Microcosm study of indigenous microorganisms in bentonite and its effect on the corrosion of cast iron

INTRODUCTION

Bentonite is a potential barrier material in deep geological repositories (DGR) for nuclear waste and spent fuel [1] and it is critical to maintain its functionality for long periods of time. Bacteria, that can originate from the bentonite itself, can affect important properties of the engineered barrier system, including bentonite’s swelling capability and integrity of the container material [2]. Cast iron is often considered as a suitable material for constructing the containers for the radioactive waste storage [3]. But the container material could be unstable and can corrode to insoluble corrosion products, which react with the bentonite barrier. In a DGR, anaerobic corrosion and microbially influenced corrosion are dominant forms of corrosion and the interactions at the metal/bentonite interface determine the long-term performance of bentonite-based radioactive waste barriers [4].

DESCRIPTION OF THE WORK

Microcosm-type setup described in [5] was used for the current study. Three types of bentonite with different indigenous microorganisms were chosen for the setup: B25, Calcigel, MX-80. Incubation of the microcosms, containing GGG40 cast iron coupons, synthetic Opalinus Clay porewater (OPA) and bentonite, was performed in N2/CO2 atmosphere at 30 °C. Some of the microcosms were supplemented with 5 mM sodium lactate or 0.5 bar of hydrogen to stimulate microbial activity. After a 271-day incubation period, the microcosms were investigated by various geochemical analyses (as e.g. ICP-MS, ion and high-performance liquid chromatography), DNA isolation and amplification of the ribosomal RNA (rRNA) intergenic spacer (RISA) for microbial community analysis, SEM-EDX and RAMAN spectroscopy to characterize the surface structure of the cast iron coupons.
In addition, a similar 3-component experiment was set up with Calcigel including 4 time points to study in more detail the microbial-induced process of cast iron corrosion in Calcigel-microcosms.

RESULTS AND DISCUSSION

After 271 days of incubation under anaerobic conditions at 30 °C, the presence of black precipitants in microcosms containing Calcigel and sodium lactate, and all substrate-containing MX-80 samples became apparent. Geochemical investigation of the respective samples showed a decrease in sulphate concentration which was dominant in microcosms containing MX-80.
Surface analysis with SEM-EDX showed severe damage for all the samples, except B25 without substrates. Two types of crystalline structures were found: iron and/or calcium carbonates and iron sulphide. The presence of the latter could be an indication of the activity of sulphate-reducing bacteria.
Different microbial community structures were observed by RISA analysis depending on the used bentonite and the applied conditions.
Overall, the results show that the reactivity at the bentonite/metal interface and the microbial activity are bentonite-type dependent and the selection of the bentonite for the DGR is highly important for preventing possible microbial implications that could lead to a faster deterioration of the metal container.

ACKNOWLEDGEMENT

Funding was provided by the German Federal Ministry of Education and Research (BMBF, Grant 02NUK053) and the Helmholtz Association (Grant SO-093).

REFERENCES

1. P. Sellin et al., “The Use of Clay as an Engineered Barrier in Radioactive-Waste Management – A Review” Clays and Clay Minerals, 61(6), pp. 477-498 (2014).
2. F. King, “Container Materials for the Storage and Disposal of Nuclear Waste” Corrosion, 69(10), pp. 986-1011 (2013).
3. F. King et al., “Nature of the near-field environment in a deep geological repository and the implications for the
corrosion behaviour of the container” Corrosion Engineering, Science and Technology, 52, 1, pp. 25-30 (2017).
4. S. Kaufhold et al., “About the Corrosion Mechanism of Metal Iron in Contact with Bentonite” ACS Earth Space Chem., 4, 5, pp. 711–721 (2020).
5. N. Matschiavelli et al., “The Year-Long Development of Microorganisms in Uncompacted Bavarian Bentonite Slurries at 30 and 60 °C” Environ. Sci. Technol., 53, 17, 10514–10524 (2019)