The scientific and practical objectives have been to develop a new process to displace soil extraction/high temperature incineration as a technology for PCB destruction. A new technology for PCB and chlorinated pesticide detoxification by low temperature homogeneous catalytic chlorine displacement has been proposed. It is envisioned that a one-step PCB soil extraction/soil detoxification process would be conceivable in conventional PCB extraction solvents. If proven, this new process could be demonstrated in environmentally-benign supercritical fluid carbon dioxide. This new concept should be applicable to real-world soil and sediment contamination sites. An estimated 900 million metric tons of soil are contaminated with PCBs in the United States.
The scientific approach is based upon the industrial use of zero valent nickel/organophosphorus (ligand) complexes as established industrial catalysts for synthesis of useful organic intermediates. These compounds are also known to activate or insert into the carbon chlorine bonds of aliphatic and aromatic halocarbon compounds. Alkoxyborohydride reagents are established compounds for reducing nickel to the zero-valent state and are also a source of hydide (H) or hydrogen for hydrogenolysis of carbon-metal-chorine bonds. By coupling these known independent scientific facts, a catalytic process for reductive dechlorination of PCBs was proposed. Based upon Industrial precedents, soluble low-valent nickel/ ligand complexes are also extractable in conventional solvents used for PCB extraction from soils. Hence, a one-step soil extraction/PCB detoxification process was proposed as part of the scientific approach.
This approach to PCB detoxification would expect to result in lower molecular weight dechlorinated PCB congeners with mainly ortho substitution patterns and, in theory, 100% biphenyl from commercial Aroclor compositions. Sodium chloride (salt) would be a by-product and final chorine sink. Biphenyl is non-toxic and completely biodegradable. Chlorine could be recovered and recycled by the well established industrial process for caustic-chorine production by Naflon membrane/electrolysis of brine. Ortho-substituted low molecular weight PCB congeners are known to be much less toxic. It has been demonstrated in the first year of research that five homogeneous nickel/ligand complexes are all active for reductive dechlorination of PCB congeners, using non-toxic decachlorobiphenyl as a model compound. In particular, catalysts for selective meta-para chlorine displacement have been established. A distinct correlation exists for percent chlorine displacement versus the molecular size and stereochemistry of the nickel/ligand fragment. Nickel/ triethylphosphine complexes are, to date, the most active for the RD process giving a lower molecular weight product distribution of mostly ortho products and a 15% yield of biphenyl (complete dechlorination product). The RD process has been demonstrated to be active even at room temperature, owing to the high reactivity of the soluble nickel complexes. In addition, we have been able to extract and simultaneously dechlorinate the model congener in synthetic soll (sillica), using a conventional Soxhlet apparatus typically used for PCB extraction of small environmental samples. Aroclors are now being tested in one step Soil Extraction/ PCB Detoxification (the UGASEPD process).
This new approach would displace conventional extraction/high temperature incineration as method of destroying PCBs. High temperature incineration is controversial since it is costly and leads to incompletely oxidized products as well as the highly toxic chlorinated dibenzo-furans and chlorinated dibenzo-dioxins. This new technology should be applicable to contaminated soils from the controlled state-operated PCB landfills and United States Army, Navy, and Air Force PCB landfills at military bases. The established chemistry should be applicable to the highly toxic dioxin-like congeners, which are only meta-para substituted derivatives, the tetrachlorodibenzofurans (only meta-para substituted) and the tetrachlorodibenzodioxins (only meta-para substituted). This interesting furan/dioxin idea will be demonstrated in future, funded research.
Supplemental Keywords: waste management, hazardous waste treatment, PCB detoxification, PCB removal by dehalogenation, soil extraction/PCB detoxification
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Last Updated: March 25, 2002