Page 179 - Soil and water contamination, 2nd edition

166 Soil and Water Contamination

petroleum products, such as MTBE (methyl tertiary-butyl ether) that has replaced lead as
petrol additive. In humans, ingestion of mineral oil , the major by-product in the distillation
of petroleum to produce petrol, can hamper the absorption of vitamin A. Mineral oils used
as lubricants for metal workers have been associated with enhanced risk of occupational
skin cancer (Irwin et al., 1998). Because of the broad variety of constituents in petroleum
and petroleum products, which vary in density, mobility, degradability, and toxicity, the
composition of each individual oil or oil product must be taken into consideration in order
to determine the environmental impact of the oil in question.
The environmental fate of spilled oil is controlled by a variety of natural processes that
reduce the amount and toxicity of oil in soil or water and, consequently, mitigate the severity
of an oil spill. These processes include evaporation , emulsification , oxidation, adsorption ,
and biodegradation (EPA, 1999a). Evaporation occurs when the lighter substances within
the oil mixture volatilise. This process leaves behind a residue consisting of the heavier oil
components, which may undergo further weathering . Some of these components are denser
than water, so, in surface waters, they may sink to the bottom. Lighter refined petroleum -
based products such as kerosene and petrol may volatilise completely within a number of
hours, thereby reducing the toxic effects to the environment. The rate of volatilisation
increases with increasing wind speed. In surface water, this effect is further promoted by
waves and currents. The action of waves also causes emulsification , i.e. the formation of
water-in-oil emulsions. The uptake of water in oil can increase the volume of a floating oil
layer up to four times. Heavy, very viscous oils tend to take up water more slowly than light,
more liquid oils. The formation of emulsions reduces the rate of other weathering processes,
making water-in-oil emulsions very persistent oil slicks. Oxidation of the oil leads to the
formation of water-soluble compounds or persistent tar. Oxidation is promoted by sunlight,
but compared to other weathering processes its overall effect on dissipation is small. Even
under intense sunlight, oil oxidises relatively very slowly: usually less than 0.1 percent per
day (ITOPF, 2011). Thick slicks may only partially oxidise, which results in the formation of
tar balls. These dense, sticky, black spheres can collect in the bed sediments of lakes or slow
moving streams and may persist in the environment for a long time. In sediments and soils,
oil is particularly adsorbed by organic matter . As the ability of sediments to bind oil increases
with increasing organic matter content , the phytotoxicity of oil is less in organic soils than in
soil with low organic matter content. Biodegradation occurs when microorganisms feed on
oil. The biodegradation rate of oil is optimal under warm conditions with sufficient supply
of oxygen and nutrients (particularly nitrogen and phosphorus).

9.3 MONOCYCLIC AROMATIC HYDROCARBONS

Monocyclic aromatic hydrocarbons are compounds consisting solely of carbon and hydrogen
and containing one benzene ring. The most common monocyclic aromatic hydrocarbons are
benzene, toluene , ethylbenzene, and the three isomers of xylene (or dimethylbenzene): ortho-
xylene, meta-xylene, and para-xylene (see Box 9.I), which all exist as clear, colourless, non-
corrosive, volatile liquids with a sweet odour. These compounds are collectively referred to
by the acronym BTEX . BTEXs can make up a significant percentage of petroleum products:
about 18 percent on a weight basis in a standard petrol blend. Benzene is used in the
production of synthetic materials and consumer products, such as synthetic rubber, plastics,
nylon, insecticides, and paints. Toluene is used as a solvent for paints, coatings, glues, oils,
and resins. Ethylbenzene may be present in consumer products such as paints, inks, plastics,
and pesticides , and as an additive in petrol and aviation fuel. Xylene is used as a solvent in
the printing, rubber, and leather industries. It is also used as a cleaning agent and as a paint
thinner.

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