While on missions without access to clean water, U. S. Marines face the challenge of procuring and storing enough drinking water to sustain them. Penn State researchers are working toward a realistic water purification option that is portable, lightweight and easy to operate.
Chris Arges and co-principal investigator Christopher Gorski will use a $570,000, three-year grant from the Office of Naval Research to advance a water purification method known as membrane capacitive deionisation (MCDI).
“Although the bulk of global desalination utilises reverse osmosis, it is unsuitable for military teams. 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-powered electricity, MCDI utilises ion-exchange membranes and porous electrodes to separate ions, such as sodium and chloride, from water. According to Arges, the method is effective for ground or brackish water but fails to sufficiently purify more highly concentrated water sources, such as seawater.
MCDI to support water purification
“The electricity triggers the sodium ions to migrate across the cation exchange membrane to a negatively charged electrode. Chloride ions migrate across the anion exchange membrane to a positively charged electrode. This is a process known as the principle of electrosorption,” Arges said. “Capturing the ions from the liquid leads to deionised, drinkable water.”
As more and more water is treated in the MCDI unit, the electrodes become saturated with salt, rendering them unable to remove as much salt from the water. At that point, Arges said, the electrodes can be regenerated by slowing down the flow of water and flipping the cell’s polarity.
“This process wastes some water. It also produces recoverable electrical energy for the next desalination cycle ,” Arges said. “This allows MDCI to remain energy-efficient.”
Water purification in saltier waters
The team will redesign the electrochemical cell module used in MCDI to improve its effect on concentrated water sources. With tools from the Nanofabrication Lab in the Penn State Materials Research Institute, the researchers will fabricate microscopic wells in an interlocking pattern on the membrane surface. This increases the interfacial area between the membrane and electrodes. This approach should improve contact and reduce the distance sodium and chloride ions travel to cross the membrane-electrode interface.
Additionally, the wells enable the electrode material to store more sodium and chloride ions. This allows users to purify water for longer periods before resorting to regeneration. If successful, the improved MCDI unit could purify ground and brackish water and seawater, too, Arges said.
In previous research, Arges and his team successfully used similar membrane patterning to separate hydronium and hydroxide ions from water in bipolar membranes to make oxygen and hydrogen in an electrolysis cell.
“The proposed approach for this grant has worked for us in the past. However, we believe the increased interfacial area will reduce ionic transport resistance. That will lead to cleaner water in greater quantities,” Arges said.
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