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A study on the immobilization of selenium oxyanions by H2/Pd(s) in aqueous solution: Confirmation of the one-electron reduction barrier of selenate
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
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
2010 (English)In: Journal of Contaminant Hydrology, ISSN 0169-7722, E-ISSN 1873-6009, Vol. 116, no 1-4, 16-23 p.Article in journal (Refereed) Published
Abstract [en]

Selenium is a trace element of concern in several geochemical contexts, due to the potentially high mobility of the selenium oxyanions and the narrow range between deficiency and toxicity of the element. For high level nuclear waste repositories the long-lived fission product 79Se has been identified as a potential key dose contributor for the long-term safety. This paper deals with the catalytic effect of Pd(s) on the H2 reduction of selenium oxyanions which was studied experimentally in aqueous solutions containing bicarbonate and chloride. Pd-catalysts and hydrogen have been proposed for the remediation of various groundwater contaminants and can also serve as a model substance for catalytic noble metal inclusions present in spent nuclear fuel. In this study selenite (SeO32) was found to adsorb on Pd. In the presence of hydrogen the rate of selenite removal increased yielding elemental Se. However, no adsorption or reduction of selenate (SeO42) was observed. A simple radiation chemical experiment revealed a notable barrier towards stepwise one-electron reduction of selenate to selenite. This provides an explanation for the lower reactivity of selenate in systems where reductive immobilization of selenite as well as selenate is thermodynamically favorable.

Place, publisher, year, edition, pages
2010. Vol. 116, no 1-4, 16-23 p.
Keyword [en]
Selenium, Immobilization, Redox, Hydrogen, Palladium, Catalyst
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-26990DOI: 10.1016/j.jconhyd.2010.04.007ISI: 000280532600002Scopus ID: 2-s2.0-77955661068OAI: oai:DiVA.org:kth-26990DiVA: diva2:373736
Note
QC 20101208Available from: 2010-12-01 Created: 2010-12-01 Last updated: 2017-12-12Bibliographically 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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