PCB Desorption Kinetics
An important consideration when invoking linear free energy relationships is whether thermodynamic equilibrium is attained between sediment and pollutants. Kinetic rates of sorption can indicate the time required for pollutants to reach sorption equilibrium in sediment. Previously published desorption data (2,13,14) were therefore interpreted with a two compartment diffusion kinetic release model described by Werner et al. (21). Apparent, first-order diffusion kinetic rates (Dapp,fast/R2) for the readily released PCB mass fraction ranged from 10−5 to 10−8 (s−1). As shown in Figure S1, the logarithms of the faster release rates log(Dapp,fast/R2) are comparably inversely related to log Kd for different PCB congeners in the sediments for which diffusion kinetic rates were fitted. Decreasing desorption rate with increasing compound hydrophobicity is expected and has been documented previously (22). Slower release rates Dapp,slow/R2 fitted by the two compartment model were mostly between 10−7 and 10−10 (s−1) and in some cases were too small to be reliably determined from the experimental desorption data, as it was obtained over at most a two month period. The majority of the total PCB mass is generally associated with the faster of the two fitted diffusion kinetic release rates, and the mass fraction released over a one month period is mostly greater than 75%, as shown in Table 1. The exception to this is Hunters Point (HP) sediment where only 43% of the PCB mass is associated with the faster release rate. Furthermore, for the 57% PCB mass associated with the more slowly released fraction, many PCB congeners have sorption kinetic rates Dapp,slow/R2 below 10−10 (s−1), which indicate equilibration times of more than a decade. The slower release of PCBs from HP sediment is likely due to three factors: a dominance of more hydrophobic PCB congeners (mostly heptachlorobiphenyls), a coarser grain size, and/or a high abundance of BC. Given several decades in the field, it seems, however, reasonable to assume that most PCB congeners and sediments have approached thermodynamic equilibrium, with the possible exception of a portion of the PCBs associated with coarse, strong sorbents in HP sediment. Linear free energy relationships between solid phase and free aqueous phase PCB concentrations may therefore be discussed.