Multicomponent NAPL Solidification Thermodynamics

Nonaqueous phase liquid (NAPL) contaminants that are chemical mixtures often contain compounds that are solids in their pure states. In the environment, weathering processes cause shifts in multicomponent NAPL composition, thereby enriching the NAPL in the less soluble compounds which may result in their eventual solidification. In this paper, we review the thermodynamic theory governing solid–liquid phase equilibria for the multicomponent NAPLs, and we present experimental observations of such phase equilibria for binary, ternary, and quaternary mixtures of polycyclic aromatic hydrocarbons (PAHs). If the NAPL phase behaves as an ideal solution and if the solid precipitate is pure, then a compound’s mole fraction solubility limit in the NAPL phase equals its solid–liquid reference fugacity ratio. This value is a constant at the temperature of the system. If the NAPL phase is a non-ideal solvent or if the solid is a solid solution, prediction of NAPL solidification in the environment is considerably more difficult. Experimental results indicate that for compounds such as naphthalene and acenaphthene, the solid–liquid reference fugacity ratio serves as a good indicator of the solubility limits in the NAPL phase. For phenanthrene, the solids that form when this compound exceeds its solubility limit are solid solutions that consistently include large portions of 2-methylnaphthalene. These results suggest that the independent behavior implied by ideal solubility theory may not be an accurate descriptor of NAPL solidification phenomena for all PAH-containing NAPLs.