Biotransforming the "Forever Chemicals": Trends and Insights from Microbiological Studies on PFAS

Abstract

Per- and polyfluoroalkyl substances (PFAS) are recalcitrant contaminants of emerging concern. Research efforts have been dedicated to PFAS microbial biotransformation in the hopes of developing treatment technologies using microorganisms as catalysts. Here, we performed a meta-analysis by extracting and standardizing quantitative data from 97 microbial PFAS biotransformation studies and comparing outcomes via statistical tests. This meta-analysis indicated that the likelihood of PFAS biotransformation was higher under aerobic conditions, in experiments with defined or axenic cultures, when high concentrations of PFAS were used, and when PFAS contained fewer fluorine atoms in the molecule. This meta-analysis also documented that PFAS biotransformation depends on chain length, chain branching geometries, and headgroup chemistry. We found that the literature is scarce or lacking in (i) anaerobic PFAS biotransformation experiments with well-defined electron acceptors, electron donors, carbon sources, and oxidation-reduction potentials, (ii) analyses of PFAS biotransformation products, and (iii) analyses to identify microorganisms and enzymes responsible for PFAS biotransformation. To date, most biotransformation research emphasis has been on 8:2 fluorotelomer alcohol (8:2 FTOH), 6:2 fluorotelomer alcohol (6:2 FTOH), perfluorooctanesulfonic acid (PFOS), and perfluorooctanoic acid (PFOA). A wide array of PFAS remains to be tested for their potential to biotransform.

Keywords: PFAS; bacteria; biodegradation; bioremediation; biotransformation; defluorination; meta-analysis; microbial; perfluoroalkyl; polyfluoroalkyl.

Figure 1

Seminal PFAS and selected microbial cultures used in PFAS biotransformation or biodegradation experiments. (δ) GenX or PFBS has not yet been reported to biodegrade or biotransform. (γ) indicates that the claim of PFAS biotransformation has been disputed and not convincingly demonstrated and/or biotransformation results have not been reproduced by others.

Figure 2

(A) Publications in which fluorinated compounds (PFAS and non-PFAS) were investigated for their potential to biotransform. Outcome of fluorinated compounds (mostly PFAS) biotransformation under (B) aerobic conditions (aerobic studies) and (C) anaerobic conditions (anaerobic studies).

Figure 3

Experimental outcomes for biotransformation of PFAS and other fluorinated compounds as a function of (A-D) aerobic/anaerobic conditions, (E) source of microbial inoculum (either sludge or soil), and (F) absence/presence of an additional carbon (C) source in anaerobic experiments. p > 0.05 = not significant (ns), p < 0.05 > 0.01 = *, p < 0.01 > 0.001 = **, p > 0.001 = ***. The corresponding references for each panel from which the data were extracted for analyses are shown in Table S4.

Figure 4

PFAS categorization (levels 1 through 4) and their state of microbial biotransformation evaluation. Biotransformation studies conducted under aerobic conditions (blue lines), anaerobic conditions (light green lines), or both (dark green lines) are shown. Black lines signify the absence of published attempts at microbial biotransformation. Categories (level 1 through 4) adhere to chemical nomenclature literature. (γ) signals the inclusion of data from studies where the claim of PFAS biotransformation has been disputed or not convincingly demonstrated and/or biotransformation results have not been reproduced by others. Acronyms are defined as follows: BuFASA = N-butyl perfluoroalkane sulfonamide, BuFASAA = N-butyl perfluoroalkane sulfonamido acetic acid, BuFASE = N-butyl perfluoroalkane sulfonamido ethanol, BUFA(M)AC = N-butyl perfluoroalkane sulfonamidoethyl acrylate and methacrylate, EtFASA = N-ethyl perfluoroalkane sulfonamide, EtFASAA = N-ethyl perfluoroalkane sulfonamido acetic acid, EtFASE = N-ethyl perfluoroalkane sulfonamido ethanol, EtFAS(M)AC = N-ethyl perfluoroalkane sulfonamidoethyl acrylates and methacrylates, FASA = perfluoroalkane sulfonamide, FASAA = perfluoroalkane sulfonamido acetic acid, FASE = perfluoroalkane sulfonamido ethanol, MeFASA = N-methyl perfluoroalkane sulfonamide, MeFASAA = N-methyl perfluoroalkane sulfonamido acetic acid, MeFASE = N-methyl perfluoroalkane sulfonamido ethanol, n:2 FTAC = n:2 fluorotelomer acrylate, n:2 FTAL = n:2 fluorotelomer aldehyde, n:2 FTCA = n:2 fluorotelomer carboxylic acid, n:2 FTI = n:2 fluorotelomer iodide, n:2 FTMAC = n:2 fluorotelomer methacrylate, n:2 FTO = n:2 fluorotelomer olefin, n:2 FTOH = n:2 fluorotelomer alcohol, n:2 FTUAL = n:2 fluorotelomer unsaturated aldehyde, n:2 FTUCA = n:2 fluorotelomer unsaturated carboxylic acid, n:2 FTUOH = n:2 fluorotelomer unsaturated alcohol, n:2 PAP = n:2 fluorotelomer phosphate, n:3 acids = n:3 saturated acid, n:3 uacid = n:3 unsaturated acid, PASF = perfluoroalkane sulfonyl fluoride, PFAA = perfluoroalkyl acid, PFAI = perfluoroalkyl iodide, PFAL = perfluoroalkyl aldehyde, PFALH2O = perfluoroalkyl aldehyde hydrate, PFCA = perfluoroalkyl carboxylic acid and perfluoroalkyl carboxylate, PFPA = perfluoroalkyl phosphonic acid, PFPIA = perfluoroalkyl phosphinic acid, PFSA = perfluoroalkyl sulfonic acid and perfluoroalkanesulfonate, PFSIA = perfluoroalkyl sulfinic acid, SFA = semifluorinated n-alkane, SFAene = semifluorinated n-alkene.