Silica from rice husk ash
doi:10.1038/nindia.2011.168 Published online 21 November 2011
New research has shown that rice husk ash (RHA), an agricultural waste product, could be used to synthesize nanosized hydroxy-sodalite (HS) particles, a type of zeolite. HS particles are a rich source of silica, which is an essential material for fabricating semiconductor devices.
Rice husk is an abundantly available agricultural waste material that contains a large amount of siliceous ash. Burning rice husk in air produces RHA, which is 85–98% silica. Burnt rice husk causes environmental pollution and poses a health hazard. Although RHA is a rich source of silica, no studies have so far tried to extract silica from RHA.
To find a novel source of silica and reduce the environmental hazards of burnt rice husk, the researchers used RHA to produce HS particles in the presence of sodium aluminate and sodium hydroxide solution at 90 °C for 6, 10 and 15 h. HS particles synthesized over a period of 6 h were small but exhibited large variations in size (50–300 nm). The particles began to agglomerate through chemical reactions. Increasing the synthesis time to 15 h allowed the smaller particles to self-assemble into coral-like spherical shapes.
The coral-like HS particles were formed without using any surfactants or structure-directing agents, making them both cheap and eco-friendly. Silica from RHA may be comparable to that obtained from sodium metasilicate. The researchers say that their HS particles will find applications in optical materials, hydrogen storage, hydrogen separation and catalysts.