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.
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.
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.
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.
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