Effects of Salinity Changes and the Formation of Dissolved Organic Matter Coatings on the Sorption of Phenanthrene:  Implications for Pollutant Trapping in Estuaries

Estuaries have been reported to be sinks for hydrophobic pollutants, and sorption has been commonly attributed to be an important mechanism responsible for the observed pollutant trapping. The sorption enhancement caused by “salt effects” and dissolved organic matter (DOM) coatings were both measured and modeled, and the results were used to probe the extent to which equilibrium sorption could explain estuarine pollutant trapping. The polycyclic aromatic compound phenanthrene, an extracellular polymer from a soil bacterial isolate, and a low organic carbon kaolinite were used as models for the hydrophobic pollutant, DOM, and suspended sediment, respectively. Sorptive interactions between phenanthrene, extracellular polymer, and kaolinite were measured at pH 8 as a function of salinity. The experimentally determined binary distribution coefficients were combined using a three-component sorption model to calculate the overall sorption coefficient for phenanthrene, K O. Increasing the ionic strength to seawater levels increased the overall sorption coefficient by 55% as compared to the freshwater value while the presence of polymer coatings increased K O by 9% at all salinities. The three-component model simulation of sorption in the estuary showed that only 0.1% of available phenanthrene would be sorbed to suspended sediment given reasonable estimates of the DOM and particulate concentrations. Order of magnitude analyses carried out with other combinations of estuarine DOM and sediments also fell short of levels required to explain observed estuarine pollutant trapping. These experiments and model simulations lead to the conclu sion that equilibrium sorption of phenanthrene cannot explain the full extent of pollutant trapping in estuaries.