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sábado, 14 de julho de 2012

Add Beads, Remove Arsenic: Shrimp Shells Clean Water

BY Gwyneth K. Shaw 
The beads look like Tic Tacs, with the slightly springy texture of a gel cap full of medicine.
This isn’t a breath mint or a dose of Tylenol, but a new way to remove a toxin from water—and save countless lives.

The beads are made of chitosan, derived from the shells of crabs, shrimp and other crustaceans. Inside are ultra-fine powders of titanium dioxide and aluminum oxide. Add arsenic-tainted water to the beads, and voila.

Harold Shapiro/Yale University Photo

“They just drop out to the bottom,” said Julie Zimmerman, an associate professor of green engineering at Yale.You’ve got clean water on top, and the beads at the bottom.”

Zimmerman worked with graduate students to develop the beads in her lab (she’s a faculty member at both the School of Engineering and Applied Science and the School of Forestry and Environmental Studies); the team recently published a paper outlining the method.

The system is reusable, too: the arsenic can be rinsed away, and the beads put back into service. Zimmerman’s team proved they can be used at least five times without any loss of efficacy, although she said there’s good reason to believe many more uses are possible.

Titanium dioxide and aluminum oxide nanopowders have been in use for a while as a way to take out arsenic. But those two substances alone, while effective, need to be filtered out of the water, an extra step that was difficult to pull off in developing countries as well as being resource-intensive. The chitosan beads save that step—and also allow for the use of less titanium and aluminum, which are far more scarce than shrimp shells.
“It’s just the waste from the shellfish industry,” Zimmerman said. “People will pay you to take it.”

The end result is a method that’s both easy to use and sustainable. In places like Bangladesh, where arsenic is a strong presence in groundwater, filtration systems like this one could have a huge impact.

More than 30 years ago, health advocates pushed for the drilling of wells to move Bangladeshis away from contaminated surface water and toward groundwater. But in a horrifying twist, that well water turned out to be loaded with arsenic. A study published in The Lancet, a British medical journal, two years ago estimated that 35 to 77 million people in Bangladesh have been drinking the well water—and that one in five deaths there could be attributed to arsenic poisoning.

Arsenic is a problem in some parts of the U.S., too, Zimmerman said, particularly in the Southwest. The U.S. Environmental Protection Agency is considering lowering the acceptable level of arsenic in drinking water; it’s currently 10 parts per billion.

Zimmerman said it’s unclear exactly what form the chitosan-based system will take when it’s delivered to people. A Brita-type filter containing the beads might work, or some kind of column that lets in sunlight, which is a key part of the chemical process of removing the arsenic. At least for now, it’s probably limited to small-scale or home use.

Then there’s the question of what to do with the arsenic once it’s out of the water. Zimmerman said she’s been in touch with Intel, which needs arsenic for its computer chips, in the hopes that the company might fund clean-water research in exchange for a steady supply of arsenic.

The project is a prime example of green chemistry and engineering—in other words, solving problems without creating new ones, using renewable materials and fewer toxic chemicals. It’s also an example of how some super-small nanoparticles (a nanometer is a billionth of a meter) are making new solutions possible, even as others are scrutinized for potentially troubling health and environmental issues.

Here’s how the system works: two types of arsenic are typically found in groundwater, arsenic III, or arsenite, and arsenic V, or arsenate. The former is more toxic and not electrically charged; the latter is less toxic and positively charged. It’s also much easier to remove from water, because of that charge.

When the water and beads are exposed to UV light, the titanium dioxide oxidizes the arsenite, transforming it into arsenate, which then gloms on to the aluminum oxide. Presto—clean water.

Zimmerman, who’s also the associate director of the Center for Green Chemistry & Green Engineering at Yale, said she and her team are working to tweak the chitosan system to remove other chemicals, such as selenium, which is also under EPA scrutiny, as well as cadmium, mercury and waste that’s a mixture of contaminants.

The thinking is, “what else can we embed in there?” Zimmerman said.

There also are efforts to scale it up. One example could be spinning the chitosan into fibers that could be part of a filtration membrane, she said.

The ultimate goal is to create a spectrum of treatments, Zimmerman said, suitable for small- and large-scale use. For example, selenium is a byproduct of mountaintop mining, at enormous levels. She said it’s difficult to imagine the existing chitosan beads being able to treat that much water—but said others might be able to engineer a way to do it.

One thing is certain: The need for clean water is only rising.

“We feel like we need a lot of solutions,” she said. “I don’t feel like there’s one magic bullet.”