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Remobilization of U and Se reductively immobilized on iron oxide surfaces
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.ORCID iD: 0000-0003-0663-0751
(English)Manuscript (preprint) (Other academic)
Abstract [en]

 

 

 

 

 

 

 

In this work we have studied the possible remobilization of U and Se reductively immobilized on iron oxide surfaces experimentally. Samples consisting of reductively immobilized U and Se (carefully characterized in previous studies) we exposed to aqueous solutions containing HCO3- and HCO3- and H2O2. HCO3- increases the solubility of U(VI) and H2O2 will oxidize U(IV) to U(VI). Exposure to a HCO3- solution revealed a transient increase in the uranium concentration which can probably attributed to detachment of loosely bound UO2 particles upon addition of the new solution. Upon addition of H2O2, the concentrations of U and Se increased fairly rapidly indicating that oxidative dissolution is taking place. The rate of uranium release was well in line with what could be expected from the kinetics of oxidative dissolution of UO2 by H2O2. The Se/U ratio increases with leaching time approaching the overall original composition of the immobilized surface. Hence, oxidative dissolution of the immobilized U and Se is not congruent. Furthermore, the iron oxide surface exposed to the solution after oxidative removal of the immobilized Se and U, appears to be less reactive towards Se and U in solution compared to the original oxide surface. Consequently, the oxidative remobilization process has also affected the iron oxide. These findings suggest that oxidative re-mobilization of reductively immobilized redox sensitive radionuclides is aprocess of potential impact in a deep geological repository for spent nuclear fuel.

National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-27179OAI: oai:DiVA.org:kth-27179DiVA: diva2:375332
Note
QC 20101208Available from: 2010-12-07 Created: 2010-12-07 Last updated: 2010-12-08Bibliographically approved
In thesis
1. Near field immobilization of selenium oxyanions
Open this publication in new window or tab >>Near field immobilization of selenium oxyanions
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The topic of this doctoral thesis is the potential near field immobilization of the radionuclide 79Se after intrusion of groundwater into a spent nuclear fuel canister in a repository. 79Se is a non naturally occurring long lived selenium isotope formed as a result of fission in nuclear fuel. Given the long half life (~3 x 105 y) and that the oxyanions of selenium are expected to be highly mobile and potentially difficult toimmobilize the isotope is of interest for the long term safety assessment of high level waste repositories. In this work the near field has been limited to the study of processes at or near the UO2 surface of (simulated) spent nuclear fuel and to processes occurring at or near the surface of iron (canister material) corroding under anoxic conditions. Selenite (HSeO32-) was found to adsorb onto palladium (simulated noble metal inclusion in spent nuclear fuel). Under hydrogen atmosphere selenite was reduced to elemental selenium with a rate constant of ~2 x 10-9 m s-1 (with respect to the Pd surface, 24 bar H2) forming colloidal particles. The rate constant of selenite reduction was increased by about two orders of magnitude to ~2.5 x 10-7 m s-1 (with respect to the Pd surface, 10 bar H2) for a UO2 surface doped with Pd particles, indicating that UO2 is an efficient co-catalyst to Pd. Selenate (SeO42-) was neither adsorbed nor reduced in the presence of Pd, UO2 and hydrogen. In the iron corrosion studies selenate was found to become reduced to predominantly elemental Se in the presence of a pristine iron surface. Iron covered by a corrosion layer of magnetite did however appear inert with respect to selenate whereas selenite was reduced. The reduction of dissolved uranyl into UO2 by the corroding iron surfaces was found to significantly increase the removal rate of selenite as well as selenate. The uranyl was found to transiently transform the outer iron oxide layers on the iron, forming a reactive mixed Fe(II)/Fe(III) oxyhydroxide (Green rust). Exchanging the solution and increasing the carbonate content (from 2 mM to 20 mM NaHCO3) only resulted in a minor, transient remobilization of uranium. Addition of H2O2 did however result in a significant release of uranium as well as selenium from the iron oxide surfaces. An irradiation experiment was also performed confirming the one electron reduction barrier of selenate as an important factor in systems where selenate reduction would be thermodynamically favorable.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. 68 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2010:51
Keyword
Se-79, Selenium, Immobilization, Redox, Iron, Corrosion, Palladium, Catalyst, Spent Nuclear Fuel, Repository
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-26994 (URN)978-91-7415-819-9 (ISBN)
Public defence
2010-12-17, F3, Lindstedtsvägen 26, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC 20101208Available from: 2010-12-08 Created: 2010-12-01 Last updated: 2010-12-08Bibliographically approved

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