The folks over at Oak Ridge National Laboratory are hot on the trail of a new graphene desalination membrane, which could free up vast amounts of the world’s water resources for human use. Currently, according to the lab, more than 99% of the world’s water is undrinkable, much of that being locked up in seawater
Somewhat ironically the whole thing is based on methane, the chief component in natural gas. Those of you familiar with natural gas fracking issues might be giving it the stinkeye on that account, but let’s take a look and see what they’re up to.
The Desalination Conundrum
Conventional desalination involves a process called reverse osmosis, in which water is forced through a membrane.
Despite recent improvements, though, reverse osmosis still sucks up huge amounts of energy, and part of the problem is the membrane. Conventional membranes are based on polymers (plastics). They tend to get clogged up during the process, and they have to be cleaned regularly in order to keep operating at their personal best.
One emerging solution is solar-powered desalination. Renewable energy helps to reduce dependence on fossil fuels, but it doesn’t address the membrane issue. In an increasingly crowded world, energy efficiency is a critical factor, regardless of whether you’re using fossil fuels or renewables.
An Energy Efficient Graphene Desalination Membrane
That’s where the graphene comes in. And the methane, too.
The new Oak Ridge graphene research is still in the proof of concept stage, but things look promising. The idea is to replace conventional polymer membranes with graphene.
For those of you new to the topic, graphene is a relatively new form of carbon, first discovered in 2004. Since then it has engendered thousands of research papers as scientists dig into its unique properties.
Here’s a schematic look at graphene, showing its unique hexagonal structure (the two blue areas show the chemical bonds of impurities in the graphene sheet):
Graphene is only one atom thick but it is super-strong. A graphene membrane could be made thinner and more porous than a polymer membrane, so you would need less pressure — and therefore less energy — to push water through it.
The problem is how to make the stuff at commercial scale. Graphene is only one atom thick, so fabricating graphene is a delicate task.
The Oak Ridge team also had to figure out how to punch precisely sized holes in a sheet of graphene, large enough to let water molecules through, but too small for salt ions to pass.
Here’s how the lab describes the methane part of the process for making graphene membranes:
To make graphene for the membrane, the researchers flowed methane through a tube furnace at 1,000 degrees C over a copper foil that catalyzed its decomposition into carbon and hydrogen. The chemical vapor deposited carbon atoms that self-assembled into adjoining hexagons to form a sheet one atom thick.
That was the easy part. The next step involved putting the graphene sheet on a chip of silicon nitride, and exposing it to an oxygen plasma in order to force out selected carbon atoms. That left a hole or pore in the sheet.
The team was able to tune the number and size of the pores by varying the length of time that the carbon sheet was exposed to the plasma.
That’s a whole story in itself. To calculate the most effective pore size, the team went over to a shared science user facility at Oak Ridge called the Center for Nanophase Materials Sciences, and asked to borrow their scanning transmission electron microscopy (STEM) gear.
STEM provided the team with an atom-scale image of their graphene sheet, which they used to correlate porosity with its transport properties. That enabled them to calculate the optimal pore size, and distribution level, for desalination.
In case you’d like to try this at home, that would be pores in the range of 0.5 – 1 nanometers across, distributed at a rate of one per 100 square nanometers.
The topmost image in this article shows the red graphene membrane stabilized with yellow silicon atoms. The circular figure is an enlargement to show off the honeycomb structure. Ignore the orange areas — those are residual blotches of a polymer.
Just What The World Needs: A Methane Based Graphene Desalination Membrane — No, Really
So far the graphene desalination membrane has passed its tests with flying colors, achieving almost 100 percent rejection of salt ions while allowing water to flow through at a rapid pace.
To ice the cake, according to Oak Ridge the methane-based fabrication method could be scaled up to a commercial level.
That’s not such great news when you factor in the rapid increase in environmental, public health and quality-of-life baggage carried by oil and gas fracking operations. In the US, for example, fracking (short for hydrofracturing) was practically a non-issue when it was confined to thinly populated areas in western regions, but in recent years it has exploded into more heavily populated areas as the result of new shale discoveries.
The use of methane in water purification particularly ironic, given that one of the major issues in natural gas fracking is water contamination from both fracking fluid and fracking wastewater disposal.
On the other hand, when you consider the growth of methane-rich, renewable biogas sources, perhaps some day in the sparkling green future that super-efficient graphene desalination membrane can trace its roots to your friendly neighborhood hog farm.
Image Credits: Courtesy of Oak Ridge National Laboratory.
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