Thermogravimetric Studies for the Incineration of an Anion Exchange Resin Laden with Short- or Long-Chain PFAS Compounds Containing Carboxylic or Sulfonic Acid Functionalities in the Presence or Absence of Calcium Oxide

Abstract

Anion exchange resins are being used by the drinking water industry to remove per-and polyfluoroalkyl substances (PFAS) including perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs) from water, and spent resins are commonly incinerated by the industry. By conducting thermogravimetric analysis (TGA) studies, this investigation is aimed at filling some of the knowledge gaps that currently exists in the incineration of PFAS-laden resins. This work is aimed at understanding the impact of characteristics (i.e., carbon chain length and acid functionality) of nine PFAS compounds, in the presence or absence of a calcium oxide (CaO) additive, on the decomposition and burning characteristics of PFAS-laden resins. The TGA profiles of selected pure PFAS compounds confirmed the near-complete evaporation of these compounds below their boiling points, whereas PFAS-laden resin profiles exhibited the partial holding of PFAS compounds by resin well above their boiling points. Resins laden with PFCAs exhibited TGA profiles similar to those of the blank resins, whereas resins laden with PFSAs showed a drastic weight loss from 400 to 500 °C. These results suggested that decomposition and oxidation reactions were accelerated by sulfonic acid functionalities, and the impact of sulfonic acid groups appears to increase with the increase in the chain length of PFSAs. The presence of the CaO additive resulted in similar TGA profiles for all tested PFCA-laden resins, where all exhibited a relatively flat profile in the∼470−700 °C range as a result of the simultaneous burning of carbon char and the reaction of CO 2 with CaO, followed by a rapid weight loss at 700−800 °C from the decomposition of the formed CaCO 3 to CaO. The TGA profiles of PFSA-laden resins in the presence of CaO exhibited a similar impact of sulfonic acid functionalities accelerating the decomposition and oxidation of the resin without any clear correlation with the PFSA chain length.

Thermal Phase Transition and Rapid Degradation of Forever Chemicals (PFAS) in Spent Media Using Induction Heating

“…Thermal treatment is a vital approach for the decontamination of spent or exhausted GAC laden with PFASs. ,,,, It is a common industry practice to regenerate the spent GAC at <300 °C or reactivate the spent carbon at 700–900 °C in N 2 for approximately 30 min. The thermal disposal of spent resins containing PFASs has only recently received attention . In this study, we present induction heating as a promising means to achieve fast degradation of PFASs in the spent media.…”

“…29,31,33 We now have a moderate level of understanding regarding the fate of PFAS during conventional heating: (1) thermal decomposition of PFAS proceeds through multistep free-radical reactions, including initiation, chain propagation, and termination; 33,36 (2) the addition of GAC or a resin (Amberlite XAD-2) can substantially accelerate the thermal decomposition of perfluorocarboxylic acids (PFCAs) and HFPO-DA at low and moderate temperatures (<400 °C); 33 (3) perfluoroalkyl sulfonic acids (PFSAs) require a relatively high temperature (≥450 °C) to decompose; 29,33 (4) polyfluoroalkyl substances are subject to thermal side-chain stripping on the nonfluorinated moiety 31 and more prone to degradation than perfluoroalkyl counterparts; 31 (5) the thermal degradation of PFAS yields various transient intermediates, including perfluoroalkenes, at low and moderate temperatures; 30−33 and (6) substantial mineralization (>90%) of PFOA and potassium salt of PFOS (K-PFOS) occurs at 700 °C and above. 29 Despite the insights gained from recent studies, [29][30][31][33][34][35]40,43 much is still unknown about the phase transitions (such as melting and evaporation) of these compounds, 36 which is crucial for developing effective thermal remediation plans and understanding their behavior in other thermal processes such as cooking, baking, and firefighting in which PFAS-containing products are used. Furthermore, the rate-limiting steps or factors controlling PFAS thermal decomposition kinetics also remain elusive, and little 33 or no effort has been devoted so far for accelerating PFAS thermal decomposition.…”

“…Despite the insights gained from recent studies, − ,− ,, much is still unknown about the phase transitions (such as melting and evaporation) of these compounds, which is crucial for developing effective thermal remediation plans and understanding their behavior in other thermal processes such as cooking, baking, and firefighting in which PFAS-containing products are used. Furthermore, the rate-limiting steps or factors controlling PFAS thermal decomposition kinetics also remain elusive, and little or no effort has been devoted so far for accelerating PFAS thermal decomposition.…”

“…Thermal-based approaches have emerged as an attractive means − to decontaminate PFAS-laden GAC, ,,− resins, and other solid materials (e.g., soil, , biosolids, and biomass). Our group investigated conventional thermal treatment using regular ovens/furnaces as a means of degrading PFASs in soil and spent GAC. ,, We now have a moderate level of understanding regarding the fate of PFAS during conventional heating: (1) thermal decomposition of PFAS proceeds through multistep free-radical reactions, including initiation, chain propagation, and termination; , (2) the addition of GAC or a resin (Amberlite XAD-2) can substantially accelerate the thermal decomposition of perfluorocarboxylic acids (PFCAs) and HFPO-DA at low and moderate temperatures (<400 °C); (3) perfluoroalkyl sulfonic acids (PFSAs) require a relatively high temperature (≥450 °C) to decompose; , (4) polyfluoroalkyl substances are subject to thermal side-chain stripping on the nonfluorinated moiety and more prone to degradation than perfluoroalkyl counterparts; (5) the thermal degradation of PFAS yields various transient intermediates, including perfluoroalkenes, at low and moderate temperatures; − and (6) substantial mineralization (>90%) of PFOA and potassium salt of PFOS (K-PFOS) occurs at 700 °C and above …”

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

“…Over the past five years, research on PFAS thermal treatment has quickly evolved − ,− ,,− regarding the thermal stability of PFOA and PFOS, , the threshold temperatures that thermal degradation may occur, ,, the yield of inorganic F from PFOA and PFOS during thermal degradation, ,, and the thermal degradation of PFAS (also known as precursor compounds) . As part of this evolution, there has been a strong focus on polar , and nonpolar , thermal degradation products of PFAS that are produced when subjected to thermal treatments, such as thermal regeneration of spent GAC at temperatures of <400 °C. ,, Moreover, increasing attention has been paid to the effects of porous adsorbents, such as GAC, resin, and biochar, and approaches to accelerate PFAS thermal degradation at low temperatures. ,− …”