Transformation of inherent microorganisms in Wyoming-type bentonite and their effects on structural iron

Hanna Miettinen*, Malin Bomberg, René Bes, Mia Tiljander, Minna Vikman

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

10 Citations (Scopus)
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Abstract

Bentonite is one of the materials used to construct engineered barriers in high-level radioactive nuclear waste geological disposal with its many advantageous features such as low hydraulic conductivity, self-sealing ability, durability, adsorption and immobilization of metals and radionuclides and reduction of microbial activity. Many of these properties are linked with the bentonite swelling capability. Transformations of indigenous microorganism communities from Wyoming-type bentonite and the Finnish repository site groundwater and their effects on the bentonite structural iron over five years were studied in repository relevant anoxic and oxic slurry conditions. Active sulfate reduction (0.06 nmol mL−1 day−1) was detected in the anoxic microcosm waters after a year, however after two years sulfate reduction was not active anymore. Microbial numbers determined by quantitative PCR in the bentonite slurry of both experiment types supported the finding of decrease of overall microbial activity after a year of incubation that was not maintained anymore by the dissolving organic carbon from the bentonite. Regular electron donor additions (final concentration of 2 mM for formate and acetate each, three times per year) activated the microbiome resulting in increasing numbers of bacterial 16S rRNA gene copies and sulfate reducers (dsrB gene copies) as well as detection of sulfide in the water phase of both experiment types. After 4.9 years the structural iron in the fine portion of the montmorillonite had become completely reduced in all microbial microcosms and minor smectite illitization was detected especially in anoxic microcosms. Dominating bacterial groups at the end of the experiment were mainly known sulfur/sulfate and iron reducers. Archaea and fungi constituted a minor part of the microbiomes. In originally oxic microcosms, the bacterial 16S RNA and dsrB gene copy numbers were lower than in the anoxic experiment but started to significantly increase after the electron donor additions. Microorganisms originating from the repository environment could reduce the bentonite structural iron in a few years to an extent likely to affect the bentonite swelling ability if sufficient amounts of suitable electron donors are available in localized areas where bentonite is not at high density and pressure in the geological disposal.

Original languageEnglish
Article number106465
JournalApplied Clay Science
Volume221
DOIs
Publication statusPublished - May 2022
MoE publication typeA1 Journal article-refereed

Funding

This research was funded by the Horizon 2020 project MIND through funding from the Euratom research and training programme 2014-2018 under Grant Agreement no. 661880. The research was also funded by the Finnish Research Programme on Nuclear Waste Management 2018 (Geobiocycle) and 2022 (MiBe). Posiva Ltd. is acknowledged for providing the groundwater samples and Teollisuuden Voima Oyj the surface water from Olkiluoto. Authors thank the Center for x-ray Spectroscopy for providing experiment time and support with the HelXAS spectrometer under Proposal Numbers 2021-0009 and 2021-0005.

Keywords

  • Bentonite structural iron
  • Iron reduction
  • Microbiome
  • Sulfate reduction

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