Studies on the vapor behavior of selected polychlorinated biphenyls
Titel:
Studies on the vapor behavior of selected polychlorinated biphenyls
Auteur:
Haque, Rizwanul Kohnert, Rodger
Verschenen in:
Journal of environmental science and health. Part B, Pesticides, food contaminants, and agricultural wastes
Paginering:
Jaargang 11 (1976) nr. 3 pagina's 253-264
Jaar:
1976
Inhoud:
The vapor loss of several PCB isomers from stainless steel planchets surfaces has been studied. Experiments were carried out under two different conditions. In one case, the vapor loss was observed from 2 molecular layers of PCB, while in the other series, the loss from approximately 200 molecular layers was observed. In both cases the loss of PCB from the surface was substantial, although the controlling mechanism of vapor loss was different. In the first case, the transport was approximated by a simple first order rate law, whereas in the latter case, it was a diffusion controlled process. In general, the vapor loss decreased with increasing number of chlorine atoms present in the isomer. The 4,4'-dichlorobiphenyl showed a deviation as compared to others indicating that such factors as the electronic distribution, position of chlorine atom on the biphenyl ring and symmetry of the molecule are also important in discussing the vapor transport. Although the production of polychlorinated biphenyls (PCBs) in the U.S. reached a peak during 1970 (∼33 × 106 kg), it has now declined substantially. Some of the important uses of PCB include heat transfer fluids, plasticizers, dielectric coating for capacitors, and hydraulic lubricant. There is evidence of environmental contamination due to such chemicals (Nelson, 1970; Gustafson, 1970, 1972). A major environmental problem is the persistent nature of PCBs. It has been suggested that PCBs do accumulate in soil, ocean, fresh water, sediment, and biota (Risebrough and DeLappe, 1972; Peakall and Lincer, 1970). However, the mechanism of transport of these chemicals is not very well understood. This is especially true for the transport via vapor loss. The vapor pressure of chlorobiphenyls should depend on the number of chlorine atoms present in the isomer. The vapor pressure of the formulated product increases from Aroclor 1232 to 1254 (32% and 54% chlorine weight). Monsanto gives an approximate vapor pressure of Aroclor 1232 and 1254 as 5 × 10-3 and 6 × 10-6 mm Hg, respectively at 37.8°C as compared to 2.4 × 10-6 mm Hg for DDT at the same temperature. Although little information is available on the vapor pressure of individual chlorobiphenyls, the vapor pressure of 4,4'-dichlorobiphenyl has been reported by Smith et al. (1964) to be 1.12 × 103 mm Hg at 30°C. At room temperature the vapor pressure of PCBs in general is in the range of 10-3 to 10-6 mm Hg, but increases to 10-1 mm range at over 100°C. Thus PCBs exposed to the air are capable of entering the atmosphere at normal temperatures at moderate rates. It has been estimated that total loss of PCB in the atmosphere is approximately 1.5-2.5 × 103 tons/year (Nelson, 1972). Once a chlorobiphenyl is introduced in the atmosphere, two factors which control the fate are adsorption on particulate matter and the photolysis. In view of the hydrophobic nature of chlorobiphenyls, these chemicals show strong adsorption tendencies on surfaces (Haque et al., 1974). The photolysis of the higher chlorine molecules may result in the loss of chlorine atoms, a lower chlorine number (Hutzinger et al., 1972), and thus increasing the vapor transport. In spite of these qualitative observations, information relating vapor loss to vapor pressure, loss from different surfaces, chlorine number and position is lacking. Recent studies have shown that (Haque et al., 1974) the vapor loss of Aroclor 1254 from itself and from sand surface was significant, but was negligible from the soil surface. Apparently the adsorption of PCB on the soil was responsible for the reduced vapor loss. The vapor loss also showed a decrease for higher chlorine molecules in the formulated product Aroclor 1254. This paper describes the vapor loss of selected chlorobiphenyls from a stainless steel planchet surface at room temperature.
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