Bentonite Alteration in Batch Reactor Experiments with and without Organic Supplements: Implications for the Disposal of Radioactive Waste

Abstract

Bentonite is currently proposed as a potential backfill material for sealing high-level radioactive waste in underground repositories due to its low hydraulic conductivity, self-sealing ability and high adsorption capability. However, saline pore waters, high temperatures and the influence of microbes may cause mineralogical changes and affect the long-term performance of the bentonite barrier system. In this study, long-term static batch experiments were carried out at 25 °C and 90 °C for one and two years using two different industrial bentonites (SD80 from Greece, B36 from Slovakia) and two types of aqueous solutions, which simulated (a) Opalinus clay pore water with a salinity of 19 g·L−1, and (b) diluted cap rock solution with a salinity of 155 g·L−1. The bentonites were prepared with and without organic substrates to study the microbial community and their potential influence on bentonite mineralogy. Smectite alteration was dominated by metal ion substitutions, changes in layer charge and delamination during water–clay interaction. The degree of smectite alteration and changes in the microbial diversity depended largely on the respective bentonite and the experimental conditions. Thus, the low charged SD80 with 17% tetrahedral charge showed nearly no structural change in either of the aqueous solutions, whereas B36 as a medium charged smectite with 56% tetrahedral charge became more beidellitic with increasing temperature when reacted in the diluted cap rock solution. Based on these experiments, the alteration of the smectite is mainly attributed to the nature of the bentonite, pore water chemistry and temperature. A significant microbial influence on the here analyzed parameters was not observed within the two years of experimentation. However, as the detected genera are known to potentially influence geochemical processes, microbial-driven alteration occurring over longer time periods cannot be ruled out if organic nutrients are available at appropriate concentrations.

Thermally Induced Bentonite Alterations in the SKB ABM5 Hot Bentonite Experiment

“…The chemical composition of purified smectites was measured by energy-dispersive Xray spectroscopy (EDX). Purification and preparation of the samples was undertaken using the methodology described by Podlech et al [26]. The degree of purity of the separates was controlled by transmission electron microscopy examination using a JEOL JEM-2100Plus instrument.…”

In Situ Measurements of the Hydration Behavior of Compacted Milos (SD80) Bentonite by Wet-Cell X-ray Diffraction in an Opalinus Clay Pore Water and a Diluted Cap Rock Brine

“…Based on exchangeable cations as measured by the atomic adsorption spectroscopy (AAS) of SD80 smectite treated in long-term batch experiments at various temperatures, the amount of K + was lower than the unaltered smectite material with a ca. 20-40% decrease in concentration [16]. In contrast, the interlayer K content of untreated and treated bentonite measured by SEM-EDX analyses showed no significant difference between the two samples.…”

“…Previous quantifications by X-ray diffraction (XRD) Rietveld analyses show the raw powder material contains 89% smectite and 11% accessory minerals such as feldspar and traces of quartz, calcite, pyrite, and baryte (Table 1). Assuming a pure dioctahedral nature, compositional analyses of the purified smectite fraction by energy dispersive X-ray analyses (EDX) produced a total interlayer charge of −0.36 e/phuc with −0.06 e/phuc distributed in the tetrahedral sheet and −0.30 e/phuc in the octahedral sheet [16]. The mineral formula calculation also indicates an interlayer content of 0.01 K, 0.03 Na, 0.09 Ca, and 0.06 Mg per half-unit cell (phuc), confirming Ca as the dominant exchangeable cation.…”

Extremophilic microbial metabolism and radioactive waste disposal

“…Key spore formers of interest in GDF studies include novel isolates from natural analogues, such as the isosaccharinic acid degrading alkaliphile, Anaerobacillus isosaccharinicus (Bassil and Lloyd 2019 ) and a range of SRB and iron-reducing bacteria (IRB) (Williamson et al 2013 ). Spore presence is discussed in most papers regarding microbial clay communities, as several bentonites selected for engineered barriers contain indigenous spore-formers (Haynes et al 2018 ; Matschiavelli et al 2019 ; Grigoryan et al 2020 ; Podlech et al 2021 ). Clay materials have the capacity to act as ‘microbial seed banks’ (Lennon and Jones 2011 ), containing dormant individuals that may be resuscitated when the environment permits (Lennon and Jones 2011 ).…”