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==Abiotic Reduction of Munitions Constituents==   
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==Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)==   
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Munition compounds (MCs) often contain one or more nitro (-NO<sub>2</sub>) functional groups which makes them susceptible to abiotic reduction, i.e., transformation by accepting electrons from a chemical electron donor. In soil and groundwater, the most prevalent electron donors are natural organic carbon and iron minerals. Understanding the kinetics and mechanisms of abiotic reduction of MCs by carbon and iron constituents in soil is not only essential for evaluating the environmental fate of MCs but also key to developing cost-efficient remediation strategies. This article summarizes the recent advances in our understanding of MC reduction by carbon and iron based reductants.
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[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)|Per and polyfluoroalkyl substances (PFAS)]] contained in [[wikipedia:Firefighting foam |Class B aqueous film-forming foams (AFFFs)]] are known to accumulate on wetted surfaces of many fire suppression systems after decades of exposure<ref name="LangEtAl2022">Lang, J.R., McDonough, J., Guillette, T.C., Storch, P., Anderson, J., Liles, D., Prigge, R., Miles, J.A.L., Divine, C., 2022. Characterization of per- and polyfluoroalkyl substances on fire suppression system piping and optimization of removal methods. Chemosphere, 308(Part 2), 136254. [https://doi.org/10.1016/j.chemosphere.2022.136254 doi: 10.1016/j.chemosphere.2022.136254]&nbsp;&nbsp;[[Media:LangEtAl2022.pdf | Open Access Article]]</ref>. When replacement PFAS-free firefighting formulations are added to existing infrastructure, PFAS can rebound from the wetted surfaces into the new formulations at high concentrations<ref name="RossStorch2020">Ross, I., and Storch, P., 2020. Foam Transition: Is It as Simple as "Foam Out / Foam In?". The Catalyst (Journal of JOIFF, The International Organization for Industrial Emergency Services Management), Q2 Supplement, 20 pages. [[Media:Catalyst_2020_Q2_Sup.pdf | Industry Newsletter]]</ref><ref>Kappetijn, K., 2023. Replacement of fluorinated extinguishing foam: When is clean clean enough? The Catalyst (Journal of JOIFF, The International Organization for Industrial Emergency Services Management), Q1 2023, pp. 31-33. [[Media:Catalyst_2023_Q1.pdf | Industry Newsletter]]</ref>. Effective methods are needed to properly transition to PFAS-free firefighting formulations in existing fire suppression infrastructure. Considerations in the transition process may include but are not limited to locating, identifying, and evaluating existing systems and AFFF, fire engineering evaluations, system prioritization, cost/downtime analyses, sampling and analysis, evaluation of risks and hazards to human health and the environment, transportation, and disposal.
 
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'''Related Article(s):'''
 
'''Related Article(s):'''
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*[[Munitions Constituents]]  
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*[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]  
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*[[Munitions Constituents - Alkaline Degradation]]
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*[[PFAS Sources]]
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*[[PFAS Ex Situ Water Treatment]]
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*[[Supercritical Water Oxidation (SCWO)]]
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*[[PFAS Treatment by Electrical Discharge Plasma]]
  
 
'''Contributor(s):'''  
 
'''Contributor(s):'''  
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*Dr. Jimmy Murillo-Gelvez
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*Dr. Johnsie Ray Lang
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*Paula Andrea Cárdenas-Hernández
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*Dr. Jonathan Miles
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*Dr. Pei Chiu
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*John Anderson
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*Dr. Theresa Guillette
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*[[Craig E. Divine, Ph.D., PG|Dr. Craig Divine]]
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*[[Dr. Stephen Richardson]]
  
 
'''Key Resource(s):'''
 
'''Key Resource(s):'''
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* Schwarzenbach, Gschwend, and Imboden, 2016. Environmental Organic Chemistry, 3rd ed.<ref name="Schwarzenbach2016">Schwarzenbach, R.P., Gschwend, P.M., and Imboden, D.M., 2016. Environmental Organic Chemistry, 3rd Edition. John Wiley and Sons, Ltd, 1024 pages.  ISBN: 978-1-118-76723-8</ref>
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*Department of Defense (DoD) performance standard for PFAS-free firefighting formulation:  [https://media.defense.gov/2023/Jan/12/2003144157/-1/-1/1/MILITARY-SPECIFICATION-FOR-FIRE-EXTINGUISHING-AGENT-FLUORINE-FREE-FOAM-F3-LIQUID-CONCENTRATE-FOR-LAND-BASED-FRESH-WATER-APPLICATIONS.PDF Military Specification MIL-PRF-32725]<ref name="DoD2023">US Department of Defense, 2023. Performance Specification for Fire Extinguishing Agent, Fluorine-Free Foam (F3) Liquid Concentrate for Land-Based, Fresh Water Applications. Mil-Spec MIL-PRF-32725, 18 pages. [[Media: MilSpec32725.pdf | Military Specification Document]]</ref>
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*[[Media:LangEtAl2022.pdf | Characterization of per- and polyfluoroalkyl substances on fire suppression system piping and optimization of removal methods]]<ref name="LangEtAl2022"/>
  
 
==Introduction==
 
==Introduction==
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[[File:AbioMCredFig1.PNG | thumb |left|300px|Figure 1. Common munitions compounds. TNT and RDX are legacy explosives. DNAN, NTO, and NQ are insensitive MCs (IMCs) widely used as replacement for legacy explosives.]]
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[[File:LangFig1.png | thumb |400px|Figure 1. (A) Schematic of a typical PFAS molecule demonstrating the hydrophobic fluorinated tail in green and the hydrophilic charged functional group in blue, (B) a PFAS bilayer formed with the hydrophobic tails facing inward and the charged functional groups on the outside, and (C) multiple bilayers of PFAS assembled on the wetted surfaces of fire suppression piping.]]PFAS are a class of synthetic fluorinated compounds which are highly mobile and persistent within the environment<ref>Giesy, J.P., Kannan, K., 2001. Global Distribution of Perfluorooctane Sulfonate in Wildlife. Environmental Science and Technology 35(7), pp. 1339-1342. [https://doi.org/10.1021/es001834k doi: 10.1021/es001834k]</ref>. Due to the surfactant properties of PFAS, these compounds self-assemble at any solid-liquid interface forming resilient bilayers during prolonged exposure<ref>Krafft, M.P., Riess, J.G., 2015. Selected physicochemical aspects of poly- and perfluoroalkylated substances relevant to performance, environment and sustainability-Part one. Chemosphere, 129, pp. 4-19. [https://doi.org/10.1016/j.chemosphere.2014.08.039 doi: 10.1016/j.chemosphere.2014.08.039]</ref>. Solid phase accumulation of PFAS has been proposed to be influenced by both [[wikipedia: Hydrophobic effect|hydrophobic]] and electrostatic interactions with fluorinated carbon chain length as the dominant feature influencing sorption<ref>Higgins, C.P., Luthy, R.G., 2006. Sorption of Perfluorinated Surfactants on Sediments. Environmental Science and Technology, 40(23), pp. 7251-7256. [https://doi.org/10.1021/es061000n doi: 10.1021/es061000n]</ref>. While the majority of previous research into solid phase sorption typically focused on water treatment applications or subsurface porous media<ref>Brusseau, M.L., 2018. Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface. Science of the Total Environment, 613-614, pp. 176-185. [https://doi.org/10.1016/j.scitotenv.2017.09.065 doi: 10.1016/j.scitotenv.2017.09.065]&nbsp;&nbsp;[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5693257/ Open Access Manuscript]</ref>, recently PFAS accumulations have been identified on the wetted surfaces of fire suppression infrastructure exposed to aqueous film forming foam (AFFF)<ref name="LangEtAl2022"/> (see Figure 1).
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Legacy and insensitive MCs (Figure 1.) are susceptible to reductive transformation in soil and groundwater. Many redox-active constituents in the subsurface, especially those containing organic carbon, Fe(II), and sulfur can mediate MC reduction. Specific examples include Fe(II)-organic complexes<ref name="Naka2006">Naka, D., Kim, D., and Strathmann, T.J., 2006. Abiotic Reduction of Nitroaromatic Compounds by Aqueous Iron(II)−Catechol Complexes. Environmental Science and Technology 40(9), pp. 3006–3012.  [https://doi.org/10.1021/es060044t DOI: 10.1021/es060044t]</ref><ref name="Naka2008">Naka, D., Kim, D., Carbonaro, R.F., and Strathmann, T.J., 2008. Abiotic reduction of nitroaromatic contaminants by iron(II) complexes with organothiol ligands. Environmental Toxicology and Chemistry, 27(6), pp. 1257–1266.  [https://doi.org/10.1897/07-505.1 DOI: 10.1897/07-505.1]</ref><ref name="Hartenbach2008">Hartenbach, A.E., Hofstetter, T.B., Aeschbacher, M., Sander, M., Kim, D., Strathmann, T.J., Arnold, W.A., Cramer, C.J., and Schwarzenbach, R.P., 2008. Variability of Nitrogen Isotope Fractionation during the Reduction of Nitroaromatic Compounds with Dissolved Reductants. Environmental Science and Technology 42(22), pp. 8352–8359. [https://doi.org/10.1021/es801063u DOI: 10.1021/es801063u]</ref><ref name="Kim2009">Kim, D., Duckworth, O.W., and Strathmann, T.J., 2009. Hydroxamate siderophore-promoted reactions between iron(II) and nitroaromatic groundwater contaminants. Geochimica et Cosmochimica Acta, 73(5), pp. 1297–1311.  [https://doi.org/10.1016/j.gca.2008.11.039 DOI: 10.1016/j.gca.2008.11.039]</ref><ref name="Kim2007">Kim, D., and Strathmann, T.J., 2007. Role of Organically Complexed Iron(II) Species in the Reductive Transformation of RDX in Anoxic Environments. Environmental Science and Technology, 41(4), pp. 1257–1264. [https://doi.org/10.1021/es062365a DOI: 10.1021/es062365a]</ref>, iron oxides in the presence of aqueous Fe(II)<ref name="Colón2006">Colón, D., Weber, E.J., and Anderson, J.L., 2006. QSAR Study of the Reduction of Nitroaromatics by Fe(II) Species. Environmental Science and Technology, 40(16), pp. 4976–4982.  [https://doi.org/10.1021/es052425x DOI: 10.1021/es052425x]</ref><ref name="Luan2013">Luan, F., Xie, L., Li, J., and Zhou, Q., 2013. Abiotic reduction of nitroaromatic compounds by Fe(II) associated with iron oxides and humic acid. Chemosphere, 91(7), pp. 1035–1041.  [https://doi.org/10.1016/j.chemosphere.2013.01.070 DOI: 10.1016/j.chemosphere.2013.01.070]</ref><ref name="Gorski2016">Gorski, C.A., Edwards, R., Sander, M., Hofstetter, T.B., and Stewart, S.M., 2016. Thermodynamic Characterization of Iron Oxide–Aqueous Fe<sup>2+</sup> Redox Couples. Environmental Science and Technology, 50(16), pp. 8538–8547.  [https://doi.org/10.1021/acs.est.6b02661 DOI: 10.1021/acs.est.6b02661]</ref><ref name="Fan2016">Fan, D., Bradley, M.J., Hinkle, A.W., Johnson, R.L., and Tratnyek, P.G., 2016. Chemical Reactivity Probes for Assessing Abiotic Natural Attenuation by Reducing Iron Minerals. Environmental Science and Technology, 50(4), pp. 1868–1876.  [https://doi.org/10.1021/acs.est.5b05800 DOI: 10.1021/acs.est.5b05800]</ref><ref name="Jones2016">Jones, A.M., Kinsela, A.S., Collins, R.N., and Waite, T.D., 2016. The reduction of 4-chloronitrobenzene by Fe(II)-Fe(III) oxide systems - correlations with reduction potential and inhibition by silicate. Journal of Hazardous Materials, 320, pp. 143–149.  [https://doi.org/10.1016/j.jhazmat.2016.08.031 DOI: 10.1016/j.jhazmat.2016.08.031]</ref><ref name="Klausen1995">Klausen, J., Troeber, S.P., Haderlein, S.B., and Schwarzenbach, R.P., 1995. Reduction of Substituted Nitrobenzenes by Fe(II) in Aqueous Mineral Suspensions. Environmental Science and Technology, 29(9), pp. 2396–2404.  [https://doi.org/10.1021/es00009a036 DOI: 10.1021/es00009a036]</ref><ref name="Strehlau2016">Strehlau, J.H., Stemig, M.S., Penn, R.L., and Arnold, W.A., 2016. Facet-Dependent Oxidative Goethite Growth As a Function of Aqueous Solution Conditions. Environmental Science and Technology, 50(19), pp. 10406–10412.  [https://doi.org/10.1021/acs.est.6b02436 DOI: 10.1021/acs.est.6b02436]</ref><ref name="Elsner2004">Elsner, M., Schwarzenbach, R.P., and Haderlein, S.B., 2004. Reactivity of Fe(II)-Bearing Minerals toward Reductive Transformation of Organic Contaminants. Environmental Science and Technology, 38(3), pp. 799–807.  [https://doi.org/10.1021/es0345569 DOI: 10.1021/es0345569]</ref><ref name="Colón2008">Colón, D., Weber, E.J., and Anderson, J.L., 2008. Effect of Natural Organic Matter on the Reduction of Nitroaromatics by Fe(II) Species. Environmental Science and Technology, 42(17), pp. 6538–6543.  [https://doi.org/10.1021/es8004249 DOI: 10.1021/es8004249]</ref><ref name="Stewart2018">Stewart, S.M., Hofstetter, T.B., Joshi, P. and Gorski, C.A., 2018. Linking Thermodynamics to Pollutant Reduction Kinetics by Fe<sup>2+</sup> Bound to Iron Oxides. Environmental Science and Technology, 52(10), pp. 5600–5609.  [https://doi.org/10.1021/acs.est.8b00481 DOI: 10.1021/acs.est.8b00481]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/acs.est.8b00481 Open access article.]</ref><ref name="Klupinski2004">Klupinski, T.P., Chin, Y.P., and Traina, S.J., 2004. Abiotic Degradation of Pentachloronitrobenzene by Fe(II):  Reactions on Goethite and Iron Oxide Nanoparticles. Environmental Science and Technology, 38(16), pp. 4353–4360.  [https://doi.org/10.1021/es035434j DOI: 10.1021/es035434j]</ref>, magnetite<ref name="Klausen1995"/><ref name="Elsner2004"/><ref name="Heijman1993">Heijman, C.G., Holliger, C., Glaus, M.A., Schwarzenbach, R.P., and Zeyer, J., 1993. Abiotic Reduction of 4-Chloronitrobenzene to 4-Chloroaniline in a Dissimilatory Iron-Reducing Enrichment Culture. Applied and Environmental Microbiology, 59(12), pp. 4350–4353. [https://doi.org/10.1128/aem.59.12.4350-4353.1993 DOI: 10.1128/aem.59.12.4350-4353.1993]&nbsp;&nbsp; [https://journals.asm.org/doi/reader/10.1128/aem.59.12.4350-4353.1993 Open access article.]</ref><ref name="Gorski2009">Gorski, C.A., and Scherer, M.M., 2009. Influence of Magnetite Stoichiometry on Fe<sup>II</sup> Uptake and Nitrobenzene Reduction. Environmental Science and Technology, 43(10), pp. 3675–3680.  [https://doi.org/10.1021/es803613a DOI: 10.1021/es803613a]</ref><ref name="Gorski2010">Gorski, C.A., Nurmi, J.T., Tratnyek, P.G., Hofstetter, T.B. and Scherer, M.M., 2010. Redox Behavior of Magnetite: Implications for Contaminant Reduction. Environmental Science and Technology, 44(1), pp. 55–60.  [https://doi.org/10.1021/es9016848 DOI: 10.1021/es9016848]</ref>, Fe(II)-bearing clays<ref name="Hofstetter2006">Hofstetter, T.B., Neumann, A., and Schwarzenbach, R.P., 2006. Reduction of Nitroaromatic Compounds by Fe(II) Species Associated with Iron-Rich Smectites. Environmental Science and Technology, 40(1), pp. 235–242. [https://doi.org/10.1021/es0515147 DOI: 10.1021/es0515147]</ref><ref name="Schultz2000">Schultz, C. A., and Grundl, T.J., 2000. pH Dependence on Reduction Rate of 4-Cl-Nitrobenzene by Fe(II)/Montmorillonite Systems. Environmental Science and Technology 34(17), pp. 3641–3648.  [https://doi.org/10.1021/es990931e DOI: 10.1021/es990931e]</ref><ref name="Luan2015a">Luan, F., Gorski, C.A., and Burgos, W.D., 2015. Linear Free Energy Relationships for the Biotic and Abiotic Reduction of Nitroaromatic Compounds. Environmental Science and Technology, 49(6), pp. 3557–3565.  [https://doi.org/10.1021/es5060918 DOI: 10.1021/es5060918]</ref><ref name="Luan2015b">Luan, F., Liu, Y., Griffin, A.M., Gorski, C.A. and Burgos, W.D., 2015. Iron(III)-Bearing Clay Minerals Enhance Bioreduction of Nitrobenzene by ''Shewanella putrefaciens'' CN32. Environmental Science and Technology, 49(3), pp. 1418–1426.  [https://doi.org/10.1021/es504149y DOI: 10.1021/es504149y]</ref><ref name="Hofstetter2003">Hofstetter, T.B., Schwarzenbach, R.P. and Haderlein, S.B., 2003. Reactivity of Fe(II) Species Associated with Clay Minerals. Environmental Science and Technology, 37(3), pp. 519–528.  [https://doi.org/10.1021/es025955r DOI: 10.1021/es025955r]</ref><ref name="Neumann2008">Neumann, A., Hofstetter, T.B., Lüssi, M., Cirpka, O.A., Petit, S., and Schwarzenbach, R.P., 2008. Assessing the Redox Reactivity of Structural Iron in Smectites Using Nitroaromatic Compounds As Kinetic Probes. Environmental Science and Technology, 42(22), pp. 8381–8387.  [https://doi.org/10.1021/es801840x DOI: 10.1021/es801840x]</ref><ref name="Hofstetter2008">Hofstetter, T.B., Neumann, A., Arnold, W.A., Hartenbach, A.E., Bolotin, J., Cramer, C.J., and Schwarzenbach, R.P., 2008. Substituent Effects on Nitrogen Isotope Fractionation During Abiotic Reduction of Nitroaromatic Compounds. Environmental Science and Technology, 42(6), pp. 1997–2003.  [https://doi.org/10.1021/es702471k DOI: 10.1021/es702471k]</ref>, hydroquinones (as surrogates of natural organic matter)<ref name="Hartenbach2008"/><ref name="Schwarzenbach1990">Schwarzenbach, R.P., Stierli, R., Lanz, K., and Zeyer, J., 1990. Quinone and Iron Porphyrin Mediated Reduction of Nitroaromatic Compounds in Homogeneous Aqueous Solution. Environmental Science and Technology, 24(10), pp. 1566–1574.  [https://doi.org/10.1021/es00080a017 DOI: 10.1021/es00080a017]</ref><ref name="Tratnyek1989">Tratnyek, P.G., and Macalady, D.L., 1989. Abiotic Reduction of Nitro Aromatic Pesticides in Anaerobic Laboratory Systems. Journal of Agricultural and Food Chemistry, 37(1), pp. 248–254.  [https://doi.org/10.1021/jf00085a058 DOI: 10.1021/jf00085a058]</ref><ref name="Hofstetter1999">Hofstetter, T.B., Heijman, C.G., Haderlein, S.B., Holliger, C. and Schwarzenbach, R.P., 1999. Complete Reduction of TNT and Other (Poly)nitroaromatic Compounds under Iron-Reducing Subsurface Conditions. Environmental Science and Technology, 33(9), pp. 1479–1487.  [https://doi.org/10.1021/es9809760 DOI: 10.1021/es9809760]</ref><ref name="Murillo-Gelvez2019">Murillo-Gelvez, J., Hickey, K.P., Di Toro, D.M., Allen, H.E., Carbonaro, R.F., and Chiu, P.C., 2019. Experimental Validation of Hydrogen Atom Transfer Gibbs Free Energy as a Predictor of Nitroaromatic Reduction Rate Constants. Environmental Science and Technology, 53(10), pp. 5816–5827.  [https://doi.org/10.1021/acs.est.9b00910 DOI: 10.1021/acs.est.9b00910]</ref><ref name="Niedźwiecka2017">Niedźwiecka, J.B., Drew, S.R., Schlautman, M.A., Millerick, K.A., Grubbs, E., Tharayil, N. and Finneran, K.T., 2017. Iron and Electron Shuttle Mediated (Bio)degradation of 2,4-Dinitroanisole (DNAN). Environmental Science and Technology, 51(18), pp. 10729–10735.  [https://doi.org/10.1021/acs.est.7b02433 DOI: 10.1021/acs.est.7b02433]</ref><ref name="Kwon2006">Kwon, M.J., and Finneran, K.T., 2006. Microbially Mediated Biodegradation of Hexahydro-1,3,5-Trinitro-1,3,5- Triazine by Extracellular Electron Shuttling Compounds. Applied and Environmental Microbiology, 72(9), pp. 5933–5941.  [https://doi.org/10.1128/AEM.00660-06 DOI: 10.1128/AEM.00660-06]&nbsp;&nbsp; [https://journals.asm.org/doi/reader/10.1128/AEM.00660-06 Open access article.]</ref>, dissolved organic matter<ref name="Dunnivant1992">Dunnivant, F.M., Schwarzenbach, R.P., and Macalady, D.L., 1992. Reduction of Substituted Nitrobenzenes in Aqueous Solutions Containing Natural Organic Matter. Environmental Science and Technology, 26(11), pp. 2133–2141.  [https://doi.org/10.1021/es00035a010 DOI: 10.1021/es00035a010]</ref><ref name="Luan2010">Luan, F., Burgos, W.D., Xie, L., and Zhou, Q., 2010. Bioreduction of Nitrobenzene, Natural Organic Matter, and Hematite by Shewanella putrefaciens CN32. Environmental Science and Technology, 44(1), pp. 184–190.  [https://doi.org/10.1021/es901585z DOI: 10.1021/es901585z]</ref><ref name="Murillo-Gelvez2021">Murillo-Gelvez, J., di Toro, D.M., Allen, H.E., Carbonaro, R.F., and Chiu, P.C., 2021. Reductive Transformation of 3-Nitro-1,2,4-triazol-5-one (NTO) by Leonardite Humic Acid and Anthraquinone-2,6-disulfonate (AQDS). Environmental Science and Technology, 55(19), pp. 12973–12983.  [https://doi.org/10.1021/acs.est.1c03333 DOI: 10.1021/acs.est.1c03333]</ref>, black carbon<ref name="Oh2013">Oh, S.-Y., Son, J.G., and Chiu, P.C., 2013. Biochar-Mediated Reductive Transformation of Nitro Herbicides and Explosives. Environmental Toxicology and Chemistry, 32(3), pp. 501–508.  [https://doi.org/10.1002/etc.2087 DOI: 10.1002/etc.2087]&nbsp;&nbsp; [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/etc.2087 Open access article.]</ref><ref name="Oh2009">Oh, S.-Y., and Chiu, P.C., 2009. Graphite- and Soot-Mediated Reduction of 2,4-Dinitrotoluene and Hexahydro-1,3,5-trinitro-1,3,5-triazine. Environmental Science & Technology, 43(18), pp. 6983–6988.  [https://doi.org/10.1021/es901433m DOI: 10.1021/es901433m]</ref><ref name="Xu2015">Xu, W., Pignatello, J.J., and Mitch, W.A., 2015. Reduction of Nitroaromatics Sorbed to Black Carbon by Direct Reaction with Sorbed Sulfides. Environmental Science and Technology, 49(6), pp. 3419–3426.  [https://doi.org/10.1021/es5045198 DOI: 10.1021/es5045198]</ref><ref name="Oh2002">Oh, S.-Y., Cha, D.K., and Chiu, P.C., 2002. Graphite-Mediated Reduction of 2,4-Dinitrotoluene with Elemental Iron. Environmental Science and Technology, 36(10), pp. 2178–2184.  [https://doi.org/10.1021/es011474g DOI: 10.1021/es011474g]</ref><ref name="Amezquita-Garcia2013">Amezquita-Garcia, H.J., Razo-Flores, E., Cervantes, F.J., and Rangel-Mendez, J.R., 2013.  Activated carbon fibers as redox mediators for the increased reduction of nitroaromatics. Carbon, 55, pp. 276–284. [https://doi.org/10.1016/j.carbon.2012.12.062 DOI: 10.1016/j.carbon.2012.12.062]</ref><ref name="Xin2022">Xin, D., Girón, J., Fuller, M.E., and Chiu, P.C., 2022. Abiotic Reduction of 3-Nitro-1,2,4-triazol-5-one (NTO) and Other Munitions Constituents by Wood-Derived Biochar through Its Rechargeable Electron Storage Capacity. Environmental Science: Processes and Impacts, 24(2), pp. 316-329.  [https://doi.org/10.1039/D1EM00447F DOI: 10.1039/D1EM00447F]</ref>, and sulfides<ref name="Hojo1960">Hojo, M., Takagi, Y. and Ogata, Y., 1960. Kinetics of the Reduction of Nitrobenzenes by Sodium Disulfide. Journal of the American Chemical Society, 82(10), pp. 2459–2462.  [https://doi.org/10.1021/ja01495a017 DOI: 10.1021/ja01495a017]</ref><ref name="Zeng2012">Zeng, T., Chin, Y.P., and Arnold, W.A., 2012. Potential for Abiotic Reduction of Pesticides in Prairie Pothole Porewaters. Environmental Science and Technology, 46(6), pp. 3177–3187.  [https://doi.org/10.1021/es203584d DOI: 10.1021/es203584d]</ref>. These geo-reductants may control the fate and half-lives of MCs in the environment and can be used to promote MC degradation in soil and groundwater through enhanced natural attenuation<ref name="USEPA2012">US EPA, 2012. A Citizen’s Guide to Monitored Natural Attenuation. EPA document 542-F-12-014.  [https://www.epa.gov/sites/default/files/2015-04/documents/a_citizens_guide_to_monitored_natural_attenuation.pdf Free download.]</ref>.
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Fire suppression systems with potential PFAS impacts include fire fighting vehicles that carried AFFF and fixed suppression systems in buildings containing large amounts of flammable materials such as aircraft hangars (Figure 2). PFAS residue on the wetted surfaces of existing infrastructure can rebound into replacement PFAS-free firefighting formulations if not removed during the transition process<ref name="RossStorch2020"/>. Simple surface rinsing with water and low-pressure washing has been proven to be inefficient for removal of surface bound PFAS from piping and tanks that contained fluorinated AFFF<ref name="RossStorch2020"/>
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[[File:LangFig2.png | thumb|left|600px|Figure 2. Fixed fire suppression system for an aircraft hangar, with storage tank on left and distribution piping on right.]]
  
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[[File:AbioMCredFig2.png | thumb |450px|Figure 2. General mechanism for the reduction of NACs/MCs.]]
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In&nbsp;addition&nbsp;to&nbsp;proper methods for system cleaning to remove residual PFAS, transition to PFAS-free foam may also include consideration of compliance with state and federal regulations, selection of the replacement PFAS-free firefighting formulation, a cost benefit analysis for replacement of the system components versus cleaning, and PFAS verification testing. Foam transition should be completed in a manner which minimizes the volume of waste generated as well as preventing any PFAS release into the environment.
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[[File:AbioMCredFig3.png | thumb |450px|Figure 3. Schematic of natural attenuation of MCs-impacted soils through chemical reduction.]]
 
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Although the chemical structures of MCs can vary significantly (Figure 1), most of them contain at least one nitro functional group (-NO<sub>2</sub>), which is susceptible to reductive transformation<ref name="Spain2000">Spain, J.C., Hughes, J.B., and Knackmuss, H.J., 2000. Biodegradation of Nitroaromatic Compounds and Explosives. CRC Press, 456 pages. ISBN: 9780367398491</ref>. Of the MCs shown in Figure 1, 2,4,6-trinitrotoluene (TNT), 2,4-dinitroanisole (DNAN), and 3-nitro-1,2,4-triazol-5-one (NTO)<ref name="Harris1996">Harris, N.J., and Lammertsma, K., 1996. Tautomerism, Ionization, and Bond Dissociations of 5-Nitro-2,4-dihydro-3H-1,2,4-triazolone. Journal of the American Chemical Society, 118(34), pp. 8048–8055.  [https://doi.org/10.1021/ja960834a DOI: 10.1021/ja960834a]</ref> are nitroaromatic compounds (NACs) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and nitroguanidine (NQ) are nitramines. The structural differences may result in different reactivities and reaction pathways. Reduction of NACs results in the formation of aromatic amines (i.e., anilines) with nitroso and hydroxylamine compounds as intermediates (Figure 2)<ref name="Schwarzenbach2016"/>.  
 
  
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Although the final reduction products are different for non-aromatic MCs, the reduction process often starts with the transformation of the -NO<sub>2</sub> moiety, either through de-nitration (e.g., RDX<ref name="Kwon2008">Kwon, M.J., and Finneran, K.T., 2008. Biotransformation products and mineralization potential for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in abiotic versus biological degradation pathways with anthraquinone-2,6-disulfonate (AQDS) and ''Geobacter metallireducens''. Biodegradation, 19(5), pp. 705–715. [https://doi.org/10.1007/s10532-008-9175-5 DOI: 10.1007/s10532-008-9175-5]</ref><ref name="Halasz2011">Halasz, A., and Hawari, J., 2011. Degradation Routes of RDX in Various Redox Systems. Aquatic Redox Chemistry, American Chemical Society, 1071(20), pp. 441-462. [https://doi.org/10.1021/bk-2011-1071.ch020 DOI: 10.1021/bk-2011-1071.ch020]</ref>) or reduction to nitroso<ref name="Kwon2006"/><ref name="Tong2021">Tong, Y., Berens, M.J., Ulrich, B.A., Bolotin, J., Strehlau, J.H., Hofstetter, T.B., and Arnold, W.A., 2021. Exploring the Utility of Compound-Specific Isotope Analysis for Assessing Ferrous Iron-Mediated Reduction of RDX in the Subsurface. Environmental Science and Technology, 55(10), pp. 6752–6763. [https://doi.org/10.1021/acs.est.0c08420 DOI: 10.1021/acs.est.0c08420]</ref> followed by ring cleavage<ref name="Kim2007"/><ref name="Halasz2011"/><ref name="Tong2021"/><ref name="Larese-Casanova2008">Larese-Casanova, P., and Scherer, M.M., 2008. Abiotic Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Green Rusts. Environmental Science and Technology, 42(11), pp. 3975–3981. [https://doi.org/10.1021/es702390b DOI: 10.1021/es702390b]</ref>.
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==PFAS Assembly on Solid Surfaces==
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The self-assembly of [[Wikipedia: Amphiphile | amphiphilic]] molecules into supramolecular bilayers is a result of their structure and how it interacts with the bulk water of a solution. Single chain hydrocarbon based amphiphiles can form [[Wikipedia: Micelle | micelles]] under relatively dilute aqueous concentrations, however for hydrocarbon based surfactants the formation of more complex organized system such as [[Wikipedia: Vesicle (biology and chemistry) | vesicles]] is rarely seen, requiring double chain amphiphiles such as [[wikipedia: Phospholipid|phospholipids]]. Associations of single chain [[wikipedia: Ion#Anions_and_cations|cationic and anionic]] hydrocarbon based amphiphiles into stable supramolecular structures such as vesicles has however been demonstrated<ref>Fukuda, H., Kawata, K., Okuda, H., 1990. Bilayer-Forming Ion-Pair Amphiphiles from Single-Chain Surfactants. Journal of the American Chemical Society, 112(4), pp. 1635-1637. [https://doi.org/10.1021/ja00160a057 doi: 10.1021/ja00160a057]</ref>, with the ion pairing of the polar head groups mimicking the a double tail situation. The behavior of single chain [[wikipedia: Per-_and_polyfluoroalkyl_substances#Fluorosurfactants|fluorosurfactant]] amphiphiles has been demonstrated to be significantly different from similar hydrocarbon based analogues. Not only are [[Wikipedia: Critical micelle concentration | critical micelle concentrations (CMC)]] of fluorosurfactants typically two orders of magnitude lower than corresponding hydrocarbon surfactants but self-assembly can occur even when fluorosurfactants are dispersed at low concentrations significantly below the CMC in water and other solvents<ref name="Krafft2006">Krafft, M.P., 2006. Highly fluorinated compounds induce phase separation in, and nanostructuration of liquid media. Possible impact on, and use in chemical reactivity control. Journal of Polymer Science Part A: Polymer Chemistry, 44(14), pp. 4251-4258. [https://doi.org/10.1002/pola.21508 doi: 10.1002/pola.21508]&nbsp;&nbsp;[[Media:Krafft2006.pdf | Open Access Article]]</ref>. The assembly of fluorinated amphiphiles structurally similar to those found in AFFF have been shown to readily form stable, complex structures including vesicles, fibers, and globules at concentrations as low as 0.5% w/v in contrast to their hydrocarbon analogues which remained fluid at 30% w/v<ref>Krafft, M.P., Guilieri, F., Riess, J.G., 1993. Can Single-Chain Perfluoroalkylated Amphiphiles Alone form Vesicles and Other Organized Supramolecular Systems? Angewandte Chemie International Edition in English, 32(5), pp. 741-743. [https://doi.org/10.1002/anie.199307411 doi: 10.1002/anie.199307411]</ref><ref name="KrafftEtAl_1994">Krafft, M.P., Guilieri, F., Riess, J.G., 1994. Supramolecular assemblies from single chain perfluoroalkylated phosphorylated amphiphiles. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 84(1), pp. 113-119. [https://doi.org/10.1016/0927-7757(93)02681-4 doi: 10.1016/0927-7757(93)02681-4]</ref>.  
  
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Figure 3 illustrates a typical MC reduction reaction. A redox-active soil constituent, such as organic matter or iron mineral, donates electrons to an MC and transforms the nitro group into an amino group (R-NH<sub>2</sub>). The rate at which an MC is reduced can vary by many orders of magnitude depending on the soil constituent, the MC, the reduction potential (''E<sub>H</sub>'') and other media conditions<ref name="Borch2010">Borch, T., Kretzschmar, R., Kappler, A., Cappellen, P.V., Ginder-Vogel, M., Voegelin, A., and Campbell, K., 2010. Biogeochemical Redox Processes and their Impact on Contaminant Dynamics. Environmental Science and Technology, 44(1), pp. 15–23. [https://doi.org/10.1021/es9026248 DOI: 10.1021/es9026248]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/es9026248 Open access article.]</ref>.  
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Krafft found that fluorinated amphiphiles formed bilayer membranes with phospholipids, and that the resulting vesicles were more stable than those made of phospholipids alone<ref name="KrafftEtAl_1998">Krafft, M.P., Riess, J.G., 1998. Highly Fluorinated Amphiphiles and Collodial Systems, and their Applications in the Biomedical Field. A Contribution. Biochimie, 80(5-6), pp. 489-514. [https://doi.org/10.1016/S0300-9084(00)80016-4 doi: 10.1016/S0300-9084(00)80016-4]</ref>. The similarities in amphiphilic properties between phospholipids and the hydrocarbon-based surfactants in AFFF suggests that bilayer vesicles may form between these and the fluorosurfactants also present in the concentrate. Krafft demonstrated that both the permeability of resulting mixed vesicles and their propensity to fuse with each other at increasing ionic strength was reduced as a result of the creation of an inert hydrophobic and [[wikipedia: Lipophobicity|lipophobic]] film within the membrane, and also suggested that the fluorinated amphiphiles increased [[Wikipedia: van der Waals force | van der Waals interactions]] in the hydrocarbon region<ref name="KrafftEtAl_1998"/>. Thus this low permeability may allow vesicles formed by the surfactants present in AFFF to act as long term repositories of PFAS not only as part of the bilayer itself but also solvated within the vesicle. This prediction is supported by the observation that supramolecular structures formed from single chain fluorinated amphiphiles have been demonstrated to be stable at elevated temperature (15 min at 121&deg;C) and have been shown to be stable over periods of months, even increasing in size over time when stored at environmentally relevant temperatures<ref name="KrafftEtAl_1994"/>.
  
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The most prevalent reductants in soils are iron minerals and organic carbon such as that found in natural organic matter. It has been suggested that Fe(II)<sub>aq</sub> and dissolved organic matter concentrations could serve as indicators of NAC reducibility in anaerobic sediments<ref name="Zhang2009">Zhang, H., and Weber, E.J., 2009. Elucidating the Role of Electron Shuttles in Reductive Transformations in Anaerobic Sediments. Environmental Science and Technology, 43(4), pp. 1042–1048. [https://doi.org/10.1021/es8017072 DOI: 10.1021/es8017072]</ref>. The following sections summarize these two classes of reductants separately and present advances in our understanding of the kinetics of NAC/MC reduction by these geo-reductants.
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Formation of complex structures at relatively low solute concentrations requires the monomer molecules to be well ordered to maintain tight packing in the supramolecular structure<ref>Ringsdorf, H., Schlarb, B., Venzmer, J., 1988. Molecular Architecture and Function of Polymeric Oriented Systems: Models for the Study of Organization, Surface Recognition, and Dynamics of Biomembranes. Angewandte Chemie International Edition in English, 27(1), pp. 113-158. [https://doi.org/10.1002/anie.198801131 doi: 10.1002/anie.198801131]</ref>. This order results from electrostatic forces, [[wikipedia: Hydrogen bond|hydrogen bonding]], and in the case of fluorinated amphiphiles, hydrophobic interactions. The geometry of the amphiphile also potentially contributes to the type of supramolecular aggregation<ref>Israelachvili, J.N., Mitchell, D.J., Ninham, B.W., 1976. Theory of Self-Assembly of Hydrocarbon Amphiphiles into Micelles and Bilayers. Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics, 72, pp. 1525-1568. [https://doi.org/10.1039/F29767201525 doi: 10.1039/F29767201525]</ref>. Surfactants which adopt a conical shape (such as a typical hydrocarbon based surfactant with a large polar head group and a single alkyl chain as a tail) tend to form micelles more easily. Increasing the bulk of the tail makes the surfactant more cylindrically shaped which makes assembly into bilayers more likely.  
  
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==Carbonaceous Reductants==
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Perfluoroalkyl chains are significantly more bulky than their hydrocarbon based analogues both in cross sectional area (28-30 Å<sup>2</sup> versus 20 Å<sup>2</sup>, respectively) and mean volume (CF<sub>2</sub> and CF<sub>3</sub> estimated as 38 Å<sup>3</sup> and 92 Å<sup>3</sup> compared to 27 Å<sup>3</sup> and 54 Å<sup>3</sup> for CH<sub>2</sub> and CH<sub>3</sub>)<ref name="KrafftEtAl_1998"/><ref name="Krafft2006"/>. Structural studies on linear PFOS have shown that the molecule adopts an unusual helical structure<ref>Erkoç, Ş., Erkoç, F., 2001. Structural and electronic properties of PFOS and LiPFOS. Journal of Molecular Structure: THEOCHEM, 549(3), pp. 289-293. [https://doi.org/10.1016/S0166-1280(01)00553-X doi:10.1016/S0166-1280(01)00553-X]</ref><ref name="TorresEtAl2009">Torres, F.J., Ochoa-Herrera, V., Blowers, P., Sierra-Alvarez, R., 2009. Ab initio study of the structural, electronic, and thermodynamic properties of linear perfluorooctane sulfonate (PFOS) and its branched isomers. Chemosphere 76(8), pp. 1143-1149. [https://doi.org/10.1016/j.chemosphere.2009.04.009 doi: 10.1016/j.chemosphere.2009.04.009]</ref> in aqueous and solvent phases to alleviate [[wikipedia: Steric_effects#Steric_hindrance|steric hindrance]]. This arrangement results from the carbon chain starting in the planar all anti [[wikipedia:Conformational isomerism|conformation]] and then successively twisting all the CC-CC dihedrals by 15&deg;-20&deg; in the same direction<ref>Abbandonato, G., Catalano, D., Marini, A., 2010. Aggregation of Perfluoroctanoate Salts Studied by <sup>19</sup>F NMR and DFT Calculations: Counterion Complexation, Poly(ethylene glycol) Addition, and Conformational Effects. Langmuir 26(22), pp. 16762-16770. [https://doi.org/10.1021/la102578k  doi: 10.1021/la102578k].</ref>. The conformation also minimizes the electrostatic repulsion between fluorine atoms bonded to the same side of the carbon backbone by maximizing the interatomic distances between them<ref name="TorresEtAl2009"/>.
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[[File:AbioMCredFig4.png | thumb |600px|Figure 4. Chemical structure of commonly used hydroquinones in NACs/MCs kinetic experiments.]]
 
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The two most predominant forms of organic carbon in natural systems are natural organic matter (NOM) and black carbon (BC)<ref name="Schumacher2002">Schumacher, B.A., 2002. Methods for the Determination of Total Organic Carbon (TOC) in Soils and Sediments. U.S. EPA, Ecological Risk Assessment Support Center. [http://bcodata.whoi.edu/LaurentianGreatLakes_Chemistry/bs116.pdf Free download.]</ref>. Black carbon includes charcoal, soot, graphite, and coal. Until the early 2000s black carbon was considered to be a class of (bio)chemically inert geosorbents<ref name="Schmidt2000">Schmidt, M.W.I., and Noack, A.G., 2000. Black carbon in soils and sediments: Analysis, distribution, implications, and current challenges. Global Biogeochemical Cycles, 14(3), pp. 777–793.  [https://doi.org/10.1029/1999GB001208 DOI: 10.1029/1999GB001208]&nbsp;&nbsp; [https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/1999GB001208 Open access article.]</ref>. However, it has been shown that BC can contain abundant quinone functional groups and thus can store and exchange electrons<ref name="Klüpfel2014">Klüpfel, L., Keiluweit, M., Kleber, M., and Sander, M., 2014. Redox Properties of Plant Biomass-Derived Black Carbon (Biochar). Environmental Science and Technology, 48(10), pp. 5601–5611. [https://doi.org/10.1021/es500906d DOI: 10.1021/es500906d]</ref> with chemical<ref name="Xin2019">Xin, D., Xian, M., and Chiu, P.C., 2019. New methods for assessing electron storage capacity and redox reversibility of biochar. Chemosphere, 215, 827–834. [https://doi.org/10.1016/j.chemosphere.2018.10.080 DOI: 10.1016/j.chemosphere.2018.10.080]</ref> and biological<ref name="Saquing2016">Saquing, J.M., Yu, Y.-H., and Chiu, P.C., 2016. Wood-Derived Black Carbon (Biochar) as a Microbial Electron Donor and Acceptor. Environmental Science and Technology Letters, 3(2), pp. 62–66. [https://doi.org/10.1021/acs.estlett.5b00354 DOI: 10.1021/acs.estlett.5b00354]</ref> agents in the surroundings. Specifically, BC such as biochar has been shown to reductively transform MCs including NTO, DNAN, and RDX<ref name="Xin2022"/>.
 
  
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NOM encompasses all the organic compounds present in terrestrial and aquatic environments and can be classified into two groups, non-humic and humic substances. Humic substances (HS) contain a wide array of functional groups including carboxyl, enol, ether, ketone, ester, amide, (hydro)quinone, and phenol<ref name="Sparks2003">Sparks, D.L., 2003. Environmental Soil Chemistry, 2nd Edition. Elsevier Science and Technology Books.  [https://doi.org/10.1016/B978-0-12-656446-4.X5000-2 DOI: 10.1016/B978-0-12-656446-4.X5000-2]</ref>. Quinone and hydroquinone groups are believed to be the predominant redox moieties responsible for the capacity of HS and BC to store and reversibly accept and donate electrons (i.e., through reduction and oxidation of HS/BC, respectively)<ref name="Schwarzenbach1990"/><ref name="Dunnivant1992"/><ref name="Klüpfel2014"/><ref name="Scott1998">Scott, D.T., McKnight, D.M., Blunt-Harris, E.L., Kolesar, S.E., and Lovley, D.R., 1998. Quinone Moieties Act as Electron Acceptors in the Reduction of Humic Substances by Humics-Reducing Microorganisms. Environmental Science and Technology, 32(19), pp. 2984–2989. [https://doi.org/10.1021/es980272q DOI: 10.1021/es980272q]</ref><ref name="Cory2005">Cory, R.M., and McKnight, D.M., 2005. Fluorescence Spectroscopy Reveals Ubiquitous Presence of Oxidized and Reduced Quinones in Dissolved Organic Matter. Environmental Science & Technology, 39(21), pp 8142–8149.  [https://doi.org/10.1021/es0506962 DOI: 10.1021/es0506962]</ref><ref name="Fimmen2007">Fimmen, R.L., Cory, R.M., Chin, Y.P., Trouts, T.D., and McKnight, D.M., 2007. Probing the oxidation–reduction properties of terrestrially and microbially derived dissolved organic matter. Geochimica et Cosmochimica Acta, 71(12), pp. 3003–3015. [https://doi.org/10.1016/j.gca.2007.04.009 DOI: 10.1016/j.gca.2007.04.009]</ref><ref name="Struyk2001">Struyk, Z., and Sposito, G., 2001. Redox properties of standard humic acids. Geoderma, 102(3-4), pp. 329–346.  [https://doi.org/10.1016/S0016-7061(01)00040-4 DOI: 10.1016/S0016-7061(01)00040-4]</ref><ref name="Ratasuk2007">Ratasuk, N., and Nanny, M.A., 2007. Characterization and Quantification of Reversible Redox Sites in Humic Substances. Environmental Science and Technology, 41(22), pp. 7844–7850.  [https://doi.org/10.1021/es071389u DOI: 10.1021/es071389u]</ref><ref name="Aeschbacher2010">Aeschbacher, M., Sander, M., and Schwarzenbach, R.P., 2010. Novel Electrochemical Approach to Assess the Redox Properties of Humic Substances. Environmental Science and Technology, 44(1), pp. 87–93.  [https://doi.org/10.1021/es902627p DOI: 10.1021/es902627p]</ref><ref name="Aeschbacher2011">Aeschbacher, M., Vergari, D., Schwarzenbach, R.P., and Sander, M., 2011. Electrochemical Analysis of Proton and Electron Transfer Equilibria of the Reducible Moieties in Humic Acids. Environmental Science and Technology, 45(19), pp. 8385–8394. [https://doi.org/10.1021/es201981g DOI: 10.1021/es201981g]</ref><ref name="Bauer2009">Bauer, I., and Kappler, A., 2009. Rates and Extent of Reduction of Fe(III) Compounds and O<sub>2</sub> by Humic Substances. Environmental Science and Technology, 43(13), pp. 4902–4908.  [https://doi.org/10.1021/es900179s DOI: 10.1021/es900179s]</ref><ref name="Maurer2010">Maurer, F., Christl, I. and Kretzschmar, R., 2010. Reduction and Reoxidation of Humic Acid: Influence on Spectroscopic Properties and Proton Binding. Environmental Science and Technology, 44(15), pp. 5787–5792. [https://doi.org/10.1021/es100594t DOI: 10.1021/es100594t]</ref><ref name="Walpen2016">Walpen, N., Schroth, M.H., and Sander, M., 2016. Quantification of Phenolic Antioxidant Moieties in Dissolved Organic Matter by Flow-Injection Analysis with Electrochemical Detection. Environmental Science and Technology, 50(12), pp. 6423–6432.  [https://doi.org/10.1021/acs.est.6b01120 DOI: 10.1021/acs.est.6b01120]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/acs.est.6b01120 Open access article.]</ref><ref name="Aeschbacher2012">Aeschbacher, M., Graf, C., Schwarzenbach, R.P., and Sander, M., 2012.  Antioxidant Properties of Humic Substances. Environmental Science and Technology, 46(9), pp. 4916–4925.  [https://doi.org/10.1021/es300039h DOI: 10.1021/es300039h]</ref><ref name="Nurmi2002">Nurmi, J.T., and Tratnyek, P.G., 2002. Electrochemical Properties of Natural Organic Matter (NOM), Fractions of NOM, and Model Biogeochemical Electron Shuttles. Environmental Science and Technology, 36(4), pp. 617–624. [https://doi.org/10.1021/es0110731 DOI: 10.1021/es0110731]</ref>.  
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A consequence of the helical structure is that there is limited carbon-carbon bond rotation within the perfluoroalkyl chain giving them increased rigidity compared to alkyl chains<ref>Barton, S.W., Goudot, A., Bouloussa, O., Rondelez, F., Lin, B., Novak, F., Acero, A., Rice, S., 1992. Structural transitions in a monolayer of fluorinated amphiphile molecules. The Journal of Chemical Physics, 96(2), pp. 1343-1351. [https://doi.org/10.1063/1.462170 doi: 10.1063/1.462170]</ref>. The bulkiness of the perfluoroalkyl chain confers a cylindrical shape on the fluorosurfactant amphiphile and therefore favors the formation of bilayers and vesicles the aggregation of which is further assisted by the rigidity of the molecules which allow close packing in the supramolecular structure. Fluorosurfactants therefore cannot be regarded as more hydrophobic analogues of hydrogenated surfactants. Their self-assembly behavior is characterized by a strong tendency to form vesicles and lamellar phases rather than micelles, due to the bulkiness and rigidity of the perfluoroalkyl chain that tends to decrease the curvature of the aggregates they form in solution<ref>Barton, C.A., Butler, L.E., Zarzecki, C.J., Flaherty, J., Kaiser, M., 2006. Characterizing Perfluorooctanoate in Ambient Air near the Fence Line of a Manufacturing Facility: Comparing Modeled and Monitored Values. Journal of the Air and Waste Management Association, 56, pp. 48-55. [https://doi.org/10.1080/10473289.2006.10464429 doi: 10.1080/10473289.2006.10464429]&nbsp;&nbsp;[https://www.tandfonline.com/doi/epdf/10.1080/10473289.2006.10464429?needAccess=true Open Access Article]</ref>. The larger tail cross section of fluorinated compared to hydrogenated amphiphiles tends to favor the formation of aggregates with lesser surface curvature, therefore rather than micelles they form bilayer membranes, vesicles, tubules and fibers<ref>Krafft, M.P., Guilieri, F., Riess, J.G., 1993. Can Single-Chain Perfluoroalkylated Amphiphiles Alone form Vesicles and Other Organized Supramolecular Systems? Angewandte Chemie International Edition in English, 32(5), pp. 741-743. [https://doi.org/10.1002/anie.199307411 doi: 10.1002/anie.199307411]</ref><ref>Furuya, H., Moroi, Y., Kaibara, K., 1996. Solid and Solution Properties of Alkylammonium Perfluorocarboxylates. The Journal of Physical Chemistry, 100(43), pp. 17249-17254.  [https://doi.org/10.1021/jp9612801 doi: 10.1021/jp9612801]</ref><ref>Giulieri, F., Krafft, M.P., 1996. Self-organization of single-chain fluorinated amphiphiles with fluorinated alcohols. Thin Solid Films, 284-285, pp. 195-199. [https://doi.org/10.1016/S0040-6090(95)08304-9 doi: 10.1016/S0040-6090(95)08304-9]</ref><ref>Gladysz, J.A., Curran, D.P., Horvath, I.T., 2004. Handbook of Fluorous Chemistry. WILEY-VCH Verlag GmbH & Co. KGaA,, Weinheim, Germany. ISBN: 3-527-30617-X</ref>. Rojas ''et al.'' (2002) demonstrated that perfluorooctyl sulphonamide formed a contiguous bilayer at 50 mg/L with self-assembled aggregates present at concentrations as low as 10 mg/L<ref name="RojasEtAl2002">Rojas, O.J., Macakova, L., Blomberg, E., Emmer, A., and Claesson, P.M., 2002. Fluorosurfactant Self-Assembly at Solid/Liquid Interfaces. Langmuir, 18(21), pp. 8085-8095. [https://doi.org/10.1021/la025989c doi: 10.1021/la025989c]</ref>.
  
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Hydroquinones have been widely used as surrogates to understand the reductive transformation of NACs and MCs by NOM. Figure 4 shows the chemical structures of the singly deprotonated forms of four hydroquinone species previously used to study NAC/MC reduction. The second-order rate constants (''k<sub>R</sub>'') for the reduction of NACs/MCs by these hydroquinone species are listed in Table 1, along with the aqueous-phase one electron reduction potentials of the NACs/MCs (''E<sub>H</sub><sup>1’</sup>'') where available. ''E<sub>H</sub><sup>1’</sup>'' is an experimentally measurable thermodynamic property that reflects the propensity of a given NAC/MC to accept an electron in water (''E<sub>H</sub><sup>1</sup>''(R-NO<sub>2</sub>)):
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==Thermodynamics of PFAS Accumulations on Solid Surfaces==
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The thermodynamics of formation of amphiphiles into supramolecular species requires consideration of both hydrophobic and hydrophilic interactions resulting from the amphoteric nature of the molecule. The hydrophilic portions of the molecule are driven to maximize their solvation interaction with as many water molecules as possible, whereas the hydrophobic portions of the molecule are driven to aggregate together thus minimizing interaction with the bulk water. Both of these processes change the [[wikipedia:Enthalpy|enthalpy]] and [[wikipedia: Entropy|entropy]] of the system.
  
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:::::<big>'''Equation 1:'''&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;''R-NO<sub>2</sub> + e<sup>-</sup> ⇔ R-NO<sub>2</sub><sup>•-</sup>''</big>
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In aqueous solution, the hydrophilic portions of an amphiphile form hydrogen bonds (4 - 120 kJ/mol) and van der Waals interactions (<5 kJ/mol) with water molecules and surfaces, and electrostatic interactions (5 – 300 kJ/mol) can also occur where the amphiphile is ionic<ref name="LombardoEtAl2015">Lombardo, D., Kiselev, M.A., Magazù, S., Calandra, P., 2015. Amphiphiles Self-Assembly: Basic Concepts and Future Perspectives of Supramolecular Approaches. Advances in Condensed Matter Physics, vol. 2015, article ID 151683, 22 pages. [https://doi.org/10.1155/2015/151683 doi: 10.1155/2015/151683]&nbsp;&nbsp;[[Media: LombardoEtAl2015.pdf | Open Access Article]]</ref>. These interactions, although weak compared to intramolecular covalent bonds within a molecule are energetically favorable and increase the enthalpy of the combined solute-solvent system. Thus, the hydrophilic portion of an amphiphile will look to maximize enthalpic gain through hydrogen bond interactions with the bulk water.
  
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Knowing the identity of and reaction order in the reductant is required to derive the second-order rate constants listed in Table 1. This same reason limits the utility of reduction rate constants measured with complex carbonaceous reductants such as NOM<ref name="Dunnivant1992"/>, BC<ref name="Oh2013"/><ref name="Oh2009"/><ref name="Xu2015"/><ref name="Xin2021">Xin, D., 2021. Understanding the Electron Storage Capacity of Pyrogenic Black Carbon: Origin, Redox Reversibility, Spatial Distribution, and Environmental Applications. Doctoral Thesis, University of Delaware.  [https://udspace.udel.edu/bitstream/handle/19716/30105/Xin_udel_0060D_14728.pdf?sequence=1 Free download.]</ref>, and HS<ref name="Luan2010"/><ref name="Murillo-Gelvez2021"/>, whose chemical structures and redox moieties responsible for the reduction, as well as their abundance, are not clearly defined or known. In other words, the observed rate constants in those studies are specific to the experimental conditions (e.g., pH and NOM source and concentration), and may not be easily comparable to other studies.
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The hydrophobic portion of an amphiphile cannot form hydrogen bonds with the bulk solution, and its presence disrupts the hydrogen bond interactions between individual water molecules within the bulk water matrix. This disruption lowers the entropy of the system by reducing the degrees of translational rotational freedom available to the bulk water. The [[wikipedia:Second law of thermodynamics|second law of thermodynamics]] dictates that a system will arrange itself to maximize its entropy. With hydrophobic species this can be achieved by their spontaneous aggregation, as the reduction in solution entropy of the aggregated system is less than that which would occur if the component parts were solvated individually. These hydrophobic and hydrophilic interactions are weak, and the individual entropy gain per amphiphile upon aggregation is very small. However, taken together the overall effect on the entropy of the aggregate is sufficient to maintain it in solution, and moreover these interactions make the aggregates resistant to minor perturbations while retaining the reversibility of the self-assembled structure<ref name="LombardoEtAl2015"/>.
  
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{| class="wikitable mw-collapsible" style="float:left; margin-right:40px; text-align:center;"
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==Regulatory Drivers for Transition to PFAS-Free Firefighting Formulations==
−
|+ Table&nbsp;1.&nbsp;Aqueous&nbsp;phase one electron reduction potentials and logarithm of second-order rate constants for the reduction of NACs and MCs by the singly deprotonated form of the hydroquinones lawsone, juglone, AHQDS and AHQS, with the second-order rate constants for the deprotonated NAC/MC species (i.e., nitrophenolates and NTO<sup>–</sup>) in parentheses.
+
Regulations restricting the use and release of PFAS are being proposed and promulgated worldwide, with several enacted regulations addressing the use of aqueous film forming foams (AFFF) containing PFAS<ref name="Queensland2016">Queensland (Australia) Department of Environment and Heritage Protection, 2016. Operational Policy - Environmental Management of Firefighting Foam. 16 pages. [https://environment.des.qld.gov.au/assets/documents/regulation/firefighting-foam-policy.pdf Free Download]</ref><ref>U.S. Congress, 2019. S.1790 - National Defense Authorization Act for Fiscal Year 2020. United States Library of Congress.&nbsp;&nbsp;[https://www.congress.gov/bill/116th-congress/senate-bill/1790 Text and History of Law].</ref><ref>Arizona State Legislature, 2019. Title 36, Section 1696. Firefighting foam; prohibited uses; exception; definitions. [https://www.azleg.gov/viewdocument/?docName=https://www.azleg.gov/ars/36/01696.htm Text of Law]</ref><ref>California Legislature, 2020. Senate Bill No. 1044, Chapter 308, Firefighting equipment and foam: PFAS chemicals. [https://leginfo.legislature.ca.gov/faces/billTextClient.xhtml?bill_id=201920200SB1044 Text and History of Law]</ref><ref>Arkansas General Assembly, 2021. An Act Concerning the Use of Certain Chemicals in Firefighting Foam; and for Other Purposes. Act 315, State of Arkansas. [https://trackbill.com/bill/arkansas-house-bill-1351-concerning-the-use-of-certain-chemicals-in-firefighting-foam/2008913/ Text and History of Law].</ref><ref>Espinosa, Summers, Kelly, J., Statler, Hansen, Young, 2021. Amendment to Fire Prevention and Control Act. House Bill 2722. West Virginia Legislature. [https://trackbill.com/bill/west-virginia-house-bill-2722-prohibiting-the-use-of-class-b-fire-fighting-foam-for-testing-purposes-if-the-foam-contains-a-certain-class-of-fluorinated-organic-chemicals/2047674/ Text and History of Law]</ref><ref>Louisiana Legislature, 2021. Act No. 232. [https://trackbill.com/bill/louisiana-house-bill-389-fire-protect-fire-marshal-provides-relative-to-the-discharge-or-use-of-class-b-fire-fighting-foam-containing-fluorinated-organic-chemicals/2092535/  Text and History of Law]</ref><ref>Vermont Legislature, 2021b. Act No. 36, PFAS in Class B Firefighting Foam. [https://trackbill.com/bill/vermont-senate-bill-20-an-act-relating-to-restrictions-on-perfluoroalkyl-and-polyfluoroalkyl-substances-and-other-chemicals-of-concern-in-consumer-products/1978963/  History and Text of Law]</ref>. In addition to regulated usage, firefighting formulation users are transitioning to PFAS-free firefighting formulations to reduce environmental liability in the event of a release, to reduce the cost of expensive containment systems and management of generated waste streams, and to avoid reputational damage. In 2016, Queensland, Australia was one of the first governments to ban PFAS use in firefighting foam<ref name="Queensland2016"/>. The US 2020 National Defense Authorization Act specified immediate prohibition of controlled releases of AFFF containing PFAS and required the Secretary of the Navy to publish a specification for PFAS-free firefighting formulation use and ensure it is available for use by the Department of Defense (DoD) by October 1, 2023<ref>U.S. Congress, 2021. S.2792 - National Defense Authorization Act for Fiscal Year 2021. United States Library of Congress.&nbsp;&nbsp;[https://www.congress.gov/bill/117th-congress/senate-bill/2792/ Text and History of Law].</ref>. The National Fire Protection Association (NFPA) recently removed the requirement for AFFF containing PFAS from their Standard on Aircraft Hangars and added two new chapters to allow users to determine if AFFF containing PFAS is needed at their facility<ref name="NFPA2022">National Fire Protection Association (NFPA), 2022. Codes and Standards, 409: Standard on Aircraft Hangars. [https://www.nfpa.org/codes-and-standards/4/0/9/409?l=42 NFPA Website]</ref>.
−
|-
 
−
! Compound 
 
−
! rowspan="2" |''E<sub>H</sub><sup>1'</sup>'' (V)
 
−
! colspan="4"| Hydroquinone (log ''k<sub>R</sub>''&nbsp;(M<sup>-1</sup>s<sup>-1</sup>))
 
−
|-
 
−
! (NAC/MC)
 
−
! LAW<sup>-</sup>
 
−
! JUG<sup>-</sup>
 
−
! AHQDS<sup>-</sup>
 
−
! AHQS<sup>-</sup>
 
−
|-
 
−
| Nitrobenzene (NB) || -0.485<ref name="Schwarzenbach1990"/> || 0.380<ref name="Schwarzenbach1990"/> || -1.102<ref name="Schwarzenbach1990"/> || 2.050<ref name="Murillo-Gelvez2019"/> || 3.060<ref name="Murillo-Gelvez2019"/>
 
−
|-
 
−
| 2-nitrotoluene (2-NT) || -0.590<ref name="Schwarzenbach1990"/> || -1.432<ref name="Schwarzenbach1990"/> || -2.523<ref name="Schwarzenbach1990"/> || 0.775<ref name="Hartenbach2008"/> ||
 
−
|-
 
−
| 3-nitrotoluene (3-NT) || -0.475<ref name="Schwarzenbach1990"/> || 0.462<ref name="Schwarzenbach1990"/> || -0.921<ref name="Schwarzenbach1990"/> ||  ||
 
−
|-
 
−
| 4-nitrotoluene (4-NT) || -0.500<ref name="Schwarzenbach1990"/> || 0.041<ref name="Schwarzenbach1990"/> || -1.292<ref name="Schwarzenbach1990"/> || 1.822<ref name="Hartenbach2008"/> || 2.610<ref name="Murillo-Gelvez2019"/>
 
−
|-
 
−
| 2-chloronitrobenzene (2-ClNB) || -0.485<ref name="Schwarzenbach1990"/> || 0.342<ref name="Schwarzenbach1990"/> || -0.824<ref name="Schwarzenbach1990"/> ||2.412<ref name="Hartenbach2008"/> ||
 
−
|-
 
−
| 3-chloronitrobenzene (3-ClNB) || -0.405<ref name="Schwarzenbach1990"/> || 1.491<ref name="Schwarzenbach1990"/> || 0.114<ref name="Schwarzenbach1990"/> || ||
 
−
|-
 
−
| 4-chloronitrobenzene (4-ClNB) || -0.450<ref name="Schwarzenbach1990"/> || 1.041<ref name="Schwarzenbach1990"/> || -0.301<ref name="Schwarzenbach1990"/> || 2.988<ref name="Hartenbach2008"/> ||
 
−
|-
 
−
| 2-acetylnitrobenzene (2-AcNB) || -0.470<ref name="Schwarzenbach1990"/> || 0.519<ref name="Schwarzenbach1990"/> || -0.456<ref name="Schwarzenbach1990"/> || ||
 
−
|-
 
−
| 3-acetylnitrobenzene (3-AcNB) || -0.405<ref name="Schwarzenbach1990"/> || 1.663<ref name="Schwarzenbach1990"/> || 0.398<ref name="Schwarzenbach1990"/> || ||
 
−
|-
 
−
| 4-acetylnitrobenzene (4-AcNB) || -0.360<ref name="Schwarzenbach1990"/> || 2.519<ref name="Schwarzenbach1990"/> || 1.477<ref name="Schwarzenbach1990"/> || ||
 
−
|-
 
−
| 2-nitrophenol (2-NP) || || 0.568 (0.079)<ref name="Schwarzenbach1990"/> || || ||
 
−
|-
 
−
| 4-nitrophenol (4-NP) || || -0.699 (-1.301)<ref name="Schwarzenbach1990"/> || || ||
 
−
|-
 
−
| 4-methyl-2-nitrophenol (4-Me-2-NP) || || 0.748 (0.176)<ref name="Schwarzenbach1990"/> || || ||
 
−
|-
 
−
| 4-chloro-2-nitrophenol (4-Cl-2-NP) || || 1.602 (1.114)<ref name="Schwarzenbach1990"/> || || ||
 
−
|-
 
−
| 5-fluoro-2-nitrophenol (5-Cl-2-NP) || || 0.447 (-0.155)<ref name="Schwarzenbach1990"/> || || ||
 
−
|-
 
−
| 2,4,6-trinitrotoluene (TNT) || -0.280<ref name="Schwarzenbach2016"/> || || 2.869<ref name="Hofstetter1999"/> || 5.204<ref name="Hartenbach2008"/> ||
 
−
|-
 
−
| 2-amino-4,6-dinitrotoluene (2-A-4,6-DNT) || -0.400<ref name="Schwarzenbach2016"/> || || 0.987<ref name="Hofstetter1999"/> || ||
 
−
|-
 
−
| 4-amino-2,6-dinitrotoluene (4-A-2,6-DNT) || -0.440<ref name="Schwarzenbach2016"/>  || || 0.079<ref name="Hofstetter1999"/> || ||
 
−
|-
 
−
| 2,4-diamino-6-nitrotoluene (2,4-DA-6-NT) || -0.505<ref name="Schwarzenbach2016"/> || || -1.678<ref name="Hofstetter1999"/> || ||
 
−
|-
 
−
| 2,6-diamino-4-nitrotoluene (2,6-DA-4-NT) || -0.495<ref name="Schwarzenbach2016"/> || || -1.252<ref name="Hofstetter1999"/> || ||
 
−
|-
 
−
| 1,3-dinitrobenzene (1,3-DNB) || -0.345<ref name="Hofstetter1999"/> || || 1.785<ref name="Hofstetter1999"/> || ||
 
−
|-
 
−
| 1,4-dinitrobenzene (1,4-DNB) || -0.257<ref name="Hofstetter1999"/> || || 3.839<ref name="Hofstetter1999"/> || ||
 
−
|-
 
−
| 2-nitroaniline (2-NANE) || < -0.560<ref name="Hofstetter1999"/> || || -2.638<ref name="Hofstetter1999"/> || ||
 
−
|-
 
−
| 3-nitroaniline (3-NANE) || -0.500<ref name="Hofstetter1999"/> || || -1.367<ref name="Hofstetter1999"/> || ||
 
−
|-
 
−
| 1,2-dinitrobenzene (1,2-DNB) || -0.290<ref name="Hofstetter1999"/> || || || 5.407<ref name="Hartenbach2008"/> ||
 
−
|-
 
−
| 4-nitroanisole (4-NAN) || || -0.661<ref name="Murillo-Gelvez2019"/> || || 1.220<ref name="Murillo-Gelvez2019"/> ||
 
−
|-
 
−
| 2-amino-4-nitroanisole (2-A-4-NAN) || || -0.924<ref name="Murillo-Gelvez2019"/> || || 1.150<ref name="Murillo-Gelvez2019"/> || 2.190<ref name="Murillo-Gelvez2019"/>  
 
−
|-
 
−
| 4-amino-2-nitroanisole (4-A-2-NAN) || || || ||1.610<ref name="Murillo-Gelvez2019"/> || 2.360<ref name="Murillo-Gelvez2019"/>
 
−
|-
 
−
| 2-chloro-4-nitroaniline (2-Cl-5-NANE) || || -0.863<ref name="Murillo-Gelvez2019"/> || || 1.250<ref name="Murillo-Gelvez2019"/> || 2.210<ref name="Murillo-Gelvez2019"/>  
 
−
|-
 
−
| N-methyl-4-nitroaniline (MNA) || || -1.740<ref name="Murillo-Gelvez2019"/> || || -0.260<ref name="Murillo-Gelvez2019"/> || 0.692<ref name="Murillo-Gelvez2019"/>
 
−
|-
 
−
| 3-nitro-1,2,4-triazol-5-one (NTO) || || || || 5.701 (1.914)<ref name="Murillo-Gelvez2021"/> ||
 
−
|-
 
−
| Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) || || || || -0.349<ref name="Kwon2008"/> ||
 
−
|}
 
  
−
[[File:AbioMCredFig5.png | thumb |500px|Figure 5. Relative reduction rate constants of the NACs/MCs listed in Table 1 for AHQDS<sup>–</sup>. Rate constants are compared with respect to RDX. Abbreviations of NACs/MCs as listed in Table 1.]]
+
==Selection of Replacement PFAS-Free Firefighting Formulations==       
−
Most of the current knowledge about MC degradation is derived from studies using NACs. The reduction kinetics of only four MCs, namely TNT, N-methyl-4-nitroaniline (MNA), NTO, and RDX, have been investigated with hydroquinones. Of these four MCs, only the reduction rates of MNA and TNT have been modeled<ref name="Hofstetter1999"/><ref name="Murillo-Gelvez2019"/><ref name="Riefler2000">Riefler, R.G., and Smets, B.F., 2000. Enzymatic Reduction of 2,4,6-Trinitrotoluene and Related Nitroarenes: Kinetics Linked to One-Electron Redox Potentials. Environmental Science and Technology, 34(18), pp. 3900–3906. [https://doi.org/10.1021/es991422f DOI: 10.1021/es991422f]</ref><ref name="Salter-Blanc2015">Salter-Blanc, A.J., Bylaska, E.J., Johnston, H.J., and Tratnyek, P.G., 2015. Predicting Reduction Rates of Energetic Nitroaromatic Compounds Using Calculated One-Electron Reduction Potentials. Environmental Science and Technology, 49(6), pp. 3778–3786.  [https://doi.org/10.1021/es505092s DOI: 10.1021/es505092s]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/es505092s Open access article.]</ref>.  
+
Since they first entered the market in the 2000s, the operational capabilities of PFAS-free firefighting formulations have grown<ref>Allcorn, M., Bluteau, T., Corfield, J., Day, G., Cornelsen, M., Holmes, N.J.C., Klein, R.A., McDowall, J.G., Olsen, K.T., Ramsden, N., Ross, I., Schaefer, T.H., Weber, R., Whitehead, K., 2018. Fluorine-Free Firefighting Foams (3F) – Viable Alternatives to Fluorinated Aqueous Film-Forming Foams (AFFF). White Paper prepared for the IPEN by members of the IPEN F3 Panel and associates, POPRC-14, Rome. [https://ipen.org/sites/default/files/documents/IPEN_F3_Position_Paper_POPRC-14_12September2018d.pdf Free Download].</ref> and numerous companies are now manufacturing and delivering PFAS-free firefighting formulations for fixed systems and AFFF vehicles<ref>Ansul (Company), Ansul NFF-331 3%x3% Non-Fluorinated Foam Concentrate (Commercial Product). [https://docs.johnsoncontrols.com/specialhazards/api/khub/documents/1nbeVfynU1IW~eJcCOA0Bg/content Product Data Sheet].</ref><ref>BioEx (Company), Ecopol A+ (Commercial Product). [https://www.bio-ex.com/en/our-products/product/ecopol-aplus/ Website]</ref><ref>National Foam (Company), 2020. Avio F3 Green KHC 3%, Fluorine Free Foam Concentrate (Commercial Product). [https://nationalfoam.com/wp-content/uploads/sites/4/NMS515-Avio-Green-KHC-3-FF.pdf Safety Data Sheet]</ref>. Key factors in the selection of a PFAS-free firefighting formulation product are compatibility of the new formulation with the existing system (as confirmed by a fire protection engineer) and environmental certifications (i.e., verifying the absence of organic fluorine or PFAS or the absence of other non-fluorine environmental contaminants).
  
−
Using the rate constants obtained with AHQDS<sup>–</sup>, a relative reactivity trend can be obtained (Figure 5). RDX is the slowest reacting MC in Table 1, hence it was selected to calculate the relative rates of reaction (i.e., log ''k<sub>NAC/MC</sub>'' – log ''k<sub>RDX</sub>''). If only the MCs in Figure 5 are considered, the reactivity spans 6 orders of magnitude following the trend: RDX ≈ MNA < NTO<sup>–</sup> < DNAN < TNT < NTO. The rate constant for DNAN reduction by AHQDS<sup>–</sup> is not yet published and hence not included in Table 1. Note that speciation of NACs/MCs can significantly affect their reduction rates. Upon deprotonation, the NAC/MC becomes negatively charged and less reactive as an oxidant (i.e., less prone to accept an electron). As a result, the second-order rate constant can decrease by 0.5-0.6 log unit in the case of nitrophenols and approximately 4 log units in the case of NTO (numbers in parentheses in Table 1)<ref name="Schwarzenbach1990"/><ref name="Murillo-Gelvez2021"/>.
+
In January 2023, the US Department of Defense (DoD) published the [https://media.defense.gov/2023/Jan/12/2003144157/-1/-1/1/MILITARY-SPECIFICATION-FOR-FIRE-EXTINGUISHING-AGENT-FLUORINE-FREE-FOAM-F3-LIQUID-CONCENTRATE-FOR-LAND-BASED-FRESH-WATER-APPLICATIONS.PDF Performance Specification for Fire Extinguishing Agent, Fluorine-Free Foam (F3) Liquid Concentrate for Land-Based, Fresh Water Applications]<ref name="DoD2023"/>. This Military Performance Specification (Mil-Spec) allows PFAS-free firefighting formulations to be certified as meeting certain standardized operational goals for use in military settings. In addition to Mil-Spec requirements, PFAS-free firefighting formulations can also be certified through Underwriters Laboratories Standard for Safety, Foam Equipment and Liquid Concentrates, UL 162, which requires the new firefighting formulations be investigated for suitability and compatibility with the specific equipment with which they are intended to be used<ref>Underwriters Laboratories Inc., 2018. UL162, UL Standard for Safety, Foam Equipment and Liquid Concentrates, 8th Edition, Revised 2022. 40 pages. [https://global.ihs.com/doc_detail.cfm?document_name=UL%20162&item_s_key=00096960 Website]</ref>. Several PFAS-free foams have been certified under various parts of EN1568, the European Standard which specifies the necessary foam properties and performance requirements<ref>European Standards, 2018. CSN EN 1568-1 ed. 2: Fire extinguishing media - Foam concentrates - Part 1: Specification for medium expansion foam concentrates for surface application to water-immiscible liquids. 48 pages. [https://www.en-standard.eu/csn-en-1568-1-ed-2-fire-extinguishing-media-foam-concentrates-part-1-specification-for-medium-expansion-foam-concentrates-for-surface-application-to-water-immiscible-liquids/ European Standards Website.]</ref>. Both [https://serdp-estcp.mil/ ESTCP and SERDP] have supported (and continue to support) the development and field validation of PFAS-free firefighting formulations (e.g. [https://serdp-estcp.mil/projects/details/baa72637-e3c8-40ee-a007-f295311c72ad WP22-7456], [https://serdp-estcp.mil/projects/details/1bed98f7-dbe6-4bdd-98d2-1f9cfeb5f3d9/wp21-3465-project-overview WP21-3465], [https://serdp-estcp.mil/projects/details/bc932800-cfc8-4e86-a212-5f8c9d27f17c WP20-1535]). Both the US Federal Aviation Administration (FAA) and National Fire Protection Association (NFPA) have performed a variety of foam certification tests on numerous PFAS-free firefighting formulations<ref>Back, G.G., Farley, J.P., 2020. Evaluation of the Fire Protection Effectiveness of Fluorine Free Firefighting Foams. National Fire Protection Association, Fire Protection Research Foundation. [https://www.iafc.org/docs/default-source/1safehealthshs/effectivenessofflourinefreefoam.pdf Free Download].</ref><ref>Casey, J., Trazzi, D., 2022. Fluorine-Free Foam Testing. Federal Aviation Administration (FAA) Final Report. [https://www.airporttech.tc.faa.gov/DesktopModules/EasyDNNNews/DocumentDownload.ashx?portalid=0&moduleid=3682&articleid=2882&documentid=3054  Open Access Article]</ref>.
  
−
==Ferruginous Reductants==
+
==Selection of Flushing Agent==
−
{| class="wikitable mw-collapsible" style="float:right; margin-left:40px; text-align:center;"
+
General industry guidance has typically recommended several rinses with water to remove PFAS from impacted equipment. Owing to the unique physical and chemical properties of PFAS, the use of room temperature water to remove PFAS from impacted equipment has not been very effective. To address these recalcitrant accumulations, companies are developing new methods to remove self-assembled PFAS bilayers from existing fire-fighting infrastructure so that it can be successfully transitioned to PFAS-free formulations. Arcadis developed a non-toxic cleaning agent, Fluoro Fighter<sup>TM</sup>, which has been demonstrated to be effective for removal of PFAS from equipment by disrupting the accumulated layers of PFAS coating the AFFF-wetted surfaces.  
−
|+ Table&nbsp;2.&nbsp;Logarithm&nbsp;of&nbsp;second-order rate constants for reduction of NACs and MCs by dissolved Fe(II) complexes with the stoichiometry of ligand and iron in square brackets
 
−
|-
 
−
! Compound
 
−
! E<sub>H</sub><sup>1'</sup>  (V)
 
−
! Cysteine</br>[FeL<sub>2</sub>]<sup>2-</sup>
 
−
! Thioglycolic acid</br>[FeL<sub>2</sub>]<sup>2-</sup>
 
−
! DFOB</br>[FeHL]<sup>0</sup>
 
−
! AcHA</br>[FeL<sub>3</sub>]<sup>-</sup>
 
−
! Tiron</br>[FeL<sub>2</sub>]<sup>6-</sup>
 
−
! Fe-Porphyrin
 
−
|-
 
−
| Nitrobenzene || -0.485 j || -0.347 || 0.874 || 2.235 || -0.136 || 1.424 d/</br>4.000 e || -0.018 h</br>0.026 i
 
−
|-
 
−
| 2-nitrotoluene || -0.590 j || || || || || || -0.602 h
 
−
|-
 
−
| 3-nitrotoluene || -0.475 j || -0.434 || 0.767 || 2.106 || -0.229 || 1.999 d</BR>3.800 e || 0.041 h
 
−
|-
 
−
| 4-nitrotoluene || -0.500 j || -0.652 || 0.528 || 2.013 || -0.402 || 1.446 d</br>3.500 e || -0.174 h
 
−
|-
 
−
| 2-chloronitrobenzene || -0.485 j || || || || || || 0.944 h
 
−
|-
 
−
| 3-chloronitrobenzene || -0.405 j || 0.360 || 1.810 || 2.888 || 0.691 || 2.882 d</br>4.900 e || 0.724 h
 
−
|-
 
−
| 4-chloronitrobenzene || -0.450 j || 0.230 || 1.415 || 2.512 || 0.375 || 3.937 d</br>4.581 e || 0.431 h</br>0.289 i
 
−
|-
 
−
| 2-acetylnitrobenzene || -0.470 j || || || || || || 1.377 h
 
−
|-
 
−
| 3-acetylnitrobenzene || -0.405 j || || || || || || 0.799 h
 
−
|-
 
−
| 4-acetylnitrobenzene || -0.360 j || 0.965 || 2.771 || || 1.872 || 5.028 d</br>6.300 e || 1.693 h
 
−
|-
 
−
| RDX || -0.550 k || || || || || 2.212 d</br>2.864 f ||
 
−
|-
 
−
| HMX || -0.660 k || || || || || -2.762 d ||
 
−
|-
 
−
| TNT || -0.280 l || || || || || 7.427 d || 2.050 i
 
−
|-
 
−
| 1,3-dinitrobenzene || -0.345 m || || || || || || 1.220 i
 
−
|-
 
−
| 2,4-dinitrotoluene || -0.380 n || || || || || 5.319 d || 1.156 i
 
−
|-
 
−
| Nitroguanidine (NQ) || -0.700 o || || || || || -0.185 d ||
 
−
|-
 
−
| 2,4-dinitroanisole (DNAN) || -0.400 k || || || || || || 1.243 i
 
−
|}
 
−
{| class="wikitable mw-collapsible" style="float:left; margin-right:40px; text-align:center;"
 
−
|+ Table&nbsp;3.&nbsp;Rate constants for the reduction of MCs by iron minerals
 
−
|-
 
−
! MC
 
−
! Iron Mineral
 
−
! Iron mineral loading</br>(g/L)
 
−
! Surface area</br>(m<sup>2</sup>/g)
 
−
! Fe(II)<sub>aq</sub> initial</br>(mM) ''<sup>b</sup>''
 
−
! Fe(II)<sub>aq</sub> after 24 h</br>(mM) ''<sup>c</sup>''
 
−
! Fe(II)<sub>aq</sub> sorbed</br>(mM) ''<sup>d</sup>''
 
−
! pH
 
−
! Buffer
 
−
! Buffer</br>(mM)
 
−
! MC initial</br>(&mu;M) ''<sup>e</sup>''
 
−
! log ''k<sub>obs</sub>''</br>(h<sup>-1</sup>) ''<sup>f</sup>''
 
−
! log ''k<sub>SA</sub>''</br>(Lh<sup>-1</sup>m<sup>-2</sup>) ''<sup>g</sup>''
 
−
|-
 
−
| TNT 29 || Goethite || 0.64 || 17.5 || 1.5 || || || 7.0 || MOPS || 25 || 50 || 1.200 || 0.170
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 0.1 || 0 || 0.10 || 7.0 || HEPES || 50 || 50 || -3.500 || -5.200
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 0.2 || 0.02 || 0.18 || 7.0 || HEPES || 50 || 50 || -2.900 || -4.500
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 0.5 || 0.23 || 0.27 || 7.0 || HEPES || 50 || 50 || -1.900 || -3.600
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 1.5 || 0.94 || 0.56 || 7.0 || HEPES || 50 || 50 || -1.400 || -3.100
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 3.0 || 1.74 || 1.26 || 7.0 || HEPES || 50 || 50 || -1.200 || -2.900
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 5.0 || 3.38 || 1.62 || 7.0 || HEPES || 50 || 50 || -1.100 || -2.800
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 10.0 || 7.77 || 2.23 || 7.0 || HEPES || 50 || 50 || -1.000 || -2.600
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 1.6 || 1.42 || 0.16 || 6.0 || MES || 50 || 50 || -2.700 || -4.300
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 1.6 || 1.34 || 0.24 || 6.5 || MOPS || 50 || 50 || -1.800 || -3.400
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 1.6 || 1.21 || 0.37 || 7.0 || MOPS || 50 || 50 || -1.200 || -2.900
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 1.6 || 1.01 || 0.57 || 7.0 || HEPES || 50 || 50 || -1.200 || -2.800
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 1.6 || 0.76 || 0.82 || 7.5 || HEPES || 50 || 50 || -0.490 || -2.100
 
−
|-
 
−
| RDX 80 || Magnetite || 1.00 || 44 || 1.6 || 0.56 || 1.01 || 8.0 || HEPES || 50 || 50 || -0.590 || -2.200
 
−
|-
 
−
| NG 82 || Magnetite || 4.00 || 0.56|| 4.0 || || || 7.4 || HEPES || 90 || 226 || ||
 
−
|-
 
−
| NG 85 || Pyrite || 20.00 || 0.53 || || || || 7.4 || HEPES || 100 || 307 || -2.213 || -3.238
 
−
|-
 
−
| TNT 85 || Pyrite || 20.00 || 0.53 ||  || || || 7.4 || HEPES || 100 || 242 || -2.812 || -3.837
 
−
|-
 
−
| RDX 85 || Pyrite || 20.00 || 0.53 || || ||  || 7.4 || HEPES || 100 || 201 || -3.058 || -4.083
 
−
|-
 
−
| RDX 51 || Carbonate Green Rust || 5.00 || 36 || || || || 7.0 || || || 100 || ||
 
−
|-
 
−
| RDX 51 || Sulfate Green Rust || 5.00 || 20 || || || || 7.0 || || || 100 || ||
 
−
|-
 
−
| DNAN 83 || Sulfate Green Rust || 10.00 || || || || || 8.4 || || || 500 || ||
 
−
|-
 
−
| NTO 83 || Sulfate Green Rust || 10.00 || || || || || 8.4 || || || 500 || ||
 
−
|-
 
−
| DNAN 81 || Magnetite || 2.00 || 17.8 || 1.0 || || || 7.0 || NaHCO<sub>3</sub> || 10 || 200 || -0.100 || -1.700
 
−
|-
 
−
| DNAN 81 || Mackinawite || 1.50 || || || || || 7.0 || NaHCO<sub>3</sub> || 10 || 200 || 0.061 ||
 
−
|-
 
−
| DNAN 81 || Goethite || 1.00 || 103.8 || 1.0 || || || 7.0 || NaHCO<sub>3</sub> || 10 || 200 || 0.410 || -1.600
 
−
|-
 
−
| RDX 88 || Magnetite || 0.62 ||  || 1.0 ||  ||  || 7.0 || NaHCO<sub>3</sub> || 10 || 17.5 || -1.100 ||
 
−
|-
 
−
| RDX 88 || Magnetite || 0.62 ||  ||  ||  ||  || 7.0 || MOPS || 50 || 17.5 || -0.270 ||
 
−
|-
 
−
| RDX 88 || Magnetite || 0.62 ||  || 1.0 ||  ||  || 7.0 || MOPS || 10 || 17.6 || -0.480 ||
 
−
|-
 
−
| NTO 89 || Hematite || 1.00 || 5.7 || 1.0 || 0.92 || 0.08 || 5.5 || MES || 50 || 30 || -0.550 || -1.308
 
−
|-
 
−
| NTO 89 || Hematite || 1.00 || 5.7 || 1.0 || 0.85 || 0.15 || 6.0 || MES || 50 || 30 || 0.619 || -0.140
 
−
|-
 
−
| NTO 89 || Hematite || 1.00 || 5.7 || 1.0 || 0.9 || 0.10 || 6.5 || MES || 50 || 30 || 1.348 || 0.590
 
−
|-
 
−
| NTO 89 || Hematite || 1.00 || 5.7 || 1.0 || 0.77 || 0.23 || 7.0 || MOPS || 50 || 30 || 2.167 || 1.408
 
−
|-
 
−
| NTO 89 || Hematite ''<sup>a</sup>'' || 1.00 || 5.7 ||  || 1.01 ||  || 5.5 || MES || 50 || 30 || -1.444 || -2.200
 
−
|-
 
−
| NTO 89 || Hematite ''<sup>a</sup>'' || 1.00 || 5.7 ||  || 0.97 ||  || 6.0 || MES || 50 || 30 || -0.658 || -1.413
 
−
|-
 
−
| NTO 89 || Hematite ''<sup>a</sup>'' || 1.00 || 5.7 ||  || 0.87 ||  || 6.5 || MES || 50 || 30 || 0.068 || -0.688
 
−
|-
 
−
| NTO 89 || Hematite ''<sup>a</sup>'' || 1.00 || 5.7 ||  || 0.79 ||  || 7.0 || MOPS || 50 || 30 || 1.210 || 0.456
 
−
|-
 
−
| RDX 50 || Mackinawite || 0.45 ||  ||  ||  ||  || 6.5 || NaHCO<sub>3</sub> || 10 || 250 || -0.092 ||
 
−
|-
 
−
| RDX 50 || Mackinawite || 0.45 ||  ||  ||  ||  || 7.0 || NaHCO<sub>3</sub> || 10 || 250 || 0.009 ||
 
−
|-
 
−
| RDX 50 || Mackinawite || 0.45 ||  ||  ||  ||  || 7.5 || NaHCO<sub>3</sub> || 10 || 250 || 0.158 ||
 
−
|-
 
−
| RDX 50 || Green Rust || 5 ||  ||  ||  ||  || 6.5 || NaHCO<sub>3</sub> || 10 || 250 || -1.301 ||
 
−
|-
 
−
| RDX 50 || Green Rust || 5 ||  ||  ||  ||  || 7.0 || NaHCO<sub>3</sub> || 10 || 250 || -1.097 ||
 
−
|-
 
−
| RDX 50 || Green Rust || 5 ||  ||  ||  ||  || 7.5 || NaHCO<sub>3</sub> || 10 || 250 || -0.745 ||
 
−
|-
 
−
| RDX 50 || Goethite || 0.5 ||  || 1 || 1 ||  || 6.5 || NaHCO<sub>3</sub> || 10 || 250 || -0.921 ||
 
−
|-
 
−
| RDX 50 || Goethite || 0.5 ||  || 1 || 1 ||  || 7.0 || NaHCO<sub>3</sub> || 10 || 250 || -0.347 ||
 
−
|-
 
−
| RDX 50 || Goethite || 0.5 ||  || 1 || 1 ||  || 7.5 || NaHCO<sub>3</sub> || 10 || 250 || 0.009 ||
 
−
|-
 
−
| RDX 50 || Hematite || 0.5 ||  || 1 || 1 ||  || 6.5 || NaHCO<sub>3</sub> || 10 || 250 || -0.824 ||
 
−
|-
 
−
| RDX 50 || Hematite || 0.5 ||  || 1 || 1 ||  || 7.0 || NaHCO<sub>3</sub> || 10 || 250 || -0.456 ||
 
−
|-
 
−
| RDX 50 || Hematite || 0.5 ||  || 1 || 1 ||  || 7.5 || NaHCO<sub>3</sub> || 10 || 250 || -0.237 ||
 
−
|-
 
−
| RDX 50 || Magnetite || 2 ||  || 1 || 1 ||  || 6.5 || NaHCO<sub>3</sub> || 10 || 250 || -1.523 ||
 
−
|-
 
−
| RDX 50 || Magnetite || 2 ||  || 1 || 1 ||  || 7.0 || NaHCO<sub>3</sub> || 10 || 250 || -0.824 ||
 
−
|-
 
−
| RDX 50 || Magnetite || 2 || || 1 || 1 ||  || 7.5 || NaHCO<sub>3</sub> || 10 || 250 || -0.229 ||
 
−
|-
 
−
| DNAN 84 || Mackinawite || 4.28 || 0.25 ||  ||  ||  || 6.5 || NaHCO<sub>3</sub> || 8.5 + 20% CO<sub>2</sub>(g) || 400 || 0.836 || 0.806
 
−
|-
 
−
| DNAN 84 || Mackinawite || 4.28 || 0.25 ||  ||  ||  || 7.6 || NaHCO<sub>3</sub> || 95.2 + 20% CO<sub>2</sub>(g) || 400 || 0.762 || 0.732
 
−
|-
 
−
| DNAN 84 || Commercial FeS || 5.00 || 0.214 ||  ||  ||  || 6.5 || NaHCO<sub>3</sub> || 8.5 + 20% CO<sub>2</sub>(g) || 400 || 0.477 || 0.447
 
−
|-
 
−
| DNAN 84 || Commercial FeS || 5.00 || 0.214 ||  ||  ||  || 7.6 || NaHCO<sub>3</sub> || 95.2 + 20% CO<sub>2</sub>(g) || 400 || 0.745 || 0.716
 
−
|-
 
−
| NTO 84 || Mackinawite || 4.28 || 0.25 ||  ||  ||  || 6.5 || NaHCO<sub>3</sub> || 8.5 + 20% CO<sub>2</sub>(g) || 1000 || 0.663 || 0.633
 
−
|-
 
−
| NTO 84 || Mackinawite || 4.28 || 0.25 ||  ||  ||  || 7.6 || NaHCO<sub>3</sub> || 95.2 + 20% CO<sub>2</sub>(g) || 1000 || 0.521 || 0.491
 
−
|-
 
−
| NTO 84 || Commercial FeS || 5.00 || 0.214 ||  ||  ||  || 6.5 || NaHCO<sub>3</sub> || 8.5 + 20% CO<sub>2</sub>(g) || 1000 || 0.492 || 0.462
 
−
|-
 
−
| NTO 84 || Commercial FeS || 5.00 || 0.214 ||  ||  ||  || 7.6 || NaHCO<sub>3</sub> || 95.2 + 20% CO<sub>2</sub>(g) || 1000 || 0.427 || 0.398
 
−
|-
 
−
| colspan="13" style="text-align:left; background-color:white;" | Notes:</br>''<sup>a</sup>'' Dithionite-reduced hematite; experiments conducted in the presence of 1 mM sulfite. ''<sup>b</sup>'' Initial aqueous Fe(II); not added for Fe(II) bearing minerals. ''<sup>c</sup>'' Aqueous Fe(II) after 24h of equilibration. ''<sup>d</sup>'' Difference between b and c. ''<sup>e</sup>'' Initial nominal MC concentration. ''<sup>f</sup>'' Pseudo-first order rate constant. ''<sup>g</sup>'' Surface area normalized rate constant calculated as ''k<sub>Obs</sub>'' '''/''' (surface area concentration) or ''k<sub>Obs</sub>'' '''/''' (surface area × mineral loading).
 
−
|}
 
−
{| class="wikitable mw-collapsible" style="float:right; margin-left:40px; text-align:center;"
 
−
|+ Table&nbsp;4.&nbsp;Rate constants for the reduction of NACs by iron oxides in the presence of aqueous Fe(II)
 
−
|-
 
−
! NAC ''<sup>a</sup>''
 
−
! Iron Oxide
 
−
! Iron oxide loading</br>(g/L)
 
−
! Surface area</br>(m<sup>2</sup>/g)
 
−
! Fe(II)<sub>aq</sub> initial</br>(mM) ''<sup>b</sup>''
 
−
! Fe(II)<sub>aq</sub> after 24 h</br>(mM) ''<sup>c</sup>''
 
−
! Fe(II)<sub>aq</sub> sorbed</br>(mM) ''<sup>d</sup>''
 
−
! pH
 
−
! Buffer
 
−
! Buffer</br>(mM)
 
−
! NAC initial</br>(μM) ''<sup>e</sup>''
 
−
! log ''k<sub>obs</sub>''</br>(h<sup>-1</sup>) ''<sup>f</sup>''
 
−
! log ''k<sub>SA</sub>''</br>(Lh<sup>-1</sup>m<sup>-2</sup>) ''<sup>g</sup>''
 
−
|-
 
−
| NB 11 || Magnetite || 0.200 || 56.00 || 1.5000 ||  ||  || 7.00 || Phosphate || 10 || 50 || 1.05E+00 || 7.75E-04
 
−
|-
 
−
| 4-ClNB 11 || Magnetite || 0.200 || 56.00 || 1.5000 ||  ||  || 7.00 || Phosphate || 10 || 50 || 1.14E+00 || 8.69E-02
 
−
|-
 
−
| colspan="13" style="text-align:left; background-color:white;" | Notes:</br>''<sup>a</sup>'' The NACs are Nitrobenzene (NB), 4-chloronitrobenzene(4-ClNB), 4-cyanonitrobenzene (4-CNNB), 4-acetylnitrobenzene (4-AcNB), 4-bromonitrobenzene (4-BrNB), 4-nitrotoluene (4-MeNB). ''<sup>b</sup>'' Initial aqueous Fe(II). ''<sup>c</sup>'' Aqueous Fe(II) after 24h of equilibration. ''<sup>d</sup>'' Difference between b and c. ''<sup>e</sup>'' Initial nominal NAC concentration. ''<sup>f</sup>'' Pseudo-first order rate constant. ''<sup>g</sup>'' Surface area normalized rate constant calculated as ''k<sub>Obs</sub>'' '''/''' (surface area × mineral loading).
 
−
|}
 
  
−
Iron(II)&nbsp;can&nbsp;be&nbsp;complexed by a myriad of organic ligands and may thereby become more reactive towards MCs and other pollutants. The reactivity of an Fe(II)-organic complex depends on the relative preference of the organic ligand for Fe(III) versus Fe(II)<ref name="Kim2009"/>. Since the majority of naturally occurring ligands complex Fe(III) more strongly than Fe(II), the reduction potential of the resulting Fe(III) complex is lower than that of aqueous Fe(III); therefore, complexation by organic ligands often renders Fe(II) a stronger reductant thermodynamically<ref name="Strathmann2011">Strathmann, T.J., 2011. Redox Reactivity of Organically Complexed Iron(II) Species with Aquatic Contaminants. Aquatic Redox Chemistry, American Chemical Society,1071(14), pp. 283-313. [https://doi.org/10.1021/bk-2011-1071.ch014 DOI: 10.1021/bk-2011-1071.ch014]</ref>. The reactivity of dissolved Fe(II)-organic complexes towards NACs/MCs has been investigated. The intrinsic, second-order rate constants and one electron reduction potentials are listed in Table 2.
+
Laboratory studies have supported the optimization of this PFAS removal method in fire suppression system piping obtained from a commercial airport hangar in Sydney, Australia<ref name="LangEtAl2022"/>. Prior to removal from the hangar, the stainless-steel pipe held PFAS-containing AFFF for more than three decades. Results indicated that Fluoro Fighter<sup>TM</sup>, as well as flushing at elevated temperatures, removed more surface associated PFAS in comparison to equivalent extractions using methanol or water at room temperature. ESTCP has supported (and continues to support) the development and field validation of best practices for methodologies to clean foam delivery systems (e.g. [https://serdp-estcp.mil/projects/details/1521652f-a8b2-4c52-9232-c1018989a6b1 ER20-5364], [https://serdp-estcp.mil/projects/details/6d0750be-f20b-4765-bdfa-872adccaf37a ER20-5361], [https://serdp-estcp.mil/projects/details/0aa2fb20-b851-4b5b-ac64-e72795986b8a ER20-5369], [https://serdp-estcp.mil/projects/details/4fd2e4ab-ddb7-40f8-835e-e1d637c0d650 ER21-7229]).
  
−
In addition to forming organic complexes, iron is ubiquitous in minerals. Iron-bearing minerals play an important role in controlling the environmental fate of contaminants through adsorption<ref name="Linker2015">Linker, B.R., Khatiwada, R., Perdrial, N., Abrell, L., Sierra-Alvarez, R., Field, J.A., and Chorover, J., 2015. Adsorption of novel insensitive munitions compounds at clay mineral and metal oxide surfaces. Environmental Chemistry, 12(1), pp. 74–84.  [https://doi.org/10.1071/EN14065 DOI: 10.1071/EN14065]</ref><ref name="Jenness2020">Jenness, G.R., Giles, S.A., and Shukla, M.K., 2020. Thermodynamic Adsorption States of TNT and DNAN on Corundum and Hematite. The Journal of Physical Chemistry C, 124(25), pp. 13837–13844.  [https://doi.org/10.1021/acs.jpcc.0c04512 DOI: 10.1021/acs.jpcc.0c04512]</ref> and reduction<ref name="Gorski2011">Gorski, C.A., and Scherer, M.M., 2011. Fe<sup>2+</sup> Sorption at the Fe Oxide-Water Interface: A Revised Conceptual Framework. Aquatic Redox Chemistry, American Chemical Society, 1071(15), pp. 315–343.  [https://doi.org/10.1021/bk-2011-1071.ch015 DOI: 10.1021/bk-2011-1071.ch015]</ref> processes. Studies have shown that aqueous Fe(II) itself cannot reduce NACs/MCs at circumneutral pH<ref name="Klausen1995"/><ref name="Gregory2004">Gregory, K.B., Larese-Casanova, P., Parkin, G.F., and Scherer, M.M., 2004. Abiotic Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine by Fe<sup>II</sup> Bound to Magnetite. Environmental Science and Technology, 38(5), pp. 1408–1414.  [https://doi.org/10.1021/es034588w DOI: 10.1021/es034588w]</ref> but in the presence of an iron oxide (e.g., goethite, hematite, lepidocrocite, ferrihydrite, or magnetite), NACs<ref name="Colón2006"/><ref name="Klausen1995"/><ref name="Strehlau2016"/><ref name="Elsner2004"/><ref name="Hofstetter2006"/> and MCs such as TNT<ref name="Hofstetter1999"/>, RDX<ref name="Gregory2004"/>, DNAN<ref name="Berens2019">Berens, M.J., Ulrich, B.A., Strehlau, J.H., Hofstetter, T.B., and Arnold, W.A., 2019. Mineral identity, natural organic matter, and repeated contaminant exposures do not affect the carbon and nitrogen isotope fractionation of 2,4-dinitroanisole during abiotic reduction. Environmental Science: Processes and Impacts, 21(1), pp. 51-62.  [https://doi.org/10.1039/C8EM00381E DOI: 10.1039/C8EM00381E]</ref>, and NG<ref name="Oh2004">Oh, S.-Y., Cha, D.K., Kim, B.J., and Chiu, P.C., 2004. Reduction of Nitroglycerin with Elemental Iron:  Pathway, Kinetics, and Mechanisms. Environmental Science and Technology, 38(13), pp. 3723–3730.  [https://doi.org/10.1021/es0354667 DOI: 10.1021/es0354667]</ref> can be rapidly reduced. Unlike ferric oxides, Fe(II)-bearing minerals including clays<ref name="Hofstetter2006"/><ref name="Schultz2000"/><ref name="Luan2015a"/><ref name="Luan2015b"/><ref name="Hofstetter2003"/><ref name="Neumann2008"/><ref name="Hofstetter2008"/>, green rust<ref name="Larese-Casanova2008"/><ref name="Khatiwada2018">Khatiwada, R., Root, R.A., Abrell, L., Sierra-Alvarez, R., Field, J.A., and Chorover, J., 2018. Abiotic reduction of insensitive munition compounds by sulfate green rust. Environmental Chemistry, 15(5), pp. 259–266.  [https://doi.org/10.1071/EN17221 DOI: 10.1071/EN17221]</ref>, mackinawite<ref name="Elsner2004"/><ref name="Berens2019"/><ref name="Menezes2021">Menezes, O., Yu, Y., Root, R.A., Gavazza, S., Chorover, J., Sierra-Alvarez, R., and Field, J.A., 2021. Iron(II) monosulfide (FeS) minerals reductively transform the insensitive munitions compounds 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO). Chemosphere, 285, p. 131409.  [https://doi.org/10.1016/j.chemosphere.2021.131409 DOI: 10.1016/j.chemosphere.2021.131409]</ref> and pyrite<ref name="Elsner2004"/><ref name="Oh2008">Oh, S.-Y., Chiu, P.C., and Cha, D.K., 2008. Reductive transformation of 2,4,6-trinitrotoluene,  hexahydro-1,3,5-trinitro-1,3,5-triazine, and nitroglycerin by pyrite and magnetite. Journal of hazardous materials, 158(2-3), pp. 652–655.  [https://doi.org/10.1016/j.jhazmat.2008.01.078 DOI: 10.1016/j.jhazmat.2008.01.078]</ref> do not need aqueous Fe(II) to be reactive toward NACs/MCs. However, upon oxidation, sulfate green rust was converted into lepidocrocite<ref name="Khatiwada2018"/>, and mackinawite into goethite<ref name="Menezes2021"/>, suggesting that aqueous Fe(II) coupled to Fe(III) oxides might be at least partially responsible for continued degradation of NACs/MCs in the subsurface once the parent reductant (e.g., green rust or iron sulfide) oxidizes.
+
==PFAS Verification Testing==
 +
In general, PFAS sampling techniques used to support firefighting formulation transition activities are consistent with conventional sampling techniques used in the environmental industry, but special consideration is made regarding high concentration PFAS materials, elevated detection levels, cross-contamination potential, precursor content, and matrix interferences. The analytical method selected should be appropriate for the regulatory requirements in the site area.
  
−
The reaction conditions and rate constants for a list of studies on MC reduction by iron oxide-aqueous Fe(II) redox couples and by other Fe(II)-containing minerals are shown in Table 3<ref name="Hofstetter1999"/><ref name="Larese-Casanova2008"/><ref name="Gregory2004"/><ref name="Berens2019"/><ref name="Oh2008"/><ref name="Strehlau2018">Strehlau, J.H., Berens, M.J., and Arnold, W.A., 2018. Mineralogy and buffer identity effects on RDX kinetics and intermediates during reaction with natural and synthetic magnetite. Chemosphere, 213, pp. 602–609. [https://doi.org/10.1016/j.chemosphere.2018.09.139 DOI: 10.1016/j.chemosphere.2018.09.139]</ref><ref name="Cardenas-Hernandez2020">Cárdenas-Hernandez, P.A., Anderson, K.A., Murillo-Gelvez, J., di Toro, D.M., Allen, H.E., Carbonaro, R.F., and Chiu, P.C., 2020. Reduction of 3-Nitro-1,2,4-Triazol-5-One (NTO) by the Hematite–Aqueous Fe(II) Redox Couple. Environmental Science and Technology, 54(19), pp. 12191–12201.  [https://doi.org/10.1021/acs.est.0c03872 DOI: 10.1021/acs.est.0c03872]</ref>. Unlike hydroquinones and Fe(II) complexes, where second-order rate constants can be readily calculated, the reduction rate constants of NACs/MCs in mineral suspensions are often specific to the experimental conditions used and are usually reported as BET surface area-normalized reduction rate constants (''k<sub>SA</sub>''). In the case of iron oxide-Fe(II) redox couples, reduction rate constants have been shown to increase with pH (specifically, with [OH<sup>– </sup>]<sup>2</sup>) and aqueous Fe(II) concentration, both of which correspond to a decrease in the system's reduction potential<ref name="Colón2006"/><ref name="Gorski2016"/><ref name="Cardenas-Hernandez2020"/>.
+
==Rinsate Treatment==
 +
Numerous technologies for treatment of PFAS-impacted water sources, including rinsates, have been and are currently being developed. These include separation technologies such as [[PFAS Ex Situ Water Treatment|foam fractionation, nanofiltration, sorbents/flocculants, ion exchange resins, reverse osmosis, and destructive technologies such as sonolysis, electrochemical oxidation, hydrothermal alkaline treatment]], [[PFAS Treatment by Electrical Discharge Plasma |enhanced contact plasma]], and [[Supercritical Water Oxidation (SCWO) |supercritical water oxidation (SCWO)]]. Many of these technologies have rapidly developed from bench-scale (e.g., microcosms, columns, single reactors) to commercially available field-scale units capable of managing PFAS-impacted waters of varying waste volumes and PFAS compositions and concentrations. Ongoing field research continues to improve the treatment efficiency, reliability, and versatility of these technologies, both individually and as coupled treatment solutions (e.g., treatment train). ESTCP has supported (and continues to support) the development and field validation of separation and destructive technologies for treatment of PFAS-impacted water sources, including rinsates (e.g. [https://serdp-estcp.mil/projects/details/0c7af048-3a00-471f-9480-292aa78ecd4f ER20-5370], [https://serdp-estcp.mil/projects/details/0aa2fb20-b851-4b5b-ac64-e72795986b8a ER20-5369], [https://serdp-estcp.mil/projects/details/0d7c91a8-d755-4876-a8bb-c3e896feee0d ER20-5350], [https://serdp-estcp.mil/projects/details/790e2dda-1f7b-4ff5-b77e-08ed10a456b1 ER20-5355]).  
  
−
For minerals that contain structural iron(II) and can reduce pollutants in the absence of aqueous Fe(II), the observed rates of reduction increased with increasing structural Fe(II) content, as seen with iron-bearing clays<ref name="Luan2015a"/><ref name="Luan2015b"/> and green rust<ref name="Larese-Casanova2008"/>. This dependency on Fe(II) content allows for the derivation of second-order rate constants, as shown on Table 3 for the reduction of RDX by green rust<ref name="Larese-Casanova2008"/>, and the development of reduction potential (E<sub>H</sub>)-based models<ref name="Luan2015a"/><ref name="Gorski2012a">Gorski, C.A., Aeschbacher, M., Soltermann, D., Voegelin, A., Baeyens, B., Marques Fernandes, M., Hofstetter, T.B., and Sander, M., 2012. Redox Properties of Structural Fe in Clay Minerals. 1. Electrochemical Quantification of Electron-Donating and -Accepting Capacities of Smectites. Environmental Science and Technology, 46(17), pp. 9360–9368. [https://doi.org/10.1021/es3020138 DOI: 10.1021/es3020138]</ref><ref name="Gorski2012b">Gorski, C.A., Klüpfel, L., Voegelin, A., Sander, M., and Hofstetter, T.B., 2012. Redox Properties of Structural Fe in Clay Minerals. 2. Electrochemical and Spectroscopic Characterization of Electron Transfer Irreversibility in Ferruginous Smectite, SWa-1. Environmental Science and Technology, 46(17), pp. 9369–9377.  [https://doi.org/10.1021/es302014u DOI: 10.1021/es302014u]</ref><ref name="Gorski2013">Gorski, C.A., Klüpfel, L.E., Voegelin, A., Sander, M. and Hofstetter, T.B., 2013. Redox Properties of Structural Fe in Clay Minerals: 3. Relationships between Smectite Redox and Structural Properties. Environmental Science and Technology, 47(23), pp. 13477–13485.  [https://doi.org/10.1021/es403824x DOI: 10.1021/es403824x]</ref>, where E<sub>H</sub> represents the reduction potential of the iron-bearing clays. Iron-bearing expandable clay minerals represent a special case, which in addition to reduction can remove NACs/MCs through adsorption. This is particularly important for planar NACs/MCs that contain multiple electron-withdrawing nitro groups and can form strong electron donor-acceptor (EDA) complexes with the clay surface<ref name="Hofstetter2006"/><ref name="Hofstetter2003"/><ref name="Neumann2008"/>.
+
Remedy selection for treatment of rinsates involves several key factors. It is critical that environmental practitioners have up-to-date technical and practical knowledge on the suitability of these remedial options for different site conditions, treatment volumes, PFAS composition (e.g., presence of precursors, co-contaminants), PFAS concentrations, safety considerations, potential for undesired byproducts (e.g., perchlorate, disinfection byproducts), and treatment costs (e.g., energy demand, capital costs, operational labor).
−
 
 
−
Although the second-order rate constants derived for Fe(II)-bearing minerals may allow comparison among different studies, they may not reflect changes in reactivity due to variations in surface area, pH, and the presence of ions. Anions such as bicarbonate<ref name="Larese-Casanova2008"/><ref name="Strehlau2018"/><ref name="Chen2020">Chen, G., Hofstetter, T.B., and Gorski, C.A., 2020. Role of Carbonate in Thermodynamic Relationships Describing Pollutant Reduction Kinetics by Iron Oxide-Bound Fe<sup>2+</sup>. Environmental Science and Technology, 54(16), pp. 10109–10117.  [https://doi.org/10.1021/acs.est.0c02959 DOI: 10.1021/acs.est.0c02959]</ref> and phosphate<ref name="Larese-Casanova2008"/><ref name="Bocher2004">Bocher, F., Géhin, A., Ruby, C., Ghanbaja, J., Abdelmoula, M., and Génin, J.M.R., 2004. Coprecipitation of Fe(II–III) hydroxycarbonate green rust stabilised by phosphate adsorption. Solid State Sciences, 6(1), pp. 117–124.  [https://doi.org/10.1016/j.solidstatesciences.2003.10.004 DOI: 10.1016/j.solidstatesciences.2003.10.004]</ref> are known to decrease the reactivity of iron oxides-Fe(II) redox couples and green rust. Sulfite has also been shown to decrease the reactivity of hematite-Fe(II) towards the deprotonated form of NTO (Table 3)<ref name="Cardenas-Hernandez2020"/>. Exchanging cations in iron-bearing clays can change the reactivity of these minerals by up to 7-fold<ref name="Hofstetter2006"/>. Thus, more comprehensive models are needed to account for the complexities in the subsurface environment.
 
−
 
 
−
The reduction of NACs has been widely studied in the presence of different iron minerals, pH, and Fe(II)<sub>(aq)</sub> concentrations (Table 4)<ref name="Colón2006"/><ref name="Klausen1995"/><ref name="Strehlau2016"/><ref name="Elsner2004"/><ref name="Hofstetter2006"/>. Only selected NACs are included in Table 4. For more information on other NACs and ferruginous reductants, please refer to the cited references.
 
  
 
==References==
 
==References==
Line 348: Line 73:
  
 
==See Also==
 
==See Also==
 +
[https://portal.ct.gov/-/media/CFPC/KO/2022/Latest-News/DESPP-DEEP-AFFF-MuniFDupdate-2022-05-26.pdf  Connecticut Take-Back Program for municipal fire departments using AFFF containing PFAS]
 +
 +
[https://www.arcadis.com/en-us/knowledge-hub/blog/united-states/johnsie-lang/2021/transitioning-to-pfas-free-firefighting  Arcadis blog on Fluoro Fighter<sup>TM</sup>]
 +
 +
[https://serdp-estcp.mil/projects/details/1521652f-a8b2-4c52-9232-c1018989a6b1  Project Summary ESTCP ER20-5634: Demonstration and Validation of Environmentally Sustainable Methods to Effectively Remove PFAS from Fire Suppression Systems]
 +
 +
[https://serdp-estcp.org/projects/details/0d7c91a8-d755-4876-a8bb-c3e896feee0d  Project Summary ESTCP ER20-5350: Supercritical Water Oxidation (SCWO) for Complete PFAS Destruction]

Revision as of 14:04, 29 March 2024

Transition of Aqueous Film Forming Foam (AFFF) Fire Suppression Infrastructure Impacted by Per and Polyfluoroalkyl Substances (PFAS)

Per and polyfluoroalkyl substances (PFAS) contained in Class B aqueous film-forming foams (AFFFs) are known to accumulate on wetted surfaces of many fire suppression systems after decades of exposure[1]. When replacement PFAS-free firefighting formulations are added to existing infrastructure, PFAS can rebound from the wetted surfaces into the new formulations at high concentrations[2][3]. Effective methods are needed to properly transition to PFAS-free firefighting formulations in existing fire suppression infrastructure. Considerations in the transition process may include but are not limited to locating, identifying, and evaluating existing systems and AFFF, fire engineering evaluations, system prioritization, cost/downtime analyses, sampling and analysis, evaluation of risks and hazards to human health and the environment, transportation, and disposal.

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Introduction

Figure 1. (A) Schematic of a typical PFAS molecule demonstrating the hydrophobic fluorinated tail in green and the hydrophilic charged functional group in blue, (B) a PFAS bilayer formed with the hydrophobic tails facing inward and the charged functional groups on the outside, and (C) multiple bilayers of PFAS assembled on the wetted surfaces of fire suppression piping.

PFAS are a class of synthetic fluorinated compounds which are highly mobile and persistent within the environment[5]. Due to the surfactant properties of PFAS, these compounds self-assemble at any solid-liquid interface forming resilient bilayers during prolonged exposure[6]. Solid phase accumulation of PFAS has been proposed to be influenced by both hydrophobic and electrostatic interactions with fluorinated carbon chain length as the dominant feature influencing sorption[7]. While the majority of previous research into solid phase sorption typically focused on water treatment applications or subsurface porous media[8], recently PFAS accumulations have been identified on the wetted surfaces of fire suppression infrastructure exposed to aqueous film forming foam (AFFF)[1] (see Figure 1).

Fire suppression systems with potential PFAS impacts include fire fighting vehicles that carried AFFF and fixed suppression systems in buildings containing large amounts of flammable materials such as aircraft hangars (Figure 2). PFAS residue on the wetted surfaces of existing infrastructure can rebound into replacement PFAS-free firefighting formulations if not removed during the transition process[2]. Simple surface rinsing with water and low-pressure washing has been proven to be inefficient for removal of surface bound PFAS from piping and tanks that contained fluorinated AFFF[2]

Figure 2. Fixed fire suppression system for an aircraft hangar, with storage tank on left and distribution piping on right.

In addition to proper methods for system cleaning to remove residual PFAS, transition to PFAS-free foam may also include consideration of compliance with state and federal regulations, selection of the replacement PFAS-free firefighting formulation, a cost benefit analysis for replacement of the system components versus cleaning, and PFAS verification testing. Foam transition should be completed in a manner which minimizes the volume of waste generated as well as preventing any PFAS release into the environment.

PFAS Assembly on Solid Surfaces

The self-assembly of amphiphilic molecules into supramolecular bilayers is a result of their structure and how it interacts with the bulk water of a solution. Single chain hydrocarbon based amphiphiles can form micelles under relatively dilute aqueous concentrations, however for hydrocarbon based surfactants the formation of more complex organized system such as vesicles is rarely seen, requiring double chain amphiphiles such as phospholipids. Associations of single chain cationic and anionic hydrocarbon based amphiphiles into stable supramolecular structures such as vesicles has however been demonstrated[9], with the ion pairing of the polar head groups mimicking the a double tail situation. The behavior of single chain fluorosurfactant amphiphiles has been demonstrated to be significantly different from similar hydrocarbon based analogues. Not only are critical micelle concentrations (CMC) of fluorosurfactants typically two orders of magnitude lower than corresponding hydrocarbon surfactants but self-assembly can occur even when fluorosurfactants are dispersed at low concentrations significantly below the CMC in water and other solvents[10]. The assembly of fluorinated amphiphiles structurally similar to those found in AFFF have been shown to readily form stable, complex structures including vesicles, fibers, and globules at concentrations as low as 0.5% w/v in contrast to their hydrocarbon analogues which remained fluid at 30% w/v[11][12].

Krafft found that fluorinated amphiphiles formed bilayer membranes with phospholipids, and that the resulting vesicles were more stable than those made of phospholipids alone[13]. The similarities in amphiphilic properties between phospholipids and the hydrocarbon-based surfactants in AFFF suggests that bilayer vesicles may form between these and the fluorosurfactants also present in the concentrate. Krafft demonstrated that both the permeability of resulting mixed vesicles and their propensity to fuse with each other at increasing ionic strength was reduced as a result of the creation of an inert hydrophobic and lipophobic film within the membrane, and also suggested that the fluorinated amphiphiles increased van der Waals interactions in the hydrocarbon region[13]. Thus this low permeability may allow vesicles formed by the surfactants present in AFFF to act as long term repositories of PFAS not only as part of the bilayer itself but also solvated within the vesicle. This prediction is supported by the observation that supramolecular structures formed from single chain fluorinated amphiphiles have been demonstrated to be stable at elevated temperature (15 min at 121°C) and have been shown to be stable over periods of months, even increasing in size over time when stored at environmentally relevant temperatures[12].

Formation of complex structures at relatively low solute concentrations requires the monomer molecules to be well ordered to maintain tight packing in the supramolecular structure[14]. This order results from electrostatic forces, hydrogen bonding, and in the case of fluorinated amphiphiles, hydrophobic interactions. The geometry of the amphiphile also potentially contributes to the type of supramolecular aggregation[15]. Surfactants which adopt a conical shape (such as a typical hydrocarbon based surfactant with a large polar head group and a single alkyl chain as a tail) tend to form micelles more easily. Increasing the bulk of the tail makes the surfactant more cylindrically shaped which makes assembly into bilayers more likely.

Perfluoroalkyl chains are significantly more bulky than their hydrocarbon based analogues both in cross sectional area (28-30 Å2 versus 20 Å2, respectively) and mean volume (CF2 and CF3 estimated as 38 Å3 and 92 Å3 compared to 27 Å3 and 54 Å3 for CH2 and CH3)[13][10]. Structural studies on linear PFOS have shown that the molecule adopts an unusual helical structure[16][17] in aqueous and solvent phases to alleviate steric hindrance. This arrangement results from the carbon chain starting in the planar all anti conformation and then successively twisting all the CC-CC dihedrals by 15°-20° in the same direction[18]. The conformation also minimizes the electrostatic repulsion between fluorine atoms bonded to the same side of the carbon backbone by maximizing the interatomic distances between them[17].

A consequence of the helical structure is that there is limited carbon-carbon bond rotation within the perfluoroalkyl chain giving them increased rigidity compared to alkyl chains[19]. The bulkiness of the perfluoroalkyl chain confers a cylindrical shape on the fluorosurfactant amphiphile and therefore favors the formation of bilayers and vesicles the aggregation of which is further assisted by the rigidity of the molecules which allow close packing in the supramolecular structure. Fluorosurfactants therefore cannot be regarded as more hydrophobic analogues of hydrogenated surfactants. Their self-assembly behavior is characterized by a strong tendency to form vesicles and lamellar phases rather than micelles, due to the bulkiness and rigidity of the perfluoroalkyl chain that tends to decrease the curvature of the aggregates they form in solution[20]. The larger tail cross section of fluorinated compared to hydrogenated amphiphiles tends to favor the formation of aggregates with lesser surface curvature, therefore rather than micelles they form bilayer membranes, vesicles, tubules and fibers[21][22][23][24]. Rojas et al. (2002) demonstrated that perfluorooctyl sulphonamide formed a contiguous bilayer at 50 mg/L with self-assembled aggregates present at concentrations as low as 10 mg/L[25].

Thermodynamics of PFAS Accumulations on Solid Surfaces

The thermodynamics of formation of amphiphiles into supramolecular species requires consideration of both hydrophobic and hydrophilic interactions resulting from the amphoteric nature of the molecule. The hydrophilic portions of the molecule are driven to maximize their solvation interaction with as many water molecules as possible, whereas the hydrophobic portions of the molecule are driven to aggregate together thus minimizing interaction with the bulk water. Both of these processes change the enthalpy and entropy of the system.

In aqueous solution, the hydrophilic portions of an amphiphile form hydrogen bonds (4 - 120 kJ/mol) and van der Waals interactions (<5 kJ/mol) with water molecules and surfaces, and electrostatic interactions (5 – 300 kJ/mol) can also occur where the amphiphile is ionic[26]. These interactions, although weak compared to intramolecular covalent bonds within a molecule are energetically favorable and increase the enthalpy of the combined solute-solvent system. Thus, the hydrophilic portion of an amphiphile will look to maximize enthalpic gain through hydrogen bond interactions with the bulk water.

The hydrophobic portion of an amphiphile cannot form hydrogen bonds with the bulk solution, and its presence disrupts the hydrogen bond interactions between individual water molecules within the bulk water matrix. This disruption lowers the entropy of the system by reducing the degrees of translational rotational freedom available to the bulk water. The second law of thermodynamics dictates that a system will arrange itself to maximize its entropy. With hydrophobic species this can be achieved by their spontaneous aggregation, as the reduction in solution entropy of the aggregated system is less than that which would occur if the component parts were solvated individually. These hydrophobic and hydrophilic interactions are weak, and the individual entropy gain per amphiphile upon aggregation is very small. However, taken together the overall effect on the entropy of the aggregate is sufficient to maintain it in solution, and moreover these interactions make the aggregates resistant to minor perturbations while retaining the reversibility of the self-assembled structure[26].

Regulatory Drivers for Transition to PFAS-Free Firefighting Formulations

Regulations restricting the use and release of PFAS are being proposed and promulgated worldwide, with several enacted regulations addressing the use of aqueous film forming foams (AFFF) containing PFAS[27][28][29][30][31][32][33][34]. In addition to regulated usage, firefighting formulation users are transitioning to PFAS-free firefighting formulations to reduce environmental liability in the event of a release, to reduce the cost of expensive containment systems and management of generated waste streams, and to avoid reputational damage. In 2016, Queensland, Australia was one of the first governments to ban PFAS use in firefighting foam[27]. The US 2020 National Defense Authorization Act specified immediate prohibition of controlled releases of AFFF containing PFAS and required the Secretary of the Navy to publish a specification for PFAS-free firefighting formulation use and ensure it is available for use by the Department of Defense (DoD) by October 1, 2023[35]. The National Fire Protection Association (NFPA) recently removed the requirement for AFFF containing PFAS from their Standard on Aircraft Hangars and added two new chapters to allow users to determine if AFFF containing PFAS is needed at their facility[36].

Selection of Replacement PFAS-Free Firefighting Formulations

Since they first entered the market in the 2000s, the operational capabilities of PFAS-free firefighting formulations have grown[37] and numerous companies are now manufacturing and delivering PFAS-free firefighting formulations for fixed systems and AFFF vehicles[38][39][40]. Key factors in the selection of a PFAS-free firefighting formulation product are compatibility of the new formulation with the existing system (as confirmed by a fire protection engineer) and environmental certifications (i.e., verifying the absence of organic fluorine or PFAS or the absence of other non-fluorine environmental contaminants).

In January 2023, the US Department of Defense (DoD) published the Performance Specification for Fire Extinguishing Agent, Fluorine-Free Foam (F3) Liquid Concentrate for Land-Based, Fresh Water Applications[4]. This Military Performance Specification (Mil-Spec) allows PFAS-free firefighting formulations to be certified as meeting certain standardized operational goals for use in military settings. In addition to Mil-Spec requirements, PFAS-free firefighting formulations can also be certified through Underwriters Laboratories Standard for Safety, Foam Equipment and Liquid Concentrates, UL 162, which requires the new firefighting formulations be investigated for suitability and compatibility with the specific equipment with which they are intended to be used[41]. Several PFAS-free foams have been certified under various parts of EN1568, the European Standard which specifies the necessary foam properties and performance requirements[42]. Both ESTCP and SERDP have supported (and continue to support) the development and field validation of PFAS-free firefighting formulations (e.g. WP22-7456, WP21-3465, WP20-1535). Both the US Federal Aviation Administration (FAA) and National Fire Protection Association (NFPA) have performed a variety of foam certification tests on numerous PFAS-free firefighting formulations[43][44].

Selection of Flushing Agent

General industry guidance has typically recommended several rinses with water to remove PFAS from impacted equipment. Owing to the unique physical and chemical properties of PFAS, the use of room temperature water to remove PFAS from impacted equipment has not been very effective. To address these recalcitrant accumulations, companies are developing new methods to remove self-assembled PFAS bilayers from existing fire-fighting infrastructure so that it can be successfully transitioned to PFAS-free formulations. Arcadis developed a non-toxic cleaning agent, Fluoro FighterTM, which has been demonstrated to be effective for removal of PFAS from equipment by disrupting the accumulated layers of PFAS coating the AFFF-wetted surfaces.

Laboratory studies have supported the optimization of this PFAS removal method in fire suppression system piping obtained from a commercial airport hangar in Sydney, Australia[1]. Prior to removal from the hangar, the stainless-steel pipe held PFAS-containing AFFF for more than three decades. Results indicated that Fluoro FighterTM, as well as flushing at elevated temperatures, removed more surface associated PFAS in comparison to equivalent extractions using methanol or water at room temperature. ESTCP has supported (and continues to support) the development and field validation of best practices for methodologies to clean foam delivery systems (e.g. ER20-5364, ER20-5361, ER20-5369, ER21-7229).

PFAS Verification Testing

In general, PFAS sampling techniques used to support firefighting formulation transition activities are consistent with conventional sampling techniques used in the environmental industry, but special consideration is made regarding high concentration PFAS materials, elevated detection levels, cross-contamination potential, precursor content, and matrix interferences. The analytical method selected should be appropriate for the regulatory requirements in the site area.

Rinsate Treatment

Numerous technologies for treatment of PFAS-impacted water sources, including rinsates, have been and are currently being developed. These include separation technologies such as foam fractionation, nanofiltration, sorbents/flocculants, ion exchange resins, reverse osmosis, and destructive technologies such as sonolysis, electrochemical oxidation, hydrothermal alkaline treatment, enhanced contact plasma, and supercritical water oxidation (SCWO). Many of these technologies have rapidly developed from bench-scale (e.g., microcosms, columns, single reactors) to commercially available field-scale units capable of managing PFAS-impacted waters of varying waste volumes and PFAS compositions and concentrations. Ongoing field research continues to improve the treatment efficiency, reliability, and versatility of these technologies, both individually and as coupled treatment solutions (e.g., treatment train). ESTCP has supported (and continues to support) the development and field validation of separation and destructive technologies for treatment of PFAS-impacted water sources, including rinsates (e.g. ER20-5370, ER20-5369, ER20-5350, ER20-5355).

Remedy selection for treatment of rinsates involves several key factors. It is critical that environmental practitioners have up-to-date technical and practical knowledge on the suitability of these remedial options for different site conditions, treatment volumes, PFAS composition (e.g., presence of precursors, co-contaminants), PFAS concentrations, safety considerations, potential for undesired byproducts (e.g., perchlorate, disinfection byproducts), and treatment costs (e.g., energy demand, capital costs, operational labor).

References

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  2. ^ 2.0 2.1 2.2 Ross, I., and Storch, P., 2020. Foam Transition: Is It as Simple as "Foam Out / Foam In?". The Catalyst (Journal of JOIFF, The International Organization for Industrial Emergency Services Management), Q2 Supplement, 20 pages. Industry Newsletter
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  34. ^ Vermont Legislature, 2021b. Act No. 36, PFAS in Class B Firefighting Foam. History and Text of Law
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  37. ^ Allcorn, M., Bluteau, T., Corfield, J., Day, G., Cornelsen, M., Holmes, N.J.C., Klein, R.A., McDowall, J.G., Olsen, K.T., Ramsden, N., Ross, I., Schaefer, T.H., Weber, R., Whitehead, K., 2018. Fluorine-Free Firefighting Foams (3F) – Viable Alternatives to Fluorinated Aqueous Film-Forming Foams (AFFF). White Paper prepared for the IPEN by members of the IPEN F3 Panel and associates, POPRC-14, Rome. Free Download.
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See Also

Connecticut Take-Back Program for municipal fire departments using AFFF containing PFAS

Arcadis blog on Fluoro FighterTM

Project Summary ESTCP ER20-5634: Demonstration and Validation of Environmentally Sustainable Methods to Effectively Remove PFAS from Fire Suppression Systems

Project Summary ESTCP ER20-5350: Supercritical Water Oxidation (SCWO) for Complete PFAS Destruction

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