The effect of compaction and microbial activity on the quantity and release rate of water-soluble organic matter from bentonites

Abstract

Swelling bentonites, planned to be installed around spent nuclear fuel canisters made of copper/cast iron in geological repositories, contain organic matter. Organic matter can act as substrates for microorganisms, such as sulfate-reducing microorganisms (SRM), which produce sulfide, a copper corrosion agent. Thus, it is important to study the quantity and release rate of organic matter from bentonites. In this study, the soluble organic matter (sOM) quantity of three bentonites (Wyoming, Indian, and Bulgarian) determined by dynamic leaching assays was originally 85, 16 and >163 mg kg−1, respectively. To assess the effect of simulated repository conditions on the quantity and release rate of bentonite sOM, the original bentonites were compacted to an average dry density of 1314–1368 kg m−3 and connected to sand layers in anaerobic cell systems. The sand layers were either inoculated or uninoculated with microorganisms. Afterwards, the cells were operated on for 12–15 months. As shown by postmortem dynamic leaching assays, the bentonite sOM quantities either increased (Indian and Bulgarian) or decreased (Wyoming) relative to the initial sOM quantities in original bentonites. The release rate of sOM increased in all bentonites in comparison with the original bentonites, and the increase was higher for the bentonites of the inoculated cells than for the uninoculated cells. The findings suggest that the interaction of the microorganisms with the bentonites increased the quantity and/or release rate of the bentonite sOM. Compaction of the bentonites, leading to changes in the mineral size and microstructure of bentonites, was also hypothesized to affect the amount of sOM released. Additional processes (e.g., diffusive transport) might also be relevant for the release of sOM from bentonites, but they were beyond the scope of this study. The present study represents an advance in the understanding of the effect of different possible repository conditions on the quantity and role of bentonite sOM as a source of substrates for the microbial community, especially considering the sulfide-producing SRM.