Scientists cook up new recipes for taking salt out of seawater

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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. Now scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) 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 a liquid salt that binds to water, making them useful in forward osmosis to separate contaminants from water. (See Berkeley Lab Q&A, “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 Berkeley Lab 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.

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This solar-powered device produces energy and cleans water at the same time

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UNDER THE SUN Solar panels with water purification devices mounted on their backs (illustrated) could produce freshwater and electricity simultaneously.

 Still a prototype, the machine could one day help curb electricity and freshwater shortages

By mounting a water distillation system on the back of a solar cell, engineers have constructed a device that doubles as an energy generator and water purifier.

While the solar cell harvests sunlight for electricity, heat from the solar panel drives evaporation in the water distiller below. That vapor wafts through a porous polystyrene membrane that filters out salt and other contaminants, allowing clean water to condense on the other side. “It doesn’t affect the electricity production by the [solar cell]. And at the same time, it gives you bonus freshwater,” says study coauthor Peng Wang, an engineer at King Abdullah University of Science and Technology in Thuwal, Saudi Arabia.

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Hydrogel uses sunlight to harvest fresh water from the sea

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The research team (led by Tan Swee Ching, at right) with samples of the hydrogel

In many arid coastal regions, a great quantity of valuable fresh water is lost into the atmosphere every day, as it evaporates from the surface of the ocean. This situation prompted scientists to create a new hydrogel that’s highly effective at capturing moisture from the sea air, and then releasing it as fresh water.

Developed by a team at the National University of Singapore, the zinc-based material is claimed to be over eight times more absorbent than existing drying agents such as silica gel and calcium chloride – it can absorb more than four times its dry weight in water. Additionally, unlike the case with traditional drying agents, no electricity is required to get that water back out of it, plus the gel can be reused over 1,000 times.

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Radical desalination approach may disrupt the water industry

8A38E9FC-CBF7-40A6-99BB-155E08C50CCAIllustration describing fresh water production from hypersaline brines by temperature swing solvent extraction.

Hypersaline brines—water that contains high concentrations of dissolved salts and whose saline levels are higher than ocean water—are a growing environmental concern around the world. Very challenging and costly to treat, they result from water produced during oil and gas production, inland desalination concentrate, landfill leachate (a major problem for municipal solid waste landfills), flue gas desulfurization wastewater from fossil-fuel power plants, and effluent from industrial processes.

If hypersaline brines are improperly managed, they can pollute both surface and groundwater resources. But if there were a simple, inexpensive way to desalinate the brines, vast quantities of water would be available for all kinds of uses, from agriculture to industrial applications, and possibly even for human consumption.

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