Distribution and ultrastructure of silica on rice straw surface

Abstract: Agricultural residues are annually renewable, widely distributed, and available around the world. Each year, there is around 205 million tons of rice straws produced in China. Rice straw has been traditionally used as organic fertilizer, energy, and raw material for paper and bio-composites. Rice is a typical silicon-accumulating plant. Silica is absorbed in the form of monosilicic acid. Monosilicic acid follows water in the transpiration stream, and finally deposits as an amorphous opal (SiO2). These amorphous opals are accumulated in cell walls of shoots, intercellular regions, and epidermic cells via biosilicification. Silica in rice plants is considered to be an essential element, although the beneficial effects of silica on growth and yield are partly attributed to the effects of the element on protecting plant from fungal diseases and insects. Furthermore, rice straw is a natural cellulose-SiO2 hybrid nanocomposite. The cell wall could be a matrix for self-assembly of biosilica. Within cell wall, silica is bound in cellulose, hemicellulose and lignin, while SiO2 nanoparticles act as reinforcement partners during the synthesis of such bioinorganic nanocomposites. Bioinorganic hybrid nanocomposites have become well known owing to their unique properties and widespread potential application in different areas, such as catalysis, adsorption, separation, and drug delivery system. In this paper, the sheath, stem, node and ear of rice straw were selected respectively, and the total contents of SiO2 in tissues of rice straw were measured as follows: the weighted test samples were transferred to a crucible, carbonized gently over a burner, and then kept in a muffle furnace. The resulted ash was immersed in hydrochloric acid. The acid-insoluble residue was filtered, washed with hot water until no chlorides were detectable, ignited, and finally SiO2 was obtained and weighted. The distribution of silica on the surface of sheath and stem was investigated with the scanning electron microscopy with energy dispersive spectra (SEM-EDS). The ultrastructure of SiO2 derived from rice straw tissues was measured with the X-ray diffraction (XRD). The results of SiO2 content measurement showed that the silicon content of sheath (7.2%) was higher than that of stem (2.7%). The SEM-EDS results showed that the silicon content was 12.65%, 0.22% and 1.14% in exterior, middle and interior sheath, respectively. And the silicon content was 8.98%, 0.39% and 0.52% in exterior, middle and interior stem, respectively. Considering total SiO2 content of sheath and stem, silica was mainly distributed in the exterior surface of sheath and stem. Furthermore, silica distributed in the exterior surface of sheath mainly exhibited small granular matter, accompanied with bigger granular matter and hair-like trichome. The distribution of silica in the exterior surface of stem was more regular, however, the content of bigger granular matter and hair-like trichome was further decreased. The silica in small granular matter with the diameter of 1 μm mainly exhibited spherical and dumbbell-shaped. The XRD results showed that the crystal phase of rice straw exhibited Cellulose I, with crystallinity index of 55.3%, 59.0%, 47.8% and 57.4% in sheath, stem, node and ear, respectively. The crystallite size within them was 2.10, 2.13, 1.36 and 2.22 nm, respectively. Moreover, the SiO2 derived from rice straw exhibited amorphous.