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Bioremediation of Sediments Contaminated with Polynuclear Aromatic HydrocarbonsPrincipal InvestigatorsJ. B. Hughes and C. Herbert WardRice University E-mail: hughes@rice.edu GoalPolynuclear aromatic hydrocarbons (PAHs) are among the contaminants most frequently detected in sediments. Many PAHs are biodegradable, and bioremediation technologies are being considered to treat contaminated sediments prior to and after dredging. While laboratory studies have delineated many fundamental aspects of PAH biodegradation under ideal conditions, and several full-scale attempts to bioremediate contaminated sediments have been conducted, few controlled studies have addressed the complexities encountered in transferring bioremediation processes from the laboratory to field demonstrations at actual PAH-contaminated sites.This project on sediment bioremediation is intended to bridge the gap between existing fundamental knowledge on the metabolism of PAHs, which is already abundant and adequate, and the practical requirements for implementing bioremediation processes in the field. Special attention will be given to:
RationaleUndisturbed and dredged sediments are significant sources of environmental contamination, yet most sediment management, control, and remediation strategies currently in use are limited to no action at all, or dredging and use of unconfined land disposal, land-based Confined Disposal Facilities (CDF), unconfined aquatic disposal, and Confined Aquatic Disposal Facilities (CAD). Where applicable, bioprocesses for treatment of highly contaminated sediments, using high solids slurry reactors either in situ or ex situ, may offer significant returns. The major sources of PAHs in sediments are direct discharge of petroleum-containing wastes, coal gasification, and combustion processes. Regardless of origin, the resulting contamination consists of many individual PAHs with varying chemical characteristics. Two mechanisms of aerobic PAH degradation are known. First, the PAH can serve a s a growth substrate. Two- and three-ring PAHs such as naphthalene and phenanthrene are known growth substrates for bacteria. Second, the PAH can be metabolized as a secondary substrate in a process known as cooxidation or cometabolism. This mechanism appears to be important for low solubility four- and five-ring PAHs such as chrysene and benzo -(a)-pyrene, which have been shown to be degradable but do not support bacterial growth. Mixing helps maintain aerobic conditions in a bioremediation system used to treat PAH-contaminated sediments. In addition to increasing O2 transfer rates, this results in the suspension of sediment particles, and facilitates the release of contaminants from the sediments. Highly contaminated sediments, or sediments with high levels of red uced sulfur, may require forced aeration in addition to mixing to supply adequate levels of O2. A new product, Oxygen Release Compound (ORC), may prove to be useful in controlling toxic and odorous air emissions from anaerobic sediments when they are aerated for bioremediation.An additional operational concern in achieving adequate treatment of PAHs is that slurry reactors are typically operated in a batch mode. This introduces significant operational challenges since batch processes are by definition not steady-state. In a batch process, several parameters will change with time, including contaminant composit ion, nutrient levels, and the numbers and types of microorganisms present. These temporal changes are largely controlled by microbial processes such as lag phases, preferential utilization of substrates, cooxidation, and change of community structure. In systems containing complex mixtures of PAHs, preferential utilization of more soluble PAHs h as been observed. Physical/chemical processes (desorption and volatilization ) and operational considerations (mixing rates, dissolved oxygen levels, bioaugmentation, etc.) will also influence the performance of slurry systems. Desorption processes may control the extent of treatment possible, particularly in aged sediments. A major regulatory concern in sediment bioremediation is air emissions from the bioreactors. StatusCompleted and ongoing studies are focusing on 1) the biodegradation of high molecular weight PAHıs and the influence of PAH mixtures on their degradation, 2) quantifying the fate of PAH mixtures in slurry reactors (biodegradation, volatilization, etc.), 3) identifying bacteria from estuarine sediments capable of PAH degradation (in collab oration with Dr. Tadros, Alabama A&M, through the MAI initiative), and 4) developing slurry reactor models and cost estimates (in collaboration with Dr. Rada, Prairie View A&M through the MAI initiative). This project currently supports two graduate students who are scheduled to complete their M.S. theses this summer. It is anticipated that one manuscript for submittal in the peer-reviewed literature will result from each thesis. Both students will remain for doctoral studies upon completion of their thesis. Specific outputs of research that acknowledge HSRC support during Year I of this project are listed below: Biodegradation of PAH Mixtures: Studies were conducted to determine the effect of mixtures of PAHs on the degradation of high molecular weight PAHs in systems with and without sediments. A group of three representative PAHs were chosen - naphthalene, acenaphthene and fluoranthene. Fluoranthene degrading activity was induced in bacteria that were not previously exposed to fluoranthene after approximately 30 days of exposure to this compound. Experiments in systems without sediments were used to identify the effect of mixtures on fluoranthene degradation without the complicating factors introduced with the presence of sediments. The results of sediment free experiments showed a range of mixtures effects.
Planned ActivitiesIn addition to completing the slurry reactor model, studies are to be initiated in the fall of 1996. First is a detailed study into the rapid degradation of high molecular PAHs in slurry systems. The degradation of 4- and 5- ringed PAHs is an area of great interest since these compounds are often very persistent in the environment, yet they were effectively degraded in laboratory studies. Experiments will be initiated to address this with the hypothesis that PAH mixtures are required for degradation to occur (e.g., high molecular weight PAHs may be secondary substrates in mixtures). Most studies to date have focused on compounds fed individually where PAH concentrations are very low. As a mixture, the net concentration may in fact support degradation. If available through the MAI program, isolation of degrading strains will be attempted by Dr. Tadros, Alabama A&M.A second area of research will focus on the degradation of PAH mixtures in highly contaminated sediments, such as those associated with coal tars, etc. This will introduce an additional level of complexity in experimental systems due to the introduction of a third phase (NAPL). Sediments that contain parts per thousand levels of PAH gener ally have a oil phase in addition to the adsorbed sediment phase. Studies will investigate the potential impacts of this contaminant ³sink², again with a focus on the high molecular weight compounds.
Outputs to Date
This project focuses on requirements for application of bioremediation technology to clean up PAH-contaminated sediments. Specific areas of research include: 1) the extent of treatment possible with highly contaminated, aged sediments (the bioavailablity issue), 2) the microbial and physical-chemical parameters that control degradation of high molecular weight PAHs, 3) optimization of slurry reactor performance with respect to rates and extent of contaminant biodegradation, and 4) methods of reducing volatilization during reactor operation.
Copyright
© Georgia Tech Research Corporation, 2002.
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