Getting Arsenic Out of Water
Many people in the world lack access to clean drinking water. In places including Bangladesh, millions must drink water containing arsenic, which can cause neurological problems, organ failure, and death. Making robust water filters that can remove salt and arsenic without requiring a lot of energy has been a challenge. Researchers at IBM are developing a material used to make computer chips for more-efficient removal of salt and toxic chemicals from drinking water.
Polymer-membrane water filters have been in use since the 1970s “with no big materials innovation in a long time,” says Robert Allen, senior manager of chemistry at IBM’s Almaden Research Center, in San Jose, CA. There are problems with traditional membrane filters. The chlorine used to kill pathogens in water degrades them. They’re susceptible to fouling, or clogging up, when the water forced through them in a desalination process called reverse osmosis contains oil or proteins.
The IBM researchers have made a new membrane material that resists these problems while also screening out arsenic. The material is based on polymers developed by Allen’s lab and Central Glass, a Tokyo company. The materials, called hexafluoro alcohols, are used as a patterning material for manufacturing advanced computer chips. The IBM researchers found that these compounds also make good water filters. Other collaborators on the project are Benny Freeman, a professor of chemical engineering at the University of Texas at Austin, and the King Abdul Aziz City for Science and Technology, in Saudi Arabia, a country that has large investments in water desalination.
The key to the membrane’s performance is its pH sensitivity. At high pH, arsenic in the water becomes charged, and the fluorine groups on the polymer, which also become charged, then will repel the toxic chemical. Other membranes can’t operate in the very basic conditions needed to ionize arsenic, says Allen. There are two main forms of arsenic, one more difficult to reject than the other. Traditional filters get out only 80 percent of this form; the IBM filter removes 95 percent. Depending on the water sample, says Allen, it removes 96 to 99 percent of arsenic.
The polymer membrane should also make for more energy-efficient desalination as long as the water is brought to a high pH, which helps water move through it more quickly than through other filters. Young-Hye Na, the main research scientist on the project, says that the fluorine group on the polymer protects it from chlorine. She says that the filters would also need to be cleaned less frequently. After two hours of filtering oily water, conventional membranes plug up completely. The fluorine-containing polymer membrane has been tested over a period of two days without clogging.
The starting materials used to make the membranes are more expensive than conventional polyamides. “But the layer is very thin, just 100 nanometers–and if it works, it’s worth it,” says Allen. IBM hopes to test the membranes in Bangladesh, where tens of millions of people drink water contaminated with arsenic.
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