Physicochemical characterization of residual mineral oil contaminants in bioremediated soil

After bioremediation of mineral oil-contaminated soils, residual contaminants can remain in the soils. In this article, an analytical concept to determine the physicochemical properties of residual contaminants that can be used to assess the environmental fate and potential environmental hazards of these chemicals is presented and discussed. This concept can serve as a tool to characterize unknown compounds within a complex mixture on the basis of basic analytical data. The volatility of the residual contaminants was determined by headspace analyses, but no volatile organic compounds (with a boiling point of <280 degrees C) were present. Soxhlet extraction using tetrachloromethane was found to be the most efficient method to obtain the total solvent extractable material (TSEM). Information about the polarity of the contaminants in TSEM was obtained by hash chromatography. It was found that the majority of the residual contaminants are present in the fractions containing nonpolar components, such as branched or cyclic aliphatics and alkylated aromatics. A minor fraction consisted of compounds with polar functional groups, such as long-chain alkanals, alkanols, and alkanoic acids. Information about the octanol/water partition coefficient (K-ow) of the contaminants within the TSEM was obtained with use of a combination of two independent chromatographic separation steps, reverse-phase high-performance liquid chromatography, followed by high-resolution gas chromatography. Depending on the compound classes, K-ow values varied accordingly but were above 10(6). Considering these K-ow values, the mobility of the residual contaminants in bioremediated soils is expected to be very low.