As a new class of persistent organic pollutants, perfluoroalkyl substances (PFASs) have been of great environmental concern. (1,2) Since manufactured in the 1950s, PFASs have been widely used in many commercial and industrial products. (3?5) However, their extensive usage also caused serious environmental contamination. (6,7) Especially PFOA and PFOS are extremely persistent, bioaccumulative, and toxic, (8?11) and have thus been strictly restricted for use by the Stockholm Convention. (12) To meet these restrictions, some perfluoropolyether carboxylic acids (PFECAs), fluorotelomer carboxylic acids (FTCAs), short-chain perfluorosulfonic acid, etc., have been used to substitute PFOA and PFOS for years, some of which are even more toxic. (13) These substitutes are raising new concerns. (14,15) Recent investigations show that PFOA and PFOS are still detectable in the natural environment and food chains, while short-chain PFASs are more and more frequently being detected at even higher contamination levels in the effluents of wastewater treatment plants (WWTPs). (16,17) The persistence of PFASs is dependent on the molecular structure related to the head functional group, fluoroalkyl chain length, and branch structure. (18?20) In general, short-chain PFASs (i.e., perfluorobutanesulfonic acid, PFBS) and fluorotelomers are usually more restricted to be degraded. Therefore, there is an urgent need to develop a highly efficient degradation technology capable of treating a wide range of PFASs.

The C–F bond has an extremely high bond energy of 100–120 kcal/mol. (18,19) Thereby, traditional wastewater treatment technologies, such as advanced oxidation processes (AOPs) and microbial degradation, are inefficient in degrading PFAS. (21?24) In recent years, advanced reduction technologies that rely on hydrated electrons (eaq–) have shown high efficiency in degrading and defluorinating PFAS. (25) This is due to the strong reductive nature of ep_aq^– (?2.87 V, vs SHE), which can directly cleave the C–F bond. (26) Several compounds, such as sulfites, iodide ions, nitrilotriacetic acid (NTA), indole derivatives, etc., have been applied asep_aq^– precursors under ultraviolet light (UV, 254 nm) conditions. (19,27?29) To further improve the ep_aq^– utilization efficiency, we synthesized a new indole compound (di-indole hexadecyl ammonium, DIHA), which is decorated with a hexadecane chain and a tertiary amine center (Scheme 1). (30) It can form stable nanospheres (100–200 nm) in water via supramolecular assembly. These DIHA nanospheres can provide long-range electrostatic, hydrophobic, and van der Waals (vdW) interactions to attract and adsorb PFOA and PFOS. (30) Under UV (254 nm) irradiation, eaq– can be generated from DIHA molecules, which can then degrade and defluorinate PFOA and PFOS highly efficiently. (30) Compared to sulfite or other indole compound-sourced ep_aq^– systems, the newly developed DIHA system exhibits a 1–2 order of magnitude higher eaq– utilization efficiency. (30) However, the underlying mechanism to ensure its great performance has not yet been fully explained, and its applicability to degrade different structured PFASs needs further examination...

Scheme 1. Synthetic Route of Di-indole Hexadecyl Ammonium (DIHA,n = 15, C34H49N3)

Figure 5. One-electron reduction potential (E°1, vs SHE) for all of the C–F bonds in PFCAs (n = 1–7), DFA, PFOS, 6:3-FTCA, and HFPO–DA.

Scheme 2. Potential Degradation and Defluorination Pathways of PFCAs Induced by Surface Electrons and •OH on the DIHA Nanosphere Surface

^aDetected products are marked in the dotted box.

Investigating the DIHA nanoemulsion system of this study provides a new understanding of the degradation and defluorination mechanisms for PFASs in heterogeneous systems, despite many previous studies gaining substantial insights into the degradation/defluorination mechanisms for different structured PFASs in homogeneous eaq– systems. The self-assembled DIHA nanospheres not only serve to concentrate the contaminants but also create a highly reactive surface zone with concentrated reactive species (i.e., surface electrons and •OH). The DIHA nanoemulsion system exhibits two dominant mechanisms: (1) nonselective hydrodefluorination of the ?CF2- chain and (2) synergistic reduction and oxidation toward several categories of PFASs in a single system. Based on the new mechanisms, the UV/DIHA approach exhibited high effectiveness and stability against various feed ratios, pH levels (4–10), and the presence of natural organic matter (Figure S14). (30) Herein, it also shows great advantages for treating different structured PFASs, regardless of the headgroup, chain length, and branched structure, including PFCAs, PFSAs, FTCAs, PFECAs, PFOSA, and even perfluoroalkanes. The new mechanisms inspire the development of new organomaterials for treating mixed PFASs in water.