Sediment/water and octanol/water equilibrium partitioning of volatile organic compounds: temperature dependence in the 2–25°C range

The sediment/water (Kd) and octanol/water (Kow) equilibrium partitioning coefficients have been investigated for volatile chlorinated and monocylic aromatic hydrocarbons in the 2 to 25°C temperature range. The equilibrium partitioning in closed systems (EPICS) method has been optimized to measure both equilibrium partitioning coefficients. Sediment/water equilibrium partitioning for a riverine sediment (organic carbon fraction 4.12%) proved to increase with increasing temperature, showing changes in enthalpy between 2.5 and 12.8 kJ•mol−1. A statistical approach showed that for 5 out of 12 compounds investigated this temperature dependence is significant at α=0.05. Next, the temperature dependence of the octanol/water equilibrium partitioning was investigated for 8 compounds. The octanol/water equilibrium partitioning coefficients increased with increasing temperature for all compounds, except for toluene. Changes in enthalpies of this process proved to be between −1.0 and 5.2 kJ•mol−1. However, the temperature dependence of this process was statistically not significant (α=0.05). The log Kd–log Kow relationship showed correlation coefficients between 0.89 and 0.90 at 2.3, 6.2, 18.6 and 25.0°C (n=8). Slopes of the linear regression increased with increasing temperature. This was explained by the higher temperature dependence of sediment/water equilibrium partitioning process for compounds with higher Kd coefficients.