Preparation of Silica from Rice Husks

Preliminary leaching of rice husks with a solution of hydrochloric acid before their combustion at 600°C is shown to be required to obtain relatively pure silica (∼99.5%) with a high specific surface area (∼260 m²/g) that is maintained even after heating at 800°C. Transmission electron microscopy observations indicate that this material has a homogeneous size distribution of nanometric particles. However, if the leaching with HCl is performed on the white ashes obtained by combustion of the rice husks at 600°C, an amorphous silica with the same purity also is obtained, but its specific surface area decreases to 1 m²/g. This behavior is due to a strong interaction between the silica and the potassium contained in the rice husks, which leads to a dramatic decrease of the specific surface area if K⁺ cations are not removed prior to the heat treatment at 600°C. This finding leads to a better understanding of the effect of potassium on the morphology of silica.