Sorption of selected endocrine disrupters by synthetic membrane vesicles and effects of natural organic matter
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In this study, synthetic membrane vesicles (liposomes) were prepared from phospholipids and cholesterol as a model biological phase, and the bioavailability of four estrogens and five estrogenic compounds were evaluated by determining the sorption coefficients (Klipw values) into liposomes. Since natural organic matter (NOM) is likely to bind hydrophobic organics such as endocrine disrupters and influence their bioavailability and the fate in the environment, the effects of a wide variety of model NOM compounds, including humic substances, polysaccharides, and tannic acid, on the liposome-water system were investigated by determining the sorption coefficients onto NOM (Koc values). Reasonably reproducible preparations of liposome suspensions were characterized by quasi-elastic laser light scattering (QELS) analysis and transmission electron microscopy (TEM). Reproducible measurements of Klipw values were obtained by the equilibrium dialysis technique. The Klipw values determined strongly depend on the lipid components of the liposome. Liposome with shorter acyl-chain or increased number of double bonds had higher values of Klipw, while those including cholesterol had significantly lower Klipw values than those without. Moreover, log Klipw values of the selected endocrine disrupters did not show a strong linear correlation with their log Kow. Thus, the use of linear free energy relationships (LFER) to estimate Klipw values is limited for these compounds with a wide variety of chemical structures. Koc values for the selected NOM compounds were determined by the fluorescence quenching technique. The Koc values for the selected compounds had a good linear correlation with the absorptivity at 272 nm and the concentration of phenolic groups of each NOM. However, log Koc showed no correlation with log Kow for the selected endocrine disrupters. Nominal sorption coefficients into liposome ( ) slightly decreased with an increase of NOM concentration, but the decrease was not significant at the highest NOM concentration. Simple models developed in this study showed satisfactory agreement of the estimated K values with those measured for two humic acids. Other linear sorption models were developed for activated sludge treatment and found to agree with literature results. Simple models were also developed to estimate the relative health-related risks of endocrine disrupters.