Wednesday, June 20, 2012

Bolt from the blue in the plasma lab

A mod on the underwater plasma jet experiment turned up some unexpected chemistry.

Photo by James Rotz, Michigan Engineering
It grew invisibly in the darkened laboratory, lit only by the thimbleful of plasma that glowed purple in a beaker of water. Nuclear Engineering and Radiological Sciences (NERS) graduate student Ben Yee had turned out the lights, shaded the windows, turned off the computer monitors so that the detector would only pick up the light from the plasma. After half an hour of taking data, he turned off the plasma and turned on the lights to find that a mysterious blue jelly had taken up residence in the bottom of the beaker.

It sounds like an alternate beginning for the 1950s horror flick The Blob, but Yee isn’t worried. “I haven’t heard of any unusual deaths or missing persons in the past few days,” he said. Lifting a beaker from another trial of the experiment, he studied the algae-like green jelly at the bottom. “It seems peaceful. Family-friendly,” he added. So, Flubber’s lazy cousin?

While the jelly may not intend to harm humankind, Yee and his colleagues in professor John Foster’s lab have a more beneficent end in mind for the underwater plasma: purifying water. Plasma is a hot state of matter in which the electrons separate from their atoms, forming a cloud of negative electrons and positively charged ions. The researchers harness that cloud’s ability to send electrons through the water, breaking the H2O into hydrogen (H) and hydroxide (OH). Hydroxide is highly reactive, so it attacks contaminants in the water—including microbes—theoretically sterilizing it. “That’s the magic of the plasma: it decomposes the water so that it can react,” said Foster.

The team forms the plasma right in the water with the help of a glass straw. A metal wire coils around the outside of the bottom inch of the straw, and a needle-like tungsten electrode runs down the middle. They bubble a gas through the submerged end of the straw, something like helium, argon, or plain old air. When the team generates a large difference in electric charge between the electrode and the coil, the metals create a powerful electric field that pulls the electrons away from the atoms in the gas.

The difference they need to generate is a couple thousand volts, which seems large compared to power outlets that are typically at about 110 volts. “It’s actually on the same scale as static electricity,” Yee explained. “When you shuffle your feet across a carpet and touch a doorknob, you can easily discharge a thousand volts. But the current is very small, so it’s not going to kill you.”

A person riding a bicycle hooked up to a transformer could generate the necessary voltage, so the NERS chair Ron Gilgenbach dreams that a purification system based on plasma discharges in water could eventually serve remote villages. It might seem like jelly could scupper that effort, but preventing its formation is as simple as replacing the copper coil with another metal. Foster quickly realized that the copper, which is fairly reactive, was combining with the hydroxide. That compound absorbs water and becomes a gel.
"Anyone want to guess what this is, because I have no clue." Benjamin Yee took this mobile phone snapshot and asked the Internet for suggestions about the jelly's identity before he and professor John Foster worked out that it must be copper hydroxide.

Yee tried copper because it is cheaper and a better electrical conductor than the molybdenum and tungsten that the team had been using, but those metals proved more successful in treating water. For instance, the team used plasma to break the dye methylene blue apart. “Methylene blue is used in the textile industry to dye clothes blue,” Foster explained. “We want our clothes to stay bright, so industrial dyes are hard to break down.” When they are dumped in wastewater, microbes in the environment try to decompose them, but it goes a bit wrong. “The microbes convert them to carcinogens,” said Foster.

Hydroxide attacks methylene blue more effectively. “Over multiple interactions, the dye is converted to water and carbon dioxide,” said Foster. “It’s removed from the water completely.” If plasma water purification makes it out of the lab, breaking down dyes in textile wastewater could be one of its first applications.

In the case of drinking water, the team needs to make sure that the leftover pieces of molecules and bacteria are safe for human consumption. “If you break apart hormones from farms, you have to make sure that the smaller compounds aren’t worse than what you started with,” said Yee. “Those are the questions we’re trying to answer.”

They can find clues in the light emitted by the plasma, which Yee was capturing on the afternoon that the blue jelly appeared. When atoms and molecules in and near the plasma pick up extra energy from wayfaring electrons, they emit it again as light, and pattern of colors gives away the atom or molecule’s identity. In addition to showing what chemicals are in the plasma, Yee said that the light “might also help us identify unexpected compounds resulting from the processing or impurities in the water.”

The jelly is presently with chemists at U-M for a full analysis of its composition. It continues to show no signs of sentience.

John Foster is an Associate Professor of Nuclear Engineering and Radiological Sciences.

Ron Gilgenbach is Chair and Chihiro Kikuchi Collegiate Professor of Nuclear Engineering and Radiological Sciences.

Full text of the methylene blue paper available from the Plasma Science and Technology Laboratory website.

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