Phase Stability of Multicomponent NAPLs Containing PAHs

Multicomponent nonaqueous phase liquid (NAPL) contaminants that contain compounds that are solids in their pure states can undergo phase transformations. This paper examines the relationship between NAPL composition and liquid phase stability for mixtures of polycyclic aromatic hydrocarbons (PAHs), most of which are solids in pure form at ambient temperatures. Because any natural or engineered process that acts to selectively extract compounds will alter NAPL composition, knowledge of phase stability is an important determinant in risk assessment, remediation effectiveness, and research. Ideal solubility theory dictates that a NAPL will be a homogeneous liquid if each constituent’s mole fraction is less than the solid-liquid reference fugacity ratio at the system temperature. Through experimental observation of binary and ternary systems, it is shown that ideal solubility theory is a reliable guide for predicting PAH-NAPL phase stability. Uncertainty in melting temperature and entropy of fusion is a significant determinant that may limit predictive power. The principle of liquefaction through mixing was exploited to produce complex synthetic NAPLs which can be used as surrogate materials to simulate the behavior of contaminants such as coal tars. NAPL/aqueous phase equilibrium studies were conducted to demonstrate the utility of these materials in experimentation.