Irreversible adsorption of chlorinated benzenes to natural sediments: Implications for sediment anality criteria

Desorption of many hydrophobic compounds from natural sediments has been observed to be biphasic, containing reversible and irreversible compartments. Therefore, it is necessary to reevaluate the currently adopted sediment quality criteria (SQCs) that are based on equilibrium partitioning models. In this study, the characteristics of irreversible adsorption were further studied with five chlorinated benzenes and four natural sediments. The five compounds differ significantly in K-ow and solubility, and the four sediments cover a wide range of organic carbon content from 0.27 to 4.1%. With each chemical-sediment combination, a fixed maximum irreversible adsorption capacity, q(max)(irr), was observed. The apparent organic carbon-based partition coefficient associated with this irreversible fraction is essentially constant for different chemical-sediment combinations, with K-oc(irr) = 10(5.42+/-0.17). The desorption data were modeled with a previously proposed irreversible adsorption-desorption model, in which a Langmuirian-type expression (representing the irreversible fraction) is added to a linear model. For the five chlorinated benzenes and Dickinson sediment, there is little correlation between q(max)(irr) and K-ow. For 1,4-dichlorobenzene in different sediments, q(max)(irr) is proportional to soil organic carbon content. The potential impact of this model on sediment quality criteria is also discussed.