Chemosphere 119 (2015) 371–376

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Chemosphere

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Release of potassium accompanying the dissolution of rice strawphytolith

http://dx.doi.org/10.1016/j.chemosphere.2014.06.0590045-6535/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: minhnn@hus.edu.vn (M.N. Nguyen).

Minh Ngoc Nguyen a,⇑, Stefan Dultz b, Flynn Picardal c, Anh Thi Kim Bui d, Quang Van Pham a,Juergen Schieber e

a Department of Pedology and Soil Environment, Faculty of Environmental Science, VNU University of Science, Vietnam National University, 334-Nguyen Trai, Thanh Xuan,Hanoi, Vietnamb Institute of Mineralogy, Leibniz Universität Hannover, Herrenhäuser Straße 2, 30419 Hannover, Germanyc School of Public and Environmental Affairs, Indiana University, MSBII, Room 418 N. Walnut Grove Ave, Bloomington, IN 47405-2204, USAd Institute of Environmental Technology, Vietnam Academy of Science and Technology, 18-Hoang Quoc Viet, Hanoi, Vietname Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405-1405, USA

h i g h l i g h t s

� Potassium is present in rice stems and leaves.� Potassium can be immobilized in the mineralized silica (phytolith).� We found that potassium co-exists with organic matter in phytolith structure.� Desilification of the phytolith is a main factor regulating potassium release.� Pretreatment of the rice straw at 600 �C is optimal in providing available potassium.

a r t i c l e i n f o

Article history:Received 31 May 2014Accepted 21 June 2014

Handling Editor: X. Cao

Keywords:Rice strawPhytolithSiliconPotassiumRelease

a b s t r a c t

In rice, Si assimilated from the soil solution is deposited in inter- and intracellular spaces throughout theleaf and stems to form silicified structures (so-called phytoliths). Because K is also present in significantconcentrations in rice stems and leaves, the question arises if K is immobilized in the mineralized silicaduring the precipitation of Si. This work determined whether desilification of the phytolith is a factor reg-ulating K release by implementing batch experiments. Solubility of Si and K of the rice straw heated atdifferent temperatures were examined to identify effect of pretreatment.

Analyses of phytoliths using SEM–EDX and X-ray tomographic microscopy in conjunction with theresults from batch experiments revealed that K might co-exist with occluded organic matter inside thephytolith structure. In the kinetic experiments, corresponding increases of K and Si concentrations inthe supernatants were observed which suggested that desilification of the phytolith is a main factor reg-ulating K release. The extent of heat pretreatment of the rice straw is of significant importance withrespect to dissolution of the phytolith by affecting organic removal and surface modification. At temper-atures lower than 600 �C, corresponding increases of the soluble Si and K with heating temperature havebeen obviously observed. In contrast, the solubility of Si and K gradually decreased at temperatures above600 �C. This work provides insights into factors that control release of K and Si from phytolith and a prac-tical recommendation for practices of burning rice straw that may maximize subsequent release of Si andK for crops.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Silicon (Si) and potassium (K) are two of the most abundantnutrient elements in rice (Tiwari et al., 1992; Epstein, 1999). These

two elements are taken up from the soil solution and transportedto leaf, stem and other parts of rice. While K provides the appropri-ate ionic environment for metabolic processes in the cytosol and,as such, functions as a regulator of various processes includinggrowth regulation (Leigh and Wyn Jones, 1984), Si is known toaccumulate as silicaceous phytolith by deposition in inter- andintracellular spaces throughout the leaf and stem of rice

372 M.N. Nguyen et al. / Chemosphere 119 (2015) 371–376

(Parr and Sullivan, 2005). Several studies have reported that, dur-ing the precipitation of Si to form phytolith, organic moleculescan be trapped within the silica (Kelly et al., 1991; Elbaum et al.,2003; Piperno and Stothert, 2003). This suggests that inorganiccompounds, e.g., K in the rice xylem/phloem sap, can also beoccluded in the silica body (so-called PhytOK). It is hypothesizedthat the PhytOK can be released and contribute to the K-pool ofthe soil once desilification of the phytolith occurs.

It is generally accepted that the desilification of silica in aque-ous solutions occurs via hydrolysis of Si–O–Si bonds of the SiO2

crystal structure. Water itself is a strong promoter of hydrolysisvia orientation of the electronegative water oxygens towards theSi atom, leading to a transfer of electron density to the Si–O–Sibond, thereby increasing its length and eventually breaking it(Dove and Crerar, 1990). Recently, it has been reported byNguyen et al. (2013) that dissolution of Si is controlled by aqueoussolution chemistry and pretreatment of the rice straw. However,additional data on co-releases of Si and other occluded elementssuch as K is currently lacking. Our current studies of the releaseof K accompanying phytolith dissolution in aqueous solutionshelps address this knowledge gap.

On-site burning after harvesting is the primary method of han-dling 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, doesn’t interfere with soilpreparation, and helps to reduce pest and populations of pathogensresident in the straw biomass (Dobermann and Witt, 2000). Themode of pretreatment of rice straw and burning at different temper-atures might result in various degrees of dehydroxylation of bio-genic silica and organic matter contents. In this study, we identifyco-release of Si and K using batch experiments with rice straw phy-tolith ash samples obtained from different ashing temperatures. Afurther treatment by H2O2 to remove organic matter, which is oftenmentioned as a method for phytolith preparation (Parr et al., 2001),will be used to relate losses of K and Si that accompany the removalof organic matter. SEM–EDX spectra reflecting the presence of vari-ous elements, and X-ray tomographic microscopy providing 3D-seg-mentation and visualization of the solid phase were also used in thisstudy to evaluate distribution of K in the rice straw phytoliths.Improved knowledge of the mechanisms and release processes ofK from phytolith will open different management options and is ofimportance for the continued efficient cultivation of rice.

2. Materials and methods

2.1. Sample production

Rice straw was collected from a paddy field in the rice-growingarea of the central part of the Red River Delta (105�4401700E,20�5905700N) directly after harvesting. The rice straw was air-dried,ground in a blade grinder, and passed through a 1.0 mm sieve. Therice straw had 73.6 g kg�1 Si examined by an X-ray fluorescence ana-lyzer (XRF-1800 Shimadzu, Japan), 387 g kg�1 C, and 13.2 g kg�1 Nas determined by an Elementar Vario EL (Hanau, Germany) elemen-tal analyzer with a respective C:N ratio of 29. Dry ashing of rice strawwas performed by heating finely-ground, air-dried, rice straw in anoven at a target temperature (between 300 �C and 1000 �C) for 2 h.To avoid strong exothermic reactions during dry-ashing the weightof sample was limited to 5 g. For kinetic experiments we selectedtwo samples treated at 400 �C and 800 �C.

2.2. Methods

For determination of Si and K distribution and dissolution fromphytoliths, soluble salts from the ashes were removed by washing

with deionized water for five minutes followed by centrifugationand decantation. The procedure was repeated five times, and sam-ples were finally freeze-dried. To identify distribution of elementsin phytolith using SEM–EDX, the samples were examined using aFEI Quanta 400 FEG ESEM (FEI Company, Hillsboro, OR, USA) at20 kV under low vacuum conditions using a gaseous secondaryelectron detector. The stage was at room temperature with achamber pressure of 80 Pa. Surface point elemental analysis wasperformed while the samples were being viewed in the SEM usinga Princeton Gamma-Tech energy dispersive spectrometer. Thevisualization was performed by X-ray tomographic microscopyusing the 3D segmentation and visualization software YaDiV(Friese et al., 2013) to provide a three-dimensional image of theprincipal arrangement of silicified structures and organic matterfor a vascular bundle in a dried stem of a rice plant. The sampleswere analyzed at the synchrotron light source (SLS) of the Paul–Scherrer-Institute in Villigen, Switzerland. The TOMCAT(TOmographic Microscopy and Coherent rAdiology experimenTs)beamline receives photons from a 2.9 T superbending magnet witha critical energy of 11.1 keV, producing a monochromatic beam. Asample is fixed on a centering and rotation stage in front of amicroscope, detecting the monochromatic X-ray beam.

In all experiments, 200 mg of sample was mixed with 200 mL ofsolution in 250 mL polypropylene tubes. Suspensions were gentlyshaken by hand directly after mixing and allowed to stand for24 h at room temperature. Some of the batch experiments wereextended up to 6 d with sampling at 24 h intervals. The experi-ments were terminated by filtration of the suspension through a0.45 lm pore-size cellulose acetate filter. Si and K in solutionwas determined in triplicate using the molybdate blue method(Mortlock and Froelich, 1989) with a Spectrophotometer UV–Vis(L-VIS-400, Labnics Company, Fremont, CA, USA) and flamephotometer (PFP7-Jenway, OSA, UK), respectively.

In more detail, we performed the following experiments:

Experiment 1: We analyzed the dissolution kinetics for the400 �C-, 800 �C-, 400 �C/H2O2-, and 800 �C/H2O2-treated ricestraw samples by monitoring Si and K releases into solution.The dissolution experiments lasted 6 d and sampling was car-ried out every 24 h.Experiment 2: To evaluate the effect of heat-pretreatment of ricestraw, the rice straw samples treated at 300, 400, 500, 600, 700,800, 900 and 1000 �C were mixed with deionized water andkept at room temperature for 24 h.Experiment 3: To examine whether K sorbs on siloxane surfacesof the phytolith, we used 400 �C- and 800 �C-treated rice strawsamples. Ca2+ and NH4

+ prepared in solutions with a concentra-tion range of 10–100 mmolc L�1 from pure analyzed chloridesalts were used as exchangers. Suspensions were terminatedafter 24 h.

3. Results

3.1. Sample properties

The different ashing temperatures of straw changed organic-Ccontent of the straw to different extents (Table 1). The organic-Cwas most completely removed by heating at 800 �C, whereas onlyless than 12% of total organic-C was removed at 400 �C. With a sub-sequent treatment with H2O2, remaining organic-C contents were2.85% and 0.48% for the ashes treated at 400 �C and 800 �C, respec-tively. The total Si and K contents were 20.7% and 9.4%, respec-tively, after treatment at 400 �C, and 32.0% and 10.3% following800 �C treatment. The ashes had an alkaline reaction (pH 10–11).Soluble K of the 400 �C-and 800 �C-treated samples in a 1:10

Table 1Specific surface area (SSA), chemical composition of the original rice-straw sample (1), dry-ashed samples treated at 400 �C and 800 �C (2, 3). Soluble K was analyzed for the heat-treated samples alone.

Treatment SSA (m2 g�1) Total content (%) Soluble K (g kg�1)

C Si K Ca Mg K+

Original sample (1) – 38.7 7.36 2.37 1.13 0.53 –400 �C (2) 68.6 34.2 20.7 9.4 3.0 1.3 52800 �C (3) 1.0 0.53 32.0 10.3 4.9 2.3 15

10 20 30 40

550oC

700oC

800oC

1000oC

900oC

Treatmenttemperature

**

*

++ +

* Cristobalite+ Trydimite

M.N. Nguyen et al. / Chemosphere 119 (2015) 371–376 373

extract with deionized water were 52 and 15 g kg�1 which areequal to 55% and 15% of the total contents, respectively.

A marked decrease of the specific surface area (SSA) determinedby the N2-adsorption method (Quantachrome, NOVA 4000e,Boynton Beach, FL, USA) was obtained with increasing heatingtemperature. The SSAs of the sample treated at 400 �C and 800 �Cwere found to be 68.6 and 1.0 m2 g�1, respectively, indicating astrong condensation of silica structures (Fig. 1a and c). Tempera-tures >800 �C are known to cause formation of crystalline SiO2

phases such as cristobalite or tridymite (Fig. 2). Formation of theseminerals by pyrolysis of biogenic silica was also reported byKordatos et al. (2008). Subsequent treatments by H2O2 resultedin scabrous surfaces as shown in Fig. 1b and d. For the originalsample, SSA determination by the N2-adsorption method failedbecause N2 did not enter the micropores of OM.

2 Theta

Fig. 2. XRD patterns of rice straw ashes treated at different temperatures.

3.2. EDX characterization

The elemental composition of phytolith is shown by the EDXspectra in Fig. 3. It can be seen that Si, C, O, K and Ca are major ele-ments in the phytoliths. The relative abundance of these elementsvaried between samples based on the chosen pretreatment. Anobviously high signal of C in the 400 �C-treated sample in

Fig. 1. SEM images of a leaf fragment in treated rice-straw sample

comparison with those in the 800 �C-, 400 �C/H2O2-oxidized-,and 800 �C/H2O2-treated samples denoted that oxidation reactionsby elevated heating or H2O2 treatment removed large amount oforganic matter. In contrast, Si signals in the 800 �C-, 400 �C/H2O2-,

s: 400 �C (a), 400 �C/H2O2 (b), 800 �C (c) and 800 �C/H2O2 (d).

Fig. 3. EDX spectra of the treated-rice straw samples: 400 �C (a), 400 �C/H2O2 (b),800 �C (c) and 800 �C/H2O2 (d).

Time (day)

0 1 2 3 4 5 6

Sol

uble

ions

(m

g L

-1)

0

10

20

30

40 (b)

(a)

Soluble ions / heat treatments:

0

10

20

30

40Si / 400oCSi / 800oCK / 400oCK / 800oC

(a)

Fig. 4. Si and K release from (a) dry-ashed rice straw samples treated at 400 �C and800 �C, and (b) those with a subsequent treatment with H2O2, in a time sequence upto 6 d.

Treatment temperature ( oC)

300 400 500 600 700 800 900 1000

Sol

uble

ions

(m

g L

-1)

0

5

10

15

20

25

KSi

Fig. 5. Dependence of the solubility of Si and K on treatment temperature.

374 M.N. Nguyen et al. / Chemosphere 119 (2015) 371–376

and 800 �C/H2O2-treated samples were found to be higher thanthat of the 400 �C-treated sample. By comparing the EDX signalsof K, it can be seen that K contents of the 800 �C-treated samplesare higher than those of 400 �C-treated samples. While a subse-quent wet-ashing with H2O2 result in almost no change in K abun-dance of the 800 �C-treated sample, a significant amount of K hasbeen removed from 400 �C-treated sample by the same H2O2

treatment.

3.3. Si and K dissolution kinetics

Batch experiments carried out with the heat-treated ash sam-ples and those with a subsequent treatment with H2O2 showedthat the concentration of soluble Si and K increased with time atdifferent rates depending on the treatment (Fig. 4a and b). After6 d, Si and K releases were 32.7 and 12.8 mg kg�1 for the 400 �C-treated sample, 14.2 and 8.0 mg kg�1 for the 800 �C-treated sam-ple, 3.8 and 0.3 mg kg�1 for the 400 �C/H2O2-treated sample, and42.7 and 9.2 mg kg�1 for the 400 �C/H2O2-treated sample, respec-tively. For the 400 �C- and 800 �C-treated samples without H2O2

treatment, Si and K concentration in the supernatant increasedover the initial 3 d and stayed almost constant after day 4. Forthe samples with a subsequent treatment with H2O2, Si and K con-centrations continuously increased with time and no saturationwas reached after 6 d.

Different changes in Si and K concentration in the supernatantof the 400 �C/H2O2- and 800 �C/H2O2-treated samples were alsoobserved (Fig. 4b). Lower Si and K were released for the 400 �C-treated sample when it was subsequently treated with H2O2,whereas, in the case of Si release, an opposite trend was observedfor the 800 �C-treated sample. Nguyen et al. (2013) found that asubsequent treatment with H2O2 decreased Si dissolution from ricestraw ash phytolith. The varying response of the samples to H2O2

treatment could be a change in the efficiency of the H2O2 oxidationreaction between the 400 �C- and 800 �C-treated samples. Wefound that the 400 �C-treated sample became finer and whiterafter treatment with H2O2, while no obvious change was observedfor the 800 �C-treated sample.

3.4. Effect of heat treatment on solubility of Si and K

Solubility of Si and K of the rice straw phytolith showed a highdependence on heating temperature (Fig. 5). With a change in

ashing temperature from 300 �C to 600 �C, increases of the solubleSi and K from 6 to 15 mg L�1 and 5 to 22 mg L�1, respectively, wereobserved. At a higher temperature range, from 700 �C to 1000 �C,the solubility of Si and K were obviously decreased. Lowest valuesof soluble Si and K were 3 and 4 mg L�1 for the sample treated at1000 �C. Over the entire temperature range from 300 �C to1000 �C, K showed a lower solubility in comparison with Si. Itcan be seen that soluble Si and K have similar trends (similar ‘‘peakshape’’ as shown in Fig. 5), and the maximum values were at600 �C. This strongly suggests that a similar mechanism controlsthe dissolution of Si and K from the samples.

Soluble Si (mg L-1)0 10 20 30 40 50

Sol

uble

K (

mg

L-1

)

0

2

4

6

8

10

12

14

16

(a)

(b)

y =0.2851x + 3.6391

R2 = 0.905

y = 0.2062x - 0.2157

R2 = 0.984

Fig. 7. Correlation between soluble Si and K in the supernatant of 400 �C- and400 �C/H2O2-treated samples (a), and 800 �C- and 800 �C/H2O2-treated samples (b)in the kinetic experiments.

M.N. Nguyen et al. / Chemosphere 119 (2015) 371–376 375

4. Discussion

K is taken up from soil and transferred to different parts of therice plant, e.g., stems and leaves, to provide the appropriate ionicenvironment for metabolic processes in the cytosol, and, as such,functions as a regulator of various processes including growth(Leigh and Wyn Jones, 1984). During the growth of rice, precipita-tion of Si to build up the phytolith might result in an occlusion ofplant transport sap which contains organic molecules and dis-solved substances within the silica cells. The visualization per-formed with X-ray tomographic microscopy showed a number ofholes inside the phytolith (Fig. 6a) and these holes were containedorganic substances (Fig. 6b). K is one of the most dominant ions inthe transport sap, suggesting that it might also be trapped insidethese ‘‘closed holes’’. These holes might also exist as micropores(Mohamad Remli et al., 2014) that were not visible in Fig. 6, andwhich also some occluded organic compounds and K. The EDXspectra showed that Si, O, C and K are the most dominant elementspresent in the phytolith (Fig. 2). A low EDX signal for K of the400 �C/H2O2-treated sample suggests that both K and organic mat-ter were removed when the sample was treated with H2O2. Thisindicates that K can be a component of the occluded materialinside the phytolith structure that we described above as‘‘PhytOK’’. However, a detailed analysis of the chemistry of thisK-pool and its relationship to the organic matter coexisting inthe holes of the phytolith structure was not included in this study.

In the kinetic experiments, increases of soluble Si with timewere observed as a result of phytolith decomposition. A corre-sponding increase of the K concentration in the supernatants wasobserved and suggested a relationship between K and Si releases.In order to clarify this relationship, the kinetic data was used tocreate linear trend lines (as shown in Fig. 7). A strong correlationbetween the soluble Si and K for all the 400 �C-, 800 �C-, 400 �C/H2O2-, and 800 �C/H2O2-treated samples was evident. We cantherefore conclude that the K release is controlled by the dissolu-tion of the phytolith.

The temperature of the sample treatment may also be a factorcontrolling phytolith dissolution and K release. It can be clearly seenin Fig. 7 that line (a), representing the 400 �C-, 400 �C/H2O2-treatedsamples is clearly separated from line (b) for the 800 �C-,

Fig. 6. Three-dimensional image of the principal arrangement of silicified structures (a)substances (c) and destructed cell (d). Phytolith appear gray and occluded substances d

800 �C/H2O2-treated samples. Ratios of soluble Si and K varied forthe samples treated at different temperatures as shown in Fig. 5.We believe that differences in organic removal rate and transfor-mation of the silica-containing surface groups during the heattreatment are two of the primary factors limiting Si and K releasefrom rice straw ash phytolith. High contents of organic matter inthe samples treated at low temperature prohibited Si and Kreleases. This is in accordance with findings from other studies(Van Cappellen et al., 2002; Parr and Sullivan, 2005) in which itwas reported that organic matter strengthens the phytolith surfaceand its resistance to dissolution. With increasing pretreatmenttemperature, dehydroxylation of silanol groups will form siloxanebonds and the surface becomes hydrophobic (Zhuravlev, 2000).This reaction reduces adsorption of water molecules on the surfaceand prevents the breakage of the surface siloxane bonds. It cantherefore explain why little Si and K were released from the phyto-lith samples in this case. In addition, the formation of stable silicaat high temperature resulted in a product with low specific surfacearea and less activity (Kordatos et al., 2008; Nguyen et al., 2013),likely also resulting in a decrease of Si and K release.

and it is fulfilled by occluded substances (b). Example of a silica cell with occludedark gray. The pixel width is 0.37 lm.

376 M.N. Nguyen et al. / Chemosphere 119 (2015) 371–376

Another hypothesis is that K might be located in the siloxanehexagonal cavities and it is released as a result of the breaking ofthe surface siloxane bonds. As reported by Zhuravlev (2000), silox-ane bonds can only be formed by dehydroxylation of silanol groupsof the amorphous silica during high temperatures in the pretreat-ment process. This implies that phytolith in rice straw has no silox-ane surface, i.e., hexagonal cavities do not occur, before burning.During the ashing process, some internal K in the xylem/phloemsap might be occluded in these newly-formed hexagonal cavities.However, extractions using Ca2+ and NH4

+ as exchange ions showedthat an increase of Ca2+ or NH4

+ in the supernatant resulted in loweramounts of both Si and K released (data not shown). This suggeststhat K was released by dissolution of the phytolith rather thanexchanged from the siloxane surface.

5. Conclusion

In rice straw phytolith samples, K content was up to 55%, indi-cating that this K-pool cannot be ignored when considering cropand soil fertility management. By combining the results from batchexperiments, analysis of SEM–EDX, and X-ray tomographic micros-copy, we believe that K might co-exist with organic matter in the‘‘closed-holes’’ of the phytolith structure. This implies that thisamount of PhytOK is locked and unavailable for plants prior to dis-solution of the phytolith structure. Co-release of Si and K wasobserved and it allowed us to conclude that dissolution of the phy-tolith is a main factor regulating K release. The ashing temperatureof the rice straw can affect Si and K releases by enhancing theremoval of occluded carbon or stabilizing the silica surface of thephytolith. The highest values of soluble Si and K observed at600 �C suggests that pretreatment of the rice straw around thisashing temperature is optimal in providing available Si and K forsoils and crops.

Acknowledgements

This research was funded by the Vietnam National Foundationfor Science & Technology Development (Project 105.08-2013.01).X-ray-tomographic microscopy was performed with skilful helpby Julie Fife at the TOMCAT beamline of the synchrotron facilityof the Paul Scherrer Institute, Villigen, Switzerland. Great help ofSarah B. Cichy and Karl-Ingo Friese for morphological characteriza-

tion of phytoliths from the tomographic dataset is acknowledged.We would like to thank Ph.D. Erika Elswick, Department of Geolog-ical Sciences, Indiana University for her support during the work.

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