Sediments in coastal areas are particularly exposed to anthropogenic pollution. These sediments can constitute secondary sources by re-introducing contaminants to the environment. This thesis focused on the Baltic Sea which is severely polluted by nutrients, toxic metals, and organic contaminants such as PAHs, PCBs, and dioxins/furans. The excess nutrients cause eutrophication while other pollutants accumulate in high concentrations in fish and top predators, including humans. Strategies to restore the environment include remediation of contaminated sediments, but there are few viable options to remediate all three contaminant groups; nutrients, metals, and organic contaminants.

This thesis investigates innovative methods to remediate contaminated sediments in situ (on site) using thin-layer capping (TLC), consisting in applying a strong sorbent onto the sediment surface to reduce the contaminant release and bioavailability. The main objective was to assess new sorbents and sorbent combinations, such as activated carbon (AC), activated biochar, and calcium-silicate, to advance the knowledge of the field while elucidating both the advantages and disadvantages of the treatments. The sorbents were tested in a diverse set of experiments to determine their remediation potential and to assess their ecological impact on the benthic ecosystem. The research comprised four studies. 

In Paper 1, the efficacy of a new composite TLC of an activated calcium-silicate (Polonite^®) and AC was assessed in a mesocosm study with sediments from the eutrophic and heavily polluted Brunnsviken Bay in Stockholm, Sweden. The composite TLC of Polonite and AC significantly decreased the sediment-to-water release of phosphate, cadmium, and zinc by 70 – 90%, and of PAHs and PCBs by 40 %. These results were encouraging since they were the first to show effective immobilization of all three contaminant groups by a TLC. However, the treatments had some potentially negative effects on the microbial community in the sediment.

Paper 2 is based on a larger-scale mesocosm experiment with sediment from the industrially contaminated Oskarshamn Harbor, Sweden. A composite TLC of AC and Polonite effectively reduced the release of metals into the water column and decreased the bioaccumulation of metals and dioxins/furans in native benthic macrofauna. Moreover, the study revealed that granular AC (300 – 500 µm) did not harm benthic macro- and meiofauna, as has occurred when AC of smaller particle size (powder) has been used previously. Both sorbents altered microbial processes in the sediments.

In Paper 3, activated biochars made from waste timber were tested as novel and sustainable sorbents for sediment PCB, and their performances were compared with that of a commercial coal-based AC. The activated biochars exhibited high sorption efficiency of PCB by reducing concentrations in sediment porewater by up to 99% and decreasing bioavailability by 80%, thereby outperforming the fossil AC.

Lastly, Paper 4 aimed to further assess the applicability of the Polonite as a sorbent for sediment metals. A powdered by-product from the Polonite production (also used in Paper 1 and 2) was characterized and the sorption mechanisms of metals to Polonite were examined using an array of chemical analyses. The high sorption capacity for copper, lead, and zinc was attributed to physical sorption mechanisms associated with carbonates, silicates, and metal-hydroxides on the Polonite surface.

Altogether, this thesis laid important groundwork for future pilot tests in situ with these novel treatments. Additionally, the waste timber activated biochars and the Polonite are by-products from the industry, re-purposed for remediation, showing that the TLC technique can be improved by choosing sustainable materials. The findings provide valuable insights into the potential of innovative composite treatments to address the extremely complex and diverse nature of sediment contamination, with implications for restoring the health and function of polluted coastal aquatic ecosystems.