Preparation of High Purity Silica Originated from Rice Husks by Chemically Removing Metallic Impurities

Research on bio-based materials as potential replacement for non-renewable sources is steadily gaining momentum. Rice husk is a byproduct in the agricultural industry and its abundance cannot be questioned. Traditionally, such husks have been used for animal food and burning fuel. However, after pyrolysis, it was discovered that rice husks had the potential for diverse applications. Specifically, rice husk ash was noted to be composed of over 60% amorphous silica, 10-40% carbon and negligible contents of other metals. The high silica concentration automatically qualifies rice husk as raw material for silicon production. However, with time it has been observed that the bio-generated silica particles fabricated from the rice husks contain significant levels of impurities such as alumina, magnesia and quick lime, which prevent its many potential applications.

Jin Hyung Lee and colleagues proposed a study to chemically remove metallic impurities present in silica synthesized from rice husks. They aimed at applying three representative acidic chemicals and one ionic liquid for lignocellulosic biomass pretreatment. Their work is now published in the peer-reviewed journal, Journal of Industrial and Engineering Chemistry.

The researchers commenced their empirical procedure by, first, selecting four chemicals, three acids and one ionic liquid including: sulfuric acid, hydrogen chloride, oxalic acid and ionic liquid, which were seen effective to hydrolyze polysaccharides and possessed potential to remove metallic impurities. The research team then used two of these chemicals to dissolve both cellulose and hemicellulose of the rice husks. The team then investigated the effects of chemical treatment on silica purities and properties by analyzing chemical composition, surface properties and surface-to-volume area.

The research team observed that the ash content considerably increased in sulfuric-acid and the ionic liquid treatments with the removal of both cellulose and hemicellulose. Drastic increases of silica purity were observed in samples treated with sulfuric acid and the ionic liquid at 99.6% and 99.5%, respectively. Furthermore, the ionic liquid treatment increased the surface area and pore volume by 1.9 times and 2.4 times, respectively, with respect to the sample not subjected to chemical treatment.

The chemical treatments used in the study were effective and improved the ash content in rice husk. Typically, both sulfuric acid and the ionic liquid were effective in eliminating cellulose and hemicellulose, while hydrogen chloride and oxalic acid removed primarily only hemicellulose. It seems that the purity of bio-silica is dominantly determined by chemical treatment methods which remove metallic impurities. In addition, the type of biomass feedstocks also affects the purity of silica. Adding high value to byproducts is important in biofuel production because it improves the economic feasibility of biofuel industry. Therefore, the results of thier study can be applied to biofuel production process and thereby improve its economic feasibility.