Determining PCB Sorption/Desorption Behavior on Sediments Using Selective Supercritical Fluid Extraction. 3. Sorption from Water

Abstract

Supercritical fluid extraction (SFE) with pure CO2 was used to quantitatively remove PCBs from historically contaminated sediments without substantially disturbing their bulk organic or inorganic matrix as evidenced by only small or undetectable changes in thermal gravimetric behavior, elemental (C, H, N, S) composition, ionic conductivity, and pH determined before and after SFE. The extracted PCBs were then spiked into water with the parent sediment, and sorption was allowed to occur for up to 18 days. The selective SFE conditions developed in part 1 were used to determine the proportion of PCBs which could be extracted under four conditions of increasing stringency. Comparing the selective SFE behavior of the PCBs from the water/sediment sorption samples to the original historically contaminated sediments demonstrated that 18 days was not sufficient for PCBs to migrate to the “slower” sediment-binding sites (those sites requiring more rigorous SFE conditions), which the PCBs had occupied in the historically contaminated sediments and that the adsorbed PCBs were primarly associated with the binding sites most easily extracted (“rapidly desorbed”) by SFE. Sediment/water distribution coefficients at 18 days were similar for sediments with low contamination levels (<50 ng/g of individual congeners) and high contamination levels (∼300−4000 ng/g). Apparent distribution coefficients (Kds) at 18 days were similar (within a factor of 2) for the various sediments regardless of congener molecular weight or concentration. Kds increased by a factor of ∼4−8 when exposure was increased from 2 h to 18 days, with the most rapidly sorbed molecules corresponding to those molecules most rapidly desorbed by selective SFE.