Scientia Professor David Waite

David Waite

Each year, more than 580 million tons of rice is produced, largely by developing nations in Asia. Yet a quarter of this production mass consists of rice husks – hard, inedible shells that protect the grain, while generating a significant waste problem for growers.

There has been considerable research devoted to uncovering innovative applications for these husks so they don’t end up in landfill. One exciting possibility is water treatment, says Scientia Professor David Waite from the UNSW School of Civil and Environmental Engineering.

When burned, the ash from rice husks has a high concentration of silica, which is thought to be an excellent supporting material for ultra fine silver nanoparticles.

These nanoparticles have become well-known for their anti-bacterial properties and applications in water treatment, but at the size where these particles are most effective (diameters less than 20 nm), they have a tendency to aggregate, which decreases their disinfecting potential.

Waite is leading a team working at the intersection of nanotechnology, materials science and environmental engineering, developing new composite materialsmade from silver nanoparticles anchored onto the low-cost silica from rice husk ash (RHA). In addition to preventing aggregation of the silver nanoparticles, the rice husk ash support slows down the release time of dissolved silver, enhancing the long-term anti-bacterial applications of the particles.

Over the last two years, Tata Corporation in India has begun developing water filtration systems for households that use these innovative particles. The devices, which cost about US$40, appear to be more cost-effective than similar domestic-scale units, which use ultra-violet or reverse-osmosis technologies. However, little is known about the nature of the silver nanoparticle-impregnated RHA, or how well the nanoparticles work.

Waite says the technology is ideal for supplying clean, affordable drinking water to remote communities, and for doing so following a disaster or emergency, but also says there’s considerable room for improvement.

“Production costs of these units are still too high and the efficacy of the technology has not been demonstrated robustly. As such, the market for the technology outside of India, where the original units are manufactured, is currently quite small.”

But it doesn’t have to be. Waite and his team are investigating how these particles work, to understand and optimise their properties for water treatment applications.

In a recent study published in the journal Environmental Science and Technology, Waite and his team documented the synthesis of silver nanoparticles anchored onto black and white RHA (the type of ash is determined by the conditions in which the rice husks are burned).

They were looking specifically at the surface chemistry, preparation modes, the anti-bacterial properties against specific targets like Escherichia coli, and the mechanisms governing silver dissolution.

Waite says their results indicate “that the precise nature of the rice husk ash to which the silver nanoparticles are attached has a major impact on both the toxicity and longevity of the product as does the composition of the water being treated”.

There is still a lot of work to be done to understand the mode of disinfection, and to refine the preparation method, but the end goal of providing clean, affordable drinking water to the developing world is worth the effort.