It may seem an unlikely way to clean up drinking water, but scientists at Rice University, in Houston, have found that nanoparticles of rust can be used to remove arsenic with a simple wave of a magnet.

Arsenic sticks to rust, says Vicki Colvin, a chemist at Rice’s Center for Biological and Environmental Nanotechnology. And since rust is essentially iron oxide, it tends to be magnetic, so it can be drawn back out of the water using a low-powered magnet.

The technique holds great promise for the treatment of millions of wells currently believed to be contaminated with dangerous levels of arsenic, says Colvin.

Indeed, according to the World Bank, nearly 65 million people are at risk from arsenic-related health problems, largely due to contaminated wells. The situation is so dire that it has led to the creation of a $1 million cash prize, called the Grainger Challenge, to be awarded to whoever comes up with a practical solution to removing arsenic from wells in poor countries.

It’s not just an issue in developing countries, says Proctor Reid, director of programs at the National Academy of Engineering, in Washington, D.C., which set up the Grainger Challenge. “There are even arsenic-contaminated wells in New Mexico,” he says.

Rust is well-known for its ability to bind to arsenic, says Scott Fendorf, a soil scientist at Stanford University. “Many of the existing techniques rely on iron oxide as a scavenger,” he says.

By adding nanoparticles of rust to water, the iron oxide can be even more effective, and there’s no need for expensive hardware and complex machinery, says Colvin. “By going small, you get a lot of surface area, which means you need less material to treat the content.”

However, until now it was thought that such particles, which are around 10 nanometers in diameter, would need powerful electromagnets to generate fields strong enough to overcome local forces acting on the tiny particles.

But Colvin’s team, which published its results in the journal Science, shows that even low-power magnets can do the trick. “The particles magnetically interact,” she explains. This allows them to behave, at least magnetically, like a larger magnet, and therefore be influenced like one.

Colvin and colleagues then repeated their experiment, this time with arsenic-contaminated water. They found that when the nanoparticles were removed, the arsenic levels of the water were reduced to levels well below those deemed safe by the Environmental Protection Agency.

A number of different techniques, ranging from centrifuges to filtration systems, are currently used to remove arsenic. Given that the arsenic-remediation technologies often need to be deployed in remote areas with limited access to power, it is important that they be as simple as possible and require no electricity, says Reid.

The Rice technique could provide a far more practical approach. While Colvin’s experiments used relatively expensive nanoparticles, she is confident that ultra-small rust particles can be easily and cheaply generated. As for the magnets, these can be recovered from the hard drives of old computers, many of which end up in Southeast Asia, she says.

“This is an advance for water treatment in general,” says Fendorf. But mopping up the arsenic is only part of the problem, he says. A big question remains as to what to do with the arsenic once it’s removed from the water. After all, it remains a health hazard.