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Morphology and Structure of Biomorphous Silica Isolated from
Equisetum hyemale and Equisetum telmateia
Mike Neumann, Sandra Wagner, Robert N¨oske, Brigitte Tiersch, and Peter Strauch
University of Potsdam, Institute of Chemistry, Karl-Liebknecht-Str. 24 – 25, 14476 Potsdam,
Germany
Reprint requests to Prof. Dr. P.Strauch. Fax: +49 (0)331 977-5054. E-mail: pstrauch@uni-potsdam.de
Z. Naturforsch. 2010,65b, 1113 – 1120; received March 4, 2010
The family of horsetails (Equisetaceae) is characterized through their high content of silica (SiO2),
which is the highest in known vascular plants. This work has focussed on two species of this family,
Equisetum hyemale and Equisetum telmateia, where the biomorphous silica is deposited basically as
amorphous SiO2in the outer epidermis of the plants. As source of SiO2, the original plant material
was air-dried and carved or powdered. For the isolation process the biomaterial was pre-treated with
aceotropic HCl. This pre-treatment has the advantage of the extraction of high amounts of the natural
inorganic matrix. In a second step the organic matrix was removed by a thermal oxidative process
in the temperature range of 275 – 1200 ◦
C to isolate the biogenic silicon dioxide from the perennial
plant. Parameters of time, temperature and the thermal gradient were varied to optimize the process
and to get products with the highest possible surface area. Furthermore, the particle morphology of the
biogenic SiO2from leaves and stems was examined separately. The silica deposits were characterized
by optical microscopy, scanning electron microscopy, infrared spectroscopy, gravimetry, nitrogen
sorption analysis, and sedimentation analysis.
Key words: Equisetum hyemale,Equisetum telmateia, Horsetail, Biomorphous Silica, Silicon
Dioxide
Introduction
The family of Equisetaceae (horsetail) are the last
descendents of an earlier group of plants, the Sphenop-
sida [1]. The genus of Equisetum consists of about
30 species and has its origin in the time of the Car-
boniferous [2]. The genus is divided into marsh and
land plants [1], both being found in whole Europe. The
stems are cavernous and structured in nodes and in-
ternodes. Equisetum telmateia achieves heights up to
200 cm, Equisetum hyemale are smaller with maxi-
mum heights up to 100 cm. The plants are preferren-
tially located at calcareous and humous soil [3].
The silicon source of plants is silicic acid in soil
[4]. The availability of silica depends on the concen-
tration and the chemical structure of the silicon com-
pounds. Free silica is transported in the xylem through
the plants and has many different tasks [5]. Silicon is
quasi essential for the silicon accumulator horsetail [6].
Silicon dioxide promotes the growth and reproduction
[7, 8], enhances the efficiency of photosynthesis, de-
creases the transpiration rate and affects the reflection
of light [9, 10]. The plant stores silicon as amorphous
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2010 Verlag der Zeitschrift f ¨ur Naturforschung, T ¨ubingen ·http://znaturforsch.com
silica or as “opal” [4]. Different studies have shown
that free silica is bound at cellulose and is no more flex-
ible after bonding. Therefore, it is no more available
for the rest of the plant [11]. High concentrations of
silica are found in the outer epidermis [12]. Silica con-
centrations in horsetail can range from 0.1 % to 10 %
of dry weight [9, 13].
In history people used silica from horsetails to clean
their dishware or to polish wood [5]. Today nano-
structured biomorphous silica is an environmentally
friendly alternative for synthetically produced silica in
many ranges of materials industry [14].
The object of the present study has been the charac-
terization of silicon dioxide isolated from Equisetum
telmateia and Equisetum hyemale. We have focused
our study on the morphology and structure of biogenic
SiO2. For our experiments we separated the plant ma-
terial of Equisetum telmateia into stems and leaves and
into unground and powdered samples for comparison.
With Equisetum hyemale we varied the parameters of
time, temperature and thermal gradient of the thermal
oxidative process to remove the organic matrix of the
plant system. The silica deposits were characterized
1114 M. Neumann et al. ·Morphology and Structure of Biomorphous Silica
with optical microscopy, scanning electron microscopy
(SEM/EDX), infrared spectroscopy, gravimetry, nitro-
gen sorption measurements (BET), and sedimentation
analysis.
Experimental Section
The leaves of Equisetum telmateia were derived from a
habitat located in Zittau (Saxony, Germany) in October 2004.
One fraction of the material was milled to a diameter smaller
than 250
µ
m, dried at 105 ◦
C and stored at r. t. The second
fraction was air-dried and stored at r. t.
Equisetum hyemale was collected in the Botanical Gar-
den of the University of Potsdam, Germany, in October 2004
after three vegetation periods. The plants were air-dried and
divided into sections with a length of about 2 – 3 cm.
All materials were pre-treated in a wet-chemical labora-
tory step with aceotropic HCl for 2 h at boiling temperature
(aceotropic HCl / H2O, 1 : 1). The water-washed Equisetum
was dried at 105 ◦
C.
Equisetum telmateia was ashed at 750 ◦
C in an oxida-
tive atmosphere after HCl pre-treatment [15, modified]. The
percentage yields of residue were calculated. Furthermore
pre-treated materials were tempered at different tempera-
tures, viz. 275, 325, 375, 400, 500, 650, 700, 850, 1100, and
1200 ◦
C with a thermal gradient of 1 K min−1, and kept for
48 h at the given temperature.
Samples of Equisetum hyemale were tempered for 0.5,
1, 2, 4, 6, 18, 24, 48, 73, 120, 168, 240, 360, 480, 600,
840, 1080, 1200, 1320, 1440, 1560, 1680, 1800, 1914, and
2136 h after achieving the set temperature of 350 ◦
C. The
thermal gradient was 1 K min−1. Other samples were heated
to 350 ◦
C for 48 h with three different thermal gradients of
0.1, 1 K min−1and 8 K min−1. For the maximum heating
gradient at 350 ◦
C the sample was placed directly in the pre-
heated furnace. Furthermore, samples were tempered for 48 h
with a gradient of 1 K min−1at 300, 325, 350, 375, 400, 450,
500, 600, and 700 ◦
C.
Gravimetric analysis was used to follow the loss of the or-
ganic matrix as an overall mass loss. All gravimetric analy-
ses showed differences between the only thermolized and the
HCl pre-treated biomaterial. IR spectroscopy was employed
to characterize the silica and the biomaterial. An IR spec-
trometer 16 PC FT-IR from Perkin Elmer was used with the
KBr disc technique. The discs were pressed in vacuum. The
specific surface area was determined with nitrogen sorption
measurements at 77 K (BET; DIN ISO 9277: 2003-05) [16].
A Quantachrome Autosorb Automated Gas Sorption System
was used for the BET measurements.
The sedimentation analyses were carried out with a photo-
scanning sedimentograph “analysette 20” (Fritsch) for char-
acterization of the particle size distribution of solid mat-
ter dispersed in solution. Particles in the range from 0.5 to
500
µ
m can be measured in relation to their sedimentation
velocity. The following equation was used to calculate the
sedimentation velocity:
ν
=(2r2(D1–D2)g)/(9
ε
)(
ν
=sed-
imentation velocity, r= particle radius, D1= density of the
particle, D2= density of the solvent, g= gravitation con-
stant,
ε
= viscosity of the solvent). We used water as sol-
vent for all samples. The macroscopic structures were char-
acterized by optical microscopy. We used reflected-light mi-
croscopy (Carl Zeiss Jena), with a magnification factor of up
to 100 to describe the non-planar sample surface. For further
investigations we used transmitted-light microscopy (WILL
h500 Wilozyt, hund Wetzlar) with a magnification of up to
600 times. The microscopes were combined with a Nikon
Coolpix-995 for picture recording.
The HCl pre-treated and for 73 h at 350 ◦
C calcinated sam-
ples of Equisetum hyemale were characterized by scanning
electron microscopy (SEM) and energy dispersive X-ray
spectroscopy (EDX). For the investigations of the silicon
content a cryo-field emission scanning electron microscope
S-4800 (Hitachi) was used. The resolution is 1.0 nm at 15 kV
and 1.4 nm at 1 kV. The EDX spectroscope was built by
Thermo (Thermo-NORAN-System SIX) and equipped with
a cryo-transfer system Gatan-Acto 2500-S.
Results and Discussion
For the wet-chemical pre-treatment of the bioma-
terials we exclusively used the method of boiling
in aceotropic HCl, because this method leads to the
best results. This pre-treatment of Equisetum hyemale
and Equisetum telmateia reduces the inorganic matrix,
which means a nearly complete loss of sodium, potas-
sium, magnesium, calcium, and aluminum compounds
[15]. The main amount of the inorganic matrix was
dissolved during the following washing process with
Fig. 1. IR spectrum of Equisetum hyemale after 2 h of treat-
ment with boiling aceotropic HCl.
M. Neumann et al. ·Morphology and Structure of Biomorphous Silica 1115
Fig. 2 (color online). Left: Residue of Equisetum telmateia after thermal oxidation at 750 ◦
C: () ground biomaterial, without
HCl pre-treatment; (•) unground biomaterial, without HCl pre-treatment; ( ) ground biomaterial, after HCl pre-treatment;
() unground biomaterial, after HCl pre-treatment. Right: Decomposition of the organic matrix of Equisetum telmateia ()
ground and (•) unground, after treatment with boiling HCl and calcination for 48 h at different temperatures.
pure water, until the filtrate was neutral. The HCl pre-
treatment causes also a partial acidic hydrolysis of the
organic matrix [17]. The bonds in the cellulose net-
work partially break and give glucose and polymers
with smaller degrees of polycondensation. After boil-
ing with HCl the biomaterials consist mainly of sili-
con dioxide, cellulose (about 80 %), lignin (about 5 –
20 %) and hemicelluloses [18, 19]. For Equisetum tel-
mateia an average of 55 % of the original mass were
found. Differences between unground leaves and pow-
dered materials were not detected. For Equisetum hye-
male a mass loss of 65 % was found. Fig. 1 shows the
IR spectrum of a sample of Equisetum hyemale af-
ter HCl pre-treatment. The spectrum proves the pres-
ence of the remaining organic matrix. The absorp-
tion bands at 2923 and 2856 cm−1characterize the
symmetric and asymmetric CH2valence vibrations
[20, 21]. The band for the conjugated carbonyl vibra-
tion is found at 1712 cm−1, the non-inconjugated vi-
bration at 1630 cm−1[21]. The deformation vibrations
of the CH groups are related to a band at 1504 cm−1,
those of the CH2groups at 1453 and 1363 cm−1.All
typical vibrations of silica were found in the spectra at
1098, 802 and 464 cm−1[21, 22].
The organic components of the biomaterial Equise-
tum telmateia were completely degraded during calci-
nation at 750 ◦
C. The decomposition depends mainly
on the temperature, the calcinating time and the atmo-
sphere [23]. For the calcination we used constant tem-
peratures and normal atmosphere (air). The mass of
the remaining silica in dependency of the calcination
time is shown in Fig. 2. The main loss of the organic
matrix was observed during the first 30 min. Constant
masses were reached after 60 and 90 min. The aver-
age residue for ground biomaterial without a HCl pre-
treatment was found at 27 %, for the ground and pre-
treated plant we found a residue of 21% of the original
mass.
The biomaterial Equisetum telmateia was also cal-
cinated at different temperatures after pre-treatmant
in boiling aceotropic HCl. The mass loss is also de-
picted in Fig. 2. For a complete degradation of the
organic matrix we had chosen a calcination time of
48 h after reaching the appropriate temperature. All
samples were heated up with a defined thermal gra-
dient of 1 K min−1. The decomposition temperature
of the main organic matters are for hemicelluloses in
the range of 200 – 260 ◦
C, for cellulose 240 – 350 ◦
C
and for lignin 280 – 500 ◦
C [23]. The mass loss for the
ground and unground biomaterial is different (Fig. 2)
for two reasons. First, the ground material was dried
at a defined temperature of 105 ◦
C before milling.
Possibly the air-dried unground samples of Equisetum
might contain some more water. This would result in
a higher mass loss during the thermal process. Sec-
ond, there are differences in the structure of the sam-
ples. The powder has a larger surface than the un-
ground leaves, and the oxidation process is more ef-
fective for the ground material. The IR spectra (spec-
tra not shown) showed no bands of cellulose (2900–
2800 and 1500 – 1350 cm−1) after the oxidative ther-
mal process at 275 ◦
C. Cellulose decomposes at tem-
peratures between 240 and 350 ◦
C [23]. The spectra
showed the characteristic asymmetric stretching vibra-
1116 M. Neumann et al. ·Morphology and Structure of Biomorphous Silica
tion at 1102 cm−1, the symmetric stretching vibration
at 802 cm−1and the bending / deformation vibration at
469 cm−1of silica, and the vibration bands of lignin at
1730, 1620 and 1010 cm−1after tempering at 275 ◦
C.
For the samples calcinated at temperatures higher than
325 ◦
C a decrease of the lignin bands could be ob-
served. Beside the typical absorption bands of silica
only the vibration band of lignin at 1620 cm−1and a
shoulder at 1010 cm−1were found. Higher tempera-
tures (>400 ◦
C) lead to the disappearence of the lignin
shoulder at 1010 cm−1, and a strong band of amor-
phous silica at 566 cm−1is formed. The IR spectrum
of Equisetum telmateia calcinated at 850 ◦
Cisdiffer-
ent. The organic matrix is decomposed completely, and
only the characteristic silica vibration bands at 1100,
800 and 469 cm−1[21, 22] were found. Differences be-
tween the ground samples and the unground leaves of
Equisetum telmateia were not detected. Obviously, the
milling process had no influence on the decomposition
of the organic matrix.
The samples of Equisetum telmateia calcinated at
different temperatures in the range from 325 to 850 ◦
C
(48 h) were characterized by nitrogen sorption mea-
surement (BET) at 77 K in order to determine the
specific surface area (Fig. 3). Up to a temperature of
375 ◦
C the biomaterial still contains large quantities of
the organic matrix, resulting in a low porosity of the
material and a low specific surface. The largest val-
ues of the specific surface were detected for calcina-
tion temperatures between 375 and 575 ◦
C. The surface
of the calcinated material increases up to 400 m2g−1.
This effect implies the (complete) loss of celluloses
and the presence of amorphous silica. A further in-
crease of the calcination temperatures leads to crys-
Fig. 3. Specific surface area of Equisetum telmateia as a func-
tion of different calcination temperatures. All samples were
tempered for 48 h after HCl pre-treatment.
Tab l e 1. d50 data (particle diameter) of Equisetum telmateia
after 48 h of thermal oxidation and pre-treatment with HCl.
Comparison of d50 data after calcination for 48 h at 850, 1100
and 1200 ◦
C (HCl pre-treatment).
Temperature (◦C) d50 (
µ
m) (ground) d50 (
µ
m) (unground)
375 11 –
575 13 14
850 15 –
1100 25 23
1200 34 37
tallization, sintering and partly melting processes [10].
These effects are reflected in a significant decrease
of the specific surface. The samples of ground and
unground material of Equisetum telmateia after pre-
treatment with aceotropic HCl and a thermal oxidation
at 575 ◦
C show only small differences. For the ground
material a specific surface area of 382 m2g−1,forthe
unground material of 422 m2g−1were measured. For
these samples the milling process had no significant in-
fluence on the porosity.
The particle size represented by the d50 data and
the size distribution was determined by sedimentation
analysis. The measurements of ground Equisetum tel-
mateia after HCl pre-treatment and thermal oxidation
at temperatures between 375 and 850 ◦
Cshowedno
significant differences in the d50 data: 11.13
µ
mat
375 ◦
C, 13.35
µ
m at 575 ◦
C and 14.70
µ
m at 850 ◦
C.
Up to 850 ◦
C the temperature has no strong influence to
the particle size. However, the d50 data and the particle
size distribution differences in the temperaturerange of
850 ◦
C and 1200 ◦
C are noticeable (Table 1). For every
temperature step a significant step in the particle size of
about 10
µ
m was observed. With increasing tempera-
tures the silica particles start to sinter with formation
of agglomerates. At 1200 ◦
C the sedimentation veloc-
ity in water is too high to be measured, therefore, we
changed the solvent to ethylene glycol. The sedimen-
tation analysis showed no significant differences be-
tween ground and unground biomaterial. Furthermore
we compared the d50 data of silica from ground ma-
terial and unground leaves with and without HCl pre-
treatment. The pre-treatment with aceotropic HCl has
no observable effect on the particle size. The d50 data
are between 16 and 22
µ
m (not pre-treated: ground
material 17
µ
m, unground leaves 21
µ
m; pre-treated:
ground 18
µ
m, unground 19
µ
m).
The physiological precipitation of silica in the
leaves of Equisetum telmateia and its function can be
studied by optical microscopy. Horsetails are typical
M. Neumann et al. ·Morphology and Structure of Biomorphous Silica 1117
Fig. 4 (color online). (top) Optical reflected-light spec-
troscopy of unground leaves of Equisetum telmateia af-
ter HCl pre-treatment and calcination at 375 ◦
C for 48 h.
(bottom) Optical transmitted-light spectroscopy of particles
from ground leaves of Equisetum telmateia after HCl pre-
treatment and calcination at 400 ◦
C for 48 h.
silica accumulators [10]. They assimilate silicic acid
from soil and deposit it as demobilized, amorphous sil-
ica or as opal [24]. Our biomorphous silica is mainly
of the amorphous form and contains only little of the
opal form. The amorphous silica forms a characteristic
skeleton (Fig. 4) during the decomposition of the or-
ganic matrix [5, 12]. Linkages between silicic acid and
the organic polymer (cellulose and lignin) are partly
broken. The deposition of silicon dioxide in leaves is
mainly observed in the cells of the epidermis, the leaf
veins and the closing cells [25].
To study the influence of calcination time, samples
of Equisetum hyemale were tempered at 350 ◦
C for up
to 2136 h after achieving the appropriate temperature.
Already after 18 h of thermal oxidation no significant
changes in the mass loss were detected (Fig. 5), and
after 120 h an equilibrium was reached. Nevertheless,
we found chemical changes in the structure. An ex-
Fig. 5. Top: Gravimetric residue (•) and gravimetric average
(–) of Equisetum hyemale as a function of time after wet-
chemical pre-treatment and thermal oxidation at 350 ◦
C. Bot-
tom: Decomposition of the organic matrix (Equisetum hye-
male) as a function of temperature after 48 h of thermal oxi-
dation.
tended thermal oxidation at 350 ◦
C results in the same
decomposition of the organic matrix as a thermal pro-
cess at higher temperatures (750 ◦
C). The degradation
of hemicelluloses, celluloses and lignin proceeds pos-
sibly at lower temperatures than mentioned in the liter-
ature [23]. Samples of Equisetum hyemale were tem-
pered for 48 h at different temperatures between 300
and 750 ◦
C with a heating gradient of 1 K min−1.The
maximum of mass loss for a constant calcination time
of 48 h was detected for 600 ◦
C (Fig. 5). At higher
temperatures no further changes were observed. For
short thermal treatment times (48 h) higher temper-
atures are necessary for the complete decomposition
of all organic components [23]. The decrease of the
residue mass up to 350 ◦
C is more precipitous than at
higher temperatures. The quantities of hemicelluloses
and celluloses in the biomaterial are much higher than
the quantity of lignin, and for lignin higher tempera-
tures (280 – 500 ◦
C) are required for the decomposition
1118 M. Neumann et al. ·Morphology and Structure of Biomorphous Silica
Fig. 6 (color online). IR spectra of Equisetum hyemale after HCl pre-treatment and calcination for 0, 120, 480 as well as for
2136 h at 350 ◦
C (left); after 48 h of thermal treatment at 300, 500 and 750 ◦
C (right).
than for the hemicelluloses (200 – 260 ◦
C) and cellu-
loses (240 – 350 ◦
C) [23]. There is no significant in-
fluence of different heating gradients on the thermol-
ysis of Equisetum hyemale. The remaining residue af-
ter 48 h of calcination at 350 ◦
C with different gradi-
ents is nearly constant (0.1 K min−1:26%;1Kmin
−1:
26 %; 7.8 K min−1: 26 %, and the maximal gradient at
350 ◦
C: 25 %).
The IR spectra of the samples after calcination times
up to 2136 h (Fig. 6) show the decrease of the or-
ganic matrix. At the beginning of the calcination pro-
cess at 350 ◦
C the typical absorptions of the whole
organic matrix still can be observed. The deformation
vibrations at 1504 (CH group), 1453, and 1363 cm−1
(CH2groups) disappear first reflecting the decomposi-
tion of hemicelluloses and celluloses. The strong mass
loss at the beginning of the thermal treatment corre-
lates with the disappearence of the typical absorption
bands of the organic matrix in the IR spectra. The car-
bonyl bands at 1615 (conjugated)and 1723 cm−1(non-
conjugated) [8, 19] decrease with increasing time, and
this correlates with the decomposition of lignin. The
latter has completely disappeared after 48 h, the for-
mer reaches the minimum after 120 h. All typical vi-
brations of silica were still present at 1102, 802, and
464 cm−1[21, 22]. The IR spectrum after 2136 h of
calcination shows the presence of only traces of lignin
(1630 cm−1). All IR spectra (Fig. 6) of the samples af-
ter 48 h of thermal oxidation at different temperatures
in the range between 300 and 750 ◦
C confirm the strong
decline of the lignin bands (carbonyl bands, 1712 and
1630 cm−1). The IR spectra (spectra not shown) for
different thermal gradients show no significant differ-
ences between 1 K min−1and the maximal gradient
(the sample being placed directly into the pre-heated
furnace). Only for the sample heated with a gradient
of 0.1 K min−1a lower mass loss was observed. Obvi-
ously, very low thermal gradients inhibit the decompo-
sition of hemicelluloses, celluloses and lignin in Equi-
setum hyemale.
Nitrogen sorption measurements (BET) at 77 K for
Equisetum hyemale calcinated at 350 ◦
C, with a vari-
ation of calcination time up to 2136 h (Fig. 7) gave
specific surface areas from 121 up to 418 m2g−1.Up
to 24 h of calcination at 350 ◦
C the material still con-
tains large quantities of hemicelluloses, celluloses and
lignin, which is again reflected in a reduced specific
surface area, due to a reduced porosity. The largest
specific surface areas were measured between 24 and
Fig. 7. Specific surface area of Equisetum hyemale as a func-
tion of time. All samples were calcinated at 350 ◦
CafterHCl
pre-treatment.
M. Neumann et al. ·Morphology and Structure of Biomorphous Silica 1119
Fig. 8 (color online). Optical reflected-light spectroscopy of
Equisetum hyemale after HCl pre-treatment and calcination
at 350 ◦
C for 120 h (top), 480 h (middle) and 2136 h (bot-
tom). The measure bar is 1 mm in all images.
2136 h of calcination (339– 418 m2g−1). In this time
range the largest quantity of amorphous silicon dioxide
with a high porosity is available. Longer thermal treat-
ment times lead again to crystallization, melting and
sintering processes [10].
The structure of Equisetum hyemale was also char-
acterized by optical reflected-light microscopy after up
to 2136 h of calcination at 350 ◦
C (Fig. 8). The wet-
chemically pre-treated biomaterials change their color
shade from dark-brown at the beginning via brown,
yellow, colorless to a finally snow-white end product.
After 1 h of treatment the brown biomaterial starts to
Fig. 9. SEM/EDX measurements of different areas of Equi-
setum hyemale after HCl pre-treatment and 73 h of thermal
oxidation at 350 ◦
C. The accelerating voltage was 5.0 kV. The
bar is 100
µ
m. Top: SEM image of the sample of E. hyemale.
The marked areas are the EDX measurement points. Bottom:
EDX spectra of the measurement point 1.
brighten up, and after 73 h a colorless product is ob-
served. Interestingly, after a long time of calcination
(1800 h) the color of the silica remains unchanged. The
change of color implies the oxidativedegradiation pro-
cess of the organic matrix to leave the silica skeleton.
The best resolution was observed after longer calcina-
tion times (>1800 h).
Fig. 9 shows the SEM image and the semiquantita-
tive analysis of a sample of Equisetum hyemale.The
sample was pre-treated with hydrochloric acid and cal-
cinated at 350 ◦
C for 73 h. The image shows the ba-
sic structure of the silica deposition in the biomate-
rial. Two columns of stomata are flanked with epider-
mal layers. In EDX measurements of the stomata area
(Fig. 9) carbon, oxygen and silicon can be detected.
The stomata contain nearly pure silicon dioxide. The
quantity of carbon (only 3.2 atom-%) is marginal and
supports the gravimetric results. After 120 h of calci-
nation time an equilibrium in the gravimetric residue
was reached which contains less than 3 % carbon. For
the stomata a composition of 74 atom-% oxygen and
23 atom-% silicon was measured. This atomic ratio of
0.31 for Si to O is very close to the theoretical value
of 0.33 for SiO2. The elemental analysis of the epider-
mal layer gave similar results as for the stomata area,
viz. 3 atom-% for carbon, 77 atom-% for oxygen and
1120 M. Neumann et al. ·Morphology and Structure of Biomorphous Silica
20 atom-% for silicon. The silicon to oxygen ratio is
0.27. The accumulation of silicon dioxide in the layer
area is significantly smaller than in the stomata area.
For that reason the silica distribution in Equisetum hye-
male is not homogenous [10] and varies between the
different plant components.
Conclusion
In our studies we could demonstrate that a decom-
position of the organic and inorganic matrix of leaves
from Equisetum telmateia and stems from Equisetum
hyemale without destroying the biomorphous silica
structure by a thermal oxidation process can be ac-
complished. In the wet-chemical pre-treatment most
of the inorganic matrix is dissolved improving the pu-
rity of the remaining SiO2. The majority of the essen-
tial inorganic compounds in plants are phosphates, car-
bonates and sulfates of alkali and alkaline earth ions.
With these components removed, after thermal oxida-
tion stable skeletons or small particles of silicon diox-
ide are obtained. In Equisetum hyemale the absolute
content of pure silica is about 9 %, in Equisetum tel-
mateia about 23 %. In both species the maximum sur-
face area of silica is about 400 m2g−1.
The specific surface area of the silica from Equise-
tum hyemale depends significantly on the calcination
time. Thermal oxidation even at moderate tempera-
tures but over a long period results in crystallization
and sintering processes decreasing the specific surface
area. We have shown that thermal oxidation of hemi-
celluloses, celluloses and lignin in horsetails is influ-
enced by several parameters. By optical microscopy
the important role of silica as a structural element in
horsetails could be demonstrated. The extraction of
amorphous silica with high specific surface areas from
renewable resources may be of technological impor-
tance in the future.
Acknowledgement
The authors thank Ms. Regina Rothe from the Max Planck
Institute of Colloids and Interfaces (Potsdam/Golm, Ger-
many) for nitrogen sorption measurements.
[1] H. Weymar, Buch der Farne, B ¨arlappe und Schachtel-
halme, Neumann, Radebeul and Berlin, 1958,p.40–
44.
[2] R. Junker, Samenfarne, B ¨arlappb ¨aume, Schachtel-
halme,H¨anssler, Holzgerlingen, 2000, p. 103.
[3] W. Meusel, J. Laroche, J. Hemmerling, Die Schachtel-
halme Europas, A. Ziemsen, Wittenberg, 1971,p.5–6,
17.
[4] K. Mengel, E. A. Kirby, Principles of plant nutrition,
Kluwer Academic Publishers, Dordrecht, 2001, 643 –
649.
[5] M. G. Voronkov, G. I. Zelcan, E. Lukevits, Silicium und
Leben, Akademie-Verlag, Berlin, 1975, pp. 56, 77 – 91.
[6] E. Epstein, Annu. Rev. Plant Physiol. Mol. Bio. 1999,
50, 641 – 664.
[7] A. G. Sangster, M. J. Hodson, CIBA Foundation Sym-
posium: Silicon biochemistry, Wiley, New York, 1986,
pp. 90 – 111.
[8] L. E. Dartnoff, G. H. Synder, Plant. Dis. 1992,76,
1011 – 1013.
[9] E. Epstein, Proc. Natl. Acad. Sci. 1994,91, 11 – 17.
[10] L. Sapei, N. Gierlinger, J. Hartmann, R. N¨oske,
P. Strauch, O. Paris, Anal. Bioanal. Chem. 2007,289,
1249 – 1257.
[11] S. Yoshida, Bull. Natl. Inst. Agric. Sci. B 1965,15,1–
58.
[12] J. D. Lewin, B. E. F. Rei mann, Annu. Rev. Plant Phys-
iol. 1969,20, 289 – 304.
[13] J. F. Ma, E. Takahashi, Soil, fertilizer, and plant silicon
research in Japan, Elsevier, Amsterdam, 2002, pp. 1 –
281.
[14] L. Sapei, R. N¨oske, P. Strauch, O. Paris, Chem. Mater.
2008,20, 2020 – 2025.
[15] C. Real, M. D. Alcal´a, J. M. Criado, J. Am. Ceram. Soc.
1996,79, 2012 – 2016.
[16] P. Brunnauer, P. H. Emmett, E. Teller, J. Am. Chem.
Soc. 1938,960, 309 – 319.
[17] H. Kr¨assig, Cellulose, Structure, Acessibility and Re-
activity, Gordon and Breach Science Publishers, Yver-
don, 1993, pp. 215 – 277.
[18] H. Semke, Dissertation, Albert-Ludwigs-University
Freiburg, Freiburg i. Br., 2001,p.7.
[19] R. Pastusiak, Dissertation, Technical University of Mu-
nich, M¨unchen, 2003, pp. 21 – 29.
[20] V. Trouillet, S. Heißler, U. Geckle, E. Willin,
W. Faubel, FZKA 6468, Karlsruhe, 2000, pp. 36 – 32.
[21] K. Nakamoto, Infrared and Raman Spectra of Inor-
ganic and Coordination Compounds B, Wiley, New
York, 1997, pp. 1 – 377.
[22] R. Ashokan, V. Gopal, K. C. Chhabra, Phys. Stat. Sol.
(a) 1990,121, 533 – 537.
[23] W. Sandermann, H. Augustin, Eur. J. Wood Prod. 1963,
21, 256 – 265.
[24] E. Bode, Dtsch. Tier¨arztl. Wochenschr. 1994,101,
367 – 372.
[25] C. K. Morikawa, M. Saigusa, Plant Soil 2004,258,
1–8.
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