Large quantities of mixed low level waste (MLLW) that fall under the Toxic Substances Control Act (TSCA) exist and will continue to be generated during D&D operations at DOE sites across the country. Currently, the volume of these wastes is approximately 23,500 m3, and the majority of these wastes (i.e., almost 19,000 m3) consist of PCBs and PCB-contaminated materials. Further, additional PCB-contaminated waste will be generated during D&D operations in the future. The standard process for destruction of this waste is incineration, which generates secondary waste that must be disposed, and the TSCA incinerator at Oak Ridge has an uncertain future. Beyond incineration, no proposed process for the recovery and/or destruction of these persistent pollutants has emerged as the preferred choice for DOE cleanup. The main objective of the project was to investigate and develop a deeper understanding of the thermodynamic and kinetic reactions involved in the extraction and destruction of polychlorinated biphenyls (PCBs) from low-level mixed waste solid matrices in order to provide data that would permit the design of a combined-cycle extraction/destruction process. The specific research objectives were to investigate benign dense-fluid extraction with either carbon dioxide (USC) or hot water (CU), followed by destruction of the extracted PCBs via either electrochemical (USC) or hydrothermal (CU) oxidation. Two key advantages of the process are that it isolates and concentrates the PCBs from the solid matrices (thereby reducing waste volume greatly and removing the remaining low-level mixed waste from TSCA control), and little, if any, secondary solvent or solid wastes are generated. This project was a collaborative effort involving the University of South Carolina (USC), Clemson University (CU), and Westinghouse Savannah River Company (WSRC) (including the Savannah River Technology Center, Facilities Decommissioning Division and Regulatory Compliance). T he project was directed and coordinated by the South Carolina Universities Research and Education Foundation (SCUREF), a consortium of the four public research universities in South Carolina. The original plan was to investigate two PCB extraction processes (supercritical carbon dioxide and hot, pressurized water) and two PCB destruction processes (electrochemical oxidation and hydrothermal oxidation). However, at approximately the mid-point of the three year project, it was decided to focus on the more promising extraction process (supercritical carbon dioxide) and the more promising destruction process (supercritical water oxidation). This decision was taken because the investigation of two processes simultaneously by each university was stretching resources too thin, and because the electrochemical oxidation process needed more concentrated research before it would be ready for application to PCB destruction. The solid matrix chosen for experimental work was Toxi-dry, a commonly used adsorbent made from plant material that is used in cleanup of spills and/or liquid solvents. The Toxi-dry was supplied by the research team member from the Facilities Decommissioning Division, WSRC. This adsorbent is a major component of job control waste.
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