Measurements of octanol-air partition coefficients for PCDD/Fs: a tool in assessing air-soil equilibrium status.

Octanol−air partition coefficients (KOA) 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 KOA 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, KOA 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 KOA value for any PCDD/F at any temperature, i.e., log KOA = 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 KOA-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 (fS/fAmin) 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 fS/fAmax 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)