Factors affecting the sorption and fixation of cesium in acid organic soil

Radioactive caesium deposited on upland Britain following the Chernobyl accident in 1986 remains available for uptake by plants, despite the potential of the contaminated soils to fix Cs. The minimum concentration of Cs+ required to cause Cs+ fixation is 0.60 to 0.75 mM, and this is unlikely to be reached in any contaminated upland soil. It is suggested that the fixation is caused by interlayer collapse of the illitic clay. The observed Cs+ fixation in lowland mineral soils and its absence from acidic upland soils is explained by the action of K+ ions, which can also induce interlayer collapse. Although Cs+ ions are unlikely to be fixed in acid organic soils, they can be strongly sorbed on any unoccupied Cs-specific sorption sites in the narrow parts of illitic wedge zones. A method for determining the number of such sites is described. For two of the soils studied the number of sites ranged between 8x10 (-8) and 1x10 (-5) mmol kg(-1); for two others there appeared to be no unoccupied Cs-specific sites. Although Cs+ ions sorbed on such sites do not exchange with other ions, they can be desorbed if the concentration of Cs+ in solution is decreased. Thus, radioactive Cs in such soils will remain available for plant uptake, unless interlayer collapse can be induced. (UK)