Nano-solution to a mega-problem
http://www.hindu.com/seta/2007/01/11/stories/2007011100081500.htm
BANGLADESH HAS been battling with a silent public health disaster for the past thirty years. The culprits are the deadly arsenic compounds present in the country's tube-well water.
Earlier to the 1970s, health authorities there found an epidemic of gastrointestinal diseases, due to the contamination of surface water from the lakes and rivers by disease-carrying microbes.
Well-intentioned move
In a well-intentioned move, they embarked on a programme, in collaboration with UNICEF and private parties, of digging tube-wells, so as to provide safe drinking water. By the end of 1997, over 80 per cent of its population had access to tube-well water.
Alas, tube-well water is not safe either. It contains arsenic salts at levels far higher than permissible. The problem is not a directly man-made one. Silt from rivers upstream has been, over the centuries, collecting and depositing arsenic in subterranean layers.
A silent killer
Tube-well water arsenic contamination is thus not restricted to Bangladesh alone, but is also seen in Bihar and Eastern UP, but it is Bangladesh that has been hit so calamitously.
Arsenic is a silent killer. It causes skin lesions, affects the stomach, liver, lung, kidney, and blood, disabling them over time. It combines with proteins and enzymes, inactivating them and thus causing slow metabolic disorders.
Past examples
At the extreme, it causes cancers. History is replete with examples of arsenic-induced poisoning and death. Two Popes and even Napoleon Bonaparte are thought to have been murdered through arsenic poisoning. But the scale in Bangladesh is massive, over 40 million of its 130 million are affected. The first case was detected in 1983 by Dr. K. C. Saha of the dermatology department of the Calcutta School of Tropical Medicine. Since then, thousands of cases have been reported.
Professor Dipankar Chakraborty of Jadavpur University, Kolkata, has written about the problem in detail in the Indian Journal of Medical Research, Nature and other journals.
Some methods have been suggested and used to help clear the body of ingested arsenic and prevent skin lesions. Selenium intake appears to remove some arsenic.
Some have suggested that iron sulphate be used in order to help flush arsenic out of the system. Others have suggested that the amino acid methionine may help in reducing the lesion.
How does one plan to remove the offending arsenic from water? Boiling the water to precipitate the arsenic does not work, since it does not come out of solution, as calcium does from hard water.
Nor does boiling convert arsenic into any harmless form, as happens with water contaminated by microbes. What we are looking for is an efficient, inexpensive method.
Applicable for all
The method should be applicable at all scales, from the individual families to the city water supply agencies. A group of researchers from Rice University at Houston, Texas, U.S. has been working for the last several years on precisely this problem.
They have now come out with a workable solution that appears to satisfy many of the above requirements. And their solution, published in the 10 November 2006 issue of Science, makes use of magnetic nanoparticles of iron oxide.
We humans, with our height and girth in metres, are `metre-particles'.
Tiny ants, fruitflies and lice are `millimetre particles' or `milliparticles', while bacteria, which are a thousand-fold smaller, are `microparticles'. Scaling equally down, we reach molecules and atoms whose sizes are in nanometres or even less.
As we reach this nano-scale, the properties of materials change remarkably. Size matters here; it becomes the determinant of the property. Gold glitters as a nugget, as a millimetre speck, and even as a micron particle. Cut it down to the nano scale; it loses the glitter; even its electrical conductivity changes.
New laws
New laws of physics, of the quantum world, begin to operate here. Chemically it is the same, but in various physical properties, nano-gold is quite different from macro-gold.
Even in the nano-dimension, size matters. A 3 nanometre (nm) particle of cadmium selenide shines green, but emits red when its size increases to a bit more than 5 nm. Take the example of magnets. Magnetite, a composite oxide of iron, is a good magnet.
But its magnetic property changes as we cut chunks of it into smaller and smaller pieces. Below 40 nm in size, its magnetic properties actually become more pronounced, and it becomes what physicists call a superparamagnet.
At the same time, as the particle size reduces, the proportion of surface area it exposes also increases. This allows it to `stick' to material more avidly than in the bulk phase.
What the Rice university researchers have done is to exploit this nano-size behaviour of magnetite. They prepared 16-nm size magnetite particles, stirred up a bit of this material in a beaker-full of arsenic-contaminated water.
Large surface area
Two things happened. Magnetite, being an iron-containing material, has an affinity to bind to arsenic salts, and it did so very avidly, thanks to the large surface area it presents at this nm size.
This removed the dissolved arsenic very efficiently from the water. Secondly, they placed an external magnet under the beaker.
This external magnetic field induced the aggregation or clumping of the magnetite into large chunks, which could be decanted or filtered out, leaving arsenic-free water. What does it mean to Bangladesh, and other areas affected by arsenic-contaminated water?
The use of nano-magnetite and a small magnet helps remove the arsenic quickly and efficiently. Here then appears a method worth trying both at the small scale and at the larger community level. Nano-Davids for Mega-Goliaths!
D. BALASUBRAMANIAN
dbala@lvpei.org
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