Phytoremediation of uranium contaminated soils: Role of organic acids in triggering uranium hyperaccumulation in plants

Uranium phytoextraction, the use of plants to extract U from contaminated soils, is an emerging technology. We report on the development of this technology for the cleanup of U-contaminated soils. In this research, we investigated the effects of various soil amendments on U desorption from soil to soil solution, studied the physiological characteristics of U uptake and accumulation in plants, and developed techniques to trigger U hyperaccumulation in plants. A key to the success of U phytoextraction is to increase soil U availability to plants. We have found that some organic acids can be added to soils to increase U desorption from soil to soil solution and to trigger a rapid U accumulation in plants. Of the organic acids (acetic acid, citric acid, and malic acid)tested, citric acid was the most effective in enhancing U accumulation in plants. Shoot U concentrations of Brassica juncea and Brassica chinensis grown in a U-contaminated soil (total soil U, 750 mg kg(-1)) increased from less than 5 mg kg(-1) to more than 5000 mg kg(-1) in citric acid-treated soils. To our knowledge, this is the highest shoot U concentration reported far plants grown on U-contaminated soils. Using this U hyperaccumulation technique, we are now able to increase U accumulation in shoots of selected plant species grown in two U-contaminated soils (total soil U, 280 and 750 mg kg(-1)) by more than 1000-fold within a few days. Our results suggest that U phytoextraction may provide an environmentally friendly alternative for the cleanup of U-contaminated soils.