Wrinkled graphene sheets for improved water purification
Graphene is a material that is unfolding new surprises for the scientific community with each passing day. It has been offering surprising solutions to social and technological problems since it was first isolated from graphite in 2004.
Researchers from Brown University have demonstrated that water can be purified very efficiently by taking advantage of tiny channels between stacked graphene sheets that can be aligned in a way that makes them perfect for water filtration.
Graphene is a two-dimensional material (a single atom thick layer of carbon) having many useful properties – one of them being its ability to filter water by being used as a membrane allowing the passage of water molecules while trapping impurities.
When graphene sheets are stacked on top of each other, it develops nanochannels in between them that can be utilized for filtration. When water is fed into the stack, it runs through the material along its length with contaminants filtered out along the path.
But the problem with this method is that the liquid has to travel a relatively long way to get from one end of a channel to the other because graphene stacks are thinner in the vertical direction compared to their horizontal length and width. Therefore, it’s not the best possible method for filtration.
The better way would be to have channels perpendicular to the orientation of the graphene sheets. In that case, the liquid would only need to cross the relatively thin vertical height of the stack rather than the significantly longer length and width. But creating desired nanochannels in the vertical orientation has proven to be very difficult.
To overcome the difficulty associated with forming channels in the vertical orientation, the researchers found an intelligent way.
Graphene sheets are stacked onto an elastic substrate that has already been stretched out. By releasing the tension in the substrate, the sheets wrinkle up into sharp peaks and valleys due to the contraction of the substrate.
The team found a way that could form wrinkles with almost vertical channels.
“When you start wrinkling the graphene, you’re tilting the sheets and the channels out of the plane,” said Liu, a former postdoctoral research fellow at Brown University’s Hurt’s Lab and now a researcher at Massachusetts Institute of Technology. “If you wrinkle it a lot, the channels end up being aligned almost vertically.”
Once the channels are nearly vertical, the assembly is encased in epoxy, and the tops and bottoms are trimmed away, making the channels open at either end. The researchers call this assembly as VAGMEs (vertically aligned graphene membranes).
In testing, water vapor was able to pass through the vertical channels easily while a larger molecule, called hexane, was successfully filtered out.
Proof-of-concept testing demonstrated that water vapor could pass easily through a VAGME, while hexane — a larger organic molecule — was filtered out.
The researchers have plans to develop this technology further to make it more suitable for industrial or household filtering applications.
To Wrap Up
A low cost and efficient water treatment and purification solution (both for domestic and industrial applications) is already a need of the market. The current state of the technology could be used by startups or entrepreneurs to validate the business idea.
Graphene Sheets for water purification
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