Radiation Induced Degradation of PCBs in Various Media

P. Neta, D.L. Poster (839), and M. Chaychian, J. Silverman and M. Al-Sheikhly (Univ. of Maryland)

Objective: To obtain kinetic and mechanistic information on the radiolytic dechlorination of polychlorinated biphenyls (PCBs) in various media and to develop strategies for detoxification of polluted sites by electron beam irradiation.

Problem: The widespread use of polychlorinated biphenyls in various industrial applications presents a major environmental issue because of the toxicity and long lifetime of these compounds in ambient conditions. The method currently used to destroy most PCB containing materials is incineration. This method, however, suffers from two disadvantages: it incinerates the medium along with the PCB, and it converts some of the PCB into more toxic materials, namely dioxins.

Approach: Radiolytic degradation of PCBs is expected to overcome both of these disadvantages. The PCBs in organic solvents such as transformer oils may be reduced into benign inorganic chloride and practically non-toxic biphenyl, without formation of any dioxins. Such treatment leaves the solvents practically unchanged so that they can be recycled instead of incinerated. This approach may be adapted to removal of PCBs in sediments and soils by combining it with extraction or other treatment methods.

Results and Future Plans: We have examined the radiolytic degradation of several PCBs in water/alcohol mixtures, in micellar aqueous mixtures and in transformer oil. We analyzed the various intermediate and final products, investigated the mechanism of reduction, and determined the rate constants for the relevant reactions by pulse radiolysis. Irradiation of PCBs in oil is not expected to lead to dechlorination by direct reaction of the solvated electrons with the PCB, because the oil contains substantial quantities of other aromatic compounds, which also react with solvated electrons very rapidly. Yet, complete degradation of tetrachlorobiphenyl (PCB 54) in transformer oil was achieved by ionizing radiation (200 kGy for 0.27 mg/g PCB 54). By analyzing samples irradiated with various doses, the gradual degradation of PCB 54 and the successive formation and degradation of trichloro-, dichloro-, and monochlorobiphenyl were demonstrated. The final products were mainly biphenyl and inorganic chloride. The mechanism of reductive degradation in transformer oil was established by pulse radiolysis. Irradiation of the oil leads to production of radical anions from the main aromatic components: biphenyl, fluorene, phenathrene, and pyrene. These radical anions transfer an electron to chlorinated biphenyls relatively rapidly, leading to dechlorination. The rate constants for several such reactions, determined individually in 2-propanol solutions, are found to be in the range of 10⁷ to 10⁸ L mol^-1 s^-1. Electron transfer between the various polycyclic aromatics is a reversible process for which the equilibrium depends on the reduction potential of each compound. Electron transfer to a chlorinated compound, however, is irreversible because of dechlorination. Thus, the reaction can proceed to complete dechlorination even if the rate constants for dechlorination are relatively low.

Future experiments are aimed at demonstrating complete radiolytic degradation of PCBs in actual oil samples of varying PCB contents and in sediments contaminated with PCBs. Irradiation of solid sediments is expected to have low radiolytic efficiency. Attempts will be made to enhance the efficiency by using aqueous suspensions with various additives and by combining ultrasonic treatment along with the irradiation to enhance the transfer of the PCB into the liquid component, where it will be efficiently dechlorinated.

Publications:

Schmelling, D. C., Poster, D. L., Chaychian, M., Neta, P., Silverman, J., and Al-Sheikhly, M., "Degradation of Polychlorinated Biphenyls Induced by Ionizing Radiation in Aqueous Micellar Solutions," Environ. Sci. Technol. 32, 270 (1998).