Getting access to drinking water is a daily challenge for more than one billion people in the world. Desalination may help relieve such water-stressed populations by filtering salt from abundant seawater, and there are more than 7,000 desalination plants worldwide, 250 operating in the United States alone. However, the membranes that these plants use to filter out salt tend to break down when exposed to an essential ingredient in the process: chlorine.
Streamlining desalination: Researcher Ho Bum Park holds two samples of the chlorine-tolerant desalination membrane. The one on the left is one-tenth of a micrometer thick and is made of a porous support with a thin coating of the membrane. The blue membrane is about 50 micrometers thick.
Now researchers at the University of Texas at Austin (UT Austin) and Virginia Polytechnic Institute have engineered a chlorine-tolerant membrane that filters out salt just as well as many commercial membranes. The researchers say that such a membrane would eliminate expensive steps in the desalination process and eventually be used to filter salt out of seawater. The results of their study appear in the most recent issue of the journal Angewandte Chemie.
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The majority of desalination plants today use polyamide membranes to effectively separate salt from seawater. Since seawater harbors a variety of organisms that can form a thick film over membranes and clog the filter, plants use chlorine to disinfect incoming water before it is sent through membranes. The problem is, these membranes degrade after continuous chlorine exposure. So the desalination industry added another step, quickly dechlorinating water after it’s been treated with chlorine and before it’s run through the membrane. Once the water has been desalinated, chlorine is added again, before the water enters the drinking-water supply.
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Benny Freeman, a professor of chemical engineering at UT Austin, says that a chlorine-tolerant membrane may help significantly streamline the desalination process. Freeman and James McGrath, a professor of chemistry at Virginia Polytechnic Institute, engineered a water-filtering membrane that stands up to repeated exposures of chlorine.
The new membrane is made from polysulfone, a sulfur-containing thermoplastic that is highly resistant to chlorine. Previous researchers have attempted to design chlorine-tolerant membranes using polysulfone but have been hampered because the material is extremely hydrophobic, and doesn’t easily let water through. Scientists have tried to chemically alter the polymer’s composition by adding hydrophilic, or water-attracting, compounds. However, timing is everything, and Freeman says that when researchers add such compounds after they synthesize the polymer, “eventually, you break the backbone of the polymer chain … to the point where it’s not useful.”
Instead, Freeman and McGrath added two hydrophilic, charged sulfonic acid groups during the polymerization process and found that they were able to synthesize a durable and reproducible polymer. They then performed a variety of experiments to gauge the material’s ability to tolerate chlorine and filter out salt, compared with commercial membranes.
First, the team carried out salt permeability tests, measuring the amount of salt passing through a membrane in a given amount of time. The less salt found in the filtered water, the better. Freeman and McGrath found that the new membrane performed just as well as many commercial membranes in filtering out water with low to medium salt content. For saltier samples comparable to seawater, the team’s membrane was slightly less permeable.
“We have materials that are competitive today with existing nano filtration and some of the brackish water membranes,” says Freeman. “We are now pushing the chemistry to get further into the seawater area, which is a significant market we’d like to access.”
The researchers also tested the polymer’s chlorine sensitivity. They found that, after exposure to concentrated solutions of chlorine for more than 35 hours, the new membrane suffered little change in composition, compared with commercial polyamide membranes, which were “eaten away by the chlorine.”
Currently, Freeman and his colleagues are further manipulating the polymer composition to try to tune various properties, in hopes of designing a more selective and chlorine-resistant membrane. They are also in talks with a leading manufacturer of desalination membranes, with the goal of bringing the new membrane to market.
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“These membranes may represent a reasonable route to commercialization,” says Freeman. “If we’re successful, we’ll have the possibility of basically making these membranes on the same equipment that people use today.”
Eric Hoek, an assistant professor of civil and environmental engineering at the University of California, Los Angeles, works on engineering new desalination membranes at the California Nanosystems Institute. He says that the chlorine-tolerant membrane developed by Freeman’s team may be a promising alternative to today’s industrial counterparts.
“This work is among the most innovative and interesting research on membrane materials in the past decade,” says Hoek. “While the chlorine tolerance exhibited by these membranes is impressive, the basic separation performance is not yet where it needs to be for these materials to be touted as immediate replacements of commercial seawater membrane technology.”
