Sorption of Cs(I) on Fe-montmorillonite relevant to geological disposal of HLW

摘要

Bentonite is the candidate buffer and backfill material in the deep geological repositories. Montmorillonite (Mt) is the major clay minerals of bentonite. Over a long period of time, the interaction of corrosion products from overpack and/or cansister with clay minerals is expected to result in formation of Fe(III)-Mt, a plausible alterated product. In this context, it is important to understand the properties of Fe-Mt in comparison with original clay mineral, Na-Mt. In the present study, sorption behavior of Cs(I), long lived fission product (Cs-135, t(1/2) =2.3 x10(6) years; Cs-1(37), t(1/2) =30.1 years) with high fission yield, on Fe(III)-Mt is investigated. Batch sorption studies are conducted at varying pH (3-9), ionic strength (0.001-1 M) and Cs(I) concentration (10(-10) to 0.05 M). The distribution coefficient (K-d) of Cs(I) on Fe(III)-Mt was found to be independent of pH except at low pH, indicating ion exchange mechanism as dominant interaction mode for Cs(I). It was further verified by ionic strength variation which depicted decrease in Cs(I) sorption with increasing ionic strength. Adsorption isotherm of Cs(I) was found to be linear over the concentration range of 10(-10) to 10(-3) M Cs(I). The Fe released from Fe(III)-Mt during the Cs(I) sorption was found to be not more than 0.2 ppm. However, on lowering the pH and increasing the ionic strength, the Fe release increased. Furthermore, the apparent diffusion coefficient for Cs(I) in Fe(III)-Mt has been determined.