Rates of mass transfer of PAHs from complex NAPLs to water and implications on bioavailability

Polynuclear Aromatic Hydrocarbons (PAHs) are toxic and carcinogenic compounds that often exist as components of complex non-aqueous phase liquid (NAPL) contaminants, e.g. coal tars and creosotes. The objectives of this work were to conduct a quantitative study on component mass transfer from NAPLs and to elucidate bioavailability; i.e., microbial uptake mechanisms. Mass transfer impacts both the extent of contamination and the success of water-based remediation schemes, potentially including bioremediation (depending on PAH bioavailability issues). Model NAPLs were designed to permit a rigorous study since environmental NAPLs are typically variable in composition and incompletely characterized.; Ideal solubility criteria (i.e., mole fraction less than solid-liquid fugacity ratio) were successfully utilized in designing four stable NAPLs, containing toluene and eight PAHs, including naphthalene. These NAPLs were denser than water and differed primarily in their naphthalene content. The rates and extents of mass transfer of the individual NAPL components were studied in segregated phase reactors (SPRs) having a relatively stable interface between internally mixed but segregated aqueous and NAPL phases. Two parameters, the aqueous phase concentration at equilibrium and the film transfer coefficient, were quantified by simulating aqueous concentration profiles with a mass-transfer-limited rate model. NAPL-phase activity coefficients for the various compounds derived from equilibrium aqueous phase concentrations were typically within a factor of two of Raoult's law prediction of unity; refinement of fugacity ratio estimates brought the values even closer to unity. Film transfer coefficients for all components studied were similar in magnitude, in the range {dollar}0.8{lcub}-{rcub}3times10sp{lcub}-3{rcub}{dollar} cm/s, and were independent of NAPL phase component mole fractions, as demonstrated for naphthalene over the mole fraction range of 0.05 to 0.25.; SPR bioavailability studies were conducted using a pure culture and a mixed culture of naphthalene-degrading bacteria, in systems that were mass transfer limited with respect to biodegradation. Comparison of NAPL-phase mass profiles during biodegradation to predicted profiles based on mass transfer parameters measured before and after biodegradation, and assuming a maximum mass transfer driving-force, indicated no evidence of direct interfacial uptake. Biodegradation occurred only after dissolution into the aqueous phase. The formation of biofilms at NAPL-water interfaces hindered mass transfer and biodegradation.