Iowa Superfund Research Program

Research Project 5

Phytoremediation to Degrade Airborne PCB Congeners from Soil and Groundwater Sources

2010-2015 Funding Period:

Research Progress April 2008 – March 2009

The goal of Project #5 is to provide engineering research (non-biomedical) to determine whether plants can be used for the in situ bioremediation of PCB congeners from airborne sources and other sources like the proposed Confined Disposal Facility in East Chicago, Indiana. Plants can uptake PCB congeners from soil and groundwater, as well as from air. Microbes in the rhizosphere of plants, together with the plants themselves, can break-down PCBs to much less toxic or completely non-toxic products. Confined Disposal Facilities (CDFs) represent one possible source for contamination of air and for exposure of nearby human populations. To summarize, the highlights of the research to date are the following:
• Hybrid poplar plants uptake PCBs (PCB-3, 15, 28, 52, and 77) and translocate some PCB congeners to stem and leaf tissues (PCB-3 and -15)
• Transformation of PCB-77 within poplar plant roots was documented for the first time
• Some genes involved in poplar detoxification of PCBs have been identified (cytochrome monoxygenases and glutathione-S-transferase genes)
• Dechlorination in the root zone was demonstrated as a strategy toward complete degradation (mineralization) of PCBs

In the third year, we have made progress on three of four specific aims listed in the original proposal.

Specific Aim #1. To test the hypothesis that plants can uptake and transform lower-chlorinated PCB congeners to much less toxic residuals. This aim is essentially completed. It represents a “proof-of-concept” for phytoremediation of selected airborne PCB congeners. We selected lesser-chlorinated congeners that were present in Chicago air and which were of interest from a toxicological or biodegradation standpoint. PCB congeners 3, 15, 28, 52, and 77 were chosen and PCB-14 was utilized as an internal standard. PCB-3 and -15 were uptaken by roots of hybrid poplar, the species of choice for phytoremediation of the CDF, and a small fraction was translocated to plant stems and leaves. PCB-28, -52, and -77 were uptaken but not translocated appreciably; they did, however, sorb to the bark of the poplar plant from the gas phase. A paper was published on the phenomenon in Chemosphere (J. Liu and J.L. Schnoor, “Uptake and translocation of lesser-chlorinated polychlorinated biphenyls (PCBs) in whole hybrid poplar plants after hydroponic exposure”, Chemosphere (2008), doi:10.1016/j.chemosphere.2008.08.009).

Earlier, we determined that cytochrome P450 genes and glutathione-S-transferase genes were involved in the detoxification of PCB congeners by hybrid poplar. These results indicate that glutathione-S-transferase GST173 may catalyze the conjugation of hydroxy- and di-hydroxy PCBs in the metabolism and detoxification of these congeners. In the third year, we proved that PCB-77 could be metabolized by plants to the 6-OH-PCB77, a metabolite different than those reported for mammals and of relatively unknown toxicity. This is the first report of metabolism of PCB-congeners inside plants, and a paper is being prepared on the novel finding. Research will continue on this metabolite to determine its ultimate fate in the plant-soil system.

Specific Aim #2. To test the hypothesis bacteria in the rhizosphere of plants can reductively dechlorinate higher PCB congeners and can mineralize resulting intermediates under oxidizing conditions. In the progress on metabolism of PCB-77 by plants, we also detected dechlorination of PCB-77 in the root zone of the plants under hydroponic exposure. Reductive dechlorination, under low oxygen conditions, produced a variety of lesser-chlorinated intermediates; some with only one remaining chlorine atom like PCB-3 were detected. In addition, we have used terminal-restriction fragment length polymorphism (T-RFLP) to identify a number of soil organisms capable or degrading (mineralizing) PCB congeners. We have not performed this experiment in soils from the site yet, and we have not detected complete mineralization so far. But future experiments will concentrate on the sequential cycling of redox conditions (through manipulation of soil moisture) to determine if complete mineralization to innocuous products is possible (see specific aim #3).

Specific Aim #3. To test the hypothesis that phytoremediation will allow for significant reductions in the airborne transfer of PCBs from waste disposal sites and will mitigate exposure to humans and ecosystems. Research on this specific aim is on-going using real soils and potted plants. In addition, we will use a proteomic approach to identify and quantify the expression of genes induced in response to PCB metabolites in selected bacteria and plant strains, so that we can understand the mechanism of PCB-detoxification.

Specific Aim #4. To test the hypothesis that residues of PCB transformation products in plant tissues are non-toxic or of greatly reduced toxicity. We have detected the intermediate 6-OH-PCB77 and we are continuing to assess its relative toxicity in plants and to other organisms. We have utilized two assays so far (OECD acute toxicity testing with earthworms E. fetida and nematodes C. elegans). PCB parent congeners and plant residue products are not toxic in these assays, and not significantly different than controls. However, in soil microcosms, we determined that 4’-OH-PCB35 was highly specific in its growth inhibition of E. Coli bacteria.

Project Leader: Jerry L. Schnoor, PhD
Dr. Schnoor will manage the project and guide the research as the Principal Investigator (PI). He has managed over $25 million of research projects since 1980, and has considerable experience as the Editor-in-Chief of Environmental Science and Technology, and serves as the Chair of the EPA-ORD Board of Scientific Counselors. Dr. Schnoor is an international leader in the field of phytoremediation, co-editor of the book Phytoremediation - Transformation and Control of Contaminants (2003), and is the PI of the W.M. Keck PhytoTechnologies Laboratory at the University of Iowa.

Co-Project Leader: Benoit Van Aken, PhD
Dr. Van Aken, the University of Iowa, leads our effort in the laboratory on molecular biological methods for analysis of catabolic enzymes and metabolic pathways of plants and microorganisms in phytoremediation. He has a background in environmental biotechnology and has published several key papers in the area of metabolite identification and enzymatic pathways. Dr. Van Aken led the discovery of a new endosymbiotic bacteria living inside hybrid poplar trees which mineralizes nitramine explosive compounds, Methylobacteriumpopuli. This research was published in the International Journal of Systematics Evolutionary Microbiology and Applied and Environmental Microbiology

Timothy Mattes, PhD
Associate Professor, Civil and Environmental Engineering, The University of Iowa

Hans J. Lehmler, PhD
Associate Professor (Research), College of Public Health, Department of Occupational and Environmental Health, The University of Iowa

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