Breakdown Products from Perfluorinated Alkyl Substances (PFAS) Degradation in a Plasma-Based Water Treatment Process

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.

Fundamental data for modeling electron-induced processes in plasma remediation of perfluoroalkyl substances

Plasma treatment of per- and polyfluoroalkyl substances (PFAS) contaminated water is a potentially energy efficient remediation method. In this treatment, an atmospheric pressure plasma interacts with surface-resident PFAS molecules. Developing a reaction mechanism and modeling of plasma-PFAS interactions requires fundamental data for electron-molecule reactions. In this paper, we present results of electron scattering calculations, potential energy landscapes and their implications for plasma modelling of a dielectric barrier discharge in PFAS contaminated gases, a first step towards modelling of plasma-water-PFAS intereactions. It is found that the plasma degradation of PFAS is dominated by dissociative electron attachment with the importance of other contributing processes varying depending on the molecule. All molecules posses a large number of shape resonances - transient negative ion states - from near-threshold up to ionization threshold. These states lie in the region of the most probable electron energies in the plasma (4-5 eV) and consequently are expected to further enhance the fragmentation dynamics in both dissociative attachment and dissociative excitation.

IMPACT: Innovative (nano)Materials and processes for advanced catalytic technologies to degrade PFOA in water

We hereby report the development of a novel electrochemical method to degrade perfluorooctanoic acid (C7F15COOH, PFOA). At the center of the approach are bimetallic Pd-Ru nano-catalyst materials called IMPACT: Innovative (nano)Materials and Processes for Advanced Catalytic Technologies. IMPACT uses flavonoid-sequestered Pd-Ru, allowing the development of specialized electrodes with tunable properties to sequentially degrade PFOA in wastewater samples into a sustainable byproduct via an indirect electrochemical method. Electron transfers at RuOxHy species stabilize the Pd component of the nano-catalysts, enabling the degradation process via PFOA deprotonation, chain shortening, decarboxylation, hydrolysis, fluoride elimination, and CF2 flake-off mechanism. IMPACT enabled the observation of redox peaks at -0.26 V and 0.56 V for the first time, with accompanying reduction peaks at -0.5V and 0.29 V, respectively. These redox peaks, which correlated with the concentrations of PFOA (20, 50, 100, 200, and 400. mg L-1), were verified and confirmed using electrochemical simulations. Control experiments did not show degradation of PFOA in the absence of Pd-Ru nano-catalyst. The degradation in wastewater was obtained within 3 h with an efficiency of 98.5%. The electrochemical degradation products of PFOA were identified using High-resolution desalting paper spray mass spectrometry (DPS-MS) and collision-induced dissociation (CID) analysis. The results yielded C2F5COOH, C3F7COOH, and C6F13OH with dissociation losses of CF2O or CO2. IMPACT introduces a novel nano-catalyst with high efficiency and a reliable capability that defluorinates strong C-F bonds that are components of recalcitrant organics in myriad environmental matrices.

Structural investigation of the complexation between vitamin B12 and per- and polyfluoroalkyl substances: Insights into degradation using density functional theory

Environmental remediation of per- and polyfluoroalkyl substances (PFAS) has become a significant research topic in recent years due to the fact that these materials are omnipresent, resistant to degradation and thus environmentally persistent. Unfortunately, they have also been shown to cause health concerns. PFAS are widely used in industrial applications and consumer products. Vitamin B12 (B12) has been identified as being catalytically active towards a variety of halogenated compounds such as PFAS. It has also been shown to be effective when using sulfide as a reducing agent for B12. This is promising as sulfide is readily available in the environment. However, there are many unknowns with respect to PFAS interactions with B12. These include the reaction mechanism and B12's specificity for PFAS with certain functionalization(s). In order to understand the specificity of B12 towards branched PFAS, we examined the atomistic interactions between B12 and eight different PFAS molecules using Density Functional Theory (B3LYP/cc-pVDZ). The PFAS test set included linear PFAS and their branched analogs, carboxylic acid and sulfonic acid headgroups, and aromatic and non-aromatic cyclic structures. Conformational analyses were carried out to determine the lowest energy configurations. This analysis showed that small chain PFAS such as perfluorobutanoic acid interact with the cobalt center of B12. Bulkier PFAS prefer to interact with the amine and carbonyl groups on the sidechains of the B12 ring system. Furthermore, computed complexation energies determined that, in general, branched PFAS (e.g. perfluoro-5-methylheptane sulfonic acid) interact more strongly than linear molecules (e.g. perfluorooctanesulfonic acid). Our results indicate that it may be possible to alter the interactions between B12 and PFAS by synthetically modifying the sidechains of the ring structure.

Dielectric Barrier Discharge (DBD) enhanced Fenton process for landfill leachate nanofiltration: Organic matter removal and membrane fouling alleviation

This study investigated a sustainable approach through dielectric barrier discharge (DBD) enhanced Fenton technology coupling nanofiltration (NF) process for landfill leachate treatment. The DBD/Fe(II)/H2O2 system exhibited significant synergistic effects, removing 55.07 % of TOC and 53.79 % of UV254 within 60 min, respectively. Additionally, the DBD/Fe(II)/H2O2 system demonstrated exceptional performance in removing fluorescent substances and large molecular organic compounds, thereby reducing the formation of cake layer on the nanofiltration membrane. Moreover, membrane flux increased by 2.34 times, with reversible and irreversible resistances decreasing by 75.79 % and 81.55 %, respectively. Quenching experiments revealed ·OH as the primary active species for perfluorooctanoic acid (PFOA) degradation in the DBD/Fe(II)/H2O2 process. The degradation pathway of PFOA was also elucidated via capillary electrophoresis-quadrupole time-of-flight mass spectrometry analysis. Correlation analysis indicated that TOC and EEM were the primary fouling factors. Lastly, through an assessment of energy consumption, economic costs, and carbon dioxide emissions, the advantages and practical application potential of the DBD/Fe(II)/H2O2 system were demonstrated. In summary, the DBD/Fe(II)/H2O2 system emerges as a feasible strategy for NF pretreatment, holding immense potential for treating landfill leachate.

PFOA-contaminated soil remediation: a comprehensive review

Soil and groundwater contamination has been raised as a concern due to the capability of posing a risk to human health and ecology, especially in facing highly toxic and emerging pollutants. Because of the prevalent usage of perfluorooctanoic acid (PFOA), in industrial and production processes, and subsequently the extent of sites contaminated with these pollutants, cleaning up PFOA polluted sites is paramount. This research provides a review of remediation approaches that have been used, and nine remediation techniques were reviewed under physical, chemical, and biological approaches categorization. As the pollutant specifications, environmental implications, and adverse ecological effects of remediation procedures should be considered in the analysis and evaluation of remediation approaches, unlike previous research that considered a couple of PFAS pollutants and generally dealt with technical issues, in this study, the benefits, drawbacks, and possible environmental and ecological adverse effects of PFOA-contaminated site remediation also were discussed. In the end, in addition to providing sufficient and applicable understanding by comprehensively considering all aspects and field-scale challenges and obstacles, knowledge gaps have been found and discussed.

Natural mineral and industrial solid waste-based adsorbent for perfluorooctanoic acid and perfluorooctane sulfonate removal from surface water: Advances and prospects

PER: and polyfluorinated alkyl substances, especially perfluorooctanoic acid and perfluorooctane sulfonic acid (PFOX), have attracted considerable attention lately because of their widespread occurrence in aquatic environment and potential biological toxicity to animals and human beings. The development of economical, efficient, and engineerable adsorbents for removing PFOX in water has become one of the research focuses. This review summarized the recent progress on natural mineral and industrial solid based adsorbent (NM&ISW-A) and removal mechanisms concerning PFOX onto NM&ISW-A, as well as proposed the current challenges and future perspectives of using NM&ISW-A for PFOX removal in water. Kaolinite and montmorillonite are usually used as model clay minerals for PFOX removal, and have been proved to adsorb PFOX by ligand exchange and electrostatic attraction. Fe-based minerals, such as goethite, magnetite, and hematite, have better PFOX adsorption capacity than clay minerals. The adsorbent prepared from industrial solid waste by high temperature roasting has great potential application prospects. Fabricating nanomaterials, amination modification, surfactant modification, fluorination modification, developing versatile composites, and designing special porous structure are beneficial to improve the adsorption performance of PFOX onto NM&ISW-A by enhancing the specific surface area, positive charge, and hydrophobicity. Electrostatic interaction, hydrophobic interaction, hydrogen bond, ligand and ion exchange, and self-aggregation (formation of micelle or hemimicelle) are the main adsorption mechanisms of PFOX by NM&ISW-A. Among them, electrostatic and hydrophobic interactions play a considerable role in the removal of PFOX by NM&ISW-A. Therefore, NM&ISW-A with electrostatic functionalities and considerable hydrophobic segments enables rapid, efficient, and high-capacity removal of PFOX. The future directions of NM&ISW-A for PFOX removal include the preparation and regeneration of engineerable NM&ISW-A, the development of coupling technology for PFOX removal based on NM&ISW-A, the in-depth research on adsorption mechanism of PFOX by NM&ISW-A, as well as the development of NM&ISW-A for PFOX alternatives removal. This review paper would be helpful the comprehensive understanding of NM&ISW-A potential for PFOX removal and the PFOX removal mechanisms, and identifies the gaps for future research and development.

Anthropogenic stressors and the marine environment: From sources and impacts to solutions and mitigation

Human-released contaminants are often poorly understood wholistically in marine ecosystems. This review examines the sources, pathways, impacts on marine animals, and mitigation strategies of five pollutants (plastics, per- and polyfluoroalkyl substances, bisphenol compounds, ethynylestradiol, and petroleum hydrocarbons). Both abiotic and biotic mechanisms contribute to all five contaminants' movement. These pollutants cause short- and long-term effects on many biological processes genetically, molecularly, neurologically, physiologically, reproductively, and developmentally. We explore the extension of adverse outcome pathways to ecosystem effects by considering known inter-generational and trophic relations resulting in large-scale direct and indirect impacts. In doing so, we develop an understanding of their roles as environmental stressors in marine environments for targeted mitigation and future work. Ecosystems are interconnected and so international collaboration, standards, measures preceding mass production, and citizen involvement are required to protect and conserve marine life.

Rapid degradation of perfluorooctane sulfonic acid (PFOS) and perfluorononanoic acid (PFNA) through bimetallic catalyst of Fe2O3/Mn2O3 and unravelling the effect of support SiO2

Perfluoroalkyl substances (PFAS) are emerging contaminants present in various water sources. Their bioaccumulation and potential toxicity necessitate proper treatment to ensure safe water quality. Although iron-based monometallic photocatalysts have been reported to exhibit rapid and efficient PFAS degradation, the impact of bimetallic photocatalysts is unknown. In addition, the mechanistic effects of utilizing a support are poorly understood and solely based on physicochemical properties. This study investigates the efficacy of bimetallic photocatalysts (Fe2O3/Mn2O3) in inducing the photo-Fenton reaction for the degradation of perfluorooctane sulfonate (PFOS) and perfluorononanoic acid (PFNA) under various conditions. The rapid removal of both PFAS was observed within 10 min, with a maximum efficiency exceeding 97 % for PFOS under UV exposure, aided by the photocatalytic activation (photo-Fenton) of the oxidant (H2O2). Contrary to expectations, the use of the SiO2 support material did not significantly improve the removal efficiency. The efficacy of PFNA decreased despite SiO2 providing larger surface areas for Fe2O3/Mn2O3 loading. Further analysis revealed that the adsorption of PFAS onto the catalyst surfaces owing to electrostatic interactions contributed to the removal efficiency, where the degradation efficacy was worse than that of the catalyst with SiO2. This is because adsorption hindered the effective contact of H2O2 with catalytic reaction sites, thereby impeding the generation of hydroxyl (·OH) radicals. This study indicates the importance of considering chemical properties, including surface charge, in catalyst design to ensure effective degradation, focusing on physicochemical properties, such as surface area might overlook crucial factors.

Ultrasonic degradation of per-and polyfluoroalkyl substances (PFAS), aqueous film-forming foam (AFFF) and foam fractionate (FF)

The ubiquitousness of per- and polyfluoroalkyl substances (PFAS) is a big concern and PFAS remediation is urgently needed such as via degradation. While previous studies have explored ultrasonic degradation of PFAS, work evaluating the operational parameters is rare, especially concerning real wastes such as aqueous film-forming foam (AFFF) and foam fractionate (FF). This study investigates the key operational parameters affecting the degradation efficiency of PFAS, encompassing ultrasonication frequency (580-1144 kHz), power intensity (125-187.5 W), initial concentration (0.08-40 ppm), treatment duration (0.5-3 h), sample volume (100-500 mL), and PFAS structure (perfluorooctanoic acid or PFOA; perfluorooctane sulfonate or PFOS; 6:2 fluorotelomer sulfonate or 6:2 FTS). The defluorination kinetics is different from the removal/degradation kinetics due to the generation of degradation intermediates, suggesting the complex degradation mechanism, which should be evaluated to close the mass balance effectively. Notably, the optimised ultrasonic system achieves ∼125%/∼115% defluorination in AFFF/FF example wastes (compared to ∼65%/∼97% removal) despite their complex composition and the involvement of total oxidizable precursor (TOP) assay. In the meantime, a few new PFAS are detected in the post-treatments, including perfluorohexane sulfonic acid (PFHxS) and 10:2 fluorotelomer sulfonate (10:2 FTS) in the AFFF, and perfluorooctane sulfonamide (FOSA) and 8:2 fluorotelomer sulfonate (8:2 FTS) in the FF, again suggesting the complex degradation mechanism. Overall, ultrasonication is effective to degrade PFAS real example wastes, advancing its potential for scale-up applications.

Enhancing the Thermal Mineralization of Perfluorooctanesulfonate on Granular Activated Carbon Using Alkali and Alkaline-Earth Metal Additives

Thermal treatment has emerged as a promising approach for either the end-of-life treatment or regeneration of granular activated carbon (GAC) contaminated with per- and polyfluoroalkyl substances (PFAS). However, its effectiveness has been limited by the requirement for high temperatures, the generation of products of incomplete destruction, and the necessity to scrub HF in the flue gas. This study investigates the use of common alkali and alkaline-earth metal additives to enhance the mineralization of perfluorooctanesulfonate (PFOS) adsorbed onto GAC. When treated at 800 °C without an additive, only 49% of PFOS was mineralized to HF. All additives tested demonstrated improved mineralization, and Ca(OH)2 had the best performance, achieving a mineralization efficiency of 98% in air or N2. Its ability to increase the reaction rate and shift the byproduct selectivity suggests that its role may be catalytic. Moreover, additives reduced HF in the flue gas by instead reacting with the additive to form inorganic fluorine (e.g., CaF2) in the starting waste material. A hypothesized reaction mechanism is proposed that involves the electron transfer from O2- defect sites of CaO to intermediates formed during the thermal decomposition of PFOS. These findings advocate for the use of additives in the thermal treatment of GAC for disposal or reuse, with the potential to reduce operating costs and mitigate the environmental impact associated with incinerating PFAS-laden wastes.

Efficient Mineralization of Lithium Bis(pentafluoroethanesulfonyl)imide and Related Electrolyte Fluorochemicals Using Superheated Water

Lithium bis(pentafluoroethanesulfonyl)imide, Li[N(SO2C2F5)2], a typical fluorochemical aimed at better electrochemical performance of battery electrolytes, in superheated water was studied for its waste treatment. When Li[N(SO2C2F5)2] was reacted in pure superheated water at 300 °C, little F- ions were produced. In contrast, complete mineralization of the fluorine, sulfur, and nitrogen atoms in Li[N(SO2C2F5)2] was achieved when the reaction was performed in the presence of KMnO4. Specifically, when Li[N(SO2C2F5)2] was treated for 18 h with 158 mM of KMnO4, the F- and SO42- yields were 101 and 99%, respectively, and the sum of the NO3- and NO2- yields was 101%. In the gas phase, trace CO2 was detected and no CHF3, which has high global warming potential, was formed. Furthermore, the fluorine, sulfur, and nitrogen atoms in the analogues K[N(SO2C4F9)2] and K[N(SO2CF2)2CF2] also underwent complete mineralization using the same approach.

Hydrothermal Destruction and Defluorination of Trifluoroacetic Acid (TFA)

Per- and polyfluoroalkyl substances (PFAS) have received increased attention due to their environmental prevalence and threat to public health. Trifluoroacetic acid (TFA) is an ultrashort-chain PFAS and the simplest perfluorocarboxylic acid (PFCA). While the US EPA does not currently regulate TFA, its chemical similarity to other PFCAs and its simple molecular structure make it a suitable model compound for studying the transformation of PFAS. We show that hydrothermal processing in compressed liquid water transforms TFA at relatively mild conditions (T = 150-250 °C, P < 30 MPa), initially yielding gaseous products, such as CHF3 and CO2, that naturally aspirate from the solution. Alkali amendment (e.g., NaOH) promotes the mineralization of CHF3, yielding dissolved fluoride, formate, and carbonate species as final products. Fluorine and carbon balances are closed using Raman spectroscopy and fluoride ion selective electrode measurements for experiments performed at alkaline conditions, where gas yields are negligible. Qualitative FTIR gas analysis allows for establishing the transformation pathways; however, the F-balance could not be quantitatively closed for experiments without NaOH amendment. The kinetics of TFA transformation under hydrothermal conditions are measured, showing little to no dependency on NaOH concentration, indicating that the thermal decarboxylation is a rate-limiting step. A proposed TFA transformation mechanism motivates additional work to generalize the hydrothermal reaction pathways to other PFCAs.

Influence of water chemistry and operating parameters on PFOS/PFOA removal using rGO-nZVI nanohybrid

Graphene and zero-valent-iron based nanohybrid (rGO-nZVI NH) with oxidant H2O2 can remove perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) through adsorption-degradation in a controlled aquatic environment. In this study, we evaluated how and to what extent different environmental and operational parameters, such as initial PFAS concentration, H2O2 dose, pH, ionic strength, and natural organic matter (NOM), influenced the removal of PFOS and PFOA by rGO-nZVI. With the increase in initial PFAS concentration (from 0.4 to 50 ppm), pH (3 to 9), ionic strength (0 to 100 mM), and NOM (0 to 10 ppm), PFOS removal reduced by 20%, 30%, 2%, and 6%, respectively, while PFOA removal reduced by 54%, 76%, 11%, and 33% respectively. In contrast, PFOS and PFOA removal increased by 10% and 41%, respectively, with the increase in H2O2 (from 0 to 1 mM). Overall, the effect of changes in environmental and operational parameters was more pronounced for PFOA than PFOS. Mechanistically, •OH radical generation and availability showed a profound effect on PFOA removal. Also, the electrostatic interaction between rGO-nZVI NH and deprotonated PFAS compounds was another key factor for removal. Most importantly, our study confirms that rGO-nZVI in the presence of H2O2 can degrade both PFOS and PFOA to some extent by identifying the important by-products such as acetate, formate, and fluoride.

Controllable chemical redox reactions to couple microbial degradation for organic contaminated sites remediation: A review

Global environmental concern over organic contaminated sites has been progressively conspicuous during the process of urbanization and industrial restructuring. While traditional physical or chemical remediation technologies may significantly destroy the soil structure and function, coupling moderate chemical degradation with microbial remediation becomes a potential way for the green, economic, and efficient remediation of contaminated sites. Hence, this work systematically elucidates why and how to couple chemical technology with microbial remediation, mainly focused on the controllable redox reactions of organic contaminants. The rational design of materials structure, selective generation of reactive oxygen species, and estimation of degradation pathway are described for chemical oxidation. Meanwhile, current progress on efficient and selective reductions of organic contaminants (i.e., dechlorination, defluorination, -NO2 reduction) is introduced. Combined with the microbial remediation of contaminated sites, several consideration factors of how to couple chemical and microbial remediation are proposed based on both fundamental and practical points of view. This review will advance the understanding and development of chemical-microbial coupled remediation for organic contaminated sites.

Dielectric barrier discharge plasma promotes disinfection-residual-bacteria inactivation via electric field and reactive species

Traditional disinfection processes face significant challenges such as health and ecological risks associated with disinfection-residual-bacteria due to their single mechanism of action. Development of new disinfection processes with composite mechanisms is therefore urgently needed. In this study, we employed liquid ground-electrode dielectric barrier discharge (lgDBD) to achieve synergistic sterilization through electric field electroporation and reactive species oxidation. At a voltage of 12 kV, Pseudomonas fluorescens (ultraviolet and ozone-resistant) and Bacillus subtilis (chlorine-resistant) were completely inactivated within 8 and 6 min, respectively, surpassing a 7.0-log reduction. The lgDBD process showed good disinfection performance across a wide range of pH values and different practical water samples. Staining experiments suggest that cellular membrane damage contributes to this inactivation. In addition, we used a two-dimensional parallel streamer solver with kinetics code to fashion a representative model of the basic discharge unit, and discovered the presence of a persistent electric field during the discharge process with a peak value of 2.86 × 106 V/m. Plasma discharge generates excited state species such as O(1D) and N2(C3Πu), and further forms reactive oxygen and nitrogen species at the gas-liquid interface. The physical process, which is driven by electric field-induced cell membrane electroporation, synergizes with the bactericidal effects of reactive oxygen and nitrogen species to provide effective disinfection. Adopting the lgDBD process enhances sterilization efficiency and adaptability, underscoring its potential to revolutionize physicochemical synergistic disinfection practices.

Sequential C-F Bond Transformation of the Difluoromethylene Unit in Perfluoroalkyl Groups: A Combination of Fine-Tuned Phenothiazine Photoredox Catalyst and Lewis Acid

A sequential process via photoredox catalysis and Lewis acid mediation for C-F bond transformation of the CF2 unit in perfluoroalkyl groups has been achieved to transform perfluoroalkylarenes into complex fluoroalkylated compounds. A phenothiazine-based photocatalyst promotes the defluoroaminoxylation of perfluoroalkylarenes with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) under visible light irradiation, affording the corresponding aminoxylated products. These products undergo a further defluorinative transformation with various organosilicon reagents mediated by AlCl3 to provide highly functionalized perfluoroalkyl alcohols. Our novel phenothiazine catalyst works efficiently in the defluoroaminoxylation. Transient absorption spectroscopy revealed that the catalyst regeneration step is crucial for the photocatalytic aminoxylation.

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.

pKa as a Predictive Descriptor for Electrochemical Anion Adsorption

The adsorption of anions onto metal surfaces is important in many applications including effective (electro)catalyst design, metal surface modification, and contaminant removal in wastewater treatment. In electrocatalysis, anions can be both reactive intermediates or site-blocking spectators, where their adsorption strength therefore dictates the rate of reaction. In this work, we have measured the adsorption energy of a series of carboxylic acids on a Pt (111) single-crystal electrode surface from aqueous solution. We find that the adsorption strength of the carboxylate anion is linearly correlated with its acid-dissociation constant (pKa) and therefore the heterolytic O-H bond dissociation strength in solution. Using density functional theory modeling, we split the anion adsorption energy into a sum of the adsorption energy and electron affinity of a neutral (carboxyl) radical. Surprisingly, the adsorption energy of the carboxyl radicals are similar and therefore the large difference in electron affinity is what dictates anion adsorption strength; the greater the cost in energy to remove the electron from the anion upon adsorption, the weaker its binding. Therefore, at least within a class of anions with similar structure and surface binding atoms, both electron affinity and acidity are predictive descriptors of adsorption strength.

Fluorinated Propionic Acids Unmasked: Puzzling Fragmentation Phenomena of the Deprotonated Species

Environmental contamination by per- and polyfluorinated substances (PFAS) is an emerging concern for the public. In this study, short-chain PFAS such as deprotonated per- and polyfluorinated propionic acids are investigated using a combination of infrared multiple-photon dissociation (IRMPD) spectroscopy, collision-induced dissociation (CID), and density functional theory calculations. IRMPD and CID proceed via multiple competing pathways: (1) production of fluoroformate (FCO2-) and the associated ethylene derivative, (2) production of HF and the associated carbanion, or (3) loss of CO2 and the associated carbanion. Fluorinated propionic acids with at least one fluorine atom bound to the terminal carbon yield FCO2-, whereas loss of HF is observed in polyfluorinated species with at least one fluorine atom bound to the α-carbon. To explore the reaction pathways of the various fluorinated propionic acids, the nudged elastic band method is employed. The relative energy of the four-membered ring transition state leading to FCO2- dictates which product channel is observed in dissociation.

Defluorination of perfluorooctanoic acid and perfluorooctane sulfonic acid by heterogeneous catalytic system of Fe-Al2O3/O3: Synergistic oxidation effects and defluorination mechanism

In this study, catalytic ozonation by Fe-Al2O3 was used to investigate the defluorination of PFOA and PFOS, assessing the effects of different experimental conditions on the defluorination efficiency of the system. The oxidation mechanism of the Fe-Al2O3/O3 system and the specific degradation and defluorination mechanisms for PFOA and PFOS were determined. Results showed that compared to the single O3 system, the defluorination rates of PFOA and PFOS increased by 2.32- and 5.92-fold using the Fe-Al2O3/O3 system under optimal experimental conditions. Mechanistic analysis indicated that in Fe-Al2O3, the variable valence iron (Fe) and functional groups containing C and O served as important reaction sites during the catalytic process. The co-existence of 1O2, OH, O2- and high-valence Fe(IV) constituted a synergistic oxidation system consisting of free radicals and non-radicals, promoting the degradation and defluorination of PFOA and PFOS. DFT theoretical calculations and the analysis of intermediate degradation products suggested that the degradation pathways of PFOA and PFOS involved Kolbe decarboxylation, desulfonation, alcoholization and intramolecular cyclization reactions. The degradation and defluorination pathways of PFOA and PFOS consisted of the stepwise removal of -CF2-, with PFOS exhibiting a higher defluorination rate than PFOA due to its susceptibility to electrophilic attack. This study provides a theoretical basis for the development of heterogeneous catalytic ozonation systems for PFOA and PFOS treatment.

Degradation and mechanism of PFOA by peroxymonosulfate activated by nitrogen-doped carbon foam-anchored nZVI in aqueous solutions

Perfluorooctanoic acid (PFOA) is an emerging pollutant that is non-biodegradable and presents severe environmental and human health risks. In this study, we present an effective and mild approach for PFOA degradation that involves the use of nitrogen-doped carbon foam anchored with nanoscale zero-valent iron (nZVI@NCF) to activate low concentration peroxymonosulfate (PMS) for the treatment. The nZVI@NCF/PMS system efficiently removed 84.4% of PFOA (2.4 μM). The active sites of nZVI@NCF including Fe0 (110) and graphitic nitrogen played crucial roles in the degradation. Electrochemical analyses and density functional theory calculations revealed that nZVI@NCF acted as an electronic donor, transferring electrons to both PMS and PFOA during the reaction. By further analyzing the electron paramagnetic resonance and byproducts, it was determined that electron transfer and singlet oxygen were responsible for PFOA degradation. Three degradation pathways involving decarboxylation and surface reduction of PFOA in the nZVI@NCF/PMS system were determined. Finding from this study indicate that nZVI@NCF/PMS systems are effective in degrading PFOA and thus present a promising persulfate-advanced oxidation process technology for PFAS treatment.

The use of a fluorine mass balance to demonstrate the mineralization of PFAS by high frequency and high power ultrasound

High-frequency ultrasound (sonolysis) has been shown as a practical approach for mineralizing PFAS in highly concentrated PFAS waste. However, a fluorine mass balance approach showing complete mineralization for ultrasound treatment has not been elucidated. The impact of ultrasonic power density (W/L) and the presence of co-occurring PFAS on the degradation of individual PFAS are not well understood. In this research, the performance of a 10L sonochemical reactor was assessed for treating synthetic high-concentration PFAS waste with carboxylic and sulfonic perfluoroalkyl surfactants ranging in chain length from four to eight carbons at three different initial concentrations: 6, 55, 183 μM. The mass balance for fluorine was performed using three analytical techniques: triple quadrupole liquid chromatography-mass spectrometry, a fluoride ion selective electrode, and 19F nuclear magnetic resonance. The test results showed near complete mineralization of PFAS in the waste without the formation of intermediate fluorinated by-products. The PFAS mineralization efficiency of the sonolysis treatment at two different power densities for similar initial concentrations were almost identical; the G value at 145 W/L was 9.7*10-3 g/kWh, whereas the G value at 90 W/L was 9.3*10-3 g/kWh. The results of this study highlight the implications for the scalability of the sonolytic process to treat high-concentration PFAS waste.

Characterizing Volatile Emissions and Combustion Byproducts from Aqueous Film-Forming Foams Using Online Chemical Ionization Mass Spectrometry

Aqueous film-forming foams (AFFFs) are used in firefighting applications and often contain per- and polyfluoroalkyl substances (PFAS), which can detrimentally impact environmental and biological health. Incineration is a potential disposal method for AFFFs, which may produce secondary PFAS and other air pollutants. We used online chemical ionization mass spectrometry (CIMS) to measure volatile PFAS emissions from incinerating AFFF concentrate solutions. We quantified perfluorinated carboxylic acids (PFCAs) during the incineration of legacy and contemporary AFFFs. These included trifluoroacetic acid, which reached mg m-3 quantities in the incinerator exhaust. These PFCAs likely arose as products of incomplete combustion of AFFF fluorosurfactants with lower peak furnace temperatures yielding higher PFCA concentrations. We also detected other short-chain PFAS, and other novel chemical products in AFFF combustion emissions. The volatile headspace above AFFF solutions contained larger (C ≥ 8), less oxidized PFAS detected by CIMS. We identified neutral PFAS resembling fluorotelomer surfactants (e.g., fluorotelomer sulfonamide alkylbetaines and fluorotelomer thioether amido sulfonates) and fluorotelomer alcohols in contemporary AFFF headspaces. Directly comparing the distinct chemical spaces of AFFF volatile headspace and combustion byproducts as measured by CIMS provides insight toward the chemistry of PFAS during thermal treatment of AFFFs.

Enhanced electrochemical degradation of perfluorooctanoic acid by ligand-bridged PtII at Pt anodes

A new mechanism for the electro-oxidation (EO) degradation of perfluorooctanoic acid (PFOA) by Pt anode was reported. Using bridge-based ligand anions (SCN-, Cl- and N3-) as electrolytes, the degradation effect of PFOA by Pt-EO system was significant. Characterization of the Pt anode, the detection and addition of dissolved platinum ions, and the comparison of Pt with DSA anodes determined that the Pt- ligand complexes resulting from the specific binding of anodically dissolved PtII with ligand ions and C7F15COO- ((C7F15-COO)PtII-L3, L = SCN-, Cl- and N3-) on the electrode surface played a decisive role in the degradation of PFOA. Density functional theory (DFT) calculations showed that inside (C7F15-COO)PtII-L3 complexes, the electron density of the perfluorocarbon chain (including the F atom) compensated toward the carboxyl group and electrons in the PFOA ion transferred to the PtII-Cl3. Moreover, the (C7F15-COO)PtII-Cl3, as a whole, was calculated to migrate electrons toward the Pt anode, leading to the formation of PFOA radical (C7F15-COO•). Finally, with the detection of a series of short chain homologues, the CF2-unzipping degradation pathway of PFOA was proposed. The newly developed Pt-EO system is not affected by water quality conditions and can directly degrade alcohol eluent of PFOA, which has great potential for treating industrial wastewater contaminated with PFOA.

Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) in water and edible fish species of Karun River, Ahvaz, Iran: spatial distribution, human health, and ecological risk assessment

Per- and polyfluoroalkyl substances (PFASs) such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) are environmental contaminants with unfavorable impacts on human health and nature. This study aimed to determine the PFOA and PFOS concentration in water and fish samples from Karun, the largest river in Iran. According to the results, the PFOA and PFOS in water samples were 5.81-69.26 ng/L and not detected (n.d.)-35.12 ng/L, respectively. The dry season displayed higher concentrations in water samples than in the wet season. The maximum PFOS concentration measured was related to Barbus barbules sp. (27.89 ng/g). The human health risk assessment indicated minor risks (hazard ratio, HR < 1) from PFOA and PFOS through consuming contaminated drinking water and fish. Only HR value of PFOS in downstream area exceeded slightly 1.0, indicating potential health risk due to consumption of the river fish. Considering the average PFASs concentration, the risk quotients (RQs) showed low ecological risk.

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.

Aqueous and solid phase photocatalytic degradation of perfluorooctane sulfonate by carbon- and Fe-modified bismuth oxychloride

In this study, we prepared and tested a carbon-modified, Fe-loaded bismuth oxychloride (Fe-BiOCl/CS) photocatalyst for photocatalytic degradation of perfluorooctane sulfonate (PFOS). Structural analyses revealed a (110) facet-dominated sheet-type BiOCl crystal structure with uniformly distributed Fe and confirmed carbon modification of the photocatalyst. The presence of d-glucose facilitated the growth control of BiOCl particles and enhanced the adsorption of PFOS via added hydrophobic interaction. Adsorption kinetic and equilibrium tests showed rapid uptake rates of PFOS and high adsorption capacity with a Langmuir Qmax of 1.51 mg/g. When used for directly treating PFOS in solution, Fe-BiOCl/CS was able to mineralize or defluorinate 83% of PFOS (C0 = 100 μgL-1) under UV (254 nm, intensity = 21 mW cm-2) in 4 h; and when tested in a two-step mode, i.e., batch adsorption and subsequent photodegradation, Fe-BiOCl/CS mineralized 65.34% of PFOS that was pre-concentrated in the solid phase under otherwise identical conditions; while the total degradation percentages of PFOS were 83.48% and 80.50%, respectively, for the two experimental modes. The photoactivated electrons and/or hydrated electrons and superoxide radicals primarily initiated the desulfonation of PFOS followed by decarboxylation and defluorination, through a stepwise chain-subsiding mechanism. The elevated photocatalytic activity can be attributed to the effective separation of e-/h+ pairs facilitated by the (110) interlayer electrostatic field, Fe doping, and the presence of oxygen vacancies. This work reveals the potential of carbon-modified and Fe-co-catalyzed BiOCl for concentrating and degrading PFOS and possibly other persistent organic pollutants.

Degradation of Pesticide Residues in Water, Soil, and Food Products via Cold Plasma Technology

Water, soil, and food products contain pesticide residues. These residues result from excessive pesticides use, motivated by the fact that agricultural productivity can be increased by the use of these pesticides. The accumulation of these residues in the body can cause health problems, leading to food safety concerns. Cold plasma technology has been successfully employed in various applications, such as seed germination, bacterial inactivation, wound disinfection, surface sterilization, and pesticide degradation. In recent years, researchers have increasingly explored the effectiveness of cold plasma technology in the degradation of pesticide residues. Most studies have shown promising outcomes, encouraging further research and scaling-up for commercialization. This review summarizes the use of cold plasma as an emerging technology for pesticide degradation in terms of the plasma system and configuration. It also outlines the key findings in this area. The most frequently adopted plasma systems for each application are identified, and the mechanisms underlying pesticide degradation using cold plasma technology are discussed. The possible factors influencing pesticide degradation efficiency, challenges in research, and future trends are also discussed. This review demonstrates that despite the nascent nature of the technology, the use of cold plasma shows considerable potential in regards to pesticide residue degradation, particularly in food applications.

Inputs, amplification and sinks of perfluoroalkyl substances at coastal Antarctica

The sources, biogeochemical controls and sinks of perfluoroalkyl substances, such as perfluoroalkyl acids (PFAAs), in polar coastal regions are largely unknown. These were evaluated by measuring a large multi-compartment dataset of PFAAs concentrations at coastal Livingston and Deception Islands (maritime Antarctica) during three austral summers. PFAAs were abundant in atmospheric-derived samples (aerosols, rain, snow), consistent with the importance of atmospheric deposition as an input of PFAAs to Antarctica. Such PFAAs deposition was unequivocally demonstrated by the occurrence of PFAAs in small Antarctic lakes. Several lines of evidence supported the relevant amplification of PFAAs concentrations in surface waters driven by snow scavenging of sea-spray aerosol-bound PFAAs followed by snow-melting. For example, vertical profiles showed higher PFAAs concentrations at lower-salinity surface seawaters, and PFAAs concentrations in snow were significantly higher than in seawater. The higher levels of PFAAs at Deception Island than at Livingston Island are consistent with the semi-enclosed nature of the bay. Concentrations of PFOS decreased from 2014 to 2018, consistent with observations in other oceans. The sink of PFAAs due to the biological pump, transfer to the food web, and losses due to sea-spray aerosols alone are unlikely to have driven the decrease in PFOS concentrations. An exploratory assessment of the potential sinks of PFAAs suggests that microbial degradation of perfluoroalkyl sulfonates should be a research priority for the evaluation of PFAAs persistence in the coming decade.

Electric Field-Assisted Nanofiltration for PFOA Removal with Exceptional Flux, Selectivity, and Destruction

Per- and polyfluoroalkyl substances (PFAS) pose significant environmental and human health risks and thus require solutions for their removal and destruction. However, PFAS cannot be destroyed by widely used removal processes like nanofiltration (NF). A few scarcely implemented advanced oxidation processes can degrade PFAS. In this study, we apply an electric field to a membrane system by placing a nanofiltration membrane between reactive electrodes in a crossflow configuration. The performance of perfluorooctanoic acid (PFOA) rejection, water flux, and energy consumption were evaluated. The reactive and robust SnO2-Sb porous anode was created via a sintering and sol-gel process. The characterization and analysis techniques included field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), ion chromatography, mass spectroscopy, porosimeter, and pH meter. The PFOA rejection increased from 45% (0 V) to 97% (30 V) when the electric field and filtration were in the same direction, while rejection capabilities worsened in opposite directions. With saline solutions (1 mM Na2SO4) present, the induced electro-oxidation process could effectively mineralize PFOA, although this led to unstable removal and water fluxes. The design achieved an exceptional performance in the nonsaline feed of 97% PFOA rejection and water flux of 68.4 L/m2 hr while requiring only 7.31 × 10-5 kWh/m3/order of electrical energy. The approach's success is attributed to the proximity of the electrodes and membrane, which causes a stronger electric field, weakened concentration polarization, and reduced mass transfer distances of PFOA near the membrane. The proposed electric field-assisted nanofiltration design provides a practical membrane separation method for PFAS removal from water.

Hydrothermal Alkaline Treatment (HALT) of Foam Fractionation Concentrate Derived from PFAS-Contaminated Groundwater

While foam fractionation (FF) process has emerged as a promising technology for removal of per- and polyfluoroalkyl substances (PFASs) from contaminated groundwater, management of the resulting foam concentrates with elevated concentrations of PFASs (e.g., >1 g/L) remains a challenge. Here, we applied hydrothermal alkaline treatment (HALT) to two foam concentrates derived from FF field demonstration projects that treated aqueous film-forming foam (AFFF)-impacted groundwater. Results showed >90% degradation and defluorination within 90 min of treatment (350 °C, 1 M NaOH) of all 62 PFASs (including cations, anions, and zwitterions) identified in foam concentrates. Observed rate constants for degradation of individual perfluoroalkyl sulfonates (PFSAs, CnF2n+1-SO3-), the most recalcitrant class of PFASs, in both foam concentrates were similar to values measured previously in other aqueous matrices, indicating that elevated initial PFAS concentrations (e.g., PFHxSinit = 0.55 g/L), dissolved organic carbon (DOC; up to 4.5 g/L), and salt levels (e.g., up to 325 mg/L chloride) do not significantly affect PFAS reaction kinetics. DOC was partially mineralized by treatment, but a fraction (∼15%) was recalcitrant. Spectroscopic characterization revealed molecular features of the HALT-recalcitrant DOC fraction, and nontarget high-resolution mass spectrometry tentatively identified 129 nonfluorinated HALT-recalcitrant molecules. Analysis of process energy requirements shows that treating PFAS-contaminated foam concentrates with HALT would add minimally (<5%) to the overall energy requirements of an integrated FF-HALT treatment train.

Products, reactive species and mechanisms of PFOA degradation in a self-pulsing discharge (SPD) plasma reactor

Non-thermal plasma is a promising tool for novel technologies to treat water contaminated by recalcitrant pollutants. We report here on products, reactive species and mechanisms of the efficient degradation of perfluorooctanoic acid (PFOA) achieved with a self-pulsing discharge developed previously in our lab. Air or argon were used as plasma feed gas, ultrapure or tap water as aqueous medium. Identified organic intermediate products arise from chain-shortening and defluorination reactions, the latter achieving not only C-F to C-H exchange (hydro-de-fluorination), as reported in the literature, but also C-F to C-OH exchange (hydroxy-de-fluorination). In contrast with chain-shortening, yielding lower homologues of PFOA via selective cleavage of the C-C bond at the carboxylate group, defluorination occurs at various sites of the alkyl chain giving mixtures of different isomeric products. Plasma generated reactive species were investigated under all experimental conditions tested, using specific chemical probes and optical emission spectroscopy. Cross-analysis of the results revealed a striking direct correlation of energy efficiency for PFOA degradation and for production of plasma electrons. In contrast, no correlation was observed for emission bands of either Ar+ or OH radical. These results indicate a prevalent role of plasma electrons in initiating PFOA degradation using self-pulsing discharge plasma above the liquid.

Y-mediated optimization of 3DG-PbO2 anode for electrochemical degradation of PFOS

In our previous study, the three-dimensional graphene-modified PbO2 (3DG-PbO2) anode was prepared for the effective degradation of perfluorooctanesulfonat (PFOS) by the electrochemical oxidation process. However, the mineralization efficiency of PFOS at the 3DG-PbO2 anode still needs to be further improved due to the recalcitrance of PFOS. Thus, in this study, the yttrium (Y) was doped into the 3DG-PbO2 film to further improve the electrochemical activity of the PbO2 anode. To optimize the doping amount of Y, three Y and 3DG codoped PbO2 anodes were fabricated with different Y3+ concentrations of 5, 15, and 30 mM in the electroplating solution, which were named Y/3DG-PbO2-5, Y/3DG-PbO2-15 and Y/3DG-PbO2-30, respectively. The results of morphological, structural, and electrochemical characterization revealed that doping Y into the 3DG-PbO2 anode further refined the β-PbO2 crystals, increased the oxygen evolution overpotential and active sites, and reduced the electron transfer resistance, resulting in a superior electrocatalytic activity. Among all the prepared anodes, the Y/3DG-PbO2-15 anode exhibited the best activity for electrochemical oxidation of PFOS. After 120 min of electrolysis, the TOC removal efficiency was 80.89% with Y/3DG-PbO2-15 anode, greatly higher than 69.13% with 3DG-PbO2 anode. In addition, the effect of operating parameters on PFOS removal was analyzed by response surface, and the obtained optimum values of current density, initial PFOS concentration, pH, and Na2SO4 concentration were 50 mA/cm2, 12.21 mg/L, 5.39, and 0.01 M, respectively. Under the optimal conditions, the PFOS removal efficiency reached up to 97.16% after 40 min of electrolysis. The results of the present study confirmed that the Y/3DG-PbO2 was a promising anode for electrocatalytic oxidation of persistent organic pollutants.

Deeper Defluorination and Mineralization of a Novel PFECA (C7 HFPO-TA) in Vacuum UV/Sulfite: Unique Mechanism of H/OCF3 Exchange

C7 HFPO-TA is a newly identified alternative to PFOA, which possesses a unique structure fragment (CF3O-CF(CF3)-). In this study, we evaluated the chemical reactivity of C7 HFPO-TA in advanced oxidation and reduction processes for the first time, which revealed a series of unexpected transformation mechanisms. The results showed that reductive degradation based on hydrated electrons (eaq-) was more feasible for the degradation of C7 HFPO-TA. For oxidative degradation, the branched -CF3 at the α-position carbon posed as the spatial hindrance, shielding the attack of SO4•- to -COO-. The synergistic effects of HO•/eaq- and direct photolysis led to deeper defluorination and mineralization of C7 HFPO-TA in the vacuum UV/sulfite (VUV/SF) process. We identified a unique H/OCF3 exchange that converted the CF3O-CF(CF3)- into H-CF(CF3)- directly, and the SO3•- involved mechanism of C7 HFPO-TA for the first time. We revealed the branched -CF3 connected to the same carbon next to the CF3O- group affected the C-O bond cleavage site, preferring the H/OCF3 exchange pathway. The defluorination of C7 HFPO-TA was compared with PFOA and three PFECAs in the VUV/SF process, which was highly dependent on structures. Degradation kinetics, theoretical calculations, and products' analysis provided an in-depth perspective on the degradation mechanisms and pathways of C7 HFPO-TA.

A systematic study of the competitive sorption of per- and polyfluoroalkyl substances (PFAS) on colloidal activated carbon

Treatment of environmental media contaminated with per- and polyfluoroalkyl substances (PFAS) is crucial to mitigate mounting health risks associated with exposure. Colloidal activated carbon (CAC) has shown promise in treating contaminated soils, but understanding the interaction among PFAS during sorption is necessary for optimal remediation. This study investigated the extent to which PFAS of varying chain lengths and functional groups compete for sorption to CAC. Batch tests were conducted with natural soil and spiked water, using CAC at 0.2% w/w to remove seven PFAS with individual starting concentrations up to 0.05 mmol L-1. PFAS sorption to CAC was evaluated in three systems: a composite mixture of all studied compounds, a binary-solute system, and a single-solute system. The sorption experiments exhibited strong PFAS affinity to CAC, with removal rates between 41% and 100%, and solid/liquid partition coefficients (Kd) between 10 and 104 L kg-1. Differences were noticed among the various spiking mixtures, based on perfluorocarbon chain length, functional group, and the starting PFAS concentrations. Competition effects were detected when PFAS were in a multi-solute system, with an average 10% drop in removal, which can evidently become more relevant at higher concentrations, due to the observed non-linearity of the sorption process. The PFAS most vulnerable to competition effects in multi-solute systems were the short-chain perfluoropentanoic acid (PFPeA) and perfluorobutane sulfonic acid (PFBS), with an up to 25% reduction in removal. In bi-solute systems, perfluorooctane sulfonamide (FOSA) dominated over its ionisable counterparts, i.e. perfluorooctane sulfonic acid (PFOS) and perfluorononanoic acid (PFNA), indicating the importance of hydrophobic effects or layer formation in the sorption process. These results underscore the importance of considering competition in PFAS sorption processes when designing and implementing remediation techniques for PFAS-contaminated media.

Foam fractionation and electrochemical oxidation for the treatment of per- and polyfluoroalkyl substances (PFAS) in environmental water samples

Treatment of waters contaminated by per- and polyfluoroalkyl substances (PFAS) in large volumes remains a challenge to date. Treatment trains comprising separation and destruction technologies are promising to manage PFAS contamination. Foam fractionation (FF) and electrochemical oxidation (EO) are two cost-effective technologies for PFAS separation and destruction, respectively. This work systematically explored the performance of a treatment train of FF followed by EO (FF-EO) for treating PFAS in environmental water samples. For each treatment step, the dependence of the treatment performance on operational factors and other variables were analyzed statistically. The statistical analysis revealed PFAS enrichment and removal depend significantly on PFAS carbon chain length, solution conductivity, and PFAS concentration. Whether FF-EO treatment costs less energy than direct EO without FF mainly relies upon PFAS carbon chain length and TOC content in the sample. Both correlations were found to be linear. For all environmental water samples in this study, FF-EO is more energy-efficient than EO alone.

PFAS-CTAB Complexation and Its Role on the Removal of PFAS from a Lab-Prepared Water and a Reverse Osmosis Reject Water Using a Plasma Reactor

Electrical discharge plasma reactors with argon bubbling can effectively treat long-chain perfluoroalkyl acids (PFAAs) in contaminated water, and the addition of a cationic surfactant cetrimonium bromide (CTAB) is known to enhance the removal of short-chain PFAAs. However, the roles of PFAA chain length, functional group, and water matrix properties on PFAA-CTAB complexation are largely unknown. This work investigated the bulk liquid removal of different PFAAs by CTAB in the absence of plasma. Stepwise addition of CTAB was subsequently used to efficiently treat PFAAs in a lab-prepared water and a reverse osmosis (RO) reject water using an enhanced contact plasma reactor. The results show that CTAB inhibited the bulk liquid removal of long-chain PFAAs in the absence of plasma likely due to the formation of hydrophilic CTAB-PFAA mixed micelles and competition for interfacial access between long-chain PFAAs and CTAB. On the contrary, CTAB enhanced the removal of short- and ultrashort-chain PFAAs by forming hydrophobic complexes. After 6 h of treatment in the plasma reactor with CTAB, PFAAs were 86 to >99% removed from the lab-prepared water and 29 to >99% removed from the RO reject water. This study provides important insights for overcoming mass transfer limitations for PFAA treatment technologies.

Contributions of reactor geometry and ultrasound frequency on the efficieny of sonochemical reactor

An intermediate-scale reactor with 10L capacity and two transducers operating at 700 and 950 kHz frequencies was developed to study the scalability of the sonolytic destruction of Per and Polyfluoroalkyl substance (PFAS). The impact of frequency, height of liquid or power density, and transducer position on reactor performance was evaluated with the potassium iodide (KI) oxidation and calorimetric power. The dual frequency mode of operation has a synergistic effect based on the triiodide concentration, and calorimetric power. The triiodide concentration, and calorimetric power were higher in this mode compared to the combination of both frequencies operating individually. The sonochemical efficiency for an intermediate-scale reactor (10L) was similar that obtained from a bench-scale reactor (2L), showing the scalability of the sonolytic technology. The placement of the transducer at the bottom or side wall of the reactor had no significant impact on the sonochemical reactivity. The superposition of the ultrasonic field from the dual transducer mode (side and bottom) did not produce a synergistic effect compared to the single transducer mode (bottom or side). This can be attributed to a disturbance due to the interaction of ultrasonic fields of two frequencies from each transducer. With the encouraging results scaling up is in progress for site implementation.

Metal-free electro-Fenton degradation of perfluorooctanoic acid with efficient ordered mesoporous carbon catalyst

Heterogeneous electro-Fenton with in situ generated H₂O₂ and ^•OH is a cost-effective method for the degradation of refractory organic pollutants, in which the catalyst is an important factor affecting its degradation performance. Metal-free catalysts can avoid the potential risk of metal dissolution. However, it remains great challenge to develop efficient metal-free catalyst for electro-Fenton. Herein, ordered mesoporous carbon (OMC) was designed as a bifunctional catalyst for efficient H₂O₂ and ^•OH generation in electro-Fenton. The electro-Fenton system showed fast perfluorooctanoic acid (PFOA) degradation with kinetics constant of 1.26 h^-1 and high total organic carbon (TOC) removal efficiency of 84.0 % after 3 h reaction. The ^•OH was the main species responsible for PFOA degradation. Its generation was promoted by the abundant oxygen functional groups such as C-O-C and the nano-confinement effect of mesoporous channels on OMCs. This study indicated that OMC is an efficient catalyst for metal-free electro-Fenton system.

Destruction and defluorination of PFAS matrix in continuous-flow supercritical water oxidation reactor: Effect of operating temperature

Cleanup and disposal of stockpiles and waste streams containing per- and polyfluoroalkyl substances (PFAS) require effective end-of-life destruction/mineralization technologies. Two classes of PFAS, perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs), are commonly found in legacy stockpiles, industrial waste streams, and as environmental pollutants. Continuous flow supercritical water oxidation (SCWO) reactors have been shown to destroy several PFAS and aqueous film-forming foams. However, a direct comparison of the SCWO efficacy for PFSAs and PFCAs has not been reported. We show the effectiveness of continuous flow SCWO treatment for a matrix of model PFCAs and PFSAs as a function of operating temperature. PFSAs appear to be significantly more recalcitrant than PFCAs in the SCWO environment. The SCWO treatment results in a destruction and removal efficiency of 99.999% at a T > 610 °C and at a residence time of ∼30 s. Fluoride recovery lags destruction PFAS at 510 °C and reaches >100% above 610 °C, confirming the formation of liquid and gaseous phase intermediate product during lower temperature oxidation. This paper establishes the threshold for destroying PFAS-containing liquids under SCWO conditions.

Pilot-Scale Thermal Destruction of Per- and Polyfluoroalkyl Substances in a Legacy Aqueous Film Forming Foam

The destruction of per- and polyfluoroalkyl substances (PFAS) is critical to ensure effective remediation of PFAS contaminated matrices. The destruction of hazardous chemicals within incinerators and other thermal treatment processes has historically been determined by calculating the destruction efficiency (DE) or the destruction and removal efficiency (DRE). While high DEs, >99.99%, are deemed acceptable for most hazardous compounds, many PFAS can be converted to other PFAS at low temperatures resulting in high DEs without full mineralization and the potential release of the remaining fluorocarbon portions to the environment. Many of these products of incomplete combustion (PICs) are greenhouse gases, most have unknown toxicity, and some can react to create new perfluorocarboxylic acids. Experiments using aqueous film forming foam (AFFF) and a pilot-scale research combustor varied the combustion environment to determine if DEs indicate PFAS mineralization. Several operating conditions above 1090 °C resulted in high DEs and few detectable fluorinated PIC emissions. However, several conditions below 1000 °C produced DEs >99.99% for the quantifiable PFAS and mg/m3 emission concentrations of several non-polar PFAS PICs. These results suggest that DE alone may not be the best indication of total PFAS destruction, and additional PIC characterization may be warranted.

Plasma-assisted degradation of a short-chain perfluoroalkyl substance (PFAS): Perfluorobutane sulfonate (PFBS)

This study investigates the degradation of perfluorobutane sulfonate (PFBS), a chemical compound belonging to a group of per- and polyfluoroalkyl substances (PFAS), by gas-phase electrical discharge plasma. Plasma alone was ineffective in degrading PFBS due to its poor hydrophobicity, which inhibited the compound from accumulating at the plasma-liquid interface, the region of chemical reactivity. To overcome bulk liquid mass transport limitations, a surfactant, hexadecyltrimethylammonium bromide (CTAB), was introduced to interact with and transport PFBS to the plasma-liquid interface. In the presence of CTAB, ∼99% of PFBS was removed from the bulk liquid and concentrated at the interface, where 67% of the concentrate was degraded and 43% of that amount was defluorinated within one hour. PFBS degradation was further improved by optimizing the surfactant concentration and dosage. Experiments with a range of cationic, non-ionic, and anionic surfactants revealed that the PFAS-CTAB binding mechanism is predominantly electrostatic. A mechanistic understanding of the PFAS-CTAB complex formation, its transport to and destruction at the interface is proposed, alongside the chemical degradation scheme, which includes the identified degradation byproducts. This study shows that surfactant-assisted plasma treatment is one of the most promising techniques for destroying short-chain PFAS in contaminated water.

Degradation of micropollutants in secondary wastewater effluent using nonthermal plasma-based AOPs: The roles of free radicals and molecular oxidants

Emerging micropollutants (µPs) appearing in water bodies endanger aquatic animals, plants, microorganisms and humans. The nonthermal plasma-based advanced oxidation process is a promising technology for eliminating µPs in wastewater but still needs further development in view of full-scale industrial application. A novel cascade reactor design which consists of an ozonation chamber preceding a dielectric barrier discharge (DBD) plasma reactor with a falling water film on an activated carbon textile (Zorflex®) was used to remove a selection of µPs from secondary municipal wastewater effluent. Compare to previous plasma reactor, molecular oxidants degraded micropollutants again in an ozonation chamber in this study, and the utilization of different reactive oxygen species (ROS) was improved. A gas flow rate of 0.4 standard liter per minute (SLM), a water flow rate of 100 mL min^-1, and a discharge power of 25 W are identified as the optimal plasma reactor parameters, and the µP degradation efficiency and electrical energy per order value (EE/O) are 84-98% and 2.4-5.3 kW/m³, respectively. The presence of ROS during plasma treatment was determined in view of the µPs removal mechanisms. The degradation of diuron (DIU), bisphenol A (BPA) and 2-n-octyl-4-isothiazolin-3-one (OIT) was mainly performed in ozonation chamber, while the degradation of atrazine (ATZ), alachlor (ALA) and primidone (PRD) occurred in entire cascade system. The ROS not only degrade the µPs, but also remove nitrite (90.5%), nitrate (69.6%), ammonium (39.6%) and bulk organics (11.4%). This study provides insights and optimal settings for an energy-efficient removal of µPs from secondary effluent using both free radicals and molecular oxidants generated by the plasma in view of full-scale application.

Adsorption of PFOS onto graphite intercalated compound and analysis of degradation by-products during electro-chemical oxidation

Perfluorooctane sulfonate (PFOS) is a highly recalcitrant perfluoro chemical belonging to the family of per- and polyfluoroalkyl substances (PFAS). Its adsorption and degradation was demonstrated in a novel PFAS remediation process involving the adsorption onto graphite intercalated compounds (GIC) and the electrochemical oxidation. The Langmuir type of adsorption was characterized by a loading capacity of 53.9 μg PFOS g^-1 GIC and a second order kinetics (0.021 g μg^-1 min^-1). Up to 99% of PFOS was degraded in the process with a half-life of 15 min. The breakdown by-products included short chain perfluoroalkane sulfonates such as Perfluoroheptanesulfonate (PFHpS), Perfluorohexanesulfonate (PFHxS), Perfluoropentanesulfonate (PFPeS) and Perfluorobutanesulfonate (PFBS), but also short chain perfluoro carboxylic acids such as perfluorooctanoic acid (PFOA), perfluorohexanoic acid (PFHxA) and perfluorobutanoic acid (PFBA) indicating different degradation pathways. These by-products could also be broken down but the shorter the chain the slower the degradation rate. This novel combined adsorption and electrochemical process offers an alternative treatment for PFAS contaminated waters.

Plasma-Induced Selective Propylene Epoxidation Using Water as the Oxygen Source

Propylene oxide (PO) is a critical gateway chemical used in large-scale production of plastics and many other compounds. In addition, PO is also used in many smaller-scale applications that require lower PO concentrations and volumes. These include its usage as a fumigant and disinfectant for food, a sterilizer for medical equipment, as well as in producing modified food such as starch and alginate. While PO is currently mostly produced in a large-scale propylene epoxidation chemical process, due to its toxic nature and high transport and storage costs, there is a strong incentive to develop PO production strategies that are well-suited for smaller-scale on-site applications. In this contribution, we designed a plasma-liquid interaction (PLI) catalytic process that uses only water and C₃H₆ as reactants to form PO. We show that hydrogen peroxide (H₂O₂) generated in the interactions of water with plasma serves as a critical oxidizing agent that can epoxidize C₃H₆ over a titanium silicate-1 (TS-1) catalyst dispersed in a water solution with a carbon-based selectivity of more than 98%. As the activity of this plasma C₃H₆ epoxidation system is limited by the rate of H₂O₂ production, strategies to improve H₂O₂ production were also investigated.

Ultrasound-enhanced Magnéli phase Ti4O7 anodic oxidation of per- and polyfluoroalkyl substances (PFAS) towards remediation of aqueous film forming foams (AFFF)

Per-and polyfluoroalkyl substances (PFAS) remediation is still a challenge. In this study, we propose a hybrid system that combines electrochemical treatment with ultrasound irradiation, aiming for an enhanced degradation of PFAS. Equipped with a titanium suboxide (Ti₄O₇) anode, the electrochemical cell is able to remove perfluorooctanoic acid (PFOA) effectively. Under the optimal conditions (50 mA/cm² current density, 0.15 M Na₂SO₄ supporting electrolyte, and stainless steel/Ti₄O₇/stainless steel electrode configuration with a gap of ∼10 mm), the electrochemical process achieves ∼100 % PFOA removal and 43 % defluorination after 6 h. Applying ultrasound irradiation (130 kHz) alone offers a limited PFOA removal, with 33 % PFOA removal and 5.5 % defluorination. When the electrochemical process is combined with ultrasound irradiation, we observe a significant improvement in the remediation performance, with ∼100 % PFOA removal and 63.5 % defluorination, higher than the sum of 48.5 % (43 % achieved by the electrochemical process, plus 5.5 % by the ultrasound irradiation), implying synergistic removal/oxidation effects. The hybrid system also consistently shows the synergistic defluorination during degradation of other PFAS and the PFAS constituents in aqueous film forming foam (AFFF). We attribute the synergistic effect to an activated/cleaned electrode surface, improved mass transfer, and enhanced production of radicals.

A critical review of the occurrence, fate and treatment of per- and polyfluoroalkyl substances (PFASs) in landfills

The aim of this critical review is i) to summarize the occurrence of Per- and polyfluoroalkyl substances (PFASs) in landfills; ii) to outline the environmental fate and transport of PFASs in landfills; iii) to compare the treatment technologies of PFASs in landfill leachate and remediation methods of PFASs in surrounding groundwater; iv) to identify the research gaps and suggest future research directions. In recent years, PFASs have been detected in landfills around the world, among which Perfluoroalkyl acids (PFAAs) especially Perfluorooctanoic acid (PFOA) and Perfluorooctane sulfonic acid (PFOS) are mostly studied due to their long-term stability. Short-chain PFASs (<8 carbons) are more common than long-chain PFASs (≧8 carbons) in landfill leachate. PFASs in landfill leachate are eventually transported to the surrounding groundwater, surface water and soil. Some PFASs evaporate from landfills to the ambient air. To avoid the environmental and health risks of PFASs in landfills, new technologies and combined use of existing technologies have been implemented to treat PFASs in landfill leachate. Integrated remediation methods are applied to control the diffusion of PFASs in groundwater surrounding landfills. In future, the mechanisms of PFAAs precursors degradation, the correlation among PFASs in different environmental media around landfills, as well as the environmental behavior and toxic effect of combined pollutants together with PFASs in landfill leachate and surrounding groundwater should be studied.

Destruction of PFAS in AFFF-impacted fire training pit water, with a continuous hydrothermal alkaline treatment reactor

As regulations are being established to limit the levels of per- and polyfluoroalkyl substances (PFAS) in drinking water and wastewater, effective treatment technologies are needed to remove or destroy PFAS in contaminated liquid matrices. Many military installations and airports have fire training ponds (FTPs) where PFAS-containing firefighting foams are discharged during training drills. FTP water disposal is expensive and challenging due to the high PFAS levels. Hydrothermal alkaline treatment (HALT) has previously been shown to destroy a wide range of PFAS compounds with a high degree of destruction and defluorination. In this study, we investigate the performance of a continuous flow HALT reactor for destroying PFAS in contaminated FTP water samples. Processing with 5 M-NaOH and 1.6 min of continuous processing results in >99% total PFAS destruction, and 10 min processing time yields >99% destruction of every measured PFAS species. Operating with 0.1 M-NaOH or 1 M-NaOH shows little effect on the destruction of measured perfluorosulfonic acids, while all measured perfluorocarboxylic acids and fluorotelomer sulfonates are reduced to levels below the method detection limits. Continuous HALT processing with sufficient NaOH loading appears to destroy parent PFAS compounds significantly faster than batch HALT processing, a positive indicator for scaling up HALT technology for practical applications in environmental site remediation activities.

A new on-line SPE LC-HRMS method for simultaneous analysis of selected emerging contaminants in surface waters

In recent years emerging contaminants (ECs) have received significant attention due to their widespread detection in surface waters and concerns that these compounds can cause adverse ecological and/or human health effects. Therefore, accurate methods for determining and quantifying ECs in surface water are essential for estimating their environmental impact. This work describes the development, validation and application of a sensitive multiclass method for simultaneous determination of 22 per and polyfluorinated alkyl substances (PFASs), 3 pharmaceuticals, 15 pesticides, and 2 bisphenols in surface water using on-line solid phase extraction (SPE) coupled with ultra-performance liquid chromatography-high-resolution mass spectrometry (UPLC-HRMS). The method allows simultaneous sample clean-up from interfering matrices and lower limits of detection (LODs) by injecting a large sample volume into the LC system without compromising chromatographic efficiency and resolution. Linearity of response over several orders of magnitude was demonstrated for all tested compounds (R2 > 0.99), with the LODs ranging from 0.8 and 33.7 pg mL-1, allowing detection of ECs at trace levels in surface water. The method showed acceptable accuracy and precision (CV, % and RE below 20%) for all tested ECs. It also provided recoveries between 60% and 130% for all tested ECs. The validated method was successfully applied for analysis of surface water samples from three rivers (Cam, Ouse and Thames) in England. Several ECs, including perfluorooctanesulfonic acid (PFOS), perfluorobutanesulfonic acid (PFBS), perfluorohexanoic acid (PFHxA), perfluorohexane sulfonic acid (PFHxS), dimethyl-metatoluamide (DEET) and ibuprofen were observed in analysed surface water above the method's limit of quantitation (LOQ), with concentrations ranging between 3.5 and 460 pg mL-1.