Wires turn salt water into freshwater

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.

Enlarge
(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

I find it intriguing as well. It’s going to be expensive for a long time, whereas using a couple of carbon wires and 1.2Volts is already cheap.

Desalination: Energy Hog No More?

http://www.forbes.com/sites/ericagies/2012/05/31/desalination-energy-hog-no-more-part-1/

Porifera Inc. – Innovation: Carbon Nanotubes

Desalination has long been the domain of arid, energy-rich states like those in the Persian Gulf thanks to its high-energy footprint. But as populations have boomed, as climate change makes water supplies more uncertain, and as clean water regulations are tightening on industry, more governments and businesses have been looking to desalination.

Efficiency and reallocation are much cheaper and sustainable options for obtaining new water supplies for municipalities. Water reuse in cities is also up and coming, as I will discuss in a future column. But the market to clean industrial wastewater of all kinds is booming.

For all these reasons, innovation in desalination has been continuing apace. At the Blue Tech Forum in San Francisco yesterday, Tyler Algeo, a research analyst at BlueTech Research, said that patents for desalination technologies in 2010 were double the number filed in 2005. Desalination energy inputs have been reduced more than 50 percent in the past decade.

Many entities are interested in these technology breakthroughs, said Algeo: large water technology corporations, venture capital firms, Fortune 500 companies, research groups, consulting engineering practices, and government agencies.

In addition to the Middle East, Australia, Algeria, and Spain now have major desalination programs, he said.

The Forum selected three companies with various approaches to desalination: carbon nanotubes, radial deionization, and biomimetic aquaporin membranes. Today I report highlights from the company that is innovating with carbon nanotubes. Look for the other two technologies in subsequent days.

Porifera Inc. is a two-year-old company based in Hayward, Calif. Vice president of business development, Jeffrey Mendelssohn, said it has achieved a breakthrough in forward osmosis membranes by using carbon nanotubes.

Reverse osmosis is a common filtration technology that uses a pump to push water through a filter. It can be energy intensive, which is why a lot of innovation is happening in forward osmosis.

Forward osmosis uses the thermodynamic law of entropy to separate solids from a fluid. Using a vessel separated into two compartments by a membrane filter, you put a dirty liquid on one side and clean water on the other. The water passes through the membrane until the percentage of solids on both sides is the same.

“Forward osmosis works better in high fouling environments [severely contaminated water] much better than pressure-driven membrane processes [reverse osmosis] because particulate matter in high fouling environments will scrunch up against the membrane under pressure and cause membrane performance to fail,” said Mendelssohn.

Porifera’s membrane isn’t just for desal, although Mendelssohn claims that it can perform salt rejection 10 times better than existing forward-osmosis processes. The company claims it has superior permeability, durability, and selectivity for water purification to other membranes. This innovation was reported in Science magazine in 2006 and was discovered at the Lawrence Livermore National Laboratory by a group of scientists led by Porifera’s principal R&D team.

From its website:

Carbon nanotubes are seamless, atomically smooth carbon “straws” whose diameters range from less than a nanometer to tens of nanometers (a water molecule is ~0.3nm). Water flows through these unique pores 1,000 times faster than through any other pore of similar diameter. Moreover, gases also flow through the nanotubes pores more than 100 times faster than through any other nanometer scale pore. This reduction in flow resistance manifests itself in large enhancements of the membrane permeability and in drastic reduction of viscous losses.

Other applications include dewatering and water treatment of all kinds.

Markets include the Department of Defense, said Mendelssohn, which needs portable on-site desalination to reduce the number of water and fuel resupply convoys in Afghanistan.

The developing world is also a potent marketplace. Dubai’s famous Burj Khalifa skyscraper has no wastewater service, said Mendelssohn, like many newly developed areas of the city. Instead, tanker trucks remove sewage regularly. By installing an on-site water treatment and reuse facility, Porifera could reduce those truck trips by 90 percent, said Mendelssohn.

On the domestic market, the company could clean water from oil and gas hydrofracking at half the cost of current forward osmosis systems and also recycle the water for reuse onsite, said Mendelssohn.

Commercial elements will begin shipping in July, he said.

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