Disintegrating Water Pollution with a Green Light-Emitting Diode

By on March 6, 2018

Researchers at Rensselaer Polytechnic Institute have developed a new method for manufacturing green LEDs with greatly enhanced light output. Led by Professor Christian Wetzel, the research team etched a nanoscale pattern at the interface between the LED's sapphire base and the layer of gallium nitride (GaN) that gives the LED its green color. (Credit: Rensselaer/Robbins)

Until now, decomposing harmful, stubborn chlorides and fluorides polluting water has been a serious chore. The complex laser systems that are needed for the task are very costly, and using them also requires safety equipment, strict precautions, and expertise, because they present notable risks. However, a research team of chemists from Martin Luther University Halle-Wittenberg (MLU) in Germany have discovered a way to disintegrate these tough pollutants in water in a cost-effective, simple process with just a green light-emitting diode, vitamin C, and a catalyst.

Photochemistry and pollution remediation

Light can pierce molecules and cause chemical reactions inside them. When vitamin C is in the mix, light energy can release electrons from the molecule, freeing these highly reactive “hydrated electrons” up to break down even the most stable pollutants reliably. Chemist Professor Martin Goez corresponded with EM about the process and what inspired this research.

“Chloroorganics are at the same time highly toxic to organisms and extremely stable against ‘normal’ chemical reactants, so they accumulate in aqueous environments,” explains Dr. Goez. “But the hydrated electron is reactive enough to crack them. So on one hand, these pollutants provide stringent benchmarks for testing our electron generators, and on the other hand as bulk chemicals they constitute real remediation issues.”

The new system uses the standard green LED, vitamin C, along with a catalyst in the form of traces of a metal complex. How does the process work?

“A catalyst is driven through a cycle, consuming, in that order, one green photon, one molecule of ascorbate (vitamin C), and a second green photon,” details Dr. Goez. “The last photon sets free the electron and restores the catalyst to its starting state. A micelle formed by a common detergent protects the catalyst and makes it much more long-lived.”

Until the MLU team successfully applied photochemistry to the pollution remediation problem, complex laser systems were required for this task. The LED process is not only safer and cheaper, but also has the advantage of removing pollutants without producing byproducts any more harmful than detergent, “contained in the waste water of every household possessing a washing machine.”

“A suitable laser sets you back by 20,000 to 30,000 EUR, while the LED comes at one percent of that cost in its high-power version, including the power supply; otherwise, even less,” remarks Dr. Goez. “And operating a class 4 laser incurs health-and-safety issues; eye protection is the main concern.”

Class 4 laser. (Credit: By P.A.W.N. LASER – P.A.W.N. RECORDINGS – P.A.W.N. EVENTS: [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)])

Testing and improving the process

The team tested the technique on a substance that is both very stable and highly toxic: chloroacetic acid. The method broke the compound into harmless components and was just as effective as the high-power laser at generating electrons. The MLU process is suitable for breaking down harmful fluorides and chlorides, but that isn’t the limit of its applications; other reluctant photochemical reactions can be initiated by the technique as well.

“It can be used for every pollutant that can be broken down by reduction,” Dr. Goez confirms. “It cannot be used when the pollutant needs to be remediated by oxidation.”

The process works exactly the same in the environment as it does in the lab. It is safe and simple enough that undergraduate students can learn the process easily. Even so, the team is not satisfied to leave the process as it is.

“We are currently working on improving the catalyst to make the whole process even more efficient and versatile,” adds Dr. Goez, confirming that we’re likely to see an even more effective version of the method in the future.

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