Control factors of the marine nitrogen cycle
Capping with activated carbon affects, among other things, the geochemical processes in the sediments.

Adding strong sorbents, such as activated carbon (AC), to the surface of contaminated sediments is a cost-effective and less intrusive remediation technique compared to dredging. Over the last decade thin-layer capping with AC has proved successful in decreasing the bioavailability of organic contaminants and their release to the water column by up to 90 %.

Several studies have, however, also shown that these thin-layer capping techniques have unwanted side effects on benthic organisms, such as a decrease in growth, reproduction or diversity. It has now been observed for the first time that thin-layer capping with AC has negative effects also on sediment geochemical processes and may thus affect, at least initially, essential benthic ecosystem functions. This is concluded in a study made by researchers at DEEP, Stockholm University in cooperation with Aarhus University, Denmark and Vrije Universiteit Brussel, Belgium, which has been partly funded by the Baltic Sea Center.

Experimental study

In a recently published article Stefano Bonaglia and colleagues point out that the side effects of capping with activated carbon affects denitrification, and reduces meiofaunal community and diversity. The study builds on a 4-week laboratory experiment investigating the influence of two different powdered activated carbon materials, derived from bituminous coal and from coconut shell respectively. The materials have been added as a thin surface layer cap on sediment cores collected from the Baltic Sea and compared to the effects of thin cap of clay only.

The observations demonstrate that capping with powdered activated carbon significantly affects benthic biogeochemical fluxes, microbial processes and abundance of benthic meiofauna (organisms < 1mm). The authors suggest that the effects may be due to an increase in porewater pH and to the sequestration of organic matter by the activated carbon particles.

Less invasive methods preferable

Pending longer term studies and field pilot studies to be done the researchers advise caution. More research is needed to better understand negative side effects of these capping materials before thin-layer capping can be used to remediate contaminated sediments in Sweden and elsewhere.

Less invasive methods like monitored natural recovery, i.e. let the system recover by itself, while monitoring for contaminant decrease, may be a better alternative in sediments that do not pose an acute toxicity risk to aquatic organisms.

Dredging or capping with activated carbon should be restricted to heavily contaminated sites where the ecosystem is significantly threatened by the local sediment pollution.

Read the whole article Capping with activated carbon reduces nutrient fluxes, denitrification and meiofauna in contaminated sediments