Superfund Research Program
Human Cell Mutagen Formation During the Thermal Destruction of Hazardous Wastes
Project Leader: Gregory Rutledge
Grant Number: P42ES004675
Funding Period: 1995 - 2000
Semi-volatile organic compounds (SOC) are present in significant fractions in both gas and particle phases, and become increasingly difficult to characterize at higher molecular weight/lower volatility. The fate and transport of such compounds in the environment is complicated by the redistribution of SOCs among airborne particulates of various sizes, groundwater, soil and sediment. From a health standpoint, the extent of airborne risk depends on whether crucial mutagenic SOCs enter the lungs in the gas phase or on particulates, and whether the particles are of a size range to deposit in the lungs for long term exposure. Fluid mechanical models used to investigate the fate and transport of these compounds depend on accurate and complete descriptions for all the important mechanisms contributing to partitioning between the various gas, liquid and solid phases in the environment. For this purpose, sound thermodynamic models of the various partitioning modes relevant to both absorption and adsorption are required. The complications which attend efforts to work with known hazardous wastes make alternative, accurate methods of estimation valuable for understanding and predicting fate and transport, and in the evaluation of competing technologies to remediate Superfund sites.
To address this issue, investigators developed first principles computational chemistry and molecular simulation approaches to describe the properties of SOCs, in particular PAHs, under environmental conditions. The first effort explored the use of simple geometric characterization of PAHs having five and six rings, based on semi-empirical quantum chemistry calculations. These calculations showed that, given a sufficiently broad class of compounds, commonly used geometric descriptors result in multiple correlation curves. The results of this work demonstrate the inadequacy of such molecular field descriptors to model retention in chromatographic separations and, by implication, in environmental media.
The second approach has been to develop molecular simulations that account for both enthalpic and entropic factors in determining partition coefficients. For this purpose, a state-of-the-art Monte Carlo (MC) molecular simulation based on the Expanded Ensemble Method (EEM) was developed to estimate the chemical potential of aromatic hydrocarbons in liquids of various compositions. From this information, the vapor-liquid partitioning of such compounds under thermodynamic conditions can be accurately assessed over a wider range of temperatures and pressures than is possible using common analytical equations of state and ad hoc mixing rules. Due to the paucity of the basic force field information required for such simulations in the case of aromatic hydrocarbons, researchers conducted preliminary validation studies on benzene and naphthalene, both in their pure liquids and in oily liquid phases consisting of linear alkanes of up to C24. Investigators have also begun to develop simpler force fields of the Gay-Berne type for aromatic hydrocarbons to permit more rapid estimation of the thermodynamic behavior of larger PAHs involving up to five and six rings.