Using solid-phase microextraction to evaluate the role of different carbon matrices in the distribution of PAHs in sediment-porewater systems of the Baltic Sea

Purpose The speciation of polycyclic aromatic hydrocarbons (PAHs) in sediment-porewater systems affects both the chemical fate and bioavailability of these compounds. PAHs may be dissolved or sorbed to sediment particles or dissolved organic carbon (DOC). Furthermore, soot carbon has been shown to control the sorption of PAHs onto particles in natural waters. The present study investigates the distribution of individual PAHs among these three phases by examining sediments from the western Baltic Sea, focusing on a highly contaminated former dumping area and evaluating the importance of soot-carbon partitioning. Materials and methods Freely dissolved concentrations of nine PAHs were measured in the porewater of sediment samples using solid-phase microextraction, a cost- and time-efficient method with detection limits in the lower nanogram per liter range. Additionally, total sediment and total porewater concentration were measured in sediments to calculate the distribution of PAHs between the three compartments. Dissolved organic carbon, total organic carbon, and soot content were analyzed for calculation of partitioning coefficients. Results and discussion Soot-carbon control partitioning between the dissolved and particulate phases and the partition coefficients are orders of magnitude higher than would be expected on the basis of organic carbon/water partitioning alone. The carbon-normalized partition coefficient for the distribution of individual PAHs between DOC and the dissolved phase exhibit no more than a weak correlation with hydrophobicity. This indicates that there are different sorption mechanisms at work in DOC than in particulate organic carbon and soot. Conclusions The sediment-water partition coefficient is a key parameter in the risk assessment of PAHs in sediments, as it determines the chemical activity of the PAHs in water, which in turn determines bioconcentration. The PAH partitioning observed in this study suggest that partitioning models are of limited value in PAH risk assessment if the sorbents under study differ markedly from those used to develop the model. In such cases, PAH risk assessment should rely on measurements of PAH activity. In this work, we showed how such measurements can be performed in a comparatively simple way.