 and hydroxylated (b) graphene pores, and (c) side view of the computational system described in this research.)
GOING WITH THE FLOW: Hydrogenated (a) and hydroxylated (b) graphene pores, and (c) side view of the computational system described in this research.
Image: Courtesy of the Massachusetts Institute of Technology (M.I.T.)
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The earth harbors about 1.4 billion cubic kilometers of water. Unfortunately, the vast majority of that water comes from the sea and is not potable unless treated by expensive, energy-hungry desalination plants. Those problems stem largely from inefficiency in the way salt ions are separated from water molecules, and the solution, says a team of materials scientists from the Massachusetts Institute of Technology, lies in fundamentally revising that process.
The predominant desalination method today—reverse osmosis (RO)—relies on polymer-based membranes to remove salt and requires great pressure to push water through a semipermeable film.The more pressure applied, the higher the cost. The M.I.T. researchers, led by Jeffrey Grossman and David Cohen-Tanugi, propose that films made of graphene could filter out salt without inhibiting the water flow as much. Graphene, a superstrong sheet of carbon that is only one atom thick, has mostly been seen as a material for improving electronics and optical communications.
Reverse osmosis requires less energy than other desalination approaches—such as thermal distillation—but graphene membranes containing nanoscale pores that are more permeable than the polymers currently used would further cut energy requirements, the researchers reported online last month in Nano Letters.
The idea is to discriminate between water molecules and salt ions based on size. "Reverse osmosis uses size exclusion, except it excludes everything," says Grossman, an associate professor of power engineering.
A graphene membrane would provide well-defined channels that allow water molecules to flow through at lower pressures while blocking salt ions, Grossman says.
Using software simulations, the M.I.T. researchers experimented with different pore sizes to desalinate seawater with a salt concentration of 72 grams per liter, about twice the salinity normally found in the ocean. They found that, theoretically at least, pores 0.7 to 0.9 nanometer in diameter were most effective at passing water molecules while blocking sodium ions. "That's the sweet spot," Grossman says. "If it's bigger, salt's going to flow through. If it's smaller, nothing flows through."
Grossman and his team are trying to determine whether chemical reactions might be used to tweak desalination performance. The researchers programmed their digital graphene pores to be coated with either hydrophobic (water-repelling) or hydrophilic (water-loving) atoms. The former slowed the flow but cut down on the salt ions passing through, while the latter allowed faster flow but blocked fewer salt ions. The type of coating may ultimately depend on conditions at a given facility. Still, the scientists report, simulations indicate that graphene nanopores could reject salt ions with a water permeability two-to-three orders of magnitude higher than RO membranes.
Of course, working with graphene in reality is more challenging than filtering pixilated salt from digital water molecules on a computer. For starters, although chemical etching and ion beams can be used to create holes in graphene, it is difficult to produce holes of a specific size in an even configuration, Grossman acknowledges. Nor does graphene eliminate the quandary of how much leftover brine can be safely returned to the ocean without hurting underwater habitats. Toxicity could also be an important issue, he says, "although there are no real answers right now in terms of [graphene's] potential impact on [the safety of] drinking water."
Grossman does not know when graphene-based desalination might be ready for commercial use. He and his team, though, continue to run simulations and have begun testing actual membranes in the lab to study flow rates and salinity.
Demand for potable water is expected to escalate worldwide in the coming years. Grossman says the key to meeting that need is not necessarily tweaking existing technology. "We looked around at who's working on desalination in the scientific community, and it's mostly mechanical engineers working at the systems level," he says. "Little is being done on the system design side using basic science and working from the bottom up."
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19 Comments
Add CommentI love graphene, I really do. We really need to figure out a way to get its cost down and produce it at a large scale. However until then, another way that may yield lower cost desalinization is by igniting salt water with radio waves; which break the water into its components, allowing the resulting freed hydrogen and oxygen to catch fire. Now as an energy source salt water is not useful since it takes more energy to ignite than what you get in return. However as a new desalinization process, with the side effect of the burning, could be used to recapture some of the energy, which make it a much more cost effective system than what we currently use.
Reply | Report Abuse | Link to this"Little is being done on the system design side using basic science and working from the bottom up."
Reply | Report Abuse | Link to thisseriously? why has this not been a major priority, given so many areas of the world suffer from fresh water shortages?
"simulations indicate that graphene nanopores could reject salt ions with a water permeability two-to-three orders of magnitude higher than RO membranes."
if this means that the efficiency of fresh water production might be improved by a factor between 100 and 1000 times(?), it would seem possible that such a technology may have the potential to transform areas of the earth that are currently desert into agriculturally productive land. this would be no small thing.
I wonder if it'd be worthwhile chemically modifying the graphene pores based on models of the inside of aquaporin water channels in cell membranes? Maybe, the pattern of the amino acids in these channels could be mimicked in the graphene nanopores? Just an idea. Thanks!
Reply | Report Abuse | Link to thisThe thermodynamic limit for efficiency is set by the concentration gradient and the work (osmotic pressure) necessary to offset that gradient. I doubt that RO is less than 1% efficient, so I don't see how the purported efficiency increases will materialize.
Reply | Report Abuse | Link to thisHenles loop, the part of the kidney that determines the salt in- and output from blood, is ubiquitous in mammals. It works at zero pressure, and has a high efficiency. The concentration of salt in the effluent is dependent upon a hormone called anti-diruetic hormome, or ADH. If one were to think not just outside of the box, but throwing the box away, one could imagine a Henles loop at a grand scale with a high concentration of ADH producing pure water from highly "polluted" (relative to pure water) liquids.
Reply | Report Abuse | Link to thisSounds like a good idea, but I suspect that the body uses a lot of energy in the kidneys to filter out our waste. I wonder if it as efficient as RO. Does anyone know more?
Reply | Report Abuse | Link to thisThe author mentions the problem of high-salt brine being returned to the original body of water. I would think that any chemist would love to have high-salt brine as a raw material. Pump it out over a large area and dry it. As it dries you can pump off different materials as others crystallize out. There are a lot of valuable resources in salt water. Just ask the Jordanians and the Israelis about their mining of the Dead Sea.
Reply | Report Abuse | Link to thisSounds cool...I wonder how they plan on keeping the "pores" from getting clogged..
Reply | Report Abuse | Link to this"The idea is to discriminate between water molecules and salt ions based on size."
Reply | Report Abuse | Link to thisWhy not based on net electrical charge, as with moving metal ions in electroplating? Charge seems like a more distinguishing feature. How about insulated grids with alternating plus and minus charges? In any case, it seems the ideal would be for water to flow horizontally and the plane of "filter" surface would be slanted relative to the water flow, so that salt would be deflected to the bottom, where its higher density would keep it.
And yes, any such process should be the first step in "mining" salt water.
If graphene gets into the water, and turns out to be harmful in the kidneys or liver, how do you filter it out of the water?
Reply | Report Abuse | Link to thisCause the is the kind of question I see skipped a little too often.
Can someone please tell me, when I check the box to be notified when someone responds, why I should get, now TWO, notifications of my own post?
Reply | Report Abuse | Link to thisGentlemen,
Reply | Report Abuse | Link to thisYou cannot say "a majority of water," since a majority means counting indivduals to make a total. You could, if I am not too obscure, however, say a "majority of waters," if you were counting different types of waters in bottles, say.
Now, more, don't Ihave the iumpression that this technique was pioneered in Israel a while ago?
Thats a good idea although you would be wasting a lot of water. The energy transfer would have to be using an extremely significant amount of water just to treat the rest. I like it but your talking about using more resource than what resource you get in return.
Reply | Report Abuse | Link to thisI do not know the energy use of the kidneys, but the level is very modest. The concept is based on an enzyme system that has a potassium/sodium pump as it's driver, with ADP/ATP oxegenation as its power source. Any larger scale operationalization of the process would require an array of such pumps in a large scale system. If this could be devised at all, the energy question would probably be a non issue.
Reply | Report Abuse | Link to thisAt low pressure, RO membranes are quite efficient, since less energy is wasted. Spain already gets 20 percent of its water from salt water using RO membranes.
Reply | Report Abuse | Link to this@Quinn the Eskimo - Very good question!
Reply | Report Abuse | Link to thisAdditional question in response to bucketofsquid and Quinn question....I wonder how much graphine is contained in a grilled hamburger or steak?
Reply | Report Abuse | Link to thisJust out of curiosity, how is the water forced through the R.O. filters? Would there be any benefit to perhaps using a simple gravity feed?
Reply | Report Abuse | Link to thisI'm pretty sure they use a prefiltering method to eliminate critters and plants. Would it be cost effective to add a solar evaporation/re-condensation component to remove the heavier salts?
Well water in my area isn't very good quality due to agro run off and salt levels. I've been toying with ideas for a few different options for removing unwanted things from the water.
A wide shallow pool with a corrugated glass or clear plastic top would let the sunlight evaporate some of the water. A second layer of half tubes under the low points would gather the recondensed water to move it to the next phase. This would be of no use in winter but could still accomplish a bit over the summer.
The pool would also allow heavy materials to settle on the bottom. These could be removed weekly via squeegee. Simple screens could also allow salts or limestone to attach to them and be cleaned when ever they get sufficiently crusted.
The question is how much would any option cost in ratio to its benefit. My guess is that it may make sense on a small scale but not be even close to viable on a commercial scale.
There is a method for creating large, inexpensive, high strength concrete spheres capable of withstanding pressures greater than 1,000 psi and higher. These concrete spheres are made from manufactured concrete block. These spheres can be sunk to depth (the only real work required) where a portal with a Reverse Osmosis membrane performs desalination. Then the sphere is hoisted to the surface and emptied and the process is repeated. This method comes very close to the thermodynamic limit of energy required for desalination. I have written about this several times on my blog. If anyone has any interest or questions, please contact me. If you look at my other blog entries you will get a better idea, and there are pictures of this masonry system being used in other applications.
Reply | Report Abuse | Link to thishttp://masonrydesign.blogspot.com/2011/09/energy-budget-for-desalination.html