812 OIL SPILLAGE INTO WATER—TREATMENT
clearly eliminated when the oil is dispersed as fine
droplets into the water column. These dispersed
droplets are placed several feet below the water
surface by the mixing process.
3) The fire hazard from the spilled oil is reduced by
dispersion of the oil several feet into the water
column. The removal of this combustible material
from the water’s surface and from contact with
the atmosphere prevent possible combustion of
the spilled oil. This is perhaps the most accepted
benefit accruing from the use of dispersants. It has
provided the motivation for many past instances
of dispersant applications.
4) The rate of biodegradation of the oil is enhanced.
This is the historical basis for the dispersion
of oil. It is perhaps the most significant contri-
bution of dispersants. The order of magnitude
increases in interfacial area that are generated
by the dispersant greatly increases the rate of
biodegradation of the oil. ZoBell^31 has reported
biodegradation rates that are one or two orders
of magnitude higher in laboratory experiments
in which the oil is emulsified. Not only is the
physical state of the oil, that is, small droplets,
more conducive to bacterial action, but it is also
made available to a much larger population of
microbial organisms. This particular reference
has been one of the most complete treatments of
the subject of oil biodegradation to date.
A study by Robichaux and Myrick^32 presented
the results of a study of the effects of chemi-
cal dispersing agents on the rate of microbial
destruction of crude oil in aqueous environ-
ments. Increased destruction rates of up to 15
times the rate of untreated oil/water mixtures
were reported.
5) The formation of persistent tar lumps from an
untreated oil spill is prevented. The tarry ag-
glomerates (up to 10 cc dia.) found on the ocean
surface, as mentioned previously represent a
small percent residue of the crude oil. If the
crude oil had been dispersed into 10 μ to 1 mm
diameter droplets, these large residue agglom-
erates would not have formed and their persis-
tence in the marine environment would have
been greatly reduced.The Concern Regarding the Chemical Dispersion of
Oil SpillsClearly then, from a consideration of the foregoing, the
removal of oil from the surface of the sea has merits in miti-
gating the damage resulting from a spill. It is more than a
cosmetic, hide-it-from view effect. What then are the nega-
tive aspects to their use? What is the ecological price for
introducing the chemical dispersant and dispersed oil into
the water column?The major concern regarding the use of dispersants are
twofold as covered in the following discussion.The Toxic Effects of the Chemical DispersantsThis has been an area of great concern since their use has
become signifi cant. There is a basis for this concern. It was
highlighted by the investigation by Smith et al. ,^33 after the
Torrey Canyon that indicated that in some areas, particularly
in the intertidal zone, the chemicals used were more toxic to
the marine life than the oil itself.
During this period (1967–1968), the chemical formula-
tions available to disperse spilled oil were derived mainly
from cleaning agents, hence the term “detergent” was used
quite commonly. To permit these agents to dissolve tar-like
residues and perform their cleaning function, an aromatic
solvent, such as heavy aromatic naphtha, was generally
employed. The short term acute toxicity of aromatic hydro-
carbon to marine life is well-known. Blumer^34 states that low
boiling aromatics are toxic to man as well as all other organ-
isms and that it was the great tragedy of the Torrey Canyon
that the detergents used were dissolved low boiling aromat-
ics. The Toxicity of these aromatic solvent constituents were
extensively studied by the Marine Biological Laboratory
of the UK. Their acute toxicity was evident since 5 ppm of
kerosene extract solvent killed 50% of the Elminius nauplius
larvae in 21 minutes. Their analyses of the more common
detergents (dispersants) used during the Torrey Canyon indi-
cated that they contained some portion of aromatics.
In addition to these toxic aromatic solvents, the surfactants
were typically selected from the class compounds formed by
the reaction of hydroxy-containing compounds (e.g. phenol or
alcohol) with ethylene oxide. A typical surfactant might be eth-
oxylated nonylphenol. The number of ethylene oxide groups
added to the nonylphenol hydro-phobe may be controlled to
any desired extent to adjust the degree of water solubility of the
material. These types of surfactants, although effective emulsi-
fi ers, were quite detrimental to marine life.
However, there has been research directed toward formu-
lating dispersants that would have little effect on marine life.
For example, water is now used as the solvent in products
where it is compatible with the particular surfactants. High
boiling saturated hydrocarbons which are similar to the type
of hydrocarbon that occur naturally in the marine environ-
ment have a low order of toxicity and are also employed as
solvents in some of the more recent dispersants. This modi-
fi cation of the solvent, and the selection of generic types of
surface-active agents that are not considered to be chemi-
cally toxic, have resulted in the development of dispersants
that have greatly reduced toxicity. This can be illustrated by
the study of J.E. Portmann,^35 summarized in Table 3. For
example, three dispersant products used during the Torrey
Canyon spill and identifi ed as Torrey Canyon Dispersants
A, B, C, have 48 hr LC 50 values of 8.8, 5.8, and 6.6 ppm,
respectively. These concentrations represent the amount of
the specifi c agent to kill 50% of the test species (Crangon
crangon) in 48 hours. The toxicity of a typical Torrey Canyon
surfactant, ethoxylated nonlphenol is shown at 89.5 ppm. ByC015_002_r03.indd 812C015_002_r03.indd 812 11/18/2005 10:56:41 AM11/18/2005 10:56:41 AM