In situ biological interactions in clay barrier systems are of great interest for deep geological disposal of spent nuclear fuel (SNF) – microorganisms may play a role in the transformation of smectite into illite (illitization) leading to clay swelling reduction. Illitization is a process known to occur abiotically with the following chemical reaction:
Smectite+ K⁺+ Al³⁺→ Illite+ Silica+ (Cation)
where the exchangeable interlayer cations in smectite (Na⁺ or Ca²⁺) are exchanged with K⁺. Additionally, microbial reduction of structural Fe³⁺ in the octahedral sheets of iron-rich smectites may promote the smectite-to-illite transformation. Previous studies have shown that microorganisms promote bioillitization by dissolving smectite through reduction of structural Fe³⁺. This study could: 1. determine if smectite can be transformed or degraded by the existing microbial environment, 2. inform the performance assessment (PA) of a geological repository clay barrier, and 3. help design effective strategies for environmental management. This work is focused on a mechanistic understanding and quantification of microbial reduction of Fe³⁺ in the smectite clay nanostructure to soluble Fe²⁺ and its effect on clay structure, specifically illitization. The work presented includes inoculation experiments on nontronite clay (NAu-1), an Fe³⁺-rich smectite, with an iron-reducing bacteria species, Shewanella oneidensis (MR-1). Batch reactor experiments were conducted anaerobically at 30°C for 14 days. Dissolved Fe²⁺ and total Fe were measured with an Fe assay. The clay was characterized via XRD to observe changes to the crystalline structure. Preliminary results reveal MR-1 adapt immediately when transferred from aerobic to anaerobic conditions by utilizing Fe-rich NAu-1 as a final electron acceptor, Na-lactate as a carbon source, and M1 media without Fe(III)-citrate. There is a detectable increase in Fe²⁺ after 14 days, and XRD shows that the clay becomes less expansive in the ethylene-glycolated (EC) state. Further analysis of the clay reaction products is needed to determine if these extent of alteration or formation of secondary minerals. Reactor conditions such as M1 media alterations, and reaction time, will be varied in further studies to determine survivability of MR-1 in repository type environments.

George Bachand

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