Destroying pollutants with ultrasound

THE HINDU [3 MAY, 2001]

ENGINEERS 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 an associate professor of civil engineering. A paper about the work appeared in the Journal of Physical Chemistry, published by the American Chemical Society.

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.

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.

The hypothesis was that intensity of the light coming from the bubbles would be different for different frequencies. 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.

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.

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 behaviour, the number of bubbles that form in solution, how they interact with each other, and so forth. 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 say researchers. 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.

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.

The research, focussed on using ultrasound to destroy a class of compounds known as polychlorinated biphenals, which are found in a variety of materials, including pesticides. Researchers 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.

They 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 they were able to correlate sonoluminescence intensity with reaction rate. The same frequency has been shown to be the most effective in degrading other compounds as well.