Migration Behavior of Selenium in the Presence of Iron in Bentonite
- PDF / 462,277 Bytes
- 7 Pages / 432 x 648 pts Page_size
- 36 Downloads / 179 Views
Migration Behavior of Selenium in the Presence of Iron in Bentonite Kazuya Idemitsu, Hikaru Kozaki, Daisuke Akiyama, Masanao Kishimoto, Masaru Yuhara, Noriyuki Maeda, Yaohiro Inagaki and Tatsumi Arima Dept. of Applied Quantum Physics and Nuclear Engineering, Kyushu Univ., Fukuoka, Japan ABSTRACT Selenium (Se) is an important element for assessing the safety of high-level waste disposal. Se is redox-sensitive, and its oxidation state varies from -2 to 6 depending on the redox conditions and pH of the solution. Large quantities of ferrous ions formed in bentonite due to corrosion of carbon steel overpack after the closure of a repository are expected to maintain a reducing environment near the repository. Therefore, the migration behavior of Se in the presence of Fe in bentonite was investigated by electrochemical experiments. Na2SeO3 solution was used as tracer solution. Dry density range of bentonite was from 0.8 to 1.4 ×103 kg/m3. Results indicated that Se was strongly retained by the processes such as precipitation reaction with ferrous ions in bentonite. Se K-edge X-ray absorption near-edge structure (XANES) measurements were performed at the BL-11 beamline at SAGA Light Source, and the results revealed that the oxidation state of Se in the bentonite remained Se(IV). INTRODUCTION Selenium-79 (79Se ) has a long half-life of 2.95×105 years and is one of the key radionuclides for examining the safety of high-level waste disposals [1]. In aqueous solution, Se is present mainly in anionic form, therefore is expected to be less affected by sorption than cations in the natural and engineered barriers. Se is redox-sensitive, and its oxidation state varies from -2 to 6 depending on the redox conditions and the pH of the solution. Carbon steel is a candidate overpack material for high-level waste disposal. In Japan, this material is expected to ensure complete containment of vitrified waste glass during an initial period of 1000 years [2]. Thus, large quantities of ferrous ions formed in bentonite by corrosion of carbon steel overpack after closure of a repository are expected to maintain a reducing environment in the vicinity of the repository. Therefore, the migration behavior of Se in the presence of Fe in bentonite was investigated by electrochemical experiments in this study. EXPERIMENTAL Materials A typical Japanese sodium bentonite, Kunipia-F, was used in this study. It contains approximately 99 wt % montmorillonite. The chemical composition of Kunipia-F is shown elsewhere [3]. The chemical formula of Kunipia-F is estimated to be (Na0.3Ca0.03K0.004)(Al1.6Mg0.3Fe0.1)Si4O10(OH)2. Bentonite powder was compacted into cylinders with a diameter of 10 mm and a height of 10 mm with a dry density of around 0.8 to 1.4 ×103
157
kg/m3. Each cylinder was inserted in an acrylic resin column and saturated with a 0.01 M NaCl solution for 30 days. Electrochemical and Diffusion Experiments An iron coupon was assembled with bentonite that was saturated by immersion in a contact solution of 0.01 M NaCl in an apparatus for electroche
Data Loading...
