Thermal Decomposition of Two Gaseous Perfluorocarboxylic Acids: Products and Mechanisms

Interplay of surface oxygen content and pore water during thermal regeneration of granular activated carbons

This study aims at unraveling the interplay between thermal regeneration and granular activated carbon (GAC) properties. First, twelve commercially available, unused, and thoroughly characterized GACs were pyrolyzed without per- and polyfluoroalkyl substances (PFAS) using a thermogravimetric analyzer at a 25 ºC/min heating rate up to 750 ºC under nitrogen. GACs with elevated oxygen content showed substantial weight loss due to the decomposition of acidic functional groups, leading to the formation of larger pores but a decrease in specific surface area and physical hardness. Additionally, pore water capacity of GAC influenced the regeneration, as water molecules sorbed to surface oxygenated groups via hydrogen bonding contributed to the formation of carboxylic acids and subsequent decomposition. Next, PFAS destruction mechanism during regeneration was demonstrated by loading GAC with perfluorooctane sulfonic acid (PFOS). Gaseous product analysis showed a catalytic effect for PFOS-laden GAC, where hydrogen fluoride formation occurred at 50 ºC lower temperatures than in pure PFOS, indicating improved PFOS thermolysis when adsorbed by GAC. This catalytic effect is likely due to chemical interactions between PFOS and the delocalized electrons on the GAC surface, warranting further investigation. The diverse GAC responses during regeneration underscore the importance of understanding regeneration conditions to maintain GAC functionality while enhancing PFAS treatment sustainability.

Roadmap to Catalytic Abatement of Gas Phase Per- and Polyfluoroalkyl Substances (PFAS)

While the outstanding stability of per- and polyfluoroalkyl substances (PFAS) paved the way for their widespread application in a huge variety of applications, it also resulted in their nickname "forever chemicals". The rising awareness for PFAS-related environmental and health concerns drives a discussion on the most effective ways to abate PFAS emissions into the environment, i.e. water, soil, and air, and remediation of contaminated matter. In order to address the knowledge gap regarding air pollution by PFAS, this minireview summarizes the current corpus of work in the field and outlines how catalysis can contribute to PFAS abatement in the gas phase. Beyond a mere collection of state-of-the-art knowledge, overarching challenges in catalytic PFAS removal are identified, spanning from fundamental organic and inorganic chemistry, i.e. C-F-bond activation, to heterogeneous catalysis, i.e. surface reactions at the gas-solid interface, to reaction engineering, i.e. scaling relations and technical hurdles. In addition, the article introduces concepts and workflows that aim at providing guidance during the design of technological solutions for the efficient control of gaseous PFAS emissions.

PFAS removal via adsorption: A synergistic review on advances of experimental and computational approaches

Per- and polyfluoroalkyl substances (PFAS), commonly known as "forever chemicals", have become a major focus of current research due to their toxicity and persistence in the environment. These synthetic compounds are notoriously difficult to degrade, accumulating in water systems and posing long-term health and environmental risks. Adsorption is one of the most investigated technologies for PFAS removal. This review comprehensively reviewed the PFAS adsorption process, focusing not only on the adsorption itself, but also on the behavior of PFAS in the aquatic environment prior to adsorption because these behaviors directly affect PFAS adsorption. Significantly, this review summarized in detail the advances made in PFAS adsorption from the computational approach and emphasized the importance of integrated experimental and computational studies in gaining molecular-level understanding on the adsorption mechanisms of PFAS. Toward the end, the review identified several critical research gaps and suggested key interdisciplinary research needs for further advancing our understanding on PFAS adsorption.

Perfluoropropionic Acid (CF3CF2C(O)OH): Three Conformations and Dimer Formation

Perfluoropropionic acid (CF3CF2C(O)OH) has been investigated with a focus on its complex structural properties. As a formal derivative of propanoic acid, the incorporation of fluorine atoms imparts unique structural features, including three distinct monomeric conformations and a dimeric structure. This study presents experimental findings, supported by computational modeling, to explore these characteristics. The analysis includes an FTIR study of the isolated species in an Ar-cryogenic matrix and the low-temperature determination of its crystalline structure using single-crystal X-ray diffraction.

Thermochemical Recycling and Degradation Strategies of Halogenated Polymers (F-, Cl-, Br-): A Holistic Review Coupled with Mechanistic Insights

Handling the waste associated with halogenated polymers is a daunting task due to the well-documented emission of halogen-bearing toxicants during the disposal or recycling operation. According to the Stockholm Convention treaty, most of these products are classified as persistent organic pollutants due to their potential health hazards. This review aims to provide a holistic overview of the recent updates for treating halogenated polymeric waste through physical, chemical and biological approaches. In the line of inquiry, critical analysis of the obstacles and prospects associated with each degradation technique on the halogenated polymer has been performed, assessing based on the degradation efficiency, treatment upscaling, pollution control, and feasibility. Though many treatments show promising results, they also entail drawbacks. Thermal treatment exploiting various metal oxides, especially calcium additives, is considered the most executable technique for halogenated polymer valorization coupled with mineralization/metal extraction due to its intuitive operational feasibility and potential scalability. Strategies for combating the soaring halogenated polymeric wastes summarized herein tap into promoting a circular economy approach for their sustainable disposal and recycling.

Direct measurement of fluorocarbon radicals in the thermal destruction of perfluorohexanoic acid using photoionization mass spectrometry

Thermal destruction is a critical cornerstone of addressing the rampant contamination of natural resources with per- and polyfluoroalkyl substances (PFAS). However, grave concerns associated with stack emissions from incineration exist because mechanistic studies have thus far relied on ex situ analyses of end products and theoretical calculations. Here, we used synchrotron-based vacuum ultraviolet photoionization mass spectrometry to study the pyrolysis of a representative PFAS-perfluorohexanoic acid-and provide direct evidence of fluorocarbon radicals and intermediates. A key reaction pathway from perfluorocarboxylic acids to ketenes via acyl fluorides is proposed. We furthermore propose CF2/CF3 radical-centered pyrolysis mechanisms and explain their roles in the formation of other products that may form in full-scale incinerators. These results have not only unveiled the role of radicals and intermediates in thermal PFAS decomposition and recombination mechanisms but also provide unique insight into improving the safety and viability of industrial PFAS incineration.

Investigation of Real-Time Gaseous Thermal Decomposition Products of Representative Per- and Polyfluoroalkyl Substances (PFAS)

The thermal decomposition of per- and poly fluoroalkyl substances (PFAS) is poorly understood. Here, we present an innovative, comprehensive analytical method to investigate their thermal decomposition, including perfluorocarboxylic acids (PFCAs), alcohol, sulfonates, and GenX (acid dimer), focusing on identifying their breakdown products. In this study, evolved gas analysis-mass spectrometry (EGA-MS) was used for fast real-time screening to determine the significant temperatures to be investigated with the thermal desorption-pyrolysis coupled with gas chromatography-mass spectrometry (TD-Py-GC-MS), which provided detailed information about evolved PFAS and their breakdown products. This approach enabled a systematic study of perfluorocarboxylic acids (PFCAs) ranging from C3 to C9 and GenX showing volatilization, followed by degradation and formation of respective perfluorinated-1-alkenes and C5F10O perfluorinated ether (from GenX). At elevated temperatures (e.g., 600 °C), the products observed included perfluorinated butene and higher molecular-weight products, likely formed by pyrolytic polymerization of perfluorinated radicals. 1H,1H,2H,2H-perfluoro-1-decanol, i.e., 8:2 FTOH, volatilized at 100 °C; however, at higher temperatures, several novel decomposition products were observed, including perfluoro-1-decene and perfluorinated compounds suggesting the presence of the hydroxylic group. Our method offers an alternative approach to studying the thermal behavior of currently regulated and emerging PFAS with a focus on application to a wide range of matrices (laboratory grade standards or environmental samples).

New Insights into Thermal Degradation Products of Long-Chain Per- and Polyfluoroalkyl Substances (PFAS) and Their Mineralization Enhancement Using Additives

The products of incomplete destruction (PIDs) of per- and polyfluoroalkyl substances (PFAS) represent a substantial ambiguity when employing thermal treatments to remediate PFAS-contaminated materials. In this study, we present new information on PIDs produced in both inert and oxidative environments from five long-chain PFAS, including three now regulated under the U.S. Safe Drinking Water Act, one cationic precursor compound, and one C10 PFAS. The data did not support the generation of tetrafluoromethane from any of the studied PFAS, and carbonyl fluoride was found only from potassium perfluorooctanesulfonate (K-PFOS) when heated in air in a narrow temperature range. Oxidative conditions (air) were observed to facilitate PFAS thermal degradation and accelerate the mineralization of K-PFOS. Spectroscopic data suggest that PFAS thermal degradation is initiated by the cleavage of bonds that form perfluoroalkyl radicals, leading to organofluorine PIDs (e.g., perfluoroalkenes). In air, perfluoroalkyl radicals react with oxygen to form oxygen-containing PIDs. The mineralization of PFAS was enhanced by adding solid additives, which were categorized as highly effective (e.g., granular activated carbon (GAC) and certain noble metals), moderately effective, and noneffective. Remarkably, simply by adding GAC, we achieved >90% mineralization of perfluorooctanoic acid at 300 °C and ∼1.9 atm within just 60 min without using water or solvents.

Pyrolysis of Two Perfluoroalkanesulfonates (PFSAs) and PFSA-Laden Granular Activated Carbon (GAC): Decomposition Mechanisms and the Role of GAC

Thermal treatment of perfluoroalkyl and polyfluoroalkyl substances (PFASs) presents a promising opportunity to halt the PFAS cycle. However, how co-occurring materials such as granular activated carbon (GAC) influence thermal decomposition products of PFASs, and underlying mechanisms remain unclear. We studied the pyrolysis of two potassium salts of perfluoroalkanesulfonates (PFSAs, CnF2n+1SO3K), perfluorobutanesulfonate (PFBS-K), and perfluorooctanesulfonate (PFOS-K), with or without GAC. PFBS-K is more stable than PFOS-K for pure standards, but when it is adsorbed onto GAC, its thermal stabilities and decomposition behaviors are similar. Temperatures and heating rates can significantly influence the decomposition mechanisms and products for pure standards, while these effects are less pronounced when PFSAs are adsorbed onto GAC. We further studied the underlying decomposition mechanisms. Pure standards of CnF2n+1SO3K can decompose directly in their condense phase by reactions: F(CF2)nSO3K → F(CF2)n-2CF═CF2 + KFSO3 or F(CF2)nSO3K → F(CF2)n- + K+ + SO3. GAC appears to facilitate breakage of the C-S bond to release SO2 at temperatures as low as 280 °C. GAC promotes fluorine mineralization through functional reactive sites. SiO2 is particularly important for the surface-mediated mineralization of PFASs into SiF4. These findings offer valuable insights into optimizing thermal treatment strategies for PFAS-contaminated waste.

Thermal transformations of perfluorooctanoic acid (PFOA): Mechanisms, volatile organofluorine emissions, and implications to thermal regeneration of granular activated carbon

Thermal treatment is effective for the removal of perfluorooctanoic acid (PFOA). However, how temperatures, heating methods, and granular activated carbon (GAC) influence pyrolysis of PFOA, and emission risks are not fully understood. We studied thermal behaviors of PFOA at various conditions and analyzed gaseous products using real-time detection technologies and gas chromatography-mass spectrometry (GC-MS). The thermal decomposition of PFOA is surface-mediated. On the surface of quartz, PFOA decomposed into perfluoro-1-heptene and perfluoro-2-heptene, while on GAC, it tended to decompose into 1 H-perfluoroheptane (C7HF15). Neutral PFOA started evaporating around 100 ℃ without decomposition in ramp heating. During pyrolysis, when PFOA was pre-adsorbed onto GAC, it was mineralized into SiF4 and produced more than 45 volatile organic fluorine (VOF) byproducts, including perfluorocarbons (PFCs) and hydrofluorocarbons (HFCs). The VOF products were longer-chain (hydro)fluorocarbons (C4-C7) at low temperatures (< 500 ℃) and became shorter-chain (C1-C4) at higher temperatures (> 600 ℃). PFOA transformations include decarboxylation, VOF desorption, further organofluorine decomposition and mineralization in ramp heating of PFOA-laden GAC. Decarboxylation initiates at 120 ℃, but other processes require higher temperatures (>200 ℃). These results offer valuable information regarding the thermal regeneration of PFAS-laden GAC and further VOF control with the afterburner or thermal oxidizer.

PFAS remediation: Evaluating the infrared spectra of complex gaseous mixtures to determine the efficacy of thermal decomposition of PFAS

Due to their widespread production and known environmental contamination, the need for the detection and remediation of per- and polyfluoroalkyl substances (PFAS) has grown quickly. While destructive thermal treatment of PFAS at low temperatures (e.g., 200-500 °C) is of interest due to lower energy and infrastructure requirements, the range of possible degradation products remains underexplored. To better understand the low temperature decomposition of PFAS species, we have coupled gas-phase infrared spectroscopy with a multivariate curve resolution (MCR) analysis and a database of high-resolution PFAS infrared reference spectra to characterize and quantify a complex mixture resulting from potassium perfluorooctanesulfonate (PFOS-K) decomposition. Beginning at 375 °C, nine prevalent decomposition products (namely smaller perfluorocarbon species) are identified and quantified.

Formation of Products of Incomplete Destruction (PID) from the Thermal Oxidative Decomposition of Perfluorooctanoic Acid (PFOA): Measurement, Modeling, and Reaction Pathways

The thermal decomposition of perfluorooctanoic acid (PFOA) under oxidative conditions was investigated using air (O2) and N2O as oxidants over temperatures ranging from 400 to 1000 °C in an α-alumina reactor. In the presence of air, PFOA was found to decompose into perfluorohept-1-ene (C7F14) and perfluoroheptanoyl fluoride (C7F14O) in addition to HF, CO, and CO2. At temperatures above 800 °C, both C7F14 and C7F14O were no longer detected. A comprehensive analysis of the reaction mechanisms through quantum chemical analysis and kinetic modeling in combination with experimental observations was utilized to identify key reaction pathways. Quantum chemical analysis led to the conclusion that oxygen atoms are crucial in decomposing perfluoroalk-1-enes, especially the stable perfluorohept-1-ene (C7F14). Under oxidative conditions, it was found that significant quantities of C2F6 and CF4 were formed. Further quantum chemical analysis suggests that the O atoms facilitate the formation of volatile fluorinated compounds (VFCs) such as tetrafluoromethane (CF4) and hexafluoroethane (C2F6), particularly at higher temperatures. By elucidating these key reactions, an improved understanding of the potential formation products of incomplete combustion (PICs) or products of incomplete destruction (PIDs) is made.

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.

Formation of volatile chlorinated and brominated products during low temperature thermal decomposition of the representative PFAS perfluorohexane sulfonate (PFHxS) in the presence of NaCl and NaBr

Per- and polyfluoroalkyl substances (PFAS) are synthetic organofluorine compounds known for their chemical and physical stability as well as their wide range of uses. Some PFAS are widely distributed in the environment, leading to concerns related to both environmental and human health. High temperature thermal treatment (i.e., incineration) has been utilized for PFAS treatment, but this requires significant infrastructure and energy, prompting interest in lower temperature approaches that may still lead to efficient destruction. Lower treatment temperatures, however, increase the potential for incomplete PFAS mineralization and formation of volatile organofluorine (VOF) products. Herein, we report the formation of novel VOF products that include chlorinated and brominated compounds during the thermal treatment of potassium perfluorohexane sulfonate (PFHxS), a representative perfluoroalkyl acid (PFAA). By comparing the gas chromatography-mass spectrometry (GC-MS) results of known VOF stocks to evolved VOF during thermal treatment of PFAS, the formation of perfluorohexyl chloride and perfluorohexyl bromide was observed when PFHxS was heated at temperatures between 275 and 475 °C in the presence of NaCl and NaBr, respectively. To our knowledge, this is the first report of chlorinated or brominated VOF products during thermal treatment of a PFAA. These findings suggest that a range of mixed halogenated VOF may form during thermal treatment of PFAS at relatively low temperature (e.g., 500 °C) and that these can be a function of salts present in the matrix.

Spontaneous Degradation of the "Forever Chemicals" Perfluoroalkyl and Polyfluoroalkyl Substances (PFASs) on Water Droplet Surfaces

Because of their innate chemical stability, the ubiquitous perfluoroalkyl and polyfluoroalkyl substances (PFASs) have been dubbed "forever chemicals" and have attracted considerable attention. However, their stability under environmental conditions has not been widely verified. Herein, perfluorooctanoic acid (PFOA), a widely used and detected PFAS, was found to be spontaneously degraded in aqueous microdroplets under room temperature and atmospheric pressure conditions. This unexpected fast degradation occurred via a unique multicycle redox reaction of PFOA with interfacial reactive species on the droplet surface. Similar degradation was observed for other PFASs. This study extends the current understanding of the environmental fate and chemistry of PFASs and provides insight into aid in the development of effective methods for removing PFASs.

Efficient and fast remediation of soil contaminated by per- and polyfluoroalkyl substances (PFAS) by high-frequency heating

This study presents a novel thermal technology (high-frequency heating, HFH) for the decontamination of soil containing per- and polyfluoroalkyl substances (PFAS) and aqueous film-forming foams (AFFFs). Ultra-fast degradation of short-chain PFAS, long-chain homologs, precursors, legacy PFAS, emerging PFAS was achieved in a matter of minutes. The concentrations of PFAS and the soil type had a negligible impact on degradation efficiency, possibly due to the ultra-fast degradation rate overwhelming potential differences. Under the current HFH experiment setup, we achieved near-complete degradation (e.g., >99.9%) after 1 min for perfluoroalkyl carboxylic acids and perfluoroalkyl ether carboxylic acids and 2 min for perfluoroalkanesulfonic acids. Polyfluoroalkyl precursors in AFFFs were found to degrade completely within 1 min of HFH; no residual cationic, zwitterionic, anionic, or non-ionic intermediate products were detected following the treatment. The gaseous byproducts were considered. Most of gaseous organofluorine products of PFAS at low-and-moderate temperatures disappeared when temperatures reached 890 °C, which is in the temperature zone of HFH. For the first time, we demonstrated minimal loss of PFAS in water during the boiling process, indicating a low risk of PFAS entering the atmosphere with the water vapor. The findings highlight HFH its potential as a promising remediation tool for PFAS-contaminated soils.

Influence of reactor composition on the thermal decomposition of perfluorooctanesulfonic acid (PFOS)

Various reactor tubes (quartz, stainless steel 316 and stainless steel 253 MA) were used to examine their influence on the thermal decomposition of perfluorooctanesulfonic acid (PFOS) between 400 and 1000 °C. Using helium as a carrier gas, with the addition of 100 - 300 ppm of PFOS to the feed gas, the influence of the reactor materials on PFOS decomposition was studied. The quartz reactor led to a notable reduction in the concentration of HF and substantial quantities of SiF4 were observed. Stainless steel 316 produced C2F4, HF, COF2 and SO2 as its primary products up to 800 °C. However, at temperatures above 800 °C, near quantitative removal of SO2 from the gas phase was observed, with the concomitant formation of a blue molybdenum sulfur complex. Stainless steel 253 MA, the composition of which contains over 1% Si produced substantial quantities of SiF4 but no significant decrease in the gas phase concentration of HF. ENVIRONMENTAL IMPLICATION: This research underscores the significant role of reactor material in the thermal treatment of PFAS, a globally widespread and enduring environmental contaminant. The findings have direct implications for the optimization of thermal treatment strategies aimed at mitigating PFAS contamination. The insight into how different reactor materials interact with PFOS during thermal treatment expands our understanding of potential destruction methods. This knowledge is crucial in the development of effective, sustainable strategies for managing persistent environmental pollutants like PFAS.

Predicting pyrolysis decomposition of PFOA using computational nanoreactors: a thermodynamic study

Per- and polyfluoroalkyl substances (PFAS) are a large, complex, environmentally persistent, and ever-expanding group of manufactured chemicals. Disposal of these compounds could produce potentially dangerous products necessitating the need to quickly predict their decomposition products. This study focuses on the thermal decomposition of perfluorooctanoic acid (PFOA) using nanoreactor simulations to find the decomposition products and their respective energies. Applying the nanoreactor method, which is novel for this system, allows for rapid prediction of thermal decomposition pathways with minimal researcher bias and it predicted PFOA to decompose at ∼650 °C, consistent with previously reported experimental studies.

Mechanistic Investigations of Thermal Decomposition of Perfluoroalkyl Ether Carboxylic Acids and Short-Chain Perfluoroalkyl Carboxylic Acids

In this study, we investigated the thermal decomposition mechanisms of perfluoroalkyl ether carboxylic acids (PFECAs) and short-chain perfluoroalkyl carboxylic acids (PFCAs) that have been manufactured as replacements for phased-out per- and polyfluoroalkyl substances (PFAS). C-C, C-F, C-O, O-H, and C═C bond dissociation energies were calculated at the M06-2X/Def2-TZVP level of theory. The α-C and carboxyl-C bond dissociation energy of PFECAs declines with increasing chain length and the attachment of an electron-withdrawing trifluoromethyl (-CF3) group to the α-C. Experimental and computational results show that the thermal transformation of hexafluoropropylene oxide dimer acid to trifluoroacetic acid (TFA) occurs due to the preferential cleavage of the C-O ether bond close to the carboxyl group. This pathway produces precursors of perfluoropropionic acid (PFPeA) and TFA and is supplemented by a minor pathway (CF3CF2CF2OCFCF3COOH → CF3CF2CF2· + ·OCFCF3COOH) through which perfluorobutanoic acid (PFBA) is formed. The weakest C-C bond in PFPeA and PFBA is the one connecting the α-C and the β-C. The results support (1) the C-C scission in the perfluorinated backbone as an effective PFCA thermal decomposition mechanism and (2) the thermal recombination of radicals through which intermediates are formed. Additionally, we detected a few novel thermal decomposition products of studied PFAS.