Sediment contamination in estuarine and coastal regions is widespread in the United States. Hydrophobic organic compounds (HOCs) such as polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) associate with fine-grained, organic-rich, sediment material. This sediment serves as a contaminant reservoir from which fish and bottom-dwelling organisms accumulate toxic compounds passed up the food chain. Common approaches to managing contaminated sediment are dredging and capping, but both options are expensive, cause habitat alteration and require large-scale material handling and long-term supervision. Dredging may not remove all contaminants, and capping becomes less efficient over time.
The objective of this project was to investigate the feasibility of using a coal-derived sorbent (e.g., coke or granular activated carbon) for in situ stabilization of persistent organic contaminants, such as PCBs and PAHs, in marine sediments.
Sediment from the inter-tidal zone of South Basin, Hunters Point Naval Shipyard, San Francisco Bay was used in this research. Sampling took place during low tide along approximately 150 yards of shoreline. PCB-contaminated sediment was contacted with different sizes and doses of coke and activated carbon for 1 and 6 months. Wet sediment, equivalent to approximately 3.8 kg dry weight, was placed in 1-gallon glass roller bottles, followed by addition of sorbent and approximately 300 mL of 17 ppt seawater. The glass bottles containing sediment and carbon were rolled at 2-3 rpm. At the end of a contact experiment, the treated and control sediments were harvested for physicochemical, and biological tests for contaminant availability.
Sediment characterization by size and density fraction revealed that the heavier density (>1.8 s.g.) mineral fraction of the sediment comprising sand/silt/clays contributed to 94% of the sediment mass but only 32% of the total PCBs and 11% of the total PAHs. In comparison, the lighter density organic fraction comprising coal/charcoal/coke/wood comprised only 6% of the sediment mass but contained 68% of the PCBs and 89% of the PAHs.
Results from three biological and three physicochemical tests showed that PCB availability was reduced significantly when contaminated sediments were treated with activated carbon and mixed for 1 month. Aqueous equilibrium PCB concentrations were reduced by 86% for activated carbon-treated sediment relative to untreated. Total PCBs released to semi-permeable membrane devices decreased by 72% for activated carbon-treated sediment. Treating sediment with activated carbon reduced PCB bioaccumulation by 69% in Macoma balthica (clam). The percent reduction in PCB uptake varied with PCB chlorination level and ranged from 83% for the pentachlorobiphenyls to 47% for the octachlorobiphenyls. The parallel test with coke amendment showed that the addition of coke had no significant impact on PCB bioaccumulation. Similar reductions in PCB bioaccumulation with activated carbon were observed in two other benthic organisms: 83% in Neanthes arenaceodentata (worm) and 72% in Leptocheirus plumulosus (amphipod).
The effect of dose on reducing the chemical availability of PCBs and PAHs and the bioavailability of PCBs was tested by contacting sediment with activated carbon at concentrations of 0.34%, 1.7%, and 3.4%, dry mass basis. Results showed that increasing carbon dose increased the effectiveness of carbon treatment. Adding 3.4% carbon was more effective than adding 1.7%, thus showing that the optimal dose has likely not been reached. A separate experiment using activated carbon crushed to smaller size (< 75μm) showed that smaller particle diameter activated carbon was significantly more effective (98% reduction) at reducing aqueous equilibrium concentrations than uncrushed (75–300 μm) (92% reduction).
Coke decreased PAH aqueous equilibrium concentrations by 38-64%, depending on coke dose and particle size. Adding coke had a negligible effect on reducing PCB bioaccumulation in benthic organisms, probably due to slow kinetics of PCB diffusion into the particles. The greater effectiveness of activated carbon compared to coke is attributed to a much greater available surface area and internal porosity and the ability to bind organic contaminants such as PCBs and PAHs. Results from the physicochemical tests suggested that adding activated carbon to contaminated field sediment reduces HOC availability to the aqueous phase. The benefit is manifested relatively quickly under optimum contact conditions and improves in effectiveness with contact time.
The addition of inexpensive activated carbon to PCB-contaminated sediment can be an effective in situ stabilization method to reduce contaminant availability to biota and surrounding water. Applying fresh, high adsorption affinity and high surface area activated carbon to sediments in the field results in the repartitioning of hydrophobic contaminants from the available sediment components to the applied sorbent phase where the contaminants become much less bioavailable due to the strong binding. These results point to a new, cost-effective concept for sediment management. A follow-on ESTCP project (ER-200510) demonstrated and validated in situ stabilization of PCBs in sediment under field conditions at Hunters Point Naval Shipyard.