Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2004
Publication Date: 8/25/2005
Citation: Wendling, L.A., Harsh, J.B., Palmer, C.D., Hamilton, M.A., Flury, M. 2005. Rhizosphere effects on cesium fixation sites of soils containing micaceous clays. Soil Science Society of America Journal. 69:1652-1657.
Interpretive Summary: Many sites within the United States are contaminated with radioactive waste like cesium-137. Removal of this waste by conventional methods (excavation and treatment or storage) is very costly, but plants may provide an alternative method of soil remediation as they are able to extract some radioisotopes from soil. This study investigated the potential of using plants for remediation of cesium-contaminated soils at the Idaho National Engineering and Environmental Laboratory (INEEL) by examining cesium behavior in the zone of soil influenced by plant roots. Although cesium-137 is strongly adsorbed by soil, plant roots and soil microbes were found to excrete organic compounds that enhanced desorption of cesium-137 from soil particles. Soil scientists must now determine how much cesium-137 can be removed from different soils and how much desorbed cesium can be taken up by plants before plants can be used by government agencies such as the Department of Energy or Department of Defense as tools to clean up contaminated soils.
Technical Abstract: Physical and chemical weathering processes in the rhizosphere may lead to the generation of a greater density of Cs-selective frayed edge sites (FES) on rhizosphere soil as compared to bulk soil. This study was undertaken to determine if there are significant differences between bulk and rhizosphere soils from the INEEL with respect to their ability to bind Cs. The concentration of FES on bulk and rhizosphere soils and conditional Cs/K selectivity of FES (cKexFES) were determined as a function of both soil type and initial exchanger composition. The FES concentration was significantly higher in untreated, Ca-treated, and K-treated rhizosphere soils compared to bulk soils. Sorption/desorption isotherms were obtained at Cs concentrations between 5 x 10-9 and 5 x 10-6 M. No difference in Cs sorption was observed between bulk and rhizosphere soils. The composition of the exchanging solution had the greatest effect on the magnitude of Cs desorption; significantly more Cs was desorbed in the presence of KCl than in either CaCl2 or a mixed cation soil solution. In addition, Cs desorption was greater from rhizosphere soils relative to bulk soils. Cesium selectivity with respect to both Ca and K was significantly suppressed by weathering in the rhizosphere. We conclude that enhanced weathering in the rhizosphere increased the concentration of FES, but also reduced Cs selectivity, indicating that low affinity sites were altered as well. Enhanced Cs desorption from rhizosphere INEEL soils is likely in the presence of actively growing plants and associated microorganisms.