Document GKQgaQd3k3NQ6Mkg20DGxmE6m

pubs.acs.org/est Article Mechanistic Investigations of Thermal Decomposition of Perfluoroalkyl Ether Carboxylic Acids and Short-Chain Perfluoroalkyl Carboxylic Acids Ali Alinezhad, Heng Shao, Katerina Litvanova, Runze Sun, Alena Kubatova, Wen Zhang, Yang Li,* and Feng Xiao* Cite This: Environ. Sci. Technol. 2023, 57, 8796-8807 Read Online Downloaded via UNIV OF EDINBURGH on July 15, 2023 at 02:53:46 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ACCESS Metrics & More Article Recommendations *si Supporting Information ABSTRACT: In this study, we investigated the thermal decomposition mechanisms of perfluoroalkyl ether carboxylic acids (PFECAs) and short-chain perfluoroalkyl carboxylic acids (PFCAs) that have been manufactured as replacements for phased-out per- and polyfluoroalkyl substances (PFAS). C-C, C-F, C-O, O-H, and CC bond dissociation energies were calculated at the M06-2X/Def2-TZVP level of theory. The -C and carboxyl-C bond dissociation energy of PFECAs declines with increasing chain length and the attachment of an electron-withdrawing trifluoromethyl (-CF3) group to the -C. Experimental and computational results show that the thermal transformation of hexafluoropropylene oxide dimer acid to trifluoroacetic acid (TFA) occurs due to the preferential cleavage of the C-O ether bond close to the carboxyl group. This pathway produces precursors of perfluoropropionic acid (PFPeA) and TFA and is supplemented by a minor pathway (CF3CF2CF2OCFCF3COOH CF3CF2CF2 + OCFCF3COOH) through which perfluorobutanoic acid (PFBA) is formed. The weakest C-C bond in PFPeA and PFBA is the one connecting the -C and the -C. The results support (1) the C-C scission in the perfluorinated backbone as an effective PFCA thermal decomposition mechanism and (2) the thermal recombination of radicals through which intermediates are formed. Additionally, we detected a few novel thermal decomposition products of studied PFAS. KEYWORDS: PFAS, thermal transformation, high-resolution mass spectrometry, GC-MS, density functional theory, spent media INTRODUCTION Major manufacturers of per- and polyfluoroalkyl substances (PFAS) have phased out the production of perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), and related substances1,2 due to public concern about their persistence,3-5 toxicity,6,7 and ubiquitous presence in the environment.8-10 Perfluoroalkyl ether carboxylic acids (PFECAs) and short-chain perfluoroalkyl carboxylic acids (PFCAs) have been introduced into the market to replace PFOA and PFOS.11-14 However, these replacement substances are also recalcitrant to degradation15,16 and environmentally mobile.17,18 PFECAs and shortchain PFCAs, including hexafluoropropylene oxide dimer acid (HFPO-DA), perfluorobutanoic acid (PFBA, C4), and perfluoropentanoic acid (PFPeA, C5), have been frequently detected in surface and groundwater worldwide.11,16,19-22 Hexafluoropropylene oxide trimer acid (HFPO-TA), a novel alternative to PFOA, has recently been observed in surface water.17,23 Acute and long-term toxicological studies have revealed the hazardous properties of HFPO-DA, HFPO-TA, and short-chain PFCAs.24-28 The U.S. Environmental Protection Agency (EPA) has recently issued the final health advisory for HFPO-DA and its ammonium salt ("GenX" chemicals) in drinking water at 10 ng/L.29 In the European Union, the recently updated Drinking Water Directive recommends limits of 100 ng/L for 20 PFAS, including PFBA and PFPeA.30 An impact of these actions will be to make the removal of PFAS from drinking water31-34 a global priority. Currently, sorption methods using granular activated carbon (GAC) or anion exchange (AIX) resins are frequently employed to remove PFAS from water in large-scale operations.34-37 More recently, single-use AIX resins have been investigated for removing PFECAs and short-chain PFAS from water in laboratory- and pilot-scale experiments.38,39 The predominant residual from GAC and AIX resin systems is the spent adsorbents containing PFAS. It has been observed that regenerating PFAS-laden spent adsorbents can be challenging using organic solvents or brine.40-42 For example, methanol Received: January 11, 2023 Revised: April 27, 2023 Accepted: May 3, 2023 Published: May 17, 2023 2023 The Authors. Published by American Chemical Society 8796 https://doi.org/10.1021/acs.est.3c00294 Environ. Sci. Technol. 2023, 57, 8796-8807 Environmental Science & Technology pubs.acs.org/est Article alone could only extract less than 50% of PFOA and PFOS from GAC.42 Liu and Sun demonstrated that brine was ineffective in regenerating PFAS-laden single-use AIX resins.40 Thermal-based approaches have emerged as an attractive means42-48 to decontaminate PFAS-laden GAC,42,46,49-51 resins,52 and other solid materials (e.g., soil,43,51 biosolids,53 and biomass54). Recent studies have demonstrated PFAS decomposition even at relatively low temperatures of 200- 500 C.42-46 These temperatures are achievable using the current in situ and ex situ heating technologies.50,55,56 Over the past five years, research on PFAS thermal treatment has quickly evolved42-44,46-48,52,57-59 regarding the thermal stability of PFOA and PFOS,42,43 the threshold temperatures that thermal degradation may occur,42,43,46 the yield of inorganic F from PFOA and PFOS during thermal degradation,42,43,46 and the thermal degradation of PFAS (also known as precursor compounds).44 As part of this evolution, there has been a strong focus on polar44,46 and nonpolar45,60 thermal degradation products of PFAS that are produced when subjected to thermal treatments, such as thermal regeneration of spent GAC at temperatures of <400 C.43,44,46 Moreover, increasing attention has been paid to the effects of porous adsorbents, such as GAC,46 resin,46 and biochar,46 and approaches to accelerate PFAS thermal degradation at low temperatures.46,60-65 HFPO-DA is resistant to degradation by ozone16 and hydroxyl radicals.66 The cleavage of C-O ether bonds in HFPO-DA is rarely observed without potent radicals, acids, and bases.67,68 Yet, the thermal decomposition of HFPO-DA has been observed at temperatures as low as 150 C.42,43,46 Compared to legacy PFAS (e.g., PFOA), the conceptual and quantitative thermal degradation models for PFECAs, including HFPO-DA, are far from being clear. Such compounds may undergo the same thermal degradation pathways as PFOA, but because they bear an ether group, they can participate in pathways that PFOA cannot. Using density functional theory (DFT), Adi and Altarawneh investigated various possible HFPO-DA thermal decomposition pathways.69 While the present study was prepared, Blotevogel et al. reported DFT calculation results on pathways, mechanisms, and kinetics of the thermal decomposition of PFOA and HFPO-DA.70 The authors suggest that the initial defluorination of HFPO-DA proceeds through HF elimination at the -C.70 We are only aware of these two studies in the scholarly literature that have investigated the thermal degradation mechanisms of HFPO-DA. It is worth mentioning that PFBA, perfluoropropionic acid (PFPrA), and trifluoroacetic acid (TFA) were not considered thermal decomposition products of HFPO-DA in these two previous studies.69,70 Likewise, experimental data of short-chain PFCAs under thermal conditions are practically nonexistent in the literature. For mechanistic considerations, there is a need for a better understanding of thermal decomposition mechanisms and pathways of PFECAs and short-chain PFCAs, including the structures and reactivities of transient intermediates. To this end, the authors employ a combined experimental and computational approach to understand the thermal degradation mechanisms of HFPO-DA and short-chain PFCAs. Thermal degradation experiments of relevant PFAS were conducted to guide the model development in computational studies. The computational data, in turn, explain the experimental results and illustrate the underlying thermal decomposition mechanisms. An in-depth analysis of the thermal decomposition pathways of HFPO-DA, HFPO-TA, PFBA, and PFPeA was performed. Qualitative and quantitative information on transient inter- mediates was obtained. High-throughput DFT calculations were carried out to compute C-C, C-F, C-O, O-H, and CC bond dissociation energies (BDEs) of studied PFAS and to further elucidate their thermal degradation pathways. Through this combined experimental and computational approach, we attempt to answer the following questions: (1) Does the side -CF3 on the -C prevent HFPO-DA and other PFECAs (e.g., HFPO-TA) from being degraded during thermal treatment? In UV/persulfate oxidation processes, the -CF3 group on the -C has been shown to inhibit the degradation of GenX by sulfate and hydroxyl radicals.68 (2) What transient organic intermediates are produced when HFPO-DA, HFPO-TA, and short-chain PFCAs are heated and through which mechanisms? (3) Is this process radical mediated? (4) Will HFPO-DA and HFPO-TA transform to perfluorinated compounds during thermal treatment? (5) Given that they are unsymmetrical ethers, which C- O ether bond in HFPO-DA and HFPO-TA is most likely to break upon heating? Answers to these questions have theoretical and practical implications for modeling PFECA degradation pathways and predicting transient and final degradation products. MATERIALS AND METHODS Thermal Treatment of PFAS-Laden Spent Media. In preliminary adsorption and thermal experiments, we included the following perfluorinated compounds in determining the effect of treatment temperature (Table S1): HFPO-DA, HFPOTA, perfluoro-2,5,8-trimethyl-3,6,9-trioxadecanoic acid (HFPO-TeA), perfluoro-3,6-dioxaheptanoic acid (PFO2HpA), perfluoro(3-methoxypropanoic) acid (PFMPA), PFBA (C4), PFPeA (C5), PFOA (C8), perfluorononanoic acid (PFNA, C9), perfluorodecanoic acid (PFDA, C10), perfluoroundecanoic acid (PFUnDA, C11), 4-(trifluoromethyl)hexafluoropent-2-enoic acid (PFMeUPA), and potassium salts of perfluorobutanesulfonic acid (PFBS), perfluorohexanesulfonic acid (PFHxS), and PFOS. PFMeUPA is an unsaturated PFCA; unsaturated PFCAs have been detected in sludge-applied soil and grass samples.71 All of these PFAS were purchased from Sigma-Aldrich (Table S1). The adsorbents used were single-use AIX resin (AmberChrom 1 8 or formerly known as Dowex 1 8) and GAC (Filtrasorb 200; N2 BET surface area, 691.4 m2/g; microporosity, 0.3 cm3/g; mesoporosity, 0.07 cm3/g). Standard batch sorption experiments were carried out to assess the adsorption of a single PFAS or a mixture of PFAS to the resin and GAC, following previously established procedures.72 The test solution was prepared with distilled water containing 1.0 10-3 mol/L NaHCO3 as a buffer and 1.0 10-3 mol/L NaCl for the ionic strength. After adsorption, the supernatant fluid was decanted. PFAS-laden adsorbent particles were freeze-dried and stored in a desiccator to reach room temperature. Then, these particles were split into two portions.42 One portion of the particles was extracted using methanol amended with 100 mol/L ammonium acetate.42 The PFAS mass before thermal treatment (MPFAS,solid,initial) was computed using eq 1 MPFAS,solid,initial = (Cextr Vextr)/E (1) where E (%) is the extraction efficiency, Cextr (mol/L) is the concentration of PFAS in the extract, and Vextr (L) is the volume of methanol. The analysis of PFAS and degradation products in the extract was performed using a Waters Acuity ultra-highpressure liquid chromatograph coupled with a high-resolution 8797 https://doi.org/10.1021/acs.est.3c00294 Environ. Sci. Technol. 2023, 57, 8796-8807 Environmental Science & Technology pubs.acs.org/est Article Figure 1. Thermal degradation of PFECAs and PFCAs on adsorbents at 200 C for up to 60 min in N2. In panels a and b, the adsorbent was laden with a mixture of the PFAS, whereas the adsorbent was laden with one PFAS chemical in panel c. Initial PFAS loading on GAC (mol/g): HFPO-DA, 5.9- 7.6; HFPO-TA, 21.2-42.6; HFPO-TeA, 18.0; PFMPA, 1.2-2.5; PFO2HpA, 1.7-2.6; PFBA, 1.5-2.1; PFPeA, 1.9-3.5; PFHpA, 3.1-4.9; and PFOA, 7.2-12.5. The generation and subsequent degradation of TFA, PFPrA, and HFPO-DA during the treatment are illustrated by arrows. Mi and MT refer to the mass of PFAS (mol/g) before and after the thermal treatment, respectively (eq 1); for TFA and PFPrA, Mi was the mass after 7 min of heating. The dashed lines represent the best linear fits, assuming a pseudo-first-order degradation. The degradation rate constants (k, min-1) are given. The residual PFECAs after heating for more than 30 min were generally lower than the method detection limits. mass spectrometer (Synapt G2-S quadrupole time-of-flight mass spectrometer, QToF-MS/MS, Waters Corporation, Milford, MA, USA).73,74 Analytical details can be found in the Supporting Information. In the commercial GAC regeneration process, spent carbon is heated for approximately 30 min with inert gases such as N2 and CO2 at relatively low temperatures (300 C).75 To simulate this process, the other portion of the PFAS-laden adsorbent particles was placed in a pre-cleaned, stainless-steel reactor (7 mL; 45 mm in height and 19 mm in outside diameter) (QAQC Lab Inc., White Stone, VA, USA). The reactor was sealed with a rubber septum and purged with nitrogen for 30 min. The septum was then replaced with a stainless-steel screw cap (QAQC Lab Inc., White Stone, VA, USA). Heating was carried out in a muffle furnace (Neytech, Vulcan 3-550, USA) at a preset temperature (up to 500 C) for 30 min. To understand the effect of treatment time, we also heated PFAS-laden adsorbent particles at a given temperature for different durations (0-60 min). After thermal treatment and cooling, the reactor was rinsed with methanol containing 100 mol/L ammonium acetate to extract residual PFAS.42 PFAS thermal decomposition was assessed by comparing the residual mass in heated samples to nonheated controls.43,46 Analysis of Decomposition Products. To capture transient intermediates of PFECAs and PFCAs upon heating, we performed additional thermal degradation experiments of individual PFAS in N2 in a borosilicate reactor following a procedure developed previously.43 After heat treatment and cooling, the reactor was rinsed with distilled water to extract F or methanol containing 100 mol/L ammonium acetate to extract residual PFAS and polar decomposition products.43,46 Inorganic F in the water extract was measured using an F-ion selective electrode, as described previously.43 The yield of F from a PFAS with n F atoms was calculated with the following equation Yield(F) = n[MPFAS,iMF MPFAS,T] 100% (2) where M is the mass of PFAS calculated by eq 1. The yield of a transient product from a parent PFAS compound was calculated by 8798 https://doi.org/10.1021/acs.est.3c00294 Environ. Sci. Technol. 2023, 57, 8796-8807 Environmental Science & Technology pubs.acs.org/est Article Figure 2. Yields of intermediates from a single PFECA preadsorbed on GAC after heating at 200 C for different durations. Yield(product) = [MPMFApSr,oi duct,MforPmFeAdS,T] 100% (3) Thermal decomposition products of PFAS in the methanol extract were analyzed by ultrahigh-pressure liquid chromatography-quadruple time-of-flight tandem mass spectrometry (UPLC-QToF-MS/MS), following previously described procedures (or see the Supporting Information for details).43-45 Chromatography was performed using a Waters Acquity UPLC BEH Shield RP18 column (100 2.1 mm; 130 ; 1.7 m) with a Waters Acquity UPLC BEH Shield RP18 VanGuard precolumn (5 2.1 mm; 130 ; 1.7 m). MS analysis was performed using the Synapt G2-S quadrupole time-of-flight mass spectrometer with an ESI source operated in a negative ion mode. MassLynx V4.1 software (Waters) was used for instrument control, acquisition, and mass analysis. Standards of two ultra-short chain-PFCAs, TFA (C2) and PFPrA (C3), were purchased to confirm the thermal decomposition products of studied PFAS. A high-resolution precursor ion scan (HRPIS) method74 was also employed for nontarget screening of possible thermal degradation products (see the Supporting Information). Thermal Desorption-Pyrolysis-Gas Chromatography-MS Experiments. To date, no accurate and sensitive method has been established for capturing gaseous thermal degradation products of PFAS. This is due to the lack of effective adsorbents that can adsorb all gaseous organofluorine species in the off-gas, as well as the lack of an efficient extraction method for extracting all adsorbed organofluorine species from the adsorbents. Rather than using the traditional adsorption- extraction-cleanup-concentration-GC-MS method, we analyzed gaseous thermal decomposition products of PFAS using thermal desorption-pyrolysis (TD-Pyr) (Frontier 3030D, Frontier Labs Inc., Japan) connected to a GC-MS system (Agilent GC 7890 and 5975C MS; Santa Clara, CA). This system has previously been utilized to analyze gaseous products emitted during the thermal treatment of long-chain PFAS43 and aqueous film-forming foams.43,45 A Frontier 30 m Ultra ALLOY capillary column (30 m; Frontier Labs Inc., Japan) was used with an inner diameter of 0.25 mm and a 5% diphenyldimethyl polysiloxane stationary phase with 0.25 m film thickness. MS analysis was performed with electron ionization in the mass range of 35-850 m/z. Ultra-pure helium (99.999%) was used as the carrier gas with a constant flow rate of 1.1 mL/min (more details can be found in the Supporting Information). Data processing included the MS fragmentation pattern evaluation of a potential pyrolyzate based on the highly curated 2005 National Institute of Standards and Technology library that contains 190,825 spectra. Compounds were considered tentatively identified with a library match quality of >70%.45 Computational Studies. A supercomputer (Intel Platinum 48 Core, Beijing Super Cloud Computing Center) was used to perform high-throughput DFT calculations of BDEs of all 8799 https://doi.org/10.1021/acs.est.3c00294 Environ. Sci. Technol. 2023, 57, 8796-8807 Environmental Science & Technology pubs.acs.org/est Article Figure 3. Yields of intermediates from a single PFCA preadsorbed on GAC after heating at 200 C for different durations. covalent bonds (C-C, C-F, C-O, O-H, and CC) in PFECAs, short-chain PFCAs, and the unsaturated PFCA (i.e., PFMeUPA). A state-of-the-art M06-2X method76 at a 3-zeta basis set of Def2-TZVP was implemented using the state-of-theart Gaussian 16 program.77 RESULTS AND DISCUSSION Degradation of PFECAs and Short-Chain PFCAs on Spent GAC and Resin. Figures S1 and 1 illustrate the thermal degradation of PFECAs and short-chain PFCAs preadsorbed on adsorbents. Detailed adsorption isotherms and mechanisms are outside the scope of this paper and will be the subject of another paper in the future. In selected experiments, a high initial mass (up to 75.9 mol/g) of PFECAs was directly added to AmberChrom 1 8 to estimate the maximum percentage removal (Figure S1). Regardless of concentrations of PFAS in the resin, all these PFAS compounds exhibited near-complete degradation (>98.59%) in as little as 30 min at 500 C. Deformation of the resin beads was seen at 300 C and above. No attempt was made to reuse the decontaminated resin beads as they are intended for single use only. Upon heating PFECAs and PFCAs at 500 C, no new noteworthy peaks emerged in UPLC-HRMS chromatograms of full scans and HRPIS, signifying rapid degradation of both parent compounds and transient intermediates at this temperature.42,45 For example, Yao et al. demonstrated that perfluoroheptene, a notable thermal decomposition product of PFOA, starts to degrade at a temperature as low as 200 C.45 PFECAs are more prone to degradation than PFCA counterparts, as evidenced by Figure S1 and the apparent yield of inorganic F species (Figure S2). HFPO-DA has a halflife of 4.3 min at 200 C compared to 13 min for PFBA (Figure 1a). The half-lives of both chemicals decreased as the heating temperature increased (e.g., 5.0 min for PFBA at 400 C).43 The relatively long half-lives of parent PFAS at low temperatures allow us to compare the thermal stability and capture transient thermal decomposition products. The thermal degradation rate constants decline in the following order: PFECAs > PFCAs > unsaturated PFCA (i.e., PFMeUPA). The most notable features of Figure 1as well as a side-by-side comparison of Figure 1a,bare the significant yields of TFA, PFPrA, and HFPO-DA when a mixture of studied PFECAs and PFCAs was heated at low temperature (Figure 1b,c). These results indicate the formation of TFA, PFPrA, and HFPO-DA during thermal treatments. Thermal Decomposition Products. HFPO-DA. We conducted additional thermal treatment experiments on individual PFECAs (Figure 1c) to capture short-lived transient intermediates of PFECAs and short-chain PFCAs. Figure 2 displays the evolution of detected transient intermediates from PFECAs, which can be grouped into two categories: small intermediates and relatively large ones with a UPLC retention time close to that of the parent compound. These transient intermediates were assigned by comparing their UPLC retention time and MS fragmentation patterns with authentic standards. Three transient organic intermediates (I, II, III) of HFPO-DA were detected by UPLC-HRMS. Compound I with m/z 212.9792 and MS fragments characteristic of PFCAs (C3F7, m/z 168.9894; C2F5, m/z 118.9926) were assigned to be PFBA. This is further substantiated by an authentic standard. Compounds II and III were identified as PFPrA and TFA, respectively, confirmed with corresponding authentic standards. Figures 1 and 2 show that they built up rapidly but decomposed within 60 min. To our knowledge, these PFCAs were identified for the first time as thermal decomposition products of HFPO-DA. A novel volatile intermediate (IV) with a molecular weight (m/z 265.9) greater than that of PFBA was also detected (Figures S3-S6). The fragments at m/z 69.0 (CF3+), 119.0 (C2F5+), and 169.0 (C3F7+) are characteristic of fluorinated fragments.44,74 The fragments at m/z 146.9 and 196.9 represent the successive loss of the fluorinated fragment CF2 from IV. We assigned the ion at m/z 146.9 to be an unsaturated fluorinated fragment with one C replaced by O (SMILES: FC(F) = [O +]C(F) = C(F)F). The ion at m/z 81.0 represents an 8800 https://doi.org/10.1021/acs.est.3c00294 Environ. Sci. Technol. 2023, 57, 8796-8807 Environmental Science & Technology pubs.acs.org/est Article Figure 4. BDEs of PFAS calculated at the M06-2X/Def2-TZVP level of theory using high-performance computing resources. Figure 5. Proposed thermal decomposition pathways of HFPO-DA. Dashed arrows: DFT predicted pathways that are not supported by experimental data. Solid arrows: pathways supported by experimental results. PFBA, PFPrA, TFA, and perfluoro(propyl vinyl ether) are detected as thermal decomposition products of HFPO-DA (Figures 2, S3-S6). The three-digit numbers (e.g., 79.6) indicate the DFT-calculated enthalpy change (kcal/ mol). HFPO-DA decomposes primarily through Pathway A, whereas Pathway B serves as a secondary, less predominant route. unsaturated fluorinated fragment (SMILES: F[C+] = C(F)F). Taken these results together, we assigned IV to be perfluoro(propyl vinyl ether) (SMILES: FC(F) = C(F)OC(F)(F)C(F)(F)C(F)(F)F) (library match quality, 74-80%). The yields of transient intermediates from HFPO-DA are shown in Figure 2, except for perfluoro(propyl vinyl ether), which was not quantified due to the lack of an authentic standard. TFA, PFPrA, and PFBA were generated quantitively from HFPO-DA (Figure 2). TFA was formed in 30% maximum yield but disappeared on the same timescale as HFPO-DA. PFBA was a minor product of HFPO-DA, which disappeared after a slight lag compared to PFPrA. 8801 https://doi.org/10.1021/acs.est.3c00294 Environ. Sci. Technol. 2023, 57, 8796-8807 Environmental Science & Technology pubs.acs.org/est Article Figure 6. Proposed thermal decomposition pathways (i.e., random-chain scission and end-chain scission) of PFPeA. PFBA, PFPrA, TFA, and perfluoro-1-butene are detected as thermal decomposition products of PFPeA (Figures 3, S9, and S10). The three-digit numbers (e.g., 70.8) indicate the DFT-calculated enthalpy change (kcal/mol). HFPO-TA and HFPO-TeA. HFPO-TA and HFPO-TeA may be thermally decomposed by a similar pathway judging by the significant quantities of PFPrA and TFA produced during their degradation (Figure 2). The yields of TFA and PFBA from HFPO-TA and HFPO-TeA were much higher than those from HFPO-DA because multiple C-O bonds can break off in HFPO-TA and HFPO-TeA (Figure 2). Interestingly, HFPODA was a minor thermal decomposition product of its longchain homologues (i.e., HFPO-TA and HFPO-TeA). PFBA. TFA and PFPrA evolved quantitatively from PFBA after 10 min in 4.6% maximum yield but disappeared on the same timescale as PFBA (Figure 3). A novel gaseous thermal decomposition product (V) of PFBA was detected with an empirical formula of C3F6. It has two fewer F atoms than perfluoropropane, with a fragmentation pattern identical to perfluoropropene (CAS no.: 116-15-4).78 Therefore, a reasonable but tentative assignment of compound V is perfluoropropene (library match quality, 74-81%) (Figures S7 and S8). PFPeA. The transformation of PFPeA was similar to that of PFBA. Within minutes of thermal treatment at 200 C, almost 100% of PFPeA was removed from spent GAC and resins (Figures S1 and 1). TFA, PFPrA, and PFBAwhose identities were confirmed with authentic standardswere formed from PFPeA but rapidly degraded with a longer treatment time (Figure 1). A novel gaseous product (VI) of PFPeA, perfluoro-1butene (C4F8), was detected (library match quality, 84-94%) (Figures S9 and S10). Where comparisons can be made, the yield of shorter-chain homologues from a PFCA declines with the chain length of this PFCA. Perfluoroheptanoic acid (C7) forms in 30 mol % yield from PFOA (C8) under the experimental conditions, whereas the yields of PFBA (C4) from PFPeA (C5) and PFPrA (C3) from PFBA are both less than 10 mol % (Figure 3). Computational Results. BDE, also known as the bond enthalpy, provides critical information on which bonds in PFAS are likely to break off upon thermal, chemical, and physical treatments. Degradation of a PFECA or PFCA molecule may occur as soon as the temperature is high enough to initiate bond breaking. The bond that is more easily broken is favored. Tables S2-S8 give the DFT calculation details of C-C, C-F, C-O, O-H, and CC BDEs. These computational results, in combination with the experimental data, yield several interesting and novel observations. (i) The C-C bonds in the perfluorinated backbone of PFPeA and PFBA are generally weaker than the bond (-C-COOH) connecting the -C and the carboxyl-C (Figure 4). Therefore, in addition to decarboxylation, the random C-C scission in the perfluorinated chain is a very likely mechanism that dissociates PFPeA and PFBA. (ii) BDE data on PFPeA, PFMPA, and HFPO-DA indicate that the -C-COOH BDE increases with an electrondonating ether O in the perfluorinated chain but declines with the electron-withdrawing -CF3 moiety attached to the -C (Figure 4). The result suggests that the side trifluoromethyl group may aid in the decarboxylation of HFPO-DA. (iii) The CC double bond strengthens the adjacent C-C bonds, including the -C-COOH bond (Figure 4). (iv) In HFPO-DA, the C-O ether bond (C-Oaway) away from the electronwithdrawing carboxyl group is stronger than the C-O ether bond (C-Onr) closer to the carboxyl group (Figure 4). The result indicates the preferential thermal cleavage of the C-Onr bond, supported by experimental data; the minor yield of PFBA (Figure 2) rules out the cleavage of the C-Oaway bond (pathway B in Figure 5) as a primary thermal degradation pathway for HFPO-DA. The results differ from a previous study by Bentel et al. which found that the C-Onr bond had a considerably higher BDE than the C-Oaway bond in HFPO-DA.67 The discrepancy could be caused by the different DFT methods (e.g., M06-2X/ Def2-TZVP in this study). (v) The difference in C-O BDEs in PFECAs decreases with increasing chain length (Figure 4). Thermal Decomposition Pathways of PFECAs. With these experimental and computational results, it is possible to develop explicit thermal degradation pathways for HFPO-DA. 8802 https://doi.org/10.1021/acs.est.3c00294 Environ. Sci. Technol. 2023, 57, 8796-8807 Environmental Science & Technology pubs.acs.org/est Article PFBA, PFPrA, and TFA were not considered thermal decomposition products of HFPO-DA in previous computational studies.69,70 Therefore, the detection of these short-chain PFCAs from HFPO-DA upon heating indicates a novel thermal degradation pathway that was not considered previously. A pathway for the thermal decomposition of HFPO-DA must take into account: (i) the practically instantaneous formation of TFA, PFPrA, and PFBA upon heating of the parent compound; (ii) the relatively large transient yields of TFA and PFPrA; and (iii) the trace yield of PFBA that rules it out as an intermediate along a primary path. A pathway consistent with these results is the thermal cleavage of the C-Onr bond, leading to the formation of a perfluoropropyl-oxidanyl radical and a precursor of PFPrA ( PFPrA-H) (pathway A in Figure 5). This result is accomplished by the recombination of the PFPrA-H and F to form PFPrA (Figure 5), followed by the thermal decomposition of the resulting PFPrA radical to give TFA (Figure 5). The thermal cleavage of the other stronger C-O ether bond is evidenced by the formation of PFBA from HFPO-DA. HFPO-DA decarboxylation (pathway C) generates perfluoro(propyl vinyl ether). The DFT simulations predict a few other pathways involving the cleavage of the trifluoromethyl (-CF3) group, which are not supported by MS spectral evidence; however, this does not preclude the formation of transient intermediates in small amounts in these DFT-predicted pathways (dashed arrows in Figure 5). There are five C-C and four C-O ether bonds in an HFPOTA molecule. We have suggested multiple concurrent HFPOTA degradation pathways (eq 4) with enthalpy changes in the range of 65-80 kcal/mol (Table S3). The thermal decomposition mechanisms developed here for HFPO-DA and HFPOTA (Figure 5 and eq 4) involve recombining radicals (e.g., F) and perfluorinated radicals to yield TFA, PFPrA, and PFBA. For example, HFPO-DA is likely formed from HFPO-TA through the recombination of F and CF2-CF2-CF2-O-CF(CF3)- COOH in pathway D (eq 4). The partially negatively charged F and O atoms form an electron shell around the partially positively charged perfluorinated carbon chain,79 which may inhibit the physical adsorption of PFAS molecules on soil/clay particles.79 However, this theoretical shell seems not to prevent the thermal recombination of F with a perfluoroalkyl radical. The thermal generation of PFOS from 8:2 fluorotelomer sulfonate also likely involves the recombination of perfluoroalkyl radicals with the sulfonate group.44 HFPO TA looooodecarboxylation ooooooo CF3 CF2 CF2 O CF(CF3) CF2 i Kcal y oooooo O CF(CF3) + COOHjjj68.4 k mol zzz { ooooooether bond cleavage oooooooCF3 CF2 CF2 O CF(CF3) CF2 ooooooo + O CF(CF3) COOHijjj75.4 Kcal yzzz oooooo k mol { ooooooo CF3 CF2 CF2 O CF(CF3) CF2 i Kcal y oooooo O + CF(CF3) COOHjjj75.4 k mol zzz { oooooooD: CF3 CF2 CF2 O + CF(CF3) oooooo CF2 O CF(CF3) ooooooooooo COOHikjjj77.9 Kmcoall y{zzz moooooo CF3 CF2 CF2 O CF2 + O CF(CF3) CF(CF3) ooooooo COOHijjj78.5 Kcal yzzz oooooooC k C scission mol { oooooooCF3+CF3 CF2 CF2 O CF(CF3) oooooo CF2 O CF() oooooo COOHijjj68.8 Kcal yzzz oooooo k mol { ooooooCF3 CF2 CF2 O CF(CF3) oooooooo + CF2 O CF(CF3) COOHijj73.2 Kcal yzz oooooo kj mol {z ooooooCF3 + CF2 CF2 O CF(CF3) ooooooo CF2 O CF(CF3) i Kcal y noooo COOHjjj76.9 k mol zzz { (4) Thermal Decomposition Pathways of Short-Chain PFCAs. Because of the strong C-F bond, the thermal decomposition of short-chain PFCAs should be initiated by the cleavage of the relatively weak C-X bond in which X stands for elements such as C other than F. For PFCAs, only two types of C-X bonds can break: (1) the -C-COOH bond connecting the perfluorinated chain and the carboxyl group and (2) the C- C covalent bond on the perfluorinated backbone. The bonds that tend to break first are the weakest in the structure. Xiao et al. previously interpreted HRMS data of PFOA and PFOS thermal degradation products in an "unzip" mechanism involving the end-chain scission and random C-C scission at seemingly random points in the perfluorinated chain.44,46 The experimental and computational data obtained in this study support this mechanism. Equation S3 in Supporting Information and Figure 6 illustrate the degradation pathways of two short-chain PFCAs (i.e., PFBA and PFPeA) through both pathways. Endchain scission occurs through decarboxylation that breaks 8803 https://doi.org/10.1021/acs.est.3c00294 Environ. Sci. Technol. 2023, 57, 8796-8807 Environmental Science & Technology pubs.acs.org/est Article PFCAs' a-C--COOH bond (Figures 5 and 6). The weakest C-- C bond in both PFPeA and PFBA, however, is the one connecting the a-C and the /3-C. The /3-C--y-C bond is also weaker than the a-C--COOH bond in PFPeA. Therefore, both computational and experimental results suggest random scission in the perfluorinated chain as a degradation mechanism for these short-chain PFCAs (eq S3 in Supporting Information and Figure 6). Little research has been geared toward developing a mechanistic picture of the thermal decomposition mechanisms of PFECAs and short-chain PFCAs that is evidence-based. Therefore, the present study makes several contributions to the field. This study provides the first evidence for a C--C scission mechanism on the perfluorinated backbone through which short-chain PFCAs decompose thermally. It gives C--C, C--F, C--O, O--H, and C=C BDEs of PFAS of interest and illustrates the effect of functionality and chain length. We find several interesting bond dissociation trends. Some BDE data are reported for the first time, adding to the current knowledge base or challenging existing knowledge. For example, we found that the C--O,,, bond in HFPO-DA is weaker than the C--Oaway bond and that the side --CF3 group on the a-C may aid in HFPO-DA decarboxylation. This study is also among the first to investigate the thermal degradation of PFAS on spent resin. Furthermore, as our understanding of the thermal stability and decomposition of PFAS has evolved, it has become clear that thermal processes are nonselective and effective for degrading PFAS. However, acquiring critical information on the formation and speciation of PFAS incomplete thermal degradation products (PIDs) is necessary. This information is vital for designing and operating thermal technologies and ensuring treatment targets are achieved while minimizing undesired product formation. This study reports the formation of PIDs from three primary PFAS-HFPO-DA, PFBA, and PFPeA, which is potentially useful for many technological applications involving their removals, such as thermal regeneration of spent GAC42,46,50 and low-temperature thermal desorption (<300 . 4 56 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/ doi/10.1021/acs.est.3c00294. UPLC--QToF-MS/MS method, HRPIS experiments, TD--Pyr--GC--MS method, DFT calculations, PFAS chemicals used in this study, BDEs of HFPO-DA, BDEs of HFPO-TA, BDEs of HFPO-TeA, BDEs of PFMeUPA, BDEs of PFMPA, BDEs of PFBA, BDEs of PFPeA, thermal decomposition of various PFAS preadsorbed on a single-use resin and GAC, apparent yield of F from PFCAs and PFECAs during thermal treatment, TD-- Pyr--GC--MS chromatograms of HFPO-DA at 200-500 C, TD-Pyr-GC-MS chromatogram and spectrum of HFPO-DA at 200 C, TD-Pyr-GC-MS chromatogram and spectrum of HFPO-DA at 300 C, TD-Pyr-GCMS chromatogram and spectrum of HFPO-DA at 500 C, TD-Pyr-GC-MS chromatogram and spectrum of PFBA heated at 200 C, TD-Pyr-GC-MS chromatogram of PFBA heated at 300 C, TD-Pyr-GC-MS chromatogram of PFPeA heated at 200 C, and TD-- Pyr--GC--MS chromatogram of PFPeA heated at 300 C (PDF) AUTHOR INFORMATION Corresponding Authors Yang Li -- Key Laboratory of Water and Sediment Sciences of Ministry of Education, State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, People's Republic o China; orcid.org/0000-0001-7287-109X; Phone: Email: @bnu.edu.cn Feng Xiao -- Department of Civil and Environmental Engineering, The University of Missouri, Columbia, Missouri 65211, United States; m orcid.org/0000-0001-5686-6055; Phone: Email: Feng.Xiao@ Missouri.edu Authors Ali Alinezhad -- Department of Civil and Environmental Engineering, The University of Missouri, Columbia, Missouri 65211, United States Heng Shao -- Key Laboratory of Water and Sediment Sciences of Ministry of Education, State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, People's Republic of China Katerina Litvanova -- Department of Chemistry, The University of North Dakota, trand Forks, North Dakota 58202, United States Runze Sun -- Department of Civil and Environmental Engineering, The University of Missouri, Columbia, Missouri 65211, United States Alena Kubatova -- Department of Chemistry, The University of North Dakota, trand Forks, North Dakota 58202, United States; orcid.org/0000-0002-2318-5883 Wen Zhang -- John A. Reif, Jr. Department of Civil and Environmental Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102, United States; orcid.org/0000-0001-8413-0598 Complete contact information is available at: https://pubs.acs.org/10.1021/ acs.est.3c00294 Author Contributions A.A. and H.S. contributed equally to this work and share first authorship. A.A. was responsible for experimental execution and data analysis, while H.S. calculated BDEs using DFT and generate plots. K.L. analyzed the gaseous products of PFAS, and R.S. assisted with experimental execution. A.K. also analyzed the gaseous products of PFAS and contributed to manuscript review and editing. W.Z. participated in manuscript evaluation, while Y.L. offered insightful feedback during the review and editing process. Finally, F.X. was responsible for the overall conception and design of the study, including experimental design, data visualization, thermal decomposition pathways of HFPO-DA, HFPO-TA, PFPeA, and PFBA, and manuscript and Supporting Information preparation. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This project was funded by grants from the U.S. National Science Foundation (CBET-2047062; F.X.), U.S. Environmental Protection Agency STAR Program (RD839660; F.X.), U.S. Department of Defense SERDP (ER21-1185; F.X.), and the National Natural Science Foundation of China (no. 8804 https://doi.org/10.1021/acs.est.3c00294 Environ. Sci. Technol. 2023, 57, 8796-8807 Environmental Science & Technology pubs.acs.org/est Article 52170024; Y.L.). The UPLC and the hybrid QToF-MS/MS systems were purchased by the Department of Biomedical Sciences at the University of North Dakota using the NIH COBRE Mass Spec Core Facility Grant 5P30GM103329-05. REFERENCES (1) EPA. 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