Interaction between smectite and bacteria: Implications for bentonite as backfill material in the disposal of nuclear waste

This study presents experimental results on the interaction between smectite clays (nontronite and MX80 bentonite) and the facultative anaerobic, heterotrophic Shewanella putrefaciens in two types of conditions: i) batch experiments with low solid to liquid ratios and agitated oxygenated conditions and ii) reaction-cell experiments with high solid to liquid ratios in anaerobic, confined volume conditions. The former was chosen to study the ability of S. putrefaciens to live on smectite as the only substrate and the latter was designed to simulate more compacted subsurface environments of underground repository waste sites. Bacterial cell counts in the batch experiments reveal the prolonged survival of S. putrefaciens in the smectite suspension compared to standard laboratory culture media. In the case of nontronite, variations in solution chemistry indicate bacterial consumption and/or partial binding of cations. Microscopic investigations show associated biofilm-smectite aggregates and Si-rich gels produced by the partial dissolution of clay mineral grains. In contrast, the MX80 bentonite was not seen to be chemically affected by bacterial activity in batch cultures. However, the confined volume experiments, using reaction-cell X-ray diffraction combined with peak calculations (CALCMIX), do indicate that S. putrefaciens has a pronounced effect on the water content of compacted MX80 bentonite. The presence of these bacteria enhances both the amount of adsorbed interlayer water and the available pore space. The anaerobic conditions were also favourable for accessory phase dissolution (notably calcite) and synchronous precipitation of lepidocrocite related to bacterially induced changes in pH and Eh. The varied response of the two studied clays to the presence of bacteria is attributed largely to the materials composition. The interlayer Ca of nontronite facilitates bacterial attachment to surfaces and Fe(III) provokes the production of chelators that enhance mineral dissolution. Although MX80 bentonite is less affected by bacterially enhanced dissolution, it is more sensitive to microstructural changes. Mechanisms involve aggregation of Na-smectite particles in voids created by cell lyses, the initial production of biofilm and the pH and Eh dependent dissolution and precipitation of accessory minerals. This investigation highlights the importance of including bacteria-mineral studies in assessing the safety issue of underground disposal of nuclear waste material.