Elucidating degradation mechanisms for a range of per- and polyfluoroalkyl substances (PFAS) via controlled irradiation studies

Tandem Mass Spectrometry of Perfluorocarboxylate Anions: Fragmentation Induced by Reactive Species Formed From Microwave Excited Hydrogen and Water Plasmas

RATIONALE

Polyfluoroalkyl substances (PFAS) like perfluorooctanoic acid have persistent environmental and physiological effects. This study investigates the degradation of CnF2n+1CO2 - (n = 1-7) with neutral radical fragmentation under oxygen attachment dissociation (OAD). Unique fragments absent from collision-induced dissociation (CID) are observed. Further, potential mechanisms are uncovered by density functional theory (DFT) calculations.

METHODS

From a standard mixture of PFAS, straight-chain perfluorinated carboxylic acids with carbon chain lengths of one to eight were separated via liquid chromatography and transferred to the gas phase via negative-mode electrospray ionisation. Each CnF2n+1CO2 - of interest was mass selected and fragmented via both CID and OAD in a quadrupole time-of-flight mass spectrometer. DFT optimisations of structures were performed at M06/6-31+g(d), and single point energy calculations were performed at M06-2X/aug-cc-pVTZ for C3F7CO2 -.

RESULTS

Decarboxylation was observed from both CID and OAD, but fluorine abstraction and hydroxyl addition only occurred with OAD. The DFT calculations suggest that C3F6 -• (m/z 150) is most likely formed from by H• attack onto a β- C-F bond, then loss of HF, finally decarboxylation. Further, C3F5O- (m/z 147) likely arises from C3F6 -• recombining with OH• to produce energised C3F6OH- ions, followed by α- or β- elimination of HF to give enolate and/or epoxide-type products.

CONCLUSIONS

OAD of C3F7CO2 - yields unique product ions C3F6 -• (m/z 150) and C3F5O- (m/z 147) absent from collision-induced dissociation. DFT calculations suggest an intricate pathway of H• attack onto a β C-F bond, then loss of HF, decarboxylation, recombination with OH•, and finally α- or β- elimination of HF to give the products.

Advancing PFAS characterization: Development and optimization of a UV-H2O2-TOP assay for improved PFCA chain length preservation and organic matter tolerance

As per- and polyfluoroalkyl substances (PFAS) infiltrate the environment via industrial, commercial, and domestic sources, the demand for robust, cost-effective, and straightforward analytical assays intensifies to enhance PFAS characterization and quantification. To address this demand, this study introduces a novel UV-H2O2-TOP assay, identifying optimal parameters such as pH (5-9), oxidant concentration (500 mM H2O2), activation rate (63 mM H2O2 h-1), and an acceptable total organic carbon (TOC) limit (~1000 mg/L TOC) to achieve maximum PFAA precursor conversion. Additional work was performed further optimizing the UV-TOP assay, by confirming its superiority to heat activation, identifying the effectiveness of different persulfate salts, and investigating different concentrations of sodium persulfate and sodium hydroxide at a 1:2.5 ratio on PFCA yield. Our investigation concluded by applying the UV-H2O2-TOP assay, using sodium persulfate as the TOP assay oxidant, to 6:2 FTS and five different AFFF samples. High-resolution mass spectrometry and an expanded analytical suite support sample analysis, facilitating direct quantification of ultra-short chain perfluoroalkyl carboxylates (PFCAs) and common fluorotelomer compounds including 5:3/5:1:2 fluorotelomer betaine and 6:2 fluorotelomer sulfonamido betaine. Results highlight several advantages of this tandem UV-activated method, including enhanced preservation of perfluoroalkyl chains (post-oxidation of 6:2 fluorotelomer sulfonate resulted in 28 % PFHpA, 47 % PFHxA, 25 % C3-C5 PFCA), capacity to handle high TOC limits (1000 mg/L TOC), and ability to incorporate higher persulfate concentrations in a single oxidation cycle.

Anaerobic biodegradation of perfluorooctane sulfonate (PFOS) and microbial community composition in soil amended with a dechlorinating culture and chlorinated solvents

Perfluorooctane sulfonate (PFOS), one of the most frequently detected per- and polyfluoroalkyl substances (PFAS) occurring in soil, surface water, and groundwater near sites contaminated with aqueous film-forming foam (AFFF), has proven to be recalcitrant to many destructive remedies, including chemical oxidation. We investigated the potential to utilize microbially mediated reduction (bioreduction) to degrade PFOS and other PFAS through addition of a known dehalogenating culture, WBC-2, to soil obtained from an AFFF-contaminated site. A substantial decrease in total mass of PFOS (soil and water) was observed in microcosms amended with WBC-2 and chlorinated volatile organic compound (cVOC) co-contaminants - 46.4 ± 11.0 % removal of PFOS over the 45-day experiment. In contrast, perfluorooctanoate (PFOA) and 6:2 fluorotelomer sulfonate (6:2 FTS) concentrations did not decrease in the same microcosms. The low or non-detectable concentrations of potential metabolites in full PFAS analyses, including after application of the total oxidizable precursor assay, indicated that defluorination occurred to non-fluorinated compounds or ultrashort-chain PFAS. Nevertheless, additional research on the metabolites and degradation pathways is needed. Population abundances of known dehalorespirers did not change with PFOS removal during the experiment, making their association with PFOS removal unclear. An increased abundance of sulfate reducers in the genus Desulfosporosinus (Firmicutes) and Sulfurospirillum (Campilobacterota) was observed with PFOS removal, most likely linked to initiation of biodegradation by desulfonation. These results have important implications for development of in situ bioremediation methods for PFAS and advancing knowledge of natural attenuation processes.

Mechanochemical degradation of per- and polyfluoroalkyl substances in soil using an industrial-scale horizontal ball mill with comparisons of key operational metrics

Horizontal ball mills (HBMs) have been proven capable of remediating per- and polyfluoroalkyl substances (PFAS) in soil. Industrial-sized HBMs, which could easily be transported to impacted locations for on-site, ex-situ remediation, are readily available. This study examined PFAS degradation using an industrial-scale, 267 L cylinder HBM. This is the typical scale used in the industry before field application. Near-complete destruction of 6:2 fluorotelomer sulfonate (6:2 FTS), as well as the non-target PFAS in a modern fluorotelomer-based aqueous film forming foam (AFFF), was achieved when spiked onto nepheline syenite sand (NSS) and using potassium hydroxide (KOH) as a co-milling reagent. Perfluorooctanesulfonate (PFOS) showed much better and more consistent results with scale-up regardless of KOH. Perfluorooctanoate (PFOA) was examined for the first time using a HBM and behaved similarly to PFOS. Highly challenging field soils from a former firefighting training area (FFTA) were purposefully used to test the limits of the HBM. To quantify the effectiveness, free fluoride analysis was used; changes between unmilled and milled soil were measured up to 7.8 mg/kg, which is the equivalent of 12 mg/kg PFOS. Notably, this does not factor in insoluble fluoride complexes that may form in milled soils, so the actual amount of PFAS destroyed may be higher. Soil health, evaluated through the assessment of key microbial and associated plant health parameters, was not significantly affected as a result of milling, although it was characterized as poor to begin with. Leachability reached 100 % in milled soil with KOH, but already ranged from 81 to 96 % in unmilled soil. A limited assessment of the hazards associated with the inhalation of PFAS-impacted dust from ball-milling, as well as the cross-contamination potential to the environment, showed that the risk was low in both cases; however, precautions should always be taken.

Application of electron beam technology to decompose per- and polyfluoroalkyl substances in water

The widespread detection of per- and polyfluoroalkyl substances (PFAS) in environmental compartments across the globe has raised several health concerns. Destructive technologies that aim to transform these recalcitrant PFAS into less toxic, more manageable products, are gaining impetus to address this problem. In this study, a 9 MeV electron beam accelerator was utilized to treat a suite of PFAS (perfluoroalkyl carboxylates: PFCAs, perfluoroalkyl sulfonates, and 6:2 fluorotelomer sulfonate: FTS) at environmentally relevant levels in water under different operating and water quality conditions. Although perfluorooctanoic acid and perfluorooctane sulfonic acid showed >90% degradation at <500 kGy dose at optimized conditions, a fluoride mass balance revealed that complete defluorination occurred only at/or near 1000 kGy. Non-target and suspect screening revealed additional degradation pathways differing from previously reported mechanisms. Treatment of PFAS mixtures in deionized water and groundwater matrices showed that FTS was preferentially degraded (∼90%), followed by partial degradation of long-chain PFAS (∼15-60%) and a simultaneous increase of short-chain PFAS (up to 20%) with increasing doses. The increase was much higher (up to 3.5X) in groundwaters compared to deionized water due to the presence of PFAS precursors as confirmed by total oxidizable precursor (TOP) assay. TOP assay of e-beam treated samples did not show any increase in PFCAs, confirming that e-beam was effective in also degrading precursors. This study provides an improved understanding of the mechanism of PFAS degradation and revealed that short-chain PFAS are more resistant to defluorination and their levels and regulation in the environment will determine the operating conditions of e-beam and other PFAS treatment technologies.

Removal of per- and polyfluoroalkyl substances from water by plasma treatment: Insights into structural effects and underlying mechanisms

Non-thermal plasma emerges as a promising technology for per- and polyfluoroalkyl substances (PFAS) decomposition due to its notable efficacy and environmentally friendly characteristics. In this study, we demonstrated the efficacy of a falling film dielectric barrier discharge (DBD) system for the removal of 10 PFAS, including perfluoroalkyl carboxylic acids (PFCAs), perfluoroalkyl sulfonic acids (PFSAs) and hexafluoropropylene oxide (HFPO) oligomer acids. Results showed that compounds with fluoroalkyl chain length>4 were effectively decomposed within 100 min, with long-chain PFAS demonstrating more pronounced removal performance than their short-chain analogues. The superior removal but low defluorination observed in HFPO oligomer acids could be ascribed to their ether-based structural features. The integration of experimental results with density functional theory (DFT) calculations revealed that the synergistic effects of various reactive species are pivotal to their efficient decomposition, with electrons, OH•, and NO2• playing essential roles. In contrast, the degradation of PFSAs was more dependent on electron attack than that of PFCAs and HFPO oligomer acids. Significantly, the most crucial degradation pathway for HFPO oligomer acids was the cleavage of ether CO, whether through radical or electron attack. Furthermore, the demonstrated effective removal in various water matrices showed the potential of the plasma system for removing PFAS in complex aquatic environments. This study provided mechanistic insights into PFAS degradation behavior in plasma processes, and it underscored the vital influence of molecular structures on degradability, thereby contributing to the further development and regulation of plasma-based technologies for treating PFAS in water.

The role of helicity in PFAS resistance to degradation: DFT simulation of electron capture and defluorination

Defluorination of perfluorinated alkyl substances (PFASs) via the direct capture of excess electrons poses a promising path to environmental decontamination. Herein we show that quantum-chemical model optimization methods can be adapted to simulate the changes to molecular geometry that result from electron capture. These reaction pathways demonstrate that the introduction of an additional electron causes a loss of the helical arrangement along linear carbon tail chains. Regaining helicity is sufficiently favourable to enable fluoride release in C7-C10 PFAS chains; shorter chains are enthalpically hindered from degradation while the additional charge is stabilized on longer chains by the greater entropy their flexibility permits. These results suggest that reductive PFAS treatment processes could be made more effective under high pressure or confined conditions.

Advancing PFAS characterization: Enhancing the total oxidizable precursor assay with improved sample processing and UV activation

Per- and polyfluoroalkyl substances (PFAS) encompass over 9000 chemicals utilized in various industrial and commercial applications. However, the quantification of PFAS using standard commercial analytical methods is currently limited to <50 selected compounds. To address this issue, the total oxidizable precursor (TOP) assay was developed, allowing for the oxidative conversion of previously undetectable PFAS precursors into measurable PFAS. This study investigated different sample processing methods to address post-oxidation PFAS loss identified in literature. Using PFOS as a probe molecule, up to 50 % loss of PFOS was identified during sample work-up. It was determined that the use of mass-labelled PFOS and methanolic acetic acid to chemically quench the sample post-oxidation improved PFOS recovery and allowed for correction of any remaining PFOS loss. The use of ultraviolet (UV) light was then investigated as an activator in contrast to the standard thermal activation method. A comparative evaluation was conducted to assess the recovery and conversion of perfluorooctanoic acid (PFOA), PFOS, and 6:2 fluorotelomer sulfonate (6:2 FTS) using both the heat-activated and UV-activated TOP assays. Results demonstrated that the UV-activated TOP assay achieved complete (100 %) oxidation of 6:2 FTS within 7.5 min, resulting in a total yield of generated perfluorinated carboxylic acids (PFCAs) at 108 ± 8 %. The study concluded by investigating the UV-activated TOP assay for its application on various aqueous film forming foam (AFFF) formulations and two AFFF samples drained from military aircraft rescue firefighting vehicles (ARFFVs). Analysis of these AFFF samples were supported by high resolution mass spectrometry and an expanded analytical suite, identifying several fluorotelomer precursors. The findings of this study provide compelling evidence that modifications in sample processing, work-up procedures, expansion of initial PFAS calibration standards, and UV-activation methods enhance the TOP assay, positioning it as a more reliable and quantitative analytical tool for PFAS characterization.

Advanced electrocatalytic redox processes for environmental remediation of halogenated organic water pollutants

Halogenated organic compounds are widespread, and decades of heavy use have resulted in global bioaccumulation and contamination of the environment, including water sources. Here, we introduce the most common halogenated organic water pollutants, their classification by type of halogen (fluorine, chlorine, or bromine), important policies and regulations, main applications, and environmental and human health risks. Remediation techniques are outlined with particular emphasis on carbon-halogen bond strengths. Aqueous advanced redox processes are discussed, highlighting mechanistic details, including electrochemical oxidations and reductions of the water-oxygen system, and thermodynamic potentials, protonation states, and lifetimes of radicals and reactive oxygen species in aqueous electrolytes at different pH conditions. The state of the art of aqueous advanced redox processes for brominated, chlorinated, and fluorinated organic compounds is presented, along with reported mechanisms for aqueous destruction of select PFAS (per- and polyfluoroalkyl substances). Future research directions for aqueous electrocatalytic destruction of organohalogens are identified, emphasizing the crucial need for developing a quantitative mechanistic understanding of degradation pathways, the improvement of analytical detection methods for organohalogens and transient species during advanced redox processes, and the development of new catalysts and processes that are globally scalable.

Research Updates on the Mechanism and Influencing Factors of the Photocatalytic Degradation of Perfluorooctanoic Acid (PFOA) in Water Environments

Perfluorooctanoic acid is ubiquitous in water bodies and is detrimental to the health of organisms. Effectively removing perfluorooctanoic acid (PFOA), a persistent organic pollutant, has been a hot topic around the world. With traditional physical, chemical, and biological methods, it is difficult to effectively and completely remove PFOA, the costs are high, and it is easy to cause secondary pollution. There are difficulties in applying some technologies. Therefore, more efficient and green degradation technologies have been sought. Photochemical degradation has been shown to be a low-cost, efficient, and sustainable technique for PFOA removal from water. Photocatalytic degradation technology offers great potential and prospects for the efficient degradation of PFOA. Most studies on PFOA have been conducted under ideal laboratory conditions at concentrations that are higher than those detected in real wastewater. This paper summarizes the research status of the photo-oxidative degradation of PFOA, and it summarizes the mechanism and kinetics of PFOA degradation in different systems, as well as the influence of key factors on the photo-oxidative degradation and defluoridation process, such as system pH, photocatalyst concentration, etc. PFOA photodegradation technology's existing problems and future work directions are also presented. This review provides a useful reference for future research on PFOA pollution control technology.

A review on superior advanced oxidation and photocatalytic degradation techniques for perfluorooctanoic acid (PFOA) elimination from wastewater

Perfluorooctanoic acid (PFOA) has been identified as the most toxic specie of the family of perfluorinated carboxylic acids (PFCAs). It has been widely distributed and frequently detected in environmental wastewater. The compound's unique features such as inherent stability, rigidity, and resistance to harsh chemical and thermal conditions, due to its multiple and strong C-F bonds have resulted in its resistance to conventional wastewater remediations. Photolysis and bioremediation methods have been proven to be inefficient in their elimination, hence this article presents intensive literature studies and summarized findings reported on the application of advanced oxidation processes (AOPs) and photocatalytic degradation techniques as the best alternatives for the PFOA elimination from wastewater. Techniques of persulfate, photo-Fenton, electrochemical, photoelectrochemical and photocatalytic degradation have been explored and their mechanisms for the degradation and defluorination of the PFOA have been demonstrated. The major advantage of AOPs techniques has been centralized on the generation of active radicals such as sulfate (SO₄^•-) hydroxyl (•OH). While for the photocatalytic process, photogenerated species (electron (e) and holes (h ⁺ _vb)) initiated the process. These active radicals and photogenerated species possessed potentiality to attack the PFOA molecule and caused the cleavage of the C-C and C-F bonds, resulting in its efficient degradation. Shorter-chain PFCAs have been identified as the major intermediates detected and the final stage entails its complete mineralization to carbon dioxide (CO₂) and fluoride ion (F^-). The prospects and challenges associated with the outlined techniques have been highlighted for better understanding of the subject matter for the PFOA elimination from real wastewaters.

Low-temperature persulfate activation by powdered activated carbon for simultaneous destruction of perfluorinated carboxylic acids and 1,4-dioxane

Carbonaceous materials have emerged as a method of persulfate activation for remediation. In this study, persulfate activation using powdered activated carbon (PAC) was demonstrated at temperatures relevant to groundwater (5-25 °C). At room temperature, increasing doses of PAC (1-20 g L^-1) led to increased persulfate activation (3.06 × 10^-6s^-1 to 2.10 × 10^-4 with 1 and 20 g L^-1 PAC). Activation slowed at lower temperatures (5 and 11 °C); however, substantial (>70 %) persulfate activation was achieved. PAC characterization showed that persulfate is activated at the surface of the PAC, as indicated by an increase in the PAC C:O ratio. Similarly, electron paramagnetic resonance (EPR) spectroscopy studies with a spin trapping agents (5,5-dimethyl-1-pyrroline N-oxide (DMPO)) and 2,2,6,6-tetramethylpiperidine (TEMP) revealed that singlet oxygen was not the main oxidizing species in the reaction. DMPO was oxidized to form 5,5-dimethylpyrrolidone-2(2)-oxyl-(1) (DMPOX), which forms in the presence of strong oxidizers, such as sulfate radicals. The persulfate/PAC system is demonstrated to simultaneously degrade both perfluorooctanoic acid (PFOA) and 1,4-dioxane at room temperature and 11 °C. With a 20 g L^-1 PAC and 75 mM persulfate, 80 % and 70 % of the PFOA and 1,4-dioxane, respectively, degraded within 6 h at room temperature. At 11 °C, the same PAC and persulfate doses led to 57% dioxane degradation and 54 % PFOA degradation within 6 h. Coupling PAC with persulfate offers an effective, low-cost treatment for simultaneous destruction of 1,4-dioxane and PFOA.

The Need for Testing Isomer Profiles of Perfluoroalkyl Substances to Evaluate Treatment Processes

Many environmentally relevant poly-/perfluoroalkyl substances (PFASs) including perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) exist in different isomeric (branched and linear) forms in the natural environment. The isomeric distribution of PFASs in the environment and source waters is largely controlled by the source of contamination and varying physicochemical properties imparted by their structural differences. For example, branched isomers of PFOS are relatively more reactive and less sorptive compared to the linear analogue. As a result, the removal of branched and linear PFASs during water treatment can vary, and thus the isomeric distribution in source waters can influence the overall efficiency of the treatment process. In this paper, we highlight the need to consider the isomeric distribution of PFASs in contaminated matrices while designing appropriate remediation strategies. We additionally summarize the known occurrence and variation in the physicochemical properties of PFAS isomers influencing their detection, fate, toxicokinetics, and treatment efficiency.

Use of a horizontal ball mill to remediate per- and polyfluoroalkyl substances in soil

There is a need for destructive technologies for per- and polyfluoroalkyl substances (PFAS) in soil. While planetary ball mill have been shown successful degradation of PFAS, there are issues surrounding scale up (maximum size is typically 0.5 L cylinders). While having lower energy outputs, horizontal ball mills, for which scale up is not a limiting factor, already exist at commercial/industrial sizes from the mining, metallurgic and agricultural industries, which could be re-purposed. This study evaluated the effectiveness of horizontal ball mills in degrading perfluorooctanesulfonate (PFOS), 6:2 fluorotelomer sulfonate (6:2 FTSA), and aqueous film forming foam (AFFF) spiked on nepheline syenite sand. Horizontal ball milling was also applied to two different soil types (sand dominant and clay dominant) collected from a firefighting training area (FFTA). Liquid chromatography tandem mass spectrometry was used to track 21 target PFAS throughout the milling process. High-resolution accurate mass spectrometry was also used to identify the presence and degradation of 19 non-target fluorotelomer substances, including 6:2 fluorotelomer sulfonamido betaine (FtSaB), 7:3 fluorotelomer betaine (FtB), and 6:2 fluorotelomer thioether amido sulfonate (FtTAoS). In the presence of potassium hydroxide (KOH), used as a co-milling reagent, PFOS, 6:2 FTSA, and the non-target fluorotelomer substances in the AFFF were found to undergo upwards of 81%, 97%, and 100% degradation, respectively. Despite the inherent added complexity associated with field soils, better PFAS degradation was observed on the FFTA soils over the spiked NSS, and more specifically, on the FFTA clay over the FFTA sand. These results held through scale-up, going from the 1 L to the 25 L cylinders. The results of this study support further scale-up in preparation for on-site pilot tests.