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DesalData Weekly - October 21st, 2015

Posted 21 October, 2015 by Mandy

Nanoporous carbonScientists at the Leibniz Institute for New Materials (INM) in Saarbrücken, Germany are further developing an emerging desalination method.  The process, referred to as capacitive deionization (CDI), extracts ions from water using electrodes, generating clean water and ions that have been enriched on the electrodes.[1] This process removes salts and heavy metals and, uniquely, it does not use conventional water treatment methods that rely on chemical reactions or sophisticated filtering methods.

In addition to desalination, Professor Volker Presser, head of the Energy Materials Group at INM, explains that emissions from power plants can be used to produce electricity using CDI—as long as the emissions are present (i.e. dissolved) in the water as ions— by taking advantage of the temperature change between the charge and discharge of the electrodes.

Nanoporous carbon materials in capacitive deionization| Credit: Uwe Bellhäuser /Volker Presser (INM)

[1] by taking advantage of the temperature change between the charge and discharge of the electrodes.  The INM’s new press release on CDI explains ion-electrosorption:

 

The water to be purified flows between two electrodes made of porous carbon to which a voltage is applied. The positively charged electrode extracts the ions which have a negative charge from the water and the negatively charged electrode, located opposite, extracts the ions with a positive charge from the water. The ions are stored in the nanopores of the electrode material and, at the end of the process, clean water flows out.[2]

At an international conference at the Institute later this month, more than 110 experts from 20 countries will gather to further understand and improve the nascent CDI process.

Meanwhile, in China, researchers have offered a solution for using carbon nanotubes (CNTs) in desalination membranes.[3]  The technology is currently not in use because nanotubes have not proved efficient in removing salt ions from water.  In theory, CNTs should work well as desalination filters: computer simulations show the tubes permit the flow of water and block the passage of salt ions.  However, as explained by Dr. Haiping Fang at the Shanghai Institute of Applied Physics, CNTs are likely to become blocked by the salt ions, thereby preventing water from flowing through them.   Dr. Fang reveals that previous simulations of the desalination process have under-examined this relationship between ions and the nanotubes.[4]  Including these interactions in their calculations—by merging the classical molecular dynamic simulations with density functional calculations of the electronic states and interaction energies—Dr. Fang and his colleagues show that the ions get bound into the carbon nanotubes, thereby obstructing water flow. 

Carbon nanotubes

Carbon nanotubes   Credit: Shutterstock

Dr. Fang’s research team proposes two methods for removing blockages.  One technique is to prevent interactions by chemically altering the carbon tube ends; and the alternative is to apply a powerful electrical field to drag the ions away.  The first option is preferred because it does not have an energy cost and does not require large equipment to generate a strong electrical field, which would enable the use of a more portable desalination device.

Nanotube expert Jayendran Rasaiah at the University of Maine in the U.S. commends this research.   However, Ben Corry of the Australian National University in Canberra suggests that there are other reasons that may explain why carbon nanotubes fail to effectively desalinate water.  Corry further states that the nanotubes would not produce great energy savings for large-scale desalination, because the “greater permeability of CNT membranes would simply reduce the required surface area of such membranes.”[5] 



[1] Water Technology Staff, “New Desalination Process can Generate Electricity,” Water Tech Online, October 20, 2015, <http://www.watertechonline.com/new-desalination-process-can-generate-electricity/> accessed October 20, 2015.

[2] “Nanocarbons Clean Water Without the Use of Chemical and Transform Waste Heat into Electricity,” Leibniz Institute for New Materials, October 15, 2015, <http://www.leibniz-inm.de/en/2015/10/nanocarbons-clean-water-without-the-use-of-chemicals-and-transform-waste-heat-into-electricity/> accessed October 20, 2015.

[3] Philip Ball, “Nanotube Desalination Could Be Put Back on Track,” Royal Society of Chemistry, Chemistry World, <http://www.rsc.org/chemistryworld/2015/10/carbon-nanotube-cnt-desalination> accessed October 20, 2015.

[4] Ibid.

[5] Ibid.

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