Polar Fuel Constituents: Compound Identification and Equilibrium Partitioning between Nonaqueous Phase Liquids and Water.
Groundwater contamination by fuel constituents from nonaq.-phase liqs. (NAPLs) on top of the groundwater table is a widespread problem. While leaching of classical fuel constituents such as benzene, toluene, ethylbenzene, and xylenes (BTEX) from NAPLs into groundwater has been studied extensively, little is known about the identity and partitioning of polar fuel components. Our work shows that gasoline commonly contains appreciable amts. of aniline, phenol, and their alkyl-substituted homologues, as well as a suite of other polar compds. To assess the potential for leaching of such compds. from NAPLs into groundwater we measured the equil. fuel/water partitioning coeffs., Kfw, of some representative constituents in batch systems. Kfw values for the investigated phenols, anilines, benzotriazoles, and S-heterocycles ranged from 0.2 to 1700. These values are up to 3 orders of magnitude lower than the Kfw of benzene. The NAPL-water partitioning of anilines and phenols strongly depends on the compds.' structure as well as on pH and the gasoline compn. (e.g., MTBE content). Linear free energy relationships (LFERs) using Kow or Cwsat (molar aq. soly.) failed to predict measured Kfw values of polar solutes. In contrast, a polyparameter approach taking into account mol. interactions (van der Waals forces, hydrogen bonds) between solutes and major gasoline components allows precise a priori predictions of Kfw values of both polar and BTEX fuel constituents without any fit parameters. Since most of the polar fuel constituents studied here are extd. from NAPLs by groundwater much more efficiently than BTEX, such compds. could form contaminant plumes threatening receiving wells before detectable concns. of BTEX are present.
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