Graphene sheets with precisely controlled pores could purify water more efficiently.
PHOTO ILLUSTRATION: DAVIS COHEN-TANUGI AND JEFFREY C. GROSSMAN, DEPARTMENT OF MATERIALS SCIENCE & ENGINEERING
Population growth and climate change threaten to increase the problem worldwide, making the oceans—which account for 97% of the water on Earth—a tempting place to seek solutions.
Desalination can make salt water potable, but significant barriers exist to its adoption at larger scales: the technology is both energy intensive and expensive. “We need to think about new ways to make clean water,” says Jeffrey Grossman, a professor of materials science and engineering.
Grossman and Evelyn Wang, an associate professor of mechanical engineering, are each tackling the problem from different technical perspectives, but with a shared desire to help solve a major challenge for humanity.
“I grew up in California where drought is a major issue, so I’ve always been aware of water scarcity and its impact on health and quality of life,” says Wang.
Both researchers believe the best hope for turning on the tap lies in transforming the central technology used in conventional desalination, reverse osmosis. This process involves pumping salt water through membranes that filter out salt and other impurities. While current technology is effective, providing 21 billion gallons of water a day to some of the world’s most arid areas, the energy required to operate the plants is expensive.
“When you look for opportunities to advance reverse osmosis, the membranes are a key challenge,” says Wang. The 1960s-era polymer used in membranes restricts how much water flows through, and this means the pumps have to work harder, drawing more energy. As important, says Wang, the membranes get clogged—or “foul”—over time from impurities in the water and need to be replaced. She wondered if there might be a way to prevent fouling, which would reduce or altogether avoid the need to replace membranes and at the same time achieve a more energy-efficient removal of salt and other impurities.
Wang has been pursuing these questions through two distinct research thrusts, collaborating with colleagues at the King Fahd University of Petroleum and Minerals in Saudi Arabia. In the first, she is using chemistry to synthesize zeolites, crystals that can be found in nature whose properties seem tailor-made for salt exclusion. Made of an aluminum silica hybrid, synthetic zeolites have uniformly spaced pores, 5.5 angstroms in diameter, just small enough to exclude salt ions, but not water molecules. And they can be more easily cleaned. “This is an attractive material, something you could really take advantage of in desalination,” says Wang.
Her other research angle dispenses with membranes altogether. Instead, Wang and her Saudi partners have been developing capacitive deionization (CDI), a method of capturing salt ions using electric fields as water spills through a channel between two electrically conductive surfaces. Carpeting these surfaces with carbon nanotubes seems a particularly promising way to pull off salt ions as water flows past, her studies show.
The CDI process, notes Wang, is aimed at desalinating brackish, rather than ocean water, and would be of particular use “in the many remote areas of the world without resources for a reverse osmosis plant.”
Approaching the membrane problem from another angle, Grossman asked, “What if you could throw out what you have today and start over: what would the ultimate filter look like?”
For the answer, he has turned to graphene, carbon that takes the form of a hexagonal lattice one atom thick. In computer simulations, Grossman’s group showed that a nanoporous graphene (NPG) membrane—graphene with regularly spaced nano-sized holes—was “off the charts” in terms of permeability, hundreds of times more permeable to water than the industry-standard polymer membrane, while maintaining full salt rejection.
Follow-up research, in collaboration with Professor John Lienhard in the Department of Mechanical Engineering, looked at the real operation of a desalination plant to quantify the potential benefits of such high permeability. “It’s anywhere from a 15–50% reduction in energy consumption,” Grossman says. “That would be a game changer.”
Grossman is racing to create a prototype NPG membrane, and contemplating a manufacturing process that would make the material competitive with the conventional polymer. “As a starting point I need to be able to do two things: make a ton of it and poke nanoholes in it, both really cheaply,” says Grossman.
His research group is currently experimenting with different ways of synthesizing NPG, including carefully calibrated ripping or puncturing of the graphene lattice to remove carbon atoms and create well-spaced holes. “I am hopeful that soon we will have a prototype based on technology that can scale,” Grossman says.
Wang is also moving toward the prototyping stage for her research. She has a centimeter-size sample of a zeolite-impregnated membrane, although moving up to meter scale and avoiding material defects “are issues we have yet to face,” she says.
Both researchers are bullish on the potential for laboratory innovations to make inroads in quenching the world’s thirst. “In an age where we can put atoms almost anywhere we want to make almost anything,” says Grossman, “we have the capability to design practically limitless new materials…that don’t just slightly improve a process, but completely change performance, offering us a chance to solve crucial problems in the world such as access to clean water.”