Introduction
Sorption is a dominant process that affects the ultimate environmental fate of persistent organic pollutants such as polychlorinated biphenyls (PCBs) in sediments. While strong sorption may extend the lifetime and persistence of organic pollutants by reducing their accessibility and hence biodegradability, sorption may also reduce pore water concentrations, ecotoxicity, and biouptake as pollutants become sequestered.
An increasing body of evidence demonstrates that the traditional paradigm used to assess the sorption of hydrophobic organic compounds to sediments, which is based on the fraction of organic carbon (fOC) and the organic carbon−water partitioning coefficient (KOC), cannot correctly explain compound availability and ecotoxicity in field-aged sediments (1−4). The presence of strongly sorbing carbonaceous particles such as soot, char, and coal is postulated to be the reason for the reduced availability and ecotoxicity (5). Thus, relationships have been proposed to improve correlations between solid phase and free aqueous phase concentrations (3,6) that consider sediment total organic carbon (TOC) and black carbon (BC) separately. However, a survey of 114 PAH-contaminated sediments was unable to confirm the broad applicability of such correlations (7). For PCBs the database for a comparable evaluation is still very limited.
In clean water batch systems, coal and charcoal were shown to adsorb PCBs much more strongly (8) than anticipated from estimates based on fOC and KOC correlations. However, when present with sediment, these black carbonaceous adsorbent materials may suffer some apparent reduction in the strength of sorption for PCBs, which may be caused by the presence of other organic matter constituents or anthropogenic compounds (9,10). Furthermore, linear free energy relationships that invoke physicochemical models of the relationship between the amount of PCB sorbed and free aqueous concentrations invoke the assumption of thermodynamic equilibrium. Yet equilibrium may not be attained in field samples, even after a number of years. For example, the great dissimilarity between the physicochemical availability of DDT versus its metabolites, indicates that nonequilibrium may persist in field-contaminated sediments where the DDT has been aged for decades (11). These issues are particularly important for assessing environmental risk and exposure, for example through the use of biota−sediment accumulation factors. These complications have begun to be addressed by quantifying the free aqueous concentration or the readily available fraction of hydrophobic organic contaminants to obtain better predictive relationships for biolipid concentrations than those based on total organic carbon (TOC)-normalized solid phase concentrations (12).
The aim of this work is to comprehensively review traditional and new relationships among solid phase, free aqueous phase, and biolipid PCB concentrations in well-characterized field-contaminated sediments. To this end we compiled data from our previous studies (2,13−17) and a study from the literature (12) to obtain a representative database comprising seven PCB-contaminated freshwater and marine sediments exhibiting a wide range of PCB concentrations, abundance of native BC, and other physicochemical characteristics.