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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.
: Compacted bentonite is currently being considered as a suitable backfill material for sealing
underground repositories for radioactive waste as part of a multi-barrier concept. Although showing
favorable properties for this purpose (swelling capability, low permeability, and high adsorption
capacity), the best choice of material remains unclear. The goal of this study was to examine and
compare the hydration behavior of a Milos (Greek) Ca-bentonite sample (SD80) in two types of
simulated ground water: (i) Opalinus clay pore water, and (ii) a diluted saline cap rock brine using
a confined volume, flow-through reaction cell adapted for in situ monitoring by X-ray diffraction.
Based on wet-cell X-ray diffractometry (XRD) and calculations with the software CALCMIX of the
smectite d(001) reflection, it was possible to quantify the abundance of water layers (WL) in the
interlayer spaces and the amount of non-interlayer water uptake during hydration using the two
types of solutions. This was done by varying WL distributions to fit the CALCMIX-simulated XRD
model to the observed data. Hydrating SD80 bentonite with Opalinus clay pore water resulted
in the formation of a dominant mixture of 3- and 4-WLs. The preservation of ca. 10% 1-WLs and
the apparent disappearance of 2-WLs in this hydrated sample are attributed to small quantities of
interlayer K (ca. 8% of exchangeable cations). The SD80 bentonite of equivalent packing density
that was hydrated in diluted cap rock brine also contained ca. 15% 1-WLs, associated with a slightly
higher concentration of interlayer K. However, this sample showed notable suppression of WL
thickness with 2- and 3-WLs dominating in the steady-state condition. This effect is to be expected for
the higher salt content of the brine but the observed generation of CO2 gas in this experiment, derived
from enhanced dissolution of calcite, may have contributed to the suppression of WL thickness. Based
on a comparison with all published wet-cell bentonite hydration experiments, the ratio of packing
density to the total layer charge of smectite is suggested as a useful proxy for predicting the relative
amounts of interlayer and non-interlayer water incorporated during hydration. Such information is
important for assessing the subsequent rates of chemical transport through the bentonite barrier.