Part 1: the method and preceding lab work
What you might think of when hearing about sediment clean-up (remediation) is the conventional method of dredging the contaminated material and depositing it somewhere else (off-site methods). But did you ever try grabbing a fistful of mud from under your feet when you’re standing in the water? Not so easy! You usually manage to get some to the surface, but what about all that slurry that stays suspended in the water? In sediment remediation, this can easily cause even more trouble, since it leads to increased dispersal of contaminated material over the water body, as well as increased exposure to everything that has to swim through the water-sediment suspension. Besides that, an excavator vessel is not the cheapest thing to rent either.
Activated carbon -based “on-site” remediation has been proposed as an alternative method. The basic idea is to add the activated carbon as a sorbent straight to a contaminated site, where it binds the contaminant so strongly, that it becomes unavailable for organisms to assimilate and accumulate. So while the pollutant is still in the sediment, it is rendered mostly harmless. It works pretty much the same way as medical activated carbon: The poison that you accidentally ate is bound and thus prevented from entering your bloodstream, from where it could cause havoc. The only difference in sediment remediation is that this sequestration of contaminants happens already before they are taken up by an organism. A more detailed description of the method and its mode of action you can find here.
In our current research we are focusing on the use of activated carbon to clean up sediments polluted with PCBs. This group of chemicals that is found in the environment of most parts of the world. Listing all the uses and potential dangers of these PCBs in the environment would probably fill another blog post. In brief: it was seen as harmful enough for a worldwide (!) ban of production and use in 2001. One of the biggest problems with PCBs in the environment is their persistency and the fact that they accumulate easily in organisms that are exposed to it.
This is where activated carbon enters the stage: many researchers, including our own group, found that already small doses of activated carbon suffice to prevent almost any of this accumulation of PCBs. So you might say: “Great! It sounds like a great alternative to the messy and laborious dredging operations”. But as Bernard Shaw once said “Science never solved a problem without creating ten more” – we also found that activated carbon itself might have negative side effects to certain organisms. Our job is now to find out if the new problems we create are actually worse than the original one, or if they are a minor trade-off. Our lab studies showed a relatively “balanced” situation, showing both high remediation efficiency accompanied by strong adverse effects. However, lab studies are always limited in their meaningfulness, because we are bound to exclude a lot of parameters that make up a natural environment.
Therefore the next logical step was to bring the tests of activated carbon based sediment remediation to the field. So in August 2015 our research group has set up the first ever field trial in Finland aimed at investigating the potential and the risks of this method. How this looked like and worked in detail, you can find out in the second part of this blog post.
Text by Sebastian Abel, photos by Sebastian Abel, Jarkko Akkanen and Inna Nybom