Measurements of Octanol−Air Partition Coefficients for PCDD/Fs: A Tool in Assessing Air−Soil Equilibrium Status

Octanol−air partition coefficients ( K OA ) were measured for nine dioxins (PCDDs) and one furan (PCDF) over the temperature range 0−50 °C using a generator column method. Temperature slopes for plots of log K OA versus inverse absolute temperature ranged from 3200 to 5541 with corresponding enthalpies of phase change associated with the transition from octanol to air of 61−106 kJ mol -1 . These values were higher than determined for polychlorinated biphenyls (PCBs) and polychlorinated naphthalenes (PCNs) having the same degree of chlorination. When plotted against the subcooled liquid vapor pressure on a log−log scale, K OA values for PCDD/Fs separated into two groupings. Group 1 ( n = 5) defined as any PCDD/F not in group 2, and group 2 ( n = 5) defined as any tetra- to hexachlorinated PCDD/F having 3 or 4 chlorine atoms in the 2,3,7,8 substitution positions. Published retention time indices (RTI) were used to develop an expression for determining the K OA value for any PCDD/F at any temperature, i.e., log K OA = a ‘ + b ‘ (RTI). For group 1: a ‘ = 1672/ T − 2.98 and b ‘ = 0.857/ T + 7e-5. For group 2: a ‘ = 986/ T + 0.55 and b ‘ = 1.714/ T − 0.0032. This was used to assess the soil−air equilibrium status of several PCDD/Fs ( n = 17) using previously measured maximum and minimum congener-specific total (i.e., gas + particulate) air concentrations at a semirural location in the northwest U.K. and mean residues for rural soil. A K OA -based absorption model was used to predict the partitioning of PCDD/Fs to atmospheric particulate matter and to estimate gas-phase concentrations. For low air concentration events, the calculated soil/air fugacity ratio ( f S / f Amin ) was less than unity for most congeners but within a factor of 5−10 indicating that soil-to-air transfer may at least partially contribute to atmospheric burdens. When maximum air concentrations were used, all congeners were far-removed from soil−air equilibrium and f S / f Amax values were much less than unity, indicating net gas-phase deposition to soil. In summary then, unlike many other persistent organic pollutants (e.g., PCBs and OC pesticides) the net flux across the air−soil interface is still into the soil at the present time. This wide range in soil−air fugacity ratios is due to the complexity of PCDD/F atmospheric burdens that are influenced by a combination of primary and secondary emissions, superimposed by a seasonally dependent primary combustion signal (domestic burning).