Distributed reactivity model for sorption by soils and sediments. 15. High-concentration co-contaminant effects on phenanthrene sorption and desorption.

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Soil and sediment materials having organic matter matrixes of different geochemical character were examined with respect to their sorption and desorption of phenanthrene in the presence of order-of-magnitude larger concentrations of trichloroethylene (TCE) and dichlorobenzene (DCB). These co-contaminants depressed phenanthrene sorption in the lowest residual solution phase concentration ranges of that target solute investigated, whereas in its highest residual concentration regions phenanthrene sorption was either not affected or was actually enhanced. In both concentration ranges, the effects observed varied with the hydrophobicity and relative concentration of the co-contaminant and with the geological maturity and associated degree of condensation and aromatization of the soil/sediment organic matter (SOM). Desorption isotherms for phenanthrene indicate the occurrence of increased hysteresis in the presence of high concentrations of DCB and TCE, the effect increasing with increased degree of associated organic condensation. Tests in which high concentrations of DCB and TCE were added after completion of the phenanthrene desorption experiments show clear evidence of partial displacement of sorbed phenanthrene to the solution phase. The results of the work support the concept of SOM glass-transition concentrations, above which matrix deformation occurs and so-called "conditioning effects" are observed.