Clay Science 12 Supplement 2, 57-62 (2006) In-situ, Real ...
Transcript of Clay Science 12 Supplement 2, 57-62 (2006) In-situ, Real ...
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: [email protected]
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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.
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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
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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
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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
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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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