Going the green way: Rice husks as an eco-friendly source of silica
Traditional production methods for silicon-based materials are highly energy-intensive and unsustainable, which is why researchers are now looking for “greener” ways to synthesize this versatile material – for example, by recycling agricultural waste products.
By Joseph C. Furgal
Silicones play an essential role in our modern world. The flexible and robust polymers are used in the production of personal care products and kitchenware, as well as in toys and solar panels. The synthesis of silicones, however, is a subject of debate due to the fact that conventional methods consume large amounts of energy, pollute the environment and are generally unsustainable. Carbothermal reduction, the most common method of converting silica (a natural compound commonly found in quartz sand) into silicon, requires heating silica to more than 1900 °C and produces carbon monoxide and carbon dioxide as by-products.
The “greening” of silicon chemistry is thus becoming increasingly important, and opens up new areas of research into the development of more sustainable methods. In that regard, the conversion of rice husks – an abundantly available agricultural waste product – into silicone building blocks has been a crucial step forward. Since rice plants absorb silica from the soil, their husks contain up to 20% of the mineral. According to a recent article, published in Physical Sciences Reviews, rice husk ash is considered to be one of the most promising and environmentally friendly sources for the production of silicon-based materials.
In their review, the researchers furthermore assessed direct low-temperature methods of generating silicone building blocks, including simple and sustainable catalytic methods to break down one of the strongest naturally occurring bonds – the Si-O bond. Thereby, silica (SiO2) from rice husk ash is broken down into its smallest components to then construct, on demand, new Si-containing materials. These “greener” silicon based materials can be used to make coatings, cosmetics, adhesives and sealants.
In addition to the adverse environmental effects caused by synthetic production methods, another important concern relates to the degradation of silicones in nature and their effects on biological systems. However, according to the review’s authors, the degradation of silicon-based materials has only a minimally negative environmental impact.
“Since silicon-based materials hold an important place in our world”, the authors comment, “it is necessary to think about the environmental impact of these materials and to develop new strategies to make their lifecycle, from synthesis to end of use, more cradle to cradle and less cradle to grave.”
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