Diffusion of Calcium Chloride in a Modified Bentonite: Impact on Osmotic Efficiency and Hydraulic Conductivity

Abstract

Abstract-Chemically modified bentonites are being developed with the aim of preserving low hydraulic conductivity in the presence of potentially aggressive permeants in pollutant-containment applications. 'Multiswellable' bentonite (MSB) has been obtained by treating standard sodium bentonite with propylene carbonate. Research on the engineering properties of MSB has focused mainly on permeability and chemical compatibility. Solute diffusion and membrane behavior in MSB have not yet been investigated. A combined chemico-osmotic/diffusion test was performed on a MSB specimen using a 5 mM CaCl 2 solution. Permeability with distilled water and with the 5 mM CaCl 2 solution was measured prior to and after the chemico-osmotic/diffusion tests. The material exhibited time-dependent membrane behavior with a peak osmotic efficiency value (o) of 0.172 that gradually shifted to zero upon breakthrough of calcium ions. Effective diffusion coefficients of calcium and chloride ions were in the range commonly described for untreated bentonite at similar porosities. After the chemico-osmotic/diffusion stage and permeation with 5 mM CaCl 2 , the hydraulic conductivity of MSB increased from 1.1610 À11 m/s to 7.0 610 À11 m/s. The MSB was apparently converted into a calcium-exchanged bentonite at the end of the test. Prehydration and subsequent permeation might have contributed to elution of the organic additive from the clay. Further investigation is recommended to clarify the effect of prehydration on the hydraulic performance of MSB in the presence of potentially aggressive permeants.

Membrane behavior of compacted sand–bentonite mixture

“…Except for the 5 mmol/L KCl stage for specimen RW-2, the difference between ave and o , ⌬, was generally ≤0.033, and tended to decrease with increasing C ot . The closeness in values between o and ave for both tests for all values of C ot likely is due, in part, to the circulation of TW across the bottom boundary of the specimens in this study relative to previous studies, where DIW generally has been circulated across the bottom boundary of the specimen (e.g., Malusis and Shackelford 2002;Shackelford and Lee 2003;Yeo et al 2005;Kang and Shackelford 2010;Mazzieri et al 2010;Bohnhoff 2012;Dominijanni et al 2013;Meier et al 2014;Tang et al 2014aTang et al , 2014bTang et al , 2015Di Emidio et al 2015). The use of TW as the bottom circulation liquid resulted in values of −⌬C o and −⌬C ave that were closer in magnitude relative to the differences in these values based on the use of DIW as the bottom circulation liquid.…”

Investigation of Reactive Transport and Coupled THMC Processes in the EBS

“…Hence, the net diffused mass ( The experimental results for the diffusive flux and cumulative mass/activity plots for Ca, Br, and tritium ( 3 H) are given in the following figures 4.19 to 4.24. For Ca, the time-frame needed to reach steady-state conditions in our Na-montmorillonite diffusion experiment (~33 days) agrees well with previously published data for calcium diffusion in bentonite (31 days (Mazzieri et al, 2010) or 35 days (Shackelford and Lee, 2003)). July 2013  Demonstration that MD simulations can help to extrapolate existing diffusion measurements to the high temperatures that occur in the near-field of high-level radioactive waste repositories while also providing molecular scale insights into the predominant diffusion pathways of water and solutes in the EBS.…”

“…Hence, a calcium concentration gradient (Figure 4.1) and the diffusive transport of Ca within barrier systems seem very likely. At this point, only a few diffusion studies have been performed for Ca in bentonite (Mazzieri et al, 2010;Shackelford and Lee, 2003); data for Ca diffusion in Na-montmorillonite are even scarcer . However, these data will be very relevant for interpreting uranium(VI) diffusion behavior in Na-montmorillonite at varying Ca concentrations.…”

Review of the fundamental geochemical and physical behaviors of organoclays in barrier applications

“…MSB is manufactured by manually mixing the prescribed amount of PC and bentonite [weight ratio of 1:5, 1:4, 1:2, 1:1 as reported in Mazzieri et al (2010), , and Fehervari et al (2016)] approximately 3 min, then resting the mixture for 24 h (Fehervari et al, 2016). PC is intercalated in the interlayer space of bentonite, attracts numerous water molecules, decreases the hydraulic conductivity of the unmodified bentonite by one to two orders of magnitude, and increases the swelling potential of the unmodified bentonite even under solutions with either high valence ions or high concentration (Table 5) Di Emidio et al, 2015).…”