Biosilica recovery from pulped rice husk by acid precipitation

Rice husk is amongst the abundant agricultural wastes in rice-producing countries, such as Indonesia. About 20% mass of the whole rice is rice husk. Furthermore, silica content in rice husk is quite high of about 20%. Therefore, rice husk has a potential to be used as an alternative source of biosilica which could be further used as adsorbent or catalyst support. In this research, 2 types of acids, i.e. HCl and H2SO4 were compared in terms of their effectiveness during biosilica recovery from pulped rice husk. Rice husk was first pulped with 1 M NaOH at elevated temperature to dissolve silica and then filtered. Precipitation of silica was induced by the addition of acids until pH of 7 was reached. The mixture was aged for 72 hours prior to silica recovery. The results showed that HCl treatment was more effective compared to H2SO4 treatment. The silica obtained from was whiter in color and has a higher purity based on SEM/ EDX analysis. Furthermore, the silica was proved to be amorphous while the silica obtained from H2SO4 treatment showed the onset of crystalline silica. Additionally, the concentration of silica in the particle obtained from HCl treatment was ~24% based on SEM/ EDX analysis, which was almost double of that obtained from H2SO4 treatment. This process enabled biosilica recovery which has an added value out of low cost rice husk wastes.