Sorption of Phenanthrene to Environmental Black Carbon in Sediment with and without Organic Matter and Native Sorbates

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Abstract
Strong sorption to soot- and charcoal-like material (collectively termed black carbon or BC) in soils and sediments is possibly the reason for recent observations of elevated geosorbent−water distribution ratios, slow desorption, limited uptake, and restricted bioremediation. We evaluated the role of environmental BC in the sorption of phenanthrene (PHE) to a polluted lake sediment from a Rhine River sedimentation area. Sorption isotherms were determined over a wide concentration range (0.0005−6 μg/L) for the original sediment (with organic matter or OM, native sorbates, and BC), sediment from which we had stripped >90% of the native sorbates (only OM and BC), and sediment combusted at 375 °C (only BC). The sorption isotherms of the original and stripped sediments were almost linear (Freundlich coefficient or nF > 0.9), whereas the isotherm of the BC remaining after the sediment combustion was highly nonlinear (nF = 0.54). At low concentrations (ng/L range), PHE sorption to BC in the combusted sediment was found to exceed the total PHE sorption in the original and stripped sediments. This implies that it may not be possible to use a BC−water sorption coefficient measured in combusted sediment to estimate total sorption to the original sediment. This “intrinsic” BC−water sorption coefficient after combustion was calculated to be 9 times larger than the “environmental” one in the untreated sediment. Competition between the added PHE and the native PAHs and/or OM may explain this difference. It appears that, at low aqueous PHE concentrations (ng/L and below), BC is the most important geosorbent constituent with respect to sorption. At higher concentrations (μg/L), BC sorption sites become saturated and BC sorption is overwhelmed by sorption to the other OM constituents. Because sorption is a central process affecting contaminant behavior and ecotoxicity, understanding this process can strongly contribute to risk assessment and fate modeling.

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