Influence of sorbate planarity on the magnitude of rapidly desorbing fractions of organic compounds in sediment.

Organic compounds in sediments are known to distribute between rapid, slowly, and very slowly desorbing sites. This distribution is relevant to bioavailability and risk assessment of organic compounds in sediment. In this study, the fraction desorbing to Tenax in 6 h was measured for a range of organic compounds in sediment differing in their extent of planarity. The aim was to determine the influence of the extent of planarity on the distribution over the rapidly desorbing sites on the one hand and the slowly and very slowly desorbing sites on the other. The magnitude of rapidly desorbing fractions, calculated from the fractions desorbed to Tenax in 6 h, decreased with increasing extent of planarity, expressed as sorbate thickness. For a thickness of less than 5.5 to 6 A, rapidly desorbing fractions are approximately equal to those for fully planar compounds, such as polycyclic aromatic hydrocarbons, which have a thickness of about 3 A. This is in agreement with previously reported differences in sediment-water distribution coefficients between planar polycyclic aromatic hydrocarbons (PAHs) and nonplanar PCBs. The present findings suggest that simple molecular modeling of the thickness of nonplanar organic compounds enables the estimation of the affinity for rapidly desorbing sites relative to planar compounds.