Wires turn salt water into freshwater

Jun 08, 2012 by Lisa Zyga feature
(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

(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.

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 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 from the salt water, as done in distillation and reverse osmosis, particularly for water with a high . The opposite approach is to remove the from the salt water to obtain freshwater, which is done in deionization and desalination techniques using, among other things, batteries and .

Here, the scientists used the second approach, in which they removed positively charged and negatively charged 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 .

“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.”

Explore further: Chemists create nanofibers using unprecedented new method

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 search and more info website

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User comments : 13

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TopherTO
5 / 5 (3) Jun 08, 2012
"Water, water everywhere so let's all have a drink"

I wait for the day Homer's brilliance becomes reality.
Mike_Massen
1.6 / 5 (7) Jun 08, 2012
This process could be made very cheaply and used in places where there is little power but need for drinking water or partially desalinated water as aid to industry etc.
Thadieus
1 / 5 (1) Jun 08, 2012
It would great if it could turn water into wine. That would be a miracle!
Mike_Massen
2.6 / 5 (10) Jun 08, 2012
Thadieus got caught in ye olde spectacle
It would great if it could turn water into wine. That would be a miracle!
Think that trick's been done.

Get opaque red wine bottle (R)
Get red wine, let it dry as paste
Coat inside of R with the paste, dry
Push sponge into R
Soak sponge with some alcohol.

Now for show:-

Hold up a flask of water,
Hold up R and upturn it, nothing flows out.

Put water into R to about 20% from top and make some
yabba dabba do spirit a shaken dance with R.

Place R down and let it settle...

Make some more yabba dabba up to sky whilst speaking various deities names...

Pour R into glass, give someone to taste :-)

Hey like Wow - how did you do that Man ? Now I can get drunk too !

And I know some religious people who won't touch wine or alcohol, does that mean their deity might think they are against Jesus - he used to drink, who knows how much though (shakes head)?

No miracle as such, all based on nature, with great entertainment potential :-)

*grin*

Sean_W
1 / 5 (4) Jun 08, 2012
If we found a complementary process which can reduce sea water to brackish levels of salinity (which would seem useless on its own) and do it cheaply and efficiently, we could put the two together. After that, the only issue is how far up hill can we afford to transport it.
Jeddy_Mctedder
1 / 5 (4) Jun 08, 2012
multi-stage desalinization of sea water has been proposed and studied. intuitively , it seems the most efficient method for producing sea water at small scale will involve multiple steps, possibly using different processes, one which feeds its output into the input of the next.

at massive scale. it is still possible that on site desalinization of water by dedicated nuclear power plants might be enormously efficient, as all the waste heat would be utilized for desalinization purposes making the project enormously efficient at scale.
sender
2 / 5 (4) Jun 08, 2012
When plasma reactor columns were invented to fractionate colloids, I asked whether another industrial space age had undergone dissonance.
Urgelt
5 / 5 (2) Jun 08, 2012
Desalinization is often not the only treatment required to convert brackish water into potable water. There are usually microorganisms and pollutants to neutralize, too.

I could see this process being of use in some industrial applications, but I don't expect it to solve the world's needs for potable water unless it can be economically combined with other treatment regimes.
Jonseer
2.3 / 5 (6) Jun 08, 2012
If we found a complementary process which can reduce sea water to brackish levels of salinity (which would seem useless on its own) and do it cheaply and efficiently, we could put the two together. After that, the only issue is how far up hill can we afford to transport it.


Brackish water is not useless at all, but it requires thinking beyond the immediate goal of making fresh water.

Should the plants be placed near natural deltas, estuaries and place where fresh water and salt water mix (and a minimum amount of fresh water allowed to flow to maintain an ecosystem) brackish water is very useful We could make a trade. Use less energy to make brackish water which is released into the natural mixing zone, and take more fresh water out above the zone.
A_Paradox
5 / 5 (3) Jun 09, 2012
I guess this idea might not be too popular but, it seems to me something like this could improve the treatment of human sewage systems which inevitably pick up salt from you, me, and all the others. As I see it, we will not be really good custodians of the world until we have all learned to clean our waste water enough so that it can be returned to the biosphere UPSTREAM from where we borrowed it.

Where I live in Perth, Western Australia, people still have not got the message. A very big crunch is coming as the population increases but the rainfall decreases. The Eastern States have been having their 'hundred year' floods so officially the droughts are over, there but ... nobody knows the future.

20 years ago I was saying that climate change will probably mean 'less climate and more weather'. I see nothing to change that viewpoint as yet.
tadchem
not rated yet Jun 11, 2012
I once considered the following: connect an electrolytic cell to the electrical output of a hydrogen-oxygen fuel cell, connect the gaseous output of the electrolytic cell to the gas supply valves of the fuel cell. Add a low voltage DC booster source to make up for resistance losses.
'Bad' water (with electrolytes) goes into the electrolyic cell (with overflow for unelectrolyzed input), pure water (reconstituted and slightly warm) comes out of the fuel cell.
But then the funding failed...
Mike_Massen
1 / 5 (4) Jun 11, 2012
tadchem needs a primer in power flows with this illustrative and comparative gem for my kids to look and chuckle at:-
I once considered the following: connect an electrolytic cell to the electrical output of a hydrogen-oxygen fuel cell, connect the gaseous output of the electrolytic cell to the gas supply valves of the fuel cell. Add a low voltage DC booster source to make up for resistance losses.
'Bad' water (with electrolytes) goes into the electrolyic cell (with overflow for unelectrolyzed input), pure water (reconstituted and slightly warm) comes out of the fuel cell.
But then the funding failed...


Quite understandable the "funding failed", these funding sources obviously and with great relief had physicists with basic understanding of thermodynamics and their skill of analytical/critical thinking, pity most applicants of funding sources dont have that, so much time and wasted energy could be saved (sometimes) !

(sigh)

MRBlizzard
1 / 5 (1) Jun 17, 2012
I don't know if brackish water is diluted sea water... but how about:

1. Dilute sea water with pure water and make brackish water,
2. Use this process to purify the brackish water, and then
3. Use the pure water for step 1, and drink the rest.

How much power does this actually take? small solar cell?