Geochemical, Geotechnical, and Microbiological Changes in Mg/Ca Bentonite after Thermal Loading at 150 °C

Abstract

Bentonite buffers at temperatures beyond 100 °C could reduce the amount of high-level radioactive waste in a deep geological repository. However, it is necessary to demonstrate that the buffer surrounding the canisters withstands such elevated temperatures, while maintaining its safety functions (regarding long-term performance). For this reason, an experiment with thermal loading of bentonite powder at 150 °C was arranged. The paper presents changes that the Czech Mg/Ca bentonite underwent during heating for one year. These changes were examined by X-ray diffraction (XRD), thermal analysis with evolved gas analysis (TA-EGA), aqueous leachates, Cs sorption, cation exchange capacity (CEC), specific surface area (SSA), free swelling, saturated hydraulic conductivity, water retention curves (WRC), quantitative polymerase chain reaction (qPCR), and next-generation sequencing (NGS). It was concluded that montmorillonite was partially altered, in terms of the magnitude of the surface charge density of montmorillonite particles, based on the measurement interpretations of CEC, SSA, and Cs sorption. Montmorillonite alteration towards low- or non-swelling clay structures corresponded well to significantly lower swelling ability and water uptake ability, and higher saturated hydraulic conductivity of thermally loaded samples. Microbial survivability decreased with the thermal loading time, but it was not completely diminished, even in samples heated for one year.

Dramatic loss of microbial viability in bentonite exposed to heat and gamma radiation: Implications for deep geological repository

“…After heating BCV powder to 150°C, just 1-month's exposure resulted in (probably complete) bentonite sterilization. These results contradict the previous data from the EURAD project 'HITEC' (Kašpar et al 2021), where BCV powder was also heated to 150°C for six-and 12-months. In this case, possible microbial proliferation was detected after 6-months exposure, and possibly also after 12-months exposure.…”

“…In our study, both enrichment cultures in different media and natural incubations of bentonite suspensions were included to test which cultivation conditions were more suitable for demonstrating effects on microbial survivability. While natural incubations were successfully used in our previous experiments (Kašpar et al 2021;Bartak et al 2023) to assess the reaction of bentonite microorganisms to elevated pressure and temperature, the enrichment culture approach was clearly superior to natural incubations for detecting microbial survivability in the present study. This nding agrees well with previous studies, where excess of available nutrients promoted microbial recovery following harsh treatment, e.g.…”

“…The overall temperature threshold for survivability of common microbes (aside from extremophile prokaryotes) is believed to be 121°C (Kashe and Lovley 2003). Yet, microbial analyses of bentonite from one of our long-term experiments indicated that microorganisms can survive in powdered Czech calcium-magnesium bentonite (BCV), even when heated to 150°C for six or even 12 months (Kašpar et al, 2021). However, these data still need to be veri ed through independent studies as the experiment was not initially designed for microbiological application and contamination is a possibility.…”

Effect of pressure and temperature on survivability and proliferation of microorganisms in bentonite

“…Each experiment was performed on a suspension of Czech BCV bentonite, mined and processed by Keramost Ltd. [24], and sterile deionised water. For each treatment and sampling point, duplicate samples were prepared.…”

On-Line Thermally Induced Evolved Gas Analysis: An Update—Part 1: EGA-MS

“…These changes were examined by X-ray diffraction, evolved gas analysis, aqueous leachates, Cs sorption, and other techniques. It was concluded that montmorillonite was partially altered [ 194 ].…”