Redox properties of MX-80 and Montigel bentonite-water systems
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Redox properties of MX-80 and Montigel bentonite-water systems Cecilia Lazo,1 Ola Karnland,2 Eva-Lena Tullborg 3 and Ignasi Puigdomenech4 1 Dept. Inorg. Chem., Royal Institute of Technology, Stockholm, Sweden. 2 Clay Technology AB, IDEON Research Center, Lund, Sweden. 3 Terralogica AB, Gråbo, Sweden. 4 Swedish Nuclear Fuel and Waste Management Co. (SKB), Stockholm, Sweden. ABSTRACT The uptake of dissolved oxygen (O2) has been studied in bentonite suspensions in 0.1 M NaCl media at (25±2)°C. MX-80 and Montigel bentonites were used in concentrations varying from 18 to 73 g/L. The experiments were performed in a magnetically stirred closed glass vessel, in an N2-glove box. Redox potentials where measured with Pt-wires, and dissolved O2 was measured both with a membrane electrode and with an optode. The experiments with MX-80 show that dissolved O2 disappears in ≈5 days under these conditions. Redox potentials decreased from ≈ +500 to ≈ +125 mVSHE (versus Standard Hydrogen Electrode). The data for the Montigel bentonite show similar time scales for O 2 uptake but lower redox potentials at the end of the experiments ≈ −175 mVSHE . Pyrite oxidation is perhaps not the main process for O2 uptake, as MX-80 contains 0.3% FeS2 while Montigel bentonite only has a negligible amount. INTRODUCTION The Swedish nuclear industry is planing to dispose spent nuclear fuel encapsulated in copper canisters surrounded by bentonite buffer in granitic bedrock at ~500 m depth. The redox conditions in the bentonite buffer are important for canister corrosion and radionuclide migration. Key issues in localized copper corrosion are the chemical environment and the electrochemical potential. The bentonite buffer in the Swedish concept for a spent fuel repository will influence and control the immediate chemical surrounding and electrochemical potential for the copper canister. Some of the questions that need to be addressed in a safety assessment for such a repository are: What happens to O2 initially present in the backfill/buffer at repository closure? How long time will it take before oxygen is consumed by the buffer? What are the equilibrium redox conditions of the porewaters of bentonite clay? Could O2-rich glacial meltwaters possibly affect the integrity of the canister? Model calculations [1] assuming that pyrite oxidation will be the dominating O2consuming process in the backfill concluded that reducing conditions should be expected in a relatively short time after repository closure (between 7 and 293 years). However an experimental confirmation has been lacking. MX-80 is the bentonite used as a reference by SKB in performance assessments, repository desing, etc. The aims of this work were: a) further characterization of the MX-80 bentonite, with special emphasis on pyrite contents; and b) experimental determination of the redox potential in bentonite clay systems, and c) to study the effect of pyrite contents on the redox properties of bentonite by comparing MX-80 with Montigel bentonite, that has negligible amounts of FeS2.
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