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In-situ RBS Studies on Dissolution of Pb Atoms from the SiO2
Surface into Water Solutions
K. Morita, J. Yuhara, R. Ishigami, B. Tsuchiya, D. Ishikawa, K. Soda,
K. Saitoh1), T. Ohnuki2), S. Yamamoto3), Y. Aoki3),
K. Narumi2) and H. Naramoto2)
Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-01, Japan
National Institute of Advanced Industrial Science and Technology (AIST Chubu)1)
2266-98 Anagahora, Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan
Tokai Establishment2) and Takasaki Establishment3)
JAERI, Watanuki-cho, Takasaki 370-12, Japan
An in-situ RBS system has been developed to study the dissolution of Pb layers deposited
physically on the SiO2 surface of Si(100) crystal into water solutions with different pH values. It is found
that Pb atoms are not dissolved into alkaline water, but into acid water, and that the dissolution in the latter
case is the zero-th order reaction kinetics and the rate constant is 0.67×1013 atoms cm-2s-1, which corresponds
to 1.04×10-11 mol. cm-2s-1. The dissolution mechanism is discussed based on the experimental results.
is irradiated with a probing MeV ion beam, and to
determine the rate constants of sorption and dissolution of
various nuclides at the liquid-solid interface [7].
In this paper, we report the experimental data on
the rate constants of dissolution of Pb atoms deposited on
the SiO2 surface into water solutions with different values
of pH in which HNO3 or NH3OH are weakly dissolved.
It is shown that Pb atoms are not dissolved into alkaline
water, but into acid water.
INTRODUCTION
Understanding of adsorption and desorption
processes of atomic and molecular species at the
liquid-solid interface is of both fundamental and applied
interest. In the technology for geologic radioactive
waste disposal, adequate containment is estimated to
result in principal from low solubility-limited release
rates of the radioactive reactor products, even if
low-velocity flowing ground water is present. The
partition coefficients of various radioactive nuclides
between the surface of minerals and the modeled ground
water have been obtained using the radioactivity
measurement of nuclear waste products [1,2]. For
estimation of the period of time for the radioactive
material to reach the biosphere, reliable data on the rate
constants for the adsorption and desorption at the
liquid-solid
interface
are
primarily
required.
Fundamentally, the information on the adsorption sites at
the solid surface in contact with a liquid is also
interesting in relation to the stability of nuclides adsorbed
there.
The ion beam analysis techniques have been
applied for in-situ measurements of elements adsorbed at
the liquid-solid interfaces. The usefulness of RBS and
NRA for analysis of metal deposition, oxide formation
and ingress of elements in membrane of liquid-solid
interfaces has been demonstrated by several authors [3~6].
In the JAERI-Universities collaboration project for
advanced application of MeV ion beam materials analysis,
we have developed an in-situ RBS system in a chamber
connected to a beam line of a 3 MV Tandem Accelerator
in TIARA facilities in order to measure the depth
distribution of stable heavy nuclides adsorbed at the inner
surface of a thin film window of a liquid container, which
EXPERIMENTAL
The thin window of the silicon specimen was
fabricated using preferentially slow etching of heavily
B-implanted silicon.
The starting materials were
(100)-oriented silicon disks of 0.4 mm in thickness and of
10mm in diameter, one face being mirror-polished. 4
MeV B ions, of which the projected range is 5.5m, were
uniformly implanted into the mirror-polished surface to a
The
dose of 1×1016 ions cm-2 at random direction.
implanted disks were annealed at 950°C for 1 hour to
reduce damage produced by the implantation and to form
an oxide layer of about 0.25m on the surface. The
thinning process was described in detail elsewhere [8].
The silicon disk with a thinner window of 3mm
in diameter was used for the wall of the sample assembly,
which is installed in the small vacuum chamber of the
experimental apparatus shown in Fig.1. The sample
assembly was made of stainless steel and quartz glass,
and sealed with O-rings. The inner surface of stainless
steel components of the sample assembly was covered
with the tubes and plates of quartz. The vacuum
chamber was connected to a beam line of the 3MV
Tandem accelerator, in TIARA facilities of JAERI,
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
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FIGURE 1. Experimental arrangement of an in-situ RBS
system for measuring nuclides absorbed at the liquid-solid
interface
through a differentially pumped line equipped with an
emergent shatter moving quickly and electrically for
vacuum protection. The sample was irradiated with a
9MeV He++ ion beam through a carbon aperture slit with
a hole of 2mm in diameter in order to reduce the
background scattering from high-Z elements compared
with carbon. The back scattered ions were measured
with an annular-type SSB detector. The introduction of
liquid into assembly was performed through an inlet pipe
form the bottom, which was some what inclined to the
horizontal level.
Deposition of Pb layers onto the SiO2 surface of
the silicon window and measurement of the Pb thickness
were done in the other chamber which is connected to a
2MeV Van de Graaff accelerator in the Nagoya
University [9]. The pH value of water solutions
contacted with the sample was varied by weak dissolution
of HNO3 or NH3OH into water in ultra high purity.
FIGURE 2. Typical RBS spectra of 9 MeV He++ ion beam
from the silicon window of the sample assembly in which
Pb layers as-deposited on the backsurface, before injection
of water solution (a) and 10min after injection water
solution with pH5(b)
the RBS technique with 2 MeV He+ ion beam. The
thickness (coverage) of the Pb layers was measured as a
function of time elapsing after injection of water
solutions with different pH values into the inner chamber.
In Fig.3 is shown the Pb coverage as a function
of time elapsing after injection of deionized water into
the inner chamber. It is clearly seen from Fig.3 that the
Pb coverage does not change at all, namely Pb atoms on
the SiO2 surface are not dissolved into deionized water.
Fig.4 shows the Pb coverage as a function of
time elapsing after injection of water solution with pH
higher than 9 which includes dilute NH3. It is also clearly
seen from Fig.4 that Pb atoms are not dissolved into
alkaline water.
Fig.5 shows the Pb coverage as a function of
time after injection of water solution with pH 5 which
includes dilute HNO3. It is clearly seen from Fig.5 that
the Pb coverage decreases linearly with elapse time from
24 ML to 3ML. The rate constant of dissolution was
obtained to be 0.67×1013 atoms cm-2·s-1. This fact
indicates that the dissolution of Pb atoms is the zero-th
order reaction kinetics. Therefore, it is concluded the Pb
atoms are dissolved uniformly from the whole surface.
EXPERIMENTAL RESULTS AND
DISCUSSION
Dissolution behavior of Pb layers deposited on
the oxidized backsurface of the silicon (100) window into
the different water solutions has been measured under the
in-situ condition. Typical RBS spectra of the 9 MeV He++
ion beam from the silicon window with Pb layers of 24
ML in thickness on the backsurface are shown in Fig.2,
where (a) represents the RBS spectrum before injection
of water solutions and (b) represents the RBS spectrum at
10 min after start of injection of water solutions with
pH=5. In Fig.2 the backscattering yields at the channel
number between 320 and 420 are from the silicon
window, of which multiple peaks are ascribed to the
resonance nuclear reactions and a sharp peak yield,
marked by PbB, at channnel number of 600 is from the Pb
layers. The total peak area corresponds to the 24
monolayers in thickness which has been determined by
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Here, the dissolution mechanism of Pb from the
SiO2 surface is discussed. The sorption of Pb on the SiO2
surface is considered to be controlled by two different
mechanisms. One is the precipitation on the surface as
PbO. The other is the surface complex formation of Pb
with the silanol expressed by (2SiO)Pb. In such cases,
The dissolution of PbO and the desorption of
Pb2+ are described as follows:
Dissolution: PbO+2H+→Pb2++H2O
(1)
Desorption: (2SiO)Pb+2H+→2SiOH+Pb2+ (2)
Direct in-situ X ray adsorption measurement shows
that lead is inner-spherically bound to the surface of
γ–Al2O3[10]. No direct evidence of the sorption of Pb2+
on SiO2 has been reported. If the sorption of Pb2+ on SiO2
is similar to that on γ–Al2O3, Pb layer on SiO2 should be
mono-layer. Since thickness of the Pb layers is 24 ML
and the specimen has been exposed to normal air, most of
Pb are precipitated as PbO.
The dissolution of PbO in the solution depends
on the thermodynamic stability of PbO [11]. Since the
thermodynamic stability is examined by the saturation
index which is expressed by log (Q/K), where Q is the
activity product of species in the PbO dissolution reaction,
and K is the stability constant of PbO. The saturation
index of PbO in the different pH water solutions are
calculated using the EQ3NR software package [12]. The
thermodynamic data for PbO is taken from the data base
in EQ3NR. If the water saturation index is higher than 0,
PbO is considered to form thermodynamically in the
solution. The relationships between the saturation index
of PbO and the Pb2+ concentration at the different values
of pH are shown in Fig.6.
The dissolution rate should depend on the
saturation index or the Gibbs free energy when the
dissolution is congruent [11]. In the present study, PbO
dissolution is congruent because of no secondary mineral
formation. The Gibbs free energy is expressed by ∆G=RT
ln (Q/K) , where R is the gas constant and T is the
absolute temperature.
With decreasing ∆G, the
dissolution of the primary mineral increases [10]. The
saturation index at pH=9 is higher than 0, even at the Pb
concentration of 10-8 mol. cm-3. Thus, no dissolution of
Pb at pH 9 results from the stability of PbO in the
solution.
The saturation index of PbO at the values of
pH=5 and 7 are lower than 0, when the concentration of
Pb2+ is below 10-6 mol. cm-3, In the present study, the rate
constants of PbO dissolution at the values of pH=5 and 7
are 1.04×10-11 and 0 mol. cm-2s-1, respectively. Since the
dissolution reaction of PbO is expressed by Eq. (1), the
dissolution rate should depend on the square of the H+
concentration. The concentration of H+ at pH=7 is lower
by 100 times than that at pH 5. If the dissolution rate is
directly related to the square of the H+ concentration, the
rate constant of PbO dissolution at pH=7 is calculated to
be 1.04×10 - 15 mol. cm - 2 s - 1 . This value leads the
dissolution rate constant of PbO layer to be 6.7×108
FIGURE 3. Pb coverage as a function of time elapsing
after injection of deionized water
FIGURE 4. Pb coverage as a function of time elapsing
after injection of alkaline water
FIGURE 5. Pb coverage as a function of time elapsing after
injection of acid water
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atoms do not dissolute into alkaline water, but acid water
and that the dissolution in the latter case is the zeroth
order reaction kinetics and the rate constant is 0.67×1014
atoms cm-2s-1.
ACKNOWLEDGMENTS
This study is partly supported by Grant-in-Aid for
Scientific Research (B) in the Ministry of Education,
Science, Sports and Culture. The authors are grateful to
Dr. S. Tajima and the staff members of Electrostatic
Accelerator Group for cooperation in the case of the
3MV Tandem accelerator in the TIARA facilities.
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Aoki, H. Takeshita and H. Naramoto, Rad. Phys.
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FIGURE 6. The relation ship between the saturation index of
PbO and the Pb++ concentration calculated at different values
of pH
atoms cm-2s-1. This indicates that it takes 280 hours for
the 1 ML PbO to be dissolved into the water solution at
pH=7. Thus, in the present study no dissolution was
observed at pH=7.
The dissolution rate constant of PbO has not
been reported. The dissolution rate constant of anorthite
(CaAlSiO) is measured at pH=4.56 by Murakami et al
[13], at the different temperatures of 90, 150 and 210°C.
Extrapolation of this result gives the dissolution rate
constant at 25°C to be 4.9x10-9 mole. cm2s. Since the
crystal chemistry of PbO differs from that of anorthite,
the rate constant of PbO may differ from anorthite.
However, the dissolution rate constant of PbO at pH=5
(1.04×10-11 mol. cm-2s-1) is lower than that of anorthite at
pH=4.56. Since lower pH gives higher dissolution rate
constants, the dissolution rate constant of PbO obtained
in the present study is reasonable.
SUMMARY
The dissolution of Pb layers deposited physically
on the SiO2 surface of Si(100) crystal into water solutions
with different pH values has been measured using the in
situ RBS system developed. It has been found that Pb
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