Treatment of PFAS-containing landfill leachate using an enhanced contact plasma reactor

Improving the purification of aqueous solutions by controlling the production of reactive oxygen species in non-thermal plasma; a systematic review

Treatment with non-thermal plasma is a reliable technology to oxidize chemical impurities that exist in polluted water, wastewater, and leachate, those degradation-resistant and cannot be removed by conventional treatment methods. In this study, the effective factors affecting in the formation ofreactive oxygen species in non-thermal plasma treatment process, as a new advanced oxidation process method explianed. In this manner, all associated manuscripts existed in the main databases including Google Scholar, Science Direct, PubMed, and Open Access Journal Directory from 1990 until 2022 were explored. The utilized keywords were involved non-thermal plasma, Cold plasma, Measurement, •OH, O3 and UV. Overall, 8,813 articles were gathered and based on the relevance titles and abstracts, 18 paper were selected for further reviewing. In several studies, plasma techniques have been used to treat water, wastewater and leachate, but few studies have evaluated the factors influencing the production of ROS species by non-thermal plasma. The non-thermal plasma destroys pollutants by reactive free radicals spices (hydroxyl, hydrogen atoms, etc.) a combination effect of strong electric fields, energetically charged particles, and ultrasound. Some factors such as water vapor, hydraulic retention time, inter-electrode spacing, discharge power density, and aeration of the effluent as well as use of catalyst have direct effect on the reactive oxygen species formation. If these factors controlled within the best ranges, it will promote the oxidizing radical production and system performance. Also, high-energy electrons and oxidizing species produced in the cold plasma system can well degrade most of pollution in water and wastewater.

Characterization of per- and polyfluoroalkyl substances (PFAS) and other constituents in MSW landfill leachate from Puerto Rico

Elevated per- and polyfluoroalkyl substance (PFAS) concentrations have been reported in municipal solid waste (MSW) landfill leachate with higher levels in wet and warmer subtropical climates. Information about landfill leachate characteristics is much more limited in tropical climates. In this study, 20 landfill leachate samples were collected from three MSW landfills on the tropical island of Puerto Rico and results were compared against landfills nationally and within Florida, USA. The samples collected in Puerto Rico underwent physical-chemical analysis, as well as a quantitative analysis of 92 PFAS. Samples described in this study include discrete leachate types, such as leachate, gas condensate, and leachate which has undergone on-site treatment (e.g., RO treatment, phytoremediation, lagoons). A total of 51 PFAS were detected above quantitation limits, including perfluorohexylphosphonic acid, a perfluoroalkyl acid (PFAA) which has not been reported previously in landfill leachate. ∑PFAS concentrations in this study (mean: 38,000 ng L-1), as well as concentrations of individual PFAS, are significantly higher than other reported MSW landfill leachate concentrations. The profiles of leachates collected from on-site treatment systems indicate possible transformation of precursor PFAS as a result of treatment processes - oxidizing conditions, for example, may facilitate aerobic transformation, increase the concentrations of PFAAs, and possibly increase the apparent ∑PFAS concentration. Extreme climate events, including rising temperatures and more frequent hurricanes, have placed additional strain on the solid waste management infrastructure on the island - adding complexity to an already challenging PFAS management issue. As concern grows over PFAS contamination in drinking water, these findings should inform solid waste and leachate management decisions in order to minimize PFAS emissions in island environments.

Evaluation of per- and polyfluoroalkyl substances (PFAS) in landfill liquids from Pennsylvania, Colorado, and Wisconsin

PER: and polyfluoroalkyl substances (PFAS) have been measured in aqueous components within landfills. To date, the majority of these studies have been conducted in Florida. This current study aimed to evaluate PFAS concentrations in aqueous components (leachate, gas condensate, stormwater, and groundwater) from four landfills located outside of Florida, in Pennsylvania, Colorado, and Wisconsin (2 landfills). The Pennsylvania landfill also provided the opportunity to assess a leachate treatment system. Sample analyses were consistent across studies including the measurements of 26 PFAS and physical-chemical parameters. For the four target landfills, average PFAS concentrations were 6,900, 22,000, 280, and 260 ng L-1 in the leachate, gas condensate, stormwater, and groundwater, respectively. These results were not significantly different than those observed for landfills in Florida except for the significantly higher PFAS concentrations in gas condensate compared to leachate. For on-site treatment at the Pennsylvania landfill, results suggest that the membrane biological bioreactor (MBBR) system performed similarly as aeration-based leachate treatment systems at Florida landfills resulting in no significant decreases in ∑26PFAS. Overall, results suggest a general consistency across US regions in PFAS concentrations within different landfill liquid types, with the few differences observed likely influenced by landfill design and local climate. Results confirm that leachate exposed to open air (e.g., in trenches or in treatment systems) have lower proportions of perfluoroalkyl acid precursors relative to leachate collected in enclosed pipe systems. Results also confirm that landfills without bottom liner systems may have relatively higher PFAS levels in adjacent groundwater and that landfills in wetter climates tend to have higher PFAS concentrations in leachate.

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.

Combined adsorption and electrochemical oxidation of perfluorooctanoic acid (PFOA) using graphite intercalated compound

Perfluorooctanoic acid (PFOA) is a bioaccumulative synthetic chemical containing strong C-F bonds and is one of the most common per- and polyfluoroalkyl substances (PFAS) detected in the environment. Graphite intercalated compound (GIC) flakes were used to adsorb and degrade PFOA through electrochemical oxidation. The adsorption followed the Langmuir model with a loading capacity of 2.6 µg PFOA g-1 GIC and a second-order kinetics (3.354 g µg-1 min-1). 99.4% of PFOA was removed by the process with a half-life of 15 min. When PFOA molecules broke down, they released various by-products, such as short-chain perfluoro carboxylic acids like PFHpA, PFHxA, and PFBA. This breakdown indicates the cleavage of the perfluorocarbon chain and the release of CF2 units, suggesting a transformation or degradation of the original compound into these smaller acids. Shorter-chain perfluorinated compounds had slower degradation rates compared to longer-chain ones. Combining these two methods (adsorption and in situ electrochemical oxidation) was found to be advantageous because adsorption can initially concentrate the PFOA molecules, making it easier for the electrochemical process to target and degrade them. The electrochemical process can potentially break down or transform the PFAS compounds into less harmful substances through oxidation or other reactions.

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.

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.

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.

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.

Per- and polyfluoroalkyl substances (PFAS) distribution in landfill gas collection systems: leachate and gas condensate partitioning

While per- and polyfluoroalkyl substances (PFAS) have been reported extensively in municipal solid waste (MSW) landfill leachate,they have rarely been quantified in landfill gas or in discrete landfill liquids such as landfill gas condensate (LGC), and the potential for PFAS to partition to the condensate has not been reported. LGC and leachate collected from within gas wells known as gas well pump-out (GWP) from three MSW landfills underwent physical-chemical characterization and PFAS analysis to improve understanding of the conditions under which these liquids form and to illuminate PFAS behavior within landfills. LGC was observed to be clear liquid containing ammonia and alkalinity while GWP strongly resembled leachate - dark in color, high in chloride and ammonia. Notably, arsenic and antimony were found in concentrations exceeding regulatory thresholds by over two orders of magnitude in many LGC samples. LGC contained a lower average concentration of ΣPFAS (19,000 ng L) compared to GWP (56,000 ng L); however, LGC contained more diversity of PFAS, with 53 quantified compared to 44 in GWP. LGC contained proportionally more precursor PFAS than GWP, including more semi-volatile PFAS which are rarely measured in water matrices, such as fluorotelomer alcohols and perfluoroalkane sulfonamido ethanols. This study provides the first detailed comparison of these matrices to inform timely leachate management decisions.

Tricks and tracks in waste management with a special focus on municipal landfill leachate: Leads and obstacles

Landfilling is the most widely used disposal method for municipal solid waste around the world. The main disadvantage of this strategy is formation of leachate, among other aspects. Landfill leachate contains highly toxic and bio-refractory substances that are detrimental to the environment and human health. Hence, the risk(s) of discharging potentially harmful landfill leachate into the environment need to be assessed and measured in order to make effective choices about landfill leachate management and treatment. In view of this, the present review aims to investigate (a) how landfill leachate is perceived as an emerging concern, and (b) the stakeholders' mid- to long-term policy priorities for implementing technological and integrative solutions to reduce the harmful effects of landfill leachate. Because traditional methods alone have been reported ineffective, and in response to emerging contaminants and stringent regulations, new effective and integrated leachate treatments have been developed. This study gives a forward-thinking of the accomplishments and challenges in landfill leachate treatment during the last decade. It also provides a comprehensive compilation of the formation and characterization of landfill leachate, the geo-environmental challenges that it raises, as well as the resource recovery and industrial linkage associated with it in order to provide an insight into its sustainable management.

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.

Poly- and Perfluoroalkyl Substances (PFAS) in Landfills: Occurrence, Transformation and Treatment

Landfills have served as the final repository for > 50 % municipal solid wastes in the United States. Because of their widespread uses and persistence in the environment, per- and polyfluoroalkyl substances (PFAS) (>4000 on the global market) are ubiquitously present in everyday consumer, commercial and industrial products, and have been widely detected in both closed (tens ng/L) and active (thousands to ten thousands ng/L) landfills due to disposal of PFAS-containing materials. Along with the decomposition of wastes in-place, PFAS can be transformed and released from the wastes into leachate and landfill gas. Consequently, it is critical to understand the occurrence and transformation of PFAS in landfills and the effectiveness of landfills, as a disposal alternative, for long-term containment of PFAS. This article presents a state-of-the-art review on the occurrence and transformation of PFAS in landfills, and possible effect of PFAS on the integrity of modern liner systems. Based on the data published from 10 countries (250 + landfills), C4-C7 perfluoroalkyl carboxylic acids were found predominant in the untreated landfill leachate and neutral PFAS, primarily fluorotelomer alcohols, in landfill air. The effectiveness and limitations of the conventional leachate treatment technologies and emerging technologies were also evaluated to address PFAS released into the leachate. Among conventional technologies, reverse osmosis (RO) may achieve a high removal efficiency of 90-100 % based on full-scale data, which, however, is vulnerable to the organic fouling and requires additional disposal of the concentrate. Implications of these knowledge on PFAS management at landfills are discussed and major knowledge gaps are identified.

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.

Concentrations of perfluoroalkyl and polyfluoroalkyl substances before and after full-scale landfill leachate treatment

Many consumer and industrial products, industrial wastes and dewatered sludge from municipal wastewater treatment plants containing per- and polyfluoroalkyl substances (PFAS) are disposed of in landfills at the end of their usage, with PFAS in these products leached into landfill leachates. On-site leachate treatment is one possible method to reduce PFAS in leachates. Many landfills are equipped with on-site leachate treatment systems, but few full-scale facilities have been systematically evaluated for PFAS concentration changes. The objective of this study was to evaluate a cross-section of full-scale on-site landfill treatment systems to measure changes in PFAS concentrations. Leachate samples were collected before and after treatment from 15 facilities and were evaluated for 26 PFAS, including 11 perfluoroalkyl carboxylic acids (PFCAs), 7 perfluoroalkyl sulfonic acids (PFSAs), and 8 perfluoroalkyl acid precursors (PFAA-precursors). Transformation of precursors was evaluated by the total oxidizable precursor (TOP) assay. Results showed no obvious reductions in total measured PFAS (∑26PFAS) for on-site treatment systems including ponds, aeration tanks, powdered activated carbon (PAC), and sand filtration. Among evaluated on-site treatment systems, only systems fitted with reverse osmosis (RO) showed significant reductions (98-99 %) of ∑26PFAS in the permeate. Results from the TOP assay showed that untargeted PFAA-precursors converted into targeted short-chain PFCAs increasing ∑26PFAS in oxidized samples by 30 %, on average. Overall, results of this study confirm the efficacy of RO systems and suggest the presence of additional precursors beyond those measured in this study.

Electrochemical oxidation combined with UV irradiation for synergistic removal of perfluorooctane sulfonate (PFOS) in water

The effect of electrochemical degradation on Magnéli phase Ti₄O₇ anode combined with UV irradiation on the removal of PFOS was systematically evaluated in the present study. A synergistic effect of electrolysis and UV irradiation rather than a simple additive effect for PFOS degradation was demonstrated experimentally and theoretically. The short wavelength irradiation within 400 nm is the main contribution to enhance the electrochemical degradation of PFOS, while the initial pH of the solution has little effect on the PFOS degradation. The increase of current density accelerates the removal of PFOS either by electrolysis treatment or the joint process. The time-dependent density functional theory (TD-DFT) calculation indicates that the synergistic effect of the electrolysis and UV irradiation is most likely due to the involvement of the excited PFOS induced under UV irradiation in the electrochemical reaction. This study provides the first mechanistic explanation for the electrochemical degradation of PFOS enhanced by UV irradiation.

A review of recent studies on toxicity, sequestration, and degradation of per- and polyfluoroalkyl substances (PFAS)

The fate, effects, and treatment of per- and polyfluoroalkyl substances (PFAS), an anthropogenic class of chemicals used in industrial and commercial production, are topics of great interest in recent research and news cycles. This interest stems from the ubiquity of PFAS in the global environment as well as their significant toxicological effects in humans and wildlife. Research on toxicity, sequestration, removal, and degradation of PFAS has grown rapidly, leading to a flood of valuable knowledge that can get swamped out in the perpetual rise in the number of publications. Selected papers from the Journal of Hazardous Materials between January 2018 and May 2022 on the toxicity, sequestration, and degradation of PFAS are reviewed in this article and made available as open-access publications for one year, in order to facilitate the distribution of critical knowledge surrounding PFAS. This review discusses routes of toxicity as observed in mammalian and cellular models, and the observed human health effects in exposed communities. Studies that evaluate of toxicity through in-silico approaches are highlighted in this paper. Removal of PFAS through modified carbon sorbents, nanoparticles, and anion exchange materials are discussed while comparing treatment efficiencies for different classes of PFAS. Finally, various biotic and abiotic degradation techniques, and the pathways and mechanisms involved are reviewed to provide a better understanding on the removal efficiencies and cost effectiveness of existing treatment strategies.

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 SO₄^2- 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.

Pilot-Scale Continuous Foam Fractionation for the Removal of Per- and Polyfluoroalkyl Substances (PFAS) from Landfill Leachate

Per- and polyfluoroalkyl substances (PFAS) are of concern for their ubiquity in the environment combined with their persistent, bioaccumulative, and toxic properties. Landfill leachate is often contaminated with these chemicals, and therefore, the development of cost-efficient water treatment technologies is urgently needed. The present study investigated the applicability of a pilot-scale foam fractionation setup for the removal of PFAS from natural landfill leachate in a novel continuous operating mode. A benchmark batch test was also performed to compare treatment efficiency. The ΣPFAS removal efficiency plateaued around 60% and was shown to decrease for the investigated process variables air flow rate (Q _air), collected foam fraction (%_foam) and contact time in the column (t _c). For individual long-chain PFAS, removal efficiencies above 90% were obtained, whereas the removal for certain short-chain PFAS was low (<30%). Differences in treatment efficiency between enriching mode versus stripping mode as well as between continuous versus batch mode were negligible. Taken together, these findings suggest that continuous foam fractionation is a highly applicable treatment technology for PFAS contaminated water. Coupling the proposed cost- and energy-efficient foam fractionation pretreatment to an energy-intensive degradative technology for the concentrated foam establishes a promising strategy for on-site PFAS remediation.

Accelerated Degradation of Perfluorosulfonates and Perfluorocarboxylates by UV/Sulfite + Iodide: Reaction Mechanisms and System Efficiencies

The addition of iodide (I^-) in the UV/sulfite system (UV/S) significantly accelerated the reductive degradation of perfluorosulfonates (PFSAs, C_nF_2n+1SO₃^-) and perfluorocarboxylates (PFCAs, C_nF_2n+1COO^-). Using the highly recalcitrant perfluorobutane sulfonate (C₄F₉SO₃^-) as a probe, we optimized the UV/sulfite + iodide system (UV/S + I) to degrade n = 1-7 PFCAs and n = 4, 6, 8 PFSAs. In general, the kinetics of per- and polyfluoroalkyl substance (PFAS) decay, defluorination, and transformation product formations in UV/S + I were up to three times faster than those in UV/S. Both systems achieve a similar maximum defluorination. The enhanced reaction rates and optimized photoreactor settings lowered the EE/O for PFCA degradation below 1.5 kW h m^-3. The relatively high quantum yield of e_aq^- from I^- made the availability of hydrated electrons (e_aq^-) in UV/S + I and UV/I two times greater than that in UV/S. Meanwhile, the rapid scavenging of reactive iodine species by SO₃^2- made the lifetime of e_aq^- in UV/S + I eight times longer than that in UV/I. The addition of I^- also substantially enhanced SO₃^2- utilization in treating concentrated PFAS. The optimized UV/S + I system achieved >99.7% removal of most PFSAs and PFCAs and >90% overall defluorination in a synthetic solution of concentrated PFAS mixtures and NaCl. We extended the discussion over molecular transformation mechanisms, development of PFAS degradation technologies, and the fate of iodine species.

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.

Enhancing Interface Reactions by Introducing Microbubbles into a Plasma Treatment Process for Efficient Decomposition of PFOA

Efficient destruction of perfluoroalkyl compounds in contaminated waters remains a challenge because of highly stable C-F bonds. In this study, mineralization of perfluorooctanoic acid (PFOA) with high concentration (∼30 mg/L) was realized in a needle-plate pulsed discharge reactor integrated with a water jet (NPDW) to which microbubbles (MBs) with different carrier gases (air, N₂, and Ar) were introduced to enhance interfacial reactions. MBs effectively enrich dispersed PFOA from a bulk solution to a liquid surface to allow enhancing contact with reactive species and also expanding the plasma discharge area and channels. The PFOA removal efficiency in air and Ar discharge reached 81.5 and 95.3% in 2 h, respectively, with a defluorination ratio of no less than 50%. Energy requirements (EE/O) ranged from 216.49 to 331.95 kWh/m³. Aside from fluoride, PFOA was degraded to a range of short-chain perfluoroalkyl acids and, to a minor extent, at least 20 other fluorinated transformation products. PFOA degradation mechanisms were proposed, including decarboxylation, hydroxylation, hydrogenation reduction, and defluorination reactions. Real water matrices (groundwater, tap water, wastewater effluent, and surface water) showed moderate impact on treatment outcomes, demonstrating the robustness of the treatment process. The study demonstrated an environmentally friendly nonthermal plasma technology for effective PFOA degradation.

Occurrence and removal of poly/perfluoroalkyl substances (PFAS) in municipal and industrial wastewater treatment plants

The presence of poly- and perfluoroalkyl substances (PFAS) has caused serious problems for drinking water supplies especially at intake locations close to PFAS manufacturing facilities, wastewater treatment plants (WWTPs), and sites where PFAS-containing firefighting foam was regularly used. Although monitoring is increasing, knowledge on PFAS occurrences particularly in municipal and industrial effluents is still relatively low. Even though the production of C8-based PFAS has been phased out, they are still being detected at many WWTPs. Emerging PFAS such as GenX and F-53B are also beginning to be reported in aquatic environments. This paper presents a broad review and discussion on the occurrence of PFAS in municipal and industrial wastewater which appear to be their main sources. Carbon adsorption and ion exchange are currently used treatment technologies for PFAS removal. However, these methods have been reported to be ineffective for the removal of short-chain PFAS. Several pioneering treatment technologies, such as electrooxidation, ultrasound, and plasma have been reported for PFAS degradation. Nevertheless, in-depth research should be performed for the applicability of emerging technologies for real-world applications. This paper examines different technologies and helps to understand the research needs to improve the development of treatment processes for PFAS in wastewater streams.

Defluorination of Omega-Hydroperfluorocarboxylates (ω-HPFCAs): Distinct Reactivities from Perfluoro and Fluorotelomeric Carboxylates

Omega-hydroperfluorocarboxylates (ω-HPFCAs, HCF₂-(CF₂)_n-1-COO^-) are commercially available in bulk quantities and have been applied in agrochemicals, fluoropolymer production, and semiconductor coating. In this study, we used kinetic measurements, theoretical calculations, model compound experiments, and transformation product analyses to reveal novel mechanistic insights into the reductive and oxidative transformation of ω-HPFCAs. Like perfluorocarboxylates (PFCAs, CF₃-(CF₂)_n-1-COO^-), the direct linkage between HC_nF_2n- and -COO^- enables facile degradation under UV/sulfite treatment. To our surprise, the presence of the H atom on the remote carbon makes ω-HPFCAs more susceptible than PFCAs to decarboxylation (i.e., yielding shorter-chain ω-HPFCAs) and less susceptible to hydrodefluorination (i.e., H/F exchange). Like fluorotelomer carboxylates (FTCAs, C_nF_2n+1-CH₂CH₂-COO^-), the C-H bond in HCF₂-(CF₂)_n-1-COO^- allows hydroxyl radical oxidation and limited defluorination. While FTCAs yielded PFCAs in all chain lengths, ω-HPFCAs only yielded ^-OOC-(CF₂)_n-1-COO^- (major) and ^-OOC-(CF₂)_n-2-COO^- (minor) due to the unfavorable β-fragmentation pathway that shortens the fluoroalkyl chain. We also compared two treatment sequences-UV/sulfite followed by heat/persulfate and the reverse-toward complete defluorination of ω-HPFCAs. The findings will benefit the treatment and monitoring of H-containing per- and polyfluoroalkyl substance (PFAS) pollutants as well as the design of future fluorochemicals.

Managing and treating per- and polyfluoroalkyl substances (PFAS) in membrane concentrates

Per- and polyfluoroalkyl substances (PFAS), which are present in many waters, have detrimental impacts on human health and the environment. Reverse osmosis (RO) and nanofiltration (NF) have shown excellent PFAS separation performance in water treatment; however, these membrane systems do not destroy PFAS but produce concentrated residual streams that need to be managed. Complete destruction of PFAS in RO and NF concentrate streams is ideal, but long-term sequestration strategies are also employed. Because no single technology is adequate for all situations, a range of processes are reviewed here that hold promise as components of treatment schemes for PFAS-laden membrane system concentrates. Attention is also given to relevant concentration processes because it is beneficial to reduce concentrate volume prior to PFAS destruction or sequestration. Given the costs and challenges of managing PFAS in membrane concentrates, it is critical to evaluate both established and emerging technologies in selecting processes for immediate use and continued research.