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  Bench-Scale Evaluation of Gas Ebullition on the Release of Contaminants From Sediments, Final Report (EPA/600/R-10/062) February 2010

This report describes the performance of microcosm and a bench-scale column studies to attempt to understand and quantify the release of COCs from uncapped and capped sediments. The gas ebullition through the sediment bed was simulated by sparging mixed anaerobic gas at two flow rates (6.5 and 18.5 mL/min).

The microcosm experiments indicated that the serum bottles tested maintained anaerobic conditions. Higher percentages of CH4 and CO2 were contained in the headspace of the Lake Hartwell serum bottles than the Eagle Harbor samples at 37°C. No detectable level of gas was measured at the lower temperatures (10°C and 25°C) for either the Eagle Harbor or Lake Hartwell sediments. Higher concentrations of PAHs (ng/g) were observed in the Eagle Harbor sediment as the temperature increased from 10°C to 37°C. The concentrations of PCBs (ng/g) in the serum bottles containing Lake Hartwell sediment with an incubation temperature of 10°C were higher than those incubated at 25°C and 37°C. However, the PCB concentrations (ng/L) in water increased as the incubation temperature increased from 10°C to 37°C. The PCBs in the Lake Hartwell sediment partitioned into the water phase more strongly at higher temperatures than lower temperatures.

The results of the simulated gas ebullition column experiments showed that the total polycyclic aromatic hydrocarbon (tPAH) captured by the polyurethane foam (PUF) during the 6th and 19th week sampling events from the uncapped Eagle Harbor columns at low gas flow rate (6.5 mL/min) conditions was more than the capped columns. The uncapped PUFs also recovered PAHs with higher molecular weights, which were not detected in the capped PUF. At high gas flow rates (18.7 mL/min), the PUFs captured more tPAH from the uncapped sediment columns than the capped columns. After 19-weeks of gas sparging, the PUFs for the capped column sorbed lower molecular weight PAH compounds, such as 1-methylnaphthalene and C1-naphthalenes, than the uncapped column. However, the PUFs from the uncapped columns consistently sorbed higher molecular weight PAH compounds than the capped column.

The PUFs at the outlet of columns containing Lake Hartwell spiked and unspiked sediment captured 1041 and 164 ng of tPCB, respectively, at low gas flow conditions. The PUFs also captured higher molecular weight PCBs (such as Cl5(110)) from the PCB spiked sediment.

During high gas sparging, the PUFs sorbed more PCBs from columns that were packed with unspiked Lake Hartwell sediment in comparison to the low flow columns. The transfer of PCBs from the sediment to the water column and thereafter to the air appeared to be more dependent on the sparging flow rate than the concentration of PCB in the sediment. Higher concentrations of PCBs (hydrophobic) could be sorbed in the sediment with a low risk of escape as long as the gas ebullition rate was low. Higher gas sparging also resulted in the release of higher molecular weight PCBs.


Terrence Lyons


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