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==Abiotic Reduction of Munitions Constituents==
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==Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions==  
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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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The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective in situ approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.
 
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<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
  
 
'''Related Article(s):'''
 
'''Related Article(s):'''
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*[[Munitions Constituents]]
 
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*[[Munitions Constituents - Alkaline Degradation]]
 
  
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'''Contributor(s):'''
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*[[Monitored Natural Attenuation (MNA)]]
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*Dr. Jimmy Murillo-Gelvez
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*[[Monitored Natural Attenuation (MNA) of Chlorinated Solvents]]
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*Paula Andrea Cárdenas-Hernández
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*[[Monitored Natural Attenuation - Transitioning from Active Remedies]]
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*Dr. Pei Chiu
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*[[Matrix Diffusion]]
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*[[REMChlor - MD]]
  
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'''Key Resource(s):'''
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'''Contributors:''' Dani Tran, Dr. Charles Schaefer, Dr. Charles Werth
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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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'''Key Resource:'''
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*Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils (3)
  
 
==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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Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.
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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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For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as [[REMChlor - MD]]<ref>Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. [https://doi.org/10.1016/j.jconhyd.2016.12.007 doi: 10.1016/j.jconhyd.2016.12.007]&nbsp; [[Media: FaltaWang2017.pdf | Open Access Manuscript]]</ref><ref>Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. [https://doi.org/10.1016/j.jconhyd.2022.103986 doi: 10.1016/j.jconhyd.2022.103986]&nbsp; [[Media: KulkarniEtAl2022.pdf | Open Access Manuscript]]</ref> to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.
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==Recommended Approach==
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[[File: TranFig1.png | thumb | 600 px | Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions (data from this study and prior research)]]
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[[File: TranFig2.png | thumb | 600 px | Figure 2: Flowchart diagram of field screening procedures]]
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The recommended approach builds upon the methodology and findings of a recent study<ref name="SchaeferEtAl2025">Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. [https://doi.org/10.1111/gwmr.12709 doi: 10.1111/gwmr.12709]</ref>, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:
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#<u>Zone Identification:</u> The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
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#<u>Ferrous Mineral Quantification:</u> Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
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#<u>Mineralogical Characterization:</u> Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.  
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#<u>Reduced Gas Analysis:</u> Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.
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Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.
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Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)<sub>r</sub>) is estimated as shown in Equation 1:
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::'''Equation 1:'''&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <big>''Fe(II)<sub><small>r</small></sub> = DA + XRD<sub><small>pyr</small></sub> - XRD<sub><small>biotite</small></sub>''</big>
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where ''DA'' is the ferrous content from the dilute acid (1% HCl) extraction, ''XRD<sub><small>pyr</small></sub>'' is the pyrite content from XRD analysis, and ''XRD<sub><small>biotite</small></sub>'' is the biotite content from XRD analysis<ref name="SchaeferEtAl2025"/>.
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Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)<sub><small>r</small></sub>) concentrations are below 100 mg/kg (Figure 1). For Fe(II)<sub><small>r</small></sub> above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1<ref name="SchaeferEtAl2018">Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. [https://doi.org/10.1021/acs.est.8b04108 doi: 10.1021/acs.est.8b04108]</ref><ref>Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. [https://doi.org/10.1016/j.jconhyd.2021.103889 doi: 10.1016/j.jconhyd.2021]&nbsp; [[Media: BordenCha2021.pdf | Open Access Manuscript]]</ref>. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s ''r'' = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.
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 +
Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)<sub><small>r</small></sub>) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.
 +
 
 +
==Study Design Considerations==
 +
===Diagnostic Resin Treatments===
 +
Several commercially available resins have been verified for use in the iTIE system. Investigators can select resins based on stressor classes of interest at each site. Each resin selectively removes a CoC class from site water prior to organism exposure.
 +
*[https://www.dupont.com/products/ambersorb560.html DuPont Ambersorb 560] for removal of 1,4-dioxane and other organic chemicals<ref>Woodard, S., Mohr, T., Nickelsen, M.G., 2014. Synthetic media: A promising new treatment technology for 1,4-dioxane. Remediation Journal, 24(4), pp. 27-40. [https://doi.org/10.1002/rem.21402 doi: 10.1002/rem.21402]</ref>
 +
*C18 for nonpolar organic chemicals
 +
*[https://www.bio-rad.com/en-us Bio-Rad] [https://www.bio-rad.com/en-us/product/chelex-100-resin?ID=6448ab3e-b96a-4162-9124-7b7d2330288e Chelex] for metals
 +
*Granular activated carbon for metals, general organic chemicals, sulfide<ref>Lemos, B.R.S., Teixeira, I.F., de Mesquita, J.P., Ribeiro, R.R., Donnici, C.L., Lago, R.M., 2012. Use of modified activated carbon for the oxidation of aqueous sulfide. Carbon, 50(3), pp. 1386-1393. [https://doi.org/10.1016/j.carbon.2011.11.011 doi: 10.1016/j.carbon.2011.11.011]</ref>
 +
*[https://www.waters.com/nextgen/us/en.html Waters] [https://www.waters.com/nextgen/us/en/search.html?category=Shop&isocode=en_US&keyword=oasis%20hlb&multiselect=true&page=1&rows=12&sort=best-sellers&xcid=ppc-ppc_23916&gad_source=1&gad_campaignid=14746094146&gbraid=0AAAAAD_uR00nhlNwrhhegNh06pBODTgiN&gclid=CjwKCAiAtLvMBhB_EiwA1u6_PsppE0raci2IhvGnAAe5ijciNcetLaGZo5qA3g3r4Z_La7YAPJtzShoC6LoQAvD_BwE Oasis HLB] for general organic chemicals<ref name="SteigmeyerEtAl2017"/>
 +
*[https://www.waters.com/nextgen/us/en.html Waters] [https://www.waters.com/nextgen/us/en/search.html?category=All&enableHL=true&isocode=en_US&keyword=Oasis%20WAX%20&multiselect=true&page=1&rows=12&sort=most-relevant Oasis WAX] for PFAS, organic chemicals of mixed polarity<ref>Iannone, A., Carriera, F., Di Fiore, C., Avino, P., 2024. Poly- and Perfluoroalkyl Substance (PFAS) Analysis in Environmental Matrices: An Overview of the Extraction and Chromatographic Detection Methods. Analytica, 5(2), pp. 187-202. [https://doi.org/10.3390/analytica5020012 doi: 10.3390/analytica5020012]&nbsp; [[Media: IannoneEtAl2024.pdf | Open Access Article]]</ref>
 +
*Zeolite for ammonia, other organic chemicals
  
−
[[File:AbioMCredFig2.png | thumb |450px|Figure 2. General mechanism for the reduction of NACs/MCs.]]
+
Resins must be adequately conditioned prior to use. Otherwise, they may inadequately adsorb toxicants or cause stress to organisms. New resins should be tested for efficacy and toxicity before being used in an iTIE system.  
−
[[File:AbioMCredFig3.png | thumb |450px|Figure 3. Schematic of natural attenuation of MCs-impacted soils through chemical reduction.]]
 
−
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"/>.  
 
  
−
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>.
+
===Test Organism Species and Life Stages===
 +
Practitioners can also select different organism species and life stages for use in the iTIE system, depending on site characteristics and study goals. The iTIE system can accommodate various small test organisms, including embryo-stage fish and most macroinvertebrates. The following common toxicity tests can be adapted for application within iTIE systems<ref>U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, 1994. Catalogue of Standard Toxicity Tests for Ecological Risk Assessment. ECO Update, 2(2), 4 pages. Publication No. 9345.0.05I [https://www.epa.gov/sites/default/files/2015-09/documents/v2no2.pdf Free Download]&nbsp; [[Media: usepa1994.pdf | Report.pdf]]</ref>.
 +
<ul><u>Freshwater acute toxicity:</u></ul>
 +
*[[Wikipedia: Daphnia magna | ''Daphnia magna'']] or [[Wikipedia: Daphnia pulex | ''Daphnia pulex'']] 24-, 48-, and 96-hour survival
 +
<ul><u>Freshwater chronic toxicity:</u></ul>
 +
*[[Wikipedia: Ceriodaphnia dubia | ''Ceriodaphnia dubia'']]  7-day survival and reproduction
 +
*''D. magna'' 7-day survival and reproduction
 +
*[[Wikipedia: Fathead minnow | ''Pimephales promelas'']] 7-day embryo-larval survival and teratogenicity
 +
*[[Wikipedia: Hyalella azteca | ''Hyalella Azteca'']] 10- or 30-day survival and reproduction
 +
<ul><u>Marine acute toxicity:</u></ul>
 +
*[[Wikipedia: Americamysis bahia | ''Americamysis bahia'']] 24- and 48-hour survival
 +
<ul><u>Marine chronic toxicity:</u></ul>
 +
*''Americamysis'' survival, growth and fecundity
 +
*[[Wikipedia: Topsmelt silverside | ''Atherinops affinis'']] embryo-larval survival and growth
  
−
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>.  
+
Acute toxicity is quantifiable via organism survival rates immediately following the termination of an iTIE system field deployment. Chronic toxicity can be quantified by continuing to culture and observe test organisms in-lab. Common chronic endpoints include stunted growth, altered development such as teratogenicity in larval fish, decreased reproduction rates, and changes in gene expression.  
  
−
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.
+
Several gene expression endpoints have been detectable in bioassays following an iTIE system deployment and in-lab culturing period. Steigmeyer ''et al.''<ref name="SteigmeyerEtAl2017"/> were able to detect changes in the expression of two genes in ''D. magna'' after a 24-hour exposure to bisphenol A. In a separate study, Nichols<ref>Nichols, E., 2023. Methods for Identification and Prioritization of Stressors at Impaired Sites. Masters thesis, University of Michigan. University of Michigan Library Deep Blue Documents. [https://deepblue.lib.umich.edu/bitstream/handle/2027.42/176142/Nichols_Elizabeth_thesis.pdf?sequence=1 Free Download]&nbsp; [[Media: Nichols2023.pdf | Report.pdf]]</ref> found a significant decline in acetylcholinesterase activity in ''H. azteca'' after a 24-hour exposure to chlorpyrifos. These results indicate a potential to adapt other gene expression bioassays for use in conjunction with iTIE system field exposures to prove stressor-causality linkages.
  
−
==Carbonaceous Reductants==
+
===Cost Effectiveness Study===
−
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"/>.
+
Burton ''et al.''<ref name="BurtonEtAl2020"/> conducted a cost effectiveness study comparing the iTIE technology with the traditional US EPA Phase 1 TIE method. Comparisons were based on the estimated time required to complete various sub-tasks within each method. Sub-tasks included organism care, equipment preparation, mobilization and deployment, test maintenance, test termination, demobilization, and test termination analyses. It was ultimately estimated that the iTIE protocol requires 47% less time (67 fewer hours) to complete than the Phase 1 TIE method, with the largest time differences in equipment preparation, deployment, test maintenance, and demobilization. It is important to note that the iTIE method may require additional initial costs for equipment and training.
  
−
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>.  
+
==Field Application==
 +
[[File: CraneFig6.png | thumb | left | 400px | Figure 6. iTIES deployment at the Rouge River, Detroit, MI.  In the foreground is the iTIE Cooler Sub-System, which contains iTIE resin treatments and test organism groups, as well as the oxygenation coil and sample collection bottles. Next to the iTIE Cooler are the two pump cases. The Trident can be seen above the pump cases, installed in the river channel near shore.]]
 +
The&nbsp;iTIE&nbsp;system&nbsp;has&nbsp;been successfully deployed at a variety of marine and freshwater sites during the proof-of-concept phase of prototype development. One example is the 2024 iTIE system deployment completed near the mouth of the Rouge River in Detroit, MI (Figure 6). The Rouge River watershed has a long history of industrialization, with a legacy of chemical dumping, channelization, damming, and urban runoff<ref>Ridgway, J., Cave, K., DeMaria, A., O’Meara, J., Hartig, J. H., 2018. The Rouge River Area of Concern—A multi-year, multi-level successful approach to restoration of Impaired Beneficial Uses. Aquatic Ecosystem Health and Management, 21(4), pp. 398-408. [https://doi.org/10.1080/14634988.2018.1528816 doi: 10.1080/14634988.2018.1528816]</ref>. This has led to degraded environmental conditions, with previous detections of a wide range of chemicals including heavy metals and various organics.
  
−
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
+
[[File: CraneFig7.png | thumb | 300px | Figure 7. Survival and healthy development of ''P. promelas'' embryos and larvae following a 48-hour iTIE exposure near the mouth of the Rouge River. Organisms were exposed to site porewater as embryos for 48 hours and cultured post-exposure for an additional 5 days.]]
 +
[[File: CraneFig8.png | thumb | 300px | Figure 8. Survival of ''C. dilutus'' larvae after an iTIE exposure near the mouth of the Rouge River. Organisms were exposed to site porewater for 48 hours and cultured post-exposure for an additional 5 days. Error bars show standard deviation.]]
 +
An&nbsp;iTIE&nbsp;system&nbsp;deployment&nbsp;was designed and completed to determine which chemical classes are most responsible for causing toxicity at the site. Resin treatments included glass wool (inert, non-fractionating substance), Chelex (metals sorption), Oasis HLB (general organics sorption), and Oasis WAX (organics sorption, with a high affinity for PFAS). The study utilized fathead minnow (''P. promelas'') embryos, due to their relative sensitivity to metals and PAHs, as well as second-instar midge ([[Wikipedia: Chironomus |''Chironomus dilutus'']]) larvae due to their relative sensitivity to PFAS.
  
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==Advantages and Limitations of the Technology for PFAS Treatment==
+
The test organisms were exposed to fractionated porewater ''in situ'' for 48 hours. Following exposure, organisms were cultured for an additional five days, and survival was recorded (Figures 7 and 8). Moderate declines in survival were seen in both species in the glass wool treatment, indicating toxicity at the site. For ''P. promelas'', the highest proportion of healthy development occurred in the Chelex treatment, supporting the hypothesis that metals are a dominant cause of toxicity. ''C. dilutus'' had the greatest survival in the Oasis WAX treatment, suggesting that an organic stressor class like PFAS is also present at harmful concentrations in the river.
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===Advantages:===
 
−
* High removal rates of long-chain PFAS (C5-C8) due to the production of versatile reactive species
 
−
* Requires no chemical additions and produces no residual waste
 
−
* Total organic carbon (TOC) concentration and other non-surfactant co-contaminants do not influence the process efficiency
 
−
* The process is mobile and scalable
 
−
* Versatile: can be used in batch and continuous systems
 
  
−
===Limitations:===
+
Water chemical analyses of fractionated and unfractionated water samples were completed to support biological results. Analyses were conducted for a range of stressor classes including metals, PAHs, PCBs, an organophosphate pesticide (chlorpyrifos), a PFAS compound (PFOS) and a pyrethroid insecticide (permethrin). Of these analytes, only heavy metals and PFOS were detected. Some chemical classes including PAHs and PCBs were not detected at the site.
−
* Limited removal of short-chain PFAS due to their inability to concentrate at plasma-liquid interfaces. Addition of surfactants such as CTAB improves their removal and degradation rates.
+
To reach similar conclusions using traditional Phase 1 TIE methods, one would need to complete the following tests: baseline toxicity, filtration, aeration, EDTA, C18 SPE, and methanol elution of C18 SPE. The iTIE method allows the same conclusions to be drawn with significantly less time and effort required.
−
* Excessive foaming caused by bubbling argon gas through a solution containing high (>10 mg/L) concentrations of long-chain (surfactant) PFAS may interfere with the formation of plasma.
 
  
 
==Summary==
 
==Summary==
−
PFAS are susceptible to plasma treatment because the hydrophobic PFAS accumulates at the gas-liquid interface, exposing more of the PFAS to the plasma. Plasma-based treatment of PFAS contaminated water successfully degrades PFOA and PFOS to below the EPA health advisory level of 70 ppt and accomplishes the near complete destruction of other PFAS within a short treatment time. PFAS concentration reductions of ≥90% and post-treatment concentrations below laboratory detection levels are common for long chain PFAS and precursors. 
+
The ''in situ'' Toxicity Identification Evaluation technology and protocol is a powerful tool that investigators can use to strengthen causal linkages between chemical stressors and ecological toxicity. By fractionating sampled water and exposing test organisms ''in situ'', investigators can gather toxicity response data while minimizing sample manipulation and accurately representing environmental conditions.
−
The lack of sensitivity of plasma to co-contaminants, coupled with high PFAS removal and defluorination efficiencies, makes plasma-based water treatment a promising technology for the remediation of PFAS-contaminated water. The plasma treatment process is currently developed for ''ex situ'' application and can also be integrated into a treatment train<ref name="Richardson2021">Richardson, S., 2021. Nanofiltration Followed by Electrical Discharge Plasma for Destruction of PFAS and Co-occurring Chemicals in Groundwater: A Treatment Train Approach. Environmental Security Technology Certification Program (ESTCP), Project Number ER21-5136.  [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/ER21-5136  Project Overview]</ref>.
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<br clear="right"/>
  
 
==References==
 
==References==

Latest revision as of 15:58, 14 April 2026

Estimating PCE/TCE Abiotic First-Order Reductive Dechlorination Rate Constants in Clayey Soils Under Anoxic Conditions

The U.S. Department of Defense (DoD) faces many challenges in restoring aquifers at contaminated sites, often due to back-diffusion of tetrachloroethene (PCE) and trichloroethene (TCE) from low-permeability clay zones. The uptake, storage, and subsequent long-term release of these dissolved contaminants from clays are key processes in understanding the longevity, intensity, and risks associated with many persistent chlorinated ethene groundwater plumes. Although naturally occurring abiotic and biotic dechlorination processes in clays may reduce stored contaminant mass and significantly aid natural attenuation, no standardized field method currently exists to verify or quantify these reactions. It is critical to remediation design efforts to demonstrate and validate a cost-effective in situ approach for assessing these dechlorination processes using first-order rate constants. An approach was developed and applied across eight DoD sites to support Remedial Project Managers (RPMs) and regulators in evaluating natural attenuation potential in clay-rich environments.

Related Article(s):

Contributors: Dani Tran, Dr. Charles Schaefer, Dr. Charles Werth

Key Resource:

  • Schaefer, C.E, Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils (3)

Introduction

Cost-effective methods are needed to verify the occurrence of natural dechlorination processes and quantify their dechlorination rates in clays under ambient in situ conditions in order to reliably predict their long-term influence on plume longevity and mass discharge. However, accurately determining these rates is challenging due to slow reaction kinetics, the transient nature of transformation products, and the interplay of biotic and abiotic mechanisms within the clay matrix or at clay-sand interfaces. Tools capable of quantifying these reactions and assessing their role in mitigating plume persistence would be a significant aid for long-term site management.

For reductive abiotic dechlorination under anoxic conditions, a 1% hydrochloric acid (HCl) extraction of a sample of native clay coupled with X-ray diffraction (XRD) data can be used as a screening level tool to estimate reductive dechlorination rate constants. These rate constants can be inserted into fate and transport models such as REMChlor - MD[1][2] to quantify abiotic dechlorination impacts within clay aquitards on chlorinated solvent plumes. Thus, determination of the abiotic reductive dechlorination rate constant for a particular clayey soil can be readily utilized to provide a more accurate assessment of aquifer cleanup timeframes for groundwater plumes that are being sustained by contaminant back-diffusion.

Recommended Approach

File:TranFig1.png
Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions (data from this study and prior research)
File:TranFig2.png
Figure 2: Flowchart diagram of field screening procedures

The recommended approach builds upon the methodology and findings of a recent study[3], emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:

  1. Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
  2. Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
  3. Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
  4. Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)r) is estimated as shown in Equation 1:

Equation 1:       Fe(II)r = DA + XRDpyr - XRDbiotite

where DA is the ferrous content from the dilute acid (1% HCl) extraction, XRDpyr is the pyrite content from XRD analysis, and XRDbiotite is the biotite content from XRD analysis[3].

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1[4][5]. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s r = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

Study Design Considerations

Diagnostic Resin Treatments

Several commercially available resins have been verified for use in the iTIE system. Investigators can select resins based on stressor classes of interest at each site. Each resin selectively removes a CoC class from site water prior to organism exposure.

  • DuPont Ambersorb 560 for removal of 1,4-dioxane and other organic chemicals[6]
  • C18 for nonpolar organic chemicals
  • Bio-Rad Chelex for metals
  • Granular activated carbon for metals, general organic chemicals, sulfide[7]
  • Waters Oasis HLB for general organic chemicals[8]
  • Waters Oasis WAX for PFAS, organic chemicals of mixed polarity[9]
  • Zeolite for ammonia, other organic chemicals

Resins must be adequately conditioned prior to use. Otherwise, they may inadequately adsorb toxicants or cause stress to organisms. New resins should be tested for efficacy and toxicity before being used in an iTIE system.

Test Organism Species and Life Stages

Practitioners can also select different organism species and life stages for use in the iTIE system, depending on site characteristics and study goals. The iTIE system can accommodate various small test organisms, including embryo-stage fish and most macroinvertebrates. The following common toxicity tests can be adapted for application within iTIE systems[10].

    Freshwater acute toxicity:
    Freshwater chronic toxicity:
    Marine acute toxicity:
    Marine chronic toxicity:
  • Americamysis survival, growth and fecundity
  • Atherinops affinis embryo-larval survival and growth

Acute toxicity is quantifiable via organism survival rates immediately following the termination of an iTIE system field deployment. Chronic toxicity can be quantified by continuing to culture and observe test organisms in-lab. Common chronic endpoints include stunted growth, altered development such as teratogenicity in larval fish, decreased reproduction rates, and changes in gene expression.

Several gene expression endpoints have been detectable in bioassays following an iTIE system deployment and in-lab culturing period. Steigmeyer et al.[8] were able to detect changes in the expression of two genes in D. magna after a 24-hour exposure to bisphenol A. In a separate study, Nichols[11] found a significant decline in acetylcholinesterase activity in H. azteca after a 24-hour exposure to chlorpyrifos. These results indicate a potential to adapt other gene expression bioassays for use in conjunction with iTIE system field exposures to prove stressor-causality linkages.

Cost Effectiveness Study

Burton et al.[12] conducted a cost effectiveness study comparing the iTIE technology with the traditional US EPA Phase 1 TIE method. Comparisons were based on the estimated time required to complete various sub-tasks within each method. Sub-tasks included organism care, equipment preparation, mobilization and deployment, test maintenance, test termination, demobilization, and test termination analyses. It was ultimately estimated that the iTIE protocol requires 47% less time (67 fewer hours) to complete than the Phase 1 TIE method, with the largest time differences in equipment preparation, deployment, test maintenance, and demobilization. It is important to note that the iTIE method may require additional initial costs for equipment and training.

Field Application

Figure 6. iTIES deployment at the Rouge River, Detroit, MI. In the foreground is the iTIE Cooler Sub-System, which contains iTIE resin treatments and test organism groups, as well as the oxygenation coil and sample collection bottles. Next to the iTIE Cooler are the two pump cases. The Trident can be seen above the pump cases, installed in the river channel near shore.

The iTIE system has been successfully deployed at a variety of marine and freshwater sites during the proof-of-concept phase of prototype development. One example is the 2024 iTIE system deployment completed near the mouth of the Rouge River in Detroit, MI (Figure 6). The Rouge River watershed has a long history of industrialization, with a legacy of chemical dumping, channelization, damming, and urban runoff[13]. This has led to degraded environmental conditions, with previous detections of a wide range of chemicals including heavy metals and various organics.

Figure 7. Survival and healthy development of P. promelas embryos and larvae following a 48-hour iTIE exposure near the mouth of the Rouge River. Organisms were exposed to site porewater as embryos for 48 hours and cultured post-exposure for an additional 5 days.
Figure 8. Survival of C. dilutus larvae after an iTIE exposure near the mouth of the Rouge River. Organisms were exposed to site porewater for 48 hours and cultured post-exposure for an additional 5 days. Error bars show standard deviation.

An iTIE system deployment was designed and completed to determine which chemical classes are most responsible for causing toxicity at the site. Resin treatments included glass wool (inert, non-fractionating substance), Chelex (metals sorption), Oasis HLB (general organics sorption), and Oasis WAX (organics sorption, with a high affinity for PFAS). The study utilized fathead minnow (P. promelas) embryos, due to their relative sensitivity to metals and PAHs, as well as second-instar midge (Chironomus dilutus) larvae due to their relative sensitivity to PFAS.

The test organisms were exposed to fractionated porewater in situ for 48 hours. Following exposure, organisms were cultured for an additional five days, and survival was recorded (Figures 7 and 8). Moderate declines in survival were seen in both species in the glass wool treatment, indicating toxicity at the site. For P. promelas, the highest proportion of healthy development occurred in the Chelex treatment, supporting the hypothesis that metals are a dominant cause of toxicity. C. dilutus had the greatest survival in the Oasis WAX treatment, suggesting that an organic stressor class like PFAS is also present at harmful concentrations in the river.

Water chemical analyses of fractionated and unfractionated water samples were completed to support biological results. Analyses were conducted for a range of stressor classes including metals, PAHs, PCBs, an organophosphate pesticide (chlorpyrifos), a PFAS compound (PFOS) and a pyrethroid insecticide (permethrin). Of these analytes, only heavy metals and PFOS were detected. Some chemical classes including PAHs and PCBs were not detected at the site. To reach similar conclusions using traditional Phase 1 TIE methods, one would need to complete the following tests: baseline toxicity, filtration, aeration, EDTA, C18 SPE, and methanol elution of C18 SPE. The iTIE method allows the same conclusions to be drawn with significantly less time and effort required.

Summary

The in situ Toxicity Identification Evaluation technology and protocol is a powerful tool that investigators can use to strengthen causal linkages between chemical stressors and ecological toxicity. By fractionating sampled water and exposing test organisms in situ, investigators can gather toxicity response data while minimizing sample manipulation and accurately representing environmental conditions.

References

  1. ^ Falta, R., and Wang, W., 2017. A semi-analytical method for simulating matrix diffusion in numerical transport models. Journal of Contaminant Hydrology, 197, pp. 39-49. doi: 10.1016/j.jconhyd.2016.12.007  Open Access Manuscript
  2. ^ Kulkarni, P.R., Adamson, D.T., Popovic, J., Newell, C.J., 2022. Modeling a well-charactized perfluorooctane sulfate (PFOS) source and plume using the REMChlor-MD model to account for matrix diffusion. Journal of Contaminant Hydrology, 247, Article 103986. doi: 10.1016/j.jconhyd.2022.103986  Open Access Manuscript
  3. ^ 3.0 3.1 Schaefer, C.E., Tran, D., Nguyen, D., Latta, D.E., Werth, C.J., 2025. Evaluating Mineral and In Situ Indicators of Abiotic Dechlorination in Clayey Soils. Groundwater Monitoring and Remediation, 45(2), pp. 31-39. doi: 10.1111/gwmr.12709
  4. ^ Schaefer, C.E., Ho, P., Berns, E., Werth, C., 2018. Mechanisms for abiotic dechlorination of trichloroethene by ferrous minerals under oxic and anoxic conditions in natural sediments. Environmental Science and Technology, 52(23), pp.13747-13755. doi: 10.1021/acs.est.8b04108
  5. ^ Borden, R.C., Cha, K.Y., 2021. Evaluating the impact of back diffusion on groundwater cleanup time. Journal of Contaminant Hydrology, 243, Article 103889. doi: 10.1016/j.jconhyd.2021  Open Access Manuscript
  6. ^ Woodard, S., Mohr, T., Nickelsen, M.G., 2014. Synthetic media: A promising new treatment technology for 1,4-dioxane. Remediation Journal, 24(4), pp. 27-40. doi: 10.1002/rem.21402
  7. ^ Lemos, B.R.S., Teixeira, I.F., de Mesquita, J.P., Ribeiro, R.R., Donnici, C.L., Lago, R.M., 2012. Use of modified activated carbon for the oxidation of aqueous sulfide. Carbon, 50(3), pp. 1386-1393. doi: 10.1016/j.carbon.2011.11.011
  8. ^ 8.0 8.1 Cite error: Invalid <ref> tag; no text was provided for refs named SteigmeyerEtAl2017
  9. ^ Iannone, A., Carriera, F., Di Fiore, C., Avino, P., 2024. Poly- and Perfluoroalkyl Substance (PFAS) Analysis in Environmental Matrices: An Overview of the Extraction and Chromatographic Detection Methods. Analytica, 5(2), pp. 187-202. doi: 10.3390/analytica5020012  Open Access Article
  10. ^ U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, 1994. Catalogue of Standard Toxicity Tests for Ecological Risk Assessment. ECO Update, 2(2), 4 pages. Publication No. 9345.0.05I Free Download  Report.pdf
  11. ^ Nichols, E., 2023. Methods for Identification and Prioritization of Stressors at Impaired Sites. Masters thesis, University of Michigan. University of Michigan Library Deep Blue Documents. Free Download  Report.pdf
  12. ^ Cite error: Invalid <ref> tag; no text was provided for refs named BurtonEtAl2020
  13. ^ Ridgway, J., Cave, K., DeMaria, A., O’Meara, J., Hartig, J. H., 2018. The Rouge River Area of Concern—A multi-year, multi-level successful approach to restoration of Impaired Beneficial Uses. Aquatic Ecosystem Health and Management, 21(4), pp. 398-408. doi: 10.1080/14634988.2018.1528816

See Also

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