Degradation of hexafluoropropylene oxide oligomer acids as PFOA alternatives in simulated nanofiltration concentrate: Effect of molecular structure

Hexafluoropropylene oxide (HFPO) oligomer acids are an important class of perfluorooctanic acid (PFOA) alternatives used in fluoropolymer production. Their chemical structure presents a periodical -CF3 branch and oxygen bridge. In particular, hexafluoropropylene oxide dimer, trimer and tetramer acid (HFPO-DA, HFPO-TA and HFPO-TeA) have been globally detected in waters, and are thought to exert an even greater potential ecological risk than PFOA. Moreover, different to PFOA, HFPO-DA has been demonstrated to be strongly oxidation-resistant, and thus its removal is more challenging. In this study, the degradability of HFPO-TA and HFPO-TeA, as well as a perfluoropolyether carboxylic acid without a -CF3 branch (TeA-w), was investigated. Unlike HFPO-DA, HFPO-TA and HFPO-TeA could be oxidized under UV/persulfate, which was attributed to the changed molecular configuration and weakened protective effect of the -CF3 branch on a-carbon atoms against SO4- attack. As a result, the oxidability of these HFPO acids increases with the number of oxygen bridges (i.e. HFPO-TeA > HFPO-TA > HFPO-DA). However, oxidation is found to be a risky approach for HFPO-TA and HFPO-TeA removal owing to significant generation and accumulation of HFPO-DA. On the contrary, HFPO-TA and HFPO-TeA were effectively degraded without accumulation of (by)products in a strong reductive system (UV/sulfite). HFPO oligomer acid degradation in both oxidative and reductive systems always follows a similar pathway; namely, HFPO-TeA -> HFPO-TA -> HFPO-DA. Nanofiltration is potentially one of the best available technologies for HFPO oligomer removal from drinking water, but safe concentrate disposal should be a consideration. Given this, we performed degradation of HFPO-DA/TA/TeA mixture in simulated nanofiltration concentrate. More than 99.7% of the three substances were removed within 60 min, and a complete fluoride recovery was obtained 120 min later. This corroborated that reduction by UV/sulfite is a potential complementary technology for nanofiltration in drinking water deep treatment aimed at removing HFPO oligomer acid contamination.