Modeling The Impact Of Sediment Resuspension And Flocculation On The Fate Of Polychlorinated Biphenyls
Chang, Chihwei Andrew
Baker, Joel E
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Hydrophobic organic contaminants (HOCs) are important pollutants in urban estuaries. HOCs include polycyclic aromatic hydrocarbon (PAH), polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs). Sorption to resuspended particles and sediments plays an important role controlling the water column residence times and spatial distributions of HOC in aquatic environments. Pollutant residence times and the time required to reach sorptive equilibrium are highly dependent on the chemical character, the surrounding environment, and particle types and compositions. If rates of sorption are slow relative to particle residence times, HOC behavior may be described using kinetically-limited partitioning behavior. In this study, a flocculation model that simulates flocculation of activated carbon, organic carbon, and inorganic solids ranging in diameter from 2 to 1000 μm has been developed. A multi-class flocculation-based contaminant fate model is adapted to describe desorption kinetics for contaminants associated with flocculated particles during a resuspension event. The model is effective in predicting transport of hydrophobic organic contaminants among different size flocs, water, and two sediment layers. The model also demonstrates the impact of fractal geometry, bottom shear stress, particle composition, floc size, fraction of organic carbon (f<sub>OC</sub>), fraction of activated carbon (f<sub>AC</sub>), organic carbon partition coefficient (K<sub>OC</sub>), and total suspended solids (TSS) on contaminant desorption rate and residence time. Under different scenarios, this model's results support the importance of multi-floc cluster, sediment-water interaction, and of flocculation for the contaminant desorption rate in the water column. In a floc-rich environment flocculation is an important mechanism redistributing contaminants among flocs. When flocculation is considered in a dynamic particle environment that includes sediment resuspension, settling, and kinetic-limited HOC partitioning, the steady state total PCB concentration in the water column is decreased by 20 % and water column HOC residence time decreased by 36%. When activated carbon is added to contaminated sediments, the total PCB concentration in the water column decreases by 90% (123.4 to 11.4 ng/L). If the activated carbon coagulates with the resuspended sediment, this decrease is partially offset by some activated carbon being entrained in slowly-settling flocs, and the steady-state PCB concentration is 61 ng/L.