Bentonite pore distribution based on SAXS, chloride exclusion and NMR studies

Abstract

AB ST R ACT : Water-saturated bentonite is planned to be used in many countries as an important barrier component in high-level nuclear waste (HLW) repositories. Knowledge about the microstructure of the bentonite and the distribution of water between interlayer (IL) and noninterlayer (non-IL) pores is important for modelling of long-term processes. In this work the microstructure of water-saturated samples prepared from MX-80 bentonite was studied with nuclear magnetic resonance (NMR) and small-angle X-ray scattering spectroscopy (SAXS) coupled with chloride exclusion modelling. The sample dry densities ranged between 0.7 and 1.6 g/cm 3 . The NMR technique was used to get information about the relative amounts of different water types. Water in smaller volume domains has a shorter relaxation time than that in larger domains due to the average closer proximity of the water to the paramagnetic Fe at the layer surfaces. The results were obtained using 1 H NMR T 1r relaxation time measurements with the short inter-pulse CPMG method. The interpretation of the NMR results was made by fitting a sum of discrete exponentials to the observed decay curves. The SAXS measurement on bentonite samples was used to get information about the size distribution of the IL distance of montmorillonite. The chloride porosity measurements and Donnan exclusion calculations were used together with the SAXS results to evaluate the bentonite microstructure. In the model, the montmorillonite layers were organized in stacks having IL water between the layers and non-IL water between the stacks. In the modelling, the number of layers in the stacks was used as fitting parameters which determined the IL and non-IL surface areas. The fitting parameters were adjusted so that the modelled chloride concentration was equal to the measured one. The NMR studies and SAXS studies coupled with the Cl porosity measurements provided very similar pictures of how the porewater is divided in two phases in bentonite.

Impact of microstructure on anion exclusion in compacted clay media

“…This simple calculation may underestimate measured values of the anion-accessible porosity, however, because the anion exclusion distance, d e , is sensitive to pore width in small pores (in conditions where the diffuse layers overlap) but not in larger pores. A closer agreement between experimental results and MGC model predictions can be obtained by assuming that clay media have a non-homogeneous pore-size distribution, which implies that anionaccessible porosity is sensitive to the pore-scale microstructure of clay media (Tournassat and Appelo, 2011;Muurinen et al, 2013). 4. Sensitivity of anion-exclusion predictions to microstructural parameters 4.1.…”

Second ISSMGE R. Kerry Rowe Lecture: On the intrinsic, state, and fabric parameters of active clays for contaminant control

“…Considering the previous scheme, with reference to the effective volume of the conducting pores (Manassero et al 2016;Dominijanni et al 2017), the thickness of the Stern layers, d Stern , ranging between 1.2 nm and 2.0 nm and consisting of the first row of hydrated cations in direct contact with the negatively charged surface of the solid particles, can be taken into account. The hydrated cations of the Stern layer are characterized by a significantly reduced mobility (Tournassat et al 2009;Muurinen et al 2013;Shackelford and Moore 2013;Tinnacher et al 2016) and, therefore, can be considered as a part of the solid phase with a consequent restriction of the micro void space directly involved by the solvent and solute fluxes. Based on the previous assumptions and considerations, the void space within the platelets of the tactoids, together with the thickness of the hydrated cations in the Stern layer surrounding the external surface of the tactoid, should be subtracted from the total void space, e, to obtain the micro-void space, e m , where almost the entire solute and solvent flow takes place.…”

“…NMR previously has been used successfully to determine the distribution of pore water in water-saturated bentonites (e.g. Ohkubo et al 2008;Montavon et al 2009;Muurinen et al 2013;Ohkubo et al 2016). Through an interpretation of the differences in observed proton mobilities, Montavon et al (2009) were able to distinguish between three different populations, i.e.…”

“…the selective freezing of the bulk water and the assumption of the relaxation time of a three-hydrated layer space as the threshold between the bulk and surface water) to investigate the salt concentration effect of a NaCl solution on the pore-size distribution of compacted montmorillonite. By fitting discrete exponentials to the decay curves obtained from NMR measurements, Muurinen et al (2013) found that the observed relaxation was composed of two major relaxation processes, and that assuming a third relaxation process produced a marginal effect. On the basis of these results, Muurinen et al (2013) suggest the presence of only two water phases in water-saturated smectitic clays, viz., interlayer and noninterlayer water.…”

“…By fitting discrete exponentials to the decay curves obtained from NMR measurements, Muurinen et al (2013) found that the observed relaxation was composed of two major relaxation processes, and that assuming a third relaxation process produced a marginal effect. On the basis of these results, Muurinen et al (2013) suggest the presence of only two water phases in water-saturated smectitic clays, viz., interlayer and noninterlayer water.…”