Effect of gamma-irradiation on the surface and adsorption properties of bentonite clay

Deep geological repository (DGR) is the best solution for confining high-level radioactive waste (HLRW). DGRs are multibarrier systems, where a metallic container of carbon steel or other metals is surrounded by a compacted bentonite barrier confining the waste, and placed in adequate geological formation. The purpose of DGRs is to delay the migration of radionuclides (RN) into the biosphere until the radiation was inoffensive.

Bentonite is a clay with high stability. Its durability depends on the physical, chemical, thermal and mechanical conditions of the repository and may be affected by the radiation emitted by the waste.

The objective of this research is to compare the surface properties of gamma-irradiated and unirradiated bentonite (Ca-Mg FEBEX bentonite) to evaluate if the irradiation from the radioactive waste encapsulated in the metallic container modifies.

To evaluate the effect of the accumulated dose, the bentonite was subjected to two rounds of irradiations during 9 days with a ⁶⁰Co source in pool like installation achieving cumulative gamma doses of 14 kGy and 140 kGy.

The structural characteristics of irradiated and unirradiated bentonite samples are analysed by infrared spectroscopy [1] to identify changes in functional groups of the clays. Zeta potential measurements as well as potentiometric titrations to obtain information on the density of SOH hydroxyl surface groups on the edges of clay particles. All these properties are essential for the adsorption capacity of the material.

Lastly, the sorption capacity of irradiated bentonite was evaluated by measuring Sr and Se sorption isotherms at fixed pH and ionic strength conditions, representative elements with different sorption mechanism on bentonite, to be compared to the unirradiated bentonite [2], [3].

Results showed that the analysed properties were slightly affected by gamma-irradiation in the doses investigated.

 

 

ACKNOWLEDGEMENTS: The authors would like to thank the finance of this research to the CIEMAT predoctoral fellowship (209-PRECIE-PDE22) through the European Union Project, EURAD-WP ConCorD, Grant Agreement no. 847593.

 

REFERENCES

 

[1] Madejova, J., Komadel, P. 2001. Baseline studies of the clay minerals society source clays: infrared methods. Clays and clay minerals, 49(5), 410-432.

[2] Mayordomo, N., Alonso, U., & Missana, T. (2016). Analysis of the improvement of selenite retention in smectite by adding alumina nanoparticles. Science of The Total Environment, 572, 1025-1032.

[3] Missana, T., Garcia-Gutierrez, M., & Alonso, U. (2008). Sorption of strontium onto illite/smectite mixed clays. Physics and Chemistry of the Earth, Parts A/B/C, 33, S156-S162.