Mechanisms affecting the dissolution of nonaqueous phase liquids into the aqueous phase in slow stirring batch systems

Understanding the kinetics of the exchange processes between nonaqueous pha se liquids (NAPLs) and water is important in predicting the fate of anthrop ogenic compounds such as petroleum hydrocarbons, i.e., benzene, toluene, et hylbenzene, and xylene (BTEX) as well as polynuclear aromatic hydrocarbons (PAHs). Exchange processes occurring in the environment resemble the experi mental setup of the slow-stirring method (SSM) designed to determine solubi lities and octanol-water partition coefficients. Data obtained from SSM exp eriments for diesel fuel compounds are interpreted by a linear transfer mod el that is characterized by an aqueous molecular boundary layer and the wat er/NAPL equilibrium partition coefficient. For the chosen experimental setu p, the boundary layer thickness is 2.42 x 10(-2) cm. Typical equilibration times lie between 1 and 2 d. Due to the temperature dependence of the aqueo us diffusivity, this time increases with decreasing temperature. Transport within the NAPL phase can slow down the exchange process for the more water -soluble compounds (e.g., benzene) provided that the stirring rate exceeds a critical value.