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Making Saltwater Drinkable With Graphene
An anonymous reader writes "Graphene once again proves that it is quite possibly the most miraculous material known to man, this time by making saltwater drinkable. The process was developed by a group of MIT researchers who realized that graphene allowed for the creation of an incredibly precise sieve. Basically, the regular atomic structure of graphene means that you can create holes of any size, for example the size of a single molecule of water. Using this process scientist can desalinate saltwater 1,000 times faster than the Reverse Osmosis technique."
So how durable is this membrane when it comes to dealing with impurities?
Using this process scientist can desalinate saltwater 1,000 times faster than the Reverse Osmosis technique.
Well isn't that swell for 'scientist', but does scientist plan to share?
How does this filter work on bacteria and viruses? The standard of living in the 3rd world would go up dramatically with free access to clean water.
If they've found a way to desalinate water with much less energy, practically, that's huge.
Bruce Perens.
The TFA is just a BS article that says nothing.
A better link (and is in the TFA) is Nanoporous Graphene Could Outperform Best Commercial Water Desalination Techniques
However that references Nanoporous graphene could outperform best commercial water desalination techniques
Now we finally we get to the actual link Water Desalination across Nanoporous Graphene (which unfortunately you need to have the right credentials to see - which I don't)
How come I can follow those links and the TFS can't?
I am Slashdot. Are you Slashdot as well?
A couple of people have raised this issue, and it relies on a fundamental mis-understanding of how the universe works on a molecular scale.
Suppose that I have my colander and I wash some vegetables in it. Gunk can get stuck in the holes and it has to be washed off, which requires a fair amount of work because I have to break the interaction between the gunk and the surface. That's your macroscopic intuition about how filters and such work.
But your macroscopic intuition will lead you astray in this case. The individual holes in graphene do not work that way; yes, occasionally, molecules of one kind or another will spend some time stuck to the graphene (a useful phenomenon in other circumstances - http://en.wikipedia.org/wiki/High-performance_liquid_chromatography) but, on the scale of atoms, they are effectively in a high-powered washing machine ALL THE TIME.
Can't find quite the movie I want... this'll do:
http://protonsforbreakfast.wordpress.com/2012/04/11/brownian-motion-observed-in-milk/
So you see those oil bubbles wiggling around? Given that amount of constant wiggle, are you worried about having them "stuck" anywhere? That's thermal vibration from being at room temperature. Those milk bubbles are over 1,000 water molecules across, so each of those "wiggles" is 10 or 100 times the size of an individual graphene pore; are you worried about anything another 1000x smaller being "stuck" anywhere? It would be like worrying about gunk stuck in your colander while your colander was sitting in a fire-hose 24/7.
Anyway- to cut to the chase:
obviously you could have you take the graphene and you run the sea water *past* it at high pressure. Occasionally some gunk gets in there but it washes away sooner or later; and nothing spends any appreciable amount of time stuck in an individual graphene hole.
The good and new comes from no quarter where it is looked for, and is always something different from what is expected.
The abstract: "We show that nanometer-scale pores in single-layer freestanding graphene can effectively filter NaCl salt from water. Using classical molecular dynamics, we report the desalination performance of such membranes as a function of pore size, chemical functionalization, and applied pressure. Our results indicate that the membrane’s ability to prevent the salt passage depends critically on pore diameter with adequately sized pores allowing for water flow while blocking ions. Further, an investigation into the role of chemical functional groups bonded to the edges of graphene pores suggests that commonly occurring hydroxyl groups can roughly double the water flux thanks to their hydrophilic character. The increase in water flux comes at the expense of less consistent salt rejection performance, which we attribute to the ability of hydroxyl functional groups to substitute for water molecules in the hydration shell of the ions. Overall, our results indicate that the water permeability of this material is several orders of magnitude higher than conventional reverse osmosis membranes, and that nanoporous graphene may have a valuable role to play for water purification." Emphasis added for why, and the introduced problem
Occasionally some gunk gets in there but it washes away sooner or later; and nothing spends any appreciable amount of time stuck in an individual graphene hole.
She was a real hot-shooter, that bubble. I should have known she'd be trouble from the get go; she was naturally "charged" as they say when they're trying to be polite.
With her bouncing around all over the place even at room temperature, I guess I should have seen it coming. But, as will happen to palookas and wishful thinkers, my hopes and processes got the best of me. I was convinced that any trouble would wash away as soon as it cropped She didn't even say goodbye, just left a note saying she'd thought she had found a solution with me, but couldn't stand the suspension and was afraid of becoming just another precipitate.
That was three years ago. I took the tube directly to this here graphene hole; it was the closest one I could find. I've been stuck here ever since.
That's not as big a problem as you'd think. In solution, you don't have molecules of NaCl; you have dissociated ions of Na+ and Cl-, each of which is surrounded by a cluster of rather tightly-bound water molecules. Those clusters are much larger than bare ions or single water molecules, so there's a fair range of pore sizes that will separate the ions from the water.
Bugrit! Millenium hand and shrimp!
Figure 8 on Page 6 of the actual paper shows what they're measuring. They're comparing filter materials by Salt rejection % vs Water permeability measured in L/cm2/day/MPa. That unit incorporates all the energy-efficeny goodness you want in a filter without looking at what pump technology is actually used to provide the energy input. It says that more filtered water (L) per square centimeter of filter (/cm2) per day (/day) per MegaPascal of pressure (/MPa, the energy input) is more good. Assuming any particular pump technology would give you a number for MPa/MJ that you could apply, but it doesn't help you understadn the performance of the filter itself. The figure for improvement vs existing technology they actually give is 2-3 orders of magnitude (100-1000x) so TFS is taking the optimistic side.
The bottom line is that this has a huge potential but is still a ways from practical application.
You can purify water with activated carbon ("purify" is highly subjective, unless a governmental authority has taken the time to define it; otherwise, it's up to the marketing department). If you want to remove chlorine and objectionable tastes and odors, a simple activated carbon cartridge works great. If you want to remove heavier VOC's (volatile organic compounds) and THM's (trihalomethanes), you can use a compressed carbon block. And you can use a 1 micron absolute carbon block if you want to do all of the above, as well as achieve five log reduction (99.999%) in Giardia and Cryptosporidium cysts, as well as removing 95% of lead in water (most lead found in water is particulate and not ionic).
Desalinating is a little more complicated than this. Currently, there are three (fairly simple) methods of desalinating water: reverse osmosis, steam (or vapor compression) distillation, and de-ionization. RO is usually the preferred method, because a commercial RO unit can purify a high volume of sea water at around 70-90% efficiency.
Steam or vapor compression distillation requires a lot of energy, leaves a massive amount of residue, and depending on mineral concentrations of the feed water, requires constant cleaning to prevent the equipment breaking down.
De-ionization requires no energy, but depending on the type of DI resins used, can quickly exhaust the filter bed, requiring regeneration, which again, doesn't require a lot of energy, but it does have a chemical cost to strip and regenerate the Cation/Anion resins.
Regardless of which method of desalination is being used, the feed water should be filtered to remove sediment and volatile organics (or post-filtration, in the case of DI).
The graphene method is essentially creating a thin film membrane like RO. If you jump past the original article, and go to Water Online, the method proposed would be actually be using a thin film scaffolding to support the nano layer of graphene. At that point, you might as well use RO, unless the actual production models (the graphene method proposed is still highly theoretical as the authors admit that consistently producing graphene with a uniform pore diameter is not practical yet) would allow greater pure water production at higher efficiencies than currently available with RO.
If you want to make ultra-pure water (say USP water-for-injection grade) you need to use a combination of all the above. What results you want will determine the method or number of steps required.
Oh please. For one thing, we already have desalination plants in some places dumping brine back into the sea; obviously it's not a big problem. There's a lot of water in the oceans.
(trying really hard to not be snipe-y or sarcastic here) :)
Actually, dealing with the by-products of plants operations, which are not limited to the 'brine', are a big problem. Older plants create deadzones. Newer plants do better at defusing the saline concentrations, but that's still only one consideration. Check out the Wikipedia page on Desalination to actually learn something.
http://en.wikipedia.org/wiki/Desalination
Also, if you want to convert desalination outflow to usable table salt you have to clean it first. Economically undesirable in most cases. (But not all)
Desalination, as a solution to fresh water needs, is expenSive, complicated and (generally) damaging. It is a "big problem". However, societies generally overlook big problems when they find a way to get things that they want (more). See: fracking.
When thinking of water filtration, a lot of you automatically conjure up a mental picture of a conventional water filter -- ie, dirty water poured from the top, and impurities get trapped in between, and clean clear water drips out from the bottom
In large scale water filtration operation, that traditional top-down model does not work
Instead, raw water is pumped into the inner tube of a double-layered pipe, which is slanted upwards, at a 30-60 degree angle
Sections of wall of the inner tube are made up of filtering membrane - such as Graphene
As the raw water flows upstream , and because of the smaller diameter of the inner tube , pressure building up inside the inner tube of the double layered pipe.
Because of the higher pressure inside the inner tube, molecules of clean water flows out of the inner tube, through Graphene (or other filtration membrane), into the larger pipe on the outer layer of the double-layered pipe
And because the pipe is slanting upward, gravity causes the filtered (clean) water in the outer pipe to flow down and eventually it gathers at a collecting point (usually a tank, or a pool) at the bottom
At the top of the double-layered pipe, there is an opening for the inner-pipe for the impure-water to exit
Because of the outlet, there is no need to do any "back flushing" since impurities, including salt, are continuously being flushed away
Hope this helps
Muchas Gracias, Señor Edward Snowden !