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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."
Graphene membranes are highly durable. The main problem would be clearing the inlet side of the filter from the buildup of blocked particles.
Prevous Slashdot article here: http://science.slashdot.org/story/12/01/27/1354240/graphene-membranes-superpermeable-to-water
http://www.manchester.ac.uk/research/news/display/?id=7895
http://arxiv.org/ftp/arxiv/papers/1112/1112.3488.pdf
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?
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
http://www.wateronline.com/doc.mvc/nanoporous-graphene-could-outperform-best-commercial-water-desalination-techniques-0001
Article linked from summary links to the above article as a source.
A fool throws a stone into a well and a thousand sages can not remove it.
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!
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.
Actually, you don't really get many NaCl molecules in water (until you reach saturation, but then they drop to the bottom) but Na+ and Cl- ions surrounded by water molecules. As such, individual water molecules can go through the right-size holes while water surrounded ions can't (since they would have to "let go" of the water molecules surrounding them).
Graphite is a mixture of all kinds of carbon molecules including buckyballs, carbon nanotubes and graphene. You can eat fistfuls of graphite without serious problems. It's not great for your lungs if inhaled, of course, but getting some in your drinking water isn't going to hurt you.
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 !