2003 Progress Report: Phytoremediation in Wetlands and CDFs

EPA Grant Number: R828773C003
Subproject: this is subproject number 003 , established and managed by the Center Director under grant R828773
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: HSRC (2001) - South and Southwest HSRC
Center Director: Reible, Danny D.
Title: Phytoremediation in Wetlands and CDFs
Investigators: Pardue, J. , Moe, William
Institution: Louisiana State University - Baton Rouge
EPA Project Officer: Lasat, Mitch
Project Period: October 1, 2001 through September 30, 2006 (Extended to September 30, 2007)
Project Period Covered by this Report: October 1, 2002 through September 30, 2003
Project Amount: Refer to main center abstract for funding details.
RFA: Hazardous Substance Research Centers - HSRC (2001) Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management


The overall objective of this research project is to develop a scientific basis for a plant-based remedial approach for sediments contaminated with chlorinated organic compounds.

Hydrophobic chlorinated organics are common sediment contaminants that pose a threat to sensitive receptors. These compounds often 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 the degradation of chlorinated organics primarily via rhizospheric biodegradation processes; principally, reductive dechlorination. Previously, it was widely considered that the rhizosphere was primarily an aerobic environment because of the leakage of O2 from the aerenchymal tissues of vegetation. However, recent studies have demonstrated that the root surface is an area of intense methanogenesis, and O2 leakage only occurs at well-defined locations along the root (i.e., the root tip). Therefore, the possibility exists that a specific microbial-plant interaction exists that can be exploited to better remediate sediments.

Based on this rationale, two hypotheses are considered:

• Reductive dechlorination is enhanced in vegetated sediments because the root surface serves as a location of enhanced activities of dehalorespiring and other degrading microbial populations.

• By vegetating sediment contaminated with chlorinated organic compounds, below-ground root matter will serve as a source of H2, overcoming redox potential limitations in sediments.

The specific objectives of this research project are as follows:

1. We will define the biodegradation potential of chlorobenzenes and chlorinated solvents by quantifying biogeochemical conditions in the rhizosphere. Key conditions include the specific detrital decomposition products (organic acids and hydrogen) and microbial populations that develop on and adjacent to the plant root.

2. We will define other potential fate mechanisms: plant uptake and volatilization by studying the dynamics of plant uptake of chlorobenzenes in wetland sediments.

Progress Summary:


Vegetated sediment core and serum bottle microcosm studies have been utilized to investigate the dechlorination of chlorinated benzenes and ethenes in sediments. The methodology includes the measurements of detrital decomposition products (organic acids and ambient H2 concentrations), parent and daughter dechlorination products, and microbial populations using molecular techniques such as denaturing gradient gel electrophoresis (DGGE) and real-time and qualitative polymerase chain reaction (PCR) techniques.

Year One

• Laboratory microcosm studies conducted during the first year of the study established the kinetics of tetrachlorobenzene dechlorination in sediments with a range of organic matter content. In addition, the studies identified the role of H2 as an electron donor, the expected daughter products of dechlorination, and the relative role of methanogens in dechlorination. Results indicated that the ability to dechlorinate tetrachlorobenzene is widespread in sediments. 1,2- and 1,3-dichlorobenzene readily forms, and, subsequently, these are dechlorinated to chlorobenzene and benzene.

• Microcosm studies were conducted with root material from Phragmites communis (common reed) and Typha latifolia (cat tail). In sediments, amendments of fresh-root material increased dechlorination rates in direct proportion to the amount of root matter added. Ambient H2 and methane also increased. Root turnover was, therefore, identified as a potential driver for enhancing reductive dechlorination.

• A factorial core study experiment also was performed to examine dechlorination and microbial interactions in a more realistic setting. Sediment from the PPI site was spiked with 1,2,3,4-tetrachlorobenzene, and T. latifolia and P. communis were grown in the cores. Results demonstrated that dechlorination of 1,2,3,4-tetrachlorobenzene was observed throughout the core, but more complete dechlorination was observed near the root.

Year Two

• Characterization of microbial communities dechlorinating tetrachlorobenzene revealed similarities and differences in microbial populations across a range of sediment types. DGGE revealed differences in the structure of eubacterial and archae populations. Primer-based detection of 16s rDNA genes demonstrated that dehalorespiring Dehalococcoides populations were present in every sediment type. Certain types of these organisms have been found to link dechlorination of chlorobenzenes with the production of energy.

• Microcosm experiments revealed that measurement of H2 concentrations coupled with methane concentrations could effectively identify dehalorespiring microbial activity against a background of other H2-utlizing bacteria, such as methanogens, in sediments. The method effectively identified complete dechlorination of cis-1,2-dichloroethene in wetland sediments via dehalorespiration, and complete dechlorination of 1,2-dichloroethane was identified as cometabolic, either by dehalorespirers or methanogens.

• A real-time PCR method was developed for measurement of Dehalococcoides sp. and archae bacteria on the root surface and in bulk sediment. Methods were tested on vegetated cores that have been exposed to chloroethenes for more than 2 years. Method development for several type II methanotrophs is progressing.

Summary of Results to Date

• Demonstrated the stimulatory effects of root matter on reductive dechlorination of chlorobenzenes.

• Showed that hydrogen measurements coupled with methane measurements could identify dehalorespiration in a complex matrix of anaerobic microbial processes.

• Demonstrated rapid and complete dechlorination of chlorobenzenes and chloroethenes in planted sediments.

Year 1 of the project was utilized to perform some basic experiments on the effect of vegetation on degradation of chlorobenzenes and chloroethenes. These experiments identified some interesting trends. Freshly added root matter stimulated the reductive dechlorination of chlorobenzenes, enhancing H2 production and methanogenesis. In sediments spiked with chlorobenzenes that were planted, more rapid and complete dechlorination of chlorobenzenes was observed in sediments near the root versus the bulk soil. In upflow core experiments and microcosm studies, rapid dechlorination of chloroethenes and ethanes was observed in the rhizosphere and appeared to be linked with large populations of dehalorespiring bacteria. Although these studies suggested a role for vegetation, our existing techniques lacked the resolution to determine the nature of the interaction. It was unclear whether vegetation simply provided a source of H2 for dehalorespiring organisms in the soil via root exudates or root turnover, or whether the roots provided important intensely surfaces for dehalorespiring bacteria to grow.

Based on suggestions from the Scientific Advisory Committee to begin method development of higher resolution, quantitative techniques were performed, including real-time PCR techniques that could more directly quantify the population size of dehalorespiring and methanogenic populations. This will allow us to better separate indirect vegetation effects from direct changes in microbial populations in the rhizosphere and on the root surface. Other indirect methods also were developed to identify activities of dehalorespirers based on known H2 thresholds for different anaerobic H2-utilizing processes.

Future Activities:

The future activities of this research project will combine techniques developed in Year 2 of the project with the basic experimental setups from Year 1 of the project. Two sets of studies are proposed.

• A factorial core study will be performed with several treatments: sediments vegetated with Typha, sediments vegetated with Phragmites, and unvegetated sediments. The sediment is mineral-dominated and prepared with known quantities of labile and desorption-resistant tetrachlorobenzene. Cores will be sacrificed, and roots and bulk soil will be separated by careful sectioning and sieving. Bulk soil and root tissue will be analyzed for parent tetrachlorobenzene and daughter dechlorination products. DNA extracts of bulk soil and root matter will be performed, followed by the application of several molecular techniques, including DGGE and real-time PCR of Dehalococcoides and methanogens. The extent of dechlorination will be compared with the relative size of labile and desorption-resistant pools.

• A supporting set of microcosm studies also will be performed. Replicate cores will be used to prepare microcosms prepared separately with bulk soil and root matter. Repeated additions of chlorobenzenes will be made, and daughter products such as H2 and methane will be monitored over time. Relative magnitude of chlorobenzene dechlorination will be compared between the bulk soil and root material and various measures of the population size and composition (DGGE and real-time PCR).

The expected results of the studies proposed above would be a better understanding of the scientific basis behind the use of vegetation to remediate sediments contaminated with chlorinated benzenes, polychlorinated biphenyls, and dioxins. If the live root surface serves as a locus for higher reductive dechlorination, vegetating sediments in combined disposal facilities with species with dense root mats may serve as a passive, but effective, approach to remediating portion of the bed and minimizing flux.

Journal Articles on this Report : 2 Displayed | Download in RIS Format

Other subproject views: All 7 publications 3 publications in selected types All 3 journal articles
Other center views: All 279 publications 92 publications in selected types All 63 journal articles
Type Citation Sub Project Document Sources
Journal Article Kassenga G, Pardue JH, Moe WM, Bowman KS. Hydrogen thresholds as indicators of dehalorespiration in constructed treatment wetlands. Environmental Science & Technology 2004;38(4):1024-1030. R828773 (2004)
R828773 (Final)
R828773C003 (2003)
  • Abstract from PubMed
  • Abstract: ACS-Abstract
  • Journal Article Lee S, Pardue JH, Moe WM, Valsaraj KT. Mineralization of desorption-resistant 1,4-dichlorobenzene in wetland soils. Environmental Toxicology and Chemistry 2003;22(10):2312-2322. R828773 (2004)
    R828773 (Final)
    R828773C003 (2003)
  • Abstract from PubMed
  • Abstract: Wiley-Abstract
  • Supplemental Keywords:

    marshes, natural attenuation, wetlands, waste, water, analytical chemistry, environmental microbiology, microbiology, molecular biology, genetics, bioremediation, contaminated sediments, hazardous, hazardous waste, hexachlorobenzene, bioavailability, biochemistry, biodegradation, bioremediation of soils, chlorinated organics, contaminants in soil, contaminated sediment, contaminated soil, contaminated soils, degradation, microbial degradation, natural recovery, phytoremediation, wetland sediments., RFA, Scientific Discipline, Waste, Water, Contaminated Sediments, Microbiology, Analytical Chemistry, Environmental Microbiology, Hazardous Waste, Bioremediation, Ecology and Ecosystems, Molecular Biology/Genetics, Hazardous, degradation, Hexachlorobenzene, microbial degradation, wetland vegetation, bioavailability, biodegradation, contaminated sediment, contaminated soil, contaminants in soil, bioremediation of soils, natural recovery, wetland sediments, biochemistry, chlorinated organics, phytoremediation

    Relevant Websites:

    http://www.hsrc.org Exit
    http://www.sediments.org Exit

    Progress and Final Reports:

    Original Abstract
  • 2002 Progress Report
  • 2004 Progress Report
  • 2005
  • 2006
  • Final

  • Main Center Abstract and Reports:

    R828773    HSRC (2001) - South and Southwest HSRC

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R828773C001 Bioturbation and Bioavailability of Residual, Desorption-Resistant Contaminants
    R828773C002 In-Situ Containment and Treatment of Contaminated Sediments: Engineering Cap Integrity and Reactivity
    R828773C003 Phytoremediation in Wetlands and CDFs
    R828773C004 Contaminant Release During Removal and Resuspension
    R828773C005 HSRC Technology Transfer, Training, and Outreach