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commonly condensed. Mono-, di-, and trinuclear aromatics, for example, naphthalene and phenan-
threne, are the main components of the aromatic portion. Lubricating oils possess a high additive
content. These compounds are included in an oil blend to improve both physical and chemical prop-
erties. Additive content can be as high as 20%, the most important being detergents and dispersants
(Table 19.1) (Vazquez-Duhalt, 1989). The technology used in the fabrication of different lubricat-
ing oils, including the type and quantity of additives, is unique for each manufacturer.

19.3 CHEMICAL COMPOSITION OF USED MOTOR OIL
Used motor oil may originate from monograde automotive engine oil, multigrade automotive
engine oil, mineral-based crankcase oil, and railway diesel oil. There is no consistent chemical
composition of used motor oil since it is a function of the crude oil source, refining processes, pres-
ence of additives, and length of time in use (ATSDR, 1997a). In general, used motor oil contains
small amounts of gasoline, additives (detergents, dispersants, oxidation inhibitors, rust inhibitors,
and viscosity improvers), N and S compounds, metals such as Pb, Zn, Ca, Ba and Mg, and a broad
range of aromatic and aliphatic hydrocarbons with chain lengths ranging from C 15 to C 50
(Dominguez, 2002). Cotton et al. (1977) were unable to determine a single predominant organic
compound in 30 samples of used motor oil examined. However, several broad classes of compounds
were commonly present in the oil including saturated aliphatic, mono-, di-, tri- and polyaromatic
ring products. Aliphatic compounds comprise about 73 to 80% of the total weight of used motor oil.
This fraction is composed of alkanes and cycloalkanes of 1 to 6 rings. Monoaromatics and diaro-
matics make up another 11 to 15% and 2 to 5% of the weight, respectively (Vazquez-Duhalt, 1989).
The percentages of hydrocarbons in crankcase oil are shown in Figure 19.1.

TABLE 19.1
Typical Formulation of Engine Oils
Ingredient % by Volume
Base oil (solvent 150 neutral) 86
Detergent inhibitor (zinc dialkyl dithiophosphate) 1
Detergent (barium and calcium sulfonates) 4
Multifunctional additive (polymethyl-methacrylates) 4
Viscosity improver (polyisobutylene) 5
Source: U.S. EPA, 1974. With permission.

Polar aromatic
Diaromatic
and polar 4-8% compounds
2-5%

Mono-
aromatic
compounds
11-15%
Aliphatics
73-80%

FIGURE 19.1 Chemical composition of hydrocarbons in used motor oil (Vazquez-Duhalt, R., Sci. Total
Environ., 79, 1-23, 1989. Reproduced with kind permission of Elsevier, Oxford, UK).