Purifying Water With Nanotech

Members are working on ways to make water potable

6 December 2013
When water molecules [red and white] and sodium and chlorine ions [green and purple] in saltwater encounter a sheet of graphene with holes of the right size [center], the water passes through from right to left, but the sodium and chlorine from the salt are blocked.
Image: David Cohen-Tanugi

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About 1 of every 6 people around the world has no adequate access to water, and more than twice that number lack basic sanitation, for which water is essential, according to the U.S. National Academy of Engineering. One of the Grand Challenges for Engineering set forth by the academy aims to develop technology that will make polluted water potable.

It’s not that the world doesn’t have enough water. Globally, water is abundant, but most of it is in the oceans, where it’s unsuitable for drinking without expensive desalination.

Another problem for some developing countries is that contaminated drinking water contains bacteria and other pollutants. The application of nanotechnology to purify water is the focus of many papers presented at IEEE conferences and published in the IEEE Xplore Digital Library. Two are described here.

Jeffrey C. Grossman, MIT associate professor of power engineering, and his graduate students David Cohen-Tanugi and Shreya Dave are developing a filtration material made of a sheet of nanoporous graphene. The holes in the graphene—a one-atom thick form of carbon—are small enough to block salt ions while letting water molecules through. Smithsonian magazine called this nanoporous form of carbon one of the top five surprising scientific milestones of 2012.

Cohen-Tanugi presented their paper, “Water Desalination Across Nanoporous Graphene,” at the IEEE Conference on Technology for Sustainability, held in August, in Portland, Ore.

“It’s essentially a single layer of carbon atoms shaped like a honeycomb,” he explains. “Produced with holes in it, graphene is a much thinner, more porous, and efficient membrane than the polymers generally used for filtering water. It’s extremely strong and has very interesting physical properties, but it is only now being looked at for water applications.”

The most efficient process today for desalination is reverse osmosis, which relies on semipermeable membranes to filter salt from water. But such systems demand high pressure to force water through the membranes, which are about a thousand times thicker than ones made of graphene. The process remains very energy-intensive and expensive, notes Cohen-Tanugi.

As an example, he points to a reverse-osmosis desalination plant providing fresh water in Almeria province, in southeastern Spain, that consumes about one-third of all the electric power in the region.

“That’s a lot of energy,” Cohen-Tanugi says. “And these are expensive plants to build, typically costing hundreds of millions of dollars, a price most developing countries can’t afford. Also, the water ends up being too expensive for agriculture. It is suitable for drinking or high-value manufacturing, but you wouldn’t dream of using it to irrigate a field.

“How the physical properties of graphene translate into a more energy-efficient process and cheaper water is still an open question,” he continues. “Some assume that if you have a material like graphene that is 500 times more permeable, then you can reduce your energy costs by a factor of 500, but that’s not correct and has been misinterpreted. But if we can, with nanotechnology, make the desalination process more efficient, affordable, and available to more people, we will have made a big contribution.”

BACTERIA BE GONE

Jin-Woo Kim’s system uses a magnet and clusters of multiwalled carbon nanotubes. The clusters capture bacteria such as E. coli, and as illustrated above, the rare earth element neodymium magnet attracts the nanotubes. Photo: American Chemical Society

Developing cheaper water-treatment processes to remove pathogens from water supplies would be a huge step in reducing the number of deaths caused by drinking contaminated water. According to the NAE, nearly 5000 children worldwide die each day from diarrhea-related diseases.

IEEE Senior Member Jin-Woo Kim and his colleagues at the University of Arkansas, in Fayetteville, and the University of Arkansas for Medical Sciences, in Little Rock, say that using carbon nanotubes might be a lifesaver. Their paper, “Highly Effective Bacterial Removal System Using Carbon Nanotube Clusters,” was presented at the 2009 IEEE International Conference on Nano/Micro Engineered and Molecular Systems.

What’s more, the same process could be used for other applications including sampling water for an environmental quality-control program, disinfecting medical instruments, and purifying human skin being used for skin grafts. Kim currently is exploring how to commercialize the system.

Conventional purification methods include chlorination, filtration, UV radiation, and the infusion of water with ozone gas. Those methods aren’t that cheap, nor are they always practical for developing countries.

A professor of biological and agricultural engineering, Kim says his process is less expensive than the other methods because bacteria such as E. coli can be captured and killed on-site and the carbon nanotubes can be reused later, cutting down on the cost of materials.

The carbon nanotubes, just nanometers wide, are essentially smooth pipes of water-repelling graphite. At the same time, they attract bacteria.

The researchers tested an approach using clusters of nanotubes in a vial of water to remove waterborne bacteria, which included both the gram-positive and gramnegative bacterial strains. Gram-positive bacteria are those that are stained dark blue or violet by gram staining. This is in contrast to gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain, appearing as red or pink. These two types were chosen because the properties of their surfaces could affect the interaction between the bacteria and the nanotubes.

The system consisted of large clusters of multiwalled carbon nanotubes (MWNTs) for capturing the bacteria and a magnet for attracting the nanotubes. The contaminated water is placed in the vial along with the MWNTs.

The magnet, a cube 2.54 centimeters on a side made of the rare earth element neodymium, was placed close to the vial’s outer wall [see photo above]. In less than five minutes, the MWNT clusters were completely separated from the bulk solution. No MWNT debris or bacteria were found in the residual water.

“The results clearly demonstrate the excellent potential of MWNT clusters as highly effective bacterial adsorbents of any type of bacteria,” says Kim.

This article originally appeared in print as "Cleaner Water."

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