High-purity amorphous silica from rice husk: Preparation and characterization

Abstract

Amorphous silica has a wide range of applications in chemical, biological and environmental sectors. In this work, we studied the preparation of amorphous silica from rice husk, an abundant silica-rich agricultural waste in Vietnam. We investigated and compared two methods of preparation: (1) via selective removal of alkaline and alkaline-earth oxides from rice husk ash to yield amorphous silica; and (2) via selective precipitation of silica from aqueous solutions which had been prepared by dissolving rice husk ask in NaOH Solutions. The obtained silica was chạracterized by using XRD, EDX, SEM and BET techniques. The results show that the best obtained silica is a white amorphous and porous powder with high purity of SiO₂, in the form of aggregates of 50nm-sized primary particles, and a specific surface area of 186 m² /g. The SiO₂ recovery is around 8.3 (g) per 100 (g) rice husk. Moreover, in the context of the present work, method (1) can be a more straightforward route for the preparation of amorphous silica in terms of silica’s purity, porousity and amorphousness. In contrast, method (2) is shown for a lower grade of silica purity which could be due to strong adsorption of ions from the solution onto the amorphous SiO₂ precipitate during precipitation process. The obtained silica has promising applications as adsorbents, fillers and fertilizers.