Assessment of phytoremediation potential of herbaceous and woody species with further application in brownfields

Date of Completion

January 2009

Keywords

Biogeochemistry|Agriculture, Horticulture|Environmental Sciences

Degree

Ph.D.

Abstract

Phytoextraction is defined as the use of green plants to extract toxic metals from contaminated soil. In contrast to the traditional methods of excavation and physical removal of soil, this emerging technology is recognized as a realistic, integrated, low-cost, ecologically-sound and sustainable strategy to remove contaminants. The Mukluk site in the town of Sprague, Connecticut, has been identified as a contaminated area that contains very high concentrations of lead (Pb2+) (138–82,000 mg/kg) as a result of its former use as a skeet range. The goal of this study was to determine the most promising plant species for lead phytoextraction at this site and similar sites in the Northeast region that are contaminated with high levels of lead (Pb2+). Prior to the field trials, plants were screened in greenhouse trials for their tolerance to Pb and their ability to accumulate the metal in their tissues. The initial screening included Indian mustard (Brassica juncea), blue fescue (Festuca ovina) and 6 willow species (Salix spp.). Salix demonstrated resistance to low pH and high lead accumulation compared to the herbaceous species. In parallel, using nutrient film technique (NFT) we determined critical toxicity levels of Pb based on effective concentration (EC50 ) for 5 woody and 5 herbaceous plant species that are suitable for growth in the northern U.S. Among herbaceous species, B. napus, B. juncea and B. oleracea were found to be the most efficient in uptake and accumulation of Pb in both roots and shoots. Contrary to previous reports that B. juncea and B. napus are able to accumulate Pb >1000 mg/kg, we found that these plant species did not extract sufficiently high quantities of Pb to be considered hyperaccumulators. In the hydroponic study, woody (Salix) species accumulated about 3 times more Pb in aboveground tissue than the herbaceous species. Among Salix species, the highest EC50, shoot and root biomass, tolerance index, and Pb content were recorded for S. ×61, indicating that this clone holds promise for phytoextraction of lead. In pot experiments, five Salix species were evaluated for plant resistance, metal mobility and patterns of lead translocation within the plant using field soil. Willows showed tolerance to very high soil Pb concentrations (21,360 mg/kg), and with the addition of EDTA were able to take up and translocate significant amounts of Pb into aboveground tissues. The highest aboveground Pb content was found in S. ×67 and S. eriocephala. In the field experiment, all willow species showed tolerance to heterogeneous high soil Pb concentrations. Plants were also able to take up and translocate Pb into aboveground tissues. However, lead content in plant tissues was low and no significant removal of Pb from the soil was found based on postharvest soil analysis. More field research is required to confirm the phytoextraction abilities of willows over a period of 3-4 years. ^

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