Electrochemical Oxidation of 6:2 Polyfluoroalkyl Phosphate Diester—Simulation of Transformation Pathways and Reaction Kinetics with Hydroxyl Radicals

Abstract

Polyfluoroalkyl phosphate diesters (diPAPs) are widely used for paper and cardboard impregnation and discharged via waste streams from production processes and consumer products. To improve the knowledge about the environmental fate of diPAPs, electrochemical oxidation (EO) was used to characterize the transformation pathways and reaction kinetics. 6:2 diPAP was transformed electrochemically to perfluorocarboxylic acids (C 5 −C 7 PFCAs) and two intermediates (6:2 fluorotelomer carboxylic acid, FTCA, and 6:2 fluorotelomer unsaturated carboxylic acid, FTUCA). EO of potential intermediates 6:2 monoPAP and 6:2 fluorotelomer alcohol (FTOH) showed similar transformation products but with different ratios. We show that 6:2 diPAP is initiated by OH radical ( • OH) reactions, as evidenced by the measured steady-state concentrations of • OH with the probe molecule terephthalic acid, quenching experiments, and pH dependency of the reaction. PFHpA was the main product of 6:2 diPAP oxidation, and it was formed in a pseudo-first-order reaction for which a bimolecular rate constant was estimated to be • k OH,diPAP form PFHpA = 9.4(±1.4) × 10 7 M −1 s −1 by an initial rate approach. This can be utilized to estimate the environmental half-life of 6:2 diPAP for the reaction with • OH and the formation kinetics of persistent PFCAs.

Electrochemical Transformations of Perfluoroalkyl Acid (PFAA) Precursors and PFAAs in Landfill Leachates

“…Lastly, n :2 FTCAs are transformed to PFCAs with their decarboxylation. The latter is supported by the observations of Zweigle et al, where the electrochemical oxidation of the intermediate 6:2 FTCA led to the generation of PFCAs and low levels of 6:2 UFTCA . Moreover, oxidative conversion of fluorotelomer precursor compounds reported a mixture of C 4 –C n +1 perfluorinated carboxylates as degradation products .…”

“…The latter is supported by the observations of Zweigle et al, where the electrochemical oxidation of the intermediate 6:2 FTCA led to the generation of PFCAs and low levels of 6:2 UFTCA. 56 Moreover, oxidative conversion of fluorotelomer precursor compounds reported a mixture of C 4 − C n+1 perfluorinated carboxylates as degradation products. 53 In this work, low levels of 6:2 UFTCA (0.2 mol, Figure S5i) were also generated, but they were negligible relative to the concentration of 6:2 FTCA present in untreated L1 (11 mol) that could be transformed to 6:2 UFTCA.…”

PhotoTOP: PFAS Precursor Characterization by UV/TiO2 Photocatalysis

“…Moreover, additional attempts were made with O 3 as oxidant in the TOP assay, 31 a modified version that targets hydrolyzable precursors (THP) 32 and electrochemistry to study the transformation of precursors. 33 While the TOP assay is conveniently applicable in most laboratories due to the low requirements on equipment and chemicals it also has drawbacks. 34,35 During TOP there is a limited amount of oxidant in form of persulfate and hence OH radicals (•OH) available.…”

“…Therefore, further, partly more selective, techniques were developed to gain relevant information about the chemical structure of PFAA precursors in various matrices due to their complexity. , The most prominent technique is the total oxidizable precursor (TOP) assay, which uses peroxodisulfate to chemically convert precursors to PFAAs. Moreover, additional attempts were made with O 3 as oxidant in the TOP assay, a modified version that targets hydrolyzable precursors (THP) and electrochemistry to study the transformation of precursors …”

Environmental Exposure to 6:2 Polyfluoroalkyl Phosphate Diester and Impaired Testicular Function in Men

“…The production volume of PAPs together with other fluorotelomers had already reached 12,000 tons annually in both North America and Europe between 2000 and 2004, and this number worldwide is estimated to be 14,000 tons per year or higher since the restrictions of legacy PFAS . However, compared to legacy PFAS, PAPs are less mobile in the environment due to their comparatively stronger hydrophobicity (e.g., log K oc for 6:2 diPAP vs PFOA: 4.5 vs 2.06) . More importantly, PAPs are unstable and can be partly biotransformed into perfluorinated carboxylic acids (PFCAs), providing an additional indirect source of exposure to these legacy PFAS. , This indicates that the potential adverse health effects may be associated with not only PAPs themselves but also their metabolites and biotransformed chemicals in the human body, , such as perfluorohexanoic acid (PFHxA), perfluoroheptanoic acid (PFHpA), and perfluoropentanoic acid (PFPeA), ,, although the relative contribution is unknown.…”