Graphene-based sieve turns seawater into drinking water

[lw395]

Watermakers run at around 40-45bar?
OSmotic pressure of seawater is around 28bar?
So there is a possibility of using less energy, but no more than around 30%.

A graphene RO device was demosntrated two years ago.
This is different it's Graphene Oxide.

 

[agurney]

- and it's not a ruddy sieve It's a semi-permeable membrane, which is a different thing altogether.

It could have been dumbed down, but the scientist who was interviewed on the beeb called it a sieve that would let water molecules through, but nothing larger, such as salts.

This article in Nature gives the detail on the "tunable sieving of ions using graphene oxidemembranes".

The statement "..membranes were then trimmed off to make sure that all the nanochannels were open.." suggests (to me as a lay person) that it resembles a very fine honeycomb filter.

I suppose my kitchen sieve is a semi-permeable membrane, in that it only lets the runny stuff through.

 

[savageseadog]

- and it's not a ruddy sieve It's a semi-permeable membrane, which is a different thing altogether.

It seems a reasonable enough analogy for lay people

 

[Blue Sunray]

You could try reading the article before commenting.

+1

 

[alant]

Why would the pressure be low unless the membrane is very thin i.e. 1 or 2 molecules thick in which case it would be remarkably fragile.

Its the hole size in any membrane, regardless of material used, which determines how much pressure is needed to force X amount of water through.

 

[alant]

- and it's not a ruddy sieve It's a semi-permeable membrane, which is a different thing altogether.

No it isnt.

 

[alant]

Well enlighten us. Give us some examples of semi permeable membranes in nature performing reverse osmosis.

If you put too much fertiliser on growing potatoes, the water inside will perform a reverse osmosis to dilute the fertiliser, causing the potato to shrink.

 

[Motor_Sailor]

If you put too much fertiliser on growing potatoes, the water inside will perform a reverse osmosis to dilute the fertiliser, causing the potato to shrink.

That's osmosis. Water is passing from the less concentrated solution (inside the potatoe) to the more concentrated solution outside.

 

[macd]

That's osmosis. Water is passing from the less concentrated solution (inside the potatoe) to the more concentrated solution outside.

Precisely. Just as a semi-p membrane between fresh water and salt water will pass water from the fresh to the salt side, until either the concentrations were balanced or all the fresh water had gone. This wouldn't exactly give you more drinking water. Reverse osmosis does the opposite.

 

[elton]

It's not a sieve because a sieve uses gravity or pressure whereas osmosis does not. Osmosis can just as easily occur in zero gravity.

 

[Motor_Sailor]

It's not a sieve because a sieve uses gravity or pressure whereas osmosis does not. Osmosis can just as easily occur in zero gravity.

Spurious straw man argument. Only you describe this as osmosis despite that being no help desalinating water. The scientists behind the invention describe it as a 'sieve' to illustrate the fact that it removes the salt without the need for high pressures as with reverse osmosis.

 

[alant]

That's osmosis. Water is passing from the less concentrated solution (inside the potatoe) to the more concentrated solution outside.

But normal osmosis would be the other way, thats how potatoes grow.

 

[macd]

I've no idea whether it qualifies as a sieve or not, but I imagine most applications for it will be in places with gravity. Equally, pressure is easily applied, even in the Third World. All you need is a hill and some pipe. Whether it would be practicable to raise sufficient pressure by such a means remains to be seen.

 

[Fr J Hackett]

Its the hole size in any membrane, regardless of material used, which determines how much pressure is needed to force X amount of water through.

You haven't read the article have you

 

[Daydream believer]

It's not a sieve

I am not interested in potatoes unless boiled, roasted, or fried into chips so got a bit bored with the the bit about osmosis.
However, I suggest that you had better tell the scientists who have designed it, because they refer to it as a sieve & even use the term "sieving out" in the article.
I would further suggest that pressure required to push a liquid through a sieve has nothing to do with the definition of sieve. It has holes in it to separate materials of differing dimension
The article make interesting reading.
The breakthrough in the design is how the membrane is manufactured & then coated to prevent swelling in water which would clog the holes.
What they do not state is how the extracted salts will be removed to prevent the sieve becoming clogged very quickly

 

[Motor_Sailor]

But normal osmosis would be the other way, thats how potatoes grow.

Indeed, because the solution is stronger inside the potato than the water (plus dissolved salts) that normally surround it in the soil. However if you make the solution stronger outside the potato by over dosing with fertilizer, the process is still normal osmosis, albeit now going in the reverse direction. This is very distinct from 'reverse osmosis where one has to get the water to flow against the osmotic head, that is from the stronger solution (salt water) to the less strong solution (the desalinated water). This requires a huge energy input in the form of pressure.

 

[alant]

You haven't read the article have you

What has that got to do with common sense?
The bigger the hole, the less pressure needed to force water through, than if the hole is smaller.
What changes just by reading something?

 

[alant]

I am not interested in potatoes unless boiled, roasted, or fried into chips so got a bit bored with the the bit about osmosis.
However, I suggest that you had better tell the scientists who have designed it, because they refer to it as a sieve & even use the term "sieving out" in the article.
I would further suggest that pressure required to push a liquid through a sieve has nothing to do with the definition of sieve. It has holes in it to separate materials of differing dimension
The article make interesting reading.
The breakthrough in the design is how the membrane is manufactured & then coated to prevent swelling in water which would clog the holes.
What they do not state is how the extracted salts will be removed to prevent the sieve becoming clogged very quickly

The report on BBC TV, was the exact opposite of your statement. They said that the graphene holes were getting larger, so were coated to prevent that.

 

[Blue Sunray]

The report on BBC TV, was the exact opposite of your statement. They said that the graphene holes were getting larger, so were coated to prevent that.

Leaving aside the obvious disconnect between his point and your reply. The standard of science reporting on the BBC News being what it is I'd almost put more trust in your postings or those of the Daily Mail that in any BBC News (as opposed to a coupe of specialist programmes) report on something scientific or technological.

 

[Daydream believer]

The report on BBC TV, was the exact opposite of your statement. They said that the graphene holes were getting larger, so were coated to prevent that.

Extract from the article:-
Previous work had shown that graphene oxide membranes became slightly swollen when immersed in water, allowing smaller salts to flow through the pores along with water molecules.

Sorry you are correct. The water causes the product to swell as i said, but I incorrectly stated that this caused the holes to clog. I was sure that the person describing the process said that the swelling closed the holes not opened them, which if you think about it would seem correct. However, your quote would seem the correct one, although one might debate the theory behind that