Oxidation of Per- and Polyfluoroalkyl Ether Acids and Other Per- and Polyfluoroalkyl Substances by Sulfate and Hydroxyl Radicals: Kinetic Insights from Experiments and Models

Abstract

Per- and polyfluoroalkyl substances (PFAS) are widely used anthropogenic chemicals. Because of the strength of the carbon–fluorine bond, PFAS are not destroyed in typical water treatment processes. Sulfate (SO4 •–) and hydroxyl (•OH) radicals can oxidize some PFAS, but the behavior of per- and polyfluoroalkyl ether acids (PFEAs) in processes involving SO4 •– and •OH is poorly understood. In this study, we determined second-order rate constants (k) describing the oxidation of 18 PFAS, including 15 novel PFEAs, by SO4 •– and •OH. Among the studied PFAS, 6:2 fluorotelomer sulfonate reacted most readily with •OH [k •OH = (1.1–1.2) × 107 M–1 s–1], while polyfluoroalkyl ether acids containing an -O-CFH- moiety reacted more slowly [k •OH = (0.5–1.0) × 106 M–1 s–1]. In the presence of SO4 •–, polyfluoroalkyl ether acids with an -O-CFH- moiety reacted more rapidly [k SO4 •– = (0.89–4.6) × 106 M–1 s–1] than perfluoroalkyl ether carboxylic acids (PFECAs) and a chloro-perfluoro-polyether carboxylic acid (ClPFPECA) [k SO4 •– = (0.85–9.5) × 104 M–1 s–1]. For homologous series of perfluoroalkyl carboxylic acids, linear and branched monoether PFECAs, and multiether PFECAs, PFAS chain length had little impact on second-order rate constants. SO4 •– reacted with the carboxylic acid headgroup of perfluoroalkyl carboxylic acids and PFECAs. In contrast, for polyfluoroalkyl ether carboxylic and sulfonic acids with an -O-CFH- moiety, the site of SO4 •– attack was the -O-CFH- moiety. Perfluoroalkyl ether sulfonic acids were not oxidized by SO4 •– and •OH under the conditions evaluated in this study.

Oxidative Transformation of Nafion-Related Fluorinated Ether Sulfonates: Comparison with Legacy PFAS Structures and Opportunities of Acidic Persulfate Digestion for PFAS Precursor Analysis

“…18 A previous TOP assay of various fluoro ether structures by Zhang et al 19 found that only H-containing structures could be converted by HO • , but the product information remained largely unknown. 20 The findings presented above reflect several limitations (but not necessarily disadvantages) of the current TOP assay. First, HO • oxidation requires H atoms in the precursor and does not transform either PFCAs or PFSAs.…”

“…We recommend (i) pretreating the samples with both HO • and SO 4 – • (and sequential treatment, if needed) before target PFAS analysis, (ii) adding more known structures (e.g., – OOC–C n F 2 n –COO – , ether carboxylates/sulfonates, and other commercially available PFAS chemicals) to the target list of TOP assay, (iii) using advanced mass spectrometry methodologies, data processing algorithms, and additional spectroscopy methods (e.g., 19 F NMR) to assist structural determination, and (iv) modifying the existing methodologies with new structural transformation mechanisms. In order to satisfy the imminent need for the detection, monitoring, and treatment of “non-legacy” PFAS pollutants, it is imperative to expand our understanding of structure-transformation relationships for emerging PFAS chemicals. − , The elucidation of additional transformation pathways for SO 4 – • oxidation is equally important and intriguing to environmental chemists. The currently unknown SO 4 – • oxidation TPs might soon become the “signature” of certain PFAS pollutants.…”

“…In order to satisfy the imminent need for the detection, monitoring, and treatment of "non-legacy" PFAS pollutants, it is imperative to expand our understanding of structure-transformation relationships for emerging PFAS chemicals. [18][19][20]36 The elucidation of additional Lastly, PFAS analytical works have not adopted the widely used acidic persulfate digestion for environmental samples. We expect this report to trigger interest in testing SO 4 −• oxidation (simply adjust the S 2 O 8 2− solution pH to 2.0 and run a few additional LC−MS samples) in specific PFAS analytical scenarios 14 to obtain more critical information on pollutant structures and profiles.…”

“…The H atoms in the “precursor” molecules allow oxidation by hydroxyl radicals (HO • , generated from persulfate at pH > 12 at 85 °C), yielding one or multiple PFCAs for targeted analysis. , Our lab further modified the reaction condition (e.g., [NaOH]:[K 2 S 2 O 8 ] = 5:1 and 120 °C), extended the substrate scope (e.g., C n F 2 n +1 –(C H 2 ) 2 –COO – , C n F 2 n +1 –(C H 2 ) 2 –SO 3 – , and H C n F 2 n –COO – with variable n ) and quantified short-chain products (e.g., CF 3 –COO – , C 2 F 5 –COO – , and – OOC–CF 2 –COO – ). , Notably, structures with only one C–H bond in H CF 2 – and multiple C–H bonds in −(C H 2 ) 2 – showed distinct transformation patterns . A previous TOP assay of various fluoro ether structures by Zhang et al found that only H-containing structures could be converted by HO • , but the product information remained largely unknown …”

“…For example, the measurement of total phosphorus uses acidic persulfate digestion to convert various organic phosphate esters into orthophosphate. , SO 4 – • can also oxidize PFCAs via decarboxylation. − Second, “novel” PFAS structures do not necessarily produce linear PFCAs, , which the current TOP assay relies on. However, while the reaction kinetics under oxidative conditions have been extensively quantified, the structural transformation of emerging PFAS has not been adequately understood. , Hence, it is important to elucidate the transformation of novel PFAS “precursors” by both SO 4 – • and HO • for environmental detection and source tracking.…”