Intrinsic Biodegradation Research Articles

Field and laboratory evidence for intrinsic biodegradation of vinyl chloride contamination in a Fe(III)-reducing aquifer

Intrinsic bioremediation of chlorinated ethenes in anaerobic aquifers previously has not been considered feasible, due, in large part, to 1) the production of vinyl chloride during microbial reductive dechlorination of higher chlorinated contaminants and 2) the apparent poor biodegradability of vinyl chloride under anaerobic conditions. In this study, a combination of field geochemical analyses and laboratory radiotracer ([1,2- 14C] vinyl chloride) experiments was utilized to assess the potential for intrinsic biodegradation of vinyl chloride contamination in an Fe(III)-reducing, anaerobic aquifer. Microcosm experiments conducted under Fe(III)-reducing conditions with material from the Fe(III)-reducing, chlorinated-ethene contaminated aquifer demonstrated significant oxidation of [1,2- 14C] vinyl chloride to 14CO 2 with no detectable production of ethene or other reductive dehalogenation products. Rates of degradation derived from the microcosm experiments (0.9–1.3% d −1) were consistent with field-estimated rates (0.03–0.2% d −1) of apparent vinyl chloride degradation. Field estimates of apparent vinyl chloride biodegradation were calculated using two distinct approaches; 1) a solute dispersion model and 2) a mass balance assessment. These findings demonstrate that degradation under Fe(III) reducing conditions can be an environmentally significant mechanism for intrinsic bioremediation of vinyl chloride in anaerobic ground-water systems.

Leaching of PCB compounds from untreated and biotreated sludge-soil mixtures

Column experiments were conducted to evaluate the aqueous leachability of polychlorinated biphenyls (PCBs) from sludge-soil mixtures contaminated with PCBs and hydraulic oils. Untreated and biotreated sludge-soil samples were obtained from a two-month, pilot-scale field test of PCB biodegradation in land treatment units. Column leachate comprised mainly di- and trichlorobiphenyls. In comparison to untreated samples, column leachate from biotreated samples showed 34 to 63% reduction in PCBs; average values of total PCB concentrations in the column effluent ranged from 0.36 to 0.82 μg 1 −1 for untreated samples and 0.25 to 0.30 μg 1 −1 for biotreated samples. Only a small fraction, less than 6%, of the PCB mass in the untreated sludge-soil mixtures was removed by prolonged flushing with up to 2400–3000 pore volumes of water, suggesting long-term, slow release of PCBs at low aqueous concentrations. It should be noted that flushing rates for the column experiments were much higher than typical ground-water flow rates and the two-month period for column experiments can be qualitatively compared to a greatly extended duration for ground-water flow. The leaching of PCBs from a nonaqueous phase liquid (NAPL) in the column experiments was modelled as a predominantly dissolution-governed process based on first-order kinetics. The estimated dissolution rate coefficients ranged from 0.02 to 0.04 min −1 for various PCB homolog groups. This investigation helps support the concept of biostabilization, in which contaminated materials may be actively biotreated to remove the potentially mobile organic contaminants leaving a residual material that is more stable against leaching. Natural attenuation processes like sorption and intrinsic biodegradation further reduce the PCB leachability potential. Additional long-term monitoring of the land treatment units under passive conditions is under progress to evaluate such reductions.

Simulations of intrinsic biodegradation using a non‐linear modification of first‐order reaction kinetics

The use of mathematical models as tools to more effectively manage risks and remediation costs is becoming more common at contaminated sites. Analytical models commonly use first‐order reaction kinetics to predict the effectiveness of intrinsic biodegradation of organic contaminants at fuel contaminated sites. The equation commonly used is effectively the same decay model used to predict radioactive decay (a physical process). The unqualified use of the first‐order reaction kinetic model requires the user to make a number of unrealistic assumptions concerning the lack of ecological controls on the biologically mediated degradation process and absolute effectiveness of the Intrinsic biodegradation process on organic chemicals. A modification of the first‐order reaction kinetics model is made to accommodate the logistic equation as a means of introducing variability in the degradation rate as a function of ecologically controlled population dynamics in microbial consortia. The proposed modification also gives the user more flexibility in fitting predicted results from the first‐order kinetic model to observed trends in site‐specific data.

Intrinsic biodegradation of MTBE and BTEX in a gasoline‐contaminated aquifer

Three‐dimensional field monitoring of a gasoline plume showed rapid decay of toluene and ethylbenzene during downgradient transport with slower decay of xylenes, benzene, and MTBE under mixed aerobic‐denitrifying conditions. Decay was most rapid near the source but slower farther downgradient. Effective first‐order decay coefficients varied from 0 to 0.0010 d−1 for MTBE, from 0.0006 to 0.0014 d−1 for benzene, from 0.0005 to 0.0063 d−1 for toluene, from 0.0008 to 0.0058 d−1for ethylbenzene, from 0.0012 to 0.0035 d−1 for m‐, p‐xylene, and from 0.0007 to 0.0017 d−1 for o‐xylene. Laboratory microcosm studies confirmed MTBE biodegradation under aerobic conditions; however, the extent of biodegradation was limited.

Evaluation of Respirometric Data: Identification of Features That Preclude Data Fitting with Existing Kinetic Expressions

The use of respirometric data for the evaluation of intrinsic biodegradation kinetic parameters for single organic compounds is discussed. Emphasis is placed on the preliminary assessment of the data set to determine whether it is suitable for kinetic parameter estimation. Careful preliminary examination of the data avoids attempting parameter estimation with unacceptable data. Furthermore, the use of unbiased respirometric data helps ensure that the estimated parameters truly reflect the intrinsic kinetics for biodegradation of a single substrate by the culture tested. Both experimental and theoretical oxygen uptake curves are used to illustrate how various conditions can limit the utility of a given data set. The effect of substrate inhibition, dual or multiple substrate limited growth, inaccuracies in the initial conditions assumed for curve fitting, and the use of poorly acclimated cultures are discussed. Techniques are presented which allow identification of whether a data set is unsuitable and should not be used for parameter estimation. In addition, experimental procedures which can help avoid the collection of aberrant data are discussed.

Monoaromatic hydrocarbon transformation under anaerobic conditions at seal beach, California: Laboratory studies

AbstractAnaerobic biotransformation of several aromatic hydrocarbons found in gasoline including benzene, toluene, ethylbenzene, m‐xylene, p‐xylene, and o‐xylene (BTEX) was studied in batch anaerobic laboratory microcosms. Aquifer sediment and ground water were obtained from the site of a historic gasoline spill at Seal Beach, California. Sulfate is present in the site ground water at 80 mg/ L, and sulfate‐reducing activity appears to be the dominant intrinsic BTEX bioremediation process where nitrate is absent. In the laboratory, the microcosms were set up with different electron acceptors (sulfate and nitrate) in site ground water and various defined anaerobic media to estimate intrinsic biodegradation rates and to suggest conditions under which anaerobic bioremediation could be enhanced. In unamended microcosms, anaerobic biotransformation of toluene and m+p‐xylene (m‐xylene and p‐xylene were measured as a summed parameter) occurred at a rate of 7.2 and 4.1 μg L−1 h−1, respectively, with sulfate as the apparent electron acceptor. Addition of nitrate stimulated nitrate‐reducing conditions and increased rates of toluene and m+p‐xylene biotransformation to 30.1 and 5.4 μg L−1 h−1, respectively. The catabolic substrate range was altered to include ethylbenzene in the nitrate‐amended microcosms, suggesting an apparent preferential use of different BTEX compounds depending on the electron acceptor available. Under all the conditions studied, more than twice the amount of nitrate or sulfate was used than could be accounted for by the observed BTEX degradation. Benzene transformation was not observed under the conditions studied. Although methane was detected in microcosms prepared with anaerobic media lacking nitrate and sulfate, methanogenic biotransformation of BTEX compounds was not observed. The results of these experiments indicate that indigenous microorganisms from the Seal Beach aquifer have significant capability to degrade BTEX hydrocarbons and that intrinsic processes in the Seal Beach aquifer may remediate a portion of the hydrocarbon contamination in situ without intervention. However, the data also suggest that intervention by nitrate addition would enhance the rate and extent of anaerobic BTEX biotransformation.

Biodegradation of sorbed chemicals in soil.

Rates of biodegradation of sorbed chemicals are usually lower in soil than in aqueous systems, in part because sorption reduces the availability of the chemical to microorganisms. Biodegradation, sorption, and diffusion occur simultaneously and are tightly coupled. In soil, the rate of biodegradation is a function of a chemical's diffusion coefficient, sorption partition coefficient, the distance it must diffuse from the site of sorption to microbial populations that can degrade it, and its biodegradation rate constant. A model (DSB model) was developed that describes biodegradation of chemicals limited in the availability by sorption and diffusion. Different kinetics expressions describe biodegradation depending on whether the reaction is controlled by mass transfer (diffusion and sorption) or the intrinsic biodegradation rate, and whether biodegradation begins during or after the majority of sorption has occurred. We tested the hypothesis that there is a direct relationship between how strongly a chemical is sorbed and the chemical's biodegradation rate. In six soils with different organic carbon contents, there was no relationship between the extent or rate of biodegradation and the sorption partition coefficient for phenanthrene. Aging of phenanthrene residues in soil led to a substantial reduction in the rate of biodegradation compared to biodegradation rates of recently added phenanthrene. Considerable research has focused on identification and development of techniques for enhancing in situ biodegradation of sorbed chemicals. Development of such techniques, especially those involving inoculation with microbial strains, should consider physical mass transfer limitations and potential decreases in bioavailability over time.