Wednesday, July 30, 2008

Desalination Made Simpler

A new chlorine-tolerant material may streamline desalination processes.

By Jennifer Chu

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. Credit: Beverly Barrett/University of Texas at Austin

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.

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.

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.

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.