Posted in | Graphene

Graphene Could Help Make Desalination More Productive When Used in Filtration Membranes

On the left, an atomic-force microscopy image shows a nanoporous graphene membrane after a burst test at 100 bars. The image shows that failed micromembranes (the dark black areas) are aligned with wrinkles in the graphene. On the right, two zoomed-in scanning electron microscopy images of graphene membranes show the before (top) and after of a burst test at pressure difference of 30 bars. The images illustrate that membrane failure is associated with intrinsic defects along wrinkles. Credit: Courtesy of the researchers

A single sheet of graphene, comprising an atom-thin lattice of carbon, may appear to be rather delicate. MIT engineers, however, have discovered that the ultrathin material is remarkably sturdy and remains intact under applied pressures of at least 100 bars. This is equivalent to almost 20 times the pressure generated by a typical kitchen faucet.

The researchers found that the key to withstanding such high pressures refers to the pairing of graphene with a thin underlying support substrate that is pocked with small holes, or pores. The graphene becomes more resilient under high pressure when the substrate’s pores are smaller.

Rohit Karnik, an associate professor in MIT’s Department of Mechanical Engineering, states that the study’s results, recently reported in Nano Letters, are used as a guideline for designing tough, graphene-based membranes, especially for applications like desalination, in which it is essential for filtration membranes to withstand high-pressure flows in order to efficiently remove salt from seawater.

We’re showing here that graphene has the potential to push the boundaries of high-pressure membrane separations. If graphene-based membranes could be developed to do desalination at high pressure, then it opens up a lot of interesting possibilities for energy-efficient desalination at high salinities.

Rohit Karnik, Associate Professor, Department of Mechanical Engineering, MIT

Karnik’s co-authors are lead author and MIT postdoc Luda Wang, former undergraduate student Christopher Williams, former graduate student Michael Boutilier, and postdoc Piran Kidambi.

Water stressed

The membranes currently existing desalinate water through reverse osmosis, a process that helps applying pressure to one side of a membrane containing saltwater, in order to push water across the membrane while salt and other molecules are in fact prevented from filtering through

A number of commercial membranes desalinate water under applied pressures of about 50 to 80 bars, above they are likely to get compacted or otherwise suffer in performance. Membranes can actually enable more effective desalination of seawater by recovering more fresh water if only they were capable of withstanding higher pressures, of 100 bars or greater. It could also be possible for high-pressure membranes to purify water that is extremely salty, such as the leftover brine from desalination that is typically too concentrated for membranes to push pure water through.

It’s pretty clear that the stress on water sources is not going away any time soon, and desalination forms a major source of fresh water,. Reverse osmosis is among the most efficient methods of desalination in terms of energy. If membranes could operate at higher pressures, this would allow higher water recovery at high energy efficiency.

Rohit Karnik, Associate Professor, Department of Mechanical Engineering, MIT

Turning the pressure up

Karnik and his colleagues carried out experiments to study how far they could push graphene’s pressure tolerance. Earlier simulations have predicted that graphene is capable of remaining intact under high pressure when placed on porous supports. However, these predictions have until now not been supported by direct experimental evidence.

The team used a technique called chemical vapor deposition to grow sheets of graphene. Single layers of graphene were then placed on thin sheets of porous polycarbonate. Each sheet was designed with pores of a specific size, ranging from 30 nm to 3 μm in diameter.

The team gauged graphene’s sturdiness by concentrating on micromembranes, referring to graphene areas that were suspended over the underlying substrate’s pores, similar to fine meshwire lying over Swiss cheese holes.

The graphene-polycarbonate membranes were placed by the team in the middle of a chamber, into the top half of which argon gas was pumped by using a pressure regulator to control the flow rate and pressure of the gas. The team also measured the gas flow rate in the bottom half of the chamber, pointing out that any increase in the bottom half’s flow rate would in fact indicate that parts of the graphene membrane had failed, or “burst,” from the pressure produced in the top half of the chamber.

It was discovered that graphene, when placed over pores that were 200 nm wide or smaller, was capable of withstanding pressures of 100 bars — almost twice that of pressures commonly encountered in desalination. An increase in the number of micromembranes that remained intact was observed by the team as the size of the underlying pores decreased. Karnik explains that this pore size is vital for determining the sturdiness of graphene.

“Graphene is like a suspension bridge, and the applied pressure is like people standing on that bridge,” Karnik explains. “If five people can stand on a short bridge, that weight, or pressure, is OK. But if the bridge, made with the same rope, is suspended over a larger distance, it experiences more stress, because a greater number of people are standing on it.”

Porous design

“We show graphene can withstand high pressure,” says lead author Luda Wang. “The other part that remains to be shown on large scale is, can it desalinate?”

The question at this point is whether graphene can withstand high pressures while selectively filtering out water from seawater. The group answered this question by first fabricating nanoporous graphene to serve as an extremely simple graphene filter. The team used a technique they had earlier developed to etch nanometer-sized pores in sheets of graphene. These sheets were then exposed to increasing pressures.

In general, the researchers discovered that wrinkles in the graphene had a lot to do with whether micromembranes burst or not, despite the pressure applied. Parts of the porous graphene that lay along wrinkles burst or failed, even at pressures as low as 30 bars, while those that were not wrinkled remained intact at pressures up to 100 bars. When the underlying substrate’s pores are smaller, the micromembranes in the porous graphene are more likely to survive, even in wrinkled regions.

As a whole, this study tells us single-layer graphene has the potential of withstanding extremely high pressures, and that 100 bars is not the limit — it’s comfortable in a sense, as long as the pore sizes on which graphene sits are small enough. Our study provides guidelines on how to design graphene membranes and supports for different applications and ranges of pressures.

Rohit Karnik, Associate Professor, Department of Mechanical Engineering, MIT

Baoxa Mi, an assistant professor of civil and environmental engineering at the University of California at Berkeley, explains that graphene is widely considered to be one of the strongest materials present in the world. The possibility of whether porous graphene is capable of exhibiting similar strength remains to be uncertain till now.  

“This [study] definitely reassures [graphene’s] potential applications in filtration, chemical/pharmaceutical separation, water purification and desalination,” says Mi, who was not involved in the research. “There are more challenges to overcome to really get there, such as creating small uniform pores on the graphene and being able to scale up. If successful, this technology will be a game changer in desalination.”

This research was partially supported by the MIT Energy Initiative and the U.S. Department of Energy.

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