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Show Once fully developed, the P D U should also find applications in other areas including the control of organic air toxics. In this paper w e present a very brief review of the literature on conventional photochemical treatments for hazardous organic wastes, an overview of the theoretical basis for photothermal detoxification, and selected examples of the results to date from the laboratory investigations. Background In 1981 the U.S. Department of Agriculture ( U S D A ) developed a photochemical reactor designed to treat waste water from agricultural activity contaminated with various pollutants related to pesticides.[8] Specifically, polychlorinated biphenyls (PCBs), polychlorinated phenols, and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8- T C D D ) were target compounds selected for testing. In this device, waste water contaminated with about 1 p p m of these materials were exposed to U V radiation supplied by a xenon arc lamps surrounding the reactor, while oxygen was sparged through the reactor. It was found that while the reactor was unable to completely mineralize (i.e. conversion to products of complete oxidation) the waste it could convert the material to compounds amenable to biodegradation. Therefore, the process was completed by spraying the treated water on soil where microbes would digest the photolysis products and the remaining parent wastes. It was found that the overall destruction rate increased from approximately 4-fold for the 2,4,5-T ( 8 0 % destroyed using the combined photochemical and biological treatment vs. 1 5 % by biological treatment alone) and polychlorinated phenols ( 7 5 % vs. 2 0 % ) to 20 for P C B s (20% vs. - 1 % ) . Since the process was unable to directly destroy the wastes it was proposed as a pretreatment operation in biodegradation of pesticide residues. In 1983, the Atlantic Research Corporation further refined the USDA technology into a process called the Light Activated Reduction of Chemicals (LARC).[9] The L A R C reactor emerged as a process for treating P C B contaminated transformer oils. In this application the oil was passed through the L A R C reactor which was illuminated with U V radiation with wavelengths as short as 200 nm. Like the U S D A process, the L A R C reactor was unable to destroy the PCBs directly in a reasonable period of time. To assist the photochemical reactions, sodium metal and hydrogen sparging was added to the reactor to act as chlorine accepters. The process proved very successful at reducing P C B concentrations to less than 2 p p m in the treated oils. The overall approach first demonstrated with the USDA and LARC reactors has been further refined by several firms (i.e., Peroxidation Systems, Solarchem Environmental Systems, Ultrox International, etc.) which have produced commercially available devices for treating groundwater contaminated with low levels of organic wastes.[10] While the various systems differ in their details, they share a c o m m o n operating scheme. Specifically, they destroy the waste compounds through an indirect photochemical process that uses the U V energy provided by either i 111-19 |