Characterization of anatomical features and silica distribution in rice husk using microscopic and micro-analytical techniques

Characterization of anatomical features and silica distribution in rice husk using microscopic and micro-analytical techniques

Available online at www.sciencedirect.com Biomass and Bioenergy 25 (2003) 319 – 327 Characterization of anatomical features and silica distribution ...

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Available online at www.sciencedirect.com

Biomass and Bioenergy 25 (2003) 319 – 327

Characterization of anatomical features and silica distribution in rice husk using microscopic and micro-analytical techniques Byung-Dae Parka;∗ , Seung Gon Wib , Kwang Ho Leeb , Adya P. Singhb , Tae-Ho Yoonc , Yoon Soo Kimb a Department

of Forest Products, Korea Forest Research Institute, 207 Cheonyangni-2 dong, Dongdaemun-ku, Seoul 130-712, Republic of Korea b Department of Wood Science and Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea c Department of Materials Science and Engineering, Kwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea Received 11 September 2002; received in revised form 30 December 2002; accepted 7 January 2003

Abstract Rice husk is a by-product of rice milling process, and a great resource as a raw biomass material for manufacturing value-added composite products. One of the potential applications is to use rice husk as 7ller for manufacturing lignocellulosic 7ber–thermoplastic composites. This study was conducted to examine the silica distribution in rice husk in preparation to use it as reinforcing 7ller for thermoplastic polymers. Microscopic techniques, such as light microscopy, scanning electron microscopy and 7eld-emission SEM (FE-SEM) were used to observe the surface and internal structure of rice husk. Microscopic examination showed that two main components of husk, lemma and palea consisted of outer epidermis, layers of 7bers, vascular bundles, parenchyma cells, and inner epidermis, in sequence from the outer to the inner surface. Histochemical staining showed that epidermal and 7ber cell walls were ligni7ed, and the walls of parenchyma and lower epidermal cells were not ligni7ed. The outer epidermal walls were extremely thick, highly convoluted and ligni7ed. The outer surface of both lemma and palea were conspicuously ridged. The energy dispersive X-ray micro-analysis attached to the FE-SEM provided information on the distribution of silica in the husk. Most of the silica was present in the outer epidermal cells, being particularly concentrated in the dome-shaped protrusions. These observations provided valuable background information on the organization of husk tissues and the distribution of silica, which will help optimize processes related to the use of rice husk for making lignocellulosic 7ber–thermoplastic composites in our future work. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Rice husk; FE-SEM; EDXA; Silica distribution

1. Introduction Rice husk is a by-product of the rice milling process, and is produced about one million ton per year ∗

Corresponding author. Tel.: +82-2-961-2577; fax: +82-2961-2597. E-mail address: [email protected] (B.-D. Park).

in the Republic of Korea. Most of the rice husk is used as a bedding material for animals [1]; the industrial applications of this material are limited. The rest is burned or used for land7lling. However, land7lling is becoming expensive because of the increasing demand for land in urban and agricultural usage. Therefore, more eDcient utilization of rice husk is urgently needed. Fortunately, eEorts are already underway for

0961-9534/03/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0961-9534(03)00014-X

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producing value-added products, such as composite materials, from this important bio-resource. These applications include: use in making cement and cementious materials to increase their compressive strength [1–3], for particleboards [4], and as reinforcing agent for thermoplastics [5–7]. Rice husk is also being used for fuel [8,9]. The use of silica from rice husks in the manufacture of high value materials has recently been reviewed [10]. Silica occurs as a component of cells or cell walls in virtually all aerial parts of rice plant, and is most abundant in the husk. Several techniques have been used to obtain silica from rice husks, and processes have been re7ned for optimizing the recovery of pure silica [11,12]. Stroeven et al. reported that the silica in rice husk was mostly distributed under the husk’s outer surface [13]. Interest in the use of entire rice husk for the manufacture of composite products is also growing. The use of rice husk in making composite products such as 7berboard and lignocellulosic 7ber–thermoplastic composites is also attracting much attention because of the potential for enormous gains in certain important properties of these products [14,15]. The performance of composite products is determined by the structural, chemical, physical, and engineering properties of their individual components. Thus, the properties of composite products made from rice husks will be related to the characteristics of fragments and 7bers derived from husks. For example, it has been observed that the use of rice husk increased tensile modulus and hardness of natural rubber/linear low density polyethylene (LLDPE) composite owing to the reinforcing eEect of its particles [15]. Use of a coupling agent, which improved the interfacial adhesion between rice husk and thermoplastic matrix, further improved these properties. These observations suggest that chemical characteristics of the surface of rice husk are crucial in developing the interfacial adhesion. Unfortunately, one of the main obstacles in preparing rice husk as 7ller is the presence of abundant silica in rice husk, occurring in high concentrations in outer layers. The methods of processing rice husk as 7ller for thermoplastics are also important, because diEerent processing methods produce surfaces of rice husk, which vary in their characteristics, and thus interaction with thermoplastic matrix. Probably, dry grinding is the easiest method of preparing 7ller from rice

husk. Other possible option is steam explosion treatment, which has been used to produce pulps from wood chips. These processes are likely to expose both external surfaces and internal tissues of husk, which will vary in their composition and texture. It is therefore necessary to closely examine silica distribution and internal structure of rice husk in order to select a proper processing method for preparing 7ller from rice husk, As noted above, some of these properties of rice husk have been investigated in detail. We present here detailed information on silica distribution and internal tissue organization of rice husk, using a range of microscopic and micro-analytical techniques. 2. Materials and methods The rice husk, which was obtained from a local rice milling plant was washed to remove rice particles, which had precipitated on the bottom. The rice husk was air dried before being used for microscopic examination. For observation with scanning electron microscope (SEM) and 7eld-emission SEM (FE-SEM), dried rice husk was soaked in distilled water, critical-point dried, and then coated with gold in a sputter coater. The samples were observed with a SEM (Hitachi, Model S2400) at an acceleration voltage of 15 kV. In order to observe silica grains using FE-SEM, rice husk was also carbonized at 600◦ C until it became ash. The ash particles were then cooled and sprinkled on double-sided tape, and mounted on stubs. For light microscopy (LM), samples were dehydrated in acetone and embedded in Spurr’s low viscosity resin. Transverse sections were cut at 2–3 m thickness with a RMC MTX ultramicrotome using glass knives. The sections were examined after staining with 0.05% toluidine blue (prepared in 1.5% borax) with a Zeiss Axiolab photomicroscope. Hand sections from un-embedded husks were also examined with this microscope after staining with phloroglucinol–HCl, a lignin speci7c stain, although illustrations of this material are not provided. Rice husk tissues were also macerated with Franklin solution (1:1 hydrogen peroxide and acetic acid) at 60◦ C for 1 h. The separated tissues were observed with SEM and FE-SEM (Hitachi 4700 at the Korea Basic Science Institute, Gwangju) at 5 kV. Elemental

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analysis was undertaken using energy dispersive X-ray micro-analysis (EDXA) at 20 kV.

3. Results The main components of rice husk are lemma and palea, which tightly interlock with each other. Although there are some signi7cant diEerences in their internal organization [16], the surface morphology of lemma and palea was similar, and therefore the surface features of lemma only are described here. Similarly, FE-SEM–EDXA information is provided only for lemma. SEM views of surface features and internal tissue organization of husk are provided in Figs. 1 and 2. The outer surface of lemma is highly ridged, and the ridged structures have a linear pro7le (Fig. 1). The epidermal cells of lemma are arranged in linear ridges and furrows, and the ridges are punctuated with prominent conical protrusions. The outer surface of lemma also contains papillae and hairs of varying sizes, but they were often broken at their bases in the material examined, and therefore are not illustrated. Cross sections taken through the entire thickness of the husk provided information on its internal tissue organization. SEM view of an entire cross section of lemma is shown in Fig. 2. The outer epidermis appears to be highly undulated due to the presence of regularly

Fig. 2. High magni7cation view of cross section through central region of lemma showing highly thick-walled outer epidermal cells and a vascular bundle. Bar = 20 m.

Fig. 3. Light micrograph of rice husk cross section, showing that ligni7ed thick cell walls are greenish-blue color when stained with toluidine blue. Bar = 25 m.

Fig. 1. Low magni7cation view of the outer surface of lemma, which appears highly ridged. Bar = 120 m.

spaced protrusions. Underlying the outer epidermis are two layers of thick walled 7bers. The outer epidermis is extremely thick-walled. The lower epidermis is not well de7ned because of collapse. Among the internal tissues, the vascular bundle region is well de7ned. Xylem and phloem tissues are clearly distinguishable. They are enclosed by a bundle sheath. Fig. 3 shows a light micrograph of rice husk cross section, which has been stained with toluidine blue. Toluidine blue is a polychromatic stain, which

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Fig. 4. High magni7cation FE-SEM view of outer surface of lemma. The points indicated by letters (a–f) were analyzed by X-ray point analysis for silica concentration, as shown in Table 1. Bar = 25 m.

has been widely used to diEerentiate ligni7ed tissues from non-ligni7ed tissues. Ligni7ed cell walls generally stain greenish-blue and those which are unligni7ed stain bluish in color. The outer epidermal walls are extremely thick, and much of wall is stained greenish-blue in color. The underlying 7bers are also thick-walled and stained greenish in color. Parenchyma cells and inner epidermal cells are thin walled and stained blue in color. The FE-SEM image and elemental mapping of silica of the outer surface of lemma is shown in Figs. 4 and 5, respectively. The brightness indicates presence of silica, with the higher intensity of brightness representing greater silica concentration. Silica is likely to be present in much smaller amounts in those regions, which show little or no brightness. Thus, it appears that silica is more highly concentrated in regions corresponding to dome-shaped protrusions and adjoining sloping areas (i.e. protruding inter-dome regions). It is interesting that there is little or no brightness in the

Fig. 5. X-ray map of silica of the lemma surface shown in Fig. 4. Silica appears to be most concentrated in regions corresponding to dome-shaped protrusions and shouldering regions.

dome region, which is broken at the top. Other regions of epidermis show less or no brightness. This suggests that silica concentration is likely to vary within outer epidermis, with silica being mainly concentrated in the tips of domes and their shoulders. The FE-SEM image and elemental mapping of lemma across its thickness are shown in Figs. 6 and 7, respectively. Again, a close correspondence between speci7c regions of lemma and silica concentration is apparent. The brightness is most intense in the outer regions of the dome-shaped outer epidermal cells. In other regions of the cross section of lemma, which show little or no brightness, the concentration of silica may not be high enough to be detected by the rather short period-elemental mapping employed in our work. The results of X-ray point analysis are shown in Figs. 8 and 9 and the Table 1. Fig. 8, which is based on X-ray counts taken on the surface of lemma, shows a dominant silica peak. The result from X-ray point

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analysis at various locations on the surface (Fig. 4) and across the cross section (Fig. 6) of lemma are shown in the Table 1 and Fig. 9. Based on the atomic percentage of element concentration (Table 1), silica appears to be present throughout the outer surface of lemma and also in its internal tissues corresponding to sub-epidermal 7bers. The values of atomic percentage are much lower in the regions where the domes are broken, which supports the information obtained from elemental mapping, and provides evidence that silica present in the outer epidermal layer is most concentrated in outer regions of this layer. Interestingly, the values for other inorganic elements, such as chloride (Cl) and calcium (Ca), are signi7cantly higher for the regions where the domes are broken as compared to other surface regions. The values based on weight percentage of element concentration (Table 1) support the results obtained using atomic percentage of element concentration for silica distribution. An agreement between the atomic and weight percentages also exists for Cl and Ca.

Fig. 7. X-ray map of silica of lemma cross section shown in Fig. 6. Silica appears to be most concentrated in regions corresponding to outer epidermis. Bar = 25 m.

5000 Si Ka1 4000

3000

Counts

Fig. 6. FE-SEM view of cross section of lemma. The points indicated by letters (g and h) were analyzed by X-ray point analysis for silica concentration, as shown in Table 1. Bar = 25 m.

323

2000

1000 O Ka1 C Ka1 0 0

200

400

600

800

Channel

Fig. 8. X-ray point analysis showing high concentration of silica on the outer surface of rice husk.

Outer epidermal cells contained abundant silica, which was revealed after fracturing of these cells (Fig. 10). Silica grains were clearly visible as discrete

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B.-D. Park et al. / Biomass and Bioenergy 25 (2003) 319 – 327 40 Weight basis Element basis

35

Intensity (%)

30 25 20 15 10 5 0 A

B

C

D

E

F

Surface

G

H

Cross section

Fig. 10. FE-SEM micrograph showing the presence of silica just underneath the outer epidermal cell surface fractured. Bar=10 m.

Fig. 9. Silica distribution using X-ray point analysis at various locations on the outer surface and within internal tissues of rice husk.

particles after the husk had been carbonized (Fig. 11). Silica particles vary greatly in their size, ranging from less than 100 nm to 1 m. Larger particles appear to be formed from the aggregation of smaller particles. Quantitative analysis showed the silica in the husk to be about 12% as ash content. The particle sizes of silica observed in this study are within the size range (0.03–100 m) reported [16]. These workers also reported that the structure of silica was amorphous with a purity of 99.66% SiO2 . Since silica is largely concentrated in epidermal cells, the amount and distribution of silica in the husk is likely to be an important factor for composite products, for example, in interfacial adhesion between rice husk and

Fig. 11. SEM micrograph showing silica grains from carbonized husk. The grains are highly variable in their size. Bar = 0:5 m.

Table 1 Atomic and weight percentages of the element concentration from the X-ray point analysis at the speci7ed locations Element Outer surface type A AP C O Si S Cl Ca

WP

Cross section B AP

C WP

AP

D WP

AP

E WP

AP

F WP

AP

G WP

AP

H WP

AP

WP

66.34 46.17 49.85 36.03 45.13 32.63 70.29 55.19 61.61 46.31 74.84 59.99 59.09 35.16 80.07 61.95 5.26 4.88 28.56 27.49 34.9 33.62 12.48 13.06 19.6 19.63 8.88 9.48 9.18 7.27 6.29 6.48 23.77 28.69 21.59 36.48 19.96 33.75 17.04 31.28 17.02 29.91 16.29 30.53 1.82 2.53 1.4 2.53 N/D N/D N/D N/D N/D N/D N/D N/D N/D N/D N/D N/D 2.82 4.49 1.64 3.39 1.73 3.55 N/D N/D N/D N/D N/D N/D 0.92 2.03 N/D N/D 14.11 24.79 5.82 13.29 2.89 6.71 N/D N/D N/D N/D 0.18 0.48 0.85 2.12 N/D N/D 12.98 25.76 4.78 12.35

AP and WP represent atomic percentage and weight percentage, respectively. N/D means no detection of an element.

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Fig. 12. Low magni7cation view of several types of cells, including smooth walled and sculptured cells. Bar = 100 m.

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Fig. 14. Several smooth walled 7bers and a sculptured cell, which is also long and relatively narrow. Bar = 50 m for both Figs. 13 and 14.

cells are sculptured with periodic and more or less uniform projections occurring on one or two sides of the wall. The cell wall with one-sided projections has a serrated appearance. Projections present on two sides of the cell wall are distinctly longer and also of a diEerent form. A higher magni7cation SEM view of smooth walled 7bers is shown in Fig. 13. The 7bers are slender and long with pointed ends. Parts of both smooth walled 7bers and sculptured cell are shown at a higher magni7cation in Fig. 14. The sculptured cell is distinctly larger in diameter than smooth walled 7bers. The wall projections of the sculptured cell are variable in their length as well as form. Fig. 13. Typical smooth walled 7bers, which are long and narrow, with pointed ends.

4. Discussion

thermoplastic matrix in the manufacture of rice husk– thermoplastic composites. The morphology of cells obtained after maceration of husks is shown in SEM micrographs in Figs. 12–14. Fig. 12 is a low magni7cation SEM view of cells, which vary in their length and cell wall features. The small diameter and thin, smooth walled cells are probably 7bers, which vary greatly in their length. Although the length of 7bers was not quanti7ed, random measurements suggest their lengths to be within a range of 300 –1000 m. The walls of other 7ber-like

Rice husk is a valuable natural resource not only as an excellent source of high quality silica [12,17–19], but also as a source of lignocellulosic material which can be potentially used to produce a range of valuable composite products. However, product development will require greater understanding of rice husk. The information provided here should form both an useful background on the compositional and morphological characteristics of the husk surface as well as its internal tissues. The microscopy and micro-analytical techniques employed provided complementary information on

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husk surface topography, tissue organization, and silica distribution. It is apparent that rice husk has unique surface features, with a highly irregular outer surface, which is rich in silica. The surface features of rice husk described here are similar to those of rice husk surfaces examined earlier [18,19]. As the characteristics of the surface of particulate components are known to greatly inSuence both the properties and performance of composite products [20], the surface features of rice husk have to be kept in mind when using either the whole husk or its fragments for making composite products. Additionally, tissue composition and stiEness are other important factors that require close consideration. Little eEort has been made to understand how silica can aEect the properties and performance of composite products, and developments need to be vigorously pursued in this area in order to eEectively use rice husk in the manufacture of composite products. From the standpoint of topography and composition of cell walls the two surfaces of husk, i.e. the outer epidermis and inner epidermis are likely to diEer markedly in their interaction with each other as well as with adhesives used in the manufacture of composite products. Also, these features of two surfaces of husk have to be considered in relation to other factors, such as the temperature and pressure used during the manufacture of composite products based on rice husk. Major organic constituents such as cellulose, lignin and hemicellulose of rice husk have been previously determined [18]. The highly thick, ligni7ed and heavily silici7ed outer epidermal cells together with the underlying layers of ligni7ed 7bers undoubtedly provide strength, rigidity and stiEness to husks to combat adverse environmental factors, such as high wind, which may be particularly critical during the stage leading to caryopsis maturation. Greater understanding is needed as to the relevance of these properties of husk to the properties of composite products made from this material. After grinding and steam explosion treatment, internal tissues of husk are also likely to be exposed. As microscopy and histochemical staining have shown, the tissues vary greatly in the thickness and composition of their walls. Thick walled 7bers are likely to be less Sexible than thin walled parenchyma. Thus, composition as well as physical properties of tissues will be important factors in the use of rice husks as

7llers and in the manufacture of varied composite products. Among the separated tissues, 7bers would probably be the most suitable types of cells for use in composite products from the standpoint of strength, but separating them from the rest of the tissues may prove too costly for the products to be cost eEective. Therefore, use of entire or fragmented husk may be a preferred option, although it might be necessary to remove silica from the tissues prior to use in some applications. Potentially, silica itself can be used as a by-product of high value, knowing that the use of silica in engineered products is rapidly increasing [21]. However, it has to be kept in mind that if husk fragments are to be used for composite products, composition and topography of both husk surfaces and their internal tissues are likely to be important factors for the properties and performance of composite products. The observations on silica distribution in rice husk suggest that diEerent coupling agents have to be used for diEerent state of rice husk. In situation where silica is exposed on the surface of husk, a coupling agent that contains silicon in its chemical structure like silane compounds, would improve the compatibility between the silica of husk and thermoplastics. However, other coupling agents like maleic anhydride propylene would be compatible with lignocellulosic components in processes where separated 7bers, or internal tissues of husk are to be used. Acknowledgements This work was supported by the Korea Research Foundation Grant (2001-042-G00016) to YSK. References [1] Sung CY. An experimental study on the development and engineering performance of rice-husk concrete. Journal of Korean Agricultural Engineering 1997;39(5):55–63. [2] Islam S. Grinding methods and its eEect on reactivity of rice husk ash. M. Eng. Thesis, Bangladesh University of Engineering & Technology, Dacca, Bagladesh, 1981. [3] Ariyawansa J. Current stage of research on reactivity of rice husk ash cement. M. Eng. Thesis, University of Sri Lanka, Sri Lanka, 1980. [4] Gerardi V, Minell F, Viggiano D. Steam treated rice industry residues as an alternative feedstock for the wood

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