Abiotic oxidation of petroleum bitumens under natural conditions

Five series of crude oil samples exposed to atmospheric conditions have been analysed at the molecular level, each series comprising several samples originating from the same crude oil but altered to different extents. The aim of our investigation was to compare the specific impact of abiotic oxidation to other alteration processes such as biodegradation, evaporation and water washing. Bulk analyses revealed that increasing alteration is accompanied by an increase in oxygen content which parallels a relative increase of the proportions, as well as of the molecular weights of the macromolecular constituents of the bitumens. Gas chromatographic-mass spectrometric analyses of polar fractions showed the presence of oxygen-containing compounds (steroid ketones, benzothiophenic acids and sulfones) which result from oxidation of petroleum lipids. The hypothesis that part of these oxygenated compounds results from abiotic oxidation processes rather than from biodegradation is supported, notably, by the fact that oxygen incorporation generally occurred without any diastereomeric discrimination. This is also supported by simulation experiments performed on petroleum lipids, which showed that abiotic oxidation induces cleavage reactions affecting C-C and C-S bonds which may intervene in the transformation of geomacromolecules in the environment by degradation ("depolymerization''). Thus abiotic oxidation may play a major role in the fate of petroleum pollutants in the environment by transforming lipidic organic matter from petroleum into more water soluble and, therefore, more biodegradable constituents. However, these can be more toxic to the environment as the water-soluble fraction may be easily taken up by biota. (C) 2000 Elsevier Science Ltd. All rights reserved.