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Effects of Natural Organic Matter on PCB-Activated Carbon Sorption Kinetics: Implications for Sediment Capping Applications
Citation:
Fairey, J. L., D. G. Wahman, AND G. V. Lowry. Effects of Natural Organic Matter on PCB-Activated Carbon Sorption Kinetics: Implications for Sediment Capping Applications. JOURNAL OF ENVIRONMENTAL QUALITY. American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Madison, WI, 39(4):1359-1368, (2010).
Impact/Purpose:
To inform the public.
Description:
In-situ capping of polychlorinated biphenyl (PCB) contaminated sediments with layers of sorbents such as activated carbon has been proposed, but several technical questions remain regarding long-term effectiveness. An activated carbon amended sediment cap was mimicked in laboratory-scale fixed-bed (FB) experiments. Sorption of eleven PCB congeners was studied in two background waters – an organic-free water (referred to as OFW) and a synthetic “natural” water made by reconstituting freeze-dried natural organic matter (NOM) from the Suwannee River (referred to as SRW) at a concentration of 10 mg/L as C. In the OFW FB test, no PCBs were detected in the column effluent over the 390-day study, indicating that equilibrium capacities controlled sorption at a hydraulic loading rate (HLR) of 3.1 m/hr. In the SRW FB test, partial breakthrough of the PCBs occurred over the entire 320-day test (HLRs of 3.1-, 1.5-, and 0.8 m/hr). A modified pore and surface diffusion model (PSDM) was used to interpret the SRW FB data. Model simulations indicted that external (film diffusion) mass transfer was the dominant rate-limiting step but that internal (pore diffusion) mass transfer limitations were also present. The external mass transfer limitation could have been a manifestation of PCB-NOM complexes through a combination of: (1) film diffusion, (2) size exclusion effects and/or (3) electrostatic repulsions with NOM-coated activated carbon. Further work (e.g. physicochemical characterizations of PCB-NOM complexes) is needed to elucidate the mechanism(s) responsible for the simulated external mass transfer limitation and ultimately determine if activated carbon is a suitable amendment for sediment-capping applications where NOM is present.
