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Migration Behavior of Alkali Earth Ions in Compacted Bentonite With Iron Corrosion Product Using Electrochemical Method

Published online by Cambridge University Press:  01 February 2011

Kazuya Idemitsu ,
Daisuke Akiyama ,
Akira Eto ,
Yoshihiko Matsuki ,
Yaohiro Inagaki  and
Tatsumi Arima
Kazuya Idemitsu
Affiliation:
[email protected], Kyushu university, Applied Quantum Physics and Nuclear Engineering, Fukuoka, Japan
Daisuke Akiyama
Affiliation:
[email protected], Kyushu university, Applied Quantum Physics and Nuclear Engineering, Fukuoka, Japan
Akira Eto
Affiliation:
[email protected], Kyushu university, Applied Quantum Physics and Nuclear Engineering, Fukuoka, Japan
Yoshihiko Matsuki
Affiliation:
[email protected], Kyushu university, Applied Quantum Physics and Nuclear Engineering, Fukuoka, Japan
Yaohiro Inagaki
Affiliation:
[email protected], Kyushu university, Applied Quantum Physics and Nuclear Engineering, Fukuoka, Japan
Tatsumi Arima
Affiliation:
[email protected], Kyushu university, Applied Quantum Physics and Nuclear Engineering, Fukuoka, Japan
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Abstract

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 interlayer 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 CaCl2 or 3.0 M of SrCl2 or 1.5 M BaCl2 were sspiked 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 × 10-12 m2/s for calcium, 2 - 4 × 10-12 m2/s for strontium, 5 - 10 × 10-12m2/s for barium and 3 - 9 × 10-12 m2/s for iron, respectively.

Keywords

electromigrationdiffusionCa
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1265: Symposium AA – Scientific Basis for Nuclear Waste Management XXXIV , 2010 , 1265-AA06-07
Copyright
Copyright © Materials Research Society 2010

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References

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