Clay Science 12 Supplement 2, 160-165 (2006)

Anisotropy and Effect of Salinity in Diffusion and Activation Energy of

Cs+ Ions in Compacted Smectite

HARUO SATO*

Innovative Research Promotion Office, Japan Nuclear Cycle Development Institute (JNC),4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki 319-1194, Japan

(Received August 22, 2005. Accepted December 28, 2005)

ABSTRACT

In-diffusion experiments for Cs+ ions in compacted Na-smectite were performed in the directions parallel

and perpendicular to the orientation of the smectite particles as a function of smectite dry density (between

0.9 and 1.4Mg/m3), salinity ([NaCl]=0.01 and 0.51M) and temperature (295-333K).

The apparent diffusion coefficient (Da) tended to be higher in the direction parallel than in the direction

perpendicular to the orientation of the smectite particles. Higher salinity leads to an increase of the Da-values

for all conditions. The activation energies (ĢEa) are independent of the orientation of the smectite particles.

Higher dry densities correspond to higher ĢEa-values. For a dry density of 1.0Mg/m3, the corresponding

ƒ¢Ea-value agrees well to a predicted value ƒ¢Ea-value=24.66•}0.96kJ/mol. This value is obtained as the

difference of the activation energy for diffusion of cesium in pure water (16.47kJ/mol) and the Cs/Na ion

exchange enthalpy ƒ¢H•‹ (-11.10kJ/mol). For a dry density of 1.4Mg/m3, ƒ¢Ea is 35.20•}0.83kJ/mol. The

higher ƒ¢Ea-values found at higher dry densities are attributed to complex effects of ƒ¢H•‹ and the lowering of

porewater activity at high-dry densities.

Key words: Radioactive waste disposal, Bentonite, Smectite, Diffusion, Cesium, Activation energy

INTRODUCTION

In the safety assessment of the geological disposal forhigh-level radioactive waste (HLW) in Japan, the multi-barriersystem controls the release of radionuclides to the geosphere. Animportant barrier is the bentonite buffer. Therefore, severalstudies have been undertaken to investigate it1,2). Sinceradionuclides leached from vitrified wastes, enter the compactedbentonite, and diffuse through the bentonite towards the

geosphere, it is clear that their diffusion properties in thebentonite are regarded as one of the most importantcharacteristics in the safety assessment.

It is shown in previous studies that retardation in the diffusion

process of radionuclides in compacted bentonite is affected byvarious physico-chemical properties such as porosity, thebentonite's dry density1,3-6), sorption properties, exchangeablecations in the interlayer of smectite7), porewater chemistry8,9),additives to bentonite (e.g., silica sand)10), initial bentonite grainsize10,11), temperature5,12-17), etc. The authors have reported inprevious studies that clay particles orientate in the directionperpendicular to the direction of compaction for a bentonite withhigh-smectite content such as Kunipia-F(R) which is almost 100wt.% smectite18-20). Similarly, the diffusion coefficients oftritium (HTO) and deuterium (HDO) are entirely differentbetween directions parallel and perpendicular to the orientation

of the clay particles18-21).

In terms of activation energy (ĢEa) for diffusion, ĢEa-values

for the apparent diffusion coefficients (Da) of HTO, Na+, Cs+,

Sr2+ and Cl- ions in compacted Na-montmorillonite, which is a

major constituent of bentonite have been reported so far12-17).

The diffusion mechanisms for those ions have been discussed

based on the dependencies of the ĢEa-values on the

montmorillonite's dry density. Particularly, for Na+ ions, the

effects of silica sand mixture and salinity on ĢEa also have been

studied. It has been reported that the basal spacing of

montmorillonite decreases with increasing salinity15,22). Even at

the same montmorillonite partial density for a bentonite with

silica sand15-17), the ĢEa-values for Na+ ions and the interlayer

aperture of montmorillonite are different. In a previous study,

Suzuki et al.21) have measured ĢEa-values for the effective

diffusion coefficient (De) of HDO in compacted smectite where

the orientation of clay particles was controlled. The ĢEa-values

increased while raising with the smectite dry density and they

were slightly higher than ĢEa of the ionic diffusion coefficient in

free water (D•‹) for HDO. This indicates that the nature of

porewater near solid-liquid interfaces possibly differs from that

of free water.

In this study, the anisotropies and the effect of salinity in the

Da-and ĢEa-values for Cs+ ions in compacted Na-smectite are

discussed.*E-mail address of the corresponding author:sato.haruo@jaea.go.jp

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EXPERIMENTAL

Preparation of purified Na-smectite

In this study, Na-smectite of which the exchangeable cations

were perfectly exchanged with Na+ ions, was used. The

Na-smectite was prepared from Kunipia-F(R) (Kunimine

Industries Co. Ltd.), a Na-bentonite with a smectite content

higher than 99 wt.%. Smectite in Kunipia-F(R) is montmorillonite,

so Kunipia-F(R) can be considered as equivalent to

montmorillonite. Since Kunipia-F(R) is a purified bentonite with

about 20% Ca-smectite and 80% Na-smectite, ion-exchange

and purification were performed before starting the diffusion

experiments in order to obtain smectite with only perfectly

exchangeable Na+ cations in its interlayer.

During a week, the bentonite powder was put in contact with

a 1M-NaCl solution at a solid-liquid ratio of 100g/5 liter. Then

the bentonite was separated from the suspension. This operation

was repeated three times. At the end of the third immersion with

NaCl, the solid-liquid separation was made by centrifugation at

3,000 rpm for 20 minutes. To remove perfectly the remaining

liquid of the immersion, the bentonite was washed three times

with 80% ethyl alcohol, and then dried in air. Also in these

washings, the solid-liquid separation was made by

centrifugation at 3,000rpm for 20 minutes. The ion-exchanged

bentonite was dispersed in distilled water and bentonite particles

with•…0.5ƒÊm in diameter were collected by centrifugation at

3,000rpm for 5 minutes. The suspension containing only

particles with•…0.5ƒÊm in diameter was dried at 383K. Thus,

purified Na-smectite was obtained. The dried smectite was

finally crushed into particle sizes under 200 meshes for diffusion

experiments.

Diffusion experiments

The diffusion experiments were carried out by in-diffusion

method8,23,24). Figure 1 illustrates the experimental procedure of

the diffusion experiments and Table 1 shows the experimental

conditions. The purified smectite powder dried at 383K was

filled into a stainless steel sample holder. There it was

compacted with a special punching tool (Fig. 1) and a hydraulic

press, in order to obtain the desired dry densities.

After the compaction, the smectite block was transferred to

an acrylic sample holder. The diffusion experiments in the

direction perpendicular to the orientation of the smectite

particles were performed without changing the sample direction

after compaction. On the other hand, for diffusion tests in the

direction parallel to the orientation of the smectite particles, the

smectite block was rotated over an angle of 90•‹. The authors

have reported in previous studies that in bentonite clay particles

orientate in the direction perpendicular to the direction of

compaction if a bentonite with high-smectite content is

compacted from one direction. This has been evidenced by

scanning electron microscope observations as well as by the

anisotropy of the De-values of HTO and HDO18-20). All smectite

samples were prepared in the same way as in previous studies.

For accelerating saturation and removing air bubbles, the

smectite in the sample holder was initially saturated with

distilled water for a month in a vacuum chamber. After being

saturated with distilled water, the smectite sample was contacted

Fig. 1. Experimental procedure of diffusion experiments.

to 0.01 or 0.51 M-NaCl during 54 to 150days in order to adjust

the salinity of porewater.

After the saturation by NaCl, a small amount of tracer

solution (50ƒÊL) was pipetted on the surface of one end of each

smectite sample and allowed to diffuse at temperatures of 295 to

333K for 6h to 11d. After respective diffusion periods, each

smectite sample was cut with a knife into 1mm-pitched slices.

Each slice was immediately weighed to obtain the thickness of

Table 1. Experimental conditions.

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the slice and immersed in a 20mL KCl solution with 0.5 or 1M

for 2 to 35d to recover Cs from the slice. After being filtered

through 0.2ƒÊm membrane filters, the concentration of Cs was

analyzed with a High Resolution ICP-MS (Quantitative limit:

0.01ppb, Plasma Trace 2, Micromass, UK).

In parallel with the diffusion experiments, blank diffusion

experiments were conducted in order to obtain the background

concentration of Cs in the compacted smectite.

Determinations of Da and ĢEa

The Da-values were determined based on Fick's 2nd law25). If

Da is a constant independent of position and tracer concentration,

the diffusion equation for one-dimensional non-steady state in a

sample of infmite length is generally expressed by the following

equations;

(1)

where C is the concentration of tracer in unit smectite volume, t

the diffusing time, ƒÔ the distance from the tracer source, Da the

apparent diffusion coefficient, De the effective diffusion

coefficient, ƒ¿ rock capacity factor (ƒ¿=np+ƒÏd•E•Kd), np the porosity,

ƒÏd the smectite's dry density, and Kd the distribution coefficient.

The Da-values were determined by the analytical solution of

the diffusion equation shown above. Initially, all soluble tracer

was at the origin of the system, leading to next initial and

boundary conditions25):

with Qo the total quantity of tracer per unit surface in the system

and ƒÂ(ƒÔ) the delta-function.

The analytical solution of this problem is25):

(2)

The ĢEa was determined from Arrhenius plot as shown

below26):

(3)

where T is the absolute temperature, R the gas constant (8.314

J/mol/K), and ĢEa the activation energy.

RESULTS AND DISCUSSION

Effects of dry density, salinity and diffusion direction relative to

the orientated direction of smectite particles on Da

Figure 2 shows the measured Da-values for Cs+ ions in

compacted smectite versus the dry density for compacted

smectite (at 295K), Kunipia-F(R)1) and Na-montmorillonite15) (at

298K). All data were obtained in the distilled water-saturatedsystem. The Da-values obtained in this study are also shown inTable 2. The previous studies1,15) concern diffusion in thedirection perpendicular to the orientation of the particles (=parallel to the direction of compaction).

The Da-values for Cs+ ions obtained in the present tests aresimilar as those reported in the past1,15). The trends are the sameas well: increasing dry density leads to smaller Da-values, ahigher salinity raises the Da-values, the Da-values in thedirection parallel to the orientation of the clay particles (=perpendicular to the direction of compaction) are larger thanthose in the direction perpendicular to the orientation of clay

particles (=parallel to compaction direction).Next, the diffusion of path of the Cs+ ions in compacted

smectite is discussed. Due to its permanent negative charge inthe smectite (montmorillonite) crystal lattice, the surface ofsmectite sheets is negatively charged. The interlayer aperture ofsmectite is quite narrow. Depending on the dry density andsalinity, it consists only of two or three water layers. Increasingdry density and salinity lead to a smaller interlayer aperture15,22).Since the surface of smectite sheets is negatively charged,cations like Cs+ ions are attracted to this surface. In this smectiteinterlayer, Cs+ ions are sorbed by ion exchange with Na+ ions3).Consequently, Cs+ ions diffuse in both interlayer and interstitialpores. Tortuosity, which is the increase of the length of (along)the diffusion path (with respect to the end to end distance of thediffusion path), increases with dry density, but should notdepend significantly on salinity. The reason is that at highersalinity, due to the decrease in electrostatic repulsion, smectitesheets and aggregates coagulate, changing part of the interstitial

Fig. 2. Measured Da-values for Cs+ ions in compacted smectite versus thedry density for compacted smectite (at 295K), Kunipia-F(R)1) andNa-montmorillonite15) (at 298K). Perpendicular means perpendicular to theorientation of the particles (=parallel to the direction of compaction).Similarly, parallel is parallel to the orientation of the particles (or,equivalently, perpendicular to the direction of compaction).

163

pores into interlayer. Studies show indeed that the basal spacing

of smectite decreases with increasing salinity16,17,22).

Summarizing, the dependence of the Da-values of Cs+ ions on

dry density and salinity, can be interpreted as the result of

increase in tortuosity at higher dry densities, and the decrease in

sorption with increasing salinity.

Effects of dry density, salinity and diffusion direction on ĢEa

Figure 3 shows Arrhenius plots for the Da-values of Cs+ ions

in compacted smectite. The correlations of the logarithm of the

Da-values on the reciprocal of absolute temperature (1/T) are

linear. Figure 4 shows the dependence of the ĢEa-values,

calculated from Arrhenius plots, of Cs+ ions on the dry density.

It concerns the present results as well as those reported before12,15) The values obtained from the present study

, as well as the

Da-values, are summarized in Table 2. At a low-dry density of

1.0Mg/m3, the ƒ¢Ea-value of Cs+ ions is 24.66•}0.96kJ/mol. This

value is clearly higher than ĢEp=16.47kJ/mol, which is the

notation introduced for the ĢEa-value of the ionic diffusion

coefficient of Cs+ ions in free water (D•‹). The ƒ¢Ea-value tends to

increase with higher compaction (see Fig. 4). At a high-dry

density of 1.4Mg/m3, it is 35.20•}0.83kJ/mol. No anisotropy is

found in the ĢEa-values. For similar conditions, Kozaki et al.12,

Fig. 3. Arrhenius plots for Da-values of Cs+ ions in compacted smectite.

Fig. 4. A dependency of ĢEa-values for Cs+ ions in compacted smectite on

dry density obtained in this study together with ĢEa-data of Cs+ ions in

Na-montmorillonite reported in the past12,15)•

15) find approximately the same values as in this study.

The ĢEa-values should include the effect of ion exchange

between Cs+ and Na+ ions in the smectite interlayer (ion

exchange enthalpy ƒ¢H•‹), as well as the effect of diffusion in

water (activation energy ĢEp=16.47kJ/mol). The relationships

between Da and Kd, between Da and ƒ¢Ea, between Kd and ƒ¢H•‹

and between De and ĢEp are respectively expressed as follows:

(4)

(5)

(6)

(7)

where Df is the frequency factor for Da, Kdf the frequency factor

for Kd, ƒ¢H•‹ the ion exchange enthalpy, Df•‹ the frequency factor

for D•‹, FF the formation factor, and ƒ¢Ep the ƒ¢Ea for D•‹

Since Cs+ ions are strongly sorbed onto smectite, in

expression (4) is np<<ƒÏd Kd. With this approximation, the

substitution of expressions (5), (6) and (7) in expression (4)

gives

164

(8)

Since the constants Df, Kdf, D•‹f and FF do not depend on

temperature, expression (8) leads to the following relationship.

(9)

Since ĢEp=16.47kJ/mol and in a Na-smectite (Chambers

montmorillonite) ƒ¢H•‹=-11.10kJ/mol27) for the Cs+/Na+ ion

exchange, expression (9) gives ĢEa=27.57kJ/mol. This is

relatively consistent with ĢEa-values obtained at low-dry

densities (see Fig. 4).

The much higher ĢEa-values at high-dry densities are

explained by a change in porewater activity. At increasing high

densities, interlayer aperture and interstitial pore aperture as well

as liquid-solid ratio decrease. Water near the surface of

montmorillonite has a lower activity, a high-moisture potential

and a high suction than free water28,29,30). One layer of water

near the montmorillonite surface is thermodynamically

isostructual with ice at 273K. Similar to Cs+ ions,

measurements of ĢEa-values for De-values of HDO, which is

non-sorbing and not reactive, in compacted smectite show

ĢEa-values in the range of 23-25kJ/mol at a dry density of 1 .35

Mg/m3 21), while in pure water ĢEp=19.29kJ/mol. The

difference, about 4-7kJ/mol, is similar as for cesium (see the

value for 1.35Mg/m3 in Fig. 4). It is attributed to the lower

activity of porewater at higher dry densities. Additional

measurements allowing to verify this hypothesis are going on.

CONCLUSIONS

The Da and ĢEa-values of Cs+ ions in the directions parallel

as well as perpendicular to the orientattion of smectite particles

were measured in compacted smectite as a function of dry

density for different salinities. Increasing dry density leads to

smaller Da-values. This is attributed to higher tortuosity.

Similarly, sorption lowers at increasing salinity. Contrary to the

Da-values, the ĢEa-values do not depend on the orientation of

the smectite sample. The increase of ĢEa-values at higher dry

densities is attributed to the ion exchange enthalpy ƒ¢H•‹ of Cs+

and Na interchange in smectite, as well as to the lowering of

the porewater activity.

ACKNOWLEDGEMENT

The author would like to thank Mr. S. Kibe of PESCO Co.

Ltd. for supporting the analyses of Cs by HR ICP-MS.

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