SITE REMEDIATION AND
EMERGENCY RESPONSE NEWSLETTER
February 2010
No. 137
Activated Carbon Amendment Found Effective in Treating Sediment Contaminated by PCBs in a River
How effective is adding activated carbon (AC) as an amendment to treat sediment contaminated by PCBs in a river? Dr. Uphal Ghosh of the University of Maryland Baltimore County presented the results of a field demonstration he conducted in the Grasse River in New York at the Partners in Environmental Technology Technical Symposium and Workshop held last December, sponsored by the Department of Defense offices SERDP and ESTCP. According to Ghosh, “it appears feasible to apply AC under fifteen feet of water on a large scale in a river. The carbon is preserved in the surficial sediments two years after application and it remains effective in reducing contaminant bioavailability.”
Ghosh points out that contaminated sediment sites are large. Current technologies are both expensive and disruptive. There is a need for innovative techniques that reduce risks. He asks, “How does one clean up an ecologically sensitive site without destroying it?” Capping and dredging PCBs in sediments at a U.S. Army base in Virginia, for example, would ruin the ecosystem.
Sediments contain mineral grains (sand, silt and clay), natural organic matter, and black carbons (char, coke and soot). PCBs and PAHs bound to black carbons are less bioavailable, says Ghosh. In the field demonstration in Grasse River, the sediments were mechanically amended with AC in both mixed and unmixed treatment areas. Adding a layer of AC amendment led to more than 70 percent reduction in PCB uptake after brief mixing of AC or no mixing of AC. The longer the AC contact time, the greater the reduction in PCB biouptake. Finer AC works better than coarser AC. AC is slowly worked into the sediment through bioturbation. Over time, clean sediment layers build up.
Results of the Grasse River AC study show that in addition to the fact that applied AC remains in place two years after placement, most samples collected for aqueous and biological uptake testing had AC concentrations at or above the target dosage. The application of AC reduced the extent and rate of sediment PCB desorption. There were only slight community shifts in benthic order distributions. One and two-year post treatment monitoring data were consistent. In sum, monitoring results are positive and support the continued evaluation of the use of AC amendment to treat PCBs in sediments. It is a promising and innovative sediment cleanup technology, according to Ghosh.
As for the cost of using AC amendment, Ghosh points out that five percent AC by dry weight in the top four inches of sediment is equal to six pounds per square yard, the equivalent of 30,000 pounds per acre. (That is equivalent to a mass deposition rate for 2 millimeters per year of sedimentation.) A 100-acre site would require three million pounds of AC. That equals less than one percent of the U.S. annual production of AC. The material cost is $1 per pound of AC. That amounts to $30,000 per acre. As for the application cost, that depends on the method utilized.
He finds the following fields worthy of consideration:
The application of
small increments of AC over multiple years to incorporate into annually
deposited sediments.
The use of
activated biochars
from agricultural residue to provide an additional opportunity for carbon
sequestration.
The application of low impact methods to deliver treatment materials to sediments for in-situ remediation. SediMite™, for example, is a low-impact system for delivering such materials.
Currently, other AC demonstration projects are underway at Hunter’s Point, CA; Trondheim Harbor, Norway; Aberdeen Proving Ground, MD; U.S. Army Installation in Virginia; and Frierfjord, Norway. The contaminants of concern include PCBs, PAHs, PBDEs, DDT, mercury, methylmercury, chlorobenzenes and dioxins.
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