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

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+, K+, Mg2+, Ca2

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

Minh Ngoc Nguyen 1 *, Stefan Dultz 2 , and Georg Guggenberger 2

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

phy-toliths, 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

treat-ment 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+,

K+, Mg2+, Ca2+, Al3+) and anions (Cl–, NO3, SO2 , acetate, oxalate, citrate) in the concentration

range from 0.1 to 10 mmolcL–1 After burning rice straw at 400°C, phytoliths and biochar were

major compounds in the ash At an electrolyte background of 0.01 molcL–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

dis-solution 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 pro-tective 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 in-creased, 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 cul-tivation 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: minhnn@hus.edu.vn

served (Fleck et al., 2011) Ma (2004) summarized the effects

of Si on rice, and the general conclusion was that the

resis-tance 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

con-tamination (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

com-plete 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

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

aqu-eous solutions occurs via hydrolysis of Si–O–Si bonds of the

SiO2network 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

even-tually 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 –PO2 , may react with Si

cen-ters 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

_

20 µm

Figure 1: Three-dimensional image

of the principal arrangement of silicified structures and organic mat-ter for a vascular bundle in a dried leaf of a rice plant The visualization was performed by X-ray tomo-graphic microscopy using the 3D segmentation and visualization

soft-ware 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.

paddy soils, which are in the Red River Delta from pH 5–6

(Nguyen et al., 2009) is a necessity.

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

deproto-nated≡Si–O– sites might occur and accelerate

polymeriza-tion (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

phy-toliths 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

per-formed 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

con-sidered for the management of silicon in paddy soils

Usually, dry and wet ashing techniques used for the

extrac-tion 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

para-meter 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 H2O2resulting in, e.g., different degrees of

dehydroxila-tion 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

par-ticular 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

inter-face 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

2 Materials 2.1 Sample production

Rice straw was collected from a paddy field of a research sta-tion 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

(Gunt-zer et al., 2010) Si, 387 g kg–1C, and 13.2 g kg–1N as deter-mined by an Elementar Vario EL (Hanau, Germany) elemen-tal 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 reac-tions 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 H2O2until the end of reaction For wet ashing 25 mL of a 15% H2O2solution 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 con-tent drastically (Table 1) Organic C was most completely removed by heating at 800°C, whereas treatments of rice straw with H2O2in 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–1in the original rice-straw sample to 193 g kg–1 in the dry-ashed produced at 600°C (Table 1) Treatment of samples with H2O2resulted 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 sam-ples 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 ob-served (Table 1) For the anions, besides Cl–and SO2 also

PO3 was found in solution

A marked decrease of the specific surface area (SSA) deter-mined by the N2-adsorption method (Quantachrome, NOVA 4000e, Boynton Beach, FL, USA) was obtained with increas-ing heatincreas-ing 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 SiO2phases such as tridimite or

cris-tobalite (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 ana-lyses focus was given on the straw sample ashed at 400°C For the original sample, SSA determination by the N2 -adsorp-tion method failed because N2did not enter the micropores of OM

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

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–1and 800 cm–1,

attribu-ted 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–1proposes a full

con-densation of Si surrounded by four Si–O–Si linkages,

where-as 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.

(2005) stated that the absorption band at 950 cm–1of 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 stretch-ing vibration of Si–OH groups

3 Methods 3.1 Determination of dissolution kinetics

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 fol-lowed 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–1for 600°C treatment to 2 g kg–1for the 800°C treatment (Table 1), subsequent wet ashing with

H2O2of the sample treated at 400°C was carried out as

sug-gested by Parr et al (2001) The relatively high stability of

OM matter in rice straw against burning is thought to be

20 µm

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).

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

/ m 2 g –1

Si

/ g kg –1

C

/ g kg –1

Soluble ions / mg kg–1

K + Na + Ca 2+ Mg 2+ Cl – SO 2–

4

(1) original sample – 42.1 387 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.

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

reac-tion and resulted in a marked decrease of C content, which

was 5 g kg–1in 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

sha-ken 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

spectrophot-ometer (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

solu-bility, 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 mmolcL–1NaCl were prepared pH values from 2 to

10 were adjusted according to Fraysse et al (2009) by

add-ing correspondadd-ing 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

solu-bility Solutions of different cation composition and

concentra-tions 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

Experi-ments 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

concentra-tion of all caconcentra-tions under investigaconcentra-tion 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

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 mmolcL–1) on Si release was determined at pH 3, 4, and 5

Experiment 4: Analysis of anion effects on solubility of Si

and f Solutions with 10 mmolcL–1of Cl–, NO3, SO2 , acet-ate, oxalacet-ate, and citrate were prepared from pure analyzed Na salts The pH of the solutions was adjusted to pH 5 by drop-wise 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 sam-ples 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 sec-tion 3.1 for experiments 1–5 After gentle shaking of the sus-pension, 1.6 mL of the suspension was sampled with a pi-pette 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 parti-tioned 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 concentra-tion In addition, surface charge was quantified by polyelec-trolyte 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 sur-face 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

be-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

coordi-nated Si species is formed Consequently, Si–O bonds are

weakened and Si release is facilitated at higher pH

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

superna-tants 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)

dissolution requires deprotonation of Si–OH groups and poly-merization of Si–O–Si bonds before Si is released by nucleo-philic attack by OH–groups For this reason, increasing num-bers 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 ob-served from pH 4–10, whereas at pH 2 and 3 at very low Si solubility, no marked differences were obtained The in-creased 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 mmolcL–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–1at 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 mmolcL–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

sep-arated from bamboo where the OM was removed by combus-tion at 450°C for 6 h No clear changes in f were found be-tween suspension of pH 9 and 10 At the EB of 50 mmolcL–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

pro-cess (Bickmore et al., 2006) However, the observed

differ-ences 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 inor-ganic and orinor-ganic 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 mmolcL–1led 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 concentra-tion-dependent effects on the thickness of the electrical

dou-Days

0

10

20

30

40

3.0 4.5

6.0 pH: 6.5

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.

-80 -60 -40 -20 0

Electrolyte background:

NaCl / mmolc L-1

pH

0

2

4

6

8

10

12

14

10

50

Si release

Zeta potential

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.

ble layer and f Al3+was most effective in increasing f

result-ing 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

con-trolled first of all by the valency and secondly by ionic radius/

hydrated-ion size

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 mmolcL–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–1in the

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

high-er EB, shown in Fig 4

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 concentra-tions of 19 mg L–1being obtained after 7 d Hence, the effect

of cations on depressing Si release decreased in the order:

Al3+> K+> Na+≥ Mg2+> Ca2+ 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

adsorp-tion 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 mmolcL–1, Si concen-trations after 7 d were 21.2, 19.0, and 7.3 mg L–1,

respective-ly In comparison with deionized water, where a Si concentra-tion of 35 mg L–1was observed, clear indication was obtained that Al3+acts as a prohibitor for phytolith dissolution, whereby the effect is enhanced by increasing Al3+concentration Indi-cation 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 concentra-tion within the first 72 h was obtained whereas after 3 d Si in

solution kept almost constant Also Wilding et al (1979)

ob-served a strong reaction of Al3+with phytoliths resulting in a

reduced solubility Dixit and Van Cappellen (2002) reported

Cation concentration / mmolc L-1

-30

-20

-10

10

20

30

Na +

K+

Mg 2+

Ca 2+

Al 3+

Figure 5: Change of zeta potential of ashed rice straw treated at

400°C due to the addition of the cations Na + , K + , Mg 2+ , Ca 2+ , and Al 3+

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.

Days

0

10

20

30

40

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 molcL –1 for Na + ,

K + , Mg 2+ , and Ca 2+ , and 0.001 molcL –1 for Al 3+

Days

0 10 20 30

40

H2O

0.5

1.0

Al concentration / mmolc L -1 : 0.1

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 Al 3+ concentrations from 0 to 0.001 molcL –1

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

electro-lytes 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

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

con-centration range from 1–2 mmolcAl3+L–1no marked change

of Si concentration in solution was obtained

An increase in the Al3+concentration from 0 to 2.0 mmolcL–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 mmolcL–1) and markedly higher at pH 3 (1.7 mmolcL–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

effect of Al3+in increasing f might be due to competition of H+

on the deprotonated Si–O– sites On the other hand, in-creases 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

4.3 Anion effects on the release of Si

The batch experiment carried out at pH 5 and with anion con-centrations of 10 mmolcL–1revealed 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–, SO2 , 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 consid-ered 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

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 cen-ters 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 SO2 A weaker effect of the Na-salt solu-tions in comparison with deionized water is probably due to the EB As discussed in section 4.2, the adsorption of mono-valent cations such as Na+and K+onto deprotonated Si–O–

groups of the phytoliths can prohibit the attack of water

mole-Al3+ concentration (mmol L-1)

0

2

4

6

8

10

12

14

16

Al3+ concentration / mmolc L-1

-40

-20

0

20

40

60

pH 3

pH 4 pH 5

pH 3

pH 4 pH 5

(a)

(b)

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 Al 3+ concentrations

of 0–0.002 molcL –1

Days

0 10 20 30

40

H2O

Cl

-SO4

2-Acetate, Citrate Oxalate

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 molcL –1 solutions of Na salts with Cl – , SO 2 , acetate, citrate, and oxalate.

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 mmolcL–1showed 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

domi-nance 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 SO2 Si concentration

in solution was higher at presence of Cl– than of SO2

(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

effi-ciency 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

ob-tained 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

phy-toliths For instance, Van Cappellen et al (2002) and Parr

and Sullivan (2005) reported that OM strengthens the

phyto-lith surface and its resistance against dissolution

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 acceler-ating 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 H2O2only showed a high resistance against dissolution in comparison with sam-ples treated by heating For this sample, the increase of f with time might be due to the adsorption of cations from solu-tion onto deprotonated Si–O– sites on the external surface Despite the fact that the experiments were performed in deio-nized 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 rice-straw samples (Table 1) It can be inferred that heat treat-ments resulted in robust destruction of the rice straw and pro-duced a structure with low resistance to dissolution In con-trast, the sample treated by wet ashing with H2O2showed a more resistant structure, which protected the phytoliths and

pH

-80

-60

-40

-20

0

Cl

-SO4 2-Acetate Oxalate Citrate

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 – , SO 2 , acetate,

citrate, and oxalate (Na salts), determined by batch experiments with

a reaction time of 24 h in 0.01 molcL –1 solutions at a pH range of

2–10.

0 10 20 30

40

C

400

o C and H

2 O 2

Treatment:

(a)

Days

-60 -50 -40 -30 -20 -10

H 2 O 2

Treatment:

(b)

C-content / g kg-1 (ii) dry ashing at 400oC: 95

(v) wet ashing with H2O2: 374 (vi) dry ashing at 400o

C/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 rice-straw samples treated at 400°C (ii), 400°C treatment with subsequent wet-ashing with H2O2(v), and wet-ashing treatment (vi).

in addition occluded C within from chemical attacks In

agree-ment 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

sam-ples, 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 H2O2treatment (Table 1) It has to be

consid-ered that charred rice straw in the ash sample might

contrib-ute 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 mmolckg–1for the sample with

combin-ed treatment versus –15.5 mmolckg–1for 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

mat-ter 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 mmolckg–1for the burned and

chemi-cally oxidized sample Here, adsorption of cations from

solu-tion onto negative surface sites of the OM may decrease

neu-tralization of deprotonated sites of phytoliths and as a

conse-quence, 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

impor-tance 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

con-trast, fresh rice straw treated by H2O2only, 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

rela-tively strong effect of organic anions in suppressing Si

release, it is suggested that the presence of dissolved OM in

pore solution of paddy soils might counteract phytolith disso-lution In the conducted experiments, there are no other sorp-tion sites which may compete for the added casorp-tions 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 avail-able 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

Founda-tion for Science & Technology Development (Project 105.09-2010.03) 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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