Migration behavior of alkali earth ions in compacted bentonite with iron corrosion product using electrochemical method

Carbon steel overpack will corrode by consuming oxygen introduced during repository construction after closure of repository, that will keep the environment in the vicinity of repository reducing. The iron corrosion products can migrate in bentonite as ferrous cations (Fe2+) through the interlay er of montmorillonite replacing the exchangeable sodium ions in the interlayer. This replacement of sodium may affect the migration behavior in the altered bentonite not only for redox-sensitive elements but also the other ions. Therefore we have carried out electrochemical analysis, of calcium, strontium or barium with the ferrous ion supplied by anodic corrosion of iron coupons in compacted bentonite. Fifteen micro liters of tracer solution containing 8.6 M of CaCl ₂ or 3.0 M of SrCl₂ or 1.5 M BaCl₂ were spiked on the interface between the iron coupon and bentonite, for which the dry density was in the range of 1.4 to 1.5 Mg/m³, before assembling. The iron coupons were connected as working electrodes to the potentiostat and held at a constant supplied potential between - 500 to +300 mV (vs. Ag/AgCl reference electrode) for up to 7 days. Calcium and strontium could migrate faster and deeper into the bentonite than iron in each condition, while barium could migrate slower than iron. A model using dispersion and electromigration can explain the measured profiles in the bentonite specimens. The fitted value of electromigration velocity was a function of applied electrical potential and 10 to 23 nm/s for calcium, 11 to 19 for strontium, around 4 nm/s for barium and 5 to 15 nm/s for iron, respectively. Alternatively, the fitted value of the dispersion coefficient was not a function of applied potential, and the values were 3-8 × 10^-12m²/s for calcium, 2-4 × 10^-12 m²/s for strontium, 5-10×10 ^-12m²/s for barium and 3-9×10^-12m ²/s for iron, respectively.