For the first time, researchers have designed an electrochemical cell that can desalinate seawater (Nano Lett., DOI: 10.1021/nl203889e). They think that its cost and efficiency eventually could improve on standard techniques of purifying seawater.
Worldwide demand for freshwater is skyrocketing as the population increases. Many of today’s desalination plants use reverse osmosis or evaporation, both of which require enormous amounts of energy to supply heaters or high-pressure pumps. To find cheaper, room-temperature, energy-efficient solutions, many researchers are looking to nanomaterials and electrochemistry.
The new system uses both. It first draws ions from seawater into a pair of electrodes. As the researchers pass current through the electrodes, electrochemical reactions drive chloride ions into a silver electrode and sodium ions to an electrode made from manganese oxide nanorods. Next, the researchers remove the desalinated water and release the trapped ions into a separate stream of waste seawater by reversing the direction of the electrical current. Although the pilot experiments were not automated, the researchers say that a pump could automate the process.
The desalination system is a spinoff from a Stanford University project to create new sources of clean energy. Last year, Fabio La Mantia, now of Ruhr University Bochum, in Germany, Yi Cui, of Stanford University, and colleagues showed that they could generate electrical energy by flowing streams of water with varying salinity through an electrochemical cell (Nano Lett., DOI: 10.1021/nl200500s). “The desalination battery is essentially the same device, but reversed,” explains La Mantia, who worked again with Cui and other researchers, on the new study.
The desalinated water that comes from the battery still contains too much salt for drinking, La Mantia says: “We removed up to 50% of the original salt, but we need to arrive at 98%.”
Doing several cycles of ion removal with the battery would further desalinate the water, but those extra cycles cost energy, so La Mantia hopes to improve the efficiency enough so that the battery can remove the salt in a single pass.
John H. Lienhard, of the Massachusetts Institute of Technology, applauds the work but cautions, “There’s still some way to go before this technique could be deployed for large-scale seawater processes.” He says the researchers need to find ways to remove sulfates from seawater, lower the cost of the electrodes, and protect the system from deposits of biofilm and scale that could cripple the device.