Findings may boost efforts to destroy pollutants with ultrasound

provided by Purdue University

ngineers at Purdue University, in efforts to develop an effective system that uses ultrasound to clean polluted water, have pinpointed the frequency that degrades certain kinds of pollutants most efficiently. The findings could be used to design better ultrasonic systems for destroying pollutants in water, said Inez Hua, an associate professor of civil engineering.

    A paper about the work appeared in the April 19 issue of the Journal of Physical Chemistry A, published by the American Chemical Society. The paper was written by Hua and doctoral student Michael Beckett.

    Ultrasound causes bubbles to form and collapse in water, a process known as “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,” Hua said. “The temperatures and pressures are such that organic contaminants can degrade, and there are such extreme conditions in the bubbles that they emit light.”

    The process is known as sonoluminescence, or the emission of light by bubbles in a liquid that is bombarded by sound. The phenomenon can be used as a way of measuring the pollution-destroying efficiency of different ultrasound frequencies. The frequencies that produce the most intense flashes of light are the most efficient pollution busters.

    “Our hypothesis was that intensity of the light coming from the bubbles would be different for different frequencies,” Hua said. “The reason it would be different is that the nature of the bubble collapse as well the number of bubbles in solution are going to depend on frequency.”

    While other researchers have carried out similar experiments, the Purdue research produced a wider range of results for two reasons:

• The same “reactor” a glass container within which the reactions take place was used for all the experiments. In most previous research, different reactors were used to test different frequencies. This means the results could not entirely be attributed to a particular frequency but could also be influenced by which reactors were used. Because the Purdue engineers used the same reactor for all of the experiments, the differing results could reliably be attributed to the particular frequencies being tested.
• Previous research had not tested a range of frequencies while keeping the sound intensity at the same level. Sound intensity can be likened to volume.

    “In most of the experimental work that you see in the sonochemistry literature, all the intensities are different,” Hua said. “It's very hard to compare data when the intensities are different. What we've done in this study is use the same intensity at all of the frequencies in the same reactor. So we are basically trying to isolate the effects of frequency alone.”

    Ultrasound is used for imaging and industrial processes that harness sonochemistry, or using sound waves to drive reactions. But the technique won't be practical for environmental remediation until scientists can figure out how to improve its efficiency.

    “To enhance the efficiency of the sonochemical processes we need to learn more about the hydrodynamics of the system, or the bubble behavior, the number of bubbles that form in solution, how they interact with each other, and so forth,” Hua said. “Ultimately, we are trying to find a reactor configuration that will optimize the efficiency.”

    Ultrasound techniques could provide better alternatives to conventional methods that add chemicals like chlorine to water to get rid of organic contaminants. “The advantage of ultrasound is that you don't have to add reagents,” Hua said. “It's very easy to use. It doesn't require highly trained operators. You just turn on a switch, the power starts transmitting through the solution, and your process begins.

    “It's also a very robust system. Ultrasonic systems operate under a wide variety of conditions. They can tolerate large ranges in temperatures.”

    Hua is working with engineering doctoral students at Purdue to investigate how to improve the performance of the reactors used in the ultrasound treatment of contaminated water.

    The reactor, a glass vessel containing about a liter of water, sits on top of a steel transducer, a speaker-like vibrating device that produces the ultrasound waves transmitted through the water. The resulting cavitation breaks down organic contaminants, such as the gasoline additive methyl tertiary butyl ether, or MTBE.

    “The contaminants are transformed into more innocuous compounds,” Hua said.

    The research, which was funded by the US Department of Energy, focused on using ultrasound to destroy a class of compounds known as polychlorinated biphenals, which are found in a variety of materials, including pesticides.

    Hua and Beckett studied the efficiency of four ultrasound frequencies: 205, 358, 618 and 1071 kilohertz. Ultrasound in those frequencies was used to degrade the chemical 1,4-Dioxane, an organic contaminant that is structurally similar to MTBE.

    The researchers found that the frequency of 358 kilohertz had the fastest reaction rate, meaning it degraded the compound faster than the other frequencies.

    “The point is that we were able to correlate sonoluminescence intensity with reaction rate,” Hua said. “So now we know that, if we are going design a reactor, in terms of efficiency we are going to get good performance with 358.”

    The same frequency has been shown to be the most effective in degrading other compounds as well, she said. Future work could concentrate on designing a larger reactor or a system in which there are two or three parallel reactors.