J Mater Sci (2008) 43:7084–7090
DOI 10.1007/s10853-008-3060-9

High-purity amorphous silica originated in rice husks via
carboxylic acid leaching process
Junko Umeda Æ Katsuyoshi Kondoh

Received: 27 April 2008 / Accepted: 15 October 2008 / Published online: 30 October 2008
Ó Springer Science+Business Media, LLC 2008

Abstract The environmentally benign, harmless tohuman, and economically effective process to produce highpurity amorphous SiO2 materials from rice husks has been
established by using a carboxylic acid leaching, not theconventional strong acids. TG-DTA measurement and
GCMS analysis indicated that the leaching was effective forthe hydrolysis of celluloses and hemicelluloses contained inrice husks at 473–873 K, which produced the same results inusing the conventional sulfuric acid. In particular, the formation of furfurals and levoglucosans occurred via thehydrolysis at 473 K and 673 K, respectively, when using ricehusks leached by citric acid solutions. The metallic impurities could be also removed from the husks via a chelate
reaction between carboxyl groups (–COOH) and the metalelements. Concerning the burning conditions of rice husks
after the acid leaching, it was necessary to supply a suitableamount of air to completely combust organics; for example,
it required air supplement of 50 mL/min or more. Highpurity amorphous silica materials with 99 wt% or more wereprepared from rice husks by applying the citric acid leachingtreatment and burning process at 1073 K in air.

Introduction
Rice husks and straws are agricultural wastes produced in alarge amount, and their annual world production amounts
to about 80 million tons [1]. The major constituents of rice

J. Umeda  K. Kondoh (&)
Joining and Welding Research Institute, Osaka University,
Ibaraki 567-0047, Japane-mail: kondoh@jwri.osaka-u.ac.jp

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husks are ashes and organics such as cellulose, hemi-cellulose, and lignin. The former mainly consists of silica andsome metal impurities. The silicon (Si) atoms exist in the
protuberances and hairs on the outer and inner epidermisof the rice husks [2]. Extensive researches have been
carried out to extract SiO2 elements from rice husks [3–5],
because high-purity silica is useful raw materials forindustrial uses. In the previous studies, the strong acidleaching treatment was carried out on rice husks to remove
metallic impurities and organics contained in them. Sulfuric acid (H2SO4), hydrochloric acid (HCl), and nitricacid (HNO3) solutions are conventionally used in leaching[1, 6]. They are, however, significantly hazardous to theenvironment and humans. The strong acid leaching treatment also has an economical problem due to a necessary
use of expensive materials with corrosion resistance tostrong acids, water rinsing of husks, and a special disposal
treatment of used strong acids. In this study, the environmentally benign, harmless to humans, and economical
process to produce high-purity amorphous SiO2 from ricehusks has been established by using the carboxylic acidleaching treatment. TG-DTA (thermogravimetry-differential thermal analysis) measurement and GCMS (gaschromatograph mass spectrometer) analysis were used toclarify the effect of the hydrolysis of the organic contents
and its mechanism. The effect of air supplement in burning
acid-leached husks was investigated to purify the silica
element of ashes. XRD analysis was carried out on ricehusk ashes to optimize the burning temperature to prepare
amorphous structured silica. For the evaluation of the
carboxyl group (–COOH) of organic acids to remove
metallic impurities via a chelate reaction, the citric acidleaching treatment was applied to rice straws including a
few cadmium (Cd) elements, and the reduction effect was
discussed.

J Mater Sci (2008) 43:7084–7090

Experimental procedure
The raw materials of rice husks harvested in Niigata, Japan,
were employed, and subjected to the carboxylic acidleaching treatment by using citric acid (C6H8O7) solution.
Sulfuric acid solution was also used as the comparison in
the leaching treatment. Both acid materials are made by
Kishida Chemical Co., Ltd, and the purity of the carboxylic
acid and sulfuric acid is 99.5% and 98% or more, respectively. The concentration of each acid solution was 5 wt%,
and temperature was controlled at 323 K. About 20 g ofhusks were immersed in the acid solution of 500 mL for
900 s. The water rinsing treatment on acid-leached husks
was carried out to remove the acid solution from rice husks.
Water rinsing after the leaching was repeated three andeight times, respectively, by citric acid and sulfuric acid.
The water-rinsed materials were dried at 373 K for 3.6 ks inthe electric furnace. As mentioned above, the main content
of rice husks was cellulose and hemicellulose. Therefore,
the cellulose powders made by Merck KGaA INC., having amean particle size of 58 lm and purity of 94.8 wt%, wereprepared, and their thermal resolution temperature wasmeasured by TG-DTA equipment (Shimadzu DTG-60)
under air or argon gas supplement with a flowing rate of
150 mL/min. In the case of rice husks, the same analysis
was applied to evaluate the thermal resolution behavior. In
particular, from a viewpoint of the hydrolysis of someorganics in husks, its reactivity could be discussed quantitatively by the comparison of the exothermic heat in the
TG-DTA profile. This is indicated when the cellulose andhemicellulose atomic structure changed from the polysaccharide to monosaccharide via a hydrolysis, the exothermicheat was reduced. The optimization of the air supplement in
the burning process of rice husks was also evaluated by theexothermic heat of TG-DTA profiles. The flowing rate in
the air supplement was controlled from 0 to 150 mL/min,and argon (Ar) gas was used for the comparison in thisanalysis. It was measured from 293 K to 1273 K withheating ratio of 10 K/min. To investigate the atomic
structure changes of cellulose and hemicellulose by the acidleaching, GCMS analysis (Agilent 5973 N) was applied tothe specimens after heating at 473 K for 360 s, 673 K for
60 s, and 873 K for 6 s. The hydrolysis behavior of theorganics contained in the acid-leached husks at each sampling temperature could also be discussed by theidentification of the mass spectra and their intensities. The
effect of the citric acid with carboxyl groups on the removalof metal impurities contained in rice husks and straws isinvestigated. Three kinds of rice straws, which include
Cadmium (Cd) content of 0, 23, and 82 ppm, are preparedas raw materials. They are also subjected to the citric acidleaching treatment at 323 K for 900 s. The concentrationsof the acid solution are 5 wt% and 20 wt%. Cd contents of7085

acid-leached straws and used solutions are measured by ICP
optical emission spectrometer (HORIBA, ULTIMA-2). The
burning process of dried husks took place at 873–1423 K in
air to remove the organics from rice husks. A dependence ofthe crystallization of the silica elements on the burningtemperature was examined by X-ray diffraction (XRD)
analysis (Shimadzu XRD-6100). The content of SiO2, carbon, and other metal impurities included in the rice husk
ashes were investigated by XRF (X-ray fluorescence) andICP (inductively coupled plasma) analysis.

Result and discussion
Hydrolysis of organics by citric acid leaching
Rice husks and straws comprise cellulose, hemicellulose,
and lignin. The previous work indicates that hemicellulose
and lignin are rapidly digested by a short time acid leachingtreatment while cellulose is only partially digested [7]. It
means a high degree of resistance of the residual cellulose in
the husks toward the acid digestion. Therefore, in this study,
the hydrolysis of cellulose of rice husks by citric acidleaching treatment is discussed in detail. Figure 1 indicates
TG-DTA profiles of the conventional cellulose powders
under the air and Ar gas flow condition both with a flow
ratio of 150 mL/min. As shown in Fig. 1a, the DTA curveunder the air supplement indicates two exothermic heatswith the maximum peak temperatures at 628 K and 778 K
by the complete combustion, which corresponds well withthe previous results [7]. At the same time, Fig. 1b shows thedrastic weight loss due to their thermal degradation. In
particular, the reduction of weight at the temperature range
from 573 to 613 K is significantly large, and the following
second weight loss until 813 K is small. The latter is causedby the second exothermic heat due to the thermal resolutionof cellulose powders in burning shown in (a). On the other
hand, when using Ar gas atmosphere, the DTA curve
reveals an endothermic heat at the peak temperature of
618 K due to its carbonization without combustion. The TG
profile shows a weight loss at the same temperature as theabove endothermic. In comparison to that under the air
supplement, it reveals no second weight loss from 613 to813 K. In other words, these results suggest that the burningover 613 K with a suitable air supplement is important tocompletely combust celluloses contained in rice husks.
Figure 2 reveals DTA profiles of rice husks with the citricacid (a), sulfuric acid (b) leaching, and no treatment (c). The
initial temperature of the first exothermic heat due to thethermal degradation of organics in all materials is about
473–493 K, and lower than that of the cellulose (543 K) as
shown in Fig. 1a. This may be attributed to the content ofhemicellulose and lignin elements in rice husks that have

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J Mater Sci (2008) 43:7084–7090

Exo.

250

Table 1 Comparison of exothermic heat values of DTA profiles ofeach rice husks (kJ/g)(a)200
Air supplement(kJ/g)

Citric acidleachedSulfuric acidleachedAs-received(raw material)

Ave.

28.1

32.4

38.8

N=1

27.3

31.4

39.5

N=2

28.9

33.3

38.0

150
100

DTA / µV

Ar gas

50
0
273
-50

473

673

873

1073

1273

Temperature, T / K-100

TGA / mg

8(b)

6
Ar gas

4
Air supplement

measurement of the total exothermic heat of each specimen
is shown in Table 1. The reduction of the exothermic heat inusing acid-leached rice husks is about 17–28% against thatof raw materials without any leaching treatment. The colorof every specimen after TG-DTA analysis is completely
white, that is, the ashes are high-purity silica materials.
These results mean that the acid leaching treatment has apossibility to accelerate the hydrolysis of organics contained in rice husks. For the investigation of this reaction
behavior, GCMS analysis is carried out on each specimen.
The heating conditions before the analysis are 473 K for
360 s, 673 K for 60 s, and 873 K for 6 s in air. Figure 3

2(a) 12
473

673

873

1073

Temperature, T / K

Exo.

Fig. 1 TG-DTA profiles of cellulose measured in air and argon gas
atmosphere300

Air supplement᧷150ml/min.

250

Hot water leached

1273

Abundance / rel.u

0
273

9

Furfural
O

6

O

O
O

3

O

(c) As-received husks(with no acid leaching)

DTA / µV

2000(b) 5% sulfuric acid leached

0

2

4

150

6

8

10

12

14

16

Retention time, t / min(b) 12100(a) 5% citric acid leached

Citric acid leached

0
273
-50

473

673

873

1073

1273

1473

Temperature, T / K

Fig. 2 DTA profile of rice husks with citric acid (a) and sulfuric acidleaching (b) compared to as-received raw materials (c)significant influences on the initial temperature of thethermal degradation. They show two exothermic heatpeaks, which have the same maximum points at 608–623 K
and 723–743 K, respectively. While comparing their heatvalues, the difference between (a) and (b) is very little, but(c) has a significantly small heat compared to another. The

123

Abundance / rel.u

50
9Furfural
O

6

O

O
O

3

0

O

0

2

4

6

8

10

12

14

16

Retention time, t / minFig. 3 GCMS results of rice husks via citric acid leaching and hot
water rinsing (473 K for 360 s)

J Mater Sci (2008) 43:7084–7090

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shows GCMS results of rice husks after hot water rinsing at
323 K (a), and citric acid solution (concentration; 5 wt%)
leaching at 323 K (b). The heating condition in GCMS
analysis of each sample was 473 K for 360 s. Spectra , `,and ´ correspond to furfurals (C4H3O), and levoglucosans,respectively. In both specimens, the spectrum ´ of levoglucosans is very small compared to the furfural peaks. When
applying the above heating condition to each sample, thefurfurals were mainly formed, and a very little levoglucosans were also produced in each material with even acidleaching treatment.
Table 2 indicates the ion intensity of peaks  and ` ofeach rice husk. The maximum intensity of both peaks inusing the citric acid-leached materials is about 4–10 times
as those of hot water rinsed specimens. In general, hemicelluloses, composing the cell wall, change to D-xylose(C5H10O5) via a hydrolysis, and it forms furfurals by adehydration reaction. In the TG-DTA profile of as-received
raw rice husks, no weight reduction and no heat is detecteduntil 473 K. That means no effect of the heat treatment lessthan 473 K on the above hydrolysis of organics in ricehusks. Accordingly, the citric acid leaching treatment ofhusks is effective to accelerate the atomic structure changeof their hemicellulose to furfural via the hydrolysis anddehydration.
Figure 4 indicates GCMS results of hot water rinsed (a),and acid-leached materials (b) and (c) after heating 673 K
for 60 s. In the all specimens, the peaks  and ` aredetected, and the difference of their ion intensities is verysmall. That is, the above hydrolysis to produce furfurals iscarried out when heating rice husks at 673 K. With regard
to the peak ´, the formation of levoglucosans shown in (a)indicates an extremely small peak intensity, and very few
products of levoglucosans. On the other hand, the peak isclearly detected in the specimens (b) and (c). The ion
intensity, however, is not so different between these twovalues. It means that the citric acid leaching is also effective for the hydrolysis of the cellulose contained in husks
to produce levoglucosans, and its effect is almost same asthat via the sulfuric acid leaching. Figure 5a shows DTA
profiles of the citric acid-leached rice husks (acid concentration, 5 wt%) when changing the air supplement in theanalysis. In the case of 50 mL/min or more, there is nochange in the profile with the typical two exothermic heats

Table 2 Ion intensity of GCMS analysis on rice husks via citric acidleaching and hot water rinsing
Max. intensityof peak 

Max. intensityof peak `2.35*106

3.82*105

8.51*106

4.22*1066

4.06*106

Hot water rinsed at 323 K
Citric acid leached at 323 K
N=1
2.16*109N=29

2.15*10

Fig. 4 GCMS results of ricehusks via citric acid leaching
and hot water rinsing (673 K for
60 s)

8.22*10(a) 4Abundance / rel.u

1.35*109(c) 4

Hot water leached3
Levoglucosan
O

2

O
OH
OH

1
0
0

2

4

6

8

10

12

OH

14

16

Retention time, t / min

Sulfuric acid leached3
Levoglucosan
O

2

O
OH
OH

1
0
0

2

4

6

8

10

12

14

OH

16

Retention time, t / min(b) 4
Citric acid leached
Abundance / rel.u

N=1

Abundance / rel.u

Intensity of
total ion

3
Levoglucosan

2

O
O
OH

1
0
0

OH

2

4

6

8

10

12

14

OH

16

Retention time, t / min

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J Mater Sci (2008) 43:7084–7090

Reduction of metallic impurities by chelate reaction ofcarboxyl groups(a) 200
50ml/min.

Exo.

150ml/min.
150

DTA / µV

100
50
0
273
-50

473

673

873

1073

1273

Temperature, T / K
0ml/min.

20ml/min.

-100
-150

5% citric acid leached rice husks

-200

(b)

7

5

150 ml/min.
50 ml/min.
20 ml/min.
0 ml/min.

4

Ar gas(150 ml/min.)TGA / mg632
15% citric acidleached rice husks

0
273

473

673

873

1073

1273

Temperature, T / K

Fig. 5 TG-DTA profiles of citric acid-leached rice husks measured
under various air supplement flowing rate

as mentioned above. When reducing the air supplement to
20 mL/min, the exothermic heat value is reduced. Finally,
the profile reveals the endothermic heat at 623 K with noair supplement in the analysis, and the color of the burned
specimen remaining in the alumina cup is black. The others
indicate pure white ashes. This result suggests that a suitable air supplement with 50 mL/min or more is necessary
to completely combust the organics of rice husks, and theinsufficient air in burning causes the carbonization oforganics at 573–673 K. In addition, the same result as no
air supplement is obtained by using argon gas in TG-DTA
analysis. With regard to the TG profiles shown in Fig. 5b,
the total weight reduction after heating until 1,273 K
gradually increases with increase in the air flow rate from 0
to 150 mL/min. Every curve except for Ar gas flow condition shows two-step reduction profile, which are same asthat of cellulose powders under the air supplement shownin Fig. 1b. When using the Ar gas flow in TG-DTA analysis, the total weight reduction remarkably decreases, andthe profile is similar to that of celluloses under Ar gas.

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In general, rice husks and straws contain the metallic
impurities of about 0.8–1 wt% such as potassium (kalium/K), calcium (Ca), sodium (natrium/Na), phosphorus (P),aluminum (Al), etc. According to the soil contents, some ofhazardous metal elements such as cadmium (Cd) and lead(Pb) are included. It is reported that alkali metal impurities
of K and Na cause the carbons to remain in ashes, which
are originated in organics of husks. This is because the
eutectic reaction between the alkali metals and SiO2 element occurs during burning, and the carbon remains in the
SiO2 melt [8, 9]. Furthermore, ‘‘chemical activators’’ suchas KOH, K2CO3, and Na2CO3 are easily produced byheating rice husks including the alkali metal impurities,
and function as dehydrating agents and inhibit the formation of tar [10]. Then, K2O may act as the ‘‘chemical
activator’’ during the combustion, and unburned carbon
remains in the ashes. It is well known the chelate reactionof carboxyl groups (–COOH) with metal elements easily
occurs, and their metal complexes are formed [11]. The
other previous studies show that citric acid and tartaric acidhave a great potential to remove Cu2? from aqueous
solutions by using its carboxyl groups [12, 13]. Table 3
shows Cd contents of raw materials, used citric acid solutions, and rice straw ashes by ICP analysis. Material A
containing no Cd is referred in this study. With increase in
the Cd content of raw straws, that of used citric acidsolution after leaching gradually increases. The color of thesolutions changes to dark orange due to the eluted Cd
complexes. After the leaching treatment, Cd content ofeach straw is less than 0.001 ppm, which is the lowest
limitation by ICP analysis when using 5 wt% and 20 wt%
concentration. That is, the citric acid leaching is significantly effective to remove Cd elements via chelate reaction
from rice straws. By using the chelate of carboxyl groupsof organic acids, the metallic impurities of rice husks are
reduced. For example, the leaching treatment by citric acidsolution with 5 wt% concentration is carried out at 323 K
on rice husks. After water rinsing and drying in air, theacid-leached materials are burned in air at 1,073 K for

Table 3 Cd contents of raw materials, used citric acid solutions andrice straw ashes by ICP analysis (ppm)
Material A Material B Material C
Raw materials (ppm)

*0

23

82

Used citric acid solution 5%

*0

0.085

0.383

20% *0

0.087

0.411

*0

*0

*0

20% *0*0*0Rice straw ashes

5%

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Table 4 Chemical compositions of rice husk ashes burned at 1073 K via citric acid and sulfuric acid leaching, compared to no acid treatment
(wt%)
(wt%)

SiO2

Al2O3

MgO

Na2O

P2O5

SO2

K2O

CaO

MnO

Fe2O3

BaO

CarbonWithout leaching

94.58

0.02

0.31

0.11

0.41

0.11

3.69

0.56

0.08

0.04

0.04

0.61

Citric acid leaching

99.14

0.03

0.08

0.06

0.29

0.03

0.12

0.16

0.01

0.03

0.03

0.09

Sulfuric acid leaching

99.09

0.01

0.06

0.07

0.31

0.05

0.11

0.37

0.01

0.02

0.03

0.12

Combustion temperature of rice husks
It is important to prepare high-purity SiO2 ashes withamorphous structures from rice husks, because IARC
(International Agency for Research on Cancer), one part of
WHO (World Health Organization), strongly points thatthe crystal SiO2 particles have carcinogenic risks tohumans [14], and belong to Group 1. The structure of ricehusk ashes strongly depends on the burning temperature.
Figure 6 shows XRD profiles of the ashes of citric acidleached rice husks after burning at various temperatures inair. It indicates that the change from originally amorphous
structures to crystal ones occurs by burning the husks at
1,323 K or more. The rice husk ashes include crystallized
silica materials with tridymite and cristobalite structures[15]. In the case of the ashes of as-received rice husks, thecristobalite peaks are detected at 1,073 K, and theirintensity gradually increases with increase in the combustion temperature. The transformation to tridymite starts at
1,273 K. These results correspond well with the previous
study [16]. As mentioned above, the metallic contaminations of rice husks are effective for the eutectic reaction
with SiO2 during heating, and the crystallization occurs
when the melted SiO2 elements are slowly solidified.

5% citric acid leached husk

SiO2

Intensity (a.u.)

3.6 ks. The air supplement is 150 mL/min. As-received
rice husks without any leaching treatment is also used toprepare their ashes as a comparison. Table 4 indicateschemical compositions of rice husk ashes via each leachingprocess. When using no acid leaching treatment, a lot of
metallic oxides remain in the ashes as impurities. In particular, it is difficult to reduce K2O and CaO, which cause
the carbon to remain after burning as mentioned above,from rice husk ashes. As a result, SiO2 content of ashes is
94.6 wt%, and their color is black and dark gray. On theother hand, a citric leaching treatment is significantly
useful to remove these impurities and remained carbons
less than about 0.1 wt%. K2O content is about 1/30 of theashes with no treatment, and the CaO content is 50% or
less. The leaching effect by citric acid solution is almost
same as that of the conventional sulfuric acid. As a result,
the white ashes, having a high-purity of silica with 99 wt%
or more, are obtained by burning the acid-leached ricehusks under the suitable conditions.

1423K
1373K
1273K
1173K
1073K
973K
873K

20

30

40

50

60

Diffraction Angle, 2θ/°Fig. 6 XRD profiles of rice husk ashes of citric acid-leached ricehusks after burning at various temperatures in air

Therefore, the rice husks with very few metallic impurities
via acid leaching treatment shows a higher crystallization
temperature of 1,323 K or more than untreated ones
without any leaching.

Conclusion
The recycling process of rice husks with a high safety andeconomical benefits was developed to produce high-purity
amorphous silica materials. It consists of a citric acidleaching and air-combustion process. The acid leachingtreatment is significantly effective to accelerate thehydrolysis of cellulose of rice husks and reduce metallic
impurities by chelation of the carboxyl groups. A suitable
air supplement from 473 to 873 K is important, and TGDTA results indicate 50 mL/min or more is required tocompletely combust the organics of husks. The optimized
burning temperature of acid-leached rice husks is less than
1,273 K to prepare amorphous silica materials with a purityof 99 wt% or more.
Acknowledgement This study was supported by ‘‘Research &Technology Development on Waste Management Research Grant’’,
Ministry of the Environment.

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