Hazardous Substance Research Centers/South & Southwest

Phytoremediation in Wetlands and Confined Disposal Facilities

Principal Investigators

John H. Pardue, Louisiana State University
Joseph B. Hughes, Rice University
Jerry Schnoor, University of Iowa
Chandra Theegala, Southern University

Related project

Objectives: Hydrophobic chlorinated organics such as hexachlorobenzene are common sediment contaminants that pose a threat to sensitive receptors. These compounds are recalcitrant in sediments and bioaccumulate through the food chain. By contrast, rapid contaminant attenuation for certain chlorinated organics is observed in vegetated sediments (i.e., wetlands). In these sediments, enhanced biological processes (aerobic and anaerobic biodegradation and plant uptake) have been observed in the root zone that drives rapid natural recovery. Previous research has indicated that herbaceous wetland vegetation stimulates degradation of chlorinated organics primarily via rhizospheric biodegradation processes. It can be hypothesized that reductive dechlorinating populations are stimulated in the rhizosphere by formation of specific organic acids (i.e., propionate) during detrital processes. The presence of certain organic acids has been shown to stimulate reductive dehalogenating organisms by favorably controlling the ambient level of H2 in porewater. The formation of high concentrations of these organic acids in marshes with Phragmites vegetation has been observed, while not in other marsh vegetation types such as Typha. This microbial-vegetation interaction may represent the "mechanism" by which rapid natural attenuation occurs in these wetland systems. Based on these hypotheses, the objectives of the proposed study are to: define the biodegradation potential of chlorobenzenes by quantifying biogeochemical conditions in the rhizosphere. Key conditions include the specific detrital decomposition products (organic acids and hydrogen) in several contrasting wetland plant types. A second objective of the study will define other potential fate mechanisms: plant uptake and volatilization by studying the dynamics of plant uptake of chlorobenzenes in wetland sediments.

Approach: Initial mechanistic studies will be conducted in greenhouse mesocosms using herbaceous wetland vegetation with known differences in detrital pathways (Phragmites and Typha). Studies will test the hypotheses above by assessing daughter product (lower chlorinated benzene) concentrations and bulk redox conditions in rhizosphere to infer whether reductive dechlorination reactions are occurring. Intensity and capacity measurements of terminal electron accepting processes will determine the potential for a range of geochemical conditions in the rhizosphere. Measurements of detrital decomposition products (organic acid and ambient H2 concentrations) will be linked with the population dynamics of reductive dechlorination organisms in the rhizosphere. To test the second hypothesis that rhizospheric biodegradation processes are the key fate process, additional experiments will be conducted in collaboration with faculty and students at Southern University. The rate and extent of wetland plant uptake of lower chlorinated benzenes from well-characterized organic wetland sediments will be assessed. with realistic contaminant loadings (known amounts of readily available and desorption-resistant ("aged") chlorobenzene fractions) Test soils will be prepared with contaminant aging techniques developed in our previous work with these compounds. Identification of chlorobenzene sorption/desorption properties on roots will also be performed. These studies represent an extension of a working relationship developed during a previous HSRC project and will provide Southern with capabilities to perform tracer plant uptake experiments, the basis for many phytoremediation treatability studies.

Expected Results: The study will provide information on a common contaminant scenario in wetland sediments in the region. Understanding the potential for wetland bioremediation would impact a number of locations where natural recovery is a proposed solution. The expected results are that specific microbial-wetland vegetation interactions will be shown to exist, for example, the stimulation of chlorobenzene dechlorinating population in the rhizosphere of Phragmites. This information may spur development of new technologies for sediment remediation (a constructed wetland approach for confined disposal facilities, for example).