Trapping pollutants

A new material could soon be used to remove pollutants from toxic wastes

Published: Thursday 15 January 1998

chemists in the us have developed a sponge-like material that is very effective at absorbing certain heavy metals. The material called mesoporous silica, can be used to clean wastewater and turn it into drinking water. Researchers say it may also be cheap and adaptable enough to use the substance in agriculture, electronics, manufacturing and medicines (Scientific American , Vol 277, No 4).

For the last five years, researchers have known the method to make mesoporous silica. The material looks like a microscopic honeycomb riddled with nanometre-wide corridors. With all those internal walls, mesoprous silica contains a very large surface area. This allows the substance to have several chemical reactions in a small space. But one problem with silica is that it does not react much will other metals.

Now, Jun Liu and his colleagues at Pacific Northwest National Laboratory in Richland, Washington, usa , have devised a way to coat the walls inside mesoporous silica with chemicals that react with others. This makes the substance more useful for practical purposes. The researchers used sulphur for coating the wall. It can lock up industrial pollutants such as mercury, silver and lead, whose consumption is dangerous to human health.

During a study conducted on water and oil wastes, the sulphur-laced silica reduced the concentrations of heavy metals below drinking-water-standards. One important feature of this material is that it does not react with other less dangerous metals such as sodium and zinc.

The researchers say that by getting right amount of water inside the tiny tubes of mesoporous silica, one can place useful chemicals inside the silica. To realise this objective, the researchers first dry them, then add water along with a solvent. As the process is very simple, the substance can be produced at a large scale. According to A Aksay, a chemical engineer at the Princeton University, usa , the material may find diverse environmental applications. Though the coated silica costs nearly 50 per cent more than commercial filter materials, it absorbs metals 30 to 10,000 times more effectively, says Liu.

On the same lines, a chemist Galen Stucky at the University of California at Santa Barbara, usa , has developed stable mesoporous silica with corridors twice as wide as those developed by the Liu group. This material is large enough to contain biological molecules. The agriculture department has shown interest in packing silica powders full of pheromones to make long-acting pesticides.

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