Reactive barriers for Cs-137 retention

Cs-137 was dispersed globally by cold war activities and, more recently, by the Chernobyl accident. Engineered extraction of Cs-137 from soils and gro undwaters is exceedingly difficult. Cs-137 is only 30.2 years, remediation might be more effective land less Because the half-life of costly) if Cs-13 7 bioavailability could be demonstrably limited for even a few decades by u se of a reactive barrier. Essentially permanent isolation must be demonstra ted in those few settings where high nuclear level wastes contaminated the environment with Cs-135 (half-life 2.3 x 10(-6) years) in Cs-137. Clays are potentially a low-cost barrier to Cs movement, though their long-term addi tion to effectiveness remains untested. To identify optimal clays for Cs re tention, Cs desorption was measured for five common clays: Wyoming Montmori llonite (SWy-1), Georgia Kaolinites (KGa-1 and KGa-2), Fithian Illite (F-II I), and K-Metabentonite (K-Mbt). Exchange sites were pre-saturated with 0.1 6 M CsCl for 14 days and readily exchangeable Cs was removed by a series of LiNO3 and LiCl washes. Washed clays were then placed into dialysis bags an d the Cs release to the deionized water outside the bags measured. Release rates from 75 to 139 days for SWy-1, K-Mbt and F-III were similar; 0.017% t o 0.021% sorbed Cs released per day. Both kaolinites released Cs more rapid ly (0.12% to 0.05% of the sorbed Cs per day). In a second set of experiment s, clays were Cs-doped for 110 days and subjected to an extreme and prolong ed rinsing process. All the clays exhibited some capacity for irreversible Cs uptake. However, the residual loading was greatest on K-Mbt (similar to 0.33 wt.% Cs). Thus, this clay would be the optimal material for constructi ng artifical reactive barriers. (C) 2001 Elsevier Science B.V. All rights r eserved.