Carbon Nanotube Use #51b: Beating Biology’s Water Filters

Your Random Science News Story From the Month of August

Thanks to the number of ways carbon can bond with itself, a solid ‘lump’ of the element can come in a variety of forms. You’ll be most familiar with two of these “allotropes” – diamond and graphite. While my favorite might just be buckminsterfullerenes (aka “buckyballs“), where 60 carbon atoms are arranged in a soccer-ball shape, graphene is probably the one that gets the most headlines.

600px-Graphen
Graphene is basically a single layer of graphite. In fact, a really MacGyver way of making graphene is to take a strip of Scotch tape and stick it to a piece of graphite, and then remove it. The bonds between each carbon layer in the graphite molecule are so weak that tape can just remove the top one, giving you graphene. Note that this is not a cost-effective method of large-scale production. Credit: AlexanderAlUS, CC BY-SA 3.0.

If you roll up a narrow layer of graphene so it sticks to itself, you’ve got the subject of this month’s news story: carbon nanotubes.

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CNT are at least 100 times stronger than steel despite being 1/6th the weight. Theoretically we could build a space elevator with these things. Credit: AJC1, CC BY-SA 2.0

The paper‘s published behind a paywall in Science. But you can read it here, thanks to one of the authors…

What happened?

Researchers from Northeastern, Lawrence Livermore National Laboratory, and UC Merced tested the feasibility of using carbon nanotubes (CNTs) to filter water. Their research suggests narrow CNTs might even be more efficient than the equivalent mechanisms existing in biological cells.

They come in a range of sizes, but the ‘tube’ part of a CNT is roughly 1 nanometer wide (one-billionth of a meter, or 50,000 times thinner than a human hair), whereas the diameter of a single water molecule is about 0.3 nanometers, so for narrow enough CNTs, water molecules traverse the tube in a single file line. Why is this important? Water molecules love sticking together, which makes it harder to filter out impurities that could hide within that bulk. It also causes the water to move more slowly through traditional filters.1

In nature, cell walls have proteins called aquaporins in order to transport water through their membranes and filter out unwanted charged particles. They’re just wider than a water molecule’s diameter.

So why not try and use aquaporins in an artificial water system? They’re too fragile. CNTs are, on the other hand, are probably most known for their incredible strength given their size. They’re also cheaper to create.

The researchers created two sizes of CNT (0.8 and 1.5 nm wide) and embedded a set of either in a spherical lipid bilayer – in science speak, we’d say they made a “vesicle” – to create a filtering membrane. Instead of testing these vesicles with any variety of salt, the researchers used a fluorescent molecule called pyranine2 to see how effectively water molecules could flow through the CNTs. With some concentration of pyranine outside the vesicle, and none inside, water molecules were forced to traverse the membrane to balance the different concentrations out. In science speak, this is known as “osmotic pressure”.

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Figure 1a from Tunuguntla et al. 2017. Note that the carbon nanotubes and the lipid bilayer are not to scale. The non-water molecules on the outside (i.e. the turquoise/orange ones) are pyranine

To test actual desalination effectiveness – that is, how the CNTs will handle the transport of ions – they also tested how individual CNTs responded to a KCl (Potassium Chloride) solution. The CNT was inserted into a 200 nm patch of lipid bilayer, as opposed to the spherical vesicle above.

What’d they find?

As one would suspect, the more CNTs embedded in the membrane, the faster the water was able to pass from inside the vesicle to outside. The 0.8 nm CNT were shown to filter water 6 times faster than aquaporins, and 11 times faster than the 1.5 nm CNTs (Computer simulations showed that the wider CNTs did not force the water molecules to filter through in that desired single file line).3

Unlike aquaporins, CNT are negatively charged. So during desalination testing, they repelled the similarly-charged chloride ions, while letting neutral water molecules and positive potassium ions through. Aquaporins will filter both both positive and negative ions, which slows travel down. It’s also slower because the insides of the aquaporins aren’t smooth like CNTs are, so water molecules much more readily interact with aquaproin walls.

For the 0.8 nm CNTs, this ion selectivity works until you hit really high salt concentrations, unlike their wider counterparts. So not only do they work faster than what biology’s managed to come up with, they work better.

The researchers also found that by making the pH of the KCl solutions on either side of the membrane different (specifically, 3.0 and 7.5),4 and then switch sides, they were able to switch the direction that the potassium ions flowed. They liken this ability to that of a diode, and suggest potential applications in “dynamically reconfigurable ionic circuits”.

So what?

Only 0.007% of the water on Earth is actually drinkable, so as our global population continues to rise we really do have to increase that value. While there are chemical reactions you can do to make water (say, combusting hydrogen and oxygen gas) attentions are best turned to cleaning the water we already have.

This research is more a proof of concept than anything else – it’s not ready for commercial application. We won’t be mass producing CNT-laden filters for turning ocean water into something drinkable just yet. The CNTs have to actually be tested with something other than a fluorescent dye or one random ionic salt that doesn’t even make up the bulk of the salt in our oceans, and research has to demonstrate they can withstand the higher pressures required for desalinating actual seawater.

But the results suggest they are heading in the right direction. The labs involved in this work have partnered with an Israel-based purification organization, and they’re not the only ones working on this kind of solution, so this certainly isn’t the last time we hear about carbon nanotubes and water filtration.

It’s good to know that CNTs are useful for things other than space elevators that will probably never get built and the paint that’s so black anything you coat with it looks like a hole.

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From Lawrence Livermore: “An artist’s depiction of the promise of carbon nanotube porins for desalination. The image depicts a stylized carbon nanotube pipe that delivers clean desalinated water from the ocean to a kitchen tap.” Credit Ryan Chen/LLNL CC BY-NC-SA 4.0

1. Consider you’re walking somewhere with a large group of friends in a crowded environment (Say, Old Town Edinburgh during the month of August); you travel more slowly if you all try to stick closely to one another instead of just giving up and follow one after another to weave around all the lost tourists.
2. You’ll probably know it best from its presence in yellow highlighters.
3. Also, they found out that the presence of urea increased the flow rate (Glucose drastically dropped it). This does not mean that you should pee in your salt water to make it filter faster. 
4. The pH of a solution is determined by the concentration of protons – i.e. hydrogen atoms lacking their one electron – in the water. These hydrogens are also found bonded to water molecules, so they’re not necessarily free-floating.

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