Ex and In Situ Reactivity and Sorption of Selenium in Opalinus Clay in the Presence of a Selenium Reducing Microbial Community

Nele Bleyen,Pierre De Cannière,Charles Wittebroodt,Elie Valcke,Maryna Surkova,Joe S Small,Kristel Mijnendonckx,Katrien Hendrix,Achim Albrecht

doi:10.3390/min11070757

Abstract

79Se is a critical radionuclide concerning the safety of deep geological disposal of certain radioactive wastes in clay-rich formations. To study the fate of selenium oxyanions in clayey rocks in the presence of a selenium reducing microbial community, in situ tests were performed in the Opalinus Clay at the Mont Terri Rock Laboratory (Switzerland). Furthermore, biotic and abiotic batch tests were performed to assess Se(VI) and Se(IV) reactivity in the presence of Opalinus Clay and/or stainless steel, in order to support the interpretation of the in situ tests. Geochemical modeling was applied to simulate Se(VI) reduction, Se(IV) sorption and solubility, and diffusion processes. This study shows that microbial activity is required to transform Se(VI) into more reduced and sorbing Se species in the Opalinus Clay, while in abiotic conditions, Se(VI) remains unreactive. On the other hand, Se(IV) can be reduced by microorganisms but can also sorb in the presence of clay without microorganisms. In situ microbial reduction of Se oxyanions can occur with electron donors provided by the clay itself. If microorganisms would be active in the clay surrounding a disposal facility, microbial reduction of leached Se could thus contribute to the overall retention of Se in clayey host rocks.

Highlights

No increase in intracellular ATP was observed, lactate was not consumed, and acetate production was not observed. These data indicate the absence of active microorganisms, confirming abiotic conditions in the currently discussed batch tests
Modeling of the abiotic experiments was performed to examine the extent of sorption Modeling of the abiotic experiments was performed to examine the extent of sorption of Se(IV) onto Opalinus Clay using the 2-site Langmuir isotherm model parameterized by of Se(IV) onto Opalinus Clay using the 2-site Langmuir isotherm model parameterized
Microbial presence and activity during the in situ test was monitored by flow cytometry (FC) and intracellular ATP measurements, respectively

Summary

Introduction

In several countries relying on nuclear energy, deep clay formations are studied as potential host rocks for geological disposal of intermediate-level and high-level long-lived waste (ILW and HLW). For both types of waste (mainly vitrified HLW and nitrate-bearing bituminized waste), 79 Se (T 1 = 327 ka [1]) is considered as one of the main radionuclides contributing to the dose-to-man after final disposal [2,3,4,5]. Part of these oxyanions could sorb onto corrosion products present on the steel canisters or steel rebars [7], the remainder is expected to reach the clay host rock surrounding the waste repository

Methods

Results

Discussion

Conclusion