Industrial Crops and Products 32 (2010) 539–544

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Industrial Crops and Products
journal homepage: www.elsevier.com/locate/indcrop

High-puriﬁcation of amorphous silica originated from rice husks by combinationof polysaccharide hydrolysis and metallic impurities removal
Junko Umeda, Katsuyoshi Kondoh ∗Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaragi, Osaka 567-0047, Japana r t i c l e

i n f oArticle history:
Received 7 May 2010
Received in revised form 1 July 2010
Accepted 4 July 2010

Keywords:
Rice husk
SilicaHydrolysis
Polysaccharide
Monosaccharide
Metallic impuritiesa b s t r a c t
Rice husks and straw, containing 70–75 mass% organics and 15–20 mass% amorphous silica (SiO2 ), arerepresentative non-eatable biomasses. After the use of the organic elements as fuels, the leavings including large amounts of silica can be used as industrial materials when their purity is 99 mass% or more.
Hence, the reuse of high-purity silica will be signiﬁcantly effective in reducing the total cost of biomassenergy in using rice husks and straws. To attain high-purity silica from the remains of the rice husks
after air combustion, the optimization of the process conditions of the citric acid leaching treatment
and water rinsing process of rice husks were conducted to remove the metallic impurities such as
Na, K, Ca, Mg, Fe, Cu, etc. from husks and promote the hydrolysis reaction of polysaccharides. Whenthe citric acid solution with a concentration of 1 mass% or more was used, alkali metal oxides of Na2 O
and K2 O were completely removed. GC–MS analysis showed the progress of the hydrolysis reaction of
their hemi-cellulose of rice husks during the leaching process. Carbon content of ashes was drasticallyreduced to 0.02–0.04 mass% after combustion at 1073–1273 K, and high-purity amorphous silica with
99.5–99.77 mass% were produced from rice husks.
© 2010 Elsevier B.V. All rights reserved.

1. Introduction
Rice husks and straws are representative agricultural wastes in
the world. The annual yield of rice husks was about 127 million tons
in 2008 (FAO, 2008). At this moment, most of them are not practically reused, and cause the environmental problems. For example,
the combustion of 1 ton rice husks in the ﬁeld causes about
0.15 kg CO2 gas emission. Approximate 0.09 kg methane (CH4 ) gas
is produced when 1 ton rice husks are left on the ground and naturally decomposed (Klass, 1998). From a viewpoint of the reuseof rice husks and straws as fuel resources, they are useful biomass
materials. This is because they contain about 65–75 mass% polysaccharide organics such as cellulose and hemi-cellulose (Aristidou
and Penttilä, 2000) and become suitable fuel resources for energy
generation (Jain et al., 1994; Fang et al., 2004). On the other hand,amorphous silica materials of about 12–20 mass% are containedin rice husks and straws (Aristidou and Penttilä, 2000; Yalçin and
Sevinç, 2001), and certainly remain in their leavings after using
the organic elements as fuels. In general, silica is useful industrial
materials such as fertilizer, reinforcement of concretes, and ceramics materials (Della et al., 2002; Sensale, 2006; Krishnarao andGodkhindi, 1992). A lot of previous studies have suggested some

∗ Corresponding author.
E-mail address: kondoh@jwri.osaka-u.ac.jp (K. Kondoh).
0926-6690/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.indcrop.2010.07.002

processes to produce silica materials by combusting rice husks(Chakraverti et al., 1988; Kalapathy et al., 2000; Patel et al., 1987;
Rozainee et al., 2008). From a high-puriﬁcation of silica originatedin rice husks, a strong acid solution washing process was used toremove the alkali metal impurities (Na and K) from the husks (Liouand Wu, 2009; Chakraverti et al., 1988; Rhaman et al., 1997). Thisis because metal impurities of Na and K elements have a eutectic reaction with SiO2 during combustion of rice husks (Allendorf
and Spear, 2001; Besmann and Spear, 2002). For example, when
Na2 O and SiO2 coexists in rice husks, the ternary oxides, Na6 Si8 O19
and Na2 Si2 O5 with a eutectic melting point of 1062 K (Morey etal., 1930; Richet et al., 2006) are formed during combustion in air,and results in a drastic decrease of the melting point of SiO2 from1986 to 1062 K occurs (Yazhenskikh et al., 2006; Haltera and Mysen,2004). Therefore, the carbon elements originated from organics ofrice husks easily dissolve and remain in such ternary Si–Na–O and
Si–K–O oxides in liquid phase during combustion at 1073–1273 K.
As a result, the purity of silica materials contained in the rice husk
leavings was 96–98 mass% or less. Furthermore, a large part ofamorphous silica changes to crystalline structures after solidiﬁcation of the above ternary oxides. The authors have established anenvironmentally benign process to produce rice husk silica witha high-purity of 99 mass% and over by using the citric acid solution leaching treatment before combustion (Umeda et al., 2007;
Umeda and Kondoh, 2008). Such silica materials also showed completely amorphous structures even after combustion at 1273 K in

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J. Umeda, K. Kondoh / Industrial Crops and Products 32 (2010) 539–544

air because the metal impurities were removed from husks during
the above acid leaching treatment. The use of the citric acid solution is effective for the chelate reaction (Norman and Earnshaw,
1997) between carboxyl groups (–COOH) and metallic impurities
contained in husks, and results in the removal of such impurities
as metal complexes from rice husks during the leaching treatment.
In the present study, from a view point of a higher puriﬁcationof amorphous silica, the optimization of the process parametersin the acid leaching treatment and combustion of rice husks was
discussed. In particular, the effect of the citric acid concentrationon the polysaccharide hydrolysis behavior of organics was investigated by using GC–MS analysis, and the removal mechanism ofmetallic impurities was also discussed.
2. Experimental
Rice husks harvested in Niigata were used as input raw materials
to produce high-purity amorphous silica materials. As mentioned
above, the previous study clariﬁes that the citric acid solution leaching treatment and air combustion of rice husks are useful for theremoval of the metallic impurities (Umeda et al., 2007; Umeda
and Kondoh, 2008). Citric acid powders, having a mean particle
size of 1.22 mm, were dissolved in the distilled water at the ambient temperature to prepare citric acid solutions. 30 g rice husks
were put into 500 ml citric acid solution in Grifﬁn beaker. In thepresent study, the concentration and temperature of the citric acidsolution, and stirring time in the solution were selected as the operating parameters. This is because the chelate reaction between–COOH groups and metallic impurities strongly depends on theabove parameters and the hydrolysis of polysaccharides of cellulose
and hemi-cellulose is also dependent on them. The concentration
was controlled from 1 to 7 mass% by changing the mixing ratioof citric acid powders (Kishida Chemical Co., purity; 99.5 mass%)
and distilled water. The beaker was placed on the stirrer (AS ONE,hot stirrer, HS-5BHSD), and the solution temperature was changed
from 298 to 353 K. The rotating speed of the magnetic stirrer bar in
the solution was controlled at 960 rpm, and the stirring time was
15–120 min. After the acid leaching process, the water rinsing treatment of the rice husks was carried out in the distilled water at 293 K
for 900 s to remove the citric acid content from the husks. The stirrer was also used in the water rinsing process, and the magnetic
bar was rotated under 960 rpm speed. The materials were driedat 373 K for 60 min in the mufﬂe furnace in atmosphere, and then
combusted at 1073 K for 30 min in the same furnace. The combustion temperature of 1073 K was used in this study to prevent the
crystallization of amorphous silica contained in the husks (Rhaman
et al., 1997). The airﬂow rate in the combustion was 0.42 ml/s byusing a small air-compressor. The content of each metallic oxide
impurity of the ashes was measured by X-ray ﬂuorescence spectroscopy (XRF, PANalytical, X-ray spectrometer PW2400). Carbon
analyzer (HORIBA, EMIA-902V) was employed to measure the carbon content of the ashes. Fourier transform-infrared spectrometer(FT-IR, Nicolet, MagnaIR-560 with Dura-ATR, Sens-IR) analysis was
also carried out to investigate structures of the remained carbides
originated from the organic elements and silica materials. In orderto quantitatively evaluate the hydrolysis behavior of the polysaccharides of rice husks by using the citric acid leaching treatment,gas chromatograph–mass spectrometer (GC–MS, Agilent Technologies, Agilent-5973N) was applied to the acid-leached specimens
after air dry at 373 K. As mentioned in the previous report (Umeda
and Kondoh, 2008), the same heat treatment of them was conducted at 473 K for 360 s before GC–MS analysis. The hydrolysis
behavior of the organics was investigated by comparing the identiﬁcation of the mass spectra and their intensities, and the effect of
the citric acid concentration on the polysaccharide hydrolysis was
discussed.

Table 1
Quantitative analysis of silica (SiO2 ) and impurities contents of rice husk ashes byusing citric acid leaching treatment with various concentrations (leaching temperature; 323 K, soaking time; 60 min, water rinsing time; 15 min, air-burning
conditions; 1073 K × 30 min).
Concentration of citric acid solution (wt.%)
Mass%

0%

1%

3%

5%

7%

SiO2
MgO
Na2 O
P2 O5
S
K2 O
CaO
Cr2 O3MnO
Fe2 O3
NiO
CuO
Mo03
C

97.25
0.29
0.13
0.09
0.03
1.39
0.46
0.00
0.13
0.02
0.00
0.00
0.00
0.15

99.52
0.04
0.00
0.11
0.02
0.03
0.16
0.00
0.02
0.02
0.00
0.00
0.00
0.08

99.54
0.03
0.00
0.12
0.02
0.02
0.13
0.01
0.02
0.05
0.00
0.00
0.00
0.06

99.56
0.02
0.00
0.13
0.02
0.01
0.11
0.00
0.01
0.02
0.00
0.00
0.00
0.04

99.47
0.03
0.00
0.13
0.01
0.02
0.12
0.04
0.02
0.04
0.02
0.00
0.00
0.03

3. Results and discussion
3.1. Effect of citric acid solution leaching and water rinsing
conditions on removal of metallic impurities
Table 1 shows chemical compositions of the rice husk ashes via
the citric acid solution leaching treatment with different concentration. The solution temperature was 323 K and the stirring timeof 60 min was used in the leaching process. The concentration of
0 mass% means the use of the warm distilled water rinsing treatment at 323 K instead of the citric acid solution. As shown in Table 1,
when the concentration of the citric acid solution is over 1 mass%,
the silica (SiO2 ) purity of each ash is approximately 99.5 mass%or more. On the other hand, a silica purity of 97.25 mass% was
obtained by using the warm distilled water leached treatment(0 mass%). It means the use of the citric acid leaching treatment
obviously caused the remarkable reduction of the total content ofmetal oxide impurities, and resulted in a high-puriﬁcation of silica
materials contained in the rice husk ashes. Concerning the content of alkali metal oxides (Na2 O and K2 O) remained in the ashes,
the application of the citric acid solution is remarkably effective in
reducing them. In particular, Na2 O has been completely removed
from the rice husks by using the citric acid solution of 1 mass% ormore concentration. When the warm distilled water (0 mass%) was
employed, the content of K2 O and carbon remained in the ashes
was 1.39 and 0.15 mass%, respectively. The previous result indicates that K2 O and carbon content of rice husk ashes was 3.69
and 0.61 mass%, respectively when raw rice husks with no leaching treatment were used (Umeda and Kondoh, 2008). This meansthat even the warm distilled water leaching treatment is effective
for the removal or reduction of K impurities from rice husks. This isbecause the hydrolysis reaction of polysaccharides such as cellulose
and hemi-cellulose occurred during leaching treatment by using
warm distilled water at 323 K for 60 min, and the chelate between
K impurities and –COOH groups easily took place. The reason for the
reduction of carbon content is due to the removal of K2 O impurities,
causing a eutectic reaction of SiO2 elements at lower temperatureand their liquid phase. The content of MgO and MnO impurities
remained in the ashes is also extremely reduced by using the citricacid solution, and their reduction ratio is 85–93% when comparing
to the oxide impurities of the ashes via the warm distilled waterleaching treatment (0 mass%). Furthermore, it is obvious that the
citric acid solution leaching treatment is also effective in removing Ca elements from rice husks. However, its reduction ratio of
about 65–76% is smaller than those of MgO and MnO as men-

J. Umeda, K. Kondoh / Industrial Crops and Products 32 (2010) 539–544

541

Fig. 1. GC–MS analysis results of rice husks via citric acid solution leaching treatment using various solution concentrations of 0 mass% (a), 3 mass% (b), 5 mass% (c), and7 mass% (d) (solution temperature; 323 K, pre-heating condition; 473 K for 6 min).

tioned above. Fig. 1 indicates GC–MS analysis results of air-driedrice husks via the citric acid leaching and the next water rinsing
treatment. The concentration of the citric acid solution used in thisanalysis is 0 mass% (a), 1 mass% (b), 3 mass% (c) and 7 mass% (d).
The spectrum corresponding to furfural, which is a monosaccharide formed from hemi-cellulose via hydrolysis and dehydration
reaction (Saha, 2003), is clearly detected in specimens of the citricacid-leached husks (b)–(d). As shown in (a), a small peak of furfural is also detected. It means the warm water leaching treatment
at 323 K is also slightly effective in the hydrolysis and dehydration
reaction of hemi-cellulose. It also reveals no obvious difference ofthe spectrum intensity between 1 and 7 mass% concentration. A
very small spectrum of levoglucosan is detected in all materials
shown in Fig. 1. Fig. 2 shows a dependence of the ion intensity of
the furfural spectrum on the concentration of the citric acid solution. When 1 mass% citric acid solution is used, the ion intensity

Fig. 2. Speciﬁc ion intensity of furfural spectrum in GC–MS results of rice husks via
citric acid leaching treatment as a function of acid solution concentration.

is 2.21 × 106 (mg−1 ) and about four times as that in the use of thewarm distilled water shown in Fig. 1(a). It slightly increases withincreasing concentration from 1 to 7 mass%. These results suggestthat the citric acid solution of 1 mass% concentration is enoughfor the progress of hydrolysis reaction of hemi-cellulose (polysaccharide) to furfural (monosaccharide) contained in rice husks. The
previous study shows that the metallic impurities such as Na, K,
Ca, Mg, Fe, Al and Mn are contained in the rice husk organics consisting of hemi-cellulose and cellulose (Horng, 2004). When thehydrolysis reaction of the polysaccharides progresses by the citricacid leaching treatment, the simple structures of monosaccharides
such as furfural and levoglucosan are formed in the rice husks(Saha, 2003). As a result of this reaction, carboxyl groups easily passthrough the monosaccharides and contact the above metallic impurities contained in rice husks. Their chelate complexes are formed
and discharged from the husks into the acid solution. Therefore,
as shown in Table 1, the content of each metallic impurity oxide of
the ashes are drastically reduced by the citric acid solution leachingtreatment. In addition, Table 1 obviously indicates that the reduction of alkali metal oxides of Na2 O and K2 O is effective to decrease
the remained carbon content of the ashes. This is because a remarkable decrease of these oxide impurities, causing a liquid phase of
SiO2 by the eutectic reaction at 1062 K as mentioned above, prevents the dissolution of carbon elements in the melt SiO2 during
combustion at 1073 K. Hence, a very few organics remain in theashes, and result in a high-puriﬁcation of silica materials.
Table 2 shows chemical compositions of the rice husk ashes
when the citric acid solution with different temperature was usedin the leaching treatment. The solution concentration and stirring time were 5 mass% and 60 min, respectively. The result inTable 1 indicates that the concentration of 1 mass% is enough for theremoval of metallic impurities from the rice husks when the citricacid solution leaching is applied. However, Fig. 2 also suggests thata slight increase of the furfural spectrum intensity with increasein the solution concentration. Accordingly, 5 mass% concentrationof the citric acid solution is applied to evaluate the effect of the542

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Table 2
Quantitative analysis of silica (SiO2 ) and impurities contents of rice husk ashes
by using citric acid leaching treatment at various temperatures (citric acid solution concentration; 5 mass%, soaking time; 60 min, water rinsing time; 15 min, air
combustion conditions; 1073 K × 30 min).
Citric acid solution temperature
Mass%

298 K

313 K

323 K

333 K

353 K

SiO2
MgO
Na2 O
p2 o5
S
K2 O
CaO
Cr2 O3MnO
Fe2 O3
NiO
CuO
MoOg
C

99.25
0.07
0.00
0.13
0.02
0.08
0.27
0.00
0.05
0.03
0.00
0.02
0.00
0.05

99.54
0.05
0.00
0.12
0.02
0.02
0.14
0.00
0.02
0.02
0.00
0.00
0.04
0.06

99.56
0.02
0.00
0.13
0.02
0.01
0.11
0.00
0.01
0.02
0.00
0.00
0.00
0.04

99.58
0.09
0.00
0.12
0.00
0.02
0.08
0.00
0.01
0.02
0.00
0.00
0.00
0.02

99.77
0.04
0.00
0.10
0.00
0.00
0.03
0.00
0.00
0.02
0.00
0.00
0.00
0.03

solution temperature on the removal of metallic impurities. With
increase in the temperature, most of the impurities are removed,
and a purity of the silica is gradually improved. In particular, itreaches 99.77 mass% by using the citric acid solution of 353 K. In thecase of Na2 O, even the acid leaching treatment at 298 K is enough toremove Na impurities from rice husks. On the other hand, CaO content of the ashes gradually decreases with increasing the solution
temperature. It means that higher temperature of the acid solution is more effective in removing Ca impurity from the rice husks.
Fig. 3(a) indicates the Ca content of the used citric acid solution after
the leaching treatment as a function of the solution temperature.
The CaO content of the ashes shown in Fig. 3(b) is originated from
Table 2. Ca elements are discharged into the acid solution duringacid leaching process, and their content of the solution gradually
increases with increasing the temperature. By using these measurements, a ratio of the content of Ca impurities discharged from ricehusks to the original Ca content is estimated. In this calculation, it

Fig. 3. Ca content in citric acid solution (a) and CaO content of ash (b) dependence on
citric acid solution temperature (citric acid solution concentration; 5 mass%, stirringtime; 60 min, air combustion temperature; 1073 K).

Table 3
Dependence of discharged rate of Ca from rice husks into citric acid solution on
solution temperature (concentration of citric acid solution; 5 mass%, stirring time;
60 min).
Temperature, T (K)

298

313

323

333

353

Discharged ratio (%)

34.0

72.0

78.9

85.3

94.5

is assumed that Ca is removed by only the acid leaching treatment,
not the next water rinsing process. Hence, the original Ca content
of rice husks is the sum of that of the ashes and the used citric acidsolution. As shown in Table 3, the ratio of Ca impurities discharged
into the acid solution is 34% by using the ambient temperature acidleaching treatment (298 K). However, it suddenly increases to 72%
when the warm acid solution of 313 K is employed. With increase in
the solution temperature, the ratio gradually increases, and reaches
94.5% at 353 K. From a viewpoint of a chemical reaction, a higher
temperature of the citric acid solution is effective for both formation of Ca complexes by the chelate and hydrolysis reaction frompolysaccharides to monosaccharides, and results in the progress ofa discharge of the complexes from rice husks.
Fig. 4 indicates the effects of the stirring time during the citricacid leaching and the next water rinsing treatment on the CaO content of the rice husk ashes. The concentration and temperature of
the citric acid solution were 5 mass% and 323 K, respectively. The
stirring time in the acid leaching and water rinsing treatment are
obviously useful to reduce the CaO impurity content. In particular,
the citric acid leaching time is much dominant on the reduction of
CaO elements. According to the above results shown in Tables 1, 2
and Fig. 1, it is concluded that not only the removal of metal impurities by the chelate reaction, but also the hydrolysis reaction ofpolysaccharides by the acid leaching treatment dominate largely a
puriﬁcation of SiO2 elements of the rice husk ashes.
3.2. Effect of combustion temperature on silica crystal structure
and carbon content
The previous studies have mentioned the effect of the combustion temperature on the crystallization behavior of silica materials
contained in rice husk ashes (Ibrahim and Helmy, 1981; Shinohara
and Kohyama, 2004). In general, the combustion temperature ofrice husks at 973 K or more causes the crystallization of amorphous SiO2 of the ashes when raw rice husks with no acid leachingtreatment are used as starting materials. However, as shown in

Fig. 4. CaO content of rice husk ashes via citric acid leaching with various stirring
time as a function of water rinsing time (citric acid solution; 5 mass% and 323 K,
water rinsing at 298 K, air combustion conditions; 1073 K × 30 min).

J. Umeda, K. Kondoh / Industrial Crops and Products 32 (2010) 539–544

Fig. 5. XRD proﬁles of rice husk ashes after air combustion at various temperatures (concentration and temperature of citric acid solution leaching treatment are
5 mass% and 323 K, respectively).

Fig. 5, the crystallization temperature of amorphous silica drastically increases from 973 to 1323 K when the citric acid leachingprocess is applied to rice husks before combustion. This is because aeutectic phenomenon of SiO2 via reaction with Na2 O or K2 O impurities is prevented by reducing the content of these alkali metal
impurities from the rice husks as mentioned above. However, theheating time in the combustion is not signiﬁcantly effective on the
crystallization of amorphous silica originated in rice husks. This isbecause the crystallization phenomenon drastically occurs whenthe materials temperature reaches the critical level in combustion (Shinohara and Kohyama, 2004). Therefore, the heating time

543

Fig. 6. Dependence of carbon content of rice husk ashes on combustion temperature(citric acid solution concentration; 5 mass%, temperature; 323 K and stirring time;
60 min).

of 1.8 ks used in this study is enough to obtain the stable state ofsilica materials after combustion in air.
At the same time, the combustion temperature also affects theremained carbon content of their ashes. Fig. 6 shows a dependenceof the carbon content of the ashes on the combustion temperature.
The content was measured by ICP analysis. 5 mass% citric acid solution was used, and the stirring temperature and time during theacid leaching treatment were 323 K and 60 min, respectively. It is
obvious that the remained carbon content remarkably decreases

Fig. 7. FT-IR spectra of rice husk ash burned at 1073 K (a) and 673 K (b), compared to commercialized mineral silica particle and raw materials (c).

544

J. Umeda, K. Kondoh / Industrial Crops and Products 32 (2010) 539–544

from 0.17 mass% to 0.02–0.03 mass% by changing the combustion
temperature from 673 to 1073 K. In addition, there is no signiﬁcant difference of the carbon content by burning the husks over1073 K. DTA proﬁles of rice husks in the previous study indicate
the exothermic heat due to a combustion of the organics was completely ﬁnished at 973 K when the husks via the citric acid leachingtreatment was used (Umeda and Kondoh, 2008). It means thatpyrolysis of the organics contained in rice husks progresses completely by combustion in air over 973 K. Fig. 7 shows the results of
FT-IR analysis of rice husk ashes combusted at 1073 K (a) and 673 K(b). The infrared spectrum of the commercial mineral silica powders and raw rice husks is also shown in (a) and (c), respectively.
The infrared spectra at 1051 and 791 cm−1 due to Si–O–Si stretching modes (Ayres et al., 2007) are detected in (a), and correspond to
those of the conventional mineral silica used as reference materials. No spectrum of about 3370–2850 cm−1 of the polysaccharides(Ayres et al., 2007; Wang and Sung, 2002) of raw rice husks is obviously observed. It means that a very few carbons originated fromthe cellulose and hemi-cellulose exist in the ashes after combustion
at 1073 K. This result corresponds well to the carbon measurement
by ICP analysis shown in Fig. 6. On the other hand, the specimen
burned at 673 K shown in Fig. 7(b) indicates not only 1051 and791 cm−1 spectra (Si–O–Si bond) but also small spectra at 3350,
2920 and 2850 cm−1 corresponding to OH stretching mode and CH2
stretch vibrations (Wang and Sung, 2002), which are also detectedin the raw rice husks (c). This means that some of original organics still exist in the ashes after combustion at 673 K, and result in
the remarkably large content of the remained carbon elements ofashes burned at 673 K as shown in Fig. 6. Therefore, the combustion temperature of 1073–1273 K is suitable for the preparation ofhigh-purity amorphous silica materials originated from rice husks
via the citric acid leaching treatment.
4. Conclusion
For a high-puriﬁcation of amorphous silica originated in ricehusks, the optimization of the operating parameters in the citric acid leaching treatment and air combustion of rice husks was
discussed. When the citric acid solution with a concentration of
1 mass% or more was used, XRF analysis indicated that the metaloxide impurities could be obviously reduced from rice husks during
the acid leaching treatment. In particular, the alkali metal oxides of
Na2 O and K2 O were completely removed. GC–MS analysis showed
the progress of the hydrolysis reaction of their hemi-cellulose ofrice husks during the leaching process. As a result, the carbon content of the ashes was drastically reduced to 0.02–0.04 mass% after
combustion at 1073–1273 K. The capability to remove Ca impurities
from rice husks by the citric acid leaching treatment was dependent
on not only the concentration of the acid solution but also the solution temperature and stirring time. The combustion temperature at1073 K or more was enough to thermally resolve the original carbohydrates of rice husks, and resulted in a very few carbon contentsof 0.02–0.03 mass%. On the other hand, when the combustion temperature of 673 K was employed, FT-IR analysis showed CH2 and OH
stretching modes with 3350–2850 cm−1 spectra originated in the
carbohydrates of the rice husks. Their ashes including 0.17 mass%
carbons were obtained. In the use of the suitable conditions of
the acid leaching and air combusting processes, high-purity amorphous silica materials with 99.5–99.77 mass% were produced fromrice husks.

Acknowledgement
This study was ﬁnancially supported in part by “The Japan Securities Scholarship Foundation” in 2008.
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