PCDDs and PCDFs are generally ubiquitous in the aquatic environment, particularly in sediments, and transported to and recycled within aquatic systems. PCDDs and PCDFs enter aquatic environments from wet and dry deposition, river inflows, groundwater flow, and direct and indirect discharges from industrial facilities. Dry and wet deposition may be the most important sources of PCDDs and PCDFs to water bodies with large surface areas (lakes and seas). Long-term or temporary sequestration of PCDDs and PCDFs in aquatic systems can occur when bound to particles that settle as sediment, or volatilized across the air-water interface, or by chemical and biological transformations. The latter two processes are possible but less significant than sedimentation.
Dioxins are highly lipophilic and hydrophobic compounds and, as such, are insoluble in water and generally have low Henry's law constant (H) values. The hydropho-bicity of dioxin-like compounds can be ascertained using the octanol-water partition coefficient (Kow), which provides insight on the relationship between bioconcentration and toxicity. Dioxin congeners are considered to be super-hydrophobic because experimentally determined log Kows are typically greater than 6, and as high as 12.
PCDDs and PCDFs partition weakly between parti-culate and dissolved phases. In contrast, PCDDs and PCDFs associate strongly with organic matter; hence, the degradation and mobility of organic carbon in sediment is a significant factor contributing to the mobility of PCDDs and PCDFs. A wide range of sorption partition coefficients have been reported for dioxins, with log Kocs ranging from approximately 4 to 7.5. In surface waters, particulate-bound PCDDs and PCDFs may be incorporated in sediment and recycled at or near the sediment-water interface. In addition, PCDD- and PCDF-bound particles may become incorporated in deeper waters where resuspension and bottom currents are not strong enough to cause further transport, making sediments an important sink for dioxins. Agglomeration (a weak association, held together by surface tension and organic cohesion) or flocculation of particles held together by electrostatic forces also can be important when evaluating PCDD and PCDF fate in surface water. Particulate-asso-ciated PCDDs and PCDFs generally have short residence times in surface waters due to sedimentation or hydraulic flushing of particulates.
PCDD/Fs may be susceptible under certain environmental conditions to microbial degradation; however, research appears to be limited. It is suggested that certain bacteria have the ability to dechlorinate dioxins but the position and rate of dechlorination are dependent on a wide number of factors such as the availability of alternative nutrients, the species of bacteria, and the oxidative state of the sediment.
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