Arsenic-contaminated water can be made drinkable cheaply and simply using tiny crystals related to rust, scientists at Rice University in Texas say.
Millions of Bangladeshis are forced to drink unsafe water
The US team says that particles of iron oxide can bind themselves to large amounts of arsenic in water.
When a strong magnet is placed above the particles, they clump together like iron filings and are simple to remove.
If confirmed it could help nearly 60 million people in Bangladesh who drink water with dangerous arsenic levels.
The researchers from Rice University's Centre for Biological and Environmental Nanotechnology report their work in the journal Science.
They produced particles that were just 12 nanometres (billionths of a metre) across, about 5,000 times smaller than the width of a human hair.
When mixed into contaminated water, the tiny crystals became coated with the poison and began behaving like iron filings.
Once the clumps were removed with a magnet, the water was well within international safety limits.
"We were surprised to find that we didn't need large electromagnets to move our nanoparticles, and that in some cases hand-held magnets could do the trick," Professor Doug Natelson, a co-author of the report, said.
"Once the hand-held magnets start gently pulling on a few nanoparticles and get things going, the nanoparticles effectively work together to pull themselves out of the water."
Professor Natelson told the BBC's Newshour programme that the technique was an adaptation of a method that has been used by chemical engineers for years:
"The idea of using magnetic particles to filter and clean water is not particularly new. The trick here is that these particles are very, very small, which means they're essentially all surface.
"So the arsenic sticks to the surface of the particles, and what we've found is that when the nanoparticles are in the right range of sizes, you can pull them out of solution with a relatively small magnetic field gradient that you could get from, say, a permanent magnet."
One of the Rice University scientists conducted experiments in which flakes of rust were made into nanoparticles by heating them in coconut or olive oil at 350C and could then be used in water purification.
Professor Natelson explained how the practice could work on a domestic level in areas where arsenic was a big problem, such as in Bangladesh.
"You'd have little packets that had this powder in it, you'd fill up your pitcher of water, you'd dump in the powder, stir it up, let it sit for a little bit, and then you'd have a permanent magnet that draws all the minerals down towards the bottom of the pot, and then you pour off the relatively clean water," he said.
"I wouldn't say it's absolutely free, but it's much freer of arsenic," Professor Natelson added.
Nanotechnology is a fast emerging science that concerns itself with the engineering of materials at the scale of individual atoms and molecules.
Nanophase materials, as they are sometimes called, will often display novel properties because of the very precise way in which their component particles have been arranged or shaped.