The isotopic compositions of selected crude oil PAHs during biodegradation

Abstract

Compound specific isotope analysis (CSIA) has been used to measure the δ13C of selected individual polycyclic aromatic hydrocarbons (PAHs) from a crude oil that was exposed to enhanced microbial biodegradation. Twenty-one liters of a crude oil were introduced to series of separate 5 mx5 m wetland plots along the San Jacinto River, Texas. Smaller grids were established within each area to serve as control plots, plots for addition with nitrogen fertilizer, or as plots for addition with petroleum degrading microbes. Samples of randomly selected, oil impacted soils were collected at 4, 28, and 56 days after oil inoculation. These samples initially were analyzed using GC/MS for identification of PAH compounds. The initial results on the soils indicated a tendency for samples to shift from low and high molecular weight PAH compounds to eventually predominantly high molecular weight compounds. Fifteen specific PAH compounds, including two containing sulfur, were selected for compound specific isotope analysis based on chromatography and expected abundance in the soils. Five alkylated components, a dimethylnaphthalene, two trimethylnaphthalenes, a methyldibenzothiophene, and a dimethylphenanthrene, were seen to be best resolved and are suggested to be the most reliable for observations of isotopic changes during biodegradation. Isotopic variability during the treatment period follows patterns with an overall trend toward enrichment in the residual PAH pool of 2-8‰. All treatment grids, including the control, underwent isotopic alteration throughout the duration of the experiments. ANOVA testing of the isotopic results indicated that the control grids showed the smallest amount of changes over time, whereas the nutrient and microbe amended grids were not significantly different from each other. Although the trends on isotopic enrichments were predictable, the magnitude of changes was not. One additional finding indicated that time trials beyond 2-month periods were needed to determine if nutrient or microbial additions are advantageous for PAH biodegradation in wetland soils. The results of these tests highlight the importance of understanding the magnitude and direction of isotopic variations in PAH compounds as part of investigations aimed at tracing these compounds from potential sources. © 2002 Elsevier Science Ltd. All rights reserved.