The effect of biodegradation on bound aromatic hydrocarbons released from intermediate-temperature gold-tube pyrolysis of severely biodegraded Athabasca bitumen

Abstract

Polycyclic aromatic hydrocarbons (PAHs) from two severely biodegraded Athabasca oil sand bitumens released by gold-tube pyrolysis at 300–525 °C with 2 °C/h heating rate were characterized by gas chromatography-mass spectrometry. Alkylnaphthalenes, alkylphenanthrenes and alkyldibenzothiophenes were fully removed by biodegradation from the free phase aromatic hydrocarbon fraction in raw bitumens, while tetracyclic aromatics were partially attacked. Sample A has suffered less extensive biodegradation influence than sample B due to different oil-water-contact situations. Pronounced changes were observed in the distribution of various compound classes after pyrolysis due to thermal cracking, aromatization and condensation. PAHs released from occluded phase at intermediate–temperature (< 425 °C) exhibit clear biodegradation impact. The proportion of alkylnaphthalenes released in initial pyrolysates is much lower than those highly fused aromatics, reflecting more biodegradation influence on lower ring number aromatic hydrocarbons. Within alkylnaphthalenes, naphthalene and methylnaphthalenes with less substituents are largely depleted in the pyrolysates, indicating preferential removal of less alkylated homologues from the occluded phase by biodegradation. Isomer selectivity from the occluded aromatic hydrocarbons follows the same sequences as those observed from the free phase, resulting in poor correlation between isomerization based maturity parameters and heating temperatures. PAHs released from extremely biodegraded bitumens in intermediate–temperature pyrolysates might differ totally from those in the non-degraded oils. Caution should be taken when molecular parameters derived from occluded phase are applied for organic source input and maturity assessment.