Use of ESI FT-ICR MS to investigate molecular transformation in simulated aerobic biodegradation of a sulfur-rich crude oil

Achieving an understanding of the biodegradation of organic sulfur compound(s) (OSC) in crude oils has been hampered by limitations in separation methods and analytical techniques. In this study, a simulated aerobic biodegradation experiment was performed on a sulfur-rich crude oil, from which a series of biodegraded oils from slight to moderate degrees of biodegradation were obtained. Molecular transformations in these biodegraded oils were investigated by gas chromatography-mass spectrometry (GC-MS) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) under both positive- and negative-ion electrospray ionization (ESI) modes. The variations in the distributions of saturated hydrocarbons, OSCs, and acidic components during aerobic biodegradation were studied. A gradual decrease of OSCs with alkyl chains during biodegradation, especially in those with few sulfur-containing or naphthenic/aromatic rings, was observed. In addition, sulfur-containing phenols with alkyl chains also exhibited a trend of gradual decrease similar to alkyl phenols, with fewer ring numbers being more susceptible to biodegradation. The biodegradation pathways for OSCs and sulfur-containing acids, as well as certain other heteroatomic compounds, are discussed. Terminal oxidation, rather than sulfur-specific degradation, is the more likely biodegradation pathway of these OSCs, as in the case of n-alkanes in this study. Sulfur-containing carboxylic acids may be the biodegradation products of these OSCs.