Crude Oils
Crude Oils - Heavy-weight Components
Heavy-weight components are characterised by (Michel, 1992):
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Hydrocarbon compounds contain more than 20 carbon atoms.
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Almost no loss by evaporation.
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Almost no water-soluble fraction.
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Potential for chronic toxicity, because they contain polynuclear aromatic hydrocarbons (phenanthrene, anthracene etc).
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Potential for bioaccumulation, via sorption onto sediments, otherwise not highly bioavailable.
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Long term persistence in sediments, as tar balls, or asphalt pavements.
These heavier compounds pose little acute toxicity risks but can cause damage due to smothering. However, these are the most persistent compounds of an oil and degradation rates are very slow.
Aromatics
Aromatics are unsaturated compounds which are cyclic in structure and are based on a benzene ring with resonating double bonds. The resonance energy of benzene makes it remarkably inert. These compounds typically make up about “10% of a crude oil” (Freemantle, 1987). Although aromatic hydrocarbons are quite resistant to biodegradation and are generally insoluble in water, some compounds with low molecular weights (such as naphthalene) readily evaporate, and can be quickly oxidised. Benzene (the simplest aromatic hydrocarbon shown below) is considered the most soluble of the aromatics although this is relative and dissolution is still extremely slow.
Polyaromatic hydrocarbons (PAHs) contain two or more fused benzene rings. The complex PAHs, with several benzene rings, are less water soluble than monoaromatic hydrocarbons and therefore prove to be less toxic.
