Characterization of Motor Lubricating Oils and Their Oil–Water Partition

Abstract

The purpose of the investigation was to determine the difference in chemical composition for unused and used (waste) motor oils and to understand their partitioning effect in water. Three brands of motor oil (unused and used) were selected for this study. Our analytical results show that the chemical composition of unused and used motor oil is significantly different, due in part to the presence of gasoline combustion residues and polycyclic aromatic hydrocarbons (PAH) in the waste motor oil. No aromatic hydrocarbons were detected in the unused motor oils, irrespective of the brand, whereas alkylbenzenes, naphthalene, and methylnaphthalenes are abundant in the waste oils. Three and four-ring PAHs present as subordinate concentrations in used lubricants are probably synthesized during exposure of the motor oil to high engine temperature. Total ion chromatogram (TIC) chromatograms of extracts of water associated with unused motor oils, are dominated by a series of polar (N, S, O) compounds, which vary in composition with different brands of motor oils. The chromatographic TIC patterns of water extracts from waste motor oils are dominated by alkylbenzenes and naphthalenes, with N, S, and O polar compounds present as subordinated components. Furthermore, it is important to note that there is no unresolved hump in TIC chromatograms of the water phase extract of motor oil (unused and waste), although both non-aqueous phase liquid (NAPL) chromatograms have prominent unresolved humps. Therefore, the absence of an unresolved hump in a groundwater sample is not a marker which can be used to identify the presence or absence of motor oil in the contaminant NAPL.