Sediment remediation using activated carbon: amending knowledge gaps

Many coastal sediments have accumulated large quantities of contaminants from past anthropogenic activities and now act as a secondary emission source of legacy pollutants to coastal ecosystems. New sediment remediation strategies are needed to address widespread sediment pollution. This thesis focuses on the harbour of Oskarshamn in the Baltic Sea, contaminated by PAHs, PCBs, TBTs, dioxins, and metals from past emissions, and aims to improve knowledge on in situ sediment remediation using activated carbon, a strong sorbent for hydrophobic organic contaminants. Thin-layer capping using activated carbon aims to sequester contaminants in sediment and reduce their bioavailability to aquatic organisms without having to remove or physically isolate the contaminated sediment from the aquatic environment. Questions remain on the efficacy and persistence of activated carbon thin-layer capping in turbulent waters, and on potential adverse effects of activated carbon on benthic communities. We studied the role of activated carbon particle size on contaminant sequestration, sorbent retention (Paper I), and adverse effects in benthic macroinvertebrates (Paper III). We also assessed effects of activated carbon amendment on nutrient cycling and meiofauna communities (Paper II), and whether granular activated carbon (GAC, >300 µm) reduces toxicity to the benthic sentinel amphipod Monoporeia affinis in Oskarshamn harbour (Paper IV). We found that powdered activated carbon (PAC, <300 µm) is much more effective in sequestering PAHs and PCBs than GAC in the short term (Paper I), but that PAC is readily resuspended in turbulent water, whereas GAC may remain on the sediment surface, leading to a higher persistence of GAC over time (Paper I). Thin-layer capping with PAC raised porewater pH and reduced meiofauna abundance, nitrate reduction, nitrate release fluxes, and phosphate release fluxes by at least 50 % (Paper II). This indicates that thin-layer capping with PAC affected both sediment microbial and meiofaunal communities, potentially through the increased pH and sequestration of dissolved organic matter onto activated carbon, rendering it less available to microbial organisms. We observed strong biological responses in macroinvertebrates, with reduced weight, carbon assimilation, and gut microvilli in the polychaete Marenzelleria spp. exposed to ingestible PAC, contrasted by increased weight and carbon assimilation in polychaetes exposed to noningestible GAC (Paper III). This indicates that amendment with PAC caused starvation, i.e., that the polychaete ceased ingesting sediment or that PAC reduced the bioaccessibility of food co-ingested with the sorbent. These effects were present but less pronounced in the clam Limecola balthica (Paper III), indicating that biological responses are species dependent. A toxicity bioassay (Paper IV) showed that sediment amendment using GAC effectively reduced mortality and reproduction impairments in the amphipod Monoporeia affinis. Overall, the thesis demonstrates that GAC may have positive effects on benthic macroinvertebrates, high persistence in turbulent water, and reduces toxicity of highly contaminated sediments. We show that PAC is a highly effective sorbent, but may cause strong adverse effects on benthic macroinvertebrates, meiofauna, and microbial nutrient cycling. Thus, non-ingestible granular activated carbon appears to be a better choice for remediation of contaminated coastal sediments