Foam suppression. Although EO provides a strong capability of mineralizing both organic and fluorinated carbons in AFFF, direct EO treatment encountered a serious foaming issue due to the vigorous gas evolution from water-splitting reactions (Fig. ). To quantitatively describe the foaming, we arbitrarily define the "foaming potential" as the ratio between the height of foam and the depth of liquid under air purging at 100 mL min -1 . Before treatment, the 100-fold diluted AFFF had a foaming potential of 6 (Fig. ). After UV/S treatment, the value decreased to 1.4 (Fig. ), allowing an easy operation of EO treatment. We only observed a thin foam layer with a height of less than 8% of the liquid in the first 4 h and no foaming thereafter. As expected from the ≥99.4%