Abstract
The heavy oil is essentially comprised of asphaltene and nitrogen, sulphur, and oxygen-bearing (NSO) compounds, which records the alteration processes of aerobic biodegradation. In this study, six biodegraded oils were geochemically examined using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in negative-ion electrospray ionization (ESI(-)) mode to decipher the NSO heteroatom compounds molecular compositions. The results reveal that the NSO compounds, especially those strongly resistant to biodegradation, i.e., alcohol/phenols, carboxylic acids with various alkyl substitution, naphthenic acids with 2 to 5 naphthenic/aromatic rings, nitrogen- and sulphur-bearing carboxylic acids, and hydroxy acids (O3) and dibasic acids (O4) are enriched in biodegraded oils. The O1, N1O1, and O1S1 classes appear in trace amounts in severely to extremely biodegraded oils (PM > 7) but are abundant in moderately biodegraded oils (PM < 7), implying the alcohol group generated by biodegradation are further oxidized to form a carboxylic acid with the progressively biodegraded process. Abnormal enrichment of O2 and O4 classes was trapped in the severely–extremely biodegraded oils, implying the generation of additional monobasic acid/dibasic acids during microbial aerobic degradation of crude oils. Besides, the relative content of monocyclic naphthenic acid (DBE = 2) gradually decreased with biodegradation level, while those of the bicyclic-, tricyclic-, and tetracyclic–naphthenic acid (DBE = 3-5) showed an increasing trend. The newly developed proxies of NSO compounds (i.e., the ratios of biodegradation products with different numbers of oxygen atoms of the specified compound, and the O2, O3, O4, and O5 classes with various DBE values and/or various carbon numbers) agreed well with the existing biodegradation indicators (i.e., C2925-NH/C30H, C2117-NT/C23TT, C27DS/C27RS, and C28TAS/C21TAS), confirming the availability of ESI(-) FT-ICR MS to distinguish the biodegraded oils with high resolution.
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