We tested different application methods of activated carbon for sediment remediation

This blog post is based on our recently accepted publication in Water Research (Vol. 114, p. 104-112; http://dx.doi.org/10.1016/j.watres.2017.02.025). It is available free of charge until 15. April 2017 under this link.

Activated carbon is a sorbent with the capability of strongly binding pretty much any organic substance to its surface (a process called adsorption). Since a large share of pollution in aquatic ecosystems concerns such organic substances (for example PCB’s), it would hence be a suitable sorbent to remediate them. Once a pollutant is adsorbed to the activated carbon, it is bound so strong that it is no longer available to organisms. This includes even the case where they eat the activated carbon particles “coated” with the pollutant. The organisms would just pass it through their digestive system, pooping it out unaltered. So, long story short, the idea is to render pollutants harmless to the environment, rather than having to remove them (which additionally leaves the question where to put the removed pollutant).

Unfortunately it has been shown that activated carbon itself can actually be quite harmful to certain animals. Therefore, it is necessary to not focus solely on developing these novel remediation methods to be as effective as possible, but to ensure that they are also safe to apply in the environment. After all, what does it help us if we treat the pollution in a place, but at the same time wreck its ecosystem?

In the paper this post is based on, we mainly examined several different methods of applying activated carbon to polluted sediments (which is where the major share of pollutants in aquatic ecosystems are). You basically have two general options: the more laborious one of mixing the sorbent into the sediment actively, or the more “crude” way of thin layer capping. In the latter method you just cover the polluted sediment with the activated carbon (see picture 1). In the field that would mean all you need to do is to take a shovel and spread the carbon. So, while we did know that thin layer capping would be the easier method to execute, what we aimed to find out in our tests was how it compares in matters of effectiveness and safety.

Picture 1: The setup of our test vessels (only thin layer cap tests shown). The activated carbon is applied as a thin layer on top of PCB polluted sediment. Underneath, the burrows of the test organism (Lumbriculus variegatus – a worm living in the sediment) are visible.

We simulated the two application methods in the laboratory in test vessels containing sediment from a PCB-polluted site (Lake Kernaalanjärvi, southern Finland). As a test organism we used Lumbriculus variegatus, small worms that burrow through the sediment. The amount of PCB’s that the worms take up from the test sediments told us how well the different treatments work for remediation, while their biological responses (things like their change in body mass) were used as parameters to measure the adverse effects of the sorbent material itself.

Picture 2: A quick graphical overview on the results of our results: Adverse effects can be comparably high, but remediation efficiency (meaning the reduction of the uptake of a pollutant (here: PCB) is best when activated carbon is mixed into the sediment.

The major results published in this paper were both promising and worrying at the same time (picture 2). We found out that both methods are effective in general. Worms living in sediment under a thin layer cap took up ~50% less PCB’s from the sediment than from the untreated, “raw” sediment. When the activated carbon was mixed into the sediment, the uptake of PCB’s was prevented almost completely. So, while thin layer capping is a method that is a lot easier to use (and hence cheaper), it is not quite as effective as mixing the sorbent into the sediment. Nevertheless, one has to also keep in mind, that animals dwelling in the sediment (and the thin layer cap) can mix the activated carbon with the underlying sediment. This process is called bioturbation and it was actually even visible in our laboratory test vessels (picture 1). It’s just a lot slower than mixing sediment and sorbent right away upon application. In addition, mixing via bioturbation of course requires animals to stay on the treated site and not to flee the site in panic when the activated carbon is applied.

And that’s exactly where our more worrying results come in: the adverse effects of the sorbent itself. With both application methods it became quite apparent that the worms did not really like our miniature-scale remediation works. They lost their appetite almost completely, stopped feeding and hence lost a lot of weight. While that may sound like a desirable achievement to some humans, for our worms that could be a serious issue.

A possible explanation for this sudden loss in appetite was found on electron microscope images that we took (picture 3). It looked like the activated carbon had quite some detrimental effect to the worms’ gut walls. Their microvilli, which are responsible for nutrient absorption from the gut content, were damaged severely in most worms exposed to sediment treated with the sorbent – no matter with what application method. The exact mechanism on how activated carbon causes this kind of damage remain obscure; one suggestion for example is mechanical abrasion (the carbon particles are quite sharp), but also the strong sorption capacity of the material might be involved.

Picture 3: Activated carbon can damage the gut walls (specifically the microvilli) of Lumbriculus variegatus. Image as seen through an electron microscope at a 6000x magnification.

One interesting thing we saw was that thin layer capping with activated carbon can have quite a devastating effect on Lumbriculus variegatus. This is not too surprising, since the organisms are exposed to a high dose of pure activated carbon at the sediment-water interface. However, when we mixed the activated carbon with clay before applying (thus creating a thin layer cap that resembles natural sediment that is enriched with the sorbent), the adverse effects were a lot less severe.  This doesn’t mean there were no more adverse effects, but rather that they were at a comparable level to our other tested application method of mixing the activated carbon into the sediment.

Picture 4: The transmission electron microscopy images (which you saw above) in the making. Photo: Inna Nybom.

From the results seen in this study we were able to draw some conclusions and implications for future field applications. To sum up, both methods are effective. What the thin layer capping method lacks in immediate effectiveness, it makes up for with its easier application and lower costs. When it comes to the adverse effects, we showed that neither one of the methods has a significant advantage over the other – if certain precautions, like avoiding to apply pure activated carbon, are made. So when deciding on a method, the important factors are mostly the available budget and equipment. Thin layer capping is a better option for sediment remediation in cases where special equipment required for other methods cannot be brought in easily (remote areas) or simply in cases where funds are limited. However, before deciding whether or not to utilize activated carbon in general (and big scale), we will have to make sure that its own adverse effects to the environment are not worse than the pollution effects!

Lastly – if you check our blog post on the first field trial of activated carbon based sediment remediation in Finland, you will probably spot some of these implications already “in action”!

Text: Sebastian Abel

Pictures: Sebastian Abel, Inna Nybom

The first Finnish field trial of on-site sediment cleanup with activated carbon

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.

Testing activated carbon for sediment remediation in the lab.
Testing activated carbon for sediment remediation in the lab.

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.

Working with activated carbon powder in the lab can be pretty messy.
Working with activated carbon powder in the lab can be pretty messy.

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

Hands-on research with Chironomus riparius

Sitting in the lab in front of a sewing machine was not exactly what I imagined of doing when I started my PhD in the group of aquatic ecotoxicology. However, sewing two hundred miniature mosquito nets was required before I finished my thesis.

Inna and Greta sewing mosquito nets. Photo by Inna Nybom
Inna and Greta sewing mosquito nets. Photo by Inna Nybom

My PhD relates to activated carbon, porous carbon material, which has been studied as a new remediation method for contaminated water ecosystems. Due to its wide surface area, activated carbon can bind numerous contaminants efficiently and lock them in the bottom sediments in a way that they are now longer available for organisms living in the area. Reduced bioavailability of the contaminants for benthic organisms reduces also their transport in the food chain and in time this can reduce the contaminant load to humans from fish consumption. However, both old and new remediation methods, where something is removed or added to the natural ecosystem, can disturb the balance in the field, at least temporarily. Therefore, in this study also direct adverse effects of activated carbon to the benthic organisms were followed.

In our resent work the effects of activated carbon were studied on midge Chironomus riparius. Chironomus riparius is a nonbiting midge, with a four-stage life cycle: egg, larva, pupa and adult midge. Larvae is living in the sediment, and they were grown in activated carbon containing contaminated sediment. After few weeks the larvae develops to flying adult stage, and this is where the mosquito nets come in handy. In order to follow the effects of different exposures to the adult stages, 180 small beakers were sorted on the table and covered with individual mosquito nets, food was provided three times a week and all beakers were monitored daily. All the effort payed off when we finally got the results! We observed that adding activated carbon to the sediment reduces the contamination level not only in the larvae stages but also in the flying adult stages exposed during the larvae stage. We know that the aquatic insects going through metamorphosis can transport the contaminants buried in the bottom sediments to the terrestrial food webs, and our result indicates that adding activated carbon to the sediment may reduce this transport of contaminants! Amongst this we discovered many other cool things, which are published in details in Environmental Science and Technology (2016, 50[10] 5252–5260, DOI: 10.1021/acs.est.6b00991). Check it out, if you are interested in finding out more!

A life cycle of Chironomus riparius
A life cycle of Chironomus riparius. Photo by Inna Nybom

I had a great time working in the croup of aquatic ecotoxicology, but now new adventures are ahead. My PhD project came to an end on December last year when I defended my thesis. The most I am going to miss all the great people I got to work with. A great group of excellent scientist with a catching enthusiasm, innovative to come up with the ideas of 200 miniature mosquito nets, and crazy enough to actually sew them with you!