Grantee Research Project Results
Final Report: Assessment of Biotic and Abiotic Processes Controlling the Fate of Chlorinated Solvents in Mixed-Waste Under Iron- and Sulfate-Reducing Conditions Using Laboratory and In Situ Microcosms
EPA Grant Number: R825958Title: Assessment of Biotic and Abiotic Processes Controlling the Fate of Chlorinated Solvents in Mixed-Waste Under Iron- and Sulfate-Reducing Conditions Using Laboratory and In Situ Microcosms
Investigators: Hayes, Kim F. , Adriaens, Peter , Barcelona, Michael J.
Institution: University of Michigan
EPA Project Officer: Aja, Hayley
Project Period: November 17, 1997 through November 16, 2000
Project Amount: $449,975
RFA: EPA/DOE/NSF/ONR - Joint Program On Bioremediation (1997) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management
Objective:
The major objective of this research project was to evaluate the relative importance of biotic and abiotic reductive dechlorination processes under iron- and sulfate-reducing conditions in both simple and mixed waste systems.
In the context of this work, an abiotic reductive transformation refers to a reaction occurring at reduced iron mineral surfaces, whereas a biotic reaction implies a reaction occurring as part of a microbiologically metabolic or co-metabolic pathway. Iron and sulfate reducing conditions refer to the anaerobic conditions where iron (as Fe(III)) or sulfate are the primary electron acceptors supporting the growth of a dissimilatory iron or sulfate reducing organism, respectively. A consequence of this type of growth is the production of reduced iron (Fe(II) and reduced sulfur (e.g., sulfide) species which subsequently produces reduced iron solids. Prior to this research, under iron and sulfate reducing conditions, reductive dechlorination (i.e., an electron transfer reaction resulting in the removal of chlorines from chlorinated compounds) in contaminated groundwater systems was considered predominately to be a biologically mediated, co-metabolic process; however this had not been well studied. In view of the known production of reduced iron and sulfur species under these conditions, and the potential for reduced iron solids to facilitate reductive dechlorination, the possibility of a significant surface-mediated abiotic pathway was considered in this research. This study was therefore undertaken to explore the relative importance of surface-mediated abiotic and co-metabolic biotic reductive dechlorination pathways under iron and sulfate reducing conditions. Waste mixtures (selected metals and chlorinated solvents) were investigated in view of the lack of data on the impact of waste mixtures on reductive dechlorination pathways and the desires of this U.S. Environmental Protection Agency (EPA) program to focus projects on waste mixtures.
Summary/Accomplishments (Outputs/Outcomes):
To achieve the stated objective, reductive dechlorination experiments were performed in three types of systems: (1) abiotic systems, comprised of a representative reduced iron solid expected under iron (Fe3O4(s)) or sulfate (FeS(s)) reducing conditions; (2) biotic systems, using either a pure culture of an iron (Geobacter metallireducens) or sulfate (Desulfovibrio vulgaris) reducer; and (3) in combined abiotic and biotic systems, in which a pure culture (G. metallireducens or D. vulgaris), and the corresponding reduced iron solid (Fe3O4(s) or FeS(s)) produced by the given organism, were both present. In addition, in situ microcosm studies, and the impact of mixed wastes (selected metals and chlorinated compounds), were evaluated for selected conditions to determine the applicability of the findings to more complex waste systems and field conditions. To distinguish abiotic transformations from biotic transformation, pH dependence, temperature dependence, and product distribution studies were performed.
Magnetite (Fe3O4(s)) and mackinawite (FeS(s)) solids were synthesized either chemically with reagent grade chemicals, or generated biogenically by dissimilatory reducing organisms (i.e., by the iron reducer, G. metallireducens, or the sulfate reducer, D. vulgaris, respectively). For systems in which only a reduced iron solid was present, detailed chemical rate studies of reductive dechlorination were performed. Similar rate studies were performed in the microbiological systems to establish a basis of comparison for the combined abiotic and biotic studies. Identification of dechlorination reaction products and product distributions, dependence of reaction rate on pH and temperature, and the range of compounds that could be transformed were approaches used to establish the different signatures for the abiotic and biotic controlled systems. Chlorinated organic compounds investigated included: hexachloroethane (HCA), pentachloroethane (PCA), tetrachloroethylene (PCE), trichloroethylene (TCE), trichloroethanes (TCA), 1,1 dichloroethylene (11 DCE), cis-DCE, trans-DCE, vinyl chloride (VC), carbon tetrachloride (CT), and chloroform (CF). Bromoform (BF) also was included in some studies. A variety of metals were investigated and used (one at a time) to stimulate mixed metal-chlorinated solvent system in some abiotic rate studies, including the metal cations Co(II), Ni(II), Cu(II), Zn(II), Ag(I), Cd(II), Hg(II), Mn(II), and Cr(III), as well as the oxyacids of As(V), Se(VI), and Cr(VI).
Abiotic Reduced Iron System Studies. The relative reactivity of the two common solids expected under iron and sulfate reducing conditions, magnetite (Fe3O4(s)), and mackinawite (FeS(s)) were investigated. Significant reductive dechlorination by magnetite (Fe3O4(s)) and mackinawite (FeS(s)) occurred, but the relative rates and range of compounds were quite different. In general, the surface area normalized pseudo first order rate constants for FeS systems were more than 1000 times faster compared to Fe3O4 for similar compounds (e.g., HCA and CT). FeS also was found to dechlorinate a much wider range of chlorinated compounds than Fe3O4, with Fe3O4 only able to dechlorinate the more fully chlorinated compounds, such as HCA and CT. Furthermore, FeS was able to completely dechlorinate compounds, such as PCE and TCE, while these same compounds showed no reaction with Fe3O4. These results indicated that sulfate reducing conditions in the presence of iron can generate a much more favorable solid (FeS) for effective dechlorination than iron reducing conditions. These results also point to the potential for utilizing sulfate reducing bacteria or chemical reducing methods, when iron or sulfate are present or can be added, for generating an effective reactive material (FeS) for in situ remediation.
Reductive Dechlorination in Biotic Systems. A demonstration of the mechanism of dechlorination by an iron reducing bacterium was performed using the dissimilatory iron-reducing bacteria, G. metallireducens in the presence of CT. The results demonstrate that the dissimilative iron-reducing bacteria G. metallireducens directly transforms CT, and that the rate of transformation is proportional to biomass. The kinetics of transformation were found to conform to a two-site Michaelis-Menten expression. The biologically mediated reaction formed CF as a minor product (14-25 percent), while the major product was an apparent tri-chlorocarbon species irreversibly bound to cell material (75-86 percent). Mutual inhibition effects observed during FeIII-citrate and CT reduction, and the specific labeling of membrane proteins by 14CT, suggested that CT is transformed by G. metallireducens via co-metabolic mechanisms at sites located within the cell's membranes. This work provided the basis for comparing the relative importance of abiotic and biotic pathways for dechlorination under iron reducing conditions, as described below in combined abiotic and biotic systems.
Combined Abiotic and Biotic Reductive Dechlorination Studies. Dechlorination studies were performed in combined abiotic and biotic systems, i.e., in systems in which biogenically produced solids and the organisms that produced them were present. In a system representing iron reducing conditions, the transformation of CT by magnetite and G. metallireducens was investigated. Using the relative yield of mineral surface area to total protein during growth as a basis of comparison, the surface area and protein normalized rate constants suggested that the predominant means by which G. metallireducens induces CT transformation was through the formation of reactive magnetite surfaces rather than via co-metabolic mechanisms. In a similar study to simulate sulfate reducing conditions involved the transformation of CT by biogenic mackinawite and the organism D. vulgaris. Under sulfate reducing conditions, as with the iron reducing conditions, the surface mediated transformations by FeS were more significant than the transformation caused by co-metabolic pathways. The implication of these studies for natural systems is that a demonstrated biological dependence for contaminant transformation does not necessarily indicate direct biological mediation. In many instances, highly reactive solids formed by biological activity may be the primary pathway for dechlorination in contaminated aquifers rather than direct co-metabolic transformations. Clearly, more studies of this type for a wider range of contaminants, organic substrates, and organisms are warranted.
pH Dependent Rate Studies. pH dependence studies were undertaken to distinguish abiotic from biotic dechlorination pathways. The reductive dechlorination of selected compounds (HCA and TCE for FeS, and CT for Fe3O4) indicated that increasing pH (e.g., from 7-10) increases the pseudo first order rate constant for the dechlorination reaction. This result has two important consequences. First, because pH dependence of biologically mediated reactions are ultimately expected to decrease over the same range, due to inactivation of enzyme processes in microorganisms, the pH dependence studies may be used to distinguish abiotic from biotic dechlorination processes. Secondly, rates were found to dramatically increase, in some cases by more than 10-fold as pH increases, indicating that pH can be an important design parameter for implementing reductive dechlorination systems under iron and sulfate reducing conditions in the subsurface. Thirdly, in the case of FeS mediate transformation of TCE and PCE, increasing pH was found to improve the branching ratio (the ratio of the first order rate constants) for producing acetylene versus cis-DCE as an end product. Due to the desire to accomplish complete dechlorination, this result further indicated the preference for designing systems with higher pH when possible.
Temperature Dependent Rate Studies. Temperature dependent dechlorination rate studies of CT by Fe3O4 also were undertaken to distinguish abiotic from biotic contributions to dechlorination rates. The effect of temperature on the kinetics of CT dechlorination was examined in microcosms containing whole culture (cells and biogenic magnetite) at temperatures ranging from 20 to 70°C. The CT depletion data fits a first order reaction model at all test temperatures. The activation energy associated with this reaction was estimated to be 60.6 kJ/mol. The fact that Arrhenius relationships are obeyed well above the temperature at which most enzymes are denatured (45-50°C) provided strong evidence that the dechlorination reaction was predominantly an abiotic process associated with the biogenic magnetite. This approach demonstrated the feasibility of using temperature dependence as a way of distinguishing abiotic from biotic dechlorination in the systems in which both solids and microorganisms were present.
Products Distribution Studies. For selected systems, a full range of dechlorination products were identified and monitored during the course of the dechlorination rate studies. It was hoped that product identification would help delineate important abiotic dechlorination pathways and may allow abiotic and biotic pathways to be distinguished in the case where non-biotic products were identified. Of particular note was the fact that FeS was a much more effective reducing agent for a wide range of compounds compared to Fe3O4. FeS is capable of completely dechlorinating HCA, PCA, PCE, and TCE through a pathway that ultimately lead to the production of acetylene as a major end product. In the case of CT, FeS also was effective, leading to the nearly complete dechlorination and the suspected but not identified CS2. In the case of magnetite, dechlorination only was accomplished for CT and HCA. Only for CT was complete dechlorination possible. In the case of CT dechlorination by Fe3O4, a dechlorination pathway leading to the formation of CO as a major product was discovered. Based on these findings, and the implied predominance of sequential dechlorination pathways and products by microorganisms, a means to distinguish abiotic from biotic pathways was implicated through the product distribution studies. For sites contaminated with chlorinated ethylenes like PCE and TCE, the presence of acetylene is a strong indication that an abiotic pathway mediated by FeS is occurring. Likewise, in the case of CT contaminated sites, the presence of CS2 or CO is strong circumstantial evidence that abiotic pathways involving FeS or Fe3O4 are playing an important role in transformation of chlorinated compounds.
Reductive Dechlorination in Mixed Metal-Organic Solvent Wastes. Inasmuch as mixed metal-organic solvent waste mixtures are common, and the impact of waste mixtures on reductive dechlorination by reduced iron mineral solids was unknown, we undertook a set of screening experiments to establish if and how such mixtures may change the rates of dechlorination in model abiotic mineral systems. FeS and HCA were selected for this study. This choice was predicated on the basis of the well-known pathways for FeS mediated reductive dechlorination, and the relatively fast rate of reaction of HCA by FeS (half-life at pH of 7 of less than 2 hours), which would allow for many systems to be screened in a relatively short period of time. The metals investigated included the metal cations Co(II), Ni(II), Cu(II), Zn(II), Ag(I), Cd(II), Hg(II), Mn(II), and Cr(III), as well as the oxyacids of As(V), Se(VI), and Cr(VI). The intermediate to soft metal acids investigated, Co(II), Ni(II), Cu(II), Zn(II), Ag(I), Cd(II), and Hg(II), with the exception of Zn(II), showed enhanced rates of the HCA reductive dechlorination. This was attributed to the formation of solid solutions of these metals, with FeS leading to an even more favorable electron transfer reaction. The hard metal acids investigated, Mn(II), Cr(III), decreased reductive dechlorination rates of HCA. This was attributed to the formation of a passivating metal hydroxide coating of FeS by these sorbed metals. Each of the oxyacids tested, SeO32-, H3AsO3, CrO 2-4, in this study resulted in a decreased dechlorination rate of HCA. This was explained as due to a competing reduction reaction between the redox active oxyacids and FeS, resulting in a depletion of the electrons available for reductive dechlorination. These results illustrated that in mixed metal-solvent systems, dramatic changes in reductive dechlorination reaction rates by FeS may occur due to changes in the surface properties and surface reactions that occur in the presence of metals. However, the impacts of metal only were significant when the concentrations of metals relatively were high (0.01M) compared to the FeS concentration, indicating that trace amounts (<0.0001M) of metals in relatively uncontaminated sites may not significantly change the rates of dechlorination by FeS.
In Situ Microcosm Field Study at the Wurtsmith Air Force Base. As part of this study, in situ microcosm (ISM) studies were performed at the FT-2 (Fire Training site) at Wurtsmith Air Force Base to determine the relative importance of abiotic and biotic pathways under sulfate reducing conditions. At this site, TCE and CT (at 2 ppm), Br (200 ppm), NaN3 (0.2 percent), sulfate (50g/L), lactate (400g/L), and a FeS slurry (10 g/L) were injected in different combinations in 8 different ISMs to simulate controls and abiotic, biotic, and combined system conditions. Two ISMs controls were set up (one shallow and one deep) with only the contaminants and bromide tracer added to assess background transformations at the two different depths. Two other ISMs were used as abiotic controls which, in addition to the contaminants and bromide tracer, NaN3 (to kill bacteria) and sulfate were added at the two different depths. Four additional ISMs were set up for two biotic at shallow depth and two combined abiotic/biotic at deep depths, adding lactate to two of the microcosms and FeS and lactate to the remaining two. NaN3 also was added to one shallow and one deep sampling location. The injection of the various reagents into the 8 ISMs was implemented on November 10, 2000. Two sampling events were performed after that, one on November 27, 2000, and a second one on January 4, 2001. The extracted samples were monitored for disappearance of TCE and CT, and for the products acetylene, CS2 (known products of abiotic transformation of TCE and CT by FeS), CO, and CH4 (known products of abiotic transformation of CT by magnetite). Br, pH, sulfide, and iron also were monitored during the two sampling times. Disappearance of both TCE and CT was indicated on all ISMs, but the results about the relative importance of abiotic and biotic dechlorination were inconclusive for both the short duration of the study and the two sampling periods. Unfortunately, Wurtsmith Air Force Base was scheduled for closure before these studies could be setup again and tested over a longer period of time.
Results of potential practical significance in groundwater remediation from this research include:
(1) FeS is an Effective Reductive Dechlorination Agent. FeS (makinawite), commonly considered the primary solid produced initially under sulfate reducing conditions, was found to be an effective reducing agent for reductive dechlorination, with surface area normalized rates comparable to zero valent iron. This work also demonstrated the potential of FeS for dechlorinating a wide range of chlorinated compounds. As a result, FeS is now being considered in permeable barrier applications for groundwater remediation. Based on this work, a new remediation process involving FeS has been approved for field-testing in a project sponsored by the Department of Defense's (DoD) Environmental Security Technology Certification Program (ESTCP).
(2) Reduced Iron Solids were Found to be More Effective at Remediation of Chlorinated Compounds than the Organisms that Produced Them. Reduced iron minerals produced as a consequence of microbiological activity under iron and sulfate reducing conditions have dechlorination capabilities that exceeded the capapbilities of the microorganisms that produced them. This work points to the feasibility and desirability for developing a groundwater remediation process that utilizes iron and sulfate reducing bacteria to produce reactive reduced iron solids for in situ cleanup of contaminated groundwater sites.
(3) Specific Products of Reductive Dechlorination Provide Evidence for Abiotic Transformations Caused by Reduced Iron Minerals. We found that acetylene is a principal product of reductive dechlorination of chlorinated ethylenes by FeS. Likewise, we confirmed that the presence of CO is indicative of the transformation of carbon tetrachloride by the reduced iron solids magnetite. Detection of these compounds in situ may be used as evidence for reductive dechlorination processes mediated by reduced mineral phases. Since these products are not expected to form from microbiological transformations they also provide a way to distinguish mineral-mediated processes from biological transformation processes.
Journal Articles on this Report : 5 Displayed | Download in RIS Format
Other project views: | All 48 publications | 5 publications in selected types | All 5 journal articles |
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Butler EC, Hayes KF. Effects of solution composition and pH on the reductive dechlorination of hexachoroethane by iron sulfide. Environmental Science & Technology 1998;32(9):1276-1284. |
R825958 (Final) |
not available |
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Butler EC, Hayes KF. Kinetics of the transformation of trichloroethylene and tetrachloroethylene by iron sulfide. Environmental Science & Technology 1999;33(12):2021-2027. |
R825958 (1998) R825958 (2000) R825958 (Final) R826235 (2000) |
Exit Exit Exit |
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Butler EC, Hayes KF. Kinetics of the transformation of halogenated aliphatic compounds by iron sulfide. Environmental Science & Technology 2000;34(3):422-429. |
R825958 (2000) R825958 (Final) |
not available |
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Butler EC, Hayes KF. Factors influencing rates and products in the transformation of trichloroethylene by iron sulfide and iron metal. Environmental Science & Technology 2001;35(19):3884-3891. |
R825958 (2001) R825958 (Final) |
not available |
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McCormick ML, Bouwer EJ, Adriaens P. Carbon tetrachloride transformation in a model iron-reducing culture: relative kinetics of biotic and abiotic reactions. Environmental Science & Technology 2002;36(3):403-410. |
R825958 (Final) |
not available |
Supplemental Keywords:
reductive dechlorination, mixed-waste, groundwater, chlorinated solvents, heavy metals, reduced iron minerals, iron sulfide minerals, iron reducing conditions, sulfate reducing conditions., RFA, Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Chemical Engineering, Bioavailability, Environmental Chemistry, Fate & Transport, Ecology and Ecosystems, Ecological Risk Assessment, Bioremediation, fate and transport, dechlorination, contaminants in soil, contaminant release, contaminated aquifers, chlorinated solvents, metal compounds, heavy metalsProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.