In situ activated carbon (AC) amendment to limit bio-uptake from PCB contaminated sediment – A mesocosm study with three organisms

Abstract

In situ AC amendment (4.3% by dry weight homogenized with sediment) was investigated in an experimental mesocosm containing PCB contaminated sediment from New Bedford Harbor (New Bedford, MA, USA). AC was applied by homogenization with sediment prior to the start of the experiment. This may be feasible in the field where dredging is performed. In other cases, AC would be applied as a cap (secured with a sand or armor layer to prevent degradation) and would become homogenized over time due to bioturbation and other processes. Experimental conditions are likely representative of field conditions 10 ± 5 years after application. As this is not a practical experimental time frame, homogenization of AC and sediment allows simulation of long term conditions. Efficacy of AC amendment was evaluated by measuring freely dissolved porewater concentrations (Cfree) and lipid normalized tissue concentrations (Clipid), compared to unamended, control mesocosms. Three organisms of varying levels of sediment interaction were tested. Cfree were deduced by low density polyethylene (LDPE) passive sampling. Clipid were deduced by homogenization and extraction of organism tissues. Overall, decreases in toxicity equivalents were achieved with AC amendment. PCB congeners ranging from 2 to 9 chlorines (Cl) were detected in organism tissue and 2 to 8 Cl in sediment porewater. Reductions in Clipid were observed with AC amendment for congeners up to 6 Cl and in Cfree up to 7 Cl. Increases in Clipid were observed across all congeners of 7 – 9 Cl and minor increases in Cfree for one 6 Cl and two 8 Cl congeners indicating mobilization of larger congeners in the presence of the AC amendment. In order to determine whether overall toxicity equivalents were decreased by AC amendment, analysis of the dioxin-like (co-planar) congeners was performed. The sums of Clipid and Cfree for the dioxin-like PCBs decreased by an order of magnitude from 3550 to 361 μg PCB/kg lipid and from 4.73 to 0.121 ng PCB/LWater respectively, indicating a decrease in expected toxicity in AC amended sediments even though increases in the Clipid and Cfree of some individual congeners occurred