Photochemical formation and degradation of polychlorinated dibenzofurans and dibenzo-p-dioxins

Polychlorinated dibenzo-p-dioxins (PCDDs) long have been recognized as by-products from the manufacture of certain chlorinated phenols (Bauer et al. 1961). For example, 2, 4, 5-trichlorophenol is prepared industrially by the hydrolysis of 1, 2, 4, 5-tetrachlorobenzene at elevated temperatures and pressure, a process which can also result in the formation of traces of heterocyclic impurities, including 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) (Crosby et al. 1971). Chlorinated phenols have widespread use as fungicides, bactericides, slimicides, herbicides, etc., and they are also utilized in the production of phenoxy acids such as 2, 4-dichloro-and 2, 4, 5-trichlorophenoxy-acetic acid (2, 4,-D and 2, 4, 5-T, respectively). The annual world production of chlorinated phenols is approximately. 100,000 tons (Nilsson et al. 1974). Likewise, for almost four decades, 2, 4, 5,-T and its derivatives have received wide use as herbicides, especially for control of brush, jungle, and aquatic weeds. For instance, the 1968 U.S. production of these compounds exceeded 60 million lb, and residues have been detected in crops, soil, the atmosphere, rainwater, and surface waters (Crosby and Wong 1973 and references cited therein). In addition to PCDDs, polychlorinated dibenzofurans (PCDFs) have been shown to be present as contaminants in commercial chlorinated phenols and related products (Dobbs and Grant 1979, Buser 1976, Choudhry and Hutzinger 1982 b, Pohland and Yang 1972). Moreover, PCDFs have also been identified as contaminants in a number of industrial products such as polychlorinated biphenyls (PCBs) and hexachlorobenzene (Refs. cited in Choudhry et al. 1977 a).