Safer by the Sip

By Jade Boyd

Clean drinking water. It’s something most Americans take for granted, but not so for millions of people in India, Bangladesh and other developing countries, where thousands of cases of arsenic poisoning each year are linked to contaminated wells. But a revolutionary, low-cost technology being developed at Rice’s Center for Biological and Environmental Nanotechnology might help change that.

Arsenic contamination in drinking water is a global problem, and while there are ways to remove arsenic, they require extensive hardware and high-pressure pumps that run on electricity. “Our process is simple and requires no electricity,” says center director and lead author Vicki Colvin. “Although the nanoparticles we are using right now are expensive, we are working on new approaches that use rust and olive oil and require no more facilities than a kitchen with a gas cooktop.”

The proposed technology is based on a newly discovered magnetic interaction that takes place between particles of rust that are smaller than viruses—“nanorust.”

“Magnetic particles this small were thought to only interact with a strong magnetic field,” Colvin says. “Because we had just figured out how to make these particles in different sizes, we decided to study just how big of a magnetic field we needed to pull the particles out of suspension. We were surprised to find that we didn’t need large electromagnets to move our nanoparticles, and that in some cases, handheld magnets could do the trick.”

The experiments involved suspending pure samples of uniform-sized iron oxide particles in water and using a magnetic field to pull the particles out of solution, leaving only the purified water. The researchers expected that a large magnetic field would be required. “As particle size is reduced,” explains co-author Doug Natelson, associate professor of physics and astronomy and in electrical and computer engineering, “the force on the particles drops rapidly, and the old models predicted that very big magnetic fields would be needed to move these particles.”

But the results puzzled the researchers because even small magnetic fields, such as those generated by handheld magnets, affected the particles, pointing to a magnetic interaction between the nanoparticles themselves. “It turns out that the nanoparticles actually exert forces on each other,” says Natelson. “Once the handheld magnets start gently pulling on a few nanoparticles and get things going, the nanoparticles effectively work together to pull themselves out of the water.”

“It’s yet another example of the unique sorts of interactions we see at the nanoscale,” Colvin says.
Because iron is well-known for its ability to bind arsenic, Colvin’s group repeated the experiments in arsenic-contaminated water and found that the particles pulled out arsenic, reducing its amount in contaminated water to levels well below the Environmental Protection Agency’s threshold for U.S. drinking water.

Arsenic, a colorless, odorless, tasteless element that can lead to skin discoloration, sickness, cancer and death, is a problem in drinking water worldwide, but is most acute in the developing world. In 2005, the World Bank estimated that 65 million people in South and East Asia are at risk for arsenic-related health problems due to contaminated water. The problem in Asia stems from the reliance of many rural communities on shallow underground “tube wells.” These wells were a saving grace in the 1970s because they reduced dependence upon bacteria-infested waterways and ponds. But many of the wells have since turned out to be a source of naturally occurring arsenic.

Colvin’s group has been collaborating with researchers in civil and environmental engineering led by Rice professor Mason Tomson to further develop the technology for arsenic remediation. Colvin says Tomson’s preliminary calculations indicate the method could be practical for settings where traditional water-treatment technologies are not possible. Because the raw materials for generating the nanorust are inexpensive—rust and fatty acids that can be obtained from olive oil or coconut oil—the cost of the materials could be quite low if manufacturing methods are scaled up.

Additional co-authors include research scientist Amy Kan; postdoctoral research associate William Yu; and graduate students John Mayo, Arjun Prakash, Cafer Yuvez, Joshua Falkner, Sujin Yean, Lili Cong and Heather Shipley. The research is sponsored by the National Science Foundation, and the new technique was described in the Nov. 10, 2006, issue of Science magazine.