Research Stories

by Melissa Crytzer Fry

Washing clothes would seem among the more mundane things we do as humans. Unless you're Paul Westerhoff.

An environmental engineer at Arizona State University, Westerhoff is interested in laundered socks. Not just any socks, though. He's taking a close look at antimicrobial-impregnated socks. The manufacturer's claim is that the socks eliminate odor with the help of nanotechnology.

The socks contain silver nanoparticles. They are just one of thousands of products that take advantage of today's nanotechnology movement in which matter is manipulated to way below microscopic scales.

Nano means one-billionth. So, one nanometer is equal to one-billionth of a meter. A single water molecule is one nanometer in size. A human hair is 100,000 nanometers wide. Yes, nano means extremely small.

The socks also are just one of the commercial products Westerhoff is studying. Nano zinc, nano aluminum, nano titanium dioxide, and nano silver are just some of the new materials on the market. They are used to make more efficient, lighter-weight, more durable, and increasingly functional products.

However, reducing the size of these materials comes with a consequence. Doing so changes the electrical and chemical properties of the materials. Their behavior with other chemicals and organisms becomes unpredictable.

Back to the socks. Westerhoff wants to know just how many silver nanoparticles are released into wash water and then into the wastewater treatment systems. After all, treated wastewater serves as drinking water during times of severe drought. It is discharged into rivers, reaching people in communities who live "downstream."

Westerhoff wants to understand the nano silver's entire lifecycle—and that of the myriad other nanomaterials that are emerging weekly.

"My research focuses on understanding a new technology and its environmental consequence," explains Westerhoff. He's one of the world's leading researchers studying the behavior, fate and transport of commercial nanoparticles in drinking water and wastewater treatment facilities. His prestigious 2006 Paul L. Busch award from the Water Environment Research Foundation also allows Westerhoff to study the toxicity effects of nanomaterials in aquatic systems and on human cells.

For example, what happens if nano silver escapes from the socks during washing? What happens to it at the wastewater treatment plant?

"If the wastewater facility can't effectively remove all the nanoparticles, where do they go?" Westerhoff asks. "And if the manipulated silver atoms escape into streams and eventually into drinking water, what happens to aquatic systems and humans who ingest them?"

A team of scientists and engineers are working to find the answers. Westerhoff works in tandem with ASU researchers Yongsheng Chen, David Capco, John Crittenden, Bruce Rittmann, Terry Alford, Pierre Herckes, and a group of graduate research assistants.

Again, back to the socks. Studies at ASU revealed that silver particles do indeed get carried away in the wash water. To find out how much, the researchers dissolved the socks in acid. The remaining quantities of nano silver were recorded.

"The quantity ranged from zero nano silver particles to 31 milligrams per sock," says Westerhoff. "It's not a pile of silver, but that is a lot for one sock." Silver is extremely toxic to fish and other aquatic organisms.

Next, the researchers used new sets of socks during successive washing cycles. They then measured the wash water for traces of nano silver.

"You start with a lot of nano silver in your sock. But when you put it in water, you get two different things," Westerhoff explains.

Half of the nano silver is unchanged. The other half dissolves in water. It becomes a silver ion—also dangerous to the environment, but without nanomaterial properties. Developing analytical detection schemes to differentiate ionic from nano silver is now one of Westerhoff's research goals.

In the final test phase, the wash water was exposed to experimental vessels that simulated a wastewater treatment plant.

"The more bacteria that exists in the wastewater plant, the more the nano silver seems to stick to it, and the less that remains in the solution," says Westerhoff. "We learned that silver nanomaterials do interact with wastewater biomaterial."

Nanomaterials that latch on to this naturally occurring wastewater biomaterial turn into solids. They are called biosolids.

Identifying those types of reactions is just one part of the nano puzzle. Westerhoff wants to understand which nanomaterials are being transported into the environment as biosolids. Typically, they are applied as fertilizers for non-food crops. He also wants to know which are entering lakes and streams in the form of liquid effluent discharge originating from the treatment plants.

Westerhoff's ASU team conducted a study with the U.S. Geological Survey. They analyzed wastewater effluent from sites in Arizona, California, Colorado, Iowa, and New York.

Specifically, they looked for fullerenes. Fullerenes are carbon-based nanomaterials. Because they are not yet widely used, only small quantities were detected. However, titanium dioxide was detected in the biosolid. The substance is already used in paint, correction fluid, plastic, cheese, toothpaste, chocolate, orange juice and other products.

Biosolids don't flow into rivers and streams. But they can pose another threat.

"Biosolids get into the environment in a much different way than if they are in a liquid discharge," Westerhoff explains. Biosolids are often applied on fields as aerosol sprays. The wind can easily pick them up, or rain may wash them into rivers and streams. Humans and animals in close proximity can then be exposed.

The ASU researchers want to know exactly how much nano titanium dioxide is in the biosolids they've collected. They use lots of methods to find out.

"We burn off the biosolid cells and see how much titanium's left," Westerhoff says. "Or we add chemical oxidants that turn all that organic matter into carbon dioxide. Then we see what's left."

Westerhoff thinks it will be years before large quantities of nanomaterials are seen in wastewater treatment plants.

"But we do see nano titanium as a sentinel," he says. Its fate today may predict where future high-use nanoparticles end up tomorrow. "We have to understand, before these nanomaterials are there, if they're likely to be there."

Armed with such knowledge, the ASU scientists think they can provide information that will help support regulations and engineering systems to remove nanoparticles, if needed.

Westerhoff is not against the use of these materials. He doesn't deny the positive impact nanotechnology will have on the future, either.

"There are a lot of beneficial uses for nanotechnology," he says. Using nanotechnology as part of targeted drug delivery for the treatment of chronic disease is already working well. He says that products with water-repellant, self-cleaning, and anti-fog capabilities will become a reality. So will stronger, lighter car parts, and smaller, more efficient computers.

But there is a need for caution.

"We have to weigh the benefits versus the risk. Decades ago, no one really considered the consequences of pesticides or pharmaceuticals in the environment. We are dealing with those problems today."

He thinks that similar mistakes can be avoided altogether.

"We are at the beginning of a new technology revolution," he says. "The government is investing money to understand the consequences."

So what does the nano revolution mean to humans? Does it mean you have to stop eating your chocolate or give up your lightweight sporting equipment?

Not according to Westerhoff. The quantity of nanomaterials currently entering the environment is negligible. New systems can be engineered to remove nanomaterials from wastewater.

"There's no evidence that these things are going to kill people. Right now, the whole area of nano development is at its infancy," explains Westerhoff. What his research does point to is a need for continued surveillance.

"It makes sense to be cautious," he adds. If we spend the money to understand the engineered nanomaterials, and be sure that there's not a mistake—there's not a new DDT or an asbestos produced to lurk among us—that would be really, really good."

Are nanomaterials toxic to humans? Read more in "Nanomaterials and humans: A deadly interaction?"

Nanoparticle research at ASU is supported by the U.S. Environmental Protection Agency. For more information, contact Paul Westerhoff, Ph.D., Civil & Environmental Engineering, 480.965.2885; Yongsheng Chen, Ph.D., 480.965.3272; or David Capco, Ph.D., 480.965.7011. Send e-mail to P.Westerhoff@asu.edu; Yongsheng.Chen@asu.edu; or dcapco@asu.edu