The chlorinated solvent trichloroethene (TCE) has been found to be an increasingly problematic contaminant in groundwater.
TCE has been widely used as a cleaning agent and solvent for many military, commercial, and industrial applications. Its widespread use, along with its improper handling, storage, and disposal, has resulted in frequent detection of TCE in the groundwater. TCE has the potential to cause liver damage, malfunctions in the central nervous system and it is considered a likely human carcinogen.
The detection of TCE recently forced the shut down of the water supply in one US city-the Greater Phoenix area municipalities of Paradise Valley and Scottsdale.
In 2002, Bruce Rittmann, PhD, director of the Biodesign Institute’s Center for Environmental Biotechnology, Arizona State University, received a patent for an innovative treatment system, called the membrane biofilm reactor (MBfR), which uses naturally occurring microorganisms to remove contaminants from water. They have now fine tuned it to capture and convert TCE into a less harmful byproduct
Scientists have discovered specialized microorganisms that can replace the chlorine in the chlorinated molecules with hydrogen, a process called reductive dechlorination. While other methods are possible, they are often more costly than reductive dechlorination on a large scale, and many do not transform TCE into a harmless end product.
The Arizona State University team utilized the MbfR and a naturally occurring group of microorganisms able to remove TCE from water. Surprisingly, these microorganisms, called dehalogenerators, have an affinity for chlorinated organics and can be found all throughout nature, even in clean water supplies, the soil, and groundwater.
“These bacteria respire TCE, that is, they can use TCE like we use oxygen to breathe,” said scientists. “They take in the TCE and they start removing the chlorines, step by step. In the ideal case, the dehalogenators remove all the chlorines, converting TCE to ethene, which is harmless.”
With this knowledge in hand, the challenge for the research team was to adapt their existing MfBR system, which can remove other water contaminants, to see if it could now handle TCE.
A version of the reactor that addresses perchlorate, a byproduct of rocket fuel, is already in the commercialization pipeline.
“A key challenge with using these bacteria is that, if they don’t dechlorinate all the way, the TCE can be converted to vinyl chloride, which is a known human carcinogen,” said a researcher. “In other words, if you don’t have complete dechlorination, you can end up having something worse than what you started with. So, it is critical to have the right mix of microorganisms for complete dechlorination.”
ETA: Next, the team hopes to drive the TCE system toward commercialization. Other oxidized contaminants that the system has been effective in reducing in the laboratory setting include perchlorate, selenate (found in coal wastes and agricultural drainage), chromate (found in industrial wastes), and other chlorinated solvents.'
In a related discovery, Indiana University Bloomington chemists have designed an organic molecule that binds negatively charged ions, a feat they hope will lead to the development of a whole new molecular toolbox for biologists, chemists and medical researchers who want to remove chlorine, fluorine and other negatively charged ions from their solutions.