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MA5 Contaminated Harbor and River Sediment
(049) Next generation in situ sediment caps: Sorbent/reagent-amended active caps to contain and treat PCB-contaminated sediments.
Lowry, G1, Murphy, P1, Johnson, K2, Smith, M1, Alanso, S1, 1 Carnegie Mellon University, Pittsburgh, PA, USA2 Quantitative Environmental Analysis, LLC, Montvale, NJ, USA
ABSTRACT- Historical releases of polychlorinated biphenyls (PCBs) have resulted in PCB-contaminated sediments. Ex situ remediation techniques (e.g. dredging plus treatment) are costly, so viable in situ technologies are needed. In situ capping (ISC) is an attractive remedial alternative to dredging. Passive caps (typically 60-100cm of sand or gravel) physically isolate PCB-contaminated sediments from the overlying water column. This reduces risk to benthic organisms and the aquatic community, but does not remove contaminant mass so PCB diffusion through the cap eventually occurs. Active caps contain sorbents and/or reagents that can sequester/destroy PCBs. These improved caps can be less extensive, better mitigate PCB diffusion, and offer the potential for in situ PCB destruction. Fe(0)-based media can dechlorinate many chlorinated organic contaminants. Recent studies indicate that coal-derived materials (e.g. coke, charcoal) strongly sequester PCBs (high Kd), and can reduce their bioavailability. Coke breeze (a coke byproduct) is an inexpensive carbon-based sorbet potentially useful in active caps. The ability of Fe(0) and coke to destroy/sequester PCBs in active cap media was assessed. Laboratory experiments were performed to i) measure PCB transformation rate constants (kr) afforded by different Fe(0)-based media, and ii) determine the PCB-coke distribution coefficients (Kd). PCB transport through different active cap media was simulated based on laboratory measurements of kr and Kd. The performance and costs of active and passive caps are compared. Nanoscale Fe(0) particles dechlorinate PCBs (t1/2 1-3yr) under ambient conditions with predominately meta and para chlorines being removed. No PCB dechlorination was observed using commercial (Peerless) 8-50 mesh Fe(0). Adding coke to sediment caps can significantly enhanced their performance. Model simulations suggest that a very thin (5cm) coke layer is as effective as a 1 meter sand cap and less expensive. Enhanced PCB sequestration in coke-amended caps can lower exposure risk, but does not provide PCB destruction. Coke toxicity appears to pose little or no environmental threat. A field scale pilot demonstration of active cap technology is underway.
Key words: sorption, remediation, carbon, PCB
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