Dioxin formation in incinerators.

ed and unspecified products, respectively. In Figure 2, the pathways of the proposed mechanism are shown schematically. The rate constants assigned to the steps in the mechanism have been given values similar to rate constants which have been reported in the literature for analogous reactions. However, where there are uncertainties, the numerical values have been adjusted somewhat to bias the mechanism in favor of PCDD formation, i.e., worse case modeling. As constructed below, the mechanism is of a general nature. That is, isomer-specific kinetic rate constants are not estimated. In this regard note the following points. Unimolecular rate constants, except for resonance stabilization effects, are not sensitive to substitutions to the bond being broken at positions greater than those corresponding to a (adjacent) substitution (28-30). In addition it is expected that an increasing degree of chlorine substitution of the precursor chlorophenols will result in markedly slower rates of hydroxyl radical attack by addition reactions (30). Worse case modeling implicitly ac722 Environ. Scl. Technol., Vol. 17, No. 12, 1983 counts for these effects in determining the overall (worse case) upper limit probability for PCDD formation as a class of molecules. The latter observation with respect to hydroxyl radical attack implies qualitatively that for gasphase processes, increasingly chlorinated PCDDs should be increasingly recalcitrant to bimolecular decomposition. That is, isomer distributions should be skewed toward higher chlorinated species. More quantitative statements regarding isomer-specific reactions require further experiments and more refined theoretical calculations. As the intent of this report is to examine the overall importance of gas-phase mechanisms for PCDD formation rather than isomer-specific differences, further refinements are not necessary here. Each of the reaction steps as given in Table I is discussed below. The assigned rate constants have been biased to favor PCDD formation. Therefore, the reader is strongly cautioned not to use these estimates out of the context of this paper, as actual values for these rates will be quite different in some cases.