Determining the microbial degradation of diPAPs and the effect of sorption on their biotransformation

Per- and polyfluoroalkyl substances (PFAS) are a class of widespread anthropogenic chemicals that have raised serious health concerns over the last decades because of their potential as endocrine disruptors and carcinogenic effects. PFAS precursors (i.e., polyfluorinated compounds that are degraded biotically to short-chained, perfluorinated end products) stand out due to their high hydrophobicity and strong sorption to soils. Their strong sorption behaviour significantly slows down degradation; half-life times (DT₅₀) can reach years. This study focuses on polyfluoroalkyl phosphate diesters (diPAPs) and their biodegradation behaviour under aerobic conditions. DiPAPs are used as coatings in paper products and are of particular interest due to their widespread occurrence in the environment, and particularly in Rastatt and Baden-Baden (South-West Germany) where the application of large amounts of contaminated paper sludge to agricultural soils led to a heavy PFAS contamination (Fabregat-Palau et al., 2025).

An uncontaminated soil was suspended in ultrapure water (liquid to solid ratio 10 L/kg) and spiked with 300 µg of 6:2 diPAP to check whether biodegradation occurred over a time span of 100 days. Known degradation products were found in an expected distribution at the end of the experiment. Perfluorohexanoic acid accounted for 23 % of spiked precursor, while perfluoropentanoic acid and perfluorobutanoic acid accounted only for 4.5 % and 0.7 % respectively. Perfluoroheptanoic acid was also detected (0.05 %) as a minor product. Although degradation products indicate defluorination steps, no significant increase in dissolved fluoride could be measured due to high background levels in soil. Concentrations of the intermediate product 5:3 fluorotelomer carboxylic acid showed a small increase at early times, but stagnated and finally decreased again over the course of the experiment, suggesting a kinetic limitation of the degradation further up the reaction chain. During the experimental run of 100 days, only 30 % of 6:2 diPAP was degraded. DT₅₀ values were 247 days, which agrees with the few other data for 6:2 diPAP in other soils. Interestingly, the degradation seemed to speed up towards the end of the experiment. To evaluate the role of sorption, a parallel experiment was set up with an aqueous soil extract (at a liquid to solid ratio of 10 L/kg) that contained the necessary microbes but where all soil particles were filtered out, hence negating interference by sorption. In this system DT₅₀ was 130 days, proving that without sorption biodegradation gets faster due to higher bioavailability of 6:2 diPAP.

Fabregat-Palau, J.; Zweigle, J.; Renner, D.; Zwiener, C.; Grathwohl, P. (2025). Assessment of PFAS Contamination in Agricultural Soils: Non-target Identification of Precursors, Fluorine Mass Balance and Microcosm Studies. J. Hazard. Mat., 490, 137798. DOI: 10.1021/acs.estlett.4c00442