Clay Science 12 Supplement 2, 57-62 (2006)

In-situ, Real Time AFM Study of Smectite Dissolution

under High pH Conditions at 25•K-50•Ž

YOSHIHIRO KUWAHARA

Department of Evolution of Earth Environments, Graduate School of Social and Cultural Studies , Kyushu University,Ropponmatsu, Fukuoka 810-8560, Japan

(Received August 23, 2005. Accepted December 28, 2005)

ABSTRACT

The dissolution behavior of smectite under alkaline conditions at 25•K to 50•Ž was investigated using

in-situ Contact mode atomic force microscopy (CMAFM) and Tapping mode AFM (TMAFM) .

Smectite particles dissolved via the retreat of the edge surfaces without the affect of the AFM tip,

except in some series of the dissolution experiment in CMAFM. No etch pits were observed forming

on the basal surface within the experimental durations. The edge surface area (ESA)-normalized

dissolution rates of smectite at a certain pH and temperature condition, therefore, have a constant value

independent of the particle size, indicating the essential dissolution rate. In contrast , the dissolutionrates normalized to the total surface area (TSA) of smectite varied with the particle size . The

activation energy of smectite dissolution under alkaline conditions appears to be dependent on pH, like

as kaolinite. A model dissolution rate equation which includes simultaneously the effect of pH and

temperature was deduced from the effect of the activation energy on pH, the rate equation of smectite

dissolution at 25•Ž, and the Arrhenius equation. The rates estimated using the model are in good

agreement with experimental dissolution rates between 20•Kand 60•Ž.

Key words: Smectite, Dissolution kinetics, AFM, Alkaline condition, High-level nuclear wastes

INTRODUCTION

Smectite-rich bentonite has been recognized as a suitablematerial for the engineering barrier designed for storage ofhigh-level nuclear wastes in underground repositories1).Smectite prevents groundwater interaction with the metalcanisters, immobilizes undesirable cations from theradioactive waste, and protects the environment from any

possible leakage, due to its swelling and cation exchangecapacity1-3). The waste canister wrapped by the clay andoverpack barrier are finally sealed with a concrete plug.The pore water - concrete reactions may give rise to high pHconditions2-5). Therefore, the durability and the dissolutionbehavior of smectite under alkaline conditions are keysubjects that must be examined.

The dissolution rates of smectite have usually been derivedfrom macroscopic wet chemical data from laboratoryexperiments4, 6-7). These data, however, do not directly

provide smectite dissolution mechanisms8). In such thestudies, the N2 BET surface area has generally been used forthe normalization of the dissolution rate of minerals.However, the reactive surface sites of phyllosilicates, likesmectite, are distributed unevenly between the basal andedge surfaces and strongly anisotropic in their response to

dissolution reactions8-9). In addition, the N2 BET surface

area can vary significantly even for the same samples due to

a different number of platelets stacked per quarsicrystal8, 10).

Such dissolution rates have little relevance to phyllosilicates

and must throw us into confusion.

In-situ AFM study on the mineral dissolution makes it

possible to characterize the reactive surfaces and to estimate

the essential dissolution rate8). Recent in-situ AFM studies

on the smectite dissolution have suggested that the

dissolution rates normalized to the ESA should be directly

comparable from clay mineral to clay mineral because the

reactive surface is only the edge surfaces, at least at room

temperature8, 11-12). These studies, however, were carried

out only by CMAFM where the dissolution process may be

altered by the AFM tip scanning across the smectite particles.

In-situ TMAFM, which results in a much weaker tip-sample

interaction, may work out this problem, although it is very

difficult to collect stable image in TMAFM in liquid11).

In this study, we examined the dissolution behavior of

single crystallites of smectite under alkaline conditions at 25•K

to 50•KC, using in-situ CMAFM and TMAFM analyses. The

main goals of this work are to comprehend the dissolution

behavior of smectite under alkaline conditions, to determine

the reliable dissolution rate, and to reveal the effects of pH

and temperature on the dissolution rate.

E-mail address of the corresponding author: ykuwa@scs.kyushu-u.ac.jp

58

Table 1. The results of the 0.01M NaOH solution experiments.

*CM: CMAFM, **TM: TMAFM, •õ: In these series the affect of the AFM tip on the dissolution was observed.

Table 2. The results of the 0.001M NaOH solution experiments.

•õ

: In these series the affect1of1the AFM tip on the dissolution was observed.

59

EXPERIMENTAL METHODS

Purified smectite Kunipia-P(R) (Kunimine Industry Co Ltd)

used in this study is Na-montmorillonite and has the

structural formula: Na0.78K0.02Ca0 .12 (Al3.02Mg0.66Fe0.18Ti0.02)

(Si7.74Al0.26)O20(OH)413). The unit cell parameters chosen

for montmorillonite are a=5.18•ð, b=8.96•ð, c=9.97•ð,

and ƒÀ=99.9•‹14), and were used to calculate dissolution rate.

The N2 BET surface area of Kunipia-P(R) is 4m2/g13, 15).

The smectite dissolution was observed by a Nanoscope III

with a Multimode SPM unit (Digital Instruments) using a

fluid cell, operating in both CMAFM and TMAFM. To fix

clay particles onto an AFM mount in solution, the

polyethyleneimine (PEI) coating method16) was used. All

smectite particles used in the dissolution experiment were

single smectite layer. The smectite particles were reacted at

25•‹, 40•‹, 50•Ž, with 0.01M (pH=11.8 at 25C•‹) and 0.001M

(pH=11.2) NaOH solutions. The solutions were flowed

through the fluid cell with a constant rate of 0.01ml/min

controlled by a peristaltic pump at 25•Ž and with a constant

rate of 0.4ml/min by the gravity flow-through system at 40•‹

and 50•Ž. The AFM imaging and the analysis of the image

were followed by our previous studies17-18).

RESULTS AND DISSCUSION

Dissolution behavior of smectite

The results of the dissolution experiments of smectite are

listed in Tables 1 and 2. We could compare the smectite

dissolution in in-situ CMAFM with that in in-situ TMAFM

only at 25•Ž. Smectite particles appeared to dissolve via

the retreat of the edge surfaces without the effect of the AFM

tip, except some experimental series in CMAFM.

Especially in in-situ TMAFM, mechanically enhanced

dissolution rates due to the affect of the AFM tip were not

observed18). Some curved edge surfaces gradually

straightened and the straightened edge surface appeared to

retreat with a constant rate during the dissolution (Figs. 1 (a)

to (c)). No etch pits were observed forming on the basal

surface within the experimental durations at any temperature.

However, the affect of the AFM tip on the dissolution rate

could not completely be excluded in some in-situ CMAFM

series, especially under higher pH and temperature

conditions (Tables 1, 2). Some edge portions of the particle

were obviously scratched by the AFM tip, although some

curved edge surfaces of the untreated particle appeared to

straighten during the dissolution (Figs. 1 (d) to (f). This

tendency may be caused by the stronger adhesive interaction

between the tip and the PEI coating with increase of

temperature and NaOH concentration in solution. It may be

risky to estimate the dissolution rate of smectite and to

interpret the dissolution mechanism only from the data by

CMAFM.

The dissolution rates normalized to the ESA had a constant

value at each experimental condition and did not show the

dependence on the particle size, whereas the

Fig. 1. CMAFM height images showing the dissolution of a "single smectite layer- particle in 0.01M NaOH solution. (a)-(c) No.CM-40-001-3 at

40•Ž after (a) 0sec, (b) 1h 7 min 31sec, and (c) 1h 58min 11sec (Scan area: 1•~1ƒÊm). (d)-(f) No.CM-50-001-2 at 50•Ž after (d) 0sec, (e) 1h 9

min 51sec, and (f) 2h 2min 10sec (Scan area: 3•~3ƒÊm). Dot lines indicate that curved or rough edge surfaces of smectite gradually straightened

during the dissolution experiments. Arrows show that some edge portions were scraped by the AFM tip

60

Fig. 2. Plots of normalized dissolution rate vs. initial TSA of individual

smectite particles in 0.01M NaOH. The dissolution rates normalized tothe ESA have a constant value while the dissolution rates normalized tothe TSA increase with the decrease of initial TSA of smectite particle.Open marks show the experimental series affected by the AFM tip

Fig. 3. Plots of normalized dissolution rate vs. initial TSA of individual

smectite particles in 0.001M NaOH. The dissolution rates normalized

to the ESA have a constant value while the dissolution rates normalized

to the TSA increase with the decrease of initial TSA of smectite particle.

Open marks show the experimental series affected by the AFM tip.

TSA-normalized dissolution rates varied with the particle

size at all temperature (Figs. 2, 3). These results show that

the reactive surface for smectite dissolution is only the edge

surfaces and the basal surfaces are unreactive, at least up to

50•Ž. The mean ESA-normalized dissolution rates were

3.7•~10-14, 9.1•~10-14, and 2.0•~10-13 (mol/m2•Es) under pH 11.8

at 25•‹, 40•‹, and 50•Ž, respectively, and 1.6•~10-14, 3.6•~10-14,

and 7.7•~10-14 (mol/m2•Es) under pH 11.2 at 25•‹, 40•‹, and 50•Ž,

respectively. The dependence of the TSA-normalized

dissolution rates on the particle size has been estimated by

Kuwahara (2004, 2005)17-18). According to them, the

dependence is shown as following equations:

log (TSA-normalized dissolution rate)=1.558 log (ESA/TSA)-9.699 (1)

(at pH 11.8 and 25•Ž)

Fig. 4. Arrhenius plot for the ESA- and TSA- normalized dissolution

rates at pH 11.2 and 11.8.

log(TSA-normalized dissolution rate)=1.512log ESA/TSA)-9.413 (2)

(at pH 11.8 and 40•Ž)log (TSA-normalized dissolution rate)=1.673log (ESA/TSA)-8.691 (3)

(at pH 11.8 and 50•Ž)log (TSA-normalized dissolution rate)=1.391log (ESA/TSA)-10.425 (4)

(at pH 11.2 and 25•Ž)log (TSA-normalized dissolution rate)=1.548log (ESA/TSA)-9.735 (5)

(at pH 11.2 and 40•Ž)log (TSA-normalized dissolution rate)=1.494log(ESA/TSA)-9.530 (6)

(at pH 11.2 and 50•Ž)

Regarding a single smectite particle as a disk-shaped one, the

ESA/TSA ratio of the particle shows implicitly the particle

size (e.g., the particle with a diameter of 1ƒÊm or 0.1ƒÊm has

an ESA/TSA ratio of about 0.004 or 0.04, respectively).

The effect of pH and temperature on the dissolution rate of

smectite

Many of the dissolution rates of smectite reported in the

previous studies have been normalized to the N2 BET surface

area4-7), although the N2 BET surface area is not adequate for

the normalization of the dissolution rate of smectite8, 11-12, 17).

The dissolution rates determined from AFM data need to be

compared with those in the previous studies, to check

whether there is a difference in dissolution rate between them

or not. Recently, Inoue et al. (2005)15) and Kuwahara

(2005)18) have attempted to compare the dissolution rates

from the in-situ AFM study with those normalized to the N2

BET surface area derived from macroscopic wet chemical

data, by renormalizing them to the estimated SSA for single

smecite layer particles. This attempt yielded the dissolution

rate raw of smectite under alkaline conditions at 25•Ž:

(7)

where r is the dissolution rate and aH+ indicates protonactivity in solution.

The activation energies for the smectite dissolution at pH11.2 and 11.8 were calculated based on the Arrheniusequation (Fig. 4). Judging from the activation energies

61

Fig. 5. The effect of pH on the activation energies of smectite (squaresand diamond) and kaolinite (circles) under alkaline conditions.

from the previous studies4, 6) as well as the present one, thedependence of the activation energy (E) on pH is obvious forthe smectite dissolution (Fig. 5) and can be estimated at:

(8)

Once the dependence of the activation energy on pH can beestimated, we can evaluate the effect of temperature as wellas pH on the dissolution rate of minerals6, 19). Combiningthe Arrhenius equation and Eq. (8), one obtains

Fig. 6. Comparison between experimental dissolution rates (smectite andillite) and calculated dissolution rates (solid lines) showing the effect oftemperature and pH on those.

(9)

where r1 and r2 stand for the dissolution rates at temperatureT1 and T2, respectively, and R is the gas constant.Substituting Eq. (7) into Eq. (9):

(10)

Eq. (10) is an empirical one that expresses simultaneouslythe effect of temperature and pH on the dissolution rates ofsmectite at alkaline conditions. Therefore, the dissolutionrate laws of smectite under alkaline conditions (pH>8.5) ateach temperature can be estimated substituting temperatureinto Eq. (10):

(11)(12)(13)(14)(15)

The Eq. (12) showing the rate law at 25•Ž is identical to the

Eq. (7).

The calculated dissolution rates of smectite under alkaline

conditions at 20•‹ to 80•Ž are shown in Fig. 6. The

dissolution rates, which were renormalized to the SSA for

single smectite layer particles as mentioned above, from the

previous studies, the dissolution rates normalized to the N2

BET surface area of illite under similar conditions21), and the

TSA-normalized dissolution rates for smectite particles

having a diameter of 1ƒÊm based on our AFM data (Eqs. (1)

-(6)) are also included in Fig. 6, to compare them with the

calculated dissolution rates. The dissolution rates estimated

from the model are in good agreement with experimental

dissolution rates between 20•‹ and 60•Ž, except the slower

rates of Sato (2004)13). The dissolution rates of illite under

similar conditions21) are very close to that of smectite by our

estimation (Fig. 6). This result is in agreement with their

suggestion that the dissolution mechanism (e.g., reactive

surface area (ESA), the rate-limiting step) of illite is identical

to that of smectite21).

In contrast, there is a large difference in the rate between

the model and experiments above 60•Ž. Bauer and Berger

(1998)4) concluded that the rate-limiting step for the smectite

dissolution at both 35•‹ and 80•Ž is identical. Their

interpretation, however, leaves room for doubt because their

experiments and analysis were carried out only at two points

of temperature, moreover by the macroscopic wet chemical

methods. The large difference in dissolution rate between

the model and experiments implies the possibility that the

rate-limiting step for the smectite dissolution changes

between 60•‹ and 80•Ž.

CONCLUSIONS

This in-site CMAFM and TMAFM studies of smectite

dissolution demonstrated that the reactive surface area under

alkaline conditions is only the edge surfaces and the

dissolution rates normalized to the ESA is independent of the

particle size, whereas the dissolution rates normalized to the

62

TSA is dependent on the particle size. We could estimate

the dependence of the dissolution rate on the particle size.

The activation energy of smectite dissolution under alkaline

conditions was dependent on pH. We propose a model

dissolution rate equation, which includes simultaneously the

effect of pH and temperature, based on the effect of the

activation energy on pH, the rate law of smectite dissolution

at 25•Ž, and the Arrhenius equation. The comparison of the

dissolution rates between the model and experiments

revealed that the rate-limiting step for the smectite

dissolution may change above 60•Ž.

ACKNOWLEDGEMENTS

The author is grateful to the staff of the committee of the

long-term stability of engineering buffer materials in the

Nuclear Safety Research Association for many discussions

and helpful suggestions. This study was supported in part

by a Research Grant from the Nuclear Safety Research

Association under contract with the Japan Nuclear Fuel

Cycle Development Institute and by the Grant-in-Aid for

Scientific Research (Y. Kuwahara, No.17540457) from the

Japan Society for the Promotion of Science.

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