Effect of Sodium Nitrate on the Diffusion of Cl− and I− in Compacted Bentonite

Effective diffusion coefficients of selenium through bentonite/sand mixtures were obtained by the through-diffusion method under reducing conditions where selenium was dissolved as HSe À and Se 4 2À . Experiments were carried out under variable bentonite content and porewater salinity. Bentonite contents in bentonite/sand mixtures varied from 20 to 70 wt%. The salinities of experimental solutions varied from 0.05 to 0.5 mol dm À3 , simulating brackish groundwater and seawater. Effective diffusion coefficients were also obtained under anaerobic conditions where the dominant selenium species was determined to be SeO 3 2À . The effective diffusion coefficients of selenium species decreased with increasing bentonite content and with decreasing salinity. This tendency is primarily due to anion exclusion that was caused by the negative surface charge of montmorillonite. A diffusion model for anionic species was developed based on the electric double layer theory and the pore diffusion model, to evaluate the diffusion behaviors of HSe À , Se 4 2À , and SeO 3 2À in bentonite porewater. In this model, pores in bentonite/sand mixtures are assumed to consist of two types of space; one is the space between mineral particles and the other is the space between layers of montmorillonite. The scope of application of this model is limited to less than or equal to 1,600 kg m À3 in dry density. The effective diffusion coefficients of selenium species predicted using this model showed good agreement with experimentally measured ones.

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“…5(a) with previously reported ones of monovalent anionic species, 36,37) Cl À (Diffusion coefficient in free water:…”

Section: àsupporting

confidence: 56%

Experimental and Modeling Study on Diffusion of Selenium under Variable Bentonite Content and Porewater Salinity

Iida

Yamaguchi

Tanaka

2011

Journal of Nuclear Science and Technology

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“…The salt effect depends on the charge of the radionuclides. For example, the effective diffusion coefficient (D e ) for anions such as 36 Cl and 125 I in bentonite increases with increasing ionic strength (Glaus et al, 2010;Ishidera et al, 2008;Molera et al, 2003; Tachi and Yotsuji, 2014;Tian et al, 2013;Van Loon et al, 2007). D e for cations such as Cs + , Na + , Sr 2+ , Co 2+ and Zn 2+ , in bentonite and illite showed the opposite dependency (Glaus et al, 2015;Glaus et al, 2010; Tachi and Yotsuji, 2014;Tachi et al, 2011),…”

Section: Introductionmentioning

confidence: 99%

Salt effects on Re(VII) and Se(IV) diffusion in bentonite

Wang

et al. 2017

Applied Clay Science

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The through-diffusion method was employed to investigate the effect of the salt concentration on the diffusion behaviour of Re(VII) and Se(IV) in compacted bentonite. The accessible porosity,  acc , of Re(VII) increased from 0.25 to 0.38 with an increase of the ionic strength from 0.1 to 1.0 mol/L. The dependency of the accessible porosity  acc on the salt concentration could be described approximately by a first-order decay exponential function. The interlayer porosity of GMZ bentonite was 21 % of the total porosity at a bulk dry density of 1600 kg/m 3 at 12 C. The D e values of Re(VII) and Se(IV) increased from 2.6  10 -11 to 6.1  10 -11 m 2 /s and from 1.6  10 -11 to 4.7  10 -11 m 2 /s in GMZ bentonite with an increase of the ionic strength from 0.1 to 1.0 mol/L, respectively. In the case of Se(IV), the adsorption showed a different ionic strength dependency in GMZ bentonite and montmorillonite. The ionic strength has no significant effect on the sorption (K d =1.8  10 -4 m 3 /kg) on GMZ bentonite, indicating the formation of inner-sphere surface complexes. In the case of synthetic montmorillonite, the K d values increased with increasing ionic strength. This can be explained by the fact that the electric potential of surface sites becomes less negative with increasing ionic strength resulting in an increase in the adsorption of Se(IV) at a pH

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“…Although an anion accessible porosity of ca. 50% ± 10% of the total porosity was observed for a majority of claystones, lower values down to 10% of the total porosity were also found. , A few studies discuss the impact of ionic strength of the pore water on the anion exclusion effect in bentonite ,,, and in clay formations, , e.g., Moors studied the effect of varying ionic strength (0.016–1 M) on the diffusion of I – in Boom Clay and observed a higher accessible porosity with increasing ionic strength. Wittebroodt et al studied the diffusion of 36 Cl – and 125 I – in Upper Toarcian argillite samples from Tournemire and could also observe a significant effect of varying ionic strength (0.012–0.12 M) on the anion accessible porosity.…”

Section: Introductionmentioning

confidence: 99%

“…Muurinen et al, Birgersson and Karnland, Appelo and Wersin, and Jougnot et al based their analyses on a Donnan approach, while Bourg et al, Sposito, Tournassat et al, and Greathouse et al interpreted the anion exclusion porosity in the framework of the Gouy–Chapman theory. The majority of the experimental and modeling studies dealing with compacted bentonites showed that the anion accessible porosity increases with increasing ionic strength. ,,,, This could be explained by a compression of the electric double layer (EDL) at high ionic strength. The EDL arises from the screening of the negative surface charge of clay minerals by accumulating cations.…”

Section: Introductionmentioning

confidence: 99%

Importance of Interlayer Equivalent Pores for Anion Diffusion in Clay-Rich Sedimentary Rocks

Wigger

Loon

2017

Environ. Sci. Technol.

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The anion exclusion behavior in two different clay stones, Opalinus Clay (OPA) and Helvetic Marl (HM), was studied using a well-established experimental through-diffusion technique. The ionic strength of the pore water was varied between 0.01 and 5 M to evaluate its effect on the diffusion of HTO and Cl. The total porosity determined by HTO-diffusion was independent of the ionic strength, while the anion accessible porosity varies with the ionic strength of the pore water. In the case of Opalinus Clay, the anion accessible porosity increases from 3% at low ionic strength (0.01 M) up to 8.4% at high ionic strength (5 M), whereas the anion accessible porosity of Helvetic Marl increases from 0.6% up to only 1.1%. The anion exclusion effect in HM is thus more pronounced than that in OPA, even at high ionic strength. This observation can be correlated to differences in mineralogy and to the fact that HM has a larger fraction of interlayer equivalent pores. Interlayer equivalent pores are small pores in compressed clay stones that are small enough to have, because of overlapping electric double layers, properties similar to those of interlayers and are therefore rather inaccessible for anions.

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Effect of Sodium Nitrate on the Diffusion of Cl− and I− in Compacted Bentonite

Cited by 16 publications

References 6 publications

Experimental and Modeling Study on Diffusion of Selenium under Variable Bentonite Content and Porewater Salinity

Experimental and Modeling Study on Diffusion of Selenium under Variable Bentonite Content and Porewater Salinity

Salt effects on Re(VII) and Se(IV) diffusion in bentonite

Importance of Interlayer Equivalent Pores for Anion Diffusion in Clay-Rich Sedimentary Rocks

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