Pathways of Congener-Specific PCB Dechlorination by Palladized Magnesium

PCB contamination in sediments remains a significant problem in many rivers, harbors, and estuarine areas in the US and around the world, and continues to provide PCBs to the food chain despite the long ban on PCB manufacture and use.
This work focuses on investigating PCB dechlorination in pure solvents by a palladium-coated magnesium bimetal (Mg/Pd, 99.9%/0.1% by weight) supplied to the University of NH by NASA. Rapid degradation of single PCB congeners (BZ 3, 170 and 197) by Mg/Pd has been demonstrated in pure solvent systems (10% methanol in distilled water). More than 90% of the initial PCB was removed in 10 to 25 minutes. In experiments conducted with Aroclor 1260, about 67% of the total initial PCBs were removed in 20 minutes. No degradation byproduct was observed in these experiments.

The behavior of biphenyl, the expected degradation byproduct, was also investigated in the same pure solvent system. Rapid removal of biphenyl was observed in these studies (97% removal in 10 minutes). It was hypothesized that biphenyl was volatilized, adsorbed to the Mg/Pd surface or transformed into some unknown organic compound.

Preliminary mass balance experiments conducted with Aroclor 1260, single congeners BZ 194 and BZ 204, and Biphenyl indicated that volatilization was not significant. In experiments with Aroclor 1260 and the single congeners increases in chloride concentrations were observed in the treated samples indicating that dechlorination was occurring. A significant amount of PCBs were extracted from the filtered Mg/Pd material suggesting that PCBs first adsorb to the surface of the bimetal and then dechlorination occurs. The mass balance still could not be closed because of the reaction of the biphenyl. Present efforts are focusing on identifying potential degradation byproducts.

Understanding the fundamentals of the dechlorination pathways of single PCB congeners and identifying the reaction end products is essential to be able to better predict treatment efficiencies in more complex sediment systems. Preliminary experiments in PCB-contaminated sediments from the Housatonic River, New Bedford Harbor, and Hunter's Point indicate that there is PCB mass reduction occurring by Mg/Pd at a slower rate, over a period of a few days. Contaminant degradation by Mg/Pd is a potential promising technology that could effectively remediate PCB-contaminated sediment to very low levels in both in-situ and ex-situ processes.

Research slides (pdf)

 

Contact Information

Dr. Kevin Gardner
Director
Center for Contaminated Sediments Research
336 Environmental Technology Building
University of New Hampshire
Durham, NH 03824
603-862-4334 [phone]
603-862-3957 [fax]
kevin.gardner@unh.edu