Release of silica-rich particles from rice husk by microbial fermentation

Abstract

Rice husk, a by-product of paddy cultivation, has been the subject of numerous investigations aimed at finding a variety of industrial uses. Rice husk essentially consists of the following layers: (i) outer epidermis coated with a thick cuticle layer of highly silicified sinuous cells, (ii) sclerenchyma of hypoderm fibres also with a thick lignified and silicified wall, (iii) spongy parenchyma cells, and (iv) inner epidermis of isodiametric cells [1, 2]. The major inorganic component of the rice husk is silica (~20%) which is known to be highly pure, amorphous with high surface area and reactivity [3]. Most of the industrial uses of rice husk pertain to the silica present in the husk. These include the development of inexpensive cement [4] and ceramic whiskers such as silicon carbide and silicon nitride as reinforcements for metal matrix composites [5, 6]. Silica from rice husk has also been explored as a source for the production of solar grade silicon [7, 8]. This letter describes the microbial fermentation of rice husk as a method to pretreat rice husk at low temperatures to release silica (SiO2) in its natural, pure and highly reactive form. Such microbiological processes carried out at room temperature are relatively inexpensive and could yield a material with many potential industrial uses. Rice husk, obtained from a local dehusking mill in Bhopal (India) was subjected to microbial fermentations using a locally isolated species of white rot fungus, Cyathus [9]. Fermentation of the rice husk was carried out for a period of 60 days under standardized conditions of substrate availability and growth temperature [9]. The fermented husk was harvested, air dried at 80 ° C and used for analysis of SiO2. The dried material was sieved through different sized sieves to separate the small particles liberated from the organic fibrous material during microbial fermentation. Ashing at 450°C produced white ash which was used for estimation of silica. Fermented husk particles were examined using a Jeol scanning electron microscope (SEM) equipped with wavelength dispersive X-ray spectroscope (WDXS). Samples were coated with a thin layer of carbon prior to SEM examination. Table I shows the results of SiO2 estimations on the fermented husk samples as well as on the untreated husk. The results show that fermentation resulted in a decrease in organic matter with a corresponding increase in SiO2 content. The S i O 2 content of the fermented husk was also found to be higher in smaller sieve size fractions. The maximum S i O 2 c o n tent of 49.0% was found in 6 0 0 #m size fraction of fermented husk as compared to 23.7% in untreated husk. Scanning electron micrographs of fermented rice husk particles are shown in Figs 1 to 6. Fig. 1 shows the dorsal surface of rice husk on which mycelial growth of Cyathus can be clearly seen. A lower magnification micrograph of a typical dorsal surface which is not highly fermented is shown in Fig. 2a wherein the nodule-like structure of the rice husk can be seen. A corresponding SiKa X-ray map is shown in Fig. 2b. Comparing Fig. 2a and b it can be noted that the nodules are rich in Si(SiO2) and in between them are regions depleted of silicon (SiO2). In the extensively fermented regions of the husk, these silicious nodules show cracks (Fig. 3a) with release of spherical particles of about 2#m diameter (Fig. 3a and b). WDXS analysis carried out in the spot mode confirmed that the spherical particles released during fermentation (Fig. 3) are rich in SiO2. In certain regions of extensively fermented husk, release of fibrous material was also observed (Fig. 4). Release of nodular-shaped particles was occasionally observed (Fig. 5). Fig. 5 also shows the X-ray line analysis corresponding to SiKa superimposed on the scanning electron micrograph. The lower line in Fig. 5 is a reference line and silicon content analysed across this line was shown in the upper line. It can be seen from Fig. 5 that there is a drop in silicon (SiO2) content in between the silicious nodules. Ventral surface of the husk also showed varying degrees of degradation on exposure to Cyathus and mycelial growth with exposure of underlying fibre bundles was also observed (Fig. 6).