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.