Donald I. Townsend

Change of isomer ratio and fate of polychlorinated-p-dioxins in the environment

Abstract Polychlorodibenzo-p-dioxin (PCDD) isomers are found in many environmental samples in proportions that are numerically consistent with values predicted by a steady state kinetic model of a red-ox reaction system. Recent data extend the geographical significance of these findings to include regions of the Western United States. In addition, extension of the red-ox hypothesis, is proposed to explain similarities and differences in the behavior of PCDDs from unrelated combustion sources, which includes the possible role of photo-initiated red-ox mechanisms in the removal of PCDDs from the environment.

Reactive chemicals hazard evaluation: Impact of thermal characteristics of transportation/storage vessels

Abstract Runaway reactions occur when the total rate of heat generated in a system exceeds the rate of heat loss by the system. The major source of heat generation is by chemical reaction; this can be quantified calorimetrically, for instance via accelerating rate calorimetry. A review of the theory of adiabatic calorimetry and its application to thermal stability analysis is presented, focusing on simple reactive chemicals safety criteria that can be applied to vessels containing potentially hazardous materials. Heat losses from a given vessel can be determined by filling it with a hot, non-reactive fluid and then measuring the temperatures inside and outside of the vessel as it cools down. Results from a cool-down experiment on a 20000 gallon (75.7 m 3 ) insulated railcar are presented. Additionally, the efficacy of removing heat from the railcar via either hosing it with cooling water or by circulating cooling water in the cars heat exchange coils was evaluated. Time constants were deter- mined for a series different sized vessels, including the railcar. Large vessels have significant thermal inertia and can be poorly mixed (the latter resulting in thermal stratification). Ramifications of this are discussed, both for the routine handling of potentially hazardous mate- rials and for emergency response if an accident were to occur. Runaway reactions were simulated by combining chemical reactions with vessel thermal characteristics with reactions in a model of the time-dependent temperature behavior of the system.

The use of isomer specific data to characterize and differentiate sources of dioxins into the environment

Abstract In a recent comprehensive study of dioxin levels in the environment with several potential historical manufacturing and combustion sources, a simple but effective pattern recognition technique was developed and successfully used to guide the study. Using isomer specific data on 20 tetra isomer groups, the resulting isomer profiles were able to identify dominant sources of dioxin in various areas. Several specific studies and multi-variant statistical procedures were used to confirm the findings.

Evaluation of potential mechanisms governing dioxin congener profiles in soils near combustion sources

Abstract Trace quantities of PCDDs are emitted on particulates from many combustion processes. The composition of the complex mixture can be expressed as congener ratios. Although the latter may vary for different sources, environmental samples appear to have a characteristic composition different from those associated with known sources. It is necessary to postulate the existence of unknown physical/chemical mechanisms to explain the transition. In this study, some of these phenomena are reviewed and evaluated. Non isomer-specific photochemical attack and thermal equilibration are identified as plausible dioxin removal mechanisms from the surface of flyash. A first-order congener ratio decay model is used to generate tentative removal rates that range from 41 h −1 for T 4 CDD/OCDD to 5 h −1 for H 7 CDD/OCDD. These rates may be compared against predictions of candidate removal mechanisms. Differential settling of particulates of various sizes may also contribute to the shift in congener ratios observed in the vicinity of combustion sources, and it is likely that a combination of processes should be considered in future work.

CHLOROPHENATE PYROLYTIC SYNTHESIS OF THE 22 TETRACHLORO-DIBENZO-p-DIOXIN ISOMERS WITH STRUCTURAL CHARACTERIZATION VIA PHOTOLYTIC PROPERTIES

ABSTRACT This paper is a synopsis of experimentation beginning with the pyrolytic condensation of specific potassium chlorophenate salts to produce limited isomeric mixtures of tetrachlorodibenzo-p-dioxins (TCDDs), their subsequent isolation and characterization by sequential application of reverse phase high performance liquid chromatography (RP-HPLC), silica normal phase adsorption-HPLC (Silica-HPLC) and packed column gas chromatography - low resolution mass spectrometry (GC-LRMS), and their final structural identification by application of pattern recognition techniques to photodecomposition rate data obtained via exposure to artificial sunlight as ~25 to 100 parts per billion (pg/μl) solutions in n-hexadecane. Also presented are photodecomposition data for each TCDD isomer under similar exposure conditions to solution data when they are present on a clean soft-glass surface.