Effects of pretreatment and solution chemistry on solubility of rice straw phytoliths

J. Plant Nutr. Soil Sci. 2014, 177, 349–359

DOI: 10.1002/jpln.201300056

349

Effects of pretreatment and solution chemistry on solubility of rice-straw
phytoliths
Minh Ngoc Nguyen1*, Stefan Dultz2, and Georg Guggenberger2
1

Department of Pedology and Soil Environment, Faculty of Environmental Sciences, VNU University of Science, Vietnam National University,
Hanoi. 334–Nguyen Trai, Thanh Xuan, Hanoi, Vietnam
2 Institute of Soil Science, Leibniz Universität Hannover, Herrenhäuser Straße 2, 30419 Hannover, Germany

Abstract
Rice is a Si-accumulator plant, whereby Si has physio-chemical functions for plant growth. Its
straw contains high shares of plant silica bodies, so-called phytoliths, and can, when returned to
the soil, be an important Si fertilizer. Release of Si from phytoliths into soil solution depends on
many factors. In order to improve prognosis of availability and management of Si located in phytoliths, in this study we analyzed the effect of pretreatment of rice straw by dry and wet ashing
and the soil-solution composition on Si release. Dry ashing of rice straw was performed at
400°C, 600°C, and 800°C and wet ashing of the original straw and the sample from 400°C treatment with H2O2. To identify the impact of soil-solution chemistry, Si release was measured on
separated phytoliths in batch experiments at pH 2–10 and in presence of different cations (Na+,
2À
K+, Mg2+, Ca2+, Al3+) and anions (Cl–, NOÀ
3 , SO4 , acetate, oxalate, citrate) in the concentration
range from 0.1 to 10 mmolc L–1. After burning rice straw at 400°C, phytoliths and biochar were
major compounds in the ash. At an electrolyte background of 0.01 molc L–1, Si released at pH
6.5 was one order of magnitude higher than at pH 3, where the zeta potential (f) was close to
zero. Higher ionic strength tended to suppress Si release. The presence of cations increased f,
indicating the neutralization of deprotonated Si-O– sites. Monovalent cations suppressed Si

release more strongly than bivalent ones. Neutralization of deprotonated Si-O– sites by cations
might accelerate polymerization, leading to smaller Si release in comparison with absences of
electrolytes. Addition of Al3+ resulted in charge reversal, indicating a very strong adsorption of
Al3+, and it is likely that Si-O-Al-O-Si bonds are formed which decrease Si release. The negative
effect of anions on Si release in comparison with deionized H2O might be due to an increase in
ionic strength. The effect was more pronounced for organic anions than for inorganic ones.
Burning of rice straw at low temperatures (e.g., 400°C) appears suitable to provide silicon for
rice in short term for the next growing season. High inputs of electrolytes with irrigation water
and low pH with concomitant increase of Al3+ in soil solution should be avoided in order to keep
dissolution rate of phytoliths at an appropriate level.
Key words: rice straw / phytolith / dry ashing / solution chemistry / Si release / zeta potential

Accepted July 30, 2013

1 Introduction
Rice (Oryza sativa) belongs to a plant group known to take
up monosilicic acid (Si(OH)4) by their roots resulting in an Si
content of 5%–10% in plant dry matter (Marschner, 1995). By
deposition in inter- and intracellular spaces throughout their
leaf and stem, silicified structures are formed consisting of
biogenic silica, so-called phytoliths (Parr and Sullivan, 2005).
Within each growing period of rice relatively large amounts of
Si are taken up from the soil solution and are cycled through
the crop back into the soil (Wickramasinghe and Rowell,
2006). In the soil Si located in phytoliths is an important pool
for supplying Si (Sommer et al., 2006). The following crops
can benefit from this pool, and it is of particular interest for
cultivation safety of rice to know the decisive factors for dissolution of phytoliths and release of Si.
The function of phytoliths in the rice plant can be deduced
from the principal arrangement of silicified structures and

organic matter (OM) in the plant material, which is shown for
a vascular bundle in a rice leaf in Fig. 1. Between the bundle
sheath and the leaf surface tightly packed bundle-sheath
cells and more loosely arranged mesophyll cells form a protective cover on leaf veins stabilized by silicified structures in
inter cellular spaces. Through the deposition of silica in the
cell walls the mechanical strength of leaves and stem is increased, which prevents plants from lodging in heavy wind.
Also transpiration rate of rice is reduced, and thus, sufficient
Si supply contributes to the reduction of drought stress (Chen
et al., 2011). Reduction of excessive transpiration and
enhanced light interception promotes also photosynthesis
(Kato and Owa, 1997). Silicon fertilization of soils for rice cultivation increased the resistance to fungal stress (Kato and
Owa, 1997) and might also increase resistance to insect
pests. Recently an active impact of Si on rice root anatomy
enhancing suberization and lignifications in roots was ob-

* Correspondence: Dr. Minh Ngoc Nguyen;
e-mail:

 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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350

Nguyen, Dultz, Guggenberger

_____
20 µm

served (Fleck et al., 2011). Ma (2004) summarized the effects
of Si on rice, and the general conclusion was that the resistance of plants to various biotic and abiotic stresses is
enhanced.
Paddy soils with a high content of plant-available Si induce
low As contents in rice plants (Bogdan and Schenk, 2008). In
presence of Si, the uptake of As by paddy rice is decreased
markedly. This is of special importance for rice, as many
paddy fields, e.g., in Bangladesh show geogenic arsenic contamination (Meharg and Rahman, 2003).
On-site burning after harvesting is the primary method of
handling rice straw to return nutrients to the soils. In recent
decades, burning of rice straw has been predominant
because it is a cost-effective method of straw disposal,
avoids interferences with soil preparation, and helps to
reduce pest and disease populations resident in the straw
biomass (Dobermann and Witt, 2000). Although burning of
rice straw causes significant emission of CO2, almost complete loss of N and S, and contributes to air pollution, it is the
easiest way of returning most nutrients to the soils, and at
present rice growers have little incentive to quit burning.
Considering the large amount of Si accumulated in rice straw,
products of straw burning are an interesting pool to serve
as a silicon source for plants. Due to a relatively low ignition
temperature, burning of straw is observed at > 300°C
(Babrauskas, 2003). Burning of biogenic silica, e.g., of
the rice husk at higher temperatures > 700°C can lead to
the formation of crystalline SiO2, where amorphous silica is
transformed to more stable tridimite or cristobalite (Kordatos
et al., 2008). The apparent reduction in reactivity of biogenic
silica is associated with changes in the surface chemical
 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

J. Plant Nutr. Soil Sci. 2014, 177, 349–359

Figure 1: Three-dimensional image
of the principal arrangement of
silicified structures and organic matter for a vascular bundle in a dried
leaf of a rice plant. The visualization
was performed by X-ray tomographic microscopy using the 3D
segmentation and visualization software YaDiV (Friese et al., 2013) for
analysis of the dataset. Phytoliths
appear gray and organic matter dark
gray. The pixel width is 0.37 lm.

structure, and in particular with progressive loss of reactive
surface sites (Dixit and Van Cappellen, 2002). Crystalline
forms are very inactive in the soil and their potential to serve
as a Si source is almost lost. For this reason, the relation of
burning temperatures and dissolution of rice-straw-burned
products has to be considered, but not much literature is
available on this issue. Burning temperature of rice straw can
be decreased, when the straw is more compacted and the
water content is high.
It is generally accepted that the dissolution of silica in aqueous solutions occurs via hydrolysis of Si–O–Si bonds of the
SiO2 network. Water itself is a strong promoter by means of
its molecules oriented with their electronegative oxygen
towards the Si atom, leading to a transfer of electron density
to the Si–O–Si bond, thereby increasing its length and eventually breaking it (Dove and Crerar, 1990). pH is understood
as an important factor driving silica-dissolution kinetics
(Fraysse et al., 2006; Loucaides et al., 2008). In particular,
the acceleration of the dissolution rate with increasing pH is

explained by the increase in concentration of deprotonated
≡Si–O– sites at the solid’s surface (Brady and Walther, 1990).
The negatively charged sites promote dissolution kinetics,
either by enhancing the nucleophilic properties of water
(Dove, 1994) or polarizing, and thus weakening, surface
Si–O–Si bonds (Brady and Walther, 1990). It seems likely
that anions can attack Si–O–Si bonds in a similar way as
OH–. Additives containing chemical groups that are strongly
anionic, such as –COO– and –PO2À
4 , may react with Si centers in Si–O–Si bonds of biogenic silica (Ehrlich et al., 2010).
Several studies have highlighted the desilification of silica
under alkaline conditions (Sauer et al., 2006; Saccone et al.,
2007). However, in this way, an investigation on anion effects
with aqueous solutions close to realistic pH conditions of
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J. Plant Nutr. Soil Sci. 2014, 177, 349–359
paddy soils, which are in the Red River Delta from pH 5–6
(Nguyen et al., 2009) is a necessity.

Effect of pretreatment and solution chemistry 351

2 Materials
2.1 Sample production

Deprotonation of the silanol groups (Si–OH) on the phytolith
surface can facilitate the water molecules to attack Si–O–Si
bonds, which is known as a first step for desilification (Dove
and Elston, 1992; Fraysse et al., 2006). On the other hand,

adsorption of cations from aqueous solution onto deprotonated ≡Si–O– sites might occur and accelerate polymerization (Weres et al., 1981). The surface of phytolith might be,
therefore, strengthened to resist dissolution. Under reducing
conditions of paddy soils, the release of bivalent cations such
as Fe2+ and Mn2+ from dissolving oxides and in consequence
Ca2+ and Mg2+ desorption from exchange sites is pronounced
(Nguyen et al., 2009). The reaction of these cations with phytoliths may have a marked effect on their solubility and can
be an important factor for Si release. Considerable amounts
of electrolytes are added to paddy fields if irrigation is performed close to the coastline with brackish water. A strong
decrease in pH is observed in paddy fields when the water
table is lowered before harvesting. Hence, Al3+ occurs in soil
solution. Al3+ is known to react with biogenic silica and to
reduce its solubility remarkably (Wilding et al., 1979; Van
Bennekom et al., 1991). These studies indicate that Al3+ has
a strong effect on phytolith dissolution, which has to be considered for the management of silicon in paddy soils.
Usually, dry and wet ashing techniques used for the extraction of phytoliths from plant material (Parr et al., 2001) do not
remove all OM present in rice-straw samples. There are still
certain amounts of OM remaining in ashes (Lai et al., 2009).
The effect of OM created by pyrolysis of biomass, so called
biochar, on dissolution of phytoliths is not yet fully known.
The organic matrix may act as a protective barrier against
hydrolysis of the silica. Like phytoliths, biochar has variably
charged surface sites, and both compounds contribute to
the total net charge of the burned products. The question
arises if surface-charge properties can be used as a parameter for predicting phytolith dissolution in presence of
unburned OM.
In this study, the effect of different parameters of solution
chemistry, including pH, ionic strength, valency, and size of
cations and anions on the solubility of phytoliths from rice
straw was determined in order to make the prediction of Si
release more reliable. The mode of pretreatment of rice

straw, burning at different temperatures, and wet ashing
using H2O2 resulting in, e.g., different degrees of dehydroxilation of biogenic silica and OM contents was also investigated.
The apparent reduction in reactivity of biogenic silica goes
along with changes in surface chemical structure, and in particular loss of reactive surface sites. Thus, besides batch
experiments for quantifying Si release also zeta potential (f),
the key electrochemical parameter of the solid–liquid interface providing information about the interfacial double layer
between the solution and the stationary layer of fluid attached
to the phytoliths, was determined. f indicates ion adsorption
and ionization of surface functional groups, and thus provides
important information on dissolution kinetics of the rice-straw
phytoliths.
 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Rice straw was collected from a paddy field of a research station close to Tay Mo commune in the rice-growing area in the
central part of the Red River Delta (105°44′17″ E, 20°59′57″
N) directly after harvesting in spring 2011. The rice straw was
air-dried, ground in a blade grinder (Clatronic KSW 3306,
Kempen, Germany), and passed through a 1.0-mm sieve.
The rice straw had 42.1 g kg–1 (d.w.) Tiron-extractable (Guntzer et al., 2010) Si, 387 g kg–1 C, and 13.2 g kg–1 N as determined by an Elementar Vario EL (Hanau, Germany) elemental analyzer with a respecitve C : N ratio of 29.
Dry ashing of rice straw was performed by heating finely
ground air-dried rice straw in an oven at 400°C, 600°C, and
800°C, respectively, for 6 h. To avoid strong exothermic reactions during dry ashing the weight of sample was limited to
5 g. For comparison, OM in air-dried finely chopped rice straw
was treated by wet ashing with H2O2 until the end of reaction.
For wet ashing 25 mL of a 15% H2O2 solution were added to
5 g straw, stirred, and kept in a water bath at 80°C.

2.2 Sample properties
The different pretreatments of straw changed organic-C content drastically (Table 1). Organic C was most completely
removed by heating at 800°C, whereas treatments of rice

straw with H2O2 in a water bath at 80°C alone removed only
less than 1.5% of total organic C, showing a high resistance
of rice straw against this oxidant. Consequently, also the
Tiron-extractable Si varies from 42 g kg–1 in the original ricestraw sample to 193 g kg–1 in the dry-ashed produced at
600°C (Table 1). Treatment of samples with H2O2 resulted in
slight increase in Tiron-extractable Si only. The ashes of the
dry-ashing treatment had an alkaline reaction (pH 10–11).
Soluble anions and cations of the dry-ashed rice-straw samples were determined by anion chromatography (Dionex,
ICS-90) and ICP-OES (Varian, 725-ES) in a 1:10 extract with
deionized water. In solution, K+ was the most abundant cation
but also marked amounts of Na+, Ca2+, and Mg2+ were observed (Table 1). For the anions, besides Cl– and SO2À
4 also
PO3À
4 was found in solution.
A marked decrease of the specific surface area (SSA) determined by the N2-adsorption method (Quantachrome, NOVA
4000e, Boynton Beach, FL, USA) was obtained with increasing heating temperature. The SSA of the sample heated at
400°C, 600°C, and 800°C were found to be 68.6, 19.8 and
1.0 m2 g–1, respectively, indicating a strong condensation of
silica structures. Temperatures > 700°C are known to inherit
formation of crystalline SiO2 phases such as tridimite or cristobalite (Kordatos et al., 2008). Because of the severe
decrease of SSA at higher burning temperatures strongly
decreasing the amount of active surface sites, for further analyses focus was given on the straw sample ashed at 400°C.
For the original sample, SSA determination by the N2-adsorption method failed because N2 did not enter the micropores of
OM.
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352

Nguyen, Dultz, Guggenberger

J. Plant Nutr. Soil Sci. 2014, 177, 349–359

Table 1: Specific surface area (SSA), Si and C content of the original rice-straw sample (1), dry-ashed samples treated at 400°C, 600°C, and
800°C (2–4), wet-ashed sample treated with H2O2 (5), and combined treatment of dry ashing at 400°C and subsequent wet ashing by H2O2
addition (6). Soluble cations and anions were analyzed for the heat-treated samples alone.
Treatment

SSA
/ m2 g–1

(1) original sample

–

Si
/ g kg–1
42.1

C
/ g kg–1

Soluble ions / mg kg–1

387

n.a.

K+

(2) 400°C

68.6

166

95

1.06 ·

(3) 600°C

19.8

193

19

> 104
104

104

Na+

Ca2+

Mg2+

Cl–

SO2–
4

PO3–
4

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

505

220

180

19.9

13.7

3.5

275

275

55

18.7

16.3

3.7

(4) 800°C

1.0

112

2

455

65

145

6.9

11.5

1.7

(5) H2O2

20.2

50

374

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

(6) 400°C/H2O2

71.2

173

5

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

>

Electron micrographs of the different samples were made on
a Fei Quanta 200 (Hillsboro, OR, USA). For this purpose,
specimen were mounted on a double adhesive tape and
sputtered with gold. Back-scattered-electron images on
ground original rice straw revealed that the fragments had an
intact outer cell wall (Fig. 2a), whereas dry ashing even at
400°C resulted in a strong degradation of the rim of straw
fragments (Fig. 2b). In all samples from dry ashing silicified
cell structures were clearly detectable.

(2005) stated that the absorption band at 950 cm–1 of fumed

silica only becomes visible for samples with a SSA > 200 m2
g–1. Dehydroxilation of OH groups upon burning of straw
samples might be another reason for weakened Si–O stretching vibration of Si–OH groups.

3 Methods
3.1 Determination of dissolution kinetics

Functional groups in the samples were determined with an
FTIR spectrometer (Bruker, Tensor 27, Karlsruhe, Germany)
using the attenuated total reflectance (ATR) mode at ambient
conditions. The bands at ≈ 1100 cm–1 and 800 cm–1, attributed to the stretching vibration mode of the SiO4 tetrahedron
and the bending vibration mode of intertetrahedral Si–O–Si
bonds, were obvious in all modes of pretreatment of rice
straw (not shown). The band at 800 cm–1 proposes a full condensation of Si surrounded by four Si–O–Si linkages, whereas the band at 950 cm–1, representing the Si–O stretching
vibration of Si–OH groups, is missing. The absence of this
band might be due to the relatively low SSA of the samples
under investigation, which is up to 71 m2 g–1. Gun’ko et al.

________
20 µm

For determination of effects of solution chemistry on Si
release from phytoliths, soluble salts from the ashes were
removed by washing with deionized water for two minutes followed by centrifugation and decantation. The procedure was
repeated twice, and finally samples were freeze-dried. As C
analysis revealed, that there were still marked amounts of
organic C in the samples, ranging from 95 g kg–1 for the
400°C treatment, 19 g kg–1 for 600°C treatment to 2 g kg–1 for
the 800°C treatment (Table 1), subsequent wet ashing with
H2O2 of the sample treated at 400°C was carried out as suggested by Parr et al. (2001). The relatively high stability of

OM matter in rice straw against burning is thought to be

________
20 µm

Figure 2: Back-scattered-electron images of a leaf in the original dried and hackled rice straw (a) and leaf fragment in dry-ashed rice-straw
sample treated at 400°C (b).

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J. Plant Nutr. Soil Sci. 2014, 177, 349–359
related with the phytolith “coating” of OM in plant residues
(Fig. 1). Subsequent treatments with H2O2 (5 g ash from the
400°C treatment, 25 mL of a 15% H2O2 solution, 24 h in a
water bath at 80°C) were performed until the end of the reaction and resulted in a marked decrease of C content, which
was 5 g kg–1 in the freeze-dried material.
We analyzed effect of pH, ionic strength, and different cations
and anions on the dissolution kinetics of phytoliths in the
400°C-treated rice-straw sample by monitoring Si release
into solution. In all experiments, 50 mg of sample was mixed
with 100 mL of solution in 250-mL polypropylene tubes.
Based on results for the specific surface area and preliminary
experiments phytoliths obtained by the 400°C treatment were
used for most of the analyses. Suspensions were gently shaken by hand directly after mixing and allowed to stand for 24 h
at room temperature. Some of the batch experiments were
extended up to 7 days with sampling at 24 h intervals. The
experiments were terminated by filtration of the suspension

through a 0.45-lm pore-size cellulose acetate filter
(Macherey-Nagel, Düren, Germany). Silicon in solution was
determined in duplicate using the molybdate-blue method
(Mortlock and Froelich, 1989) and an UV-VIS spectrophotometer (Agilent/Varian Cary-50 Scan, Böblingen, Germany),
whereby the detection limit of the method was 0.1 mg Si L–1.
In detail, we performed the following experiments:
Experiment 1: Determination of the effect of pH on solubility
of Si and f of phytoliths. To identify the effect of pH on Si solubility, the solution was adjusted to pH 3.0, 4.5, 6.0, and 6.5
with 0.1 M HCl. The dissolution experiment lasted 7 d, and
pH, f, and electrical conductivity were controlled every 24 h.
In case of an increase in pH small amounts of 0.1 M HCl
were added under continuous stirring to adjust the scheduled
pH. f was measured to get information about the interfacial
contact zone between the solution and phytoliths. Electrical
conductivity was recorded in order detect possible changes
in ion concentration during the experiment.
Experiment 2: Evaluation of the effect of ionic strength on Si
solubility. Solutions with an electrolyte background (EB) of 10
and 50 mmolc L–1 NaCl were prepared. pH values from 2 to
10 were adjusted according to Fraysse et al. (2009) by adding corresponding amounts of 0.01 or 0.05 M HCl and NaOH,
respectively. f was measured in order to get information
about the underlying process.
Experiment 3: Assessment of cation effects on f and Si solubility. Solutions of different cation composition and concentrations were prepared in the concentration range of
0.5–2.5 mmolc L–1 for Al3+ and 1.0–20 mmolc L–1 for Ca2+,
Mg2+, K+, and Na+ from pure analyzed chloride salts. Experiments were started with an initial pH of 3.5 in suspension
adjusted with 0.01 M HCl. The pH of 3.5 was adjusted to
avoid precipitation of Al hydroxides which would affect f. The
suspension was sampled after 24 h for f measurement, and
pH was controlled again. Cation effects on the release
kinetics of Si were determined at pH 5, and a fixed concentration of all cations under investigation of 10.0 mmolc L–1 for

Na+, K+, Mg2+, and Ca2+ and 1.0 mmolc L–1 for Al3+ over a
time course of 7 d. In order to specify the effect of Al3+ on Si
 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Effect of pretreatment and solution chemistry 353
release and f, in a modification of this experiment the effect
of five different concentrations of Al3+ (0.1, 0.25, 0.5, 1.0, and
2.0 mmolc L–1) on Si release was determined at pH 3, 4, and 5.
Experiment 4: Analysis of anion effects on solubility of Si
2À
and f. Solutions with 10 mmolc L–1 of Cl–, NOÀ
3 , SO4 , acetate, oxalate, and citrate were prepared from pure analyzed Na
salts. The pH of the solutions was adjusted to pH 5 by dropwise addition of 0.01 M solutions of the respective acids (HCl,
HNO3, H2SO4, CH3COOH, H2C2O4, and C6H8O7). pH and Si
concentration were determined every 24 h over a time course of
7 d. In case of an increase of pH, small amounts of the acids
were added under continuous stirring to adjust pH 5.
Experiment 5: Determination of the effect of pretreatment of
rice straw on Si release and its relation to changes of f. Rice
straw samples from dry ashing at 400°C, combined treatment
of dry ashing at 400°C and subsequent wet ashing by H2O2
addition, and wet ashing treated with H2O2 were extracted
with deionized H2O adjusted to pH 5. pH and Si concentration
were determined every 24 h over a time course of 7 d.

3.2 Zeta potential measurements
The zeta potential (f) was determined for the rice-straw samples from dry and wet ashing in suspension to characterize
properties of the solid–liquid interface as a function of pH,
cation and anion concentration, and time as described in section 3.1 for experiments 1–5. After gentle shaking of the suspension, 1.6 mL of the suspension was sampled with a pipette and transferred in a cuvette for measurement in the
zeta potential analyzer (ZetaPALS, Brookhaven, Holtsville,

NY, USA). Here, f was determined using phase-analysis light
scattering (PALS), allowing measurement of particles of very
low mobility, i.e., particle movement of a fraction of their own
diameter is sufficient to obtain a good reproducibility of data.
Measurement of f was performed with each 10 runs partitioned in 20 cycles, whereby the mean is given in the figures.
Sampling of suspensions was performed simultaneously for
analysis of zeta potential and determination of Si concentration. In addition, surface charge was quantified by polyelectrolyte titration for a dry-ashed rice-straw sample (400°C
treatment) and a dry-ashed sample with subsequent H2O2
treatment in a particle-charge detector (PCD 03, Mütek,
Herrsching, Germany) according to the procedure described
in Nguyen et al. (2009) in order to determine the effect of
included residues of OM after pyrolysis in the sample on surface net charge.

4 Results and discussion
4.1 Dependency of Si release on pH and ionic
strength
For the dry-ashed rice-straw sample heated at 400°C the
increase of pH from 3.0 to 6.5 resulted in an increase of Si
release (Fig. 3). Extraction during 7 d at pH 6.5 resulted in a
Si concentration of 40 mg L–1, which is equivalent to ≈ 46% of
the total Si introduced in the experiment. This result is in bewww.plant-soil.com


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J. Plant Nutr. Soil Sci. 2014, 177, 349–359

tween the findings of Wickramasinghe and Rowell (2006) and

Wilding et al. (1979), who measured a Si extractability of
20%–38% and 50%–75%, respectively. The Si concentration
observed at pH 6.5 is one order of magnitude higher than that
at pH 3, with the other pH values showing intermediate Si
solubilites. Such strong pH dependency was also observed in
other studies (Fraysse et al., 2009). According to Ehrlich et al.
(2010), the strong pH dependency is a result of increasing pH
deprotonation of Si–OH groups resulting in a H-bonded H2O
adsorption on the negatively charged Si–O– surface. We
further suppose that a negatively charged fivefold coordinated Si species is formed. Consequently, Si–O bonds are
weakened and Si release is facilitated at higher pH.

40

Si in solution / mg L

-1

pH: 6.5

30
6.0

20
4.5

10
3.0

0

0

1

2

3

4
Days

5

6

7

Figure 3: pH dependency of Si release from ashed rice straw treated
at 400°C determined in batch experiments at pH 3.0, 4.5, 6.0, and 6.5
in a time sequence up to 7 d.

It can also be deduced from Fig. 3 that the time to reach
close-to-equilibrium conditions in the suspensions is also
depending on pH. At pH 3.0 and 4.5, the steady state in Si
concentration was reached after 2 d, whereas in the supernatants at pH 6.0 and 6.5 marked increases of soluble Si were
observed up to 7 d. Deduced from data on quartz (Dove and
Elston, 1992) and bamboo phytoliths (Fraysse et al., 2006)

0
Si

re
le
as
e

Si in solution / mg L

12
10
8

-20

Electrolyte background:
NaCl / mmolc L-1

6

-40

10
50

4

Zet
ap
ote
ntia
l

2

-60

Zeta potential / mV

-1

14

-80

0
2

3

4

5

6

7

8

9

10

pH
Figure 4: Effect of ionic strengths on Si release and zeta potential of
ashed rice straw treated at 400°C, determined by batch experiments
at pH 2–10 in 0.01 and 0.05 mol L–1 NaCl solutions and 24 h reaction
time.

 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

dissolution requires deprotonation of Si–OH groups and polymerization of Si–O–Si bonds before Si is released by nucleophilic attack by OH– groups. For this reason, increasing numbers of OH– in solution at higher pH result in more extensive
Si release.
In the experiments with an EB it was shown, that Si solubility,
in comparison of experiments at the same pH, decreased for
the higher EB introduced (Fig. 4). This can clearly been observed from pH 4–10, whereas at pH 2 and 3 at very low Si
solubility, no marked differences were obtained. The increased solubility of phytoliths at higher pH, already shown
for deionized H2O in Fig. 3, can also clearly be deduced in
the experiments with an EB of 10 and 50 mmolc L–1. At pH 2
and 3, the amounts of Si in solution were low with ≈ 1 mg L–1
after 24 h reaction time for both EBs, and with increasing pH,
Si release increased from ≈ 1 at pH 3 to 14.2 mg L–1 at pH 9.
f was more negative for the EB at lower NaCl concentration,
indicating a higher number of deprotonated silanol groups
(≡Si–O–) in these samples. An increase in pH from 2 to 9 led
to progressive decrease of f from –2 to –76 and ≈ 0 to
–49 mV for the EB of 10 and 50 mmolc L–1, respectively. The
observations on the principal course of f referring to pH are in
line with the results of Fraysse et al. (2006) on phytoliths separated from bamboo where the OM was removed by combustion at 450°C for 6 h. No clear changes in f were found between suspension of pH 9 and 10. At the EB of 50 mmolc L–1,
higher values for f might indicate a more extended adsorption
of positive charges onto deprotonated silanol groups of the

silica surface (Fig. 4). The formation of such siloxane groups
is thought to be the rate-limiting step for the dissolution process (Bickmore et al., 2006). However, the observed differences in values of f can also be assigned to other factors.
The increased ionic strength at higher EB can shift f to higher
values because of compression of the electrical double layer.
It is also probable that at high EB more Na+ was adsorbed on
deprotonated silanol groups which would contribute to charge
neutralization. Differences in f between the two EB were
most pronounced at high pH.
f of the dry-ashed rice-straw sample was close to zero at pH
2 indicating that the point of zero charge (pzc) was near pH 2.
The progressional decrease of f with increasing pH indicated
the presence of variably charged functional groups of inorganic and organic compounds in the burned ash. At pH 2,
these were almost completely protonated and the pzc was
almost reached.

4.2 Cation effects on the release of Si
Increasing concentrations of monovalent and bivalent cations
at pH 5 resulted in some increase of negative f values of the
dry-ashed rice straw sample; i.e., raising the concentrations
of Ca2+, Mg2+, K+, and Na+ from 0 to 20 mmolc L–1 led to a
change of f from –26.7 mV in deionized water to –6.3 (Ca2+),
–7.0 (Mg2+), –10.0 (K+), and –11.5 mV (Na+) (Fig. 5). Again, it
has to be considered that the shift of f to higher values with
increasing concentrations cannot be attributed to increasing
sorption of ions on the phytoliths only because of concentration-dependent effects on the thickness of the electrical douwww.plant-soil.com


J. Plant Nutr. Soil Sci. 2014, 177, 349–359

Effect of pretreatment and solution chemistry 355

ble layer and f. Al3+ was most effective in increasing f resulting in charge reversal, whereas di- and monovalent cations
showed similar behavior with a higher preference for divalent
cations. The strength of different cations appeared to be controlled first of all by the valency and secondly by ionic radius/
hydrated-ion size.
30

Zeta potential / mV

The Si-release pattern at presence of K+ was closest to that
of Al3+, whereas the Si-release pattern at presence of Ca2+
was closer to that of deionized H2O. Amounts of Si released
in presence of Mg2+ and Na+ were similar, with Si concentrations of 19 mg L–1 being obtained after 7 d. Hence, the effect
of cations on depressing Si release decreased in the order:
Al3+ > K+ > Na+ ≥ Mg2+ > Ca2+.

Na+
K+
Mg2+
Ca2+
Al3+

20
10

supernatant after 7 d was observed for deionized water. This
Si concentration was considerably smaller in the suspensions
with added electrolytes, being most pronounced for Al3+,
where after 7 d the Si concentration in the supernatant was
8 mg L–1. These batch experiments with different electrolytes

confirm the trend of decreased solubility of phytoliths at higher EB, shown in Fig. 4.

-10
-20
-30
0

10

5

20

15
-1

Cation concentration / mmolc L

Figure 5: Change of zeta potential of ashed rice straw treated at
400°C due to the addition of the cations Na+, K+, Mg2+, Ca2+, and Al3+
in concentration range of 0–0.02 molc L–1, determined by batch
experiments at pH 3.5 and a reaction time of 24 h.

In case for Al3+, the increase of f and charge reversal of the
dry-ashed rice-straw sample treated at 400°C can clearly be
assigned to adsorption of Al3+ (Fig. 5). The pzc was reached
at relatively low concentration of Al3+ of ≈ 0.4 mmolc L–1, and
further addition of Al3+ resulted in a marked charge reversal.
At the highest Al3+ concentration applied (2.5 mmolc L–1) f
was at +25 mV, which is almost the same magnitude of f in

deionized water (–28 mV), indicating strong adsorption of
Al3+ and exposure of positively charged sites of adsorbed
Al3+ at the solid–solution interface.
Batch experiments at pH 5 showed a marked effect of an
electrolyte as well as the kind of cation in solution on the
release of Si (Fig. 6). Highest Si release of 35 mg Si L–1 in the

While knowledge on the adsorption of K+ on silica has
already been well established (Davies and Oberholster,
1988), not much is known about its relation to Si release. The
K+ ion is known to fit well with a hexagonal depression in the
siloxane surface of silica (Grim, 1968). A preferential adsorption of K+ onto siloxane surface of phytoliths could explain a
stronger effect in decelerating Si release over Na+, Mg2+, and
Ca2+.
The concentration of Al3+ in solution had a marked effect on
the release of Si from dry-ashed rice-straw sample heated at
400°C (Fig. 7). Batch experiments at pH 5 revealed that at
Al3+ concentrations of 0.1, 0.5, and 1.0 mmolc L–1, Si concentrations after 7 d were 21.2, 19.0, and 7.3 mg L–1, respectively. In comparison with deionized water, where a Si concentration of 35 mg L–1 was observed, clear indication was obtained
that Al3+ acts as a prohibitor for phytolith dissolution, whereby
the effect is enhanced by increasing Al3+ concentration. Indication for a more extended adsorption of Al3+ with increasing
concentration was obtained by f measurements (Fig. 5). For
all concentrations of Al3+ a strong increase of Si concentration within the first 72 h was obtained whereas after 3 d Si in
solution kept almost constant. Also Wilding et al. (1979) observed a strong reaction of Al3+ with phytoliths resulting in a
reduced solubility. Dixit and Van Cappellen (2002) reported

40

40

-1

H 2O

30

Ca2+
Mg

20

2+

Na

+

K+

10
Al3+

0

Si in solution / mg L

Si in solution / mg L

-1

H2O

30
Al concentration / mmolc L-1: 0.1

20
0.5

10
1.0

0
0

1

2

3

4

5

6

7

Days
Figure 6: Cation effects on Si release from ashed rice straw treated
at 400°C in a time sequence up to 7 d, determined by batch

experiments at pH 5 with ion concentrations of 0.01 molc L–1 for Na+,
K+, Mg2+, and Ca2+, and 0.001 molc L–1 for Al3+.

 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

0

1

2

3

4

5

6

7

Days
Figure 7: Effect of Al3+ on Si release from ashed rice straw treated at
400°C, determined by batch experiments at pH 5 as a function of time
at Al3+ concentrations from 0 to 0.001 molc L–1.

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Nguyen, Dultz, Guggenberger

J. Plant Nutr. Soil Sci. 2014, 177, 349–359

from investigations on silica frustules that Al3+ is structurally
associated with silica, and that it is incorporated in the solid,
being on fourfold coordination, and not surface-adsorbed.
Thus, Al3+ can prevent attacks by negatively charged electrolytes to Si centers of the tetrahedral units and thus decreases
the solubility of silica. We assume that Al3+ acts on phytoliths
in a similar way, but the possibility that Al3+ is sorbed onto
deprotonated Si–O– groups also needs to be considered.

effect of Al3+ in increasing f might be due to competition of H+
on the deprotonated Si–O– sites. On the other hand, increases in f over the entire pH range from 3 to 5 showed that
Al3+ was not only structurally associated with silica, but also
adsorbed onto deprotonated Si–O– sites resulting in a less
negative surface. It is supposed that sorption of Al3+ on
Si–O– sites can prohibit Si release.

Reduction of Si release from phytoliths by presence of Al3+
was most pronounced at pH 3 at low Al3+ concentrations
(Fig. 8a). This effect was less pronounced when pH increased
to 4 and 5. With an increase of Al3+ concentration from 0 to
2.0 mmolc L–1, Si concentrations in the supernatant of the
batch experiments at pH 3, 4, and 5 decreased from 14.0 to
1.8, 10.9 to 0.8 and 2.5 to 0.7 mg L–1, respectively. In the concentration range from 1–2 mmolc Al3+ L–1 no marked change
of Si concentration in solution was obtained.

4.3 Anion effects on the release of Si

An increase in the Al3+ concentration from 0 to 2.0 mmolc L–1
at pH 4 and 5 resulted in strong increases in f with charge
reversal, whereas at pH 3 only slightly positive f was reached
at the highest Al3+ concentration introduced (Fig. 8b). The
pzc was reached at pH 4 at lowest Al3+ concentration
(0.3 mmolc L–1), whereas it is somewhat higher at pH 5
(0.4 mmolc L–1) and markedly higher at pH 3 (1.7 mmolc L–1).
Indication was obtained that Al3+ was strongly adsorbed on
the deprotonated ≡Si–O– sites at pH 4 and 5. At pH 3, a lower

The fact that organic anions suppress the Si release more
than inorganic ones might be related with their molecular size
and reactivity of functional group. According to Ehrlich et al.
(2010), organic anions attack the surface tetrahedral Si centers belonging to deprotonated silanol groups by using their
–COO– groups in a similar way as OH–. It is reasonable to
assume that these carboxylate groups might not react as
strongly as Cl– and SO2À
4 . A weaker effect of the Na-salt solutions in comparison with deionized water is probably due to
the EB. As discussed in section 4.2, the adsorption of monovalent cations such as Na+ and K+ onto deprotonated Si–O–
groups of the phytoliths can prohibit the attack of water mole-

16
(a)

12
10
8
6
pH

4
2

4

pH

Si in solution / mg L

-1

14

5

0,5

1,0

The batch experiment carried out at pH 5 and with anion concentrations of 10 mmolc L–1 revealed for all tested anions that
Si concentration in the supernatant increased within the first
72 h and stayed almost constant in the time span from 3 to
7 d (Fig. 9). After 7 d, Si release in different aqueous solutions
containing Cl–, SO2À
4 , acetate, oxalate, and citrate was 19.0,
13.6, 6.1, 5.0, and 4.8 mg L–1, respectively, indicating that in
presence of the two inorganic anions Si concentration was
markedly higher than in the presence of the three organic
ones. Remarkably, Si in solution was highest in deionized

water (34.7 mg L–1) despite the fact that anions are considered to act in a similar way as OH– ions (Ehrlich et al., 2010).
It seems that anions and also cations (Fig. 4 and 6) suppress
Si release from phytoliths obtained from dry-ashed rice-straw
sample treated at 400°C. It can be assumed that Si release
from such samples under field conditions is favored when the
content of soluble ions in soil solution is low.

pH 3

0
0,0

40

2,0

H 2O

Si in solution / mg L

5
pH

40

-1

Al3+ concentration (mmol L-1)
pH 4

Zeta potential / mV

60

1,5

20
pH 3

0
-20

30

Cl

20

-

SO42-

10

Oxalate

(b)
Acetate, Citrate

0

-40
0,0

0,5
3+

Al

1,0

1,5

2,0
-1

concentration / mmolc L

Figure 8: pH dependency of Al3+ effects on zeta potential (a) and Si
release (b) from ashed rice straw treated at 400°C, determined by
batch experiments for 24 h at pH 3, 4, and 5, and Al3+ concentrations
of 0–0.002 molc L–1.

 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

0

1

2

3

4

5

6

7

Days
Figure 9: Anion effects on Si release from ashed rice straw treated at
400°C in time sequences up to 7 d, determined by batch experiments
at pH 5 in 0.01 molc L–1 solutions of Na salts with Cl–, SO2À
4 , acetate,
citrate, and oxalate.

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Effect of pretreatment and solution chemistry 357
and Sullivan (2005) reported that OM strengthens the phytolith surface and its resistance against dissolution.

-20

Acetate
Oxalate

Citrate

-40

-60

-80
2

3

4

5

6

7

8

9

10

pH
Figure 10: Effect of pH on the change in zeta potential of ashed rice
straw treated at 400°C in presence of the anions Cl–, SO2À
4 , acetate,
citrate, and oxalate (Na salts), determined by batch experiments with

a reaction time of 24 h in 0.01 molc L–1 solutions at a pH range of
2–10.

cules on Si–O–Si bonds in the siloxane surface and depress
Si release.
The presence of different anions at a concentration of
0.01 mmolc L–1 showed only a minor effect in lowering f in the
pH range of 2–5, whereas at higher pH different effects of
anions on f were observed (Fig. 10). Because of a dominance of H+ (H+ activity at pH 2 is 100 times than that at pH 4)
not much difference in f was detected at pH < 4. In the pH
range from 4 to 10, lower f in presence of Cl– might reflect
some stronger binding of this monovalent inorganic cation on
phytoliths in comparison with divalent SO2À
4 . Si concentration
in solution was higher at presence of Cl– than of SO2À
4
(Fig. 9). This could be explained by a more effective attack of
Cl– to siloxane surfaces which facilitates Si release, but more
clarification is needed for understanding differences in the
affinity of these anions to the surface of phytoliths. For the
organic anions, the effect on f was similar and decreased in
the order: citrate > oxalate > acetate. No marked differences
of the three organic anions on Si release were obtained. It
can be concluded that an increasing number of –COO–
groups does not have a marked effect on dissolution efficiency of the phytoliths. This conclusion is in accordance with
observations reported by Ehrlich et al. (2010).

4.4 Effect of pretreatment on Si release
Silicon release in suspensions with deionized water at pH 5
increased with time for rice-straw samples heated at 400°C

and the 400°C-treated sample combined with subsequent
treatment by H2O2 (Fig. 11a). In contrast, the sample obtained by wet ashing with H2O2 only showed a low solubility
throughout the experiment. After 7 d, Si release from samples
was 34.7 (400°C), 17.6 (400°C, H2O2-treated), and 2.6 mg
L–1 (wet ashing). Release of only small amounts of Si from
the H2O2-treated rice-straw sample is in accordance with
findings from other studies on Si release from unburned phytoliths. For instance, Van Cappellen et al. (2002) and Parr
 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Comparison between the two heat treatments revealed that
the sample with higher organic-C content showed a lower
resistance to dissolution. However, the role of OM in accelerating dissolution can still not be affirmed because the “easily
dissolvable fraction of Si” of the 400°C- and H2O2-treated
sample might have already been removed during wet ashing
with H2O2.
The sample treated by wet ashing with H2O2 only showed a
high resistance against dissolution in comparison with samples treated by heating. For this sample, the increase of f
with time might be due to the adsorption of cations from solution onto deprotonated Si–O– sites on the external surface.
Despite the fact that the experiments were performed in deionized water the presence of cations in solution can be
assumed as relatively high amounts of alkaline and earth
alkaline cations, especially K+ were found in dry-ashed ricestraw samples (Table 1). It can be inferred that heat treatments resulted in robust destruction of the rice straw and produced a structure with low resistance to dissolution. In contrast, the sample treated by wet ashing with H2O2 showed a
more resistant structure, which protected the phytoliths and
40
o
Treatment: 400 C

(a)
Si in solution / mg L-1

ClSO42-

30

20
o

400

10

Ca

O2
nd H 2

H2O2

0
-10

0

1

2

3

4

5

6

7

6

7

Treatment: 400oC and H2O2

(b)
Zeta potential / mV

Zeta potential / mV

0

-20
o

400 C

-30
-40

H 2O 2

-50

-60
0

1

2

3

4
5
Days
-1
C-content / g kg
(ii) dry ashing at 400oC: 95
(v) wet ashing with H2O2: 374

(vi) dry ashing at 400oC/wet ashing with H2O2: 5
Figure 11: Effect of pretreatment of rice straw on zeta potential (a)
and Si-release kinetics at pH 5 in deionized water. Dry-ashed ricestraw samples treated at 400°C (ii), 400°C treatment with subsequent
wet-ashing with H2O2 (v), and wet-ashing treatment (vi).

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358

Nguyen, Dultz, Guggenberger

in addition occluded C within from chemical attacks. In agreement with Parr and Sullivan (2005) this implies that mixing

rice straw into the soil by tillage instead of burning it leads to
a long-term stabilization of rice-straw phytoliths under actual
soil conditions. However, mixing masses of undecomposed
straw into the soil would generate a very low redox potential.
Determination of the changes of f with time for the differently
pretreated samples in deionized water revealed that within 7 d
f kept almost constant (20–24 mV) for both heat-treated samples, whereas an increase in f from –57 to –32 mV for the
chemically oxidized sample was observed, indicating loss of
negatively charged sites with time (Fig. 11b). The dry-ashed
sample combined with subsequent H2O2 treatment showed
slightly higher f than the sample with single treatment (dry or
wet ashing), which is probably due to strong losses of OM
matter by the H2O2 treatment (Table 1). It has to be considered that charred rice straw in the ash sample might contribute to f. In order to gain more insights on the effect of charred
rice straw on f, surface charge of these two samples was
quantified at pH 5 by polyelectrolyte titration. The results on
surface charge, –12.6 mmolc kg–1 for the sample with combined treatment versus –15.5 mmolc kg–1 for the sample with dry
ashing only confirm the trend observed by f measurements.
At all, the contribution of charred rice straw to the charge on
the external surfaces appears relatively low.
We assume that the observed increase of negative surface
charge was the result of OM removal with H2O2. Organic matter present in the sample contributes to the total negative
charge of the samples. Its contribution to the total net surface
charge of the dry-ashed sample was calculated to be –1.47
whereas it was –0.06 mmolc kg–1 for the burned and chemically oxidized sample. Here, adsorption of cations from solution onto negative surface sites of the OM may decrease neutralization of deprotonated sites of phytoliths and as a consequence, Si release is affected to a lower extent by the
presence of cations.

5 Conclusions
Batch experiments combined with analysis of f for getting
information about the underlying process showed the importance of pretreatment of rice straw and solution chemistry on
Si release. In ashed samples, soluble Si was found to be up

to 46% of total Si content in ashes, indicating that burning
rice straw can be an important measure to make Si available
for Si-accumulating crops such as rice in short term. In contrast, fresh rice straw treated by H2O2 only, is highly resistant
against dissolution, indicating that phytoliths can be stable on
the long term in the soil when rice straw is directly mixed into
the soil on site without burning. Based on f measurements,
we infer that cations are sorbed on deprotonated Si–O– sites
and mitigate water attack on Si–O–Si bonds. This leads to a
decline in the dissolution rate of phytoliths at presence of
cations. Especially Al3+ showed a marked effect to decrease
Si release. Different effects of inorganic and organic anions
on the dissolution of phytoliths were observed, with the latter
impeding Si release more than the former. Based on the relatively strong effect of organic anions in suppressing Si
release, it is suggested that the presence of dissolved OM in
 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

J. Plant Nutr. Soil Sci. 2014, 177, 349–359
pore solution of paddy soils might counteract phytolith dissolution. In the conducted experiments, there are no other sorption sites which may compete for the added cations or anions
which is very different from applying rice straw or ash to soil.
Here, experiments with synthetic soil solutions and also pot
experiments including analysis of Si content in dry matter of
rice plants are thought to provide more valuable insights in Si
management of paddy soils. Our study showed that burning
of rice straw can be an important measure to make Si available. The practical problem is, however, that actually in all
countries of SE Asia there are governmental restrictions for
burning of straw and, hence, techniques for dry ashing in a
more environmentally friendly way are needed. Here, the use
of commercial-scale systems for the production of biochar,
centralized and mobile systems with temperature control
being possible, should be considered.

Acknowledgments
This research was funded by the Vietnam National Foundation for Science & Technology Development (Project 105.092010.03). An extended part of the research was supported by
the German Academic Exchange Service (DAAD); grant A/
11/00930. X-ray-tomographic microscopy was performed
with skilful help by Julie Fife at the TOMCAT beamline of the
synchrotron facility of the Paul Scherrer Institute, Villigen,
Switzerland. Great help of Sarah B. Cichy and Karl-Ingo
Friese for morphological characterization of phytoliths from
the tomographic dataset is acknowledged. We are grateful to
two anonymous reviewers for constructive comments on the
manuscript.

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