Scientists from EPFL work on sustainable approaches to soil remediation, like the use of naturally occurring microorganisms that can "eat" pollutants found in soil and the water table.
In 2020, the City of Lausanne found that large areas of its soil had been contaminated with dioxins, which are chlorinated organic compounds. This pollution was due mainly to combustion-reliant manufacturing processes. Most soil contaminants in Switzerland are either dioxins or heavy metals. While methods are available for cleaning up this kind of contamination, they're generally expensive and involve lengthy processes. As a result, public officials have to make the difficult choice between cleaning up the soil or keeping it as is, even if it's a little polluted.
For the past few years, scientists have been working with businesses to develop more sustainable approaches to soil remediation. One possible option is bioremediation, or the use of naturally occurring microorganisms (such as fungus and bacteria) that can "eat" pollutants, including those found in soil and the water table. The microorganisms metabolize the contaminating compounds, releasing the energy they need to live and reproduce and breaking down the harmful chemicals. "It's like when we eat pasta or another kind of carbohydrate," says Christof Holliger, head of EPFL's Environmental Biotechnology Laboratory. "Our body breaks it down into the glucose our cells need to function."
Proteins that can tackle chlorine
Holliger's research group has been studying bio-remediation for years, and specifically the use of bacteria to remove contaminants resulting from the use of chlorinated solvents, such as for dry cleaning. "These solvents are the main source of pollution in Switzerland's soil and water table," says Holliger. "But we've been able to isolate highly specialized bacteria that use these chlorinated compounds for respiration and turn them into nontoxic organic substances. Right now we're analyzing the process to understand the underlying mechanisms." One thing is clear - a key step in the bacteria's process is the production of proteins.
After years of living in an environment containing chlorinated compounds, these bacteria eventually learned to make good use of them. They're capable of "inhaling" the contaminants, much as we inhale oxygen. Through a multistep process called halorespiration, the bacteria use compounds produced by other bacteria. The energy comes from hydrogen created by yet other bacteria through the fermentation of organic matter. All this is done with the help of a specific protein in the form an enzyme - technically a reductive dehalogenase - that breaks the bond between the carbon and chlorine atoms, thus destroying the contaminant.
Limited applications, for now
This naturally occurring process can't be used to remediate just any kind of soil. First, the contaminant must be present in a fairly high concentration. "Otherwise, it will enter into competition with other organic compounds that the bacteria can also use for respiration," says Holliger. In that case, the bacteria may decide not to use the contaminant and instead break down another chemical in the soil.
In addition, several different kind of bacteria are needed for halorespiration, and they must all be present in the right amounts. It's not enough for scientists to select one bacterium, cultivate (or even improve) it in a lab, and then add it to the soil. Instead, they need to reproduce an entire community and "production line." Holliger also points out that contaminated soil often contains several pollutants, complicating matters further.
Proven to be effective
But under the right conditions, this kind of bioremediation can be effective. When the process is used, scientists generally don't disturb the bacteria contained in the soil or water table. Rather, they aim to create optimal conditions for the microorganisms and ensure they have all the substances they need to thrive. "At a site contaminated with hydrocarbon, for example, the soil will obviously contain a lot of carbon," says Holliger. "We therefore need to enrich it with other chemicals like nitrogen and phosphorus so the bacteria can develop."
Bioremediation may have its limitations, but it's already being used on a large scale to clean up hydrocarbon spills and contaminated water tables (although not yet in Switzerland). Some of the incidents are famous - like the Exxon Valdez oil spill in Alaska - and others less so, and are managed by specialized firms.
