Waste incineration of Polytetrafluoroethylene (PTFE) to evaluate potential formation of per- and Poly-Fluorinated Alkyl Substances (PFAS) in flue gas

Review of per- and poly-fluoroalkyl treatment in combustion-based thermal waste systems in the United States

Per- and poly-fluoroalkyl substances (PFAS) are a class of synthetic chemicals known for their widespread presence and environmental persistence. Carbon-fluorine (C-F) bonds are major components among PFAS and among the strongest organic bonds, thus destroying PFAS may present significant challenge. Thermal treatment such as incineration is an effective and approved method for destroying many halogenated organic chemicals. Here, we present the results of existing studies and testing at combustion-based thermal treatment facilities and summarize what is known regarding PFAS destruction and mineralization at such units. Available results suggest the temperature and residence times reached by some thermal treatment systems are generally favorable to the destruction of PFAS, but the possibility for PFAS or fluorinated organic byproducts to escape destruction and adequate mineralization and be released into the air cannot be ruled out. Few studies have been conducted at full-scale operating facilities, and none to date have attempted to characterize possible fluorinated organic products of incomplete combustion (PICs). Further, the ability of existing air pollution control (APC) systems, designed primarily for particulate and acid gas control, to reduce PFAS air emissions has not been determined. These data gaps remain primarily due to the previous lack of available methods to characterize PFAS destruction and PIC concentrations in facility air emissions. However, newly developed stack testing methods offer an improved understanding of the extent to which thermal waste treatment technologies successfully destroy and mineralize PFAS in these waste streams.

Per-and polyfluoroalkyl (PFAS) eternal pollutants: Sources, environmental impacts and treatment processes

Per- and polyfluoroalkyl substances (PFAS) have become a growing environmental concern due to their tangible impacts on human health. However, due to the large number of PFAS compounds and the analytical difficulty to identify all of them, there are still some knowledge gaps not only on their impact on human health, but also on how to manage them and achieve their effective degradation. PFAS compounds originate from man-made chemicals that are resistant to degradation because of the presence of the strong carbon-fluorine bonds in their chemical structure. This review consists of two parts. In the first part, the environmental effects of fluorinated compound contamination in water are covered with the objective to highlight how their presence in the environment adversely impacts the human health. In the second part, the focus is put on the different techniques available for the degradation and/or separation of PFAS compounds in different types of waters. Examples of removal/treatment of PFAS present in either surface or ground water are presented.

Fate of perfluoroalkyl and polyfluoroalkyl substances (PFAS) through two full-scale wastewater sludge incinerators

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are an emerging issue in wastewater treatment. High-temperature thermal processes, incineration being time-tested, offer the opportunity to destroy and change the composition of PFAS. The fate of PFAS has been documented through wastewater sludge incinerators, including a multiple hearth furnace (MHF) and a fluidized bed furnace (FBF). The dewatered wastewater sludge feedstock averaged 247- and 1280-μmol targeted PFAS per sample run in MHF and FBF feed, respectively. Stack emissions (reportable for all targeted PFAS from MHF only) averaged 5% of that value with shorter alkyl chain compounds comprising the majority of the targeted PFAS. Wet scrubber water streams accumulated nonpolar fluorinated organics from the furnace exhaust with an average of 0.740- and 0.114-mol F- per sample run, for the MHF and FBF, respectively. Simple alkane PFAS measured at the stack represented 0.5%-4.5% of the total estimated facility greenhouse gas emissions. PRACTITIONER POINTS: The MHF emitted six short chain PFAS from the stack, which were shorter alkyl chain compounds compared with sludge PFAS. The FBF did not consistently emit reportable PFAS from the stack, but contamination complicated the assessment. Five percent of the MHF sludge molar PFAS load was reported in the stack. MHF and FBF wet scrubber water streams accumulated nonpolar fluorinated organics from the furnace exhaust. Ultra-short volatile alkane PFAS measured at the stack represented 0.5%-4.5% of the estimated facility greenhouse gas emissions.

Mechanistic exploration of polytetrafluoroethylene thermal plasma gasification through multiscale simulation coupled with experimental validation

The ever-growing quantities of persistent Polytetrafluoroethylene (PTFE) wastes, along with consequential ecological and human health concerns, stimulate the need for alternative PTFE disposal method. The central research challenge lies in elucidating the decomposition mechanism of PTFE during high-temperature waste treatment. Here, we propose the PTFE microscopic thermal decomposition pathways by integrating plasma gasification experiments with multi-scale simulations strategies. Molecular dynamic simulations reveal a pyrolysis-oxidation & chain-shortening-deep defluorination (POCD) degradation pathway in an oxygen atmosphere, and an F abstraction-hydrolysis-deep defluorination (FHD) pathway in a steam atmosphere. Density functional theory computations demonstrate the vital roles of 1O2 and ·H radicals in the scission of PTFE carbon skeleton, validating the proposed pathways. Experimental results confirm the simulation results and show that up to 80.12% of gaseous fluorine can be recovered through plasma gasification within 5 min, under the optimized operating conditions determined through response surface methodology.

Fluoropolymers as Unique and Irreplaceable Materials: Challenges and Future Trends in These Specific Per or Poly-Fluoroalkyl Substances

In contrast to some low-molar-mass per- and polyfluoroalkyl substances (PFASs), which are well established to be toxic, persistent, bioaccumulative, and mobile, fluoropolymers (FPs) are water-insoluble, safe, bioinert, and durable. These niche high-performance polymers fulfil the 13 polymer-of-low-concern (PLC) criteria in their recommended conditions of use. In addition, more recent innovations (e.g., the use of non-fluorinated surfactants in aqueous radical (co)polymerization of fluoroalkenes) from industrial manufacturers of FPs are highlighted. This review also aims to show how these specialty polymers endowed with outstanding properties are essential (even irreplaceable, since hydrocarbon polymer alternatives used in similar conditions fail) for our daily life (electronics, energy, optics, internet of things, transportation, etc.) and constitute a special family separate from other "conventional" C1-C10 PFASs found everywhere on Earth and its oceans. Furthermore, some information reports on their recycling (e.g., the unzipping depolymerization of polytetrafluoroethylene, PTFE, into TFE), end-of-life FPs, and their risk assessment, circular economy, and regulations. Various studies are devoted to environments involving FPs, though they present a niche volume (with a yearly production of 330,300 t) compared to all plastics (with 460 million t). Complementary to other reviews on PFASs, which lack of such above data, this review presents both fundamental and applied strategies as evidenced by major FP producers.

Efficient Mineralization of Fluoroelastomers Using Superheated Water in the Presence of Potassium Hydroxide

The mineralization of fluoroelastomers (FKMs) in superheated water in the presence of potassium hydroxide (KOH) was investigated with the aim of developing a methodology for recycling the fluorine element. Two FKMs-an "uncrosslinked FKM", representing a poly(vinylidene fluoride-co-hexafluoropropylene) (poly(VDF-co-HFP)) copolymer with a VDF/HFP molar ratio of 78/22 and a "crosslinked FKM" consisting of this copolymer (cured by peroxide) and carbon black-were treated. The fluorine content of these FKMs was efficiently transformed into F- ions in the reaction solution using low KOH concentrations (0.10-0.50 M) at 200-250 °C. When the uncrosslinked or crosslinked FKMs reacted with aqueous KOH (0.20 M) at a rather low temperature (200 °C) for 18 h, the fluorine content of these FKMs was completely mineralized (both F- yields were 100%). Although the crosslinked FKM contained carbon black, the fluorine mineralization of the FKM was not inhibited. The addition of Ca(OH)2 to the reaction solutions after the superheated water treatment at 250 °C for 6 h with aqueous KOH (0.50 M) led to the production of pure CaF2, identified using X-ray spectroscopy, with 100% and 93% yields for the uncrosslinked and crosslinked FKMs, respectively.

Emerging environmental health risks associated with the land application of biosolids: a scoping review

BACKGROUND

Over 40% of the six million dry metric tons of sewage sludge, often referred to as biosolids, produced annually in the United States is land applied. Biosolids serve as a sink for emerging pollutants which can be toxic and persist in the environment, yet their fate after land application and their impacts on human health have not been well studied. These gaps in our understanding are exacerbated by the absence of systematic monitoring programs and defined standards for human health protection.

METHODS

The purpose of this paper is to call critical attention to the knowledge gaps that currently exist regarding emerging pollutants in biosolids and to underscore the need for evidence-based testing standards and regulatory frameworks for human health protection when biosolids are land applied. A scoping review methodology was used to identify research conducted within the last decade, current regulatory standards, and government publications regarding emerging pollutants in land applied biosolids.

RESULTS

Current research indicates that persistent organic compounds, or emerging pollutants, found in pharmaceuticals and personal care products, microplastics, and per- and polyfluoroalkyl substances (PFAS) have the potential to contaminate ground and surface water, and the uptake of these substances from soil amended by the land application of biosolids can result in contamination of food sources. Advanced technologies to remove these contaminants from wastewater treatment plant influent, effluent, and biosolids destined for land application along with tools to detect and quantify emerging pollutants are critical for human health protection.

CONCLUSIONS

To address these current risks, there needs to be a significant investment in ongoing research and infrastructure support for advancements in wastewater treatment; expanded manufacture and use of sustainable products; increased public communication of the risks associated with overuse of pharmaceuticals and plastics; and development and implementation of regulations that are protective of health and the environment.

Microbial and thermal treatment techniques for degradation of PFAS in biosolids: A focus on degradation mechanisms and pathways

Per- and polyfluoroalkyl substances (PFAS) are persistent organic chemicals detected in biosolids worldwide, which have become a significant concern for biosolids applications due to their increasing environmental risks. Hence, it is pivotal to understand the magnitude of PFAS contamination in biosolids and implement effective technologies to reduce their contamination and prevent hazardous aftermaths. Thermal techniques such as pyrolysis, incineration and gasification, and biodegradation have been regarded as impactful solutions to degrade PFAS and transform biosolids into value-added products like biochar. These techniques can mineralize PFAS compounds under specific operating parameters, which can lead to unique degradation mechanisms and pathways. Understanding PFAS degradation mechanisms can pave the way to design the technology and to optimize the process conditions. Therefore, in this review, we aim to review and compare PFAS degradation mechanisms in thermal treatment like pyrolysis, incineration, gasification, smouldering combustion, hydrothermal liquefaction (HTL), and biodegradation. For instance, in biodegradation of perfluorooctane sulfonic acid (PFOS), firstly C-S bond cleavage occurs which is followed by hydroxylation, decarboxylation and defluorination reactions to form perfluoroheptanoic acid. In HTL, PFOS degradation is carried through OH^-catalyzed series of nucleophilic substitution and decarboxylation reactions. In contrast, thermal PFOS degradation involves a three-step random-chain scission pathway. The first step includes C-S bond cleavage, followed by defluorination of perfluoroalkyl radical, and radical chain propagation reactions. Finally, the termination of chain propagation reactions produces very short-fluorinated units. We also highlighted important policies and strategies employed worldwide to curb PFAS contamination in biosolids.

Per- and polyfluoroalkyl substances fate and transport at a wastewater treatment plant with a collocated sewage sludge incinerator

This study aims to understand the fate and transport of per- and polyfluoroalkyl substances (PFAS) and inorganic fluoride (IF) at an undisclosed municipal wastewater treatment plant (WWTP) operating a sewage sludge incinerator (SSI). A robust statistical analysis characterized concentrations and mass flows at all WWTP and SSI primary influents/effluents, including thermal-treatment derived airborne emissions. WWTP-level net mass flows (NMFs) of total PFAS were not statistically different from zero. SSI-level NMFs indicate that PFAS, and specifically perfluoroalkyl acids (PFAAs), are being broken down. The NMF of perfluoroalkyl sulfonic acids (PFSAs; -274 ± 34 mg/day) was statistically significant. The observed breakdown primarily occurred in the sewage sludge. However, the total PFAS destruction and removal efficiency of 51 % indicates the SSI may inadequately remove PFAS. The statistically significant IF source (NMF = 16 ± 4.2 kg/day) compared to the sink of PFAS as fluoride (NMF = -0.00036 kg/day) suggests that other fluorine-containing substances are breaking down in the SSI. WWTP PFAS mass discharges were primarily to the aquatic environment (>99 %), with <0.5 % emitted to the atmosphere/landfill. Emission rates for formerly phased-out PFOS and PFOA were compared to previously reported levels. Given the environmental persistence of these compounds, the observed decreases in PFOS and PFOA discharge rates from prior reports implies regional/local differences in emissions or possibly their accumulation elsewhere. PFAS were observed in stack gas emissions, but modestly contributed to NMFs and showed negligible contribution to ambient air concentrations observed downwind.

A Novel Plenoptic Camera-Based Measurement System for the Investigation into Flight and Combustion Behavior of Refuse-Derived Fuel Particles

In the past several decades, refuse-derived fuels (RDFs) have been widely applied in industrial combustion processes, for instance, in cement production. Since RDF is composed of various waste fractions with complex shapes, its flight and combustion behaviors can be relatively complicated. In this paper, we present a novel plenoptic camera-based spatial measurement system that uses image processing approaches to determine the dwell time, the space-sliced velocity in the depth direction, and the ignition time of various applied RDF fractions based on the obtained images. The image processing approach follows the concept of tracking-by-detection and includes a novel combined detection method, a 2.5D multiple particle tracking algorithm, and a postprocessing framework to tackle the issues in the initial tracking results. The thereby obtained complete spatial fuel trajectories enable the analysis of the flight behaviors elaborated in the paper. The acquired particles' properties (duration, velocity, and ignition time) reversely prove the availability and applicability of the developed measurement system. The adequacy and accuracy of the proposed novel measurement system are validated by the experiments of detecting and tracking burning and nonburning fuel particles in a rotary kiln. This new measurement system and the provided experimental results can benefit a better understanding of the RDF's combustion for future research.

Thermal Decomposition of Two Gaseous Perfluorocarboxylic Acids: Products and Mechanisms

The thermal decomposition products and mechanisms of per- and polyfluoroalkyl substances (PFASs) are poorly understood despite the use of thermal treatment to remediate PFAS-contaminated media. To identify the thermal decomposition products and mechanisms of perfluorocarboxylic acids (PFCAs), gaseous perfluoropropionic acid (PFPrA) and perfluorobutyric acid (PFBA) were decomposed in nitrogen and oxygen at temperatures from 200 to 780 °C. In nitrogen (i.e., pyrolysis), the primary products of PFPrA were CF₂═CF₂, CF₃CF₂H, and CF₃COF. CF₃CF═CF₂ was the dominant product of PFBA. These products are produced by HF elimination (detected as low as 200 °C). CF₄ and C₂F₆ were observed from both PFCAs, suggesting formation of perfluorocarbon radical intermediates. Pyrolysis products were highly thermally stable, resulting in poor defluorination. In oxygen (i.e., combustion), the primary product of both PFPrA and PFBA below 400 °C was COF₂, but the primary product was SiF₄ above 600 °C due to reactions with the quartz reactor. Oxygen facilitated thermal defluorination by reacting with PFCAs and with pyrolysis products (i.e., fluoroolefins and fluorocarbon radicals). Platinum improved combustion of PFCAs to COF₂ at temperatures as low as 200 °C, while quartz promoted the combustion of PFCAs into SiF₄ at higher temperatures (>600 °C), highlighting the importance of surface reactions that are not typically incorporated into computational approaches.

The Impact of Different Overlay Materials on the Tactile Detection of Virtual Straight Lines

To improve the perception of haptic feedback, materials and sense-modifier effects should be examined. Teflon, Nylon mesh, and Silicone overlays were tested in combination with lateral vibrations to study their impact on the tactile sense. A feelable point moving along a line was implemented through the use of a dynamically moving interference maximum generated via the offset actuation of four haptic exciters affixed to corners of a Gorilla Glass surface. This feedback was presented to eight participants in a series of randomized experiments. Both the Nylon mesh and Teflon covering revealed a statistically significant (p < 0.05) impact of improvement to the user performance in the task of dynamic haptic virtual straight lines localization. While Silicone covering, having three times greater friction than Gorilla Glass, has less or no impact on both decision time, the number of task repetitions, and error rate (p > 0.05). The lateral vibration modifier (60 Hz) can also successfully be used with an increase in performance by about twofold, at least that was demonstrated for both the Nylon mesh and Teflon covering.

A critical review of the application of polymer of low concern regulatory criteria to fluoropolymers II: Fluoroplastics and fluoroelastomers

Fluoropolymers are a distinct class of per- and polyfluoroalkyl substances (PFAS), high molecular weight (MW) polymers with fluorine attached to their carbon-only backbone. Fluoropolymers possess a unique combination of properties and unmatched functional performance critical to the products and manufacturing processes they enable and are irreplaceable in many uses. Fluoropolymers have documented safety profiles; are thermally, biologically, and chemically stable, negligibly soluble in water, nonmobile, nonbioavailable, nonbioaccumulative, and nontoxic. Although fluoropolymers fit the PFAS structural definition, they have very different physical, chemical, environmental, and toxicological properties when compared with other PFAS. This study describes the composition, uses, performance properties, and functionalities of 14 fluoropolymers, including fluoroplastics and fluoroelastomers, and presents data to demonstrate that they satisfy the widely accepted polymer hazard assessment criteria to be considered polymers of low concern (PLC). The PLC criteria include physicochemical properties, such as molecular weight, which determine bioavailability and warn of potential hazard. Fluoropolymers are insoluble (e.g., water, octanol) solids too large to migrate into the cell membrane making them nonbioavailable, and therefore, of low concern from a human and environmental health standpoint. Further, the study results demonstrate that fluoropolymers are a distinct and different group of PFAS and should not be grouped with other PFAS for hazard assessment or regulatory purposes. When combined with an earlier publication by Henry et al., this study demonstrates that commercial fluoropolymers are available from the seven participating companies that meet the criteria to be considered PLC, which represent approximately 96% of the global commercial fluoropolymer market. Integr Environ Assess Manag 2023;19:326-354. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Thermal kinetics of PFAS and precursors in soil: Experiment and surface simulation in temperature-time plane

Per- and polyfluoroalkyl substances (PFAS) are chemically and thermally stable due to the presence of carbon-fluorine (C-F) bond in their molecular structures, hence have been previously formulated as firefighting ingredients. During the firefighting process, however, owing to the high temperature, PFAS can be potentially degraded, particularly for PFAS precursors that contain non-C-F bonds, which is studied herein by exposing PFAS-contaminated soil in a muffle furnace oven. Different temperatures and time intervals are applied to the real soil sample to mimic the firing process and to evaluate the degradation and conversion of PFAS. This thermal treatment can not only degrade precursors (e.g. 6:2 fluorotelomer sulphonate), but also degrade perfluoroalkyl carboxylates (PFCA, e.g. perfluorooctanoic acid PFOA) and perfluoroalkyl sulfonates (PFSA, e.g. perfluorooctane sulfonate PFOS). The concentration dependence of the PFAS on temperature and time is fitted using a 2D Gaussian surface to simulate the complex thermal kinetic, and to compare with the traditional approach such as thermogravimetric analysis (TGA) (1D dependence on temperature only). The 2D simulation can directly visualise the thermal kinetic of individual or sum PFAS in the complex temperature-time plane, which depends on the sample background and particularly on the coexist PFAS precursors. Overall, this study provides a simple approach to monitor and optimise the thermal treatment of the PFAS-contaminated soil.

Enhancement mechanism of magnetite on the ball-milling destruction of perfluorooctane sulfonate by iron

Zero-valent iron (Fe) is commonly employed as an additive for the mechanochemical destruction (MCD) of organic pollutants. The poly- and perfluoroalkyl substances (e.g., perfluorooctane sulfonate, PFOS) are a class of toxic environmental pollutants that are difficult to effectively degrade due to their thermodynamic and chemical stability. In this study, magnetite (Fe₃O₄) was applied to improve the milling performance of Fe to PFOS and its promoting mechanisms were emphatically explored. The desulfurization rate was in ahead of the defluorination rate because the C-S bond is less stable than the C-F bonds in PFOS. Fe₃O₄ had an excellent reinforcement effect on the milling performance of Fe, which was mainly through accelerating the electron transfer as a conductor, reacting with Fe to produce FeO, and facilitating the formation of HO^●. During the MCD of PFOS with Fe/Fe₃O₄ as an additive, HO^● played a dominant role in the defluorination process (accounting for >67%). After the elimination of sulfonate group (-SO₃^-), the produced radical (C₇F₁₅CF₂^●) continued to react through two main pathways: one was the stepwise defluorination after hydrogenation, and the other one was oxidation reaction after alcoholization to yield the corresponding aldehydes and carboxylic acids. The optimum Fe fraction (M_Fe) was 30%, and air atmosphere was more effective than oxygen and nitrogen conditions. This study helps to comprehensively understand the role of Fe₃O₄ in defluorination and fills the gap of Fe/Fe₃O₄ application in the MCD of PFASs.

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.

Widespread Occurrence of Non-Extractable Fluorine in Artificial Turfs from Stockholm, Sweden

Per- and polyfluoroalkyl substances (PFAS) are frequently used in the production of rubber and plastic, but little is known about the identity, concentration, or prevalence of PFAS in these products. In this study, a representative sample of plastic- and rubber-containing artificial turf (AT) fields from Stockholm, Sweden, was subjected to total fluorine (TF), extractable organic fluorine (EOF), and target PFAS analysis. TF was observed in all 51 AT samples (ranges of 16-313, 12-310, and 24-661 μg of F/g in backing, filling, and blades, respectively), while EOF and target PFAS occurred in <42% of all samples (<200 and <1 ng of F/g, respectively). A subset of samples extracted with water confirmed the absence of fluoride. Moreover, application of the total oxidizable precursor assay revealed negligible perfluoroalkyl acid (PFAA) formation across all three sample types, indicating that the fluorinated substances in AT are not low-molecular weight PFAA precursors. Collectively, these results point toward polymeric organofluorine (e.g., fluoroelastomer, polytetrafluoroethylene, and polyvinylidene fluoride), consistent with patent literature. The combination of poor extractability and recalcitrance toward advanced oxidation suggests that the fluorine in AT does not pose an imminent risk to users. However, concerns surrounding the production and end of life of AT, as well as the contribution of filling and blades to environmental microplastic contamination, remain.

Design and qualification of a bench-scale model for municipal waste-to-energy combustion

This paper reports the design and qualification of the first purpose-built, bench-scale reactor system to model the municipal waste-to-energy combustion of fluorinated polymers. Using the principle of similarity, the gas-phase combustion zone of a typical municipal waste-to-energy plant has been scaled down to the bench with a focus on chemical similarity. Chemical similarity is achieved in large part through the use of methanol as a surrogate for municipal solid waste (MSW). Review of prior research shows that methanol is one of the major volatile products expected during MSW thermal conversion in the fuel bed of waste-to-energy plants. Like full-scale waste-energy plants, the design of the bench-scale model includes a flame zone and a post-flame zone. Maintaining steady methanol vapor flow premixed with air to the model reactor system ensures stable combustion resulting in bench-scale CO emission levels comparable to those of full-scale waste-to-energy plants. Since investigation of fluorinated polymer combustion includes trace analysis of exhaust gas for perfluorooctanoic acid (PFOA), qualification testing focused on PFOA collection efficiency. High PFOA collection efficiency (>90%) demonstrated the capability of the reactor system in transporting and absorbing PFOA that may be generated during high-temperature combustion testing of fluorinated polymers. Overall, the bench-scale system is qualified for its intended use to investigate potential generation of PFOA from combustion of fluorinated polymers under conditions representative of waste-to-energy combustion.Implications: Decision-makers depend on environmental researchers to provide reliable predictions of pollutant emissions from waste combustion of polymers at end of product life. Reliable predictions are especially important with regard to questions about potential PFOA emissions from municipal waste combustion of fluorinated polymers. Results from qualification testing confirm that the novel bench-scale model reactor system is capable of representing gas-phase municipal waste combustion behavior upstream of air pollution control and generating representative exhaust gas samples for off-line trace-level analysis of PFOA.

Thermal treatment of carbon-fibre-reinforced polymers (Part 2: Energy recovery and feedstock recycling)

The use of carbon fibre (CF)-reinforced plastics has grown significantly in recent years, and new areas of application have been and are being developed. As a result, the amount of non-recyclable waste containing CF is also rising. There are currently no treatment methods for this type of waste. Within this project different approaches for the treatment of waste containing CF were investigated. Main subject of the research project were large-scale investigations on treatment possibilities and limits of waste containing CF in high temperature processes, with focus on the investigation of process-specific residues and possible fibre emission. The results showed that the two conventional thermal waste treatment concepts with grate and rotary kiln firing systems are not suitable for a complete oxidation of CFs due to the insufficient process conditions (temperature and dwell time). The CFs were mainly discharged via the bottom ash/slag. Due to the partial decomposition during thermal treatment, World Health Organization (WHO) fibres occurred in low concentrations. The tests run in the cement kiln plant have shown the necessity of comminution for waste containing CF. With respect to the short testing times and moderate quantities of inserted CF, a final evaluation of the suitability of this disposal path was not possible. The use of specially processed waste containing CF (carbon-fibre-reinforced plastic (CFRP) pellets) as a carbon substitute in calcium carbide production led to high carbon conversion rates. In the unburned furnace dust, which is marketed as a by-product of the process, CFs in relevant quantities could be detected.

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.

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.

PFAS fate and destruction mechanisms during thermal treatment: a comprehensive review

Per- and polyfluoroalkyl substances (PFAS) are persistent chemicals and have been detected throughout the environment. Thermal treatment is the most common remediation approach for PFAS-contaminated solid wastes. Although various thermal treatment techniques have demonstrated the potential to destruct PFAS, the fate of PFAS, removal efficacy, potential emissions, and the formation of incomplete combustion products during thermal treatment are little known. This study provides a critical review on the behavior of PFAS based on different types of thermal treatment technologies with various PFAS-impacted environmental medias that include water, soil, sewage sludge, pure PFAS materials, and other PFAS-containing wastes. Different extents of PFAS thermal destruction are observed across various thermal treatment techniques and operating conditions. PFAS removal and destruction efficiencies rely heavily on PFAS structures, the complex combustion chemistry, the presence or absence of oxygen, temperature, and other operational conditions. This review also covers proposed PFAS thermal destruction mechanisms. Different thermal destruction mechanisms for perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS), and other PFAS are reviewed and compared. The majority of studies about PFAS thermal destruction mechanisms were focused on a specific list of PFAS and based mostly on the pyrolysis treatment. The basic pathway for PFAS destruction during pyrolysis is hydrodefluorination, which could be largely influenced by the alkaline condition. Future field-scale research that involves the characterization of PFAS destruction products and incomplete combustion products is needed to address public concerns and better emission control.

PFAS Molecules: A Major Concern for the Human Health and the Environment

Per- and polyfluoroalkyl substances (PFAS) are a group of over 4700 heterogeneous compounds with amphipathic properties and exceptional stability to chemical and thermal degradation. The unique properties of PFAS compounds has been exploited for almost 60 years and has largely contributed to their wide applicability over a vast range of industrial, professional and non-professional uses. However, increasing evidence indicate that these compounds represent also a serious concern for both wildlife and human health as a result of their ubiquitous distribution, their extreme persistence and their bioaccumulative potential. In light of the adverse effects that have been already documented in biota and human populations or that might occur in absence of prompt interventions, the competent authorities in matter of health and environment protection, the industries as well as scientists are cooperating to identify the most appropriate regulatory measures, substitution plans and remediation technologies to mitigate PFAS impacts. In this review, starting from PFAS chemistry, uses and environmental fate, we summarize the current knowledge on PFAS occurrence in different environmental media and their effects on living organisms, with a particular emphasis on humans. Also, we describe present and provisional legislative measures in the European Union framework strategy to regulate PFAS manufacture, import and use as well as some of the most promising treatment technologies designed to remediate PFAS contamination in different environmental compartments.

Per- and poly-fluoroalkyl substance remediation from soil and sorbents: A review of adsorption behaviour and ultrasonic treatment

Per- and poly-fluoroalkyl substances (PFAS) are xenobiotics, present at variable concentrations in soils and groundwater worldwide. Some of the current remediation techniques being researched or applied for PFAS-impacted soils involve solidification-stabilisation, soil washing, excavation and disposal to landfill, on site or in situ smouldering, thermal desorption, ball milling and incineration. Given the large volumes of soil requiring treatment, there is a need for a more environmentally friendly technique to remove and treat PFASs from soils. Sorbents such as granular/powdered activated carbon, ion exchange resins and silicas are used in water treatment to remove PFAS. In this work, PFAS adsorption mechanisms and the effect of pore size, pH and organic matter on adsorption efficacy are discussed. Then, adsorption of PFAS to soils and sorbents is considered when assessing the viability of remediation techniques. Sonication-aided treatment was predicted to be an effective removal technique for PFAS from a solid phase, and the effect of varying frequency, power and particle size on the effectiveness of the desorption process is discussed. Causes and mitigation strategies for possible cavitation-induced particle erosion during ultrasound washing are also identified. Following soil remediation, degrading the extracted PFAS using sonolysis in a water-organic solvent mixture is discussed. The implications for future soil remediation and sorbent regeneration based on the findings in this study are given.

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.

High-Temperature Fluorocarbon Chemistry Revisited

The thermal dissociation reactions of C₂F₄ and C₂F₆ were studied in shock waves over the temperature range 1000-4000 K using UV absorption spectroscopy. Absorption cross sections of C₂F₄, CF₂, CF, and C₂ were derived and related to quantum-chemically modeled oscillator strengths. After confirming earlier results for the dissociation rates of C₂F₄, CF₃, and CF₂, the kinetics of secondary reactions were investigated. For example, the reaction CF₂ + CF₂ → CF + CF₃ was identified. Its rate constant of 10¹⁰ cm³ mol^-1 s^-1 near 2400 K is markedly larger than the limiting high-pressure rate constant of the dimerization CF₂ + CF₂ → C₂F₄, suggesting that the reaction follows a different path. When the measurements of the thermal dissociation CF₂ (+Ar) → CF + F (+Ar) are extended to temperatures above 2500 K, the formation of C₂ radicals was shown to involve the reaction CF + CF → C₂F + F (modeled rate constant 8.0 × 10¹² (T/3500 K)^1.0 exp(-4400 K/T) cm³ mol^-1 s^-1) and the subsequent dissociation C₂F (+Ar) → C₂ + F + (Ar) (modeled limiting low-pressure rate constant 3.0 × 10¹⁶ (T/3500 K)^-4.0 exp(-56880 K/T) cm³ mol^-1 s^-1). This mechanism was validated by monitoring the dissociation of C₂ at temperatures close to 4000 K. Temperature- and pressure-dependences of rate constants of reactions involved in the system were modeled by quantum-chemistry based rate theory.

PFAS removal by ion exchange resins: A review

Per- and poly-fluoroalkyl substances (PFAS) represent a large family of anthropogenic organic compounds with a wide range of industrial and commercial applications. PFAS have become a global concern due to their toxicity and bio-accumulative properties. PFAS species have been ubiquitously detected in natural waters, wastewaters, sludge, and aquatic and terrestrial species which are anionic, zwitterionic and neutral. The ion exchange (IX) process for PFAS removal is an efficient technology for the remediation of PFAS-laden surface, ground and effluent wastewaters. This approach is more effective towards eliminating emerging short-chain PFAS which are not removed by carbon-based adsorption processes. This article presents a state-of-the-art review of PFAS removal from water via IX process. The evaluation and comparison of various IX resins in terms of kinetics and isotherms is presented. Literature data indicates that IX isotherm uptake capacity for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) can range up to 5 mmol/g on commercially available IX resins such as IRA 958 and IRA 67. The mechanism involved in the PFAS uptake process, such as diffusion, electrostatic interactions and hydrophobic effects are discussed. The effects of the eluent variability on the regeneration efficacy are also highlighted and the effect of single-use vs reuse for newly developed PFAS-specific IX resins are also examined based on the reviewed literature.

Per- and polyfluoroalkyl substances thermal destruction at water resource recovery facilities: A state of the science review

Per- and polyfluoroalkyl substances (PFAS) are a recalcitrant group of chemicals and can be found throughout the environment. They often collect in wastewater systems with virtually no degradation prior to environmental discharge. Some PFAS partitions to solids captured in wastewater treatment which require further processing. Of all the commonly applied solids treatment technologies, incineration offers the only possibility to completely destroy PFAS. Little is known about the fate of PFAS through incineration, in particular, for the systems employed in water resource recovery facilities (WRRF). This review covers available research on the fate of PFAS through incineration systems with a focus on sewage sludge incinerators. This research indicates that at least some PFAS destruction will occur with incineration approaches used at WRRFs. Furthermore, PFAS in flue gas, ash, or water streams used for incinerator pollution control may be undetectable. Future research involving full-scale fate studies will provide insight on the efficacy of PFAS destruction through incineration and whether other compounds of concern are generated. PRACTITIONER POINTS: Thermal processing is the only commercial approach available to destroy PFAS. Thermal degradation conditions required for destruction of PFAS during incineration processes are discussed. Fate of PFAS through water resource recovery facility incineration technologies remains unclear. Other thermal technologies such as smoldering combustion, pyrolysis, gasification, and hydrothermal liquefaction provide promise but are in developmental phases.

Mechanochemical remediation of perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) amended sand and aqueous film-forming foam (AFFF) impacted soil by planetary ball milling

Per- and polyfluoroalkyl substances (PFAS) are manmade, fluorinated organic chemicals which have been identified as persistent organic pollutants. PFAS have surface active properties that have made them suitable for applications in oil- and water-resistant products, as well as many firefighting foams. No on-site remediation strategies exist to treat PFAS impacted soils. Mechanochemical remediation of PFOS- and PFOA-amended sand via a planetary ball mill was studied. The effect of sand mass, KOH as a co-milling reagent, and water saturation on the degradation of PFOA and PFOS was evaluated. By 4 h of milling concentrations were reduced by up to 98% for PFOS-amended dry sand and 99% for PFOA-amended dry sand without the addition of a co-milling reagent. Water saturation was determined to be a significant hindrance on the mechanochemical destruction of PFOS and PFOA. A maximum of 89% of fluoride was recovered from PFOS-amended sand when KOH was used as a co-milling reagent. It is hypothesized that reactive particles generated from the fracture of sand grains react with PFAS molecules to initiate destruction, which can result in full defluorination. Milling experiments were also conducted on soils from a Canadian firefighting training area (FFTA), demonstrating that PFOS concentrations can be reduced by up to 96% in site soils. For the first time, ball milling for the remediation of PFAS in environmental media has been demonstrated using amended sand and legacy soils from a FFTA.

A chemical kinetic model for the decomposition of perfluorinated sulfonic acids

Perfluorinated sulfonic acids (such as perfluorooctanesulfonic, PFOS, and short-chain analogues) are notorious halogenated pollutants that exhibit severe toxicity, even at minute levels. Limited number of experimental studies addressed their thermal decomposition at elevated temperatures. Such scenarios are particularly relevant to open fires and incineration of materials laden with perfluoroalkyl compounds (PFCs). Herein, we construct a detail kinetic model that illustrates major chemical reactions underpinning initial degradation of 1-butanesulfonic acid (CF₃(CF₂)₃SO₂OH), as a model compound of PFOS, and perfluorinated sulfonic acids in general. Reaction rate parameters were estimated based on an accurate density functional theory (DFT) formalism. The kinetic model incorporates four sets of reactions, namely, unimolecular decomposition channels, hydrofluorination, hydrolysis, and fragmentation of the alkyl chain. Results are discussed considering recent experimental measurements. Temperature-dependent profiles for a large array of perfluoroalkyl acyl fluorides, short perfluorinated cuts, and perfluorinated cyclic compounds, are presented. SO₂ emerges as the main sulfur carrier, with a minor contribution from SO₃. HF addition to double carbon bonds in alkenes, and to carbonyl bonds in aldehydic structures signifies a major sink pathway for hydrogen fluoride. Addition of moisture was shown to expedite the destruction of relatively large perfluoroalkyl acyl fluorides into C₁ species. Construction of this model could aid in a better understanding of the fate and chemical transformation of PFCs under a pyrolytic environment pertinent to waste incineration and fluorine mineralization.

Disposal of products and materials containing per- and polyfluoroalkyl substances (PFAS): A cyclical problem

Per- and polyfluoroalkyl substances (PFAS), highly stable and persistent chemicals used in numerous industrial applications and consumer goods, pose an exceptionally difficult challenge for disposal. Three approaches are currently available for PFAS wastes: landfilling, wastewater treatment and incineration. Each disposal approach can return either the original PFAS or their degradation products back to the environment, illustrating that the PFAS problem is cyclical. Landfilling and wastewater treatment do not destroy PFAS and simply move PFAS loads between sites. Consumer products and various materials discarded in landfills leach PFAS over time, and landfill leachate is commonly sent to wastewater treatment plants. From wastewater treatment plants, PFAS are carried over to sludge and effluent. Sewage sludge can be landfilled, incinerated, or applied on agricultural fields, and PFAS from treated sludge (biosolids) can contaminate soil, water, and crops. Incineration of PFAS-containing wastes can emit harmful air pollutants, such as fluorinated greenhouse gases and products of incomplete combustion, and some PFAS may remain in the incinerator ash. Volatile PFAS are emitted into the air from landfills and wastewater treatment plants, and research is urgently needed on the potential presence of PFAS compounds in air emissions from commercially run incinerators. Monitoring of waste streams for PFAS, stopping PFAS discharges into water, soil and air and protecting the health of fence-line communities close to the waste disposal sites are essential to mitigate the impacts of PFAS pollution on human health.

Are Fluoropolymers Really of Low Concern for Human and Environmental Health and Separate from Other PFAS?

Fluoropolymers are a group of polymers within the class of per- and polyfluoroalkyl substances (PFAS). The objective of this analysis is to evaluate the evidence regarding the environmental and human health impacts of fluoropolymers throughout their life cycle(s). Production of some fluoropolymers is intimately linked to the use and emissions of legacy and novel PFAS as polymer processing aids. There are serious concerns regarding the toxicity and adverse effects of fluorinated processing aids on humans and the environment. A variety of other PFAS, including monomers and oligomers, are emitted during the production, processing, use, and end-of-life treatment of fluoropolymers. There are further concerns regarding the safe disposal of fluoropolymers and their associated products and articles at the end of their life cycle. While recycling and reuse of fluoropolymers is performed on some industrial waste, there are only limited options for their recycling from consumer articles. The evidence reviewed in this analysis does not find a scientific rationale for concluding that fluoropolymers are of low concern for environmental and human health. Given fluoropolymers' extreme persistence; emissions associated with their production, use, and disposal; and a high likelihood for human exposure to PFAS, their production and uses should be curtailed except in cases of essential uses.

Research and Regulatory Advancements on Remediation and Degradation of Fluorinated Polymer Compounds

Per- and polyfluoroalkyl substances (PFAS) are a class of chemicals used in various commercial industries to include food packaging, non-stick repellent, and waterproof products. International environmental protection agencies are currently looking for ways to detect and safely remediate both solid and aqueous PFAS waste due to their harmful effects. Incineration is a technique that disposes of chemicals by breaking down the chemicals at high temperatures, upwards of 1400 °C. Incineration has been used on other related compounds, but PFAS presents a challenge during thermal degradation due to the molecular stability and reactivity of fluorine. Research on the efficacy of this method is currently limited, as the degradation byproducts of PFAS are not fully characterized. Current research is mostly focused on the development of benchtop methods for the safe remediation of solid PFAS waste. Aqueous fire fighting foams (AFFFs) have garnered significant attention due to extensive use since development in the 1960s. Numerous communities that are closely located near airports have been shown to have higher than average PFAS contamination from the repeated use. Detection and remediation of surface, subsurface, and wastewater have become a primary concern for environmental agencies. Use of electrochemical techniques to remove the PFAS contaminants has shown recent promise to help address this issue. Critical to the remediation efforts is development of standardized detection techniques and the implementation of local and international regulations to control the production and use of fluorinated products. No single solution has yet been developed, but much progress has been made in recent years in governmental regulation, detection, and remediation techniques.