Breathing' bacteria clean up toxic waste
Civil engineering professor Paige Novak and her colleagues rise to the challenge
For decades, we’ve released toxic chlorine-containing chemicals into the environment, leaving a legacy of superfund sites and fouled groundwater and sediments. These chlorinated compounds are implicated in a number of health problems, including cancer, endocrine disruption, kidney damage, liver damage and possibly heart disease, obesity, diabetes and developmental illnesses.
The toxins hang around in the environment for generations. The good news is that there are naturally occurring bacteria that breathe these chemicals and in the process convert them to safer compounds–if we can get the bacteria and toxins to meet under the right conditions.
This is the kind of challenge Paige Novak and her colleagues at the University of Minnesota’s BioTechnology Institute enjoy. Novak, a professor in the department of civil engineering, focuses on applied environmental microbiology. Her research is essential to transforming today’s superfund sites into tomorrow’s park lands.
The toxins Novak studies have been in wide use for years. Two of the compounds, trichloroethylene (TCE) and perchloroethylene (PCE) have been used as industrial degreasers, in dry cleaning and as solvents that carry chemicals water won’t. (TCE became famous through the book ”A Civil Action,” by Jonathan Harr, which focused on a landmark pollution tort case.) PCBs (polychlorinated biphenyls) have been widely used in electric insulation and coolants in transformers, electric motors and other applications. Production of PCB in the U.S. was banned in 1979.
Engineers have made great strides using microbiological techniques to clean up TCE and PCE. Certain bacteria will replace the chlorine with hydrogen in a process called reductive dechlorination. Basically, the bacteria breathe (respire) by using the chlorinated compounds the same way we use oxygen. The science of using these bacteria in combination with physical processes to clean up TCE and PCE is fairly well developed. For example, these bacteria love to break down products of molasses.
“In a sense, you can just dump a bunch of molasses in the water where they are, and they will begin growing and breathing the TCE and PCE,” Novak explained.
PCB is a much tougher nut (or molecule) to crack.
A major microbial discovery
PCBs have more chlorine atoms in their molecular structure than TCE and PCE, up to 10. This resulted in a problem, explains Novak, “People thought that the best that these anaerobic microbes could do with PCBs would be to make them a little less chlorinated.” But Novak and her team uncovered heartening news.
“We discovered there are tons of these microbes in the uncontaminated environment. That’s really big news."
“We provided good evidence that a group of largely unstudied microbes, similar microbes to the ones that breathe PCE and TCE, could completely remove the chlorine from the PCBs," Novak said. "They leave behind only biphenyl, which is easily degraded and not nearly as toxic.”
This is a major discovery, says Novak. “But the problem is we don’t yet have something that can be used in the environment.”
One critical barrier is the way PCBs behave in the environment. “We know how to get the bacteria to grow and breathe PCBs, but the PCBs are stuck tightly to the soil. So these organisms are trying to breathe the chlorinated compounds in an environment where the chlorinated compounds are very hard to get to. Even if we give them food, the way we do with PCE and TCE, they can’t use it.” It’s as though, Novak adds, you had very little oxygen in a sealed, crowded room. “It wouldn’t matter how much cake you gave the people in the room,” she says. “If there’s hardly any oxygen there, they can’t eat because they can’t breathe.”
A way around this is to give them something to breathe in addition to the PCBs. Unfortunately, the compounds that work are just as toxic as PCBs. So Novak has turned the search towards environmentally safe compounds that the PCB-breathing microbes can also breathe, but which won’t leave other toxins behind.
"We think that these microbes breathe compounds that are made as fallen leaves break down. So that’s where we are now–trying to find some nontoxic, easily available natural compound, like broken down leaves, to stimulate these bacteria.”
High hurdles in the real world
As both an engineer and a microbiologist, Novak is keenly aware that any solutions she and her colleagues develop need to be applicable to the real world. About a decade ago, she conducted tests on decontaminating the TCE and PCE that pollute the shuttered Twin Cities Army Ammunition Plant in Arden Hills, Minnesota. Novak, along with colleagues from the university, had developed a special hollow fiber system that could feed hydrogen to the ground and stimulate the growth of TCE- and PCE-breathing microbes. Tests at the ammo site showed that as clever as the invention was, it was impractical in the field.
Novak took a good lesson from that experiment. “I’m an engineer. I love studying microbes, but I want to solve problems. You can’t underestimate the basic physics of the real-world site you want to clean. Can you get the food to the bacteria? Can you get the bacteria to the chlorinated compounds they need to breathe?”
“You have to understand both the biological and physical requirements of the real world if you want to win this fight.” Novak thinks the field is about eight to ten years from a workable win on PCBs.
Reprinted with permission from Research @ U of M, a publication of the Office of the Vice President for Research.