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 CC
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.
Exposure to per-and polyfluoroalkyl substances (PFAS) in drinking water poses a major public health threat. Commercial granular activated carbon (GAC) has been used for the sorptive removal of PFAS in practical applications. Biochar is a possible cheaper alternative to GAC for small-scale water treatment systems. Here, we report a strategy for employing biochar for PFAS removal that combines post-pyrolysis modification, which greatly improves performance, with a reactivation step that enables its reuse. Modification entails brief postpyrolysis air oxidation at 400 °C, which considerably enlarges pore size and specific surface area and thereby increases the solid-to-water distribution ratio, K D , of individual PFAS by as much as 3 orders of magnitude. In some cases (e.g., perfluorooctanoic acid) the K D was comparable to that of commercial GAC. The sorbed PFAS could be decomposed by brief thermal reactivation of the spent biochar at 500 °C in N 2 or air. After thermal reactivation in air, the biochars exhibited even greater PFAS K D values in a second cycle. While thermal reactivation of a GAC in air could be achieved, as well, sorption affinity for the shorter-chain PFAS was noticeably reduced. Overall, this study points to a new strategy of using biochars for PFAS removal.
In this study, we have developed
an innovative thermal degradation
strategy for treating per- and polyfluoroalkyl substance (PFAS)-containing
solid materials. Our strategy satisfies three criteria: the ability
to achieve near-complete degradation of PFASs within a short timescale,
nonselectivity, and low energy cost. In our method, a metallic reactor
containing a PFAS-laden sample was subjected to electromagnetic induction
that prompted a rapid temperature rise of the reactor via the Joule
heating effect. We demonstrated that subjecting PFASs (0.001–12
μmol) to induction heating for a brief duration (e.g., <40
s) resulted in substantial degradation (>90%) of these compounds,
including recalcitrant short-chain PFASs and perfluoroalkyl sulfonic
acids. This finding prompted us to conduct a detailed study of the
thermal phase transitions of PFASs using thermogravimetric analysis
and differential scanning calorimetry (DSC). We identified at least
two endothermic DSC peaks for anionic, cationic, and zwitterionic
PFASs, signifying the melting and evaporation of the melted PFASs.
Melting and evaporation points of many PFASs were reported for the
first time. Our data suggest that the rate-limiting step in PFAS thermal
degradation is linked with phase transitions (e.g., evaporation) occurring
on different time scales. When PFASs are rapidly heated to temperatures
similar to those produced during induction heating, the evaporation
of melted PFAS slows down, allowing for the degradation of the melted
PFAS.
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