Multicomponent NAPL solidification thermodynamics

Nonaqueous phase liquid (NAPL) contaminants that are chemical mixtures ofte n contain compounds that are solids in their pure states. In the environmen t, weathering processes cause shifts in multicomponent NAPL composition, th ereby enriching the NAPL in the less soluble compounds which may result in their eventual solidification. In this paper, we review the thermodynamic t heory governing solid-liquid phase equilibria for the multicomponent NAPLs, and we present experimental observations of such phase equilibria for bina ry, ternary, and quaternary mixtures of polycyclic aromatic hydrocarbons (P AHs). If the NAPL phase behaves as an ideal solution and if the solid preci pitate is pure, then a compound's mole fraction solubility limit in the NAP L phase equals its solid-liquid reference fugacity ratio. This value is a c onstant 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 solidificat ion in the environment is considerably more difficult. Experimental results indicate that for compounds such as naphthalene and acenaphthene, the soli d-liquid reference fugacity ratio serves as a good indicator of the solubil ity limits in the NAPL phase. For phenanthrene, the solids that form when t his compound exceeds its solubility limit are solid solutions that consiste ntly include large portions of 2-methylnaphthalene. These results suggest t hat the independent behavior implied by ideal solubility theory may not be an accurate descriptor of NAPL solidification phenomena for all PAH-contain ing NAPLs.