Skip to comments.Wires turn salt water into freshwater
Posted on 06/10/2012 10:10:32 PM PDT by Kevmo
June 8, 2012 by Lisa Zyga
(Phys.org) -- As a rising global population and increasing standard of living drive demand for freshwater, many researchers are developing new techniques to desalinate salt water. Among them is a team of scientists from The Netherlands, who have shown how to transform brackish (moderately salty) water into potable freshwater using just a pair of wires and a small voltage that can be generated by a small solar cell. The simple technique has the potential to be more energy-efficient than other techniques because of the minimal amount of mixing between the treated and untreated water.
(a) Seven pairs of graphite rods/wires are dipped into brackish water. (b) An electrical voltage difference is applied between the anode and cathode wires via copper strips, causing the electrodes to adsorb salt ions. (c) Scanning electron microscopy image of the membrane-electrode assembly. Image credit: S. Porada, et al. ©2012 American Chemical Society
The researchers, led by Maarten Biesheuvel from Wageningen University in Wageningen, The Netherlands, and Wetsus, Centre of Excellence for Sustainable Water Technology in Leeuwarden, The Netherlands, have published their study on water desalination with wires in a recent issue of The Journal of Physical Chemistry Letters.
As the researchers explain in their study, there are two main ways to desalinate salt water. One way is to remove pure water molecules from the salt water, as done in distillation and reverse osmosis, particularly for water with a high salt concentration. The opposite approach is to remove the salt ions from the salt water to obtain freshwater, which is done in deionization and desalination techniques using, among other things, batteries and microbial cells.
Here, the scientists used the second approach, in which they removed positively charged sodium ions and negatively charged chlorine ions from brackish water to produce freshwater. To do this, they designed a device consisting of two thin graphite rods or wires, which are inexpensive and highly conductive. Then they coated the outer surface of the wires with a porous carbon electrode layer so that one wire could act as a cathode and one as an anode. The wires were clamped a small distance apart in a plastic holder, with each wire squeezed against a copper strip.
To activate the electrodes, the researchers dipped seven sets of wire pairs into a container of brackish water and ran electrical wires from the copper strips to an external power source. Upon applying a small voltage difference (1-2 volts) between the two graphite wires of each wire pair, one wire became the cathode and adsorbed the positively charged sodium cations, while the other wire became the anode and adsorbed the negatively charged chlorine anions from the salty water.
(a) Multiple pairs of porous electrode wires adsorb salt ions under an applied voltage. (b) A porous electrode temporarily stores ions as the device is carried to the brine container. (c) After short-circuiting the cells, salt is released in the brine container, and the wires are transferred back to the freshwater container. Image credit: S. Porada, et al. ©2012 American Chemical Society
The ions are temporarily stored inside the nanopores of the carbon electrode coating until the wire pair is manually lifted from the once-treated solution and dipped into another container of waste water, or brine. Then the researchers removed the voltage, which caused the electrodes to release the stored ions into the waste water, increasing its salinity. By repeating this cycle eight times, the researchers measured that the salt concentration of the original brackish water, 20 mM (millimolars), is reduced to about 7 mM. Potable water is considered to have a salinity of less than roughly 15 mM. As Biesheuvel explained, this improvement could be useful for applications involving the treatment of moderately salty water.
The new technique is not so suitable for extremely salty waters, as it is based on removing the salt, and making the remaining water less salty, Biesheuvel told Phys.org, explaining that distillation and reverse osmosis are still superior for desalinating seawater (500 mM salinity and higher). The new technique is more suitable, for example, for groundwater, or for water for consumer applications that needs to be treated to remove so-called hardness ions and make it less saline. These water streams are less saline to start with, say 100 mM or 30 mM. Or this new approach can be of use to treat water in industry to remove ions (salts) that slowly accumulate in the process. In this way there is no need anymore to take in freshwater and/or to dump used water (at high financial penalty).
One of the biggest advantages of the technique is that it avoids inadvertently mixing the brine with the water being treated during the process, which limits the efficiency of other deionization techniques. By using a handheld wire-based device and producing freshwater in a continuous stream, the researchers could split the two types of water in separate containers to avoid mixing. Only a minimal amount of brine, about 0.26 mL per electrode, is transferred between containers, which does limit the degree of desalination but to a lesser extent than other techniques. Another advantage of the new technique is that it has the potential to be less expensive than other desalination methods.
This technique can be made very inexpensive, just carbon rods or wires to conduct the electrons, onto which you can simply paint the activated carbon slurry, which becomes the porous carbon electrode, Biesheuvel said. Because of its simplicity and low cost, it might out-compete state-of-the-art technologies for certain applications, and may also have advantages over the technology called capacitive deionization (CDI or cap-DI), which is beyond the development stage and commercially available. Also, the voltage required is low, just 1.2 V for instance, and DC, perfectly compatible with solar panels. Thus it can be used at off-grid or remote locations.
In addition, Biesheuvel explained that the wire pairs can be used repeatedly without degradation, which could give the device a long lifetime.
In capacitive techniques where the porous carbon electrodes are used to capture ions and release them again (in the so-called electrical double layers, or EDLs, formed in the very small pores inside the carbon), it is well-known that the cycle can be used for thousands or tens of thousands of times (until the experimenter gets tired) without any appreciable decay, he said. For the wires we only went up to six times repeat and found, as expected, no changes. This is in contrast to battery-style techniques, either for energy storage or desalination, where one would expect to lose performance (like rechargeable batteries, which can only be charged, say, 100 times successfully). That is because in those techniques there is real chemistry going on, phase changes, change of the micromorphology of the anode/cathode materials. Here, in the wire desalination technology, nothing of that kind, the EDL is a purely physical phenomenon where ions are stored close to the charged carbon in the nanopores under the action of the applied voltage, and later released again.
The researchers also found that the efficiency could be improved by adding a second membrane coating to the electrodes. For instance, a cationic membrane on the cathode wire has a high selectivity toward sodium cations while blocking the desorption of chlorine anions from within the electrode region. As a result, cationic (and, on the anode wire, anionic) membranes could enable the electrodes to adsorb and remove more ions than before.
In the future, the researchers plan to perform additional experiments using the cationic and anionic membranes. They predict that these improvements could increase the desalination factor from 3 to 4 after eight cycles, with 80% of the water being recovered (i.e., 20% of the original water becomes brine). The researchers also want to use the technique to treat large volumes of water, which they say could be done by using many wire pairs in parallel to accelerate the desalination process.
This research continues by scaling up the technology (testing larger arrays of wires), packing them more closely, and trying our hand on automation to have the rods lifted automatically from one water stream into another, Biesheuvel said. We also want to test real ground/surface waters, not only artificial simple salt mixtures as tested now.
More information: S. Porada, et al. Water Desalination with Wires. The Journal of Physical Chemistry Letters. DOI: 10.1021/jz3005514
Journal reference:Journal of Physical Chemistry Letters
Yeah, but I can make an electric pickle.
Yes, but what about the fish? The enviros want to know!
I wonder how they will tax and control this? I am sure the NY Slimes will have an article soon saying that we are going to run out of saltwater soon. /s
As a rising global population and increasing standard of living drive demand for freshwater......the only reason anyone reports about global thirst is to push the AGW agenda, rather than to actually save lives.
Why is electricity needed to get the activated carbon to absorb the Na+ and Cl- ions? Why can’t the carbon do that of its lonesome? Also how quick does the carbon load up and can it be electrically purged of the same ions into salt water?
Why is it when a lib discovers a freshman year chemistry experiment it is hyped as the discovery of the century?
It seems to me that you could use some highly reflective material developed by people smarter than me to push water from a liquid to a global warming gas and then just pipe it into a drinking well when it condenses.
The salt could be used for McDonald’s fries.
If there is a coast line or a source of gravity and water, you could use that energy to generate the power necessary to run the entire thing.
You might even be able to grow algae while you are at it.
Am I stupid or is this a simple process?
The sentence you quoted was talking about increasing population and standard of living. Neither of those mentions global warming. If large quantities of the oceans were to be converted to fresh water, we’d have deserts blooming, many more plants & food growing, converting CO2. Basically global warming would be a quirky thing of the past, like our ancestors worrying about not getting enough calories to survive winter.
On the same thoughts....how expensive are the electrodes to replace.....eventually they will need to be.
Don’t listen to the man
cause he’s filled the burning sand
I loved that as a kid and it was on a blue 45 rpm. too.
Drove my poor mother nuts.
The only problem with all that is there may be some
reason for all that salt water, we go messing with
the balance and who knows what would happen.
I was thinking the same thing.
Salt’s already banned in NYC, so it’s a cakewalk from there on in.
I find the carbon nanotube filtration more intriguing. The pores are literally too small for anything larger than water molecules to pass through. So it can remove salt from water, but it can also filter out microorganisms, poisons, metals, etc. Water flows through it 10,000 times more easily than through existing reverse osmosis membranes which require a lot of energy to force water through them.
They’ve been using membranes to convert salt water into fresh water for several decades. All these guys did was make the membrane into a tube. for convenience.
I hope the patent office wasn’t stupid enough to give them a patent on stuff that’s been around for decades.
I think you're giving the discovery too much credit. Middle School seems more likely to me. The article uses the word "could" 19 times. A REAL scientific article would use the word "did". The article also talks of electricity only using the word "volt". The critical word is "power". Voltage is irrelevant, if it takes too much power to run the operation.
Can someone help me with a half-remembered factoid?
It was many years ago and I was speed reading, and it may have been just speculative alternative history trash. But I seem to recall that there was a very low-tech way of desalinizing at sea. It had to do with dragging a small container of water alongside the ship, and the item was either made of a certain material or was designed in a certain way, and the action of passing through the sea in a moving ship was a factor.
Never could find it again but I’m positive I read it somewhere.