Rhizosphere effects on cesium fixation sites of soil containing micaceous clays

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 with bulk soil. This study was undertaken to determine if there are significant differences between bulk and rhizosphere soils from the Idaho National Laboratory (INL) with respect to their ability to bind Cs. The capacity of FES on bulk and rhizosphere soil materials and conditional Cs/K selectivity of FES (^cK_ex^FES) were determined as a function of both soil type and initial exchanger composition. The FES capacity was significantly higher in untreated, Ca-saturated, and K-saturated rhizosphere sol materials as compared with bulk soil materials. Sorption-desorption isotherms were obtained at Cs concentrations between 5 × 10^-9 and 5 × 10^-6 M. No difference in Cs sorption was observed between bulk and rhizosphere soil materials. 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 soil materials relative to bulk soil materials. 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 Cs sorption capacity of FES, but also reduced Cs selectivity on these sites. Enhanced Cs desorption from rhizosphere INL soils is likely in the presence of actively growing plants and associated microorganisms.