Chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE), are by far the most prevalent priority pollutants at hundreds of Department of Defense (DoD) sites and remain among the most difficult to remediate despite years of research and development. The relative ineffectiveness and prohibitive costs of current approaches for remediating chlorinated solvent plumes has resulted in an increased reliance on natural biological, chemical, and physical processes to treat chlorinated solvents (i.e., natural attenuation). The role of biological processes in the fate of PCE and TCE has been studied in some detail, but little had been done in the past to assess the importance of chemical (or abiotic) processes. Most naturally occurring reductants are thermodynamically capable of reducing PCE and TCE. Reduced iron, sulfur, and organic matter formed via biological (e.g., microbial respiration) and chemical (e.g., weathering) pathways have been shown to reduce a number of environmentally relevant contaminants. Although there is some evidence that laboratory-synthesized iron minerals can transform PCE and TCE, it is unclear whether this process is sufficiently sustainable under field conditions to impact PCE and TCE groundwater plumes.
The objective of this project was to identify processes and mechanisms that potentially could account for the abiotic transformation of chlorinated ethenes in contaminated aquifers and to assess their likely importance under field conditions.
The reduction of chlorinated ethenes was measured by a series of chemically and microbially generated reductants under a range of natural conditions. During the project, a variety of reductants were collected, synthesized, and characterized, and the extent and rate of PCE, TCE and cis-dichloroethene (cDCE) reduction was measured in batch reactors. The reductants included, (i) Fe(II) sorbed on Fe oxides, (ii) minerals containing Fe(II), such as iron sulfides (e.g., mackinawite) and green rusts, (iii) precipitates and supernatant collected from dissimilatory iron reducing (DIR) cultures of Shewanella species and an sulfate reducing bacterial (SRB) culture of Desulfovibrio desulfuricans, and (iii) sorbed and reduced natural organic matter (NOM) and NOM model compounds.
The chlorinated ethenes were surprisingly stable in the presence of Fe(II) sorbed on Fe oxides, as well as magnetite, and reduced and sorbed natural organic matter (NOM). The experiments were repeated over a range of pH and buffer conditions and several positive controls were conducted to make sure that (i) PCE, TCE, and cDCE transformation and the formation of products could be observed if it occurred, and (ii) whether the reductants were comprised and were indeed capable of reducing other oxidized compounds. The lack of reactivity with sorbed Fe(II) and magnetite was contrary to that expected based on thermodynamic calculations and previous work showing PCE and TCE reduction in the presence of magnetite and Fe(II).
Reduction of TCE was observed with chemically synthesized green rust minerals, as well as the iron sulfide minerals, mackinawite and pyrite. Acetylene was the major product observed for TCE reduction by mackinawite, consistent with the work by Butler and colleagues. TCE was reduced rather slowly by carbonate and sulfate green rust with half-lives on order of about a month. Carbon recoveries based on dechlorinated gas products (ethene, ethane, acetylene) varied from 48 to 89% of the initial TCE added.
Data from PCE reduction by green rusts was inconclusive. In one experiment, some products were observed after 2 months, but the data was not reproducible and controls without green rust also showed significant loss of PCE.
cDCE was reduced quite rapidly by carbonate and chloride green, however, it is suspected that an interaction between the aluminum foil used to cover the septa and the green rust may have played a role in the rapid reduction of cDCE. At this time, the nature of the interaction is unclear. Given the propensity for aluminum to substitute into Fe minerals, the rapid rate of cDCE reduction by green rusts in the presence of aluminum foil may represent an interesting abiotic reaction pathway to follow up on.
The reduction of chlorinated ethenes by these same iron species (i.e., green rusts and mackinawite) formed in the presence of anaerobic bacteria also was investigated. Specifically, precipitates formed in the presence of dissimilatory iron reducing and sulfate reducing bacteria (DIRB and SRB) were collected and characterized. Carbonate green rust was formed as a result of DIR by a diverse range of Shewanella sp. and mackinawite was formed in an SRB culture of Desulfovibrio desulfuricans. Whole cultures (cells + precipitates) of both SRB and DIRB could reduce TCE. The kinetics of TCE reduction varied among different cultures and product recoveries as dechlorinated gases varied, but appreciable amounts of acetylene was observed indicating some reduction via elimination. It is believed that the biogenic precipitates, that is, the carbonate green rust and mackinawite, were responsible for the TCE reduction observed in the whole cultures, but experiments with isolated components from the culture suggested otherwise. Specifically, the washed green rust from the Fe reducing culture was unreactive with TCE, in contrast to the significant reduction the researchers observed with chemically synthesized green rust. Some reduction of TCE with autoclaved cultures indicate that there is an abiotic component, but similar amounts of reduction also were observed in sterile medium controls containing lepidocrocite, suggesting that DIR was not necessary for abiotic reduction to occur. At this point, the researchers can only speculate that there is some interaction between a soluble microbial exudate and the green rust surface that is responsible for the TCE reduction observed in the whole cultures.
In the sulfate reducing culture, TCE was reduced to acetylene and mackinawite was identified. It appears, however, that high concentrations of sulfate, as well as soluble microbial exudates inhibit the rate of TCE reduction compared to that of chemically synthesized FeS. Images of chemically synthesized mackinawite and the biogenic FeS show that the biogenic FeS surface is coated with some cellular or precipitate material and is more heterogeneous than the chemically synthesized FeS, suggesting caution when extrapolating reactivity behavior from chemically synthesized minerals to complex biogeochemical environments.
Direct outcomes from this project include kinetic rate coefficients and product branching ratios that can be used to assess the likelihood of abiotic attenuation of PCE and TCE at DoD and Department of Energy sites. The insight derived from these investigations provides an improved basis for predicting the fate of chlorinated ethenes in anoxic soils, sediments, and aquifers.