Graphene is the sexy new wonder material that seems to have a million and one uses. From thin flexible screens to solar cells that work when it’s raining, people are finding all sorts of potential applications for this one-atom thick sheet of carbon. Now scientists have added another trick to graphene’s repertoire; making seawater drinkable. Desalination is not a new concept; Israel gets over a quarter of its fresh water from the mediterranean sea.
But graphene could make the whole process much more efficient. The main technique used for large scale desalination is called reverse osmosis. Normal osmosis is where water flows across a semipermeable membrane to areas of higher saltiness, if you were one of those kids that poured salt on snails, causing the snail to shrivel up as the water leaves its body, you’ve seen it in action and also, dude, c’mon. The reverse osmosis used for desalination, however, is when pressure is applied to saltwater to force it in the directions it doesn’t normally flow, through special membranes to areas of lower salinity, separating the H2O and telling the salt Na Na Na Na, hey hey hey, goodbye… Get it because sodium is… NA.
Anyway, this all sounds great but it’s not perfect: water from desalination is very expensive, costing anywhere from $1,000 to $2,500 per acre foot, which is about the amount of water 10 people use in a year. Generally it’s cheaper to just use less water and recycle the water you do use, but for some particularly dry places there’s no other option. Part of the expense comes from pre-treating the water, but pumping the water through the jelly-roll-like plastic membranes uses a lot of energy. Computer models have predicted that if we could use super-thin graphene sheets instead of these thicker membranes, the amount of energy needed for reverse osmosis could be reduced anywhere from 15-46%.
Less energy used for pumping means less expensive water, and that could make a huge difference to people in the world’s poorer and drier places. Unfortunately reality doesn’t always cooperate with computer models, and making graphene that can separate water from salt isn’t as straightforward as it appears. Researchers found that when their graphene oxide membranes were submerged in water, they swole a little bit. They would still filter nanoparticles, organic molecules, and even larger salts but not the common salts found in seawater, which are dangerous to drink because they just lead to more dehydration.
That’s normal osmosis at work again. But now new findings from the same group of researchers have figured out how to prevent the swelling and control the pore size. Now the dissolved salts and the water molecules they bind to can’t get through the pores, but free water molecules can, and do so surprisingly fast, which is a good thing when you’re trying to make fresh water as quickly as possible. The researchers hope that their discovery can make desalination more affordable, which would be a huge boon to poorer drought stricken countries who can’t afford to build billion-dollar desalination plants. With climate change threatening to make droughts even worse, graphene could do more than just make flexible screens, it could help saves lives.
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