Mimicking mother nature: New membrane to make fresh water

Scientists at Sandia National Laboratories and their collaborators have developed a new membrane, whose structure was inspired by a protein from algae, for electrodialysis that could be used to provide fresh water for farming and energy production.

The team shared their membrane design in a paper published recently in the scientific journal Soft Matter.

Electrodialysis uses electrical power to remove dissolved salts from water. Currently it is used to capture salt from seawater to produce table salt and remove salt from brackish water to make fresh water, but it could also be used to remove salt from wastewater to provide a new source of fresh water.

The researchers found that the addition of a common amino acid, called phenylalanine, to an electrodialysis membrane enabled it to better capture and remove positive ions, such as sodium.

“Adding phenylalanine to the electrodialysis membrane increased the selectivity for positive ions by a significant amount, to our pleasant surprise,” Susan Rempe, the lead bioengineer on the project, said.

Ensuring an adequate supply of fresh water is a national security problem, she said. Fresh water is essential for everything from drinking and farming to producing energy from nuclear-, coal- and natural-gas-based power plants.

Clean water, with less electricity

Currently, a method called reverse osmosis is used commercially to remove salt from seawater or brackish water to produce fresh water, but it has several limitations. One limitation is the need for high pressure to push freshwater out of an increasingly salty solution. The high-pressure driving force is costly and leads to the membrane getting clogged or fouled by undissolved material in the water easily, Rempe said.

The more concentrated the salty solution, the bigger the problem. As a result, there are few options for cleaning up salty wastewater. As an example, water produced by hydraulic fracturing to recover natural gas, which can be ten times as salty as seawater, generally gets buried underground instead of being returned to the environment, Rempe said.

Sodium and chloride are the two most common ions in seawater, and table salt. Of course, there are a variety of other positively and negatively charged ions in seawater and wastewater, too.

Electrodialysis is a potentially better method than reverse osmosis because it uses electrical current to draw out the salt ions, leaving behind fresh water. This requires less energy and makes the membrane less likely to get clogged, Rempe said. Electrodialysis needs a pair of membranes to produce fresh water, one that captures positively charged ions, such as sodium, and one that catches negatively charged ions, such as chloride.

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