Electricity-powered water purification method could extend to salt waters

While on missions without access to clean water, US Marines face the challenge of obtaining and storing enough drinking water to sustain them. Penn State researchers, led by Chris Arges, an associate professor of chemical engineering at Penn State, are working on a realistic purification option that is portable, lightweight and easy to operate.

He and co-principal investigator Christopher Gorski, an associate professor of civil and environmental engineering at Penn State, will use a three-year grant of $570,000 from the Office of Naval Research to advance a water purification method known as membrane capacitive deionization (MCDI).

“While the majority of global desalination uses a process known as reverse osmosis in centralized production facilities, it is not suitable for military teams because it requires high-pressure piping and hardware and is difficult to operate in the field,” Arges said. “MCDI, on the other hand, is effective, mobile and energy efficient.”

Stimulated by battery or solar energy, MCDI uses ion exchange membranes and porous electrodes to separate ions, such as sodium and chloride, from water. According to Arges, the method is effective for groundwater or brackish water, but insufficient to sufficiently purify more concentrated water sources, such as seawater.

“The electricity causes the sodium ions to migrate across the cation exchange membrane to a negatively charged electrode, while chloride ions migrate across the anion exchange membrane to a positively charged electrode, a process known as the principle of electrosorption,” Arges said. “Capturing the ions from the liquid leads to deionized, potable water.”

As more and more water is treated in the MCDI unit, the electrodes become saturated with salt, which prevents them from removing as much salt from the water. At that point, Arges said, the electrodes can be regenerated by slowing the flow of water and reversing the cell’s polarity.

“This step in the process wastes some of the water, but it also produces electrical energy that can be recovered and applied to the next desalination cycle to reduce the overall energy burden,” Arges said. “This allows MDCI to remain energy efficient.”

To improve the effect of MDCI on more concentrated water sources, Arges and his team will redesign the electrochemical cell module used in MCDI. Using tools from the Nanofabrication Lab at the Penn State Materials Research Institute, the researchers will fabricate microscopic pits in an interlocking pattern on the membrane surface. This increases the interface between the membrane and the electrodes, improves contact and decreases the distance sodium and chloride ions have to travel to cross the membrane-electrode interface.

In addition, the wells allow the electrode material to store more sodium and chloride ions. This allows users to purify water for extended periods of time before resorting to regeneration. If successful, the upgraded MCDI unit could purify not only groundwater and brackish water, but also seawater, Arges said.

In previous research, Arges and his team have successfully used similar membrane patterns to separate hydronium and hydroxide ions from water in bipolar membranes to make oxygen and hydrogen in an electrolysis cell.

“Since the proposed approach for this grant has worked for us in the past, we believe that the larger interface will reduce resistance to ionic transport, leading to cleaner water in greater quantities,” Arges said.

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Materials supplied by Penn State. Originally written by Mariah Chuprinski. Note: Content is editable for style and length.