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

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.

Mineralization of fluoropolymers from combustion in a pilot plant under representative european municipal and hazardous waste combustor conditions

The goal of this study was to provide data to support mineralization of fluoropolymer waste and insignificant generation of PFAS as products of incomplete combustion (PIC) during incineration of fluoropolymer applications at their end-of-life. Destruction efficiency is not an acceptable metric to indicate mineralization and therefore we need to look for and measure products of incomplete destruction. A mixed sample of fluoropolymers representing 80% of commercial fluoropolymers was combusted at conditions representative of municipal and industrial waste incinerators operating in EU. State-of-the-art emission sampling and analytical methods (UPLC-MS/MS, GC-MS) were used for identifying and quantifying those PFAS whose standards were available. Statistical analysis of the results confirmed non-detect to negligible levels of PFAS evidencing mineralization of fluoropolymers.

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.

Investigating the partitioning behavior of per- and polyfluoroalkyl substances (PFAS) during thermal landfill leachate evaporation

Thermal landfill leachate evaporator systems can reduce the volume of leachate by up to 97%, while releasing water vapor and producing residuals (volume-reduced leachate and sludge) that are managed on-site. On-site thermal evaporators offer landfill operators leachate management autonomy without being subject to increasingly stringent wastewater treatment plant requirements. However, little is known about the partitioning of PFAS within these systems, nor the extent to which PFAS may be emitted into the environment via vapor. In this study, feed leachate, residual evaporated leachate, sludge, and condensed vapor were sampled at two active full-scale thermal landfill leachate evaporators and from a laboratory-scale leachate evaporation experiment. Samples were analyzed for 91 PFAS via ultra-high pressure liquid chromatography - tandem mass spectrometry (UHPLC-MS/MS). Similar trends were observed from Evaporator 1, Evaporator 2, and the laboratory-scale evaporator; ∑PFAS were concentrated in the residual evaporated leachate during evaporation by a factor of 5.3 to 20. All condensed vapors sampled (n = 5) contained PFAS, predominantly 5:3 fluorotelomer carboxylic acid (5:3FTCA), (full-scale vapors 729 - 4087 ng/L PFAS; lab-scale vapor 61.0 ng/L PFAS). For Evaporators 1 and 2, an estimated 9 - 24% and 10%, respectively, of the PFAS mass entering the evaporators in leachate was released with vapor during the days of sample collection. '.

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

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

A review of sample collection and analytical methods for detecting per- and polyfluoroalkyl substances in indoor and outdoor air

Per- and polyfluoroalkyl substances (PFAS) are a unique class of chemicals synthesized to aid in industrial processes, fire-fighting products, and to benefit consumer products such as clothing, cosmetics, textiles, carpets, and coatings. The widespread use of PFAS and their strong carbon-fluorine bonds has led to their ubiquitous presence throughout the world. Airborne transport of PFAS throughout the atmosphere has also contributed to environmental pollution. Due to the potential environmental and human exposure concerns of some PFAS, research has extensively focused on water, soil, and organismal detection, but the presence of PFAS in the air has become an area of growing concern. Methods to measure polar PFAS in various matrices have been established, while the investigation of polar and nonpolar PFAS in air is still in its early development. This literature review aims to present the last two decades of research characterizing PFAS in outdoor and indoor air, focusing on active and passive air sampling and analytical methods. The PFAS classes targeted and detected in air samples include fluorotelomer alcohols (FTOHs), perfluoroalkane sulfonamides (FASAs), perfluoroalkane sulfonamido ethanols (FASEs), perfluorinated carboxylic acids (PFCAs), and perfluorinated sulfonic acids (PFSAs). Although the manufacturing of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) has been largely phased out, these two PFAS are still often detected in air samples. Additionally, recent estimates indicate that there are thousands of PFAS that are likely present in the air that are not currently monitored in air methods. Advances in air sampling methods are needed to fully characterize the atmospheric transport of PFAS.

Thermochemistry of Gas-Phase Thermal Oxidation of C2 to C8 Perfluorinated Sulfonic Acids with Extrapolation to C16

New ideal-gas thermochemistry Cp°(T), H°(T), S°(T), and G°(T) are predicted for 53 species involved in the thermal destruction of perfluorinated sulfonic acids (PFSAs) ranging from C2 to C8 in perfluorinated alkyl chain length. Species were selected by considering both the pyrolytic and oxidative pathways of PFSA destruction. After the sulfur-containing moieties are removed, subsequent reactions largely involve species from a prior set of thermochemistry for the thermal destruction of perfluorinated carboxylic acids (Ram et al., J. Phys. Chem. A, 2024, 128, 7, 1313-1326). Enthalpies of formation at 0 K are computed using a new isogyric reaction scheme. Rigid-rotor harmonic-oscillator partition functions were calculated over a 200-2500 K temperature range using rovibrational properties at G4 (≤C3S1 species) and M06-2X-D3(0)/def2-QZVPP (≥C4S1 species), employing the 1D hindered rotor approximation to correct for torsional modes. Seven-coefficient NASA polynomial fits are reported in standardized formats. Bond dissociation energies and important reaction equilibria are examined to provide insights into the reactivity of potentially persistent species. Extrapolated NASA polynomials are also systematically predicted for 126 species larger than C8/C8S1 in size, allowing reasonably accurate estimates of thermochemistry without the need for expensive electronic structure calculations.

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.

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

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

Thermochemistry of Species in Gas-Phase Thermal Oxidation of C2 to C8 Perfluorinated Carboxylic Acids

New thermochemical properties, Cp°(T), H°(T), S°(T), and G°(T), are predicted for 123 species involved in the thermal destruction of perfluorinated carboxylic acids (PFCAs) using computational quantum chemistry and ideal-gas statistical mechanics. Relevant species were identified from the development of mechanisms for the pyrolysis and oxidation of PFCAs of C2 to C8 in length. Partition functions were obtained from the results of calculations at the G4 level for species up to C4 in length and M06-2X-D3(0)/def2-QZVPP for species C5 to C8 in length. The 1D hindered-rotor approximation was used to correct for torsional modes in the larger species. Ideal-gas thermochemistry was computed and fitted to 7-parameter NASA polynomials over a 200-2500 K temperature range, and the data are provided in standardized format. To gauge the effects of both method and basis set choice, enthalpies of formation at 0 K are calculated from various other density functionals (including B3LYP and ωB97XD), basis sets, and composite model chemistries (CBS-QB3). They are benchmarked against data from the Active Thermochemical Tables, high-level ANL0 calculations from the literature, and G4 calculations from this work. The effects of internal rotations and other anharmonicities are discussed, and bond dissociation energies and reaction equilibria provide mechanistic insights.

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.