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."

Re:A foul subject.

[Anonymous Coward]

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

It is a RO membrane, just a really good one

[vlm]

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.

It is a RO membrane, just a really good one? They've described exactly how a RO membrane works. Of course this may have more "holes per sq inch" or whatever, maybe even 1000 times as many.

The real link

[OzPeter]

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 [wateronline.com]
 
However that references Nanoporous graphene could outperform best commercial water desalination techniques [phys.org]
 
Now we finally we get to the actual link Water Desalination across Nanoporous Graphene [acs.org] (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?

Re:The real link

[notjustchalk]

Here's the real article (AFAIK) from The Grossman Group @ MIT, no need for credentials.
Water Desalination across Nanoporous Graphene (Warning PDF Link): http://zeppola.mit.edu/pubs/nl3012853.pdf [mit.edu]

The main site for the Grossman Group is also pretty fascinating: http://zeppola.mit.edu/ [mit.edu]

The original paper

[jmichaelg]

Here's a link to the original paper on Grossman's website. [mit.edu]

Of courswe they wouldn't

[Kupfernigk]

"Troll" is nothing to do with fantasy monsters and everything to do with fishing. A troll is someone who floats a line with yummy bait across a discussion board and waits to see who bites, having failed to notice that the grub or worm is on a hook.

These people who didn't discover the Internet till this century...kindly remove yourself from my area of cultivated graminoids.

Re:It is a RO membrane, just a really good one

[qvatch]

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

Re:A foul subject.

[foniksonik]

http://www.wateronline.com/doc.mvc/nanoporous-graphene-could-outperform-best-commercial-water-desalination-techniques-0001 [wateronline.com]

Article linked from summary links to the above article as a source.

Re:A foul subject.

[marcosdumay]

Well, TFA brings absolutely no detail. It won't even let you know it it is about something produced in a lab, some theoretical contruction, or even if nobody has no idea how to create such a filter.

Now, graphene is pretty stable. It probably cloges with time, as other athoms get in the place of carbon, but that is an incredibly slow process. A membrane composed of a single graphene sheet should last more than any other component of your plant.

Ok, all the above is great, and etc. But when you get in the real world, membranes get old because of impurities that accumulate on its porous. A single graphene sheet has nowhere for those impurities to accumulate, if you reverse whash it, all impurities are gone (except for the mechanism at the above paragraph). But no practical membrane is composed of a single graphene sheet, thus, durability will be probably all over the scale depending on the quality of the actual membrane, from trash that can't be used on a lab to as good as ceramic filters.

Re:Um, no.

[caffeinated_bunsen]

I think they're abusing the terminology a bit, using "RO" to refer to reverse osmosis conducted with existing membrane technologies. The point at issue is that thermodynamics demands that a certain amount of energy be expended in order to reduce the entropy of a homogeneous salt solution by separating it into pure (or at least low-salinity) water and high-salinity leftovers. This is totally independent of the means by which the molecules are separated. In reverse osmosis, that manifests as a minimum pressure necessary to force salt water through any selectively permeable membrane.

Practical RO systems operate with a pressure drop (and therefore energy consumption per unit volume) that's double or triple the osmotic pressure, in order to achieve useful flow rates across thick membranes with relatively low pore densities. A better filter would allow that excess pressure to be reduced, but can't do anything about the cost of reducing the entropy.

Re:The original paper

[slug.slug]

Thanks for the link. I skimmed through the paper and realized that this is a *theoretical simulation* of the process using molecular dynamics, so although with promising predictions, please give me a call when they actually have a working device.

Re:Why stop at salt?

[jbeach]

Agreed. People can show even more denial with this than with the Peak Oil problem we're going to be facing. Not the Pentagon; they're busy making plans and releasing public papers that point out the upcoming world shortages in water AND oil. But they're a bunch of pointy-headed eco-socialists apparently.

Re:Uhmm....I sense a problem with scale.

[caffeinated_bunsen]

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.

Re:Could you boost durability by stacking several?

[I_am_Jack]

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.

Re:Holes?

[Anonymous Coward]

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.

Re:A foul subject.

[Nutria]

Pull your head out of your arse and think: water treatment plants of all sorts have been doing that for 120 years using scrapers, settling tanks, sand filters and flocking agents.

Re:A foul subject.

[manicb]

According to the other article [wateronline.com] people are posting, this is based on Molecular Dynamics simulations. MD is a theoretical technique that uses time-dependent Newtonian mechanics. It relies heavily on having good-quality data for the interactions between the atoms, but allows relatively large systems to be modelled. The wikipedia article [wikipedia.org] contains a fair bit of information (probably too much).

TLDR: This is just based on computational modelling. The model is fairly crude, but is a standard technique for this scale of system and the results should be taken seriously.

Re:Holes?

[Anonymous Coward]

The graphene sheet is only a single atom thick; erosion is not possible. Tearing (catastrophic failure) is a problem, however.

Re:A foul subject.

[Woogiemonger]

But *if* they break... what then?

Let's put this to rest. Graphene is one of the strongest materials ever: http://physicsworld.com/cws/article/news/2008/jul/17/graphene-has-record-breaking-strength [physicsworld.com]

Re:A foul subject.

[plover]

Salt dissolved in water isn't just a bunch of wet table-salt-shaped crystals. It's a bunch of individual NaCl molecules floating around. And this filter has holes small enough to pass H2O molecules, but not NaCl molecules. Most other molecules, such as those of uric acid, are much larger than NaCl, and therefore this filter will trap them, too.

It isn't breaking anything down. It's not chemically altering the substances in solution. It's simply a filter that has holes so tiny that only molecules that are three atoms or smaller will pass through them.

Re:A foul subject.

[gomiam]

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).

Don't really need to clean it as much as you think

[theshowmecanuck]

RO is not like using a traditional filter. I'll see if I can explain it quickly without the explanation getting too muddy. The last RO project I worked on was in 1990 (and wasn't for salt, but same principles apply), but I doubt the basic structure of the equipment has changed much. Probably more changes are in the actual membranes.

On an industrial scale membranes are placed in canisters and usually in large banks of them. The way the canister is built is usually a couple of sheets of membrane, sandwiching a substrate that allows a reasonable liquid flow rate through it, the whole is then spiral wound (like a roll of paper towel), or better yet, like film on a film winder that goes into a film development tank for those who remember film cameras and how to develop negatives :). The edges of the substrate and membranes are attached to a framework such that the purified liquid can be collected and channelled out either one or both ends of the spiral assembly when the assembly is inserted into a properly designed tube/canister. You put the wound membrane assembly in the tube that has one inlet and two or three outlets (depending on whether you want the purified liquid outlets at either end or just one). So say we have one feed outlet and one purified outlet. On the inlet side you flow your feed liquid at high pressure. One of the two outlets is your "purified" liquid and the other is an outlet for feed liquid.

Because of the pressure differential between the feed side of the membrane and the substrate side of it, the "pure" liquid will be forced through and then flow through the substrate and the pure liquid outlet (at a much, much lower flow rate than the pressurized feed liquid). On the feed side of the membrane, this results in a slightly higher concentration as it passes the membrane and thus, the feed outlet side has a higher concentration of solute than the inlet. But you are always maintaining a flow across the membrane at high pressure and what you end up with is the slightly higher concentration liquid flowing out the far end from the inlet. Note that the downstream line from the canister is still under pressure.

So you don't really need to backflush to clean it, or not as often as you might think. You always have a flow of material over the surface in low enough concentration to keep the salt in solution. Granted that sometimes they will chain membrane canisters, the outlet from one going into the next. Or they may have a feedback loop that keeps a set (higher) concentration on the outlet. This reduces the inlet flow and increases the concentration of the output, but it also increases the pressure required. Regardless, the membrane is usually kept from clogging from the movement of the feed.

FWIW, in some systems you might want a certain concentration on the outlet to use as feed for another process. You might be able to use it to concentrate sugars, or even the salt we're talking about. The more water you squeeze out, the less you need to evaporate. But in the case of desalination, I can think of cheaper ways to get salt (like mining), but this serves as an example of what can be done.

For maintenance in some operations (like for example, in the food industry), once the system is shut down, they will run cleaners through the system and if it needs to stay shut down for a period, they'll fill the system with purified water (if water is the output they can use that). They might add a bacterial inhibitor so that nothing could possibly grow and build up in the system. If they don't keep the canisters full of liquid they will dry out and usually become useless. And they are quite expensive.

Pure water is not always what is sought after. Lower pressure RO, usually called ultra filtration has various uses. For instance, I saw one project using it in making raspberry juice. Don't ask me what they were doing with it, I just saw it in passing at a food research place. I was seconded to a research institute in a past life to study using RO to purify waste

Re:A foul subject.

[tragedy]

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.

Water Filtration

[Taco Cowboy]

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