Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/1998
Publication Date: N/A
Citation: Interpretive Summary: Heavy metal and radioactive metal (e.g. U, Pb, Zn, and Cd) contamination of soils poses serious problems to both human health and agriculture in the U.S. Current engineering-based technologies used to remediate soils (e.g., removal of top soil for storage in landfills) are quite costly, and often dramatically disturb the landscape. Recently, there has been considerable interest focused on the use of terrestrial plants to absorb heavy metals from the soil and concentrate them in the easily harvestable shoot tissues as an alternative remediation technology. However, little is currently understood about the traits that would make certain plant species the best candidates for phytoremediation of contaminated soils. There are a number of Department of Energy sites in the U.S. that are contaminated with uranium (U), as a result of weapons research, nuclear reactor operation and nuclear fuel production. The possibility that plants could be used to clean up U-contaminated soils is an intriguing possibility. However, little is known about plant U uptake and transport to the shoot. As U can exist in solution in many forms, this study investigated the relative uptake of different U forms into plants grown in hydroponic culture. It was found that only the uranyl ion, which has a plus two charge and forms in soil solution under acid conditions, was accumulated in plant tops. Thus, we studied plant U uptake from U-contaminated soil amended with different organic compounds that both acidify the soil, and can complex U and increase its solubility. It was found that the organic acid citrate greatly stimulated U accumulation in plant shoots. This study provides important new information that can be used to develop a technology to phytoremediate U contaminated soils.
Technical Abstract: Uranium (U) uptake and translocation by plants was characterized using a computer speciation model to develop a nutrient culture system that provided U as a single predominant species in solution. A hydroponic uptake study determined that at pH 5.0, the uranyl (UO22+) cation was more readily taken up and translocated by peas (Pisum sativum) than the hydroxyl and carbonate U complexes present in the solution at pH 6.0 and 8.0, respectively. A subsequent experiment tested the extent to which various monocot and dicot species take up and translocate the uranyl cation. Of the species screened, tepary bean (Phaseolus acutifolius) and red beet (Beta vulgaris) were the species showing the greatest accumulation of U. In addition to providing fundamental information regarding U uptake by plants, the results obtained also have implications for the phytoremediation of U-contaminated soils. The initial characterization of U uptake by peas suggested that in the field, a soil pH of <5.5 would be required in order to provide U in the most plant-available form. A pot study using U-contaminated soil was therefore conducted to assess the extent to which two soil amendments, HEDTA and citric acid, were capable of acidifying the soil, increasing U solubility, and enhancing U uptake by red beet. Of these two amendments, only citric acid proved effective, decreasing the soil pH to 5.0 and increasing U accumulation by a factor of 14. The results of this pot study provide a basis for the development of an effective phytoremediation strategy for U-contaminated soils.