide distribution, biotoxicity, and bio-accumulative properties. The UV radiation under 200 nm is known as vacuum UV (VUV), an alternative technology to treat micropollutants. The process combined with VUV along with sulfite (SO₃^-) and iodide (I^-) is used to decompose recalcitrant organics by producing reactive reduction species including hydrated electron (e_aq^-), H, and SO₃^-. In this study, we investigated the degradation of PFOA in water using the VUV/sulfite/iodide process under nitrogen purging conditions. The VUV/sulfite/iodide process led to a complete PFOA removal within 15 min at pH 10, and 40% of the defluorination was achived within 360 min at pH 10 (Fig. 1 (A)). There is a synergistic effect of sulfite and iodide in VUV-based advanced reduction because sulfite can scavenge reactive iodine species (I₂, I₂^-, and I₃^-) and regenerate iodide (I^-). The presence of iodide ion accelerated the removal of PFOA, but did not enhance defluorination significantly. When initial pH increased from 4 to 10, PFOA decomposition efficiency in the VUV/sulfite/iodide process increased from 29.4% to 100%, implying that e_aq^- is effectively produced under alkaline conditions (Fig. 1 (B)). Using UPLC-MS/MS, the formation of perfluorocarboxylic acids (i.e., PFHxA, PFBA, PFPrA, and TFA) was observed as the transformation products (TPs) (Fig. 2). Based on the identified TPs, the decomposition pathways of PFOA during the VUV photolysis and VUV/sulfite/iodide processes under anoxic conditions were proposed.

Fig. 1. (A) Degradation and defluorination of PFOA during VUV-based treatment, (B) the effect of pH on the degradation of PFOA during VUV/sulfite/iodide treatment.

The time-profiles of PFOA and intermediates during PFOA degradation in the (A) VUV photolysis, (B) VUV/sulfite/iodide treatment.

Heungjoo Park

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