extraction and characterization of papilla-like biosilica from rice hulls

Chinese Journal of Chemistry, 2009, 27, 1031—1034 Full Paper

* E-mail: [email protected]; Tel.: 0086-0431-85168170; Fax: 0086-0431-85168086 Received July 6, 2008; revised December 16, 2008; accepted January 6, 2009. Project supported by the National Natural Science Foundation of China (Nos. 50572031, 50825202), and the Program for New Century Excellent

Talents in University of Chinese Ministry of Education.

© 2009 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Extraction and Characterization of Papilla-like Biosilica from Rice Hulls

ZHANG, Xiaodong(张晓冬) SUN, Jing(孙菁) ZHUANG, Jiaqi(庄家骐) YANG, Wensheng*(杨文胜)

Key Laboratory of Surface and Interface Chemistry of Jilin Province, College of Chemistry, Jilin University, Changchun, Jilin 130012, China

Papilla-like biosilica was extracted from rice hulls by using a wet oxidation method and characterized by means of transmission electron microscopy (TEM), small angle X-ray scattering (SAXS) and nuclear magnetic resonance (NMR). The experimental results show that the papilla-like biosilica was a kind of amorphous mesoporous materi-als composed of particles with a mean radius of about 3.6 nm. Nitrogen adsorption-desorption measurements indi-cate that the BET surface area of the biosilica is 355.9 m2/g with a BJH adsorption average pore size of 5.8 nm.

Keywords biosilica, mesoporous, amorphous, rice hull

Introduction

Silica, the second most abundant element in earth crust, has played many important physiological roles from single-celled organisms to higher plants and ani-mals.1-3 Aquatic organisms such as diatoms and sponges produce silica-based exo- and endo- skeletons account-ing for majority of their body mass with well-defined patterns.4 Biomolecules, such as silaffins5 and sili-cateins,6 have been identified to be responsible for the silica deposition of diatoms and sponges respectively. Many landplants employ biosilica as certain skeleton structure7-10 and rely on silica to enhance their photo-synthetic ability11 and resistance to fungus disease,12 heavy metal ions,13 heat tolerance14 etc. Rice hulls, the byproduct of rice mill industry, are known to contain about 10% amorphous SiO2,

15 which can be transferred into crystalline state by calcination.16 The good under-standings on the purification and physicochemical prop-erties of biosilica in rice hulls should be helpful for utilization of the biosilica.

In this work, papilla-like biosilica was extracted from rice hulls by using a wet oxidation method and its physicochemical properties were investigated by means of transmission electron microscopy (TEM), X-ray dif-fraction (XRD), small angle X-ray scattering (SAXS) as well as nitrogen adsorption-desorption analysis and nu-clear magnetic resonance (NMR), identifying that the biosilica was a kind of amorphous mesoporous materials with BET surface area of 355.9 m2/g and average BJH pore size of 5.8 nm composed by silica nanoparticles with a mean radius of about 3.6 nm.

Experimental

100 g of rice hulls of cultivated Chinese rice 301 were washed by deionized water and dried and then the dried rice hulls were crumbled using a pulverizer. The powder fragments were sieved with a sieve with the pore diameter of 0.3 mm. The light yellow powders of rice hulls were treated by the modified wet oxidation method.16 Firstly, the powders were heated at 65 ℃ for 48 h in a 4/1 (V/V) mixture of concentrated nitric and sulfuric acids (acid treatment 1). The generated nitrogen dioxide was absorbed by 1 mol/L NaOH. After being centrifuged and washed with deionized water, the slurry was heated at 65 ℃ for 24 h in a 1/1 (V/V) mixture of concentrated nitric acid and 60% perchloric acid (acid treatment 2). After being centrifuged, washed com-pletely by deionized water and dried in vacuum for 10 h, 1.5 g of the ultimately extracted silica was obtained as white powder.

Morphology of the papilla-like biosilica was ob-served by a Leica DMRXP optical microscope equipped with a DC-300 camera. XRD and SAXS were taken on a Rigaku D/max γA X-ray diffractometer (Cu Kα, λ＝1.5418 Å). Nitrogen adsorption-desorption measure-ments were conducted on an ASAP 2010M automatic adsorption analyzer. Total surface area, pore volume and pore size distribution were calculated based on Brun-auer-Emmett-Teller (BET), t-plot and Barrett-Joyner- Halenda (BJH) methods respectively. 29Si NMR spectra were recorded on a Varian Infinity plus 400 spectrome-ter and chemical shifts were referenced to tetramethyl-silane (TMS). TEM observations were carried out on a JEOL JEM 2010 microscope. The powder was ground

1032 Chin. J. Chem., 2009, Vol. 27, No. 6 ZHANG et al.


and dispersed in ethanol and dropped on a Formvar coated copper grid for TEM observations.

Results and discussion

Figure 1 shows the optical microscope images of the rice hulls after acid treatments 1 and 2. After the acid treatments, the papilla-like biosilica can be well recog-nized as shown by the optical microscopy observations. For the rice hulls, two kinds of typical papilla-like structures for the biosilica, papilla with single- and dou-ble-peaks can be observed. Peak distance of the dou-ble-peak papilla-like biosilica is about 40 μm and its base width is about 60 μm (Figure 1A). Peak and base widths of the single-peak one are about 10 and 20 μm, respectively (Figure 1B). It has been well documented that such parameters can be used for species identifica-tion of rice.17 Up to date, observation of the papilla-like structure of biosilica can only be carried out in vivo of rice hulls.18 Preparation of plenty of separated biosilica from rice hulls allows the further investigation of phys-icochemical properties of the biosilica and identification of possible organic species contained in the silica. It is noted that the papilla-like biosilica can not be observed only after the acid treatment 1, indicating that the other tissues of rice hulls can not be removed completely be-fore the subsequent acid treatment 2.

Figure 1 Optical microscopy images of the separated papilla-

like biosilica after the acid treatments 1 and 2. (A) Papilla with double-peak, (B) with single-peak. The scale bar is 20 μm for (A) and 10 μm for (B).

Figure 2 shows TEM images of the papilla-like biosilica. It is seen that the papilla-like biosilica is likely to be composed of small silica units from the sample edge (Figure 3A). Further observation suggests that ba-sic building block of the biosilica is silica nanoparticles with radius around 4 nm (Figure 3B). This means that the papilla-like biosilica is composed of silica nanopar-ticles, similar to those of biosilica in sponges.19

Figure 2 TEM images of the papilla-like biosilica at different magnifications.

SAXS measurements were carried out to further elu-cidate the structure of the papilla-like biosilica (Figure 3). Radius of the nanoparticles was calculated from SAXS data by the Logarithmic normal distribution the-ory. Radius distribution of the nanoparticle was obtained by the following equation:20

P(r)＝1/[(2π)1/2rln σ]exp[－(ln r－ln µ)2/(21/2ln σ)2]

where P(r) is the radius distribution function and r is the radius of the dispersed nanoparticle. µ and σ are the geometry average number of the radius distribution and the standard deviation of the radius distribution, respec-tively. Inset of Figure 3 shows radius distribution curve of the silica nanoparticles. Mean radius of the silica nanoparticles was determined to be about 3.6 nm, in consistent with the TEM result.

Figure 3 Radius distribution curve of the silica nanoparticles in rice hulls determined by small angle X-ray scattering.

Figure 4a presents nitrogen adsorption-desorption isotherm of the papilla-like biosilica. The biosilica ex-hibits a type IV adsorption isotherm for mesoporous structure.21 BET surface area of the papilla-like biosilica was calculated to be 355.9 m2/g and its BJH adsorption average pore size diameter to be 5.8 nm. These data suggest that the papilla-like biosilica is a kind of mesoporous material composed of small nanoparticles. After the calcination at 1000 ℃, the surface area of the biosilica decreased to 247.04 m2/g and the pore size re-mained unchanged (Figure 4b), indicating that the biosilica can keep its mesoporous architecture after the calcination. It is interesting that the dry ashing treatment of the rice hulls can transform amorphous biosilica structure to crystalline silica nanoparticles. Figure 5 shows solid-state 29Si NMR spectra of the rice hulls be-fore and after the treatment. Before the dry ashing treatment, the signal at －108.1 is attributed to typical Si(OSi)4 unit (Q4) and the one at －104.5 to surface (OH)Si(OSi)3 structure (Q3). The ratio of Q4/Q3 is about 3.3. This indicates the amorphous characteristic of Si—O network of the rice hull biosilica, which is similar to that of silica extracted from aquatic organisms.22 After the dry ashing treatment, only one signal at －110.2 attributed to Q4 remained, suggesting the crystalline character of the papilla-like biosilica.

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Biosilica Chin. J. Chem., 2009 Vol. 27 No. 6 1033


Figure 4 Nitrogen adsorption/desorption isotherms of the pa-pilla-like biosilica (a) before and (b) after the dry ashing treatmet.

Figure 5 Solid-state 29Si NMR spectra of the papilla-like biosilica (a) before and (b) after the dry ashing treatment.

The structure of the biosilica before and after the dry ashing treatment was further investigated by X-ray dif-fraction (XRD) analyses (Figure 6). It is evident that the biosilica is unambiguously amorphous since no charac-

Figure 6 XRD patterns of the papilla-like biosilica (a) before and (b) after the dry ashing treatment.

teristic diffraction peaks are observable before the dry ashing treatment. After the treatment, the silica presents a cristobalite structure as indicated by the XRD pattern. This finding shows that the amorphous papilla-like biosilica in the rice hulls can be directly transformed into highly crystalline silica by using the dry ashing treatment.

Conclusion

In summary, pure papilla-like biosilica was extracted from rice hulls by using a two-step wet oxidation method. It has been identified that the papilla-like biosilica is a kind of mesoporous structure composed of silica nanoparticles with a mean radius of about 3.6 nm. The biosilica presents a large BET surface area of 355.9 m2/g and an average pore size of 5.8 nm. The amor-phous silica nanoparticles can be transformed into silica nanocrystallites after the dry ashing treatment. Such a result may be helpful for utilization of the biosilica from rice hulls.

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(E0807061 Lu, Y.)

extraction and characterization of papilla-like biosilica from rice hulls

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