Effect of temperature on the distribution of polycyclic aromatic hydrocarbons in soil and sediment

The knowledge of sorption-desorption processes of polycyclic aromatic hydrocarbons (PAHs) in natural solids is essential to predict the fate, transport, and environmental risks of these pollutants. In this study, the effect was investigated of temperature on the sorption-desorption of three PAHs (naphthalene, phenanthrene, and pyrene) in two natural solids with different organic carbon contents. In all cases, the sorption isotherms obtained could be well described by the linear sorption model. The analysis based on the measured isotherms and the corresponding equilibrium partition coefficients (Kp) revealed that (1) the sorption of PAHs increased with organic carbon content of the solid and PAH hydrophobicity in the order: sediment < soil and naphthalene < phenanthrene < pyrene, respectively, and (2) the extent of PAH sorption decreased with increasing temperature from 4°C to 27°C on average by 27.3, 17.0, and 27.4% for naphthalene, phenanthrene, and pyrene, respectively. The enthalpies of sorption (δHs) calculated by van't Hoff equation were negative, relatively small, and in the range of weak forces such as van der Waals forces (0-9 kJ/mol), consistent with hydrophobic interactions and partitioning of the PAHs into soil/sediment organic matter. The desorption of naphthalene and phenanthrene showed significant hysteresis, i.e. great fraction of PAHs was resistant to desorption and somewhat increased with temperature.