Carbon tetrachloride (CT) was dechlorinated to chloroform (CF) under anoxic conditions by Fe(II) that was sorbed to the surface of goethite (α-FeOOH). No reaction occurred when Fe(II) was present and goethite was absent. Several abiotic experiments were conducted with goethite at 30 °C in which the total amount of Fe(II) in the system, the amount of sorbed Fe(II), the density of sorbed Fe(II), and the pH were varied. Regeneration of sorbed Fe(II) occurred when dissolved Fe2+ was available and maintained pseudo-first-order conditions with respect to CT. Analysis of the rates of CT loss for experiments with sorbed-Fe(II) regeneration showed the rate-determining-step to be first order with respect to CT, second order with respect to the volumetric concentration of sorbed Fe(II) (i.e., mmol sorbed Fe(II) L-1 suspension), and zero order with respect to H+ for pH between 4.2 and 7.3. The absolute rate constant for the reaction was determined to be 42 ± 5 M-2 s-1. Normalization of the observed rate constants to account for different goethite concentrations yielded reaction orders of one and zero, respectively, for CT and H+, and a second-order reaction with respect to the density of sorbed Fe(II) (i.e., mmol sorbed Fe(II) g-1 goethite). On the basis of the kinetic data, the rate-determining step is proposed to be a termolecular two-electron-transfer reaction involving two Fe2+ ions sorbed to adjacent sites on the goethite surface and a CCl4 molecule that approaches the surface. The primary role of the goethite surface, thus, is to catalyze the reaction by fixing the position of the two charged reactants in a geometry suitable for reaction with CT. In separate experiments, biogenic Fe(II) formed by the enzymatic reduction of goethite by the Fe(III)-reducing bacterium Shewanella alga, strain BrY, dechlorinated CT. Of the CT degraded by abiotic and biogenic Fe(II) on goethite, 83−90% was converted to chloroform (CF), which accumulated in the reaction vial. These results indicate that dechlorination reactions in Fe(III)-reducing environments may indirectly result from the enzymatic or chemical reduction of Fe(III)-bearing minerals such as goethite.
Nitrilotriacetic acid (NTA) is a synthetic chelating agent that can form strong water-soluble complexes with a wide range of radionuclide and metal ions and has been used to decontaminate nuclear reactors and in the processing of nuclear materials. The co-disposal of NTA or other synthetic chelating agents with radionuclides may result in increased dispersal of radionuclides in soil and subsurface environments. Understanding the influence of aqueous geochemistry on NTA degradation is essential to predict the mobility and fate of inorganic contaminant−NTA complexes in the subsurface. Chelatobacter heintzii (ATCC 29600) was shown to degrade 14C-labeled NTA to 14CO2 with first-order kinetics at concentrations ranging from 0.05 to 5.23 μM (0.01−1 μg of NTA mL-1). The degradation of various metal−NTA complexes was investigated under conditions in which the NTA was predominantly present as the metal−NTA complex. The order for the rates of degradation was HNTA2- > CoNTA- = FeOHNTA- = ZnNTA- > AlOHNTA- > CuNTA- > NiNTA-, which is not related to the order of metal−NTA stability constants. The metal concentration used was not inhibitory to glucose mineralization, suggesting that toxicity of the chelated metal was not responsible for the differences in the rates of NTA degradation. After degradation of CoNTA- and NiNTA-, <3% of the Co or Ni was associated with C. heintzii cells. This indicates that, after degradation of the metal−NTA complex, metal ions will be predominantly present in the aqueous phase. The degradability of the various metal−NTA complexes was not related to their thermodynamic stability constants, but was related to the lability of the various metal−NTA complexes or the relative rates of formation of HNTA2-.
The codisposal of synthetic chelating agents [e.g., ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), and nitrilotriacetic acid (NTA)] and radionuclides has been implicated in increased radionuclide transport in the subsurface environment. Microbial transformations of chelates in the subsurface are currently unknown, but could influence chelate persistence and thus alter the transport of radionuclides. Surface soil and subsurface sediments from five formations (36‐ to 376‐m depth) were collected near Allendale, SC. Aerobic mineralization of 14C‐labeled EDTA, DTPA, and NTA occurred in select sediments indicating that subsurface microorganisms can degrade chelates, whereas chelates may be relatively stable in strata where limited mineralization occurred. The chelates were not mineralized more rapidly or to a greater extent in the surface soil than in the subsurface sediments. The relative order of chelate persistence was EDTA > DTPA > NTA, with the maximum amount mineralized during 115 d at 15, 26, and 43%, respectively. Maximum mineralization of all three chelates did not occur in the same sediment, indicating that different microbial populations were responsible for the degradation of each chelate. Mineralization of chelates was minimal under denitrifying conditions and was reduced when additional soluble C was added. There was no relationship between chelate mineralization and the adsorption of chelates to sediments or the aqueous speciation of the chelates.
A bacterium, designated Fl99, utilized toluene, naphthalene, dibenzothiophene, salicylate, benzoate, pcresol, and all isomers of xylene as a sole carbon and energy source. This bacterium was isolated from Middendorf sediments, a Cretaceous age formation that underlies the Southeast Coastal Plain in South Carolina, at a depth of approximately 410 m. F199 is a gram-positive, irregular-shaped bacterium that has a varied cell morphology that is dependent on culture medium type and growth stage. F199 required microaerobic conditions (40 to 80 ,uM 02) for growth on hydrocarbons, glucose, acetate, and lactate in mineral salts medium but not for growth on rich media. [14C]naphthalene mineralization by F199 was induced by either naphthalene or toluene; however, [14C]toluene mineralization by this strain was induced by toluene but not naphthalene. F199 was also found to harbor two plasmids larger than 100 kb. Restricted F199 plasmid and genomic DNA did not hybridize with toluene (pWW0) or naphthalene (NAH7) catabolic plasmid DNA probes. The presence in the Middendorf formation of bacteria with the capacity for degrading a variety of aromatic compounds suggests that indigenous microorganisms may have potential for in situ degradation of organic contaminants.
The ability of a metal-reducing bacterium to microbially reduce vitamin B12 was determined to expand our understanding of the role vitamin B12 plays in the transformation of halogenated compounds in microbial systems. The subsequent transformation of chlorinated methanes catalyzed by this microbially-reduced vitamin B12 was then evaluated. When incubated in the presence of Shewanella alga strain BrY and an electron donor, the microbial reduction of vitamin B12a to B12r was observed as a shift in the vitamin B12 spectrum. In treatments containing vitamin B12 and an electron donor but without BrY, the predominant species was vitamin B12a. The introduction of BrY into the system resulted in the production of vitamin B12r. The transformation of carbon tetrachloride (CT), chloroform (CF), and dichloromethane (DCM) was examined in batch systems containing vitamin B12, Shewanella alga strain BrY, and an electron donor. Transformation of both CT and CF was observed, while no significant change in the DCM concentra tion was detected. Carbon monoxide was the major product of CT transformation. No significant transformation of CT or CF was detected when vitamin B12 was omitted from the system. This work demonstrates that a metal-reducing bacterium, with no apparent ability to transform CT or CF directly, mediates the reduction of vitamin B12, which in turn catalyzes the transformation of CT.
Microcosms containing intact soil-cores are a potential tool for assessing the risks of the release of genetically engineered microorganisms (GEMs) to the environment. Before microcosms become a standard assessment tool, however, they must first be calibrated to ensure that they adequately simulate key parameters in the field. Four systems were compared: intact soil-core microcosms located in the laboratory at ambient temperature and in a growth chamber with temperature fluctuations that simulated average conditions in the field, field lysimeters, and field plots. These four systems were inoculated with rifampicin-resistantPseudomonas sp. and planted to winter wheat. Populations of thePseudomonas sp. in soil decreased more rapidly at ambient temperature, but population size at the three-leaf stage of wheat growth was the same in all four systems. Populations of thePseudomonas sp. on the rhizoplane of wheat were the same at the three-leaf stage in all four systems, and colonization with depth at the final boot stage-sampling was also similar. In general, microcosms incubated at ambient temperature in the laboratory or in the growth chamber were similar to those in the field with respect to survival of and colonization of the rhizoplane by the introducedPseudomonas sp.
Microcosms containing intact soil-cores are a potential biotechnology risk assessment tool for assessing the ecological effects of genetically engineered microorganisms before they are released to the field; however, microcosms must first be calibrated to ensure that they adequately simulate key field parameters. Soil-core microcosms were compared with the field in terms of ecological response to the introduction of a large inoculum of a rifampicin-resistant rhizobacterium,Pseudomonas sp. RC1. RC1 was inoculated into intact soil-core microcosms incubated in the laboratory at ambient temperature (22°C) and in a growth chamber with temperature fluctuations that mimicked a verage field values, as well as into field lysimeters and plots. The effect of the introduced bacterium on ecosystem structure, including wheat rhizoplane populations of total and fluorescent pseudomonads, total heterotrophic bacteria, and the diversity of total heterotrophic bacteria, was determined. Fluorescent pseudomonads were present on the rhizoplane in significantly lower numbers in soil inoculated with RC1, in both microcosms and the field. Conditions for microbial growth appeared to be most favorable in the growth chamber microcosm, as evidenced by higher populations of heterotrophs and a greater species diversity on the rhizoplane at the three-leaf stage of wheat growth. Ecosystem functional parameters, as determined by soil dehydrogenase activity, plant biomass production, and(15)N-fertilizer uptake by wheat, were different in the four systems. The stimulation of soil dehydrogenase activity by the addition of alfalfa was greater in the microcosms than in the field. In general, growth chamber microcosms, which simulated average field temperatures, were better predictors of field behavior than microcosms incubated continuously at 22°C.
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