You are here:
THE PHOTOTOXICITY OF POLYCYCLIC AROMATIC HYDROCARBONS
Citation:
Betowski, L D., M. Enlow, AND L A. Riddick. THE PHOTOTOXICITY OF POLYCYCLIC AROMATIC HYDROCARBONS. Presented at The ACS meeting, San Diego, CA, April 1-5, 2001.
Impact/Purpose:
Provide state-of-the-science sampling, analysis, separation, and detection methods to allow rapid, accurate field and laboratory analyses of contaminated soils, sediments, biota, and groundwater to support Superfund clean-up decisions. Apply state-of-the-science methods in chemical analysis and data interpretation (e.g., mass spectral interpretation) to actual problems of OSWER, the Regions, and the States, in cooperation with the Las Vegas Technical Support Center as well as by direct contacts with Regional and State employees. Provide technical advice and guidance to OSWER using the environmental chemistry expertise (e.g., mass spectrometry, analytical methods development, clean-up methodology, inorganics, organometallics, volatile organics, non-volatile organics, semi-volatile organics, separation technologies, etc.) found within the branch.
Technical research support for various projects initiated either by Regions/Program Offices or ECB scientists. While these efforts will support the Regions and Program Offices, they cannot be predicted or planned in advance, and may serve multiple duty (e.g., solve real-world problems, serve to ground-truth analytical approaches that ECB is developing, transfer new technology). Many of the activities in this task support requests involving enforcement decisions and therefore are categorized as "environmental forensics".
Description:
The U.S. Environmental Protection Agency (EPA) continues to be interested in developing methods for the detection of polycyclic aromatic hydrocarbons (PAHS) in the environment. Polycyclic aromatic hydrocarbons (PAHS) are common contaminants in our environment. Being major products of combustion processes, they are often found in air, water, and soil. While some PAHs are directly toxic and carcinogenic, exposure to ultraviolet (UV) light can increase the toxicity of some of these compounds to aquatic organisms. Newsted and Giesy investigated many of the factors that could be contributing to this phototoxicity effect of the PAHs'. The phosphorescence lifetime of the'PAHs had the best predictive power of the parameters investigated and was also found to be correlated with the lowest triplet energy of the molecule. In their attempt to develop a predictive model for the phototoxicity of PAHS, they found that non-linear models had to be used. The curve that best fits the data for a series of PARs resembles a parabola, with the triplet energy for each PAH found to be the best descriptor to these curve-linear models for predicting median lethal time (LT50) or adjusted median lethal time (ALT). Mekenyan et al.' looked deeper into the effect of each parameter on the phototoxicity of the PAHS. They proposed a series of independent, mechanistic molecular processes, which were consistent with the parabolic relationship of Newsted and Giesy. The internal effect of light absorbance was predicated to depend on the HOMO-LUMO gap, which is the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecule. The main external factor controlling phototoxicity was the exposure intensity. If there were a constant flux of photons at each wavelength in natural sunlight, photo-induced toxicity would show a linear dependence versus energy absorbed. But this flux falls off with increasing energy. Toxicity experiments used intensity ratios of 680:120:25 to simulate natural sunlight for the regions of the spectrum, visible (400-700 nm), UVA (337-400 run), and UVB (315-336 nm). These internal and external effects relating phototoxicity to the HOMO-LUMO gap, therefore, combine to produce the parabolic relationship. More formally, the HOMO-LUMO gap can be replaced by the excited state energy in these studies. Therefore, singlet and triplet excited state ab initio calculations were performed with a 6-31 1 G(d,p) basis set, using the configuration interaction approach, modeling excited states as combinations of single substitutions out of the Hartree-Fock ground state (CI-Singles or CIS). Two other methods were used to test the agreement with experiment for test compounds. Calculations were performed using the Gaussian 94 or the Gaussian 98 program suites.