As populations boom and chronic droughts persist, coastal cities like Carlsbad in Southern California have increasingly turned to ocean desalination to supplement a dwindling fresh water supply.
Scientists at the Berkeley Lab and the Foundry are investigating how to make desalination less expensive have hit on promising design rules for making so-called “thermally responsive” ionic liquids to separate water from salt.
Ionic liquids are liquid salts that bind to water, making them useful in forward osmosis to separate contaminants from water. (See Berkeley Lab Q&A with Jeff Urban, “Moving Forward on Desalination.”) Even better are thermally responsive ionic liquids as they use thermal energy rather than electricity, which is required by conventional reverse osmosis (RO) desalination for the separation. The new study, published recently in the journal Nature Communications Chemistry, studied the chemical structures of several types of ionic liquid/water to determine what “recipe” would work best.
Foundry users, working with staff scientist Jeff Urban, combined their efforts to study the behavior of ionic liquids at the molecular level and made an unexpected finding.
It was long thought that an effective ionic liquid separation relied on the overall ratio of organic components (parts of the ionic liquid that are neither positively or negatively charged) to its positively charged ions, explained Urban. But the Berkeley Lab team learned that the number of water molecules an ionic liquid can separate from seawater depends on the proximity of its organic components to its positively charged ions.
A decades-old membrane-based reverse osmosis technology originally developed at UCLA in the 1950s is experiencing a resurgence – currently there are 11 desalination plants in California, and more have been proposed. Berkeley Lab scientists, through the Water-Energy Resilience Research Institute, are pursuing a range of technologies for improving the reliability of the U.S. water system, including advanced water-treatment technologies such as desalination.
Because forward osmosis uses heat instead of electricity, the thermal energy can be provided by renewable sources such as geothermal and solar or industrial low-grade heat.