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| − | Microbial removal of halogens from organic compounds by reductive processes forms the basis for many types of bioremediation technologies. The process was discovered within the last several decades and our understanding of how microbes perform this activity has improved significantly. Advances in our understanding of the microbiology of reductive dehalogenation have led to improvements in documenting natural attenuation and implementation of biostimulation and bioaugmentation. As a general rule, bioremediation is a lower cost approach to treatment of halogenated solvents than competing processes based on physical or chemical techniques (e.g., chemical oxidation). For practitioners, a working knowledge of microbial reductive processes is essential to successful application at hazardous waste sites.
| + | At some sites Monitored Natural Attenuation (MNA) can manage metals and metalloid contaminants (“metals”) if the contaminants can be safely held in place on aquifer materials by sorption and/or precipitation processes. The degree and permanence of precipitation and adsorption can be evaluated using the concept of '''geochemical gradients''', where key aquifer properties such as pH, redox potential, and ionic strength are different within the plume and can change over time as the plume moves through the subsurface. The U.S. Environmental Protection Agency (USEPA) developed an extensive three-volume guidance document and a Directive that can be used to determine if MNA can be applied at a site with metal contaminants in groundwater<ref name="EPA2015"> U.S. Environmental Protection Agency (USEPA), 2015. Use of Monitored Natural Attenuation for Inorganic contaminants in Groundwater at Superfund Sites. Directive 9283.1-36, Office of Solid Waste and Emergency Response, United States Environmental Protection Agency. [http://www.environmentalrestoration.wiki/images/d/dc/MNA-Guidance-2015.pdf Report.pdf]</ref><ref name= "USEPA2007V1">United States Environmental Protection Agency (USEPA), 2007. Monitored natural attenuation of inorganic contaminants in groundwater, Volume 1 Technical basis for assessment, Edited by R.G. Ford, R.T. Wilkin, and R.W. Puls. U.S. Environmental Protection Agency, EPA/600/R-07/139. [http://www.environmentalrestoration.wiki/images/c/c1/USEPA-2007-MNA_of_Inorganic_Contaminants_in_GW%2C_Vol_1_Technical_Basis_for_Assessment.pdf Report pdf]</ref><ref name="USEPA2007a">U.S. Environmental Protection Agency (USEPA), 2007. Monitored natural attenuation of inorganic contaminants in groundwater, Volume 2 Assessment for Non-Radionuclides Including Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and Selenium, Edited by R.G. Ford, R.T. Wilkin, and R.W. Puls. EPA/600/R-07/140. [http://www.environmentalrestoration.wiki/images/3/3a/USEPA-2007-MNA_of_Inorganic_Contaminants_in_GW%2C_Vol_2.pdf Report pdf]</ref><ref name="USEPA2010">U.S. Environmental Protection Agency (USEPA), 2010. Monitored natural attenuation of inorganic contaminants in groundwater, Volume 3 Assessment for Radionuclides Including Tritium, Radon, Strontium, Technetium, Uranium, Iodine, Radium, Thorium, Cesium, and Plutonium-Americium, Edited by R.G. Ford and R.T. Wilkin. U.S. Environmental Protection Agency, EPA/600/R-10/093. [http://www.environmentalrestoration.wiki/images/0/05/USEPA-2010-MNA_of_Inorganic_Contaminants_in_GW%2C_Vol_3.pdf Report pdf]</ref>. Using technical aspects of this guidance document as a foundation, we also overview a “Scenarios Approach”, developed by the U.S. Department of Energy for evaluating MNA for metals in groundwater that shows the mobility chart for a number of metals for six different geochemical scenarios<ref name="Truex2011">Truex, M., Brady, P., Newell, C.J., Rysz, M., Denham, M., Vangelas, K. 2011. The scenarios approach to attenuation-based remedies for inorganic and radionuclide contaminants. Savannah-River National Laboratory U.S. Department of Energy. [http://www.environmentalrestoration.wiki/images/e/e3/TRUEX-2011-Scenarios_Approach_to_Attenuation-Based_Remedies.pdf Report pdf]</ref>. |
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| − | '''CONTRIBUTOR(S):''' [[Dr. Gorm Heron]] | + | '''CONTRIBUTOR(S):''' [[Dr. Miles Denham]] and [[Dr. Charles Newell, P.E.]] |
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| | '''Key Resource(s)''': | | '''Key Resource(s)''': |
| − | *[https://doi.org/10.1002/jctb.1567 Enhanced Anaerobic Bioremediation of Chlorinated Solvents: Environmental Factors Influencing Microbial Activity and Their Relevance under Field Conditions]<ref name= "Aulenta2006">Aulenta, F., Majone, M. and Tandoi, V., 2006. Enhanced anaerobic bioremediation of chlorinated solvents: environmental factors influencing microbial activity and their relevance under field conditions. Journal of Chemical Technology and Biotechnology, 81(9), pp.1463-1474. [https://doi.org/10.1002/jctb.1567 doi: 10.1002/jctb.1567]</ref>. | + | *[http://www.environmentalrestoration.wiki/images/a/ac/ITRC-2010-A_Decision_Framework.pdf A Decision Framework for Applying Monitored Natural Attenuation Processes to Metals and Radionuclides]<ref name="ITRC2010">ITRC, 2010. A decision framework for applying monitored natural attenuation processes to metals and radionuclides, Interstate Technology and Regulatory Council, Technical/Regulatory Guidance AMPR-1. [http://www.environmentalrestoration.wiki/images/a/ac/ITRC-2010-A_Decision_Framework.pdf Report pdf]</ref> |
| − | *[https://doi.org/10.1007/978-3-662-49875-0 Organohalide Respiring Bacteria]<ref name= "Adrian2016">Adrian, L. and Löffler, F., 2016. Organohalide Respiring Bacteria. ISBN: 978-3-662-49873-6. [https://doi.org/10.1007/978-3-662-49875-0 doi: 10.1007/978-3-662-49875-0]</ref> | + | *[http://www.environmentalrestoration.wiki/images/8/86/EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf Use of Monitored Natural Attenuation At Superfund, RCRA Corrective Action, And Underground Storage Tank Sites]<ref name="EPA1999"> U.S. Environmental Protection Agency (USEPA), 1999. Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites. [http://www.environmentalrestoration.wiki/images/8/86/EPA-1999-Use_of_MNA_at_Superfund%2C_RCRA_and_UST_sites.pdf Report.pdf]</ref> |
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| | ==Introduction== | | ==Introduction== |
| − | Organic compounds with one or more halogens attached are referred to as halogenated organics. The halogens include chlorine (Cl), bromine (Br), fluorine (F), and iodine (I). For example, ethene (C<sub>2</sub>H<sub>4</sub>) is an organic compound. When three of the four hydrogen atoms on ethene are replaced with chlorine, the resulting compound is trichloroethene (TCE; C<sub>2</sub>HCl<sub>3</sub>).
| + | Monitored natural attenuation (MNA) of metal and metalloid contaminants in groundwater is a remediation strategy that relies on natural processes occurring in an aquifer that minimize risk to human health and the environment by attenuating contaminant migration from their source to a compliance point such as a drinking water well or property boundary. With the exception of short-lived radionuclides, metals and metalloids are not destroyed by natural attenuation processes and will remain sequestered in the aquifer when sufficiently attenuated. Therefore, gaining acceptance of MNA as a remedy for metal and metalloid contamination requires demonstrating, to an acceptable degree of uncertainty, that the contaminants will remain in place and pose a minimal risk for decades to centuries, depending on the contaminant. |
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| − | The proliferation of halogenated organic compounds in the environment is a consequence of their widespread use in industrial activities. A critical part of many manufacturing processes involves removal of oil and grease from metal, fabrics, and other commodities. Because “like dissolves like,” a common way to remove oil and grease is to soak products in a nonpolar solvent. Non-halogenated hydrocarbons serve this purpose; however, accumulation of hydrocarbon vapors creates the risk of an explosion. Organic chemists solved this problem by adding halogens to the hydrocarbons, rendering them non-flammable. Use of halogenated organic compounds grew dramatically after World War II. With increased use came increased releases to the environment. Initially, halogenated solvents were thought to be inert in the environment, and they were thought not to pose any risk to humans or wildlife. Gradually, the risks associated with chronic human exposure to halogenated solvents were revealed and concern grew about their fate in the environment. Most of the compounds on the United Nations list of persistent organic pollutants (first developed at the Stockholm Convention on Persistent Organic Pollutants) are halogenated organic compounds.
| + | ==Attenuation of Metals and Metalloids== |
| | + | Natural attenuation processes of metals and metalloids that occur in aquifers are adsorption, precipitation, radioactive decay, and dispersion. Adsorption and precipitation limit the '''mobility of metals and metalloids''' by causing them to partition from groundwater to solid phases. Dispersion dilutes the concentration of contaminants, rather than limiting their mobility. Radioactive decay applies to only those radionuclides that have short enough half-lives to prevent their migration to compliance points. “Short enough” varies with the contamination scenario. Radioactive decay may be sufficient to achieve successful MNA if the groundwater travel time to the compliance point is much longer than the half-life of the radionuclide. |
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| − | Microbes possess the ability to remove halogens from halogenated organic compounds. Dehalogenation may occur via a variety of reactions, including oxidation, reduction, or hydrolysis. The halogens are released as halides, i.e., Cl<sup>-</sup>, Br<sup>-</sup>, F<sup>-</sup>, and I<sup>-</sup>. The process of removing a halogen from a halogenated organic compound by a reductive reaction is referred to as reductive dehalogenation. Reductive reactions are ones in which electrons are transferred to the carbon-halogen bond, thereby lowering the oxidation state of the parent compound. Examples reactions are given below.
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| | + | |style="text-align:center;"|'''Geochemical Gradients''' |
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| | + | |Geochemical gradients are the spatial variations in geochemical conditions created by waste disposal or other phenomena. Geochemical gradients can evolve over time as geochemical conditions change; for instance, as neutral pH water displaces low pH water. When present, geochemical gradients can strongly affect contaminant mobility and thus identification and characterization of geochemical of gradients allows the contaminant attenuation-affecting conditions of a site to be projected into the future<ref name="Truex2011"/>. |
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| | + | Demonstrating that adsorption and/or precipitation are sufficiently limiting the mobility of a contaminant metal or metalloid is the strongest evidence that MNA is an appropriate remedy. Adsorption and precipitation may occur when a metal or metalloid enters groundwater because the contaminant, and often the composition of fluids carrying the contaminant, cause perturbations of the near steady-state condition of the groundwater system. This promotes reactions that tend to return the groundwater system toward its original state and these often result in contaminant adsorption, precipitation, or both. |
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| − | One of the earliest studies (1982) to demonstrate that microbes are capable of removing halogens from halogenated organic compounds was performed with halobenzoates (e.g., 3-chlorobenzoate)<ref>Suflita, J.M., Horowitz, A., Shelton, D.R. and Tiedje, J.M., 1982. Dehalogenation: a novel pathway for the anaerobic biodegradation of haloaromatic compounds. Science, 218(4577), pp.1115-1117. [https://doi.org/10.1126/science.218.4577.1115 doi: 10.1126/science.218.4577.1115]</ref>. Knowledge about microbial dehalogenation has since grown considerably and now forms the basis for bioremediation, a commonly applied strategy to treat halogenated organic compounds found in soils, groundwater, and wastewater.
| + | Consider the evolution of a contamination plume in an aquifer (Fig. 1). As contamination enters the aquifer, a '''geochemical gradient''' forms at the leading edge of the plume<ref name="Truex2011"/>. This leading gradient may simply be a concentration gradient of the contaminant, where the concentration is higher on the side of the plume opposite of the direction of flow. In many cases, the leading gradient also includes gradients in concentrations of other constituents associated with the contaminant source. For example, contamination plumes often have different pH, oxidation-reduction potential, or ionic strength than the native groundwater. In any event, reactions tend to occur that counter the geochemical gradient and can cause adsorption of the contaminant to aquifer mineral surfaces or precipitation of the contaminant. |
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| − | As a general rule, bioremediation is a lower cost approach to treatment of halogenated solvents than competing processes based on physical or chemical techniques (e.g., chemical oxidation). The discovery that microbes are capable of replacing the chlorines on tetrachloroethene (PCE) and TCE, the two most frequently encountered organic contaminants at hazardous waste sites, opened the door to development of current bioremediation strategies. Notably, there was a pause in interest when it was initially believed that the dechlorination process stopped at vinyl chloride (VC), which is more toxic than PCE, TCE, and dichloroethene (DCE) isomers. It was subsequently determined that microbial reduction of VC to ethene occurs<ref>Freedman, D.L. and Gossett, J.M., 1989. Biological reductive dechlorination of tetrachloroethylene and trichloroethylene to ethylene under methanogenic conditions. Applied and Environmental Microbiology, 55(9), pp.2144-2151. [http://www.environmentalrestoration.wiki/images/9/96/Freedman-1989-Biological_reductive_dechlorination.pdf Report pdf]</ref>. Ethene is an acceptable endpoint since it poses no human health risks in groundwater.
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| − | Although humans are responsible for a large influx of halogenated organics into the environment as a consequence of improper handling and disposal practices, it has also come to light that there are natural sources of halogenated organic compounds; nearly 5,000 have been catalogued to date<ref>Gribble, G. W., 2010. Naturally Occurring Organohalogen Compounds - A Comprehensive Update. SpringerWien: New York. [https://doi.org/10.1007/978-3-211-99323-1 doi: 10.1007/978-3-211-99323-1]</ref>. For example, marine algae produce chloromethane as part of a chemical defense system to dissuade predation. Consequently, it is not too surprising that natural processes exist to break down halogenated organic compounds, and those processes have been harnessed to help clean up the excessive amounts released to the environment as a consequence of human activity.
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| − | There are several types of reaction pathways that involve reduction and dehalogenation, including hydrogenolysis and dihaloelimination<ref>Vogel, T.M., Criddle, C.S. and McCarty, P.L., 1987. ES&T critical reviews: transformations of halogenated aliphatic compounds. Environmental Science & Technology, 21(8), pp.722-736. [http://dx.doi.org/10.1021/es00162a001 doi:10.1021/es00162a001]</ref>. Here, we detail each of these pathways and the conditions under which each is likely to be prevalent. It should be noted that many of the reactions are also possible via abiotic processes (e.g., via reaction with zero valent iron<ref>Arnold, W.A. and Roberts, A.L., 2000. Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe (0) particles. Environmental Science & Technology, 34(9), pp.1794-1805. [http://dx.doi.org/10.1021/es990884q doi:10.1021/es990884q]</ref>). The focus of this article is on biotic reductive processes.
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| − | ==Hydrogenolysis==
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| − | Hydrogenolysis is the process by which a carbon—halogen bond is broken and hydrogen replaces the halogen substituent, resulting in release of a halide ion (Fig. 1):
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| − | [[File:Freedman Article 1 Figure 1.PNG|center|500 px|Figure 1. Generic hydrogenolysis; R = organic compound, X = halide.]]
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| − | The process requires an input of reducing power, represented in the above equation by 2e<sup>-</sup>, or two electron equivalents. Hydrogenolysis is the most frequently observed reductive pathway, and as such it is commonly referred to as reductive dehalogenation. Nevertheless, other pathways are also reductive and result in dehalogenation, so the more correct description of the above reaction is hydrogenolysis. This descriptor derives in part from the fact that hydrogen (H<sub>2</sub>) is often a source of the electron equivalents:
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| − | [[File:Freedman Article 1 Equation 1.PNG|150px|center|]]
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| − | Hydrogenolysis applies to any organohalide, yet it is most commonly associated with removal of chlorine from organic solvents and the term ''reductive dechlorination'' is often used synonymously (mostly amongst practitioners) to describe this reaction. As mentioned above, this is not quite correct, since other pathways are also reductive and result in removal of chlorine atoms, and the correct chemical mechanism is hydrogenolysis.
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| − | Hydrogenolysis applies to numerous categories of compounds; we highlight several of the major categories below. The process typically occurs via a respiratory process referred to as organohalide respiration, which is also described below.
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| − | ===Chlorinated Ethenes===
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| − | When applied to PCE, hydrogenolysis proceeds in 2e<sup>-</sup> steps through TCE, ''cis''- or ''trans''-1,2- DCE, VC, and ethene, which is also called ethylene (Fig. 2). ''cis''-DCE is the more frequently identified of the 1,2-DCE isomers. Nevertheless, ''trans''-DCE is predominant in some environments<ref>Griffin, B.M., Tiedje, J.M. and Löffler, F.E., 2004. Anaerobic microbial reductive dechlorination of tetrachloroethene to predominately trans-1, 2-dichloroethene. Environmental Science & Technology, 38(16), pp.4300-4303. [https://doi.org/10.1021/es035439g doi: 10.1021/es035439g]</ref>. 1,1-DCE is another possible dichloroethene isomer, but it is not typically formed during microbial TCE respiration. The presence of 1,1-DCE in the environment is most typically associated with prior contamination by 1,1,1-trichloroethane, which undergoes several types of transformation, including the abiotic process of dehydrohalogenation to 1,1-DCE.
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| − | In some locations, further reduction of ethene to ethane has been reported. This is not a dechlorination reaction, but it is important to mention because ethane may be the terminal product from hydrogenolysis of chlorinated ethenes. Monitoring ethane is recommended for establishing a complete assessment of the fate of chlorinated ethenes.
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| − | [[File:Freedman Article 1 Figure 2.PNG|center|800 px|Figure 2. Stepwise reduction of PCE and TCE to ethene and ethane.]]
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| − | The oxidation state of carbon in an organic compound varies from -4 (e.g., in CH<sub>4</sub>) to +4 (e.g., in CO<sub>2</sub>). The lower the oxidation state, the easier it is for oxidation to occur, and vice versa.
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| − | The oxidation state of the carbon in PCE is +4. With each successive reduction step (via an input of 2e<sup>-</sup>), the oxidation state of the carbon decreases by 2, so that the carbon in TCE has an oxidation state of +2, DCE has 0, VC has -2, and ethene has -4. For this category of contaminants, the final step is critical from a remediation perspective, since VC is a known human carcinogen while ethene poses no human health risks in groundwater.
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| − | ===Chlorinated Ethanes===
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| − | Like chlorinated ethenes, chlorinated ethanes are reduced by hydrogenolysis. One of the most widely evaluated compounds is 1,1,1-trichloroethane, which undergoes sequential reduction to 1,1-dichloroethane and chloroethane. Although further reduction to ethane is possible, it is a much slower reaction and chloroethane is typically regarded as the terminal product. This example serves to illustrate that hydrogenolysis does not always yield complete dechlorination.
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| − | Other commonly encountered chlorinated ethanes undergo hydrogenolysis, including reduction of 1,2-dichloroethane to chloroethane and 1,2-dichloropropane to 1- or 2-chloropropane.
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| − | ===Chlorinated Methanes===
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| − | Carbon tetrachloride (tetrachloromethane) undergoes hydrogenolysis to chloroform (trichloromethane) and then methylene chloride (dichloromethane). These reactions are also catalyzed by reduced iron, which may be generated by iron-reducing bacteria.
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| − | Further reduction to chloromethane and methane is not commonly observed; other anaerobic biodegradation processes, such as organohalide fermentation are more significant for dichloromethane and chloromethane.
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| − | ===Chlorinated Aromatic Compounds===
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| − | Hydrogenolysis also occurs for chlorinated aromatic compounds, including chlorinated benzenes, polychlorinated biphenyls (PCBs), chlorinated dioxins (e.g., 2,3,7,8-tetrachlorodibenzo-''p''-dioxin, or TCDD) and furans, and polychlorinated phenols (e.g. pentachlorophenol, or PCP). For these compounds, the pathways are more complicated, since reduction proceeds through multiple isomers, depending on which position on the ring that each specific chlorine atom is removed. Like chlorinated ethanes and methanes, hydrogenolysis of chlorinated aromatic compounds is rarely complete, and the rate of reduction often decreases with a decreasing number of chlorine-carbon bonds. An important exception has been identification of cultures that reduce chlorinated benzenes completely to benzene<ref>Fung, J.M., Weisenstein, B.P., Mack, E.E., Vidumsky, J.E., Ei, T.A. and Zinder, S.H., 2009. Reductive dehalogenation of dichlorobenzenes and monochlorobenzene to benzene in microcosms. Environmental Science & Technology, 43(7), pp.2302-2307. [https://doi.org/10.1021/es802131d doi: 10.1021/es802131d]</ref><ref>Nelson, J.L., Fung, J.M., Cadillo-Quiroz, H., Cheng, X. and Zinder, S.H., 2011. A role for Dehalobacter spp. in the reductive dehalogenation of dichlorobenzenes and monochlorobenzene. Environmental Science & Technology, 45(16), pp.6806-6813. [https://doi.org/10.1021/es200480k doi: 10.1021/es200480k]</ref>.
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| − | ===Bromo- and Fluoro-Organic Compounds===
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| − | Hydrogenolysis of brominated organic compounds has been documented. Examples include reduction of 1,2-dibromoethane (ethylene dibromide) to bromoethane and reduction of polybrominated diphenyl ethers. Likewise, defluorination also occurs. For example, reduction of vinyl fluoride to ethene has been reported. Nevertheless, reductive defluorination is characterized by slow rates, if it occurs at all. This is consistent with the general expectation that when the rate-limiting step for a reaction is cleavage of the carbon halogen bond, the order of reactivity is:
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| − | <div align= "center">C—I > C—Br > C—Cl > C—F<ref>Wackett, L.P., Logan, M.S., Blocki, F.A. and Bao-Li, C., 1992. A mechanistic perspective on bacterial metabolism of chlorinated methanes. Biodegradation, 3(1), pp.19-36. [https://doi.org/10.1007/bf00189633 doi: 10.1007/BF00189633]</ref></div>
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| − | For example, hydrogenolysis of trichlorofluoromethane (CFC-11) typically results in accumulation of dichloro- and chlorofluoro-methane; further hydrogenolysis occurs at a much slower rate, if at all. Microbial reductive defluorination of perflurooctanoic acid (PFOA; used in the manufacture of Teflon) has not been reported to any appreciable extent<ref>Liou, J.C., Szostek, B., DeRito, C.M. and Madsen, E.L., 2010. Investigating the biodegradability of perfluorooctanoic acid. Chemosphere, 80(2), pp.176-183. [http://dx.doi.org/10.1016/j.chemosphere.2010.03.009 doi: 10.1016/j.chemosphere.2010.03.009]</ref>.
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| − | ==Dihaloelimination==
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| − | Dihaloelimination is the process by which two groups, e.g., a hydrogen and chlorine, are removed from adjacent carbon atoms, resulting in the formation of a double bond and release of two halide ions (Fig. 3):
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| − | [[File:Freedman A 1 Fig 3.PNG|center|]]
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| − | <div align="center">'''Figure 3. Generic dihaloelimination; X<sup>-</sup> = halide.</div>'''
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| − | Although less frequently encountered than hydrogenolysis, dihaloelimination is a critical pathway for several common groundwater contaminants, including reduction of 1,2-dichloroethane and 1,2-dibromoethane (more commonly referred to as ethylene dibromide, or EDB) to ethene (Fig. 4)<ref>Yu, R., Peethambaram, H.S., Falta, R.W., Verce, M.F., Henderson, J.K., Bagwell, C.E., Brigmon, R.L. and Freedman, D.L., 2013. Kinetics of 1, 2-dichloroethane and 1, 2-dibromoethane biodegradation in anaerobic enrichment cultures. Applied and Environmental Microbiology, 79(4), pp.1359-1367. [https://doi.org/10.1128/aem.02163-12 doi: 10.1128/AEM.02163-12]</ref>:
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| − | [[File:Freedman A 1 Fig 4.PNG|center]]
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| − | <div align="center">'''Figure 4. Dihaloelimination of 1,2-dibromoethane to ethene.</div>'''
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| − | In this example, the oxidation state of the carbon decreases from -2 in 1,2-dibromoethane to -4 in ethene, i.e., by 2 electrons. Thus, the process is both reductive and results in removal of halides. Other examples of dihaloelimination include reduction of 1- or 2-chloropropane to propene.
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| − | ==Organohalide Respiration==
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| − | A variety of microbes have developed pathways to conserve the energy made available when breaking carbon-halogen bonds via reduction (i.e., hydrogenolysis and dihaloelimination), by using the halogenated organic compounds as terminal electron acceptors<ref name= "Adrian2016"/>. This has led to the notion that certain microbes are capable of “breathing” halogenated organics, analogous to respiration involving oxygen as the electron acceptor<ref name = "McCarty1997">McCarty, P.L., 1997. Breathing with chlorinated solvents. Science, 276(5318), pp.1521-1522. [https://doi.org/10.1126/science.276.5318.1521 doi: 10.1126/science.276.5318.1521]</ref>. When halogenated organic compounds serve as terminal electron acceptors, the process is referred to as organohalide respiration. The discovery of this process adds to the extensive list of terminal electron acceptors that microbes are capable of exploiting. Notably, a few types of microbes are obligate halorespirers, meaning the only known terminal electron acceptors for the cells are halogenated organic compounds. Other types of microbes are facultative with respect to their use of halogenated organics as terminal electron acceptors.
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| − | Under some circumstances, microbes carry out reductive dehalogenation but are unable to conserve energy from the process. For example, several strains of microbes are able to use PCE, TCE and ''cis''-DCE as terminal electron acceptors, whereas reduction of VC to ethene is not a growth linked process<ref>Maymó-Gatell, X., Anguish, T. and Zinder, S.H., 1999. Reductive dechlorination of chlorinated ethenes and 1, 2-dichloroethane by “Dehalococcoides ethenogenes” 195. Applied and Environmental Microbiology, 65(7), pp.3108-3113. [http://www.environmentalrestoration.wiki/images/2/22/Maymo-Gatell-1999-Reductive_dechlorination.pdf Report pdf]</ref><ref>Maymó-Gatell, X., Nijenhuis, I. and Zinder, S.H., 2001. Reductive dechlorination of cis-1, 2-dichloroethene and vinyl chloride by “Dehalococcoides ethenogenes”. Environmental Science & Technology, 35(3), pp.516-521. [https://doi.org/10.1021/es001285i doi: 10.1021/es001285i]</ref>. These cultures are able to reduce VC to ethene when growing with the other chlorinated ethenes as electron acceptors, but when provided with only VC, they are unable to grow. Under these circumstances, the transformation of VC to ethene is referred to as cometabolic, i.e., the transformation process is not linked to growth. In general, organohalide respiration occurs at a higher rate than reduction via cometabolism. Some microbes are capable of reducing VC to ethene via respiration, others are not.
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| − | Organohalide respiration appears to be a widely-distributed process in nature, even in some environments that have not previously been contaminated by human activity<ref>Krzmarzick, M.J., Crary, B.B., Harding, J.J., Oyerinde, O.O., Leri, A.C., Myneni, S.C. and Novak, P.J., 2012. Natural niche for organohalide-respiring chloroflexi. Applied and Environmental Microbiology, 78(2), pp.393-401. [https://doi.org/10.1128/aem.06510-11 doi: 10.1128/AEM.06510-11]</ref>. The widespread distribution of organohalide respiring microbes is likely related to the natural formation of halogenated compounds, which have been generated on the planet long before human activity increased the rate and amount of halogenated compounds released to the environment. Having the capacity to dehalogenate is essential in natural systems in which organohalide compounds are also synthesized.
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| − | The process of organohalide respiration is centered on reductive dehalogenases, an iron–sulfur and coronoid containing family of enzymes that break carbon-halogen bonds. Among the best characterized are the genes involved in reductive dehalogenation of PCE, TCE, ''cis''-DCE, and VC. Several microbes and enzymes are involved in each reduction step (Fig. 5).
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| − | [[File:Freedman Article 1 Figure 5.PNG|700 px|center]]
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| − | The identification of dehalogenases has progressed to the point that quantification of key genes (e.g., ''tceA'', ''bvcA'', ''vcrA'', and others) in environmental samples is now a routine part of assessing the capacity for reductive dehalogenation to occur in the environment.
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| − | Figure 6 presents a simplified schematic for how energy may be conserved during organohalide respiration. Many details of the process still need to be resolved and likely vary among the growing list of organohalide respiring microbes<ref name = "Jugder2016">Jugder, B.E., Ertan, H., Bohl, S., Lee, M., Marquis, C.P. and Manefield, M., 2016. Organohalide Respiring Bacteria and Reductive Dehalogenases: Key Tools in Organohalide Bioremediation. Frontiers in microbiology, 7. [https://doi.org/10.3389/fmicb.2016.00249 doi: 10.3389/fmicb.2016.00249]</ref>. Reductive dehalogenases are a key component of the respiratory chain, which in the example shown culminates in development of a proton motive force and subsequent synthesis of ATP.
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| − | [[File:Freedman Article 1 Figure 6.PNG|500 px|center]]
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| − | :<div align="center">'''Figure 6. Schematic representation of a microbial cell carrying out organohalide respiration.</div>'''
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| − | :<div align="center"> Blue shape = the cell membrane; red oval = hydrogenase; yellow oval = electron carrier and</div>
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| − | :<div align="center">proton translocation; orange oval = reductive dehalogenase; green shape = ATP synthase.</div>
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| − | :<div align="center">Modified from Jugder et al.<ref name = "Jugder2016"/></div>
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| − | Among the various microbes capable of organohalide respiration, ''Dehalococcoides'' are the most frequently mentioned because of their capacity for complete reduction to ethene<ref>Löffler, F.E., Yan, J., Ritalahti, K.M., Adrian, L., Edwards, E.A., Konstantinidis, K.T., Müller, J.A., Fullerton, H., Zinder, S.H. and Spormann, A.M., 2013. Dehalococcoides mccartyi gen. nov., sp. nov., obligately organohalide-respiring anaerobic bacteria relevant to halogen cycling and bioremediation, belong to a novel bacterial class, Dehalococcoidia classis nov., order Dehalococcoidales ord. nov. and family Dehalococcoidaceae fam. nov., within the phylum Chloroflexi. International journal of systematic and evolutionary Microbiology, 63(2), pp.625-635. [https://doi.org/10.1099/ijs.0.034926-0 doi: 10.1099/ijs.0.034926-0]</ref>. At this point, members of this genus are the only known that are capable of completely dechlorinating the chlorinated ethenes to ethene, and more specifically, the steps from ''cis''-DCE to VC, and VC to ethene. Their versatility extends to use of many other organohalides as terminal electron acceptors, including polychlorinated biphenyls, chlorinated ethanes, and chlorinated benzenes. ''Dehalococcoides'' use only hydrogen as an electron donor and acetate as a carbon source. Their limited electron donor use stands in contrast to other organohalide respiring microbes, which are able to use a variety of organic compounds as electron donors. Other key types of organohalide respiring microbes (that cannot generate ethene) include ''Dehalobacter'', ''Dehalogenimonas'', ''Desulfitobacterium'', ''Sulfurospirillum'', and a number of ''Deltaproteobacteria''<ref name= "Adrian2016"/> 2. ''Dehalobacter'' includes microbes that respire chlorinated ethanes<ref>Sun, B., Griffin, B.M., Ayala-del-Rı́o, H.L., Hashsham, S.A. and Tiedje, J.M., 2002. Microbial dehalorespiration with 1, 1, 1-trichloroethane. Science, 298(5595), pp.1023-1025. [https://doi.org/10.1126/science.1074675 doi: 10.1126/science.1074675]</ref>, chlorinated ethenes<ref>Holliger, C., Hahn, D., Harmsen, H., Ludwig, W., Schumacher, W., Tindall, B., Vazquez, F., Weiss, N. and Zehnder, A.J., 1998. Dehalobacter restrictus gen. nov. and sp. nov., a strictly anaerobic bacterium that reductively dechlorinates tetra-and trichloroethene in an anaerobic respiration. Archives of Microbiology, 169(4), pp.313-321. [https://doi.org/10.1007/s002030050577 doi: 10.1007/s002030050577]</ref>, and chloroform<ref>Tang, S., Wang, P.H., Higgins, S.A., Löffler, F.E. and Edwards, E.A., 2016. Sister Dehalobacter Genomes Reveal Specialization in Organohalide Respiration and Recent Strain Differentiation Likely Driven by Chlorinated Substrates. Frontiers in Microbiology, 7. [https://doi.org/10.3389/fmicb.2016.00100 doi: 10.3389/fmicb.2016.00100]</ref>.
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| − | ==Environmental Conditions==
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| − | With few exceptions, reductive dehalogenation occurs under anoxic conditions<ref name= "Adrian2016"/>. With the exception of nitrate, reductive dehalogenation has been observed in the presence of other anaerobic terminal electron acceptors, including ferric iron and sulfate. The effect of iron and sulfate on the rate and extent of reductive dechlorination is a matter of some debate, with some observing that these compounds (or the reduced forms) are inhibitory (e.g., via competition for hydrogen or the toxicity of sulfide) while others have shown that iron reduction is beneficial to the process<ref name= "Aulenta2006"/><ref>Wei, N. and Finneran, K.T., 2011. Influence of ferric iron on complete dechlorination of trichloroethylene (TCE) to ethene: Fe (III) reduction does not always inhibit complete dechlorination. Environmental Science & Technology, 45(17), pp.7422-7430. [https://doi.org/10.1021/es201501a doi 10.1021/es201501a]</ref>. An environment in which a community of anaerobes produces an excess of cobalamin (vitamin B<sub>12</sub>) is beneficial, since this coenzyme is an essential component of several dehalogenases<ref>Yan, J., Im, J., Yang, Y. and Löffler, F.E., 2013. Guided cobalamin biosynthesis supports Dehalococcoides mccartyi reductive dechlorination activity. Phil. Trans. R. Soc. B, 368(1616), p.20120320. [https://doi.org/10.1098/rstb.2012.0320 doi: 10.1098/rstb.2012.0320]</ref>. Halogenated compounds can also be reduced in the presence of methanogens; however, methanogens compete for hydrogen as an electron donor and may at times limit the dechlorination reactions.
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| − | Circumneutral pH is considered to be optimum for complete reduction of chlorinated ethenes to ethene<ref>Robinson, C., Barry, D.A., McCarty, P.L., Gerhard, J.I. and Kouznetsova, I, 2009. pH control for enhanced reductive bioremediation of chlorinated solvent source zones. Science of the Total Environment, 407(16), pp.4560-4573. [https://doi.org/10.1016/j.scitotenv.2009.03.029 doi 10.1016/j.scitotenv.2009.03.029]</ref><ref>Vainberg, S., Condee, C.W. and Steffan, R.J., 2009. Large-scale production of bacterial consortia for remediation of chlorinated solvent-contaminated groundwater. Journal of Industrial Microbiology & Biotechnology, 36(9), pp.1189-1197. [https://doi.org/10.1007/s10295-009-0600-5 doi: 10.1007/s10295-009-0600-5]</ref>. However, organohalide respiring microbes other than ''Dehalococcoides'' tolerate lower pH levels (e.g., as low as 4). There is growing evidence to indicate that strains of ''Dehalococcoides'' exist that are also tolerant of pH levels below circumneutral. For example, the pH range for a commonly used bioaugmentation culture that includes ''Dehalococcoides'' has a reported pH range of 5.8-6.3 [http://siremlab.com/kb-1-kb-1-plus/ SIREM]. This is an important consideration for bioaugmentation in low pH aquifers, since there are significant challenges associates with adjusting groundwater pH.
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| − | ==Significance==
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| − | Our understanding of the microbial processes that result in reductive removal of halogens from halogenated organic compounds has grown remarkably over the past three decades. The field has advanced from an assumption that halogenated organic compounds are non-biodegradable to our current understanding that not only do microbes perform dehalogenation reactions, but many do so via a growth-linked reductive, respiratory process, as in “breathing with chlorinated solvents”<ref name = "McCarty1997"/>. This understanding of the underlying science has formed the basis for the practice of bioremediation, which has revolutionized the options available for cleaning up hazardous waste sites. Exciting new discoveries await that will open the door to biological treatment of emerging contaminants, as well as improvements in how to treat halogenated organics at complex sites where there are often mixtures of contaminants.
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| | ==References== | | ==References== |
