Plasma-Based Water Treatment: Efficient Transformation of Perfluoroalkyl Substances in Prepared Solutions and Contaminated Groundwater

Removal and Oxidation of Low Concentration tert-Butanol from Potable Water using Nonthermal Plasma Coupled with Metal Oxide Adsorption

Taste and odor are crucial factors in evaluating the quality of drinking water for consumers. Geosmin is an example of a pollutant commonly found in potable water responsible for earthy and musty taste, and odor even at low concentrations. We have investigated the use of a hybrid two-step adsorption-mineralization process for low-level volatile organic compounds removal from potable water using dielectric barrier discharge over common metal oxides (MO). The system proposed is a proof of principle with tert-butanol (TBA) used as a model compound for geosmin removal/degradation during wastewater treatment when combined with an appropriate metal oxide adsorbent. Initial assessments of the adsorption properties of titania by density functional theory (DFT) calculations and experimental tests indicated that the adsorption of geosmin and TBA with water present results in only weak interactions between the sorbate and the metal oxide. In contrast, the DFT results show that alumina could be a suitable adsorbent for these tertiary alcohols and were reinforced by experimental studies. We find that while there is a competitive effect between the water and TBA adsorption from gaseous/liquid feed, the VOC can be removed, and the alumina will be regenerated by the reactive oxygen species (ROS) produced by a dielectric barrier discharge (DBD). The use of alumina in conjunction with NTP leads to efficient degradation of the adsorbate and the formation of oxygenated intermediates (formates, carbonates, and carboxylate-type species), which could then be mineralized for the regeneration of the adsorbent. A reaction mechanism has been proposed based on the in-situ infrared measurements and DFT calculations, while the removal of TBA with conventional heating is indicative of a gradual desorption process as a function of temperature rather than the destruction of the adsorbate. Furthermore, steady performance was observed after several adsorption-regeneration cycles, indicating no alteration of the adsorption properties of alumina during the NTP treatment and demonstrating the potential of the approach to be applied in the treatment of high throughput of water, without the challenges faced by the biocatalysts or formation of toxic byproducts.

Efficient degradation of F-53B as PFOS alternative in water by plasma discharge: Feasibility and mechanism insights

The frequent detection of 6:2 chlorinated polyfluorinated ether sulfonate (F-53B) in various environments has raised concerns owing to its comparable or even higher environmental persistence and toxicity than perfluorooctane sulfonate (PFOS). This study investigated the plasma degradation of F-53B for the first time using a water film plasma discharge system. The results revealed that F-53B demonstrated a higher rate constant but similar defluorination compared to PFOS, which could be ascribed to the introduction of the chlorine atom. Successful elimination (94.8-100 %) was attained at F-53B initial concentrations between 0.5 and 10 mg/L, with energy yields varying from 15.1 to 84.5 mg/kWh. The mechanistic exploration suggested that the decomposition of F-53B mainly occurred at the gas-liquid interface, where it directly reacted with reactive species generated by gas discharge. F-53B degradation pathways involving dechlorination, desulfonation, carboxylation, C-O bond cleavage, and stepwise CF2 elimination were proposed based on the identified byproducts and theoretical calculations. Furthermore, the demonstrated effectiveness in removing F-53B in various coexisting ions and water matrices highlighted the robust anti-interference ability of the treatment process. These findings provide mechanistic insights into the plasma degradation of F-53B, showcasing the potential of plasma processes for eliminating PFAS alternatives in water.

Optical and Chemical Measurements of Solvated Electrons Produced in Plasma Electrolysis with a Water Cathode

It is known that glow discharges with a water anode inject and form solvated electrons at the plasma-liquid interface, driving a wide variety of reduction reactions. However, in systems with a water cathode, the production and role of solvated electrons are less clear. Here, we present evidence for the direct detection of solvated electrons produced at the interface of an argon plasma and a water cathode via absorption spectroscopy. We further quantify their yield using the dissociative electron attachment of chloroacetate, measuring a yield of 1.04 ± 0.59 electrons per incident ion, corresponding to approximately 100% faradaic efficiency. Additionally, we estimate a yield of 2.09 ± 0.93 hydroxyl radicals per incident ion. Comparison of this yield with other findings in the literature supports that these hydroxyl radicals are likely formed directly in the liquid phase rather than by diffusion from the vapor phase.

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.

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.

Effects of Halides on Organic Compound Degradation during Plasma Treatment of Brines

Plasma has been proposed as an alternative strategy to treat organic contaminants in brines. Chemical degradation in these systems is expected to be partially driven by halogen oxidants, which have been detected in halide-containing solutions exposed to plasma. In this study, we characterized specific mechanisms involving the formation and reactions of halogen oxidants during plasma treatment. We first demonstrated that addition of halides accelerated the degradation of a probe compound known to react quickly with halogen oxidants (i.e., para-hydroxybenzoate) but did not affect the degradation of a less reactive probe compound (i.e., benzoate). This effect was attributed to the degradation of para-hydroxybenzoate by hypohalous acids, which were produced via a mechanism involving halogen radicals as intermediates. We applied this mechanistic insight to investigate the impact of constituents in brines on reactions driven by halogen oxidants during plasma treatment. Bromide, which is expected to occur alongside chloride in brines, was required to enable halogen oxidant formation, consistent with the generation of halogen radicals from the oxidation of halides by hydroxyl radical. Other constituents typically present in brines (i.e., carbonates, organic matter) slowed the degradation of organic compounds, consistent with their ability to scavenge species involved during plasma treatment.

Emergence of per- and poly-fluoroalkyl substances (PFAS) and advances in the remediation strategies

A group of fluorinated organic molecules known as per- and poly-fluoroalkyl substances (PFAS) have been commonly produced and circulated in the environment. PFAS, owing to multiple strong CF bonds, exhibit exceptional stability and possess a high level of resistance against biological or chemical degradation. Recently, PFAS have been identified to cause numerous hazardous effects on the biotic ecosystem. As a result, extensive efforts have been made in recent years to develop effective methods to remove PFAS. Adsorption, filtration, heat treatment, chemical oxidation/reduction, and soil washing are a few of the physicochemical techniques that have shown their ability to remove PFAS from contaminated matrixes. However these methods also carry significant drawbacks, including the fact that they are expensive, energy-intensive, unsuitable for in-situ treatment, and requirement to be carried under dormant conditions. The metabolic products released upon PFAS degradation are largely unknown, despite the fact that thermal disintegration methods are widely used. In contrast to physical and chemical methods, biological degradation of PFAS has been regarded as efficient method. However, PFAS are difficult to instantly and completely metabolize through biological methods due to the limitations of biocatalytic mechanisms. Nevertheless, cost, easy-to-operate and environmentally safe are some of the advantages over its counterpart. The present review comprehensively discusses the occurrence of PFAS, the state-of-the science of remediation technologies and approaches applied, and the remediation challenges. The article also focuses on the future research directions toward the development of effective methods for PFAS-contaminated site in-situ treatment.

Probing time-resolved plasma-driven solution electrochemistry in a falling liquid film plasma reactor: Identification of HO2- as a plasma-derived reducing agent

Many applications involving plasma-liquid interactions depend on the reactive processes occurring at the plasma-liquid interface. We report on a falling liquid film plasma reactor allowing for in situ optical absorption measurements of the time-dependence of the ferricyanide/ferrocyanide redox reactivity, complemented with ex situ measurement of the decomposition of formate. We found excellent agreement between the measured decomposition percentages and the diffusion-limited decomposition of formate by interfacial plasma-enabled reactions, except at high pH in thin liquid films, indicating the involvement of previously unexplored plasma-induced liquid phase chemistry enabled by long-lived reactive species. We also determined that high pH facilitates a reduction-favoring environment in ferricyanide/ferrocyanide redox solutions. In situ conversion measurements of a 1:1 ferricyanide/ferrocyanide redox mixture exceed the measured ex situ conversion and show that conversion of a 1:1 ferricyanide/ferrocyanide mixture is strongly dependent on film thickness. We identified three dominant processes: reduction faster than ms time scales for film thicknesses >100 µm, •OH-driven oxidation on time scales of <10 ms, and reduction on 15 ms time scales for film thickness <100 µm. We attribute the slow reduction and larger formate decomposition at high pH to HO2- formed from plasma-produced H2O2 enabled by the high pH at the plasma-liquid interface as confirmed experimentally and by computed reaction rates of HO2- with ferricyanide. Overall, this work demonstrates the utility of liquid film reactors in enabling the discovery of new plasma-interfacial chemistry and the utility of atmospheric plasmas for electrodeless electrochemistry.

Current state of knowledge of environmental occurrence, toxic effects, and advanced treatment of PFOS and PFOA

Per- and polyfluoroalkyl substances (PFAS) are anthropogenic synthetic compounds, with high chemical and thermal stability and a persistent, stable and bioaccumulative nature that renders them a potential hazard for the environment, its organisms, and humans alike. Perfluorooctane sulfonic acid (PFOS) and Perfluorooctanoic acid (PFOA) are the most well-known substances of this category and even though they are phased out from production they are still highly detectable in several environmental matrices. As a result, they have been spread globally in water sources, soil and biota exerting toxic and detrimental effects. Therefore, up and coming technologies, namely advanced oxidation processes (AOPs) and advanced reduction processes (ARPs) are being tested for their implementation in the degradation of these pollutants. Thus, the present review compiles the current knowledge on the occurrence of PFOS and PFOA in the environment, the various toxic effects they have induced in different organisms as well as the ability of AOPs and ARPs to diminish and/or eliminate them from the environment.

Unveiling nano-empowered catalytic mechanisms for PFAS sensing, removal and destruction in water

Per- and polyfluoroalkyl substances (PFAS) are organofluorine compounds used to manufacture various industrial and consumer goods. Due to their excellent physical and thermal stability ascribed to the strong CF bond, these are ubiquitously present globally and difficult to remediate. Extensive toxicological and epidemiological studies have confirmed these substances to cause adverse health effects. With the increasing literature on the environmental impact of PFAS, the regulations and research have also expanded. Researchers worldwide are working on the detection and remediation of PFAS. Many methods have been developed for their sensing, removal, and destruction. Amongst these methods, nanotechnology has emerged as a sustainable and affordable solution due to its tunable surface properties, high sorption capacities, and excellent reactivities. This review comprehensively discusses the recently developed nanoengineered materials used for detecting, sequestering, and destroying PFAS from aqueous matrices. Innovative designs of nanocomposites and their efficiency for the sensing, removal, and degradation of these persistent pollutants are reviewed, and key insights are analyzed. The mechanistic details and evidence available to support the cleavage of the CF bond during the treatment of PFAS in water are critically examined. Moreover, it highlights the challenges during PFAS quantification and analysis, including the analysis of intermediates in transitioning nanotechnologies from the laboratory to the field.

Surface modifications of biopolymers for removal of per- and polyfluoroalkyl substances from water: Current research and perspectives

Per- and polyfluoroalkyl substances (PFAS) are highly recalcitrant organic contaminants that have attracted ever-increasing attention from the general public, government agencies and scientific communities. To remove PFAS from water, especially the enormous volume of drinking water, stormwater, and groundwater, sorption is the most practical approach. Success of this approach demands green, renewable, and sustainable materials for capturing PFAS at ng/L or µg/L levels. To meet this demand, this manuscript critically reviewed sorbents developed from biopolymers, such as chitosan (CTN), alginate (ALG), and cellulose (CEL) covering the period from 2008 to 2023. The use of different cross-linkers for the surface modifications of biopolymers were described. The underlying removal mechanism of biosorbents for PFAS adsorption from molecular perspectives was discussed. Besides reviewing and comparing the performance of different bio-based sorbents with respect to environmental factors like pH, and sorption kinetics and capacity, strategies for modifying biosorbents for better performance were proposed. Additionally, approaches for regeneration and reuse of the biosorbents were discussed. This was followed by further discussion of challenges facing the development of biosorbents for PFAS removal.

Catalytic degradation of amoxicillin from water by a combined system of ultrasound/H2O2/KI

A new combined system of ultrasound (US)/ H2O2/KI was presented in this study for the degradation of amoxicillin (AMX). Various parameters that influencing the degradation efficiency, such as ultrasonic power, pH value of the solution, concentrations of H2O2 and KI, initial concentration of AMX were investigated. The maximum degradation efficiency of 84.1% in 60 min was obtained under the optimal conditions when the ultrasonic power was 195 W, the solution pH was 3.2, the concentrations of H2O2 and KI were 120 and 2.4 mmol/L, respectively. Results also showed US/H2O2/KI system possessed better degradation efficiency compared with that by using the systems of US alone, H2O2, US/H2O2, US/KI and H2O2/KI. The degradation reaction of AMX should be a very complex process, and the corresponding degradation pathways may change at different concentrations or under different reaction conditions. The results indicate that the combined system of US/ H2O2/KI has great potential applications for the treatment of wastewater containing antibiotics with high concentrations.

Cold Plasma for Green Advanced Reduction/Oxidation Processes (AROPs) of Organic Pollutants in Water

Cold plasma is gaining increasing attention as a novel tool to activate energy demanding chemical processes, including advanced reduction/oxidation processes (AROPs) of organic pollutants in water. The very complex milieu generated by discharges at the water/plasma interface comprises photons, strong oxidants and strong reductants which can be exploited for achieving the degradation of most any kind of pollutants. Despite the complexity of these systems, the powerful arsenal of mechanistic tools and chemical probes of physical organic chemists can be usefully applied to understand and develop plasma chemistry. Specifically, the added value of air plasma generated by in situ discharge with respect to ozonation (ex situ discharge) is demonstrated using phenol and various phenol derivatives and mechanistic evidence for the prevailing role of hydroxyl radicals in the initial attack is presented. On the reduction front, the impressive performance of cold plasma in inducing the degradation of recalcitrant perfluoroalkyl substances, which do not react with OH radicals but are attacked by electrons, is reported and discussed. The widely different reactivities of perfluorooctanoic acid (PFOA) and of perfluorobutanoic acid (PFBA) underline the crucial role played in these processes by the interface between plasma and solution and the surfactant properties of the treated pollutants.

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.

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.

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.

Surface-modified biopolymers for removing mixtures of per- and polyfluoroalkyl substances from water: Screening and removal mechanisms

Green, renewable, and sustainable materials are needed for removing per- and polyfluoroalkyl substances (PFASs) in water. Herein, we synthesized and tested alginate (ALG) and chitosan (CTN) based and polyethyleneimine (PEI) functionalized fibers/aerogels for the adsorption of mixtures of 12 PFASs (9 short- and long-chain PFAAs, GenX, and 2 precursors) from water at an initial concentration of 10 μg/L each. Out of 11 biosorbents, ALGPEI-3 and GTH CTNPEI aerogels had the best sorption performance. Through detailed characterization of the sorbents before and after PFASs sorption, it was revealed that hydrophobic interaction was the dominant mechanism controlling PFASs sorption while electrostatic interactions played a minor role. As a result, both aerogels had fast and superior sorption of relatively hydrophobic PFASs from pH 2 to 10. Even at extreme pH conditions, the aerogels retained their shape perfectly. Based upon the isotherms, the maximum adsorption capacity of ALGPEI-3 and GTH-CTNPEI aerogels towards total PFASs removal was 3045 and 12,133 mg/g, respectively. Although the sorption performance of the GTH-CTNPEI aerogel toward short chain PFAS was less than satisfactory and varied between 70 and 90% in 24 h, it may find its use in removing relatively hydrophobic PFAS at high concentrations in complex and extreme environments.

Degradation of per- and polyfluoroalkyl substances in landfill leachate by a thin-water-film nonthermal plasma reactor

Per- and polyfluoroalkyl substances (PFAS) are present in landfill leachate, posing potential challenges to leachate disposal and treatment. This work represents the first study of a thin-water-film nonthermal plasma reactor for PFAS degradation in landfill leachate. Of the 30 PFAS measured in three raw leachates, 21 were above the detection limits. The removal percentage depended on the category of PFAS. For example, perfluorooctanoic acid PFOA (C8) had the highest removal percentage (77% as an average of the three leachates) of the perfluoroalkyl carboxylic acids (PFCAs) category. The removal percentage decreased when the carbon number increased from 8 to 11 and decreased from 8 to 4. The effects of various landfill leachate components, including sodium chloride, acetate, humic acids, pH, and surfactants, had no or minor impacts (<30%) on PFOA mineralization in synthetic samples. This might be explained by the plasma-generation and PFAS-degradation mainly occurring at the gas/liquid interface. Shorter-chain PFCAs were produced as intermediates of PFOA degradation, and shorter-chain PFCAs and perfluorosulfonic acids (PFSAs) were produced as intermediates of perfluorooctanesulfonic acid (PFOS). The concentrations of the intermediates decreased with decreasing carbon number, suggesting a stepwise removal of difluoromethylene (CF₂) in the degradation pathway. Potential PFAS species in the raw and treated leachates were identified at the molecular level through non-targeted Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The intermediates did not show accurate toxicity per Microtox bioassay.

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 Review of PFAS Destruction Technologies

Per- and polyfluoroalkyl substances (PFASs) are a family of highly toxic emerging contaminants that have caught the attention of both the public and private sectors due to their adverse health impacts on society. The scientific community has been laboriously working on two fronts: (1) adapting already existing and effective technologies in destroying organic contaminants for PFAS remediation and (2) developing new technologies to remediate PFAS. A common characteristic in both areas is the separation/removal of PFASs from other contaminants or media, followed by destruction. The widely adopted separation technologies can remove PFASs from being in contact with humans; however, they remain in the environment and continue to pose health risks. On the other hand, the destructive technologies discussed here can effectively destroy PFAS compounds and fully address society's urgent need to remediate this harmful family of chemical compounds. This review reports and compare widely accepted as well as emerging PFAS destruction technologies. Some of the technologies presented in this review are still under development at the lab scale, while others have already been tested in the field.

Recent advances on PFAS degradation via thermal and nonthermal methods

Per- and polyfluoroalkyl substances (PFAS) are a set of synthetic chemicals which contain several carbon-fluorine (C-F) bonds and have been in production for the past eight decades. PFAS have been used in several industrial and consumer products including nonstick pans, food packaging, firefighting foams, and carpeting. PFAS require proper investigations worldwide due to their omnipresence in the biotic environment and the resulting pollution to drinking water sources. These harmful chemicals have been associated with adverse health effects such as liver damage, cancer, low fertility, hormone subjugation, and thyroid illness. In addition, these fluorinated compounds show high chemical, thermal, biological, hydrolytic, photochemical, and oxidative stability. Therefore, effective treatment processes are required for the removal and degradation of PFAS from wastewater, drinking water, and groundwater. Previous review papers have provided excellent summaries on PFAS treatment technologies, but the focus has been on the elimination efficiency without providing mechanistic understanding of removal/degradation pathways. The present review summarizes a comprehensive examination of various thermal and non-thermal PFAS destruction technologies. It includes sonochemical/ultrasound degradation, microwave hydrothermal treatment, subcritical or supercritical treatment, electrical discharge plasma technology, thermal destruction methods/incinerations, low/high-temperature thermal desorption process, vapor energy generator (VEG) technology and mechanochemical destruction. The background, degradation mechanisms/pathways, and advances of each remediation process are discussed in detail in this review.

Highly efficient degradation of PFAS and other surfactants in water with atmospheric RAdial plasma (RAP) discharge

Atmospheric plasma offers a viable approach to new water remediation technologies, best suited for the degradation of persistent organic pollutants such as PFAS, per- and polyfluoroalkyl substances. This paper reports on the remarkable performance of a novel RAdial Plasma (RAP) discharge reactor in treating water contaminated with PFAS surfactants, notably the ubiquitous perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). RAP proved to be versatile and robust, performing very well over a wide range of pollutants concentrations. Thus, PFOA degradation was most satisfactory with regard to all critical indicators, kinetics (≥99% PFOA conversion in less than 2.5 min and 30 min in solutions with initial concentrations of 41 μg/L and 41 mg/L, respectively), byproducts, and energy efficiency (G₅₀ greater than 2000 mg/kWh for 41 μg/L - 4.1 mg/L PFOA initial concentrations). Likewise for PFOS as well as for Triton X-100, a common fluorine-free non-ionic surfactant tested to explore the scope of applicability of RAP to the degradation of surfactants in general. The results obtained with RAP compare most favourably with those reported for state-of-art plasma systems in similar experiments. RAP's excellent performance is attributed to the dense network of radial discharges it generates, randomly spread over the entire exposed surface of the liquid thus establishing an extended highly reactive plasma-liquid interface with both strongly reducing and oxidizing species. Mechanistic insight is offered based on the observed degradation products and on available literature data on the surfactants properties and on their plasma induced degradation investigated in previous studies.

Electrooxidation of per- and polyfluoroalkyl substances in chloride-containing water on surface-fluorinated Ti4O7 anodes: Mitigation and elimination of chlorate and perchlorate formation

Electrooxidation (EO) has been shown effective in degrading per- and polyfluoroalkyl substances (PFASs) in water, but concurrent formation of chlorate and perchlorate in the presence of chloride is of concern due to their toxicity. This study examined EO treatment of three representative PFASs, perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and 6:2 fluorotelomer sulfonate (6:2 FTS), in chloride-containing solutions on pristine and surface-fluorinated Ti₄O₇ anodes having different percentage of surface fluorination. The experiment results indicate that surface fluorination of Ti₄O₇ anodes slightly inhibited PFAS degradation, while significantly decreased the formation of chlorate and perchlorate. Further studies with spectroscopic and electrochemical characterizations and density functional theory (DFT) computation reveal the mechanisms of the impact on EO performance by anode fluorination. In particular, chlorate and perchlorate formation were fully inhibited when fluorinated Ti₄O₇ anode was used in reactive electrochemical membrane (REM) under a proper anodic potential range (<3.0 V vs Standard Hydrogen Electrode), resulting from slower intermediate reaction steps and short residence time of the REM system. The results of this study provide a basis for design and optimization of modified Ti₄O₇ anodes for efficient EO treatment of PFAS while limiting chlorate and perchlorate formation.

Application of photocatalytic ozonation with a WO3/TiO2 catalyst for PFAS removal under UVA/visible light

This research evaluates photocatalytic ozonation for removing 5 PFAS (PFOA/PFHxS/PFBS/6:2 FTS/GenX) from water using a WO₃/TiO₂ catalyst under UVA-visible radiation. Four catalysts of varying WO₃ content (0/1/3/5 wt%) were synthesized by sol-gel and characterized by XRD, TEM, STEM-EDS, HAADF-STEM, adsorption/desorption N₂ isotherms, and DRS-UV-vis. 5 wt% WO₃/TiO₂ was the optimal composition based on physicochemical properties and photocatalytic activity tests with methylene blue. PFAS degradation showed that photocatalytic ozonation inefficiently degraded PFAS with WO₃/TiO₂ under UVA-visible light after 4 h (ΣPFAS removal 16 %, [range 4 %-26 %]). Photocatalysis had comparable removal to photocatalytic ozonation, photolysis and ozone photolysis showed lower removal, and ozonation had no effect. Microtox analysis showed the initial acute toxicity was no longer detectable after photocatalysis and photocatalytic ozonation treatment. Low PFAS removals under tested conditions require that future work evaluate different catalysts or treatment conditions, while disparities between tested PFAS removals demonstrate the need to evaluate multiple compounds. ENVIRONMENTAL IMPLICATION: The research presented in this manuscript involves the preparation and characterization of WO₃/TiO₂ catalysts used, for the first time, to remove multiple PFAS in water via photocatalytic ozonation. This manuscript supports the development of a catalytic process for the elimination of hard to degrade environmental pollutants, provides new knowledge on aspects of photocatalytic processes, and provides insights on environmental pollution abatement.

An Energy Efficient Process for Degrading Perfluorooctanoic Acid (PFOA) Using Strip Fountain Dielectric Barrier Discharge Plasma

Perfluorooctanoic acid (PFOA) is an artificially synthesized per-fluorinated chemical widely used in industry. It is often released into the environment without treatment and causes pollution in groundwater. In this paper, we employed a strip fountain dielectric barrier discharge (SF-DBD) plasma source to degrade PFOA from the water. The effects of power supply mode, discharge gases, pH, the conductivity of the solution, concentration, etc., on the degradation efficiency were studied. For a 200 mL sample of 75 mg/L PFOA, a 99% degradation efficiency with a 204.5 μg/kJ energy production rate was achieved using an average power of 43 W negative pulse argon plasma for 50 min at atmospheric pressure. The total organic carbon concentration (TOC) decreased by 63% after a 60-minute treatment. The SF-DBD proves to be a promising and energy-saving technique to efficiently remove PFOA from water.

Arc and pulsed spark discharge inactivation of pathogenic P. aeruginosa, S. aureus, M. canis, T. mentagrophytes, and C. albicans microorganisms

There is a strong and ever-escalating need for sterilization tools that are effective against a broad range of pathogenic microorganisms. To address this issue, this study evaluates the inactivation potential of arc and pulsed spark plasma discharges on Pseudomonas aeruginosa, Staphylococcus aureus, Microsporum canis, Trichophyton mentagrophytes, and Candida albicans microorganisms. Our results show that the electrical discharge plasma systems are effective in the inactivation of pathogenic microorganisms. The inactivation of the considered strains was greatly affected by the type of microorganisms. Higher viability losses of the pathogenic strains were observed in bacterial strains than in the fungal strains. Moreover, in the case of fungal strains, the population of C. albicans was decreased the most, followed by Trichophyton mentagrophyte, while the population of Microsporum canis was decreased the least. Besides, the arc discharge system was compared with the pulsed spark discharge system. It can be obtained from the results that the pulsed spark discharge treatment successfully enhanced the reduction of the pathogenic cells more than the arc discharge treatment. The higher efficiency of the pulsed spark discharge is due to the generation of discharge streamers on the water surface. The SEM analyses showed that electrical discharge plasmas produced serious damage to pathogenic eukaryotic and prokaryotic microorganisms. Also, the plasma-induced changes in pH values and temperature values were measured. The pulsed spark discharge-treated samples have more significant changes in pH value while arc discharge-treated samples have larger temperature changes.

Rate, Efficiency, and Mechanisms of Electrochemical Perfluorooctanoic Acid Degradation with Boron-Doped Diamond and Plasma Electrodes

The removal of per- or polyfluorinated alkyl substances (PFAS) has received increasing attention because of their extreme stability, our increasing awareness of their toxicity at even low levels, and scientific challenges for traditional treatment methods such as separation by activated carbon or destruction by advanced oxidation processes. Here, we performed a direct and systematic comparison of two electrified approaches that have recently shown promise for effective degradation of PFAS: plasma and conventional electrochemical degradation. We tailored a reactor configuration where one of the electrodes could be a plasma or a boron-doped diamond (BDD) electrode and operated both electrodes galvanostatically by continuous direct current. We show that while both methods achieved near-complete degradation of PFAS, the plasma was only effective as the cathode, whereas the BDD was only effective as the anode. Compared to the BDD, plasma required more than an order of magnitude higher voltage but lower current to achieve similar degradation efficiency with more rapid degradation kinetics. All these factors considered, it was noted that plasma or BDD degradation resulted in similar energy efficiencies. The BDD electrode exhibited zero-order kinetics, and thus, PFAS degradation using the conventional electrochemical method was kinetically controlled. On the contrary, analysis using a film model indicated that the plasma degradation kinetics of PFAS using plasma were mass-transfer-controlled because of the fast reaction kinetics. With the help of a simple quantitative model that incorporates mass transport, interfacial reaction, and surface accumulation, we propose that the degradation reaction kinetically follows an Eley-Rideal-type mechanism for the plasma electrode, and an intrinsic rate constant of 2.89 × 10⁸ m⁴ mol^-1 s^-1 was obtained accordingly. The investigation shows that to realize the true kinetic potential of plasma degradation for water treatment, mass transfer to the interface must be enhanced.

Electrochemical oxidation processes for PFAS removal from contaminated water and wastewater: fundamentals, gaps and opportunities towards practical implementation

Electrochemical oxidation (EO) is emerging as one of the most promising methods for the degradation of recalcitrant per- and poly-fluoroalkyl substances (PFASs) in water and wastewater, as these compounds cannot be effectively treated with conventional bio- or chemical approaches. This review examines the state of the art of EO for PFASs destruction, and comprehensively compares operating parameters and treatment performance indicators for both synthetic and real contaminated water and wastewater media. The evaluation shows the need to use environmentally-relevant media to properly quantify the effectiveness/efficiency of EO for PFASs treatment. Additionally, there is currently a lack of quantification of sorption losses, resulting in a likely over-estimation of process' efficiencies. Furthermore, the majority of experimental results to date indicate that short-chain PFASs are the most challenging and need to be prioritized as environmental regulations become more stringent. Finally, and with a perspective towards practical implementation, several operational strategies are proposed, including processes combining up-concentration followed by EO destruction.

Determination of total reducible organofluorine in PFAS-impacted aqueous samples based on hydrated electron defluorination

Per- and polyfluoroalkyl substances (PFASs) is a large group of thousands of anthropogenic chemicals. Recently, measurement of total organic fluorine (TOF) to reflect the total PFASs has been recommended in limits and advisories. In this study, a total reducible organofluorine (TROF) assay is developed based on hydrated electron (e_aq^-) conversion of PFASs into inorganic fluorine combined with ion chromatograph, which is a common and widespread instrument. The e_aq^- is generated in UV/sulfite system with alkaline condition, and the concentration of TROF (C_{F_TROF}) is the difference of fluoride concentration before and after assay. Method validation uses perfluorooctanesulfonic acid, perfluorooctanoic acid and their main alternatives, and F^- recoveries are 76.6%-101%, except for perfluorobutanesulfonic acid (48.5%). Method application of TROF assay uses industrial surfactant products and fluorochemical industry-contaminated water, meanwhile, target PFAS analysis and total oxidizable precursors (TOP) assay are concurrently conducted. Concentrations of PFASs detected in target analysis and TOP assay were converted to fluorine equivalents concentrations (C_{F_Target} and C_{F_TOP}). ∑C_{F_Target} and ∑C_{F_TOP} account for 0.80%-36% of C_{F_TROF} in industrial samples, 0.12%-54% in environmental water and 9.7%-14% in wastewater. The TROF assay can be used to initially judge whether PFASs contamination occurred near a hotspot with known sources. The C_{F_TROF} could infer the extent of PFAS contamination in PFAS-impacted samples and estimate the fraction of uncharacterized PFAS.

Emerging technologies for PFOS/PFOA degradation and removal: A review

Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are highly recalcitrant anthropogenic chemicals that are ubiquitously present in the environment and are harmful to humans. Typical water and wastewater treatment processes (coagulation, flocculation, sedimentation, and filtration) are proven to be largely ineffective, while adsorption with granular activated carbon (GAC) has been the chief option to capture them from aqueous sources followed by incineration. However, this process is time-consuming, and produces additional solid waste and air pollution. Treatment methods for PFOS and PFOA generally follow two routes: (1) removal from source and reduce the risk; (2) degradation. Emerging technologies focusing on degradation are critically reviewed in this contribution. Various processes such as bioremediation, electrocoagulation, foam fractionation, sonolysis, photocatalysis, mechanochemical, electrochemical degradation, beams of electron and plasma have been developed and studied in the past decade to address PFAS crisis. The underlying mechanisms of these PFAS degradation methods have been categorized. Two main challenges have been identified, namely complexity in large scale operation and the release of toxic byproducts. Based on the literature survey, we have provided a strength-weakness-opportunity-threat (SWOT) analysis and quantitative rating on their efficiency, environmental impact and technology readiness.

Plasma parameters and the reduction potential at a plasma-liquid interface

Nonthermal plasmas in contact with liquids have been shown to generate a variety of reactive species capable of initiating reduction-oxidation (redox) reactions at the electrochemically active plasma-liquid interface. In conventional electrochemical cells, selective redox chemistry is achieved by controlling the reduction potential at the solid electrode-electrolyte interface by applying a bias via an external circuit. In the case of plasma-liquid systems, an analogous means of tuning the reduction potential near the interface has not clearly been identified. When treated as a floating surface, the liquid is expected to adopt a net negative charge to balance the flux of hot electrons and relatively cold positive ions. The reduction potential near the plasma-liquid interface is hypothesized to be proportional to the floating potential, which can be approximated using an analytical model provided the plasma parameters are known. Herein, we present a framework for correlating the electron density and electron temperature of a noble gas plasma jet to the reduction potential near the plasma-liquid interface. The plasma parameters were acquired for an argon atmospheric plasma jet in contact with an aqueous solution by means of laser Thomson scattering. The reduction potential was determined using identical reference electrodes to measure the potential difference between the plasma-liquid interface and bulk solution. Interestingly, the measured reduction potentials near the plasma-liquid interface were found to be in good agreement with the model-predicted values determined using the plasma parameters obtained from the Thomson scattering experiments.

Sonolysis of per- and poly fluoroalkyl substances (PFAS): A meta-analysis

Human ingestion of per- and polyfluoroalkyl substances (PFAS) from contaminated food and water is linked to the development of several cancers, birth defects and other illnesses. The complete mineralisation of aqueous PFAS by ultrasound (sonolysis) into harmless inorganics has been demonstrated in many studies. However, the range and interconnected nature of reaction parameters (frequency, power, temperature etc.), and variety of reaction metrics used, limits understanding of degradation mechanisms and parametric trends. This work summarises the state-of-the-art for PFAS sonolysis, considering reaction mechanisms, kinetics, intermediates, products, rate limiting steps, reactant and product measurement techniques, and effects of co-contaminants. The meta-analysis showed that mid-high frequency (100 - 1,000 kHz) sonolysis mechanisms are similar, regardless of reaction conditions, while the low frequency (20 - 100 kHz) mechanisms are specific to oxidative species added, less well understood, and generally slower than mid-high frequency mechanisms. Arguments suggest that PFAS degradation occurs via adsorption (not absorption) at the bubble interface, followed by headgroup cleavage. Further mechanistic steps toward mineralisation remain to be proven. For the first time, complete stoichiometric reaction equations are derived for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) sonolysis, which add H2 as a reaction product and consider CO an intermediate. Fluorinated intermediate products are derived for common, and more novel PFAS, and a naming system proposed for novel perfluoroether carboxylates. The meta-analysis also revealed the transition between pseudo first and zero order PFOA/S kinetics commonly occurs at 15 - 40 µM. Optimum values of; ultrasonic frequency (300 - 500 kHz), concentration (>15 - 40 μM), temperature (≈20 °C), and pH range (3.2 - 4) for rapid PFOX degradation are derived by evaluation of prior works, while optimum values for the dilution factor applied to PFAS containing firefighting foams and applied power require further work. Rate limiting steps are debated and F- is shown to be rate enhancing, while SO42- and CO2 by products are theorised to be rate limiting. Sonolysis was compared to other PFAS destructive technologies and shown to be the only treatment which fully mineralises PFAS, degrades different PFAS in order of decreasing hydrophobicity, is parametrically well studied, and has low-moderate energy requirements (several kWh g-1 PFAS). It is concluded that sonolysis of PFAS in environmental samples would be well incorporated within a treatment train for improved efficiency.

Removal of Per- and Polyfluoroalkyl Substances by Electron Beam and Plasma Irradiation: A Mini-Review

The global prevalence and environmental risks of per- and polyfluoroalkyl substances (PFASs) have caused increasing concern regarding their strategic elimination from aqueous environments. It has recently been recognized that advanced oxidation–reduction technologies (AO/RTs) exhibit superior removal performance for these ubiquitous pollutants. However, the detailed mechanisms and product risks have not been well summarized and systematically deciphered. In this mini-review article, the basic operating principles of two typical AO/RTs (electron beam and plasma irradiation) and their reported applications in the abatement of PFASs are described in detail. It is noteworthy that these reductive treatments induced remarkable defluorination efficiency of PFOA and PFOS with the generation of short-chain congeners in water. The reaction mechanisms mainly included desulfonization, decarboxylation, H/F exchange, radical cyclization, and stepwise losses of CF2 groups. Unexpectedly, partial degradation products manifested high potential in triggering acute and chronic aquatic toxicity, genotoxicity, and developmental toxicity. Additionally, high or even increased resistance to biodegradability was observed for multiple products relative to the parent chemicals. Taken together, both electron beam and plasma irradiation hold great promise in remediating PFAS-contaminated water and wastewater, while the secondary ecological risks should be taken into account during practical applications.

Application of Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Contaminated Groundwater and Soil

Hydrothermal alkaline treatment (HALT) can effectively degrade per- and polyfluoroalkyl substances (PFASs) present in aqueous film-forming foam (AFFF). However, information is lacking regarding the treatment of PFASs in actual groundwater and soil from AFFF-impacted sites, especially for complex soil matrices. Given the lack of studies on direct soil treatment for PFAS destruction, we herein applied HALT to two groundwater samples and three soil samples from AFFF-impacted sites and characterized the destruction of PFASs using high-resolution mass spectrometry. Results showed that the 148 PFASs identified in all collected field samples, including 10 cationic, 98 anionic, and 40 zwitterionic PFASs, were mostly degraded to nondetectable levels within 90 min when treated with 5 M NaOH at 350 °C. The near-complete defluorination, as evidenced by fluoride release measurements, confirmed the complete destruction of PFASs. While many structures, including perfluoroalkyl carboxylic acids and polyfluorinated substances, were readily degraded, perfluoroalkyl sulfonates (PFSAs, C_nF_2n+1-SO₃^-), most notably with short chain lengths (n = 3-5), were more recalcitrant. Rates of PFSA destruction in groundwater samples were similar to those measured in laboratory water solutions, but reactions in soil were slow, presumably due to base-neutralizing properties of the soil. Further, the degradation of PFASs in groundwaters and soils was found to be a function of reaction temperature, NaOH concentration, and reaction time. These findings have important implications for the remediation of AFFF-impacted sites.

Critical Review of Thermal Decomposition of Per- and Polyfluoroalkyl Substances: Mechanisms and Implications for Thermal Treatment Processes

Per- and polyfluoroalkyl substances (PFASs) are fluorinated organic chemicals that are concerning due to their environmental persistence and adverse human and ecological effects. Remediation of environmental PFAS contamination and their presence in consumer products have led to the production of solid and liquid waste streams containing high concentrations of PFASs, which require efficient and cost-effective treatment solutions. PFASs are challenging to defluorinate by conventional and advanced destructive treatment processes, and physical separation processes produce waste streams (e.g., membrane concentrate, spent activated carbon) requiring further post-treatment. Incineration and other thermal treatment processes are widely available, but their use in managing PFAS-containing wastes remains poorly understood. Under specific operating conditions, thermal treatment is expected to mineralize PFASs, but the degradation mechanisms and pathways are unknown. In this review, we critically evaluate the thermal decomposition mechanisms, pathways, and byproducts of PFASs that are crucial to the design and operation of thermal treatment processes. We highlight the analytical capabilities and challenges and identify research gaps which limit the current understanding of safely applying thermal treatment to destroy PFASs as a viable end-of-life treatment process.

Pathways of organic micropollutants degradation in atmospheric pressure plasma processing - A review

Concern of toxic compounds and their, potentially more harmful degradation products, present in aquatic environment alarmed scientific community and research on the development of novel technologies for wastewater treatment had become of great interest. Up to this date, many papers pointed out the challenges and limitations of conventional wastewater treatment and of some advanced oxidation processes. Advanced technologies based on the use of non-equilibrium or non-thermal plasma had been recognized as a possible solution for, not only degradation, but for complete removal of recalcitrant organic micropollutants. While previous review papers have been focused on plasma physics and chemistry of different types of discharges for few organic micropollutants, this paper brings comprehensive review of current knowledge on the chemistry and degradation pathways by using different non-thermal plasma types for several micropollutants' classes, such as pharmaceuticals, perfluorinated compounds, pesticides, phenols and dyes and points out some major research gaps.

Effectiveness of Non-Thermal Plasma Induced Degradation of Per- and Polyfluoroalkyl Substances from Water

Per- and polyfluoroalkyl substances (PFAS) are omnipresent synthetic chemicals. Due to their industrial importance and widespread use as a key component in various applications and a variety of products, these compounds can be found today in high concentrations (>1 μg/L) in surface and groundwater but also spread throughout the ecosystem, where they represent a serious threat to most living organisms. The removal or degradation of PFAS contaminants from water and soil is becoming a legal obligation in a growing number of countries around the globe. This, however, demands novel techniques for the degradation of PFAS since conventional water treatment techniques are either insufficient or extremely expensive due to the persistent nature of these compounds caused by their extraordinary chemical stability. The goal of this work was therefore to investigate the practical potential of the application-oriented use of atmospheric non-thermal plasma as a powerful advanced oxidation method for the purification of water contaminated with PFAS compounds. Special attention was devoted to the development of the concept that can be scaled up to the capacity level of approximately 100–200 m3 of water per hour, contaminated with PFAS and other contaminants including organic and inorganic material generally present in soil, and surface or groundwater. Our major research interest was to define the minimum required treatment time for optimal purification results, as well as to understand the influence of the initial concentration of PFAS in water and the potential presence of co-contaminants often present in situ on the efficiency of the degradation process. A chemical analysis of the treated samples demonstrated the ability of the atmospheric plasma to reduce more than 50% of the initial PFAS amount in the water samples in less than 300 s of treatment time. PFOA, however, showed more rigidity towards degradation, where a double treatment time was needed to reach similar degradation levels. The obtained results showed that the initial concentration level does not play a major role in the process. However, the PFAS degradation profiles for all tested concentrations show a strongly nonlinear behavior with time, characterized by the fast decrease of the process efficiency in the case of longer treatment times. For prolonged treatment times, a constant increase in the samples’ conductivity was measured, which might be the limiting factor for the degradation rate in the case of prolonged treatment times.

Photocatalytic degradation of perfluoroalkyl substances in water by using a duo-functional tri-metallic-oxide hybrid catalyst

The recalcitrant nature of perfluoroalkyl substances (PFASs) urges scientists to discover solutions to permanently remove PFAS contaminations from water with less energy in contrast to incineration. Herein, a duo-functional tri-metallic-oxide (f-TMO) hybrid photocatalyst was developed via a facile process, which displayed both high adsorption capacity and high defluorination rate of a series of PFASs including PFOA, PFOS, PFHpA, PFHxA and PFBA due to the generated holes/electrons (h⁺/e^-) and multi-radicals such as O₂^•- and SO₄^•-. Particularly the Langmuir adsorption capacities up to 827.84 and 714.46 mg g^-1 along with the adsorption efficiency of 99.8% and 99.4% for PFOS and PFOA were respectively achieved. A defluorination ratio of as high as 74.8% with PFOA and a ratio up to 67.6% with PFOS were respectively received. Over 98% PFOA molecules were degraded within as fast as 15 min under initial concentrations ranging from 1 ppb to 1000 ppb, which demonstrates an excellent degradation kinetics. As for the sulfonic acid of PFOS, an as high as 95.5% degradation efficiency was obtained within 300 min. The degradation rates were 4.5 mg L^-1 h^-1 for PFOA and 0.54 mg L^-1 h^-1 for PFOS, respectively. In parallel, the f-TMO photocatalyst still exhibited a >96.2% degradation efficiency after eight regeneration cycles. The high physical adsorption capacity and high defluorination rate make this f-TMO catalyst promising applications in removing various PFASs from a broad range of residential and industrial water systems.

Activated carbon versus metal-organic frameworks: A review of their PFAS adsorption performance

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are a class of fluorinated aliphatic compounds considered as emerging persistent pollutants. Owing to their adverse effects on human health and environment, efficient methods of their removal from various complex matrices need to be developed. This review focuses on recent results addressing the adsorption of PFAS on activated carbons (AC) and metal-organic frameworks (MOF). While the former are well-established adsorbents used in water treatment, the latter are relatively new and still not applied at a large scale. Nevertheless, they attract research interests owing to their developed porosity and versatile surface chemistry. While AC provide high volumes of pores and hydrophobic surfaces to strongly attract fluorinated chains, MOF supply sites for acid-base complexation and a variety of specific interactions. The modifications of AC are focused on the introduction of basicity to attract PFAS anions via electrostatic/chemical interactions, and those of MOF - on structural defects to increase the pore sizes. Based on the comparison of the performance and specifically adsorption forces provided by these two groups of materials, activated carbons were pointed out as worthy of further research efforts. This is because their surface, especially that in large pores, where dispersive forces are week and where extensive pore space might be utilized to adsorb more PFAS, can be further chemically modified and these modifications might be informed by the mechanisms of PFAS adsorption, which are specific for MOF. This review emphasizes the effects of these modifications on the adsorption mechanism and brings the critical assessment of the advantages/disadvantages of both groups as PFAS adsorbents.

Theoretical and experimental insights into the mechanisms of C6/C6 PFPiA degradation by dielectric barrier discharge plasma

As an emerging alternative legacy perfluoroalkyl substance, C6/C6 PFPiA (perfluoroalkyl phosphinic acids) has been detected in aquatic environments and causes potential risks to human health. The degradation mechanisms of C6/C6 PFPiA in a dielectric barrier discharge (DBD) plasma system were explored using validated experimental data and density functional theory (DFT) calculations. Approximately 94.5% of C6/C6 PFPiA was degraded by plasma treatment within 15 min at 18 kV. A relatively higher discharge voltage and alkaline conditions favored its degradation. C6/C6 PFPiA degradation was attributed to attacks of •OH, •O₂^-, and ¹O₂. Besides PFHxPA and C2 -C6 shorter-chain perfluorocarboxylic acids, several other major intermediates including C4/C6 PFPiA, C4/C4 PFPiA, and C3/C3 PFPiA were identified. According to DFT calculations, the potential energy surface was proposed for possible reactions during C6/C6 PFPiA degradation in the discharge plasma system. Integrating the identified intermediates and DFT results, C6/C6 PFPiA degradation was deduced to occur by stepwise losing CF₂, free radical polymerization, and C-C bond cleavage. Furthermore, the DBD plasma treatment process decreased the toxicity of C6/C6 PFPiA to some extent. This study provides a comprehensive understanding of C6/C6 PFPiA degradation by plasma advanced oxidation.

A Review of Treatment Techniques for Short-Chain Perfluoroalkyl Substances

In recent years, an increasing amount of short-chain perfluoroalkyl substance (PFAS) alternatives has been used in industrial and commercial products. However, short-chain PFASs remain persistent, potentially toxic, and extremely mobile, posing potential threats to human health because of their widespread pollution and accumulation in the water cycle. This study systematically summarized the removal effect, operation conditions, treating time, and removal mechanism of various low carbon treatment techniques for short-chain PFASs, involving adsorption, advanced oxidation, and other practices. By the comparison of applicability, pros, and cons, as well as bottlenecks and development trends, the most widely used and effective method was adsorption, which could eliminate short-chain PFASs with a broad range of concentrations and meet the low-carbon policy, although the adsorbent regeneration was undesirable. In addition, advanced oxidation techniques could degrade short-chain PFASs with low energy consumption but unsatisfied mineralization rates. Therefore, combined with the actual situation, it is urgent to enhance and upgrade the water treatment techniques to improve the treatment efficiency of short-chain PFASs, for providing a scientific basis for the effective treatment of PFASs pollution in water bodies globally.

Field-Scale Demonstration of PFAS Leachability Following In Situ Soil Stabilization

A field-scale validation is summarized comparing the efficacy of commercially available stabilization amendments with the objective of mitigating per- and polyfluoroalkyl substance (PFAS) leaching from aqueous film-forming foam (AFFF)-impacted source zones. The scope of this work included bench-scale testing to evaluate multiple amendments and application concentrations to mitigate PFAS leachability and the execution of field-scale soil mixing in an AFFF-impacted fire-training area with nearly 2.5 years of post-soil mixing monitoring to validate reductions in PFAS leachability. At the bench scale, several amendments were evaluated and the selection of two amendments for field-scale evaluation was informed: FLUORO-SORB Adsorbent (FS) and RemBind (RB). Five ∼28 m³ test pits (approximately 3 m wide by 3 m long by 3 m deep) were mixed at a site using conventional construction equipment. One control test pit (Test Pit 1) included Portland cement (PC) only (5% dry weight basis). The other four test pits (Test Pits 2 through 5) compared 5 and 10% ratios (dry weight basis) of FS and RB (also with PC). Five separate monitoring events included two to three sample cores collected from each test pit for United States Environmental Protection Agency (USEPA) Method 1315 leaching assessment. After 1 year, a mass balance for each test pit was attempted comparing the total PFAS soil mass before, during, and after leach testing. Bench-scale and field-scale data were in good agreement and demonstrated >99% decrease in total PFAS leachability (mass basis; >98% mole basis) as confirmed by the total oxidizable precursor assay, strongly supporting the chemical stabilization of PFAS.

Reactive Nitrogen Species Generated by Gas-Liquid Dielectric Barrier Discharge for Efficient Degradation of Perfluorooctanoic Acid from Water

Perfluorooctanoic acid (PFOA) poses a serious threat to the ecological environment and biological health because of its ubiquitous distribution, extreme persistence, and high toxicity. In this study, we designed a novel gas-liquid dielectric barrier discharge (GLDBD) reactor which could efficiently destruct PFOA. PFOA removal efficiencies can be obtained in various water matrices, which were higher than 98.0% within 50 min, with energy yields higher than 114.5 mg·kWh^-1. It was confirmed that the reactive species including e^-, ONOOH, •NO₂, and hydroxyl radicals (•OH) were responsible for PFOA removal. Especially, this study first revealed the crucial role of reactive nitrogen species (RNS) for PFOA degradation in the plasma system. Due to the generation of a large amount of RNS, the designed GLDBD reactor proved to be less sensitive to various water matrices, which meant a broader promising practical application. Moreover, influential factors including high concentration of various ions and humic acid (HA), were investigated. The possible PFOA degradation pathways were proposed based on liquid chromatograph-mass spectrometer (LC-MS) results and density functional theory (DFT) calculation, which further confirmed the feasibility of PFOA removal with RNS. This research, therefore, provides an effective and versatile alternative for PFOA removal from various water matrices.

Atmospheric plasma-based approaches for the degradation of dimethyl phthalate (DMP) in water

Cold plasma based treatment of contaminated water is becoming a promising novel green remediation option. This study assessed the performance of two different cold plasma reactors, using, respectively, a self-pulsing discharge (SPD) and a multipin corona discharge (MCD), in the degradation of dimethyl phthalate (DMP), a persistent and ubiquitous pollutant of the aquatic environment. The process kinetics and energy efficiency, as well as the main plasma generated reactive species were determined under various operating conditions concerning the plasma feed gas and flowrate, the voltage polarity, the input power, the DMP initial concentration, the liquid conductivity, and the aqueous matrix used to prepare DMP solutions for these experiments. The MCD reactor, operated with air as plasma feed gas and negative voltage polarity, gave the best results in terms of rate and energy efficiency. Moreover, variations in plasma input power and in the liquid conductivity have limited effect on DMP degradation rate, making this reactor suitable for treating liquids with a range of initial conductivities The effects of DMP initial concentration on its rate of degradation and on the process energy efficiency were also investigated. Differences in the efficiency of production and distribution of plasma generated reactive species, notably ^•OH and H₂O₂, observed for the two tested reactors are discussed in terms of different extension of the plasma/liquid interface and diffusion into the bulk solution. It is proposed that among the reactive species, ^•OH foremost, and O₃ to a lesser extent, play a pivotal role in DMP degradation, while the contribution of H₂O₂ appears to be limited. The rate of DMP degradation was not drastically different in Milli-Q water and in tap water, a positive outcome in view of practical applications of the technology. The lower rate observed in tap than in Milli-Q water is attributed to the presence of bicarbonate and carbonate, which are known scavengers of hydroxyl radicals.