Sorption behavior and modeling of endocrine-disrupting chemicals on natural sediments: role of biofilm covered on surface

The surfaces of natural sediments are ubiquitously coated by biofilms that increase the content of organic matter in sediments. However, it is less understood whether the biofilms act as a sorbent or a barrier of mass transfer from water column to sediment phase. This study focused on the role of biofilms coverage on sediments in the sorption of bisphenol A (BPA), 17 alpha-ethinyl estradiol (EE2), and 4-nonylphenols (4-NP) as model compounds for endocrine-disrupting chemicals (EDCs). The OC-normalized distribution coefficients (k (OC)) for BPA, EE2 and 4-NP ranged from 10(1.87) to 10(3.09) l/kg, the k (OC) of EE2 was slightly higher (10(2.23) l/kg) for sediment after H2O2 oxidation than before (10(1.93) l/kg). A two-stage model with a fast section and slow section was employed to describe the sorption process (r (2) > 0.95). The model results showed that the fast sorption section played a main role in the sorption process, while the slow section determined the extent of the reaction (the second-phase partition coefficient (k (p2)) ranged from 11.7 to 118.9 l/kg). The ratios of the mass transfer rate constant of the two stages for the natural sediment ranged from 6.0 to 7.2, which were somewhat lower than those for soil samples. These results indicated that the biofilm coverage on sediment may act as a barrier in mass transfer from water to sediment and scarcely increased the sorption capacity of sediments.