Phytoremediation of Radionuclide-Contaminated Soils

Northwestern New Mexico has many abandoned uranium mine sites, some with high levels of radionuclide contamination, such as uranium and radium. Contamination is generally highest around former ore mills and mine tailings, with addition spread from water and wind erosion of the soils.

To remediate the soil of one such mine site with moderate levels of contamination, a series of greenhouse tests were conducted using soil from the site. Three growth studies were completed: two to determine which plants grew the best in the soil and which growth inputs were needed (fertilizer, irrigation, etc.) and one to perform a radionuclide mass balance. To facilitate proper drainage, the plants were placed in a 1:1 volume mixture of inert sand and soil. The three selected plant varieties were guar (Cyamopsis tetragonoloba var. Kinman), sunflower (Helianthus annuus var. Cobalt 2), and hemp (Cannabis sativa var. Samurai.) Seeds were planted directly into the soil and allowed to grow until the plants reached maturity and both flowers and seeds presented. Aboveground biomass and seeds were harvested for analysis. Plants were washed using deionized water to remove residual soil. Plant biomass was combusted in a muffle furnace and the ash was acid-digested for total metals analysis. The seeds were processed in an oil press to separate out an oil fraction that was also acid digested. Metals, including uranium and radium, were quantified using a combination of inductively-coupled plasma optical emission spectroscopy (ICP-OES), inductively-coupled plasma mass spectrometry (ICP-MS), or multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS.)

Mass balances from the greenhouse study were used to estimate the time required for the concentration of radionuclides to be reduced to below levels required by the U.S. EPA. Results indicate that the three plant species do uptake radionuclides and fractionate the metals into the non-oil components of the plant. Soil remediation timescales are on the order of centuries; however, other benefits of phytoremediation, such as soil cover and value-added use of the biomass for renewable energy may justify continued field development.