About the possible upgrade of bentonite with respect to iodide retention capacity

Abstract

Compacted bentonites are currently investigated as geotechnical barrier in high level radioactive waste (HLRW) repositories due to their low permeability, high swelling pressure, and cation adsorption capacity. In contrast, anions are repelled by the negatively charged smectite surfaces due to electrostatic forces. Therefore anion diffusion through bentonites is particularly slow (‘anion exclusion’). However, in case of crack formation bentonites do not possess anion retention capacity. In this case anions, neutral or negatively charged molecules or colloidal particles can pass the technical barrier faster. One of the hazardous anions is 129I − which is particularly dangerous due to its long half life, mobility, and biological relevance. In this context several studies concerning iodide adsorption by a variety of solids are available. The selection of an applicable bentonite additive for the improvement of iodide retention capacity of HLRW technical barriers is still subject to discussion. The optimum bentonite additive should possess high selectivity towards iodide, thermal stability, should not affect bentonite properties, and should be inexpensive. In this study we compare different potential iodide adsorbents using standardized conditions since it is well known that experimental parameters (concentration, time, precursor material, solid liquid ratio…) significantly influence the results of adsorption experiments. Therefore, only the application of standardized conditions allow for the direct comparison of a set of potential I − adsorber materials. Obtained results indicate that organoclays (bentonites modified by organic surfactants) are supposed to represent the most suitable bentonite additive with respect to the upgrade of iodide retention capacity. Additionally, we could show that the addition of 5 wt.% hexadecylpyridinium treated bentonite did not affect the swelling pressure of compacted MX80 bentonite. However, long term tests and/or natural analogue studies have to be performed in order to be able to answer the open questions.