Speciation by XANES of copper migrated into compacted bentonite using electromigration techniques

Copper is a promising overpack material for deep underground nuclear waste disposal, although it is susceptible to corrosion following closure of the repository, and subsequent migration of the corrosion products through the buffer material may affect the migration of redox-sensitive radionuclides. Electromigration experiments were performed in which a copper coupon in contact with compacted bentonite served as the working electrode and was held at a constant potential of + 300 mV vs. Ag/AgCl for up to 48 h. The chemical state of the copper that migrated into the bentonite specimens was investigated by copper K-edge X-ray absorption near-edge structure measurements at the Saga Light Source, Japan. The results confirmed that the oxidation state of copper in the bentonite was copper(II), as indicated by the correlation between the amount of copper migrated into bentonite specimens and the corrosion current. Copper(I) was also observed on the copper coupon, but not in the bentonite samples. The obtained results indicate that the copper initially dissolves as copper(II), which then reacts with the copper metal and precipitates as Cu₂O.