Graphene oxide: A better membrane, but no 'silver bullet' for desalination
A new membrane marks a step forward for desalination technology. But it still faces a challenging road to the world’s seawater treatment plants, suggesting that the search for solutions to humanity’s thirst is far from over.
In recent decades, arid regions around the world have turned to the seas for drinking water.
Desalination – removing salt from seawater – offers dry places from the Middle East to the American Southwest an alternative to scarce rainfall and groundwater. With 1.8 billion people projected to live “in countries or regions with absolute water scarcity” by 2025, the demand for solutions to water-supply challenges looks set to grow in coming years, according to the United Nations.
Better filtration membranes could make desalination more viable as one of these solutions. Most modern plants use reverse osmosis, an energy-intensive process of pumping seawater through membranes to filter out the salt.
Since the 1960s, these membranes have almost always been made of polymers. But recent research has aimed to reduce the pumping energy by instead making them from “nanomaterials.” On Monday, British researchers at the University of Manchester’s National Graphene Institute in England announced that they had developed a membrane from one such material: graphene oxide.
Their new membrane marks a step forward for desalination technology. But it still faces a challenging road to the world’s seawater treatment plants, suggesting that the search for solutions to humanity’s thirst is far from over.
“The big cost is the energy cost of desalination and the reliability of the membranes, how long they last,” explains Peter Gleick, co-founder and president emeritus of the Pacific Institute in Oakland, Calif. “I think that the current research results are a great step forward for both of those issues … but in the end, it's really going to depend on our ability to commercialize this process.”
For more than three decades, Dr. Gleick has researched and worked to raise awareness of water availability issues. During that time, some cities have seen the cost per cubic meter of desalinated water drop by half, from $1.50 to 75 cents.
But in a phone interview, Gleick tells the Monitor that most gains have come as “incremental improvements,” with membrane costs dropping and lifetimes extending. In many locales, these gains haven’t been enough to justify the costs of desalination. In 2015, “a thousand gallons of freshwater from a desalination plant costs the average US consumer $2.50 to $5 ... compared to $2 for conventional freshwater,” reported Public Radio International.
“There hasn't been what I would describe as a revolutionary breakthrough,” Gleick says.
Small steps of progress
In recent years, graphene – a crystalline layer of carbon just one atom thick – seemed to promise such a breakthrough. Computer models by researchers at the Massachusetts Institute of Technology in Cambridge, Mass., “showed that graphene membranes could cut the energy used in reverse osmosis by 15 to 46 percent. Even better, the high permeability could mean that far less surface area is needed to get the job done, so the entire [desalination] plant could be half the size,” reports David Talbot for MIT Technology Review.
However, “It has been difficult to produce large quantities of single-layer graphene using existing methods,” says Jijo Abraham, a research scholar at the University of Manchester and co-author on the new study in an email to the Monitor. He adds that “current production routes are also quite costly.” Just four square centimeters of the substance, available on Graphenea.com, will set you back $146.
“However,” he continues, “graphene oxide,” which consists of a graphene-like lattice of carbon atoms, studded with oxygen groups, “can be produced by the simple oxidation of the parent material graphite (a naturally available material) and is very cheap to produce industrially.”
The researchers used this material to develop their membranes, explains materials scientist Ram Devanathan, who wrote a companion commentary accompanying the study in Nature Nanotechnology. Currently the acting division director of the Earth Systems Science Division at Pacific Northwest National Laboratory, Dr. Devanathan stresses in an email that the team demonstrated ways to “control layer spacing,” keeping the graphene oxide layers from drifting apart when immersed in water. That’s crucial for desalination: A membrane’s pores need to be large enough to let water molecules through, but small enough to keep salt’s potassium and chloride ions out.
To understand how the new membrane accomplishes this goal, he suggests: “Think of a stack of papers, except the individual sheets are not flat but have ripples. Now apply epoxy layers to the top and bottom of the stack to keep the stack from changing size. In this analogy, the graphene oxide layers are the individual sheets of paper.”
Mr. Abraham, currently pursuing his PhD in physics, says that the methods they tested “are quite effective for desalination in a lab scale.”
Challenges facing desalination on a large scale
But crops and consumers need water on more than just a lab scale. While Gleick of the Pacific Institute says he’s “as hopeful as the next person that there will be a big breakthrough,” he’s also clear-eyed about the challenges that such giant leaps face.
For the graphene oxide membranes to see use in treatment plants, “they have to be able to produce very large volume membranes that are very reliable in the long run.” He also cautions that any new technology will generate salty, sea-life-threatening brine as waste.
Meanwhile, desalination may be reaching the limits of its efficiency. The laws of thermodynamics set a “theoretical limit to how much energy is required to basically take salt out of water … current technology is pretty close to that.”
Can graphene oxide still do better? Abraham acknowledges that “more studies are needed to make it practically possible on an industrial scale,” adding that the team still lacks data on the membranes’ long-term performance.
“Achieving a defect-free, large area membrane will be a major challenge for us,” he continues. “We are expecting collaboration with other membrane manufacturers to tackle this issue and take this technology from the lab scale to the pilot scale in a few years’ time.”
“In principle, we do not see any major challenges” for large-scale production, he says, “but we need support from the industry.”
But in many corners of the world, utilities may see less high-tech methods as the best way to meet their customers’ needs. In California, for example, “a dollar spent on conservation or efficiency, or advanced wastewater treatment and reuse, gives you more than a dollar spent on desalination,” says Gleick.
So far, the presence or lack of other options has determined the degree to which different places desalinate. While the first reverse-osmosis plant was built in California, the Golden State’s once-abundant rivers and snowmelt discouraged investment in the technology, the Monitor’s Christa Case Bryant explained in 2015. Instead, it was Israeli firms, trying to sustain farms in the parched Middle East, who honed the method.
Now, drier climates may be making desalination a more cost-effective option, with Israeli firms bringing their expertise to California and elsewhere. This trend could give the new graphene oxide membrane a home in future treatment plants. But Gleick predicts that different places will still need different tools for navigating a drier future.
"Wherever you are," he advises, "think carefully about the economics of your options, and the environmental and social and political factors also are important. And that will determine where you want to spend money."