Partitioning of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans between the Atmosphere and Corn

Abstract

Polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) are lipophilic atmospheric contaminants that accumulate in vegetation, a process which may lead to the contamination of food. Thus, knowledge of PCDD/F accumulation in vegetation is crucial to evaluating human exposure. In an effort to determine this accumulation mechanism, we analyzed matched atmosphere, corn kernel, and corn leaf samples from a private farm in Felicity, OH. Corn is an important feed crop in the United States; both corn kernels and corn leaves (silage) are widely used. PCDD/F concentrations were below the detection limits (0.5−1 pg/g lipids) in the corn kernels. However, we found PCDD/F concentrations of 510 ± 75 fg/m3, 1300 ± 300 fg/m3, and 4.2 ± 1.2 ng/g lipids in the atmospheric gas phase, atmospheric particle phase, and corn leaves, respectively. For each set of air and corn leaf samples, we calculated corn leaf-atmosphere partition coefficients for both the atmospheric gas and particle phases, Kv,g and Kv,p, respectively. On the basis of plots of the natural logarithm of these partition coefficients versus the reciprocal of the average, atmospheric temperature, we conclude that the partitioning of PCDD/F between the atmosphere and corn leaves is dependent on temperature. We calculated the enthalpies of phase change between the vegetation and air for all PCDD/F homologues; our values are comparable to literature values for polycyclic aromatic hydrocarbons and polychlorinated biphenyls. The ratio of Kv,p to Kv,g is a log−log function of the average vapor pressure for each PCDD/F homologue, indicating that vapor pressure plays an important role in the partitioning of both the gas and particle phases to corn leaves.