Per- and polyfluoroalkyl substances in the environment

Abstract

Over the past several years, the term PFAS (per- and polyfluoroalkyl substances) has grown to be emblematic of environmental contamination, garnering public, scientific, and regulatory concern. PFAS are synthesized by two processes, direct fluorination (e.g., electrochemical fluorination) and oligomerization (e.g., fluorotelomerization). More than a megatonne of PFAS is produced yearly, and thousands of PFAS wind up in end-use products. Atmospheric and aqueous fugitive releases during manufacturing, use, and disposal have resulted in the global distribution of these compounds. Volatile PFAS facilitate long-range transport, commonly followed by complex transformation schemes to recalcitrant terminal PFAS, which do not degrade under environmental conditions and thus migrate through the environment and accumulate in biota through multiple pathways. Efforts to remediate PFAS-contaminated matrices still are in their infancy, with much current research targeting drinking water.

Fig. 1:. Summary of PFAS manufacturing, from production to consumer use.

While numerous product fluxes are reasonably documented, considerable lacunae remain. See text for details and citations. HFCs (hydrofluorocarbons); HCFOs (hydrochlorofluoroolefins); HFOs (hydrofluoroolefins); HFEs (hydrofluoroethers); PASF (perfluoroalkanesulfonyl fluoride).

Fig. 2. Intermediate and final manufacturing products.

A: Multistep multicomponent syntheses yield a complex PFAS universe. B: Multistep multicomponent syntheses yield a complex PFAS universe. Notes: * PFCAs have also been synthesised using other routes, e.g. from PFAIs or n:2 fluorotelomer iodides. Different synthesis routes may generate PFCAs with different perfluorocarbon chain lengths. ** Additional details on known synthesis routes of PASF-based and n:2 fluorotelomer derivatives can be found in Part (b). *** For many compounds such as HFP and TFE, there are different synthesis routes with different starting materials, and here shows only one of them. **** PFAIs may also be synthesized from other initial substances such as (CF₃)₂CFI. Sources: (1) Siegemund G. et al. Fluorine Compounds, Organic, 3rd ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2000; Vol. 33. (2) Banks RE et al. Organofluorine Chemistry: Principles and Commercial Applications. New York: Plenum, 1994. (3) Buck RC et al. Perfluoroalkyl and polyfluoroalkyl substances in the environment: terminology, classification, and origins, Integr Environ Assess Manag 2011, 7 (4), 513–541. (4) Wang Z et al. Global emission inventories for C4-C14 perfluoroalkyl carboxylic acid (PFCA) homologues from 1951 to 2030, Part I: production and emissions from quantifiable sources. Environ Int 2014, 70, 62–75. (5) Zhang W. et al. Manufacture of hydrofluorocarbons and hydrofluoroolefins as the CFCs-alternatives: from fundamental of catalytic reaction to commercialisation. Scientia Sinica Chimica 2017, 47 (11), 1312-1325.

Fig. 2. Intermediate and final manufacturing products.

A: Multistep multicomponent syntheses yield a complex PFAS universe. B: Multistep multicomponent syntheses yield a complex PFAS universe. Notes: * PFCAs have also been synthesised using other routes, e.g. from PFAIs or n:2 fluorotelomer iodides. Different synthesis routes may generate PFCAs with different perfluorocarbon chain lengths. ** Additional details on known synthesis routes of PASF-based and n:2 fluorotelomer derivatives can be found in Part (b). *** For many compounds such as HFP and TFE, there are different synthesis routes with different starting materials, and here shows only one of them. **** PFAIs may also be synthesized from other initial substances such as (CF₃)₂CFI. Sources: (1) Siegemund G. et al. Fluorine Compounds, Organic, 3rd ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2000; Vol. 33. (2) Banks RE et al. Organofluorine Chemistry: Principles and Commercial Applications. New York: Plenum, 1994. (3) Buck RC et al. Perfluoroalkyl and polyfluoroalkyl substances in the environment: terminology, classification, and origins, Integr Environ Assess Manag 2011, 7 (4), 513–541. (4) Wang Z et al. Global emission inventories for C4-C14 perfluoroalkyl carboxylic acid (PFCA) homologues from 1951 to 2030, Part I: production and emissions from quantifiable sources. Environ Int 2014, 70, 62–75. (5) Zhang W. et al. Manufacture of hydrofluorocarbons and hydrofluoroolefins as the CFCs-alternatives: from fundamental of catalytic reaction to commercialisation. Scientia Sinica Chimica 2017, 47 (11), 1312-1325.

Fig 3.. Reaction schemes for the (A) 8:2 FTOH (47, 53, 63) and (B) N-EtFOSE (65, 66).

Transformation products proposed by the original investigators are shown with brackets.

Fig. 4:. PFAS partitioning in environmental media (log K_d).

The environmental sorption complex varies grossly with setting, with NOM concentrated in shallow soil horizons, and ferric (oxy)hydroxides commonly dominating in subsurface media (Properties). Log K_d varies as a function of fluoroalkyl number and terminal moiety (A (95); pH=5.2 values depicted). Because of this partitioning behavior, when not complicated by precursor degradation, i) relative mobility amongst PFAS commonly varies with fluoroalkyl carbon number (B (97),D (105),E (106)), ii) terrestrial vegetation accumulation diminishes with increasing fluoroalkyl number, but accumulation in terrestrial detrital feeders increases with fluoroalkyl number (C (101)), and iii) in aquatic settings, vegetative and detrital-feeder accumulation both increase with fluoroalkyl number (F (107)). CEC/AEC (cation-/anion-exchange capacity).

Fig. 5:. Trophic transfer and environmental exposures:

Bioaccumulation factors (BAFs) in aquatic food webs are greater for long-chain perfluorocarboxylates (top panel) and perfluorosulfonates (bottom panel) than short-chains. Higher trophic-level organisms demonstrate greater bioaccumulation of PFOS than PFOA (center panel); trophic-level accumulation was estimated for data with a single-prey classification method (FishBase) and standardized bioaccumulation factor by wet weight of organism. Multiple toxicological implications (right panel) reflect the diversity of PFAS physicochemical properties and have been linked to both functional group and fluoroalkyl-carbon-chain length. Data were originally compiled by Burkhard (127).

Fig. 6:. Site management options for media streams containing PFAS:

Brown, blue, and green indicate solid/semisolid, water/liquid, and air/gas phases. PFAS, including precursors and products of incomplete destruction, cycle through the management options based on treatment and operational choices. Without informed management choices, the persistence of PFAS results in re-releases to the environment. Only complete mineralization, with HF control, offers a permanent solution for breaking the treatment cycle.

Print Fig:. The PFAS lifecycle.

PFAS product flows from primary producer, to commercial user, to consumers, to disposal. Each step is attended by atmospheric and aqueous fugitive releases. Soils constitute a long-term environmental sink, slowly releasing PFAS to the hydrosphere and allowing uptake in biota, but the ultimate reservoir is deep marine sediment.