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Cleaning Water with Ultrasound

Chemicals may not be needed to destroy water pollutants, according to a new study in the Journal of Physical Chemistry A. Instead ultrasound at just the right frequency might do the trick. Ultrasound creates bubbles in water through a process called cavitation. "When the bubbles collapse, the gas inside of them becomes very pressurized and is at high temperatures for a very short amount of time," lead author Inez Hua of Purdue University explains. "The temperatures and pressures are such that organic contaminants can degrade."

To figure out which frequency of ultrasound would be most efficient for water purification, the scientists looked at another process related to cavitation called sonoluminescence, in which water bubbles bombarded with sound actually emit light. "Our hypothesis was that intensity of the light coming from the bubbles would be different for different frequencies," Hua says. "The reason it would be different is that the nature of the bubble collapse, as well as the number of bubbles in the solution, are going to depend on frequency."

Hua's team placed a glass container of about one liter of water on a steel transducer that produced ultrasound waves. They also dropped 1,4 Dioxane, an organic contaminant, into the water. Then they zapped the mix with ultrasound frequencies of 205, 358, 618 and 1,071 kilohertz. Other scientists have conducted similar series of experiments in the past, but Hua's team was the first to leave all conditions but the ultrasound frequency consistent.

They found that at 358 kilohertz the compound reacted faster than at any other frequency. Hua hopes ultrasound will become an alternative to the conventional chemical methods of water purification. "It's very easy to use," she adds. "It doesn't require highly trained operators. You just turn on the switch, the power starts transmitting through the solution, and your process begins."

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