This study assessed the partitioning of 2,2',4,4'-tetrabromo diphenyl ether (BDE-47), 2,2',4,4',5-pentabromo diphenyl ether (BDE-99) and benzo[a]pyrene (BaP) among water, dissolved organic carbon (DOC: 4.93-8.72 mg/L), and particulate organic carbon (POC: 191-462 µg/L) along the salinity gradient (0-5.5‰) of the Baltic Sea off the coast of Finland. Equilibrium dialysis and two solid-phase extraction techniques using polyoxymethylene polymer (POM) were used to determine partitioning coefficients. Experiments using artificial coastal water (ACW) with Nordic fulvic (FAs) and humic acids (HAs) were used to assess the effect of salinity (0 and 5.5‰) on the DOC-water partitioning of the model compounds. All three compounds bound more (2.2-3.8-fold) to the HAs than to the FAs. Increasing salinity from 0 to 5.5‰ decreased sorption to dissolved humic substances in the ACW and Baltic Sea water samples. Along the salinity gradient, the sorption of compounds to organic material decreased when the salinity increased. Particulate organic matter sorbed model compounds per unit of carbon more than dissolved organic matter. Along the studied salinity gradient, the freely dissolved portion increased from 10 to 29% to 52 to 80% in the coastal water samples, mainly because of the increasing salinity and changes in DOC and quality of POC.

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