Partitioning of Polychlorinated Dibenzo-<i>p</i>-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/m 3, 1300 ± 300 fg/m 3, 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, K v,g and K v,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 K v,p to K v,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.