an electrokinetic/fe0 permeable reactive barrier system for the treatment of nitrate-contaminated...
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An electrokinetic/Fe0 permeable reactive barrier system forthe treatment of nitrate-contaminated subsurface soils
Tasuma Suzuki*, Yukinori Oyama, Mai Moribe, Masakazu Niinae
Department of Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University,
2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan
a r t i c l e i n f o
Article history:
Received 12 September 2011
Received in revised form
12 November 2011
Accepted 17 November 2011
Available online 27 November 2011
Keywords:
Nitrate
Electrokinetic remediation
Permeable reactive barrier
Zero-valent iron
Iron oxides
* Corresponding author. Tel.: þ81 836 85 969E-mail address: [email protected]
0043-1354/$ e see front matter ª 2011 Elsevdoi:10.1016/j.watres.2011.11.048
a b s t r a c t
Effective nitrate removal by Fe0 permeable reactive barriers (Fe0 PRB) has been recognized
as a challenging task because the iron corrosion product foamed on Fe0 hinders effective
electron transfer from Fe0 to surface-bound nitrate. The objectives of this study were (i) to
demonstrate the effectiveness of an electrokinetic/Fe0 PRB system for remediating nitrate-
contaminated low permeability soils using a bench-scale system and (ii) to deepen the
understanding of the behavior and fate of nitrate in the system. Bench-scale laboratory
experiments were designed to investigate the influence of the Fe0 content in the permeable
reactive barrier, the pH in the anode well, and the applied voltage on remediation effi-
ciency. The experimental results showed that the major reaction product of nitrate
reduction by Fe0 was ammonium and that nitrate reduction efficiency was significantly
influenced by the variables investigated in this study. Nitrate reduction efficiency was
enhanced by either increasing the Fe0 content in the Fe0 reactive barrier or decreasing the
initial anode pH. However, nitrate reduction efficiency was reduced by increasing the
applied voltage from 10 V to 40 V due to the insufficient reaction time during nitrate
migration through the Fe0 PRB. For all experimental conditions, nearly all nitrate nitrogen
was recovered in either anode or cathode wells as nitrate or ammonium within 100 h,
demonstrating the effectiveness of the system for remediating nitrate-contaminated
subsurface soils.
ª 2011 Elsevier Ltd. All rights reserved.
1. Introduction contaminants before contaminants extensivelymigrate in the
Nitrate contamination of drinking water sources is a world-
wide environmental issue. The conventional technologies
used in drinking water purification plants for nitrate removal
include reverse osmosis and ion exchange, which are some-
times combined with biological denitrification for the subse-
quent treatment of concentrated nitrate. Although these
technologies are capable of removing nitrate effectively, in situ
approaches are highly desirable because in situ remediation
has the advantage of removing, degrading, or immobilizing
0; fax: þ81 836 85 9601..jp (T. Suzuki).ier Ltd. All rights reserve
environment.
Permeable reactive barriers using zero-valent iron (Fe0
PRBs) as the reactive media represent a cost-effective in situ
technology for remediation of contaminated subsurface
regions (Henderson and Demond, 2007; Cundy et al., 2008;
Thiruvenkatachari et al., 2008). In fact, monitoring the long-
term performance of field-scale or pilot-scale Fe0 PRB
systems has demonstrated that Fe0 PRB systems effectively
remediate groundwater contaminated by various contami-
nants including organic compounds (McMahon et al., 1999;
d.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 7 7 2e7 7 8 773
Vogan et al., 1999; Guerin et al., 2002; Wilkin et al., 2003) and
heavy metals (Puls et al., 1999a, 1999b; Wilkin et al., 2003;
Wilkin et al., 2005; Ludwig et al., 2009). However, effective
long-term nitrate removal by traditional Fe0 PRB systems has
been recognized as a challenging task (Thiruvenkatachari
et al., 2008) because, as noted by several researchers based
on the results obtained from a series of batch experiments
performed at near neutral initial pH (Cheng et al., 1997; Huang
et al., 1998, 2003; Choe et al., 1999; Huang and Zhang, 2002;
Huang and Zhang, 2005; Huang and Zhang, 2006; Suzuki
et al., unpublished results), the iron corrosion products
foamed on Fe0 hinders effective electron transfer from Fe0 to
surface-bound nitrate (i.e., slows down the reaction).
In this study, we developed an Fe0 PRB system that was
coupled with an electrokinetic process (EK/Fe0 PRB system). In
the EK/Fe0 PRB system, the EK process was expected to play
three important roles that may help overcome the shortcom-
ings of traditional Fe0 PRB systems. First, one of the main
strengths of EK processing is its ability to induce themovement
of dissolved compounds through low permeability soils by
electroosmosis and ion migration (Acar and Alshawabkeh,
1993; Acar et al., 1995; Kim et al., 2002; Reddy and
Chinthamreddy, 2003; Reddy and Saichek, 2004; Saichek and
Reddy, 2005; Niinae et al., 2008; Alshawabkeh, 2009; Reddy
et al., 2009; Reddy, 2010). Therefore, the EK/Fe0 PRB system
could be a novel remediation system that is capable of
producing more uniform nitrate transport in heterogeneous
subsurface soils compared to traditional Fe0 PRB systems.
Second, in the EK/Fe0 PRB system, acid generated at the anode
byelectrochemical reactions couldprevent the formationof the
passive layer of iron corrosion products on Fe0 particles.
Consequently, more effective long-term nitrate removal could
be achieved by the EK/Fe0 PRB system. Finally, the EK/Fe0 PRB
system is capable not only of converting toxic nitrate into less
toxic substances but also of recovering the dissolved reaction
products.
The specific objectives of this study were (i) to demonstrate
the effectiveness of the EK/Fe0 PRB system for the treatment of
nitrate-contaminated low permeability soils using a bench-
scale system and (ii) to deepen the understanding of the
behavior and fate of nitrate in the EK/Fe0 PRB system by
ascertaining the effects of the Fe0 content of the PRB, the pH of
the anode well, and the applied voltage. Insights obtained
from this study provide fundamental information that is
useful for optimizing remediation conditions.
2. Materials and methods
2.1. Materials and chemicals
All solutions were prepared with distilled deionized (DDI)
water with a resistivity greater than 18 MU cm�1 (WA200,
Yamato Scientific Co. Ltd., Japan). All chemical reagents
(guaranteed reagent grade) were purchased either from Naca-
lai Tesque, Inc. or Wako Pure Chemical Industries, Ltd. and
used without further purification. The zero-valent ion (Fe0)
purchased from Nacalai Tesque, Inc. was in the size range of
20e60mesh, and themeasuredBET surfaceareawas 1.1m2g�1
(Jemini 2375, Micromeritics Instrument Corp., GA). Kaolinite
from the Dixie Rubber Pit was purchased from Nichika Inc.
Particle size distribution analysis using a laser diffraction
particle size analyzer (SALD-2000J, Shimadzu Corp., Japan)
showed that the kaolinite contained approximately 63% clay-
size particles (<5 mm), and the remaining material was silt.
Additionally, preliminary experiments showed that the water
content of the saturated kaolinite was 50% by weight.
2.2. EK/Fe0 PRB experimental setup and procedures
A schematic of the EK/Fe0 PRB system is shown in Fig. 1. A PVC
column measuring 2 cm in diameter and 20 cm in length was
loaded with non-contaminated saturated kaolinite (Soil A),
artificially nitrate-contaminated saturated kaolinite (Soil B),
and non-contaminated Fe0 PRB (Soil C). Then, both sides of the
PVC column were sealed with a 0.2 mm-mixed cellulose ester
typemembrane filter to prevent outflowof soil particles. Soil A
was prepared by mixing an equal weight of kaolinite and DDI
water using a shaker (SA300, Yamato Scientific Co. Ltd.,
Japan). In a similar way, Soil B was prepared by mixing an
equal weight of kaolinite and an aqueous NaNO3 solution. Soil
C was prepared bymixing kaolinite, Fe0, and excess DDI water
and then discarding the supernatant after centrifuging the
mixture at 1220 g for 20 min using a benchtop centrifuge (CT4,
Hitachi Koki Co. Ltd., Japan).
Once loaded with Soils AeC, the PVC column was con-
nected to anode and cathode wells in which graphite elec-
trodes were placed. The graphite electrodes were then
connected to a DC power supply (GP060-10R, Takasago Ltd.,
Japan) providing constant voltage across the column. Anode
and cathode wells were filled with a 0.05 mol/L NaCl solution.
To avoid the water flow driven by the difference in static fluid
pressure, thewater levels of the anode and cathodewellswere
maintained constant by overflowing the solutions to reser-
voirs. The pH in the cathodewell was held constant during the
experiments at 6.2 using an automated titrator (AUT-701,
DKK-TOA Corp., Japan). The pH in the anode well was
adjusted before the experiments to either 2.5 or 4.5 using HCl,
and there was no pH control during the experiments.
The current was monitored using a direct current (DC)
current sensor (CMD-4-DC01-SC, U.R.D. Co. Ltd., Japan) and
recorded using a data logger (XL111-M, Yokogawa Electric
Corp., Japan).Water sampleswere collected from reservoirs at
appropriate time intervals and analyzed for NO�3 , NO�
2 , and
NHþ4 using an HPLC system with conductivity detection
(Prominence series, Shimadzu Corp., Japan). After energizing
for 100 h, the power of the DC power supply was turned off,
and the final pH in the Fe0 PRB was measured using a spear
type pH electrode (GST-5724C, DKK-TOA Corp., Japan). The
experimental conditions for each experiment are summarized
in Table 1.
3. Results and discussion
3.1. Electrochemical redox reactions at graphiteelectrodes
Electrolysis under DC current creates a reducing environment
at the cathode and an oxidizing environment at the anode.
Fig. 1 e Schematic diagram of the EK/Fe0 PRB system.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 7 7 2e7 7 8774
Therefore, before NO�3 contaminated soil was treated using
the EK/Fe0 PRB system, blank experiments were performed to
investigate the possible generation of nitrogen species at
graphite electrodes (Tests 2e3 in Table 1). The Fe0 content in
the PRB and initial anode pH were 5 wt% and 2.5, respectively.
The results of these trials are shown in Fig. 2. It is clear from
Fig. 2 that the generation of NO�3 and NHþ
4 was negligible at
10 V. However, when the applied voltage was increased to
40 V, the amount of NHþ4 generated at the cathode increased
Table 1 e Experimental conditions and results for tests 1e8.a
Test Initial conditionsb
NO�3 in Soil B
(mgN/kg wet soil)Fe0 in PRB
(wt%)Applied
voltage (V)Init
anode
1 150 0 10 2.5
2 0 5 10 2.5
3 0 5 40 2.5
4 150 80 10 2.5
5 150 50 10 2.5
6 150 5 10 2.5
7 150 5 10 4.5
8 150 5 40 4.5
a For all tests, both the anode and cathode wells were filled with 0.05
energization time was 100 h.
b The initial pH value of Soil Bwas ca.5.5. The initial pH value of Fe0 PRBsw
dissolution.
c The pH in the Fe0 PRB varied depending on the location, generally the l
values shown here were obtained from the soils located at approximatel
d Energy consumption for remediation was calculated by summing the p
1.5 min. For the experiments performed at 10 V (Tests 4e7), the remediatio
from contaminated Soil B, was determined to be 100 h according to the
anode well in 20 h (Fig. 5). Therefore, the remediation time was assumed t
with time during the 100 h-experiment. The generation of
NHþ4 at the cathode was expressed by Eqn. (1) representing the
reaction between dissolved nitrogen gas and hydrogen gas
generated at the cathode via water electrolysis.
3H2ðgÞ þN2ðgÞ þ 2Hþ/2NHþ4 (1)
Consequently, although it is apparent that the NHþ4 gener-
ation rate at the cathode varies depending on electrolyte
Anode pHafter 100 h
PRB pHafter 100 hc
Energy consumptionfor remediationd (Wh)
ialpH
2.2 2.4 e
2.4 6.1 e
1.9 4.8 e
2.4 6.7 8.9 (in 100 h)
2.4 5.9 6.9 (in 100 h)
2.3 5.2 8.0 (in 100 h)
3.5 7.0 7.3 (in 100 h)
2.2 5.0 26.4 (in 40 h)
mol/L NaCl solution. The pH in the cathode well was 6.2, and the
as higher than that for Soil B (6.0e7.5) due to acid consumption by Fe0
owest for the anode side and the highest for the cathode side. The pH
y 1 cm from the anode.
roduct of the applied voltage and the current with the time interval of
n time, which was defined as the time required to recover all NO�3 eN
results shown in Figs. 3e5. For Test 8, NO�3 finished migrating to the
o be 40 h considering the time for NHþ4 migration to the cathode well.
0
2
4
6
8
10
0 20 40 60 80 100
NH4+ (10 V, cathode)
NO3- (10 V, anode)
NH4
+ (40 V, cathode)
NO3
- (40 V, anode)
HN
4+O
N ro 3-
)Ng
m( sllew ni ssa
m
Time (hours)
Fig. 2 e Mass of NOL3 and NHD
4 formed through
electrochemical redox reactions at electrodes. The Fe0
content in the PRB and initial anode pHwere 5 wt% and 2.5,
respectively. See Test 2 (for 10 V) and Test 3 (for 40 V) in
Table 1 for more detailed experimental conditions.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 7 7 2e7 7 8 775
conditions in wells, it was concluded that the generation of
NO�3 and NHþ
4 during the 100 h-experiment was negligible at
the applied voltage of 10 V, while significant amounts of NH4þ
were generated in the cathode at 40 V. Accordingly, when
experiments were performed at 40 V (Test 8 in Table 1),
0
20
40
60
80
100
0 20 40 60 80 100
Cathode NO 3 -
Cathode NH 4 +
Anode NO 3 -
AnodeNH 4
+
Total N
) %
( y r e v o c e r n e g o r t i
N
Time (h)
ba
0
20
40
60
80
100
0 20 4
)%( yrevocer negorti
N
c
Fig. 3 e Influence of the Fe0 content in the PRB on the NO3- redu
and initial pH in the anode well were 10 V and 2.5, respectively. (
content in the PRB [ 50 wt% (Test 5), (c) Fe0 content in the PRB
nitrogen recovery (NR) from the NO�3 contaminated Soil B,
which was defined by Eqn. (2), was not calculated for NHþ4 in
the cathode well.
NR ¼ Nitrogen species recovered in wells ðmgNÞTotal NO�
3 in the contaminated soil ðmgNÞ (2)
3.2. Influence of the Fe0 content in the PRB
To investigate the influence of the Fe0 content in the PRB on
NO�3 reduction efficiency and nitrogen recovery (NR), three
sets of experiments, each of which had a different Fe0 content,
were performed (Tests 4e6 in Table 1). The applied voltage
and initial pH in the anode well were 10 V and 2.5, respec-
tively. As shown in Fig. 3, when the Fe0 content was higher
than 50 wt%, nearly 100% of NO�3 eN was recovered as NHþ
4 in
the cathode well over the course of 100 h. However, when the
Fe0 content was decreased to 5 wt%, because of an insufficient
amount of Fe0, only 60% of NO�3eN was recovered as NHþ
4 in
the cathode well in 100 h. The remaining NO�3eN penetrated
through the Fe0 PRB and was recovered in the anode well.
Therefore, it was demonstrated that the Fe0 content in the PRB
was an important factor in controlling the performance of the
EK/Fe0 PRB system.
These results showed that the major reaction product of
NO�3 reduction by the Fe0 PRB was NHþ
4 , which is consistent
with the previous studies that investigated the reduction of
NO�3 by Fe0 using batch systems (Cheng et al., 1997; Huang
et al., 1998, 2003; Choe et al., 1999; Huang and Zhang, 2002,
0
20
40
60
80
100
0 20 40 60 80 100
) %
( y r e v o c e r n e g o r t i
N
Time (h)
0 60 80 100
Time (h)
ction efficiency and nitrogen recovery. The applied voltage
a) Fe0 content in the PRB[ 80 wt% (Test 4 in Table 1), (b) Fe0
[ 5 wt% (Test 6).
0
20
40
60
80
100
0 20 40 60 80 100
Cathode NO3
-
Anode NO3
-
Anode NH4
+
Nitr
ogen
rec
over
y (%
)
Time (h)
Fig. 5 e Nitrogen recovery in wells as a function of
energization time (Test 8 in Table 1). Experimental
conditions: Fe0 content in the PRB [ 5 wt%, applied
voltage [ 40 V, initial anode pH [ 4.5. The data for NHD4 in
the cathode well is not shown because a significant
amount of NHD4 was formed through electrochemical N2
reduction (Fig. 2). Correspondingly, the data for total
nitrogen is also not shown.
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 7 7 2e7 7 8776
2005, 2006; Huang and Zhang, 2004; Hwang et al., 2011; Suzuki
et al., unpublished results). These results also clarified the
behavior and fate of NO�3 in the EK/Fe0 PRB system. Under the
electric potential gradient, the negatively charged NO�3 ion
migrated to the anode, reaching the Fe0 PRB and being reduced
to NHþ4 by reacting with Fe0 in the PRB. The positively charged
NHþ4 ion then migrated to the cathode. However, if the NO�
3
reduction rate in the Fe0 PRB was not fast enough, a portion of
NO�3 penetrated the Fe0 PRB without being reduced into NHþ
4
and was recovered in the anode well.
3.3. Influence of the initial pH in the anode well
To investigate the influence of the pH in the anode well, the
initial anode pHwas increased from 2.5 to 4.5 with holding the
Fe0 content in the PRB and the applied voltage the same as in
Test 6 (Tests 7 in Table 1). The obtained results are shown in
Fig. 4. The comparison between Figs. 3(c) and 4 showed that
the percentage of NO�3 penetrating through the Fe0 PRB
increased from 30 % to 60 % accompanying the increase in the
initial anode pH from 2.5 to 4.5. This result indicated that the
reaction rate between NO�3 and Fe0 in the PRB was enhanced
by decreasing pH, which is also consistent with the results of
previous studies in which batch experiments were performed
to investigate the effects of aqueous pH on reaction kinetics
(Cheng et al., 1997; Huang et al., 1998; Choe et al., 1999; Huang
and Zhang, 2004; Suzuki et al., unpublished results).
3.4. Influence of the applied voltage
Another set of experiments (Test 8 in Table 1) was performed
to investigate the influence of the applied voltage. In this
experiment, the applied voltage was increased from 10 V to
40 V, while the Fe0 content in the PRB and the initial anode pH
0
20
40
60
80
100
0 20 40 60 80 100
Cathode NO3
-
Cathode NH4
+
Anode NO3
-
AnodeNH4
+
Total N
Nitr
ogen
rec
over
y (%
)
Time (h)
Fig. 4 e Nitrogen recovery in wells as a function of
energization time (Test 7 in Table 1). Experimental
conditions: Fe0 content in the PRB [ 5 wt%, applied
voltage [ 10 V, initial anode pH [ 4.5.
were held the same as in Fig. 4 (5 wt% and pH ¼ 4.5). The ob-
tained results are shown in Fig. 5. A comparison between Figs.
4 and 5 showed that the proportion of NO�3 penetrating
through the Fe0 PRB increased from 60 % to 90 % accompa-
nying the increase in the applied voltage from 10 V to 40 V.
It is important to discuss the reason behind the deterio-
ration in NO�3 reduction efficiency caused by the increase in
the applied voltage because increasing the applied voltage
concurrently influences several factors controlling the NO�3
reduction efficiency. Increasing the applied voltage enhances
the electrolysis of water at the anode, which results in greater
acidification in the Fe0 PRB. In fact, the pH in the Fe0 PRB after
100 h of energization for the 40 V experiment (Test 8) was 5.0,
which was lower than the corresponding value (7.0) obtained
from the 10 V experiment (Test 7). Because the NO�3 reduction
rate by Fe0 was accelerated by decreasing pH (Cheng et al.,
1997; Huang et al., 1998; Choe et al., 1999; Huang and Zhang,
2004; Suzuki et al., unpublished results), the NO�3 reduction
efficiency was expected to be improved by increasing the
applied voltage from 10 V to 40 V. However, increasing the
applied voltage also accelerated the NO�3 migration rate
toward the anode well. As illustrated in Fig. 3(c) and Fig. 4, the
NO�3eN mass recovered in the anode reached a plateau in
roughly 60 h, showing that 60 h was required for NO�3 mole-
cules located at the far left column to migrate all the way to
the anodewell at the applied voltage of 10 V. In contrast, at the
applied voltage of 40 V, 20 h was sufficient to reach a plateau
as shown in Fig. 5. This discrepancy clearly demonstrated that
NO�3 molecules migrated faster in the Fe0 PRB at higher
applied voltages and indicated that adequate reaction time
between NO�3 and Fe0 might not be obtained at 40 V.
In summary, increasing the applied voltage could lead to
improvements in the NO�3 reduction efficiency because the Fe0
wat e r r e s e a r c h 4 6 ( 2 0 1 2 ) 7 7 2e7 7 8 777
PRB is more effectively acidified, but could alternatively lead
to the deterioration of the NO�3 reduction efficiency because
the reaction time between NO�3 and Fe0 is reduced. Under the
experimental conditions investigated, the NO�3 reduction
efficiency was decreased by increasing the applied voltage
from 10 V to 40 V, indicating that the reduced contact time had
a larger negative impact compared to the positive impact
resulting from stronger soil acidification.
4. Discussion
The pH in the Fe0 PRB after 100 h of energization was in the
range of 5.0e7.0 (Tests 4e8 in Table 1), which was high
compared to the pH in the anodewell (pH 2.2e3.5). However, it
is important to note that the pH discrepancy did not mean
that the acid generated at the anode was not effectively used
to acidify Fe0 PRBs. In fact, when Fe0 was absent in the PRB
(Test 1), the pH in the PRB successfully dropped to 2.4, which
was about the same as the pH in the anode well (pH 2.2). This
result demonstrated that an acid front actually reached the
Fe0 PRB in 100 h. Thus, the observed pH discrepancy between
Fe0 PRBs and anode well in Tests 4e8 was a result of acid
consumption by dissolving Fe0 particles (Fe0 þ 2Hþ / Fe2þ þH2[). Because of the reaction, Fe0 particles in the PRB retained
ametallic glaze after 100 h of energization, and a black passive
coating that was visually confirmed in our study (Suzuki et al.,
unpublished results) was not observed. These experimental
results demonstrated that the EK/Fe0 PRB system has the
capability to effectively utilize the acid generated at the anode
to regenerate the Fe0 surface, which allows the system to treat
nitrate-contaminated subsurface water flows for a protracted
period of time. This is clearly a technical advantage of the EK/
Fe0 PRB system over traditional Fe0 PRB systems.
This study also demonstrated that the Fe0 content in the
PRB, the anode pH, and the applied voltage were all important
factors in controlling the NO�3 reduction efficiency. However,
more research is required to fully understand the complex
relationship between these factors. Furthermore, in addition
to the NO�3 reduction efficiency and nitrogen recovery (both of
which were evaluated in this study), there are several other
aspects that need to be considered in determining the reme-
diation conditions. First, this study confirmed that NH4þ was
generated at 40 V via N2 reduction (Eqn. (1) and Fig. 2).
Although NHþ4 is less toxic than NO�
3 , and for this reason, the
current study attempted to convert NO�3 into NHþ
4 , it is still
desirable to operate the EK/Fe0 PRB system at the lower
applied voltage to avoid or at least to minimize the generation
of NHþ4 . Second, ferrous ion (Fe2þ) that is released as a result of
Fe0 dissolution by acid gives the water a red rust coloring (i.e.,
red water) when exposed to oxidants (e.g., oxygen). In fact,
reddish-brown precipitation was visually observed in the
cathode well when the experiment was performed at 40 V
(Tests 3 and 8 in Table 1). Therefore, the initial anode pH and
the applied voltage need to be carefully determined to avoid
excessive acidification and subsequent Fe0 dissolution.
Finally, as shown in Table 1, the total energy consumption
strongly depended on the applied voltage. Under the experi-
mental conditions investigated in this study, the total energy
consumption was reduced by choosing a lower voltage,
although applying a higher voltage clearly shortened the
remediation time. Therefore, proper choice of the remediation
time and the applied voltage is essential for the EK/Fe0 PRB
system because these variables directly affect the total energy
consumption and the remediation cost.
Acknowledgement
The authors gratefully acknowledge the partial financial
support for this study by the Japan Society for the Promotion
of Science (JSPS), Grant-in-Aid for Scientific Research (B),
No.22360380, 2010.
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