Reversible ion-exchange fixation of cesium-137 leading to mobilization from reservoir sediments

Abstract

The radioactive fission product, 137Cs, has been observed to mobilize from bottom sediments of two South Carolina reservoirs during summer thermal stratification and hypolimnetic anoxia. Mobilization is attributed to ion-exchange displacement of 137Cs from sediments by cations such as NH + 4, Fe +2 and Mn +2 released under anaerobic conditions. Three types of 137Cs binding sites to sediment clay minerals are identified: 1) surface and planar sites from which 137Cs is generally exchangeable by all cations studied (Na +, NH + 4, H +, Cs +, Ca +2, Mg +2, Fe +2, and Mn +2); 2) wedge sites where 137Cs exchange is sterically limited to cations of similar size and charge (NH + 4, Cs +, K +, and perhaps H 3O +); 3) interlayer sites from which 137Cs is not readily exchanged. More than 15 years after final 137Cs inputs, the reservoir sediments we studied showed the following percentage distribution of sites: 2 to 9% surface sites, 6 to 13% wedge sites, and 78 to 85% interlayer sites. In contrast, lake and stream sediments near Oak Ridge, Tennessee receiving 137Cs inputs more than 20 years earlier had greater than 99% of their 137Cs associated with non-exchangeable interlayer sites. The difference is attributed to the paucity in the South Carolina sediments of weathered micaceous clay minerals with their abundant interlayer sites. Such interlayer deficient clays are dominant in the Atlantic and Gulf coastal plains of the United States and elsewhere. This suggests that 137Cs will be physically and chemically more mobile in such areas as well as more biologically available. Mobility will be enhanced in regimes where cation inputs favoring 137Cs exchange occur. Subsurface waste disposal sites where anaerobic conditions develop with NH + 4 production and Fe +2 and Mn +2 release might be such a regime.