Variations of stable carbon isotopic compositions of Iranian heavy crude oil during simulated weathering

Abstract

Oils from different sources have distinct chemical compositions, which can be used to identify the source of oil pollution. The chemical compositions of spilled oils, however, can be altered in the environment by a number of physicochemical and biological factors including evaporation, dissolution, photooxidation and microbial degradation, making source identification difficult. The stable isotope compositions of individual compounds in weathered oil remain relatively unaffected and therefore can be effectively used as a correlation parameter. To simulate natural weathering of oil, an artificial weathering experiment by rotary evaporation and a field weathering experiment (mesocosm) were conducted. Weathered oils were analyzed by GC/IRMS (gas chromatograph / isotope ratio mass spectrometry) for stable isotope ratios of n-alkanes and GC/MS or GC/FID for molecular compositions. Changes in stable carbon isotopic compositions were monitored in accordance with the degree of weathering and also with the time of field exposure. The stable isotopic compositions of n-alkanes in oils during weathering experiments did not show significant changes. This study shows that stable isotope profile of n-alkanes can be effectively used to trace the source of weathered oil.