EPA/600/R-08/037
March 2008
Dispersion of Crude Oil and Petroleum
Products in Freshwater
Prepared
By
Brian A. Wrenn
Department of Energy, Environmental, and Chemical Engineering
Washington University, Box 1180
St. Louis, MO 63130
Order No. EP06C000259
EPA Project Officer
Albert D. Venosa
Land Remediation and Pollution Control Division
National Risk Management Research Laboratory
Cincinnati, Ohio 45268
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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Foreword
The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting the
Nation's land, air, and water resources. Under a mandate of national environmental laws, the
Agency strives to formulate and implement actions leading to a compatible balance between
human activities and the ability of natural systems to support and nurture life. To meet this
mandate, EPA's research program is providing data and technical support for solving
environmental problems today and building a science knowledge base necessary to manage our
ecological resources wisely, understand how pollutants affect our health, and prevent or reduce
environmental risks in the future.
The National Risk Management Research Laboratory (NRMRL) is the Agency's center for
investigation of technological and management approaches for preventing and reducing risks
from pollution that threaten human health and the environment. The focus of the Laboratory's
research program is on methods and their cost-effectiveness for prevention and control of
pollution to air, land, water, and subsurface resources; protection of water quality in public water
systems; remediation of contaminated sites, sediments and ground water; prevention and control
of indoor air pollution; and restoration of ecosystems. NRMRL collaborates with both public and
private sector partners to foster technologies that reduce the cost of compliance and to anticipate
emerging problems. NRMRL's research provides solutions to environmental problems by:
developing and promoting technologies that protect and improve the environment; advancing
scientific and engineering information to support regulatory and policy decisions; and providing
the technical support and information transfer to ensure implementation of environmental
regulations and strategies at the national, state, and community levels.
This publication has been produced as part of the Laboratory's strategic long-term research plan.
It is published and made available by EPA's Office of Research and Development to assist the
user community and to link researchers with their clients.
Sally Gutierrez, Director
National Risk Management Research Laboratory
11
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Notice
The U.S. Environmental Protection Agency through its Office of Research and
Development funded the research described here under order number EP06C000259 to Brian A.
Wrenn at the Washington University Department of Energy, Environmental, and Chemical
Engineering. It has been subjected to the Agency's review and has been approved for publication
as an EPA document.
in
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IV
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1. Abstract
The objective of this research was to investigate the relationship between dispersion
effectiveness in freshwater and the surfactant composition for fresh and weathered crude oil.
Although limited research on the chemical dispersion of crude oil and petroleum products in
freshwater has been conducted, previous studies did not identify the dispersants that were
investigated, much less describe the chemistry of the surfactants that were used. The absence of
information on surfactant composition is a major impediment to the scientific investigation of
dispersant effectiveness because this information is necessary for the development of a more
fundamental understanding of dispersant effectiveness. Therefore, the relationship between
surfactant chemistry and dispersant effectiveness was systematically evaluated. This report
showed that, at least with Mars Blend crude oil in simulated lake water, dispersants can be
designed to drive an oil slick into the freshwater column with the same efficiency as in saltwater
as long as the hydrophilic-lipophilic balance (HLB) is optimum. Clearly, many more oils would
need to be tested under different conditions (temperature, organic content, water composition,
etc.) to enable firm conclusions that oil can be dispersed in freshwater as a response tool. The
ultimate decision to use dispersants in treating freshwater petroleum oil spills is up to the federal
on-scene coordinator, the incident command team, the regional response teams, and EPA
Headquarters, since many other factors need to be considered before rendering a decision to
disperse oil into the water column. It is beyond the scope of this report to consider such factors.
Its purpose was simply to determine if freshwater dispersion is possible and to determine
whether effective freshwater dispersants can be designed to produce stable oil droplet
distributions in such an environment.
2. Introduction
2.1. Question addressed by this report
This report attempts to answer the question about whether petroleum oils can be
dispersed in a freshwater environment. Most dispersants currently on the National Contingency
Plan Product Schedule (NCPPS) have been developed for saltwater environments, and the
literature is lacking in regards to freshwater dispersion. Research data presented herein will show
that effective freshwater dispersants can be developed as a response tool, but the decision to use
them is a policy decision that is not addressed by this technical report.
2.2. Chemical Properties of Dispersants
One response alternative for marine crude petroleum oil spills is chemical dispersion.
Dispersion is accomplished by addition of chemicals that interact with the floating oil to reduce
the oil-water interfacial tension and facilitate the formation of small oil droplets (NRC, 2005).
The individual chemicals are formulated into mixtures that are collectively known as dispersants.
The composition of most commercially available dispersants is proprietary, but in general they
consist of one or more nonionic surfactants dissolved in a solvent carrier (NRC, 1989). Some
dispersants also include one or more anionic surfactants and other additives (e.g., phosphoric
acid). The purpose of the solvent is to provide the surfactant mixture with an appropriate
viscosity, which ensures that it can be pumped through spray nozzles at environmental
temperatures. The solvent may be water miscible (e.g., 2-butoxyethanol) or immiscible (normal
alkanes) (NRC, 2005).
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Surfactants are the active (i.e., interfacial tension reducing) ingredients in dispersants.
Surfactants are compounds that have hydrophobic and hydrophilic components within the same
molecule. The amphiphilic character of surfactants causes them to accumulate at interfaces
because the hydrophilic part of the molecule interacts strongly with water, and the hydrophobic
part interferes with more thermodynamically-favorable hydrogen bonding interactions between
water molecules. These interactions are thus displaced from the water phase (Porter, 1991).
Nonionic surfactants are most common in dispersants because they have much lower aqueous
solubility than do anionic surfactants (Porter, 1991), and they are generally less toxic and less
affected by electrolyte concentration than are anionic and cationic surfactants (Porter, 1991;
Myers, 2006). This is not necessarily true for the alkyl phenol ethoxylates, which have been
linked to endocrine disrupting activity. The latter surfactants were not a part of this study. The
lower water solubility tends to increase the extent to which the surfactants partition into the oil
phase when the aqueous phase is in great excess, as it is for environmental applications. The
nonionic surfactants used in past dispersant studies are ethoxylated derivatives of fatty acids,
fatty alcohols, and fatty acid esters of sorbitan (NRC, 1989; Fingas et al, 1990; Georges-Ares
and Clark, 2000), in which the hydrophilic portion of the molecule consists of polyethylene
glycol chains of varying lengths, and the hydrophobic portion is contributed by the fatty acyl
chains, usually ranging between about 12 and 18 carbon atoms in length.
A typical measure of the relative importance of the hydrophobic and hydrophilic portions
of nonionic surfactants is the hydrophile-lipophile balance (HLB), which ranges from zero for
completely lipophilic (hydrophobic) molecules to 20 for completely hydrophilic (uncharged)
molecules. Packing arguments suggest that the dominant group, characterized by the ratio of
linear cross-sectional areas, will tend to orient into the continuous phase (NRC, 1989; Porter,
1991). So, surfactants with low HLB tend to stabilize water-in-oil emulsions, whereas those
with high HLB stabilize the more desirable oil-in-water emulsions (NRC, 1989; Clayton etal.,
1993). Commercial dispersants tend to have overall HLBs in the range of 9 to 11, which is often
achieved by combining surfactants with higher and lower HLB. Although the industry
consensus suggests that combining surfactants with different HLB improves dispersant
effectiveness (NRC, 1989; Clayton etal., 1993), others have offered alternative findings (Fingas
etal., 1990).
Although the HLB is the parameter that is most commonly used to describe the
characteristics of dispersants, the same HLB can be obtained in many different ways. For
example, a single surfactant with the desired HLB can be used, or two or more surfactants can be
mixed in proportions that give the same desired HLB. The HLB of surfactant mixtures is given
by the mass-weighted average of the individual surfactants (Myers, 2006). Furthermore,
dispersants with different chemical characteristics may have the same HLB but exert different
effects on the system (Bruheim et al., 1999; Van Hamme and Ward, 1999; Bruheim and
Eimhjellen, 2000). Therefore, the composition of the dispersant formulations used in this
research were varied systematically to produce dispersants with similarly defined HLBs but
different chemical compositions.
2.3. Effect of Salinity on Dispersion Effectiveness
The effectiveness of chemical dispersants can be strongly influenced by salinity (i.e.,
ionic strength), but the relationship between dispersion effectiveness and salinity can vary for
different dispersant-oil combinations (Lehtinen and Vesala, 1984; Belk et al., 1989; Fingas et al.,
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1991; Blondina et al, 1999). Dispersants that were optimized for marine use [e.g., Corexit 9500
(http://www.epa.gov/emergencies/docs/oil/ncp/schedule.pdf), Enersperse 700 (no longer on the
EPA Product Schedule)] are often ineffective in freshwater (Fingas et al., 1991; Blondina et al.,
1999), while those that are optimized for use in freshwater are less sensitive to salinity (Belk et
al., 1989). Although most studies vary salinity by diluting natural or artificial seawater, which is
dominated by sodium and chloride ions, minor ions such as calcium and magnesium may be
more important in determining dispersion effectiveness. One study using the Labofma method of
measuring dispersant effectiveness showed very high effectiveness of freshwater and marine
dispersants at calcium concentrations that were low relative to seawater concentrations (Belk et
al., 1989). The effectiveness of the unidentified marine dispersant increased from 6% in
deionized water to 81% in water containing calcium at a concentration of 400 mg/L as CaCOs,
which is less than half the concentration found in seawater but still high relative to most
freshwater. Dispersants optimized for freshwater were less sensitive to the calcium
concentration than those optimized for marine use.
3. Objective of Research
The objective of this research was to investigate the relationship between dispersion
effectiveness in freshwater and the surfactant composition for weathered crude oil. Although
limited research on the chemical dispersion of crude oil and petroleum products in freshwater has
been conducted, previous studies (e.g., Belk et al., 1989) did not identify the dispersants that
were investigated, much less describe the chemistry of the surfactants that were used. The
absence of information on surfactant composition is a major impediment to the scientific
investigation of dispersant effectiveness because this information is necessary for the
development of a more fundamental understanding of dispersant effectiveness. Therefore, the
relationship between surfactant chemistry and dispersant effectiveness was systematically
evaluated.
4. Materials and Methods
4.1. Surfactants
The surfactants that were used in this research are listed in Table 1. The surfactants were
selected from three chemical classes: sorbitan esters, fatty acid ethoxy esters, and fatty acyl
ethoxy ethers. Because many of the surfactants used in this research are widely distributed and
used for a variety of purposes, Table 1 provides the trade names in addition to the chemical
names of the surfactants that were used. For brevity, the structure of the hydrophilic groups is
given as POE(x), where "POE" means "polyoxyetheylene" and "x" is the total number of
ethylene oxide units in the POE chains. The size of the hydrophilic groups is also abbreviated as
"Ex" in some abbreviations. The meaning of the variable 'x' in Ex is identical to POE(x). They
both refer to the number of ethoxyl groups. (The sorbitan ester surfactants frequently have
several POE chains esterified to different hydroxyl groups of the sorbityl group, but the total
number of ethylene oxide monomers is as indicated.) Example of poly oxy ethyl ene (20) sorbitan
monolaurate is shown below:
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The chain length and chemical characteristics of the fatty acyl groups is indicated by Cy:z,
where "y" is the number of carbon atoms in the fatty acyl chain and "z" is the number of carbon-
carbon double bonds. The number of carbon-carbon double bonds is important because, for a
given chain length, the melting point is a function of the degree of unsaturation. Greater
unsaturation (i.e., number of double bonds) is correlated with lower melting point for a given
chain length and, therefore, greater tendency to exist as a liquid at ambient temperatures. Several
of the surfactants include more than one fatty acyl group in the hydrophobic portion. These are
indicated by the use of "di-" (i.e., two fatty acyl groups), "tri-" (i.e., three fatty acyl groups), or
"hexa-" (i.e., six fatty acyl groups). The Tergitols (fatty acyl ethers) are composed of mixtures
of fatty acyl groups varying from 11 to 15 carbon atoms in chain length.
The HLB of the individual surfactant molecules is determined by the balance between the
size of the hydrophilic group (i.e., "x", the number of ethylene oxide units in the
polyoxyethylene chain) and the length (i.e., "y", the number of carbon atoms) and number (i.e.,
di-, tri-) of the fatty acyl group that constitutes the hydrophobic portion.
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Table 1: List of surfactant trade names and chemical names
class
sorbitan esters
fatty acid
ethyoxy esters
fatty alcohol
ethyoxy ethers
trade name
Span 20
Span 40
Span 60
Span 80
Tween 60
Tween 65
Tween 80
Tween 85
Myrj 45
Myrj 52
Brij 52
Brij 58
Brij 76
Brij 98
Tergitol-15-S-3
Tergitol-15-S-5
Tergitol-15-S-7
chemical name
sorbitan monolaurate (S-Ci2)
sorbitan monopalmitate (S-C,6)
sorbitan monostearate (S-Ci8)
sorbitan monooleate (S-Ci8:i)
POE(20) sorbitan monostearate (E20-S-Ci8)
POE(20) sorbitan tristearate (E20-S-triCi8)
POE(20) sorbitan monooleate (E20-S-C18:i)
POE(20) sorbitan trioleate (E20-S-triCi8:i)
POE(50) sorbitan hexaoleate (E5o-S-hexaCi8:i)
POE(2) monolaurate (E2-Ci2)
POE(4) monolaurate (E4-C12)
POE(12) monolaurate (E12-C]2)
POE(12) dilaurate (E12-diC12)
POE(4) monostearate (E4-C18)
POE(8) monostearate (E8-C18)
POE(40) monostearate (E40-Ci8)
POE(4) distearate (E4-diC18)
POE(6) tridecyl ether (E6-Ci3)
POE(2) hexadecyl edier (E2-Ci6)
POE (20) hexadecyl ether (E20-C16)
POE(IO) stearyl'ether (E10-Ci8)
POE(20) oelyl ether (E,0-C18 .,)
(E3-Cn.15)
(E5-Cn.15)
(E7-Ci,.,5)
HLB
8.6
6.7
4.7
4.3
14.9
10.5
15.0
11.0
11.6
6.5
9.8
14.9
12.2
7.9
11.2
17.3
5.0
11.4
5.2
15.7
12.4
15.3
8.0
10.0
12.1
4.2. Composition of Synthetic Lake Water
Experiments were conducted in artificial freshwater that was designed to have a divalent
cation composition similar to Lake Michigan water. The artificial Lake Michigan water has the
following composition (mg/1): NaHCO3 (96), CaCl2-2H2O (128), MgSO4-7H2O (111). It is not
known to what extent this formulation differs from other freshwater sources. After dissolving the
components, the artificial lake water was aerated to reduce the pH to approximately 8 and
filtered through a 0.2 um membrane filter. The synthetic lake water and Lake Michigan water
are compared in Table 2. The alkalinity of the synthetic lake water is lower than Lake Michigan
water, and the sodium and chloride concentrations are higher, but these factors were not expected
to have a significant effect on dispersant performance. Natural organic matter might also have an
influence on dispersant performance, but such a factor is difficult to simulate in a small
laboratory study such as this.
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Table 2: Composition of synthetic lake water
Component
calcium
magnesium
sodium
chloride
sulfate
alkalinity
Concentra
Lake Michigan
34
10
5
8
20
108
tion, mg/1
Synthetic Lake Water
35
11
26
62
43
57
4.3. Experimental Design
The dispersant formulations that were tested in this research are shown in Table 3. The
dispersants were prepared by dissolving the surfactants in n-dodecane (Sigma-Aldrich, St. Louis,
MO). The total concentration of surfactants in all dispersant formulations was 70% by mass, and
the concentration of dodecane was 30%. These dispersant formulations were designed to test the
effects of three factors on performance: (A) HLB, (B) surfactant chemical characteristics, and
(C) dispersant composition. All treatment factors were tested at three levels. The dispersant
formulations shown in Table 3 encompass the HLB range of most interest (i.e., between about 8
and 12), and the dispersants were formulated using surfactants with three different chemical
characteristics: sorbitan esters, fatty acid ethoxylates, and ethoxylated fatty acyl ethers. Some
investigators have suggested that dispersants with lower HLB should be more effective in
freshwater, whereas dispersants with higher HLB should be more effective in seawater (Clark,
personal communication). The surfactants used to formulate these dispersants are similar in that
the hydrophilic part of the molecule is composed primarily of polyethoxy groups, and the
hydrophobic part is contributed by long-chain (€12 to Cig) fatty acyl chains, which may be fatty
acids (as in sorbitan ester and fatty ester surfactants) or fatty alcohols (as in fatty ether
surfactants). The biggest structural difference between these surfactants is the presence of a
sorbitan molecule (i.e., a cyclic anhydride of the sugar alcohol sorbitol), to which the fatty acid
and polyethoxy groups are connected, in the sorbitan esters. Also, the Span-type sorbitan esters
are not ethoxylated. The third factor that was tested, dispersant composition, evaluated the
benefit conferred by using mixtures of two surfactants instead of a single surfactant. Since the
superiority of surfactant mixtures over single-surfactant formulations has been rationalized based
on packing arguments (Porter, 1991; Myers, 2006), it seems reasonable to speculate that
mixtures that are dominated by one surfactant will not be as effective as those that contain equal
amounts of all surfactants. Therefore, dispersants with the same HLB were prepared in three
different ways for each surfactant chemical class: one dispersant was prepared with a single
surfactant with HLB that was sufficiently close to the target HLB, one dispersant was prepared
by adding two surfactants in different proportions, and the third was prepared by combining two
surfactants in similar proportions (i.e., close to a 1:1 ratio).
The only commercial dispersant whose composition is known is Corexit 9500 with an
HLB of 10-11, and its composition is similar to the Span80-Tween80 ratio shown in Table 3
under Sorbitan esters for HLB 10.
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Table 3: Dispersant formulations for testing in freshwater
HLBt
8
(Al)
10
(A2)
12
(A3)
irget
Cl
C2
C3
Cl
C2
C3
Cl
C2
C3
Sorbitan Esters (Bl)
Span 20
65% Span 80 +
35% Tween 80
48% Span 60 +
52% Tween 85
Tween 65
60% Span 40 +
40% Tween 60
47% Span 80 +
53% Tween 80
E5rrS-hexaCis:i
28% Span 80 +
72% Tween 80
46% Span 20 +
54% Tween 60
Surfactant Type1
Fattv Acid Etlioxv Esters
(B2)
E4-Ci8
70%diC18-E4 +
30%E12-Ci2
54%E4-diC18 +
46% Myrj 45
E4-Cn
31%E2-C,2 +
69% Myrj 45
50%E4-diCls +
50% Ei2-Ci2
E12-diC12
29%E4-diC18 +
71%E12-Ci2
49% E2-C12 +
51% Myrj 52
Fatty Alcohol Ethoxy
Ethers (B3)
Tergitol 15-S-3
73% Brij 52 +
27% Brij 98
56% Brij 52 +
44%E6-C13
Tergitol 15-S-5
34% Brij 52 +
66% Brij 76
53% Brij 52 +
47% Brij 98
Tergitol 15-S-7
33% Brij 52 +
67% Brij 98
48% Tergitol 15-S-3 +
52% Brij 5 8
'see Table 1 and Section 3.1 for description of the chemical characteristics of these surfactants and for a
definition of the abbreviations
A total of 10 g of each dispersant (7 g of surfactants plus 3 g n-dodecane) was prepared.
Each component was weighed to an accuracy of + 0.01 g. After the surfactants and solvent were
combined, the mixtures were heated (if required to make the surfactants dissolve) and mixed for
a minimum of 15 minutes at 200 RPM on an orbital shaker. Although all formulations were
evaluated, not all have physical characteristics that would be conducive to practical application
in oil-spill response. Two problems were noted with these formulations: (1) some of the
dispersants were solids at room temperature, and (2) some of the surfactants were insoluble in
the solvent used to make the dispersant (dodecane). The physical characteristics of the
dispersants are indicated in Table 4, and the problem formulations are depicted in boldface font.
Dispersant formulations that were solids at room temperature were equilibrated at 50 °C
prior to use to facilitate their use in dispersion experiments. All of the dispersant formulations
that were tested in this research were liquids at this temperature. To prevent refreezing of the
dispersants during transfer, the pipette tips were also equilibrated at 50 °C before use.
Dispersants for which one or more of the surfactants was insoluble in the solvent were well
mixed prior to use. This procedure appeared to be adequate for use in these dispersion
experiments because it produced a cloudy suspension that separated into two phases relatively
slowly.
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Table 4: Physical characteristics of dispersant formulations
riLJjtarget
8
(Al)
10
(A2)
12
(A3)
Cl
C2
C3
Cl
C2
C3
Cl
C2
C3
Sorbitan Esters (Bl)
homogenous liquid
homogenous liquid
homogenous solid
2-phase solid
homogenous solid
homogenous liquid
homogenous liquid
homogenous liquid
homogenous liquid
Surfactant Type
Fattv Acid Ethoxy Esters
(B2)
2-phase solid
homogenous solid
homogenous solid
homogenous liquid
2-phase solid
homogenous solid
homogenous liquid
2-phase solid
homogenous solid
Fatty Alcohol Ethoxy
Ethers (B3)
homogenous liquid
homogenous solid
2-phase liquid
homogenous liquid
homogenous solid
homogenous solid
homogenous liquid
homogenous solid
homogenous solid
the problem formulations are depicted in boldface font
Each dispersant formulation was tested using three independently replicated effectiveness
tests. The order of testing was randomized except that all dispersants were tested once before
any dispersant was tested a second time. Because it took several weeks to test all of the
dispersants once, this type of blocking prevented confounding of treatment effects with time
effects. That is, in a completely randomized design, it would have been possible for all three
replicates of one dispersant to be conducted at almost the same time and all three replicates of
another dispersant to be conducted at another time. If slow systematic changes in performance
occurred over time, the effect of time could have been identified as a treatment effect. Such
temporal grouping of replicates was not possible with the experimental design that was used.
4.4. Oil
Artificially weathered Mars Blend crude oil (from the Gulf of Mexico) was used in these
experiments. Approximately 200 ml of this oil, which had an initial API gravity of about 30,
was weathered by evaporation under nitrogen for 3 days. Density measurements were not made
after weathering. To minimize mass transfer limitations, the oil was mixed by magnetic stirring
during the evaporation process. The mass of oil was reduced by 18.5% by this process. This oil
is listed in Environment Canada's ETC database.
4.5. Dispersion Effectiveness Experiments
The dispersant formulations listed above in Table 3 were tested using the baffled flask
test (BFT), a bench-scale dispersion effectiveness test that was developed for testing of
dispersants for listing on the National Contingency Plan Product Schedule (Serial etal., 2004).
Dispersion effectiveness was evaluated by adding 100 ul of weathered Mars Blend crude oil to a
150-ml baffled flask containing 120 ml of synthetic freshwater using a Repeater Plus Pipette
(Eppendorf, Westbury, NY) with a 0.5 ml pipette tip. Next, 4 ul of dispersant was added to the
floating oil using the same pipette with a 0.1 ml pipette tip. This produced a dispersant-to-oil
ratio (DOR) of 1:25. This baffled flask was mixed for 10 minutes at 200 rpm using a Lab-Line
Orbit Environ Shaker (Lab-Line Instruments, Inc., Melrose Park, IL). After the mixing period,
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the flask was carefully removed from the shaker and the dispersion was allowed to settle for 10
minutes. Figure 1 shows the baffled flask during the settling period.
After the settling period, a 40-ml sample was collected through the stopcock at the
bottom of the baffled flask. A 30-ml subsample of the 40-ml sample was extracted with
Figure 1: Baffled flask during the settling period
dichloromethane (DCM) to measure the mass of dispersed oil, and a 40-ul sample from the
remaining volume was diluted with 100 ml of deionized water and analyzed using a model LS-
200 Liquilaz Optical Particle Counter (OPC) (Particle Measuring Systems, Inc, Boulder, CO) to
determine the size distribution of dispersed oil droplets.
Dispersion effectiveness was estimated from the mass concentration of dispersed oil,
which was measured by extraction of the oil into DCM. A 30-ml sample of the dispersed phase
was added to a 125-ml separately funnel containing 5 ml of a 250 g/L sodium chloride solution.
The dispersed oil was then extracted by adding 10 ml of DCM to the separatory funnel, and the
mixture was shaken for approximately two minutes. This mixture was allowed to settle until it
separated into two distinct phases, and the DCM phase was collected by draining from the
bottom of the separatory funnel. This procedure was repeated at least three times and until the
DCM phase was colorless after extraction. Addition of sodium chloride facilitated separation of
the DCM and aqueous phases into two separate phases with a distinct interface.
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The solvent recovered from all three extraction steps was combined and filtered through
DCM-rinsed anhydrous sodium sulfate to remove water. The resulting solution was transferred
to a tared weighing-vial and the solvent was evaporated under a stream of air in a N-Evap
Analytical Evaporator (Organomation Associates, Berlin, MA). After evaporation of the DCM,
the tared vials were placed in a desiccator for 24 hours to remove any residual moisture. The
vials were then weighed a second time to determine the mass of the dispersed oil.
The dispersion effectiveness (r\) was estimated from the mass of the extracted oil as
follows:
* oil, tot Poll
where M0 is the initial mass of the tared vial, Mf is the mass of the vial containing the residue
remaining after evaporating the solvent, Vaq,tot is the total aqueous-phase volume used in the
dispersion experiments (i.e., 120 ml), Vaq,extraet is the volume of aqueous phase that was extracted
(i.e., 30 ml), V0ii,tot is the volume of oil added at the beginning of the experiment (i.e., 0.1 ml),
and p0ii is the density of the weathered Mars Blend crude oil (i.e., 0.9142 g/ml). The oil density
was measured by weighing at least 10 aliquots of oil delivered by the Eppendorf Repeater Plus
Pipette fitted with the 0.5-ml tip.
The dispersion efficiency and the size distribution of the dispersed oil droplets was also
measured using an optical particle counter (OPC). The OPC counts the number of oil droplets
within 15 user-defined size ranges between 2 and 120 |_im. The droplet-size distribution was
used to estimate the total volume of dispersed oil, which was used to calculate the dispersion
efficiency, and two particle-size statistics: the number mean diameter (NMD, d ) and the
diameter of mean volume (DMV, d-). Six replicate measurements of the number of oil droplets
in each size range (n;) were averaged before calculating size-distribution statistics. (The droplet-
size distribution was actually measured nine times for each sample, but the first three
measurements were discarded to insure that the detector was completely flushed of previous
samples or wash solutions.) The average diameter for each size range was used to represent all
droplets within the bin. For example, if the lower limit of the size bin was 2 |j,m and the upper
limit was 3 urn, all droplets within the bin were assumed to have a diameter of 2.5 um.
The NMD (d , (jrn) is a simple average of the droplet sizes:
15
d = -^ (2)
N»
where d; is the average diameter of size bin "i", m is the average number of droplets in bin "i"
from six replicate measurements as described above, and Ntot is the total number concentration of
oil droplets in the sample:
10
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Ntot=][> (3)
1=1
The DMV (d-; |j,m) is the volume-weighted average droplet diameter:
(4)
The DMV is a better measure of the mean of the droplet-size distribution because droplet
volume is proportional to the mass, whereas number is not (i.e., a small droplet contains less oil
than a large droplet, but both are weighted equally in calculation of the NMD). The NMD is
always smaller than the DMV in these size distributions because a large number of small
droplets are formed during dispersion, but they represent a relatively small fraction of the total
oil mass.
The dispersion efficiency was estimated as follows:
aq.tot
-xlOO% (5)
where V
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evaporation of volatile oil components during evaporation of the solvent. Loss of extracted oil
by evaporation was observed to occur despite using evaporatively weathered crude oil in these
experiments. Due to the systematic underestimation of dispersion effectiveness by the
gravimetric method, the treatment effects associated with dispersant formulation were evaluated
using the OPC-based measurement of dispersion effectiveness.
Table 5: Comparison of dispersion effectiveness using OPC and gravimetric analysis.
HLB
8
10
12
Chemistry"
sorbitan ester
fatty acid ester
fatty alcohol ether
sorbitan ester
fatty acid ester
fatty alcohol ether
sorbitan ester
fatty acid ester
fatty alcohol ether
Composition
single
two, unequal
two, equal
single
two. unequal
two, equal
single
two, unequal
two, equal
single
two, unequal
two, equal
single
two, unequal
two, equal
single
two, unequal
two, equal
single
two, unequal
two, equal
single
two, unequal
two, equal
single
two, unequal
two, equal
dispersion eff<
OPC
72.7 + 6.18
86.2 + 3.03
96.2 + 7.07
82.2+10.6
80.3 + 4.04
78.6+16.8
74.4 + 6.60
79.4 + 8.01
71.6 + 9.28
82.8+12.0
93.7 + 6.32
78.4+13.0
79.0 + 4.03
72.4 + 8.48
79.3+11.9
71.6 + 9.28
65.0+13.1
69.6 + 7.09
96.2 + 5.61
61.1 + 15.4
79.3 + 7.91
74.7+13.9
75.4 + 6.03
57.4+11.4
58.0 + 8.04
66.5 + 4.73
82.4 ±6.28
;ctiveness (%)
gravimetric
79.7+3.40
73.3 + 0.67
68.1+7.74
68.2 + 7.74
63.8 + 2.71
60.6 + 6.53
83.4±2.16
68.5 + 2.64
83.2 + 6.49
60.2+16.9
79.9+1.38
68.4 + 4.38
83.8 + 3.54
59.6 + 8.31
58.6 + 8.31
82.5 + 4.59
66.8 ±1.66
63.6 ±7. 14
65. 6 ±6.47
53. 6 ±5.40
75.4 ± 0.25
61.2 ±5. 30
61.1 + 3.51
44.5 + 4.40
76.8 ±3.28
56.0 ±7.23
72.9 ±2.49
5.2. Size Distributions
12
-------
The droplet-size distributions can be based on either number or volume. Figure 2 shows
a comparison of the normalized number and volume distributions for three dispersions produced
by single-surfactant dispersants with HLB = 8. Normalized size distributions are obtained by
dividing the number (n^) or volume (Vj) concentration of oil droplets in each size bin by the total
number (Ntot) or volume (Vtot) concentration of the dispersion and by the size of the bin (Ad;).
These two operations ensure that the area under the size-distribution curve is equal to 1.0 (i.e.,
the normalized size distributions have properties similar to histograms). In two of three cases,
the number distribution was unimodal, but all of the volume distributions were bimodal. The
0.6
. . . I . . .Q I . . . .
0.0
Figure 2: Comparison of normalized number (left axis) and volume (right axis) distributions
for dispersions produced by single-surfactant dispersants with HLB = 8. The
dispersants used included (A) Span 20 (HLB = 8.6), (B) POE(4) monostearate
(HLB = 7.9), or (C) Tergitol 15-S-3 (HLB = 8.0) as the sole surfactant.
13
-------
mean diameter of the largest size mode was about 10 ^m, whereas the mean diameter of the
smallest size mode was only about 4 |_im. The bimodality of the volume distribution reflects the
disproportionate influence of large droplets. Whereas oil droplets in the larger size mode
constituted less than 2% of the total number of dispersed oil droplets in dispersants prepared with
Span 20 and Tergitol 15-S-3, they represented about 20% of the total dispersed oil volume.
Similarly, in the dispersion produced by application of the dispersant containing POE(4)
monostearate, the large droplet-size mode constituted less than 10% of the total number of
dispersed droplets but it represented about 40% of the total dispersed oil volume. Because the
volume of dispersed oil is more important than the number of droplets from the perspective of
ecological effects and performance in spill response, the dispersant formulations tested in this
research will be compared based on the characteristics of the volume distributions. It is
recognized, however, that smaller droplet sizes promote emulsion stability, thereby resulting in
more rapid biodegradation rates due to the resultant higher surface area of the smaller droplets.
The normalized volume distributions that were produced during this research are shown
in Figures 3, 4, and 5. Figure 3 shows the volume distributions that were obtained using single-
surfactant dispersants, whereas Figs. 4 and 5 show the droplet-volume distributions for
dispersions that were produced with dispersions prepared with two surfactants present in unequal
or approximately equal proportions, respectively. All three figures also compare the size
distributions obtained with dispersants prepared with sorbitan ester (panel A), fatty acyl ester
(panel B), and fatty acyl ether (panel C) surfactants. They also compare the size distributions
produced by dispersants with HLB of 8 (filled circles), 10 (open circles), and 12 (filled squares).
The error bars represent one standard deviation for three independent replicates (i.e., dispersions
prepared in independent experimental units at different times). These figures clearly show that
the overall characteristics of the dispersions are similar regardless of the dispersant that was
used: all of the dispersions show two main size modes with mean diameters of approximately 4
|j,m and 10 |j,m. The distribution of oil between the two modes, however, does depend on the
characteristics of the dispersant. The distribution of oil between the two modes can be quantified
using the diameter of mean volume (DMV; Eq. 4).
14
-------
HLB = 8.6
HLB= 10.5
HLB= 11.6
HLB = 7.9
HLB = 9.8
HLB= 12.2
HLB = 8.0
HLB= 10.0
HLB= 12.0
0.0
10
20
30
40
50
Figure 3: Normalized volume distributions for single-surfactant dispersants prepared from
(A) sorbitan ester surfactants, (B) fatty acyl ester surfactants, and (C) fatty acyl
ether surfactants. The error bars represent one standard deviation of three
independent dispersion experiments.
15
-------
0.4
0.3 --
0.2 --
0.1 --
0.0
^ 0.3 - -
'E
o.o
0.3 --
0.2 --
0.1 --
B
10
HLB = 8
HLB= 10
HLB= 12
20
30
40
50
Figure 4: Normalized volume distributions for dispersions produced by dispersants prepared
with two surfactants that were present in unequal proportions. The surfactants
used were (A) sorbitan esters, (B) fatty acyl esters, or (C) fatty acyl ethers.
16
-------
HLB =
HLB = 10
HLB = 12
10
20
30
40
50
Figure 5: Normalized volume distributions for dispersions produced using dispersants
prepared with two surfactants present in equal proportions. The surfactants that
were used were (A) sorbitan esters, (B) fatty acyl esters, and (C) fatty acyl ethers.
17
-------
5.3. Evaluation of Effects of Dispersant Characteristics
Treatment effects on dispersion effectiveness were evaluated by performing ANOVA on
the data shown in Table 5 (OPC; based on droplet-size distribution). The null hypothesis tested
was that none of the main effects or interactions was significant at the a = 0.05 level. The results
are shown in Table 6, which shows that surfactant HLB and chemistry exerted significant effects
on dispersion effectiveness. In addition, the three-factor interaction (i.e., HLB, dispersant
composition, surfactant chemistry) is statistically significant, indicating that the treatment main
effects are not strictly additive. A similar analysis was performed to evaluate treatment effects
on the droplet-size distribution, as indicated by the DMV. The results of this analysis are shown
in Table 7 and demonstrate that all three main effects were highly significant (p < 0.001) as were
all three two-factor interactions and the three-factor interaction (p < 0.001).
Table 6: ANOVA for treatment effects on dispersion effectiveness
Source of Variation
HLB
Composition
Chemistry
HLB x Composition
HLB x Chemistry
Composition x Chemistry
HLB x Composition x Chemistry
Error
Sum of
Squares
869.0
70.2
1743.5
524.5
122.6
884.9
3646.6
4830.4
Degrees of
Freedom
2
2
2
4
4
4
8
54
Mean Square
434.5
35.1
871.8
131.1
30.6
221.2
455.8
89.5
F-ratio
4.858
0.392
9.746
1.466
0.343
2.473
5.096
P
0.011
0.677
0.001
0.225
0.848
0.055
0.001
Table 7: ANOVA for treatment effects on diameter of mean volume (DMV)
Source of Variation
HLB
Composition
Chemistry
HLB x Composition
HLB x Chemistry
Composition x Chemistry
HLB x Composition x Chemistry
Error
Sum of
Squares
6.68
11.4
44.7
5.58
7.02
12.0
24.3
5.28
Degrees of
Freedom
2
2
2
4
4
4
8
54
Mean Square
3.34
5.71
22.4
1.40
1.76
3.01
3.04
0.098
F-ratio
34.18
58.46
228.7
14.28
17.95
30.78
31.08
P
0.001
O.001
0.001
O.001
0.001
O.001
O.001
The treatment main effects on dispersion effectiveness and diameter of mean volume are
shown in Figure 6. Significant differences between treatment means were identified using
Tukey's method for pairwise comparisons, which allows comparison of all possible pairs of
18
-------
100
75 --
50 --
25 --
75 --
(A
(A
(1)
5 so
.s
•53 25
0 -
75 -
50 -
25 -
A,B
A1
B
i i sorbitan ester
^^H fatty acyl ester
Y / A fatty acyl ether
-- 8
A1
C1
T
effectiveness
DMV
o>
o
6 >
c
ra
0)
4 £
0)
*!
0)
E
re
1-0 ^
Figure 6: Comparison of treatment main effects on dispersion effectiveness (left axis) and
droplet DMV (right axis). The top panel shows the effects of dispersant HLB on
these performance parameters, the middle panel shows the effects of surfactant
chemistry, and the bottom panel shows the effects of dispersant composition. The
error bars represent one standard deviation. Bars labeled with the same letter are
not significantly different at the 95% confidence level.
means while keeping the global Type I error at 5%. In Figure 6, dispersion effectiveness means
are compared only to other effectiveness means, and DMVs are compared only to other DMVs.
19
-------
Figure 6 shows that, whereas dispersants with HLB = 12 were less effective than those
with HLB = 8, the differences between dispersants with HLB = 8 and 10 and between
dispersants with HLB = 10 and 12 were not statistically significant. Dispersants with HLB = 12
also produced dispersions with a larger fraction of the dispersed oil in larger droplets. Also,
dispersants that contained sorbitan-ester surfactants (e.g., Tweens and Spans) were significantly
more effective than those containing fatty acyl ester and fatty acyl ether surfactants. The fatty
acyl ether surfactants, however, produced the smallest oil droplets. Although dispersant
composition (e.g., formulation with one or two surfactants) had no effect on dispersion
effectiveness, dispersants formulated with a single surfactant resulted in the formation of smaller
oil droplets.
Figures 7 and 8 show the effects of two-factor interactions on droplet DMV. In these
figures, the significant differences that are indicated are those identified using the Least-
Significant Difference (LSD) test because use of Tukey's method for pairwise comparisons
produced too many comparisons (and too many differences) to easily show graphically. The
differences identified using Tukey's method are shown in Tables 8-10. Figure 7 shows the
interactions between surfactant chemistry and dispersant composition. In general, oil droplets
formed by dispersants containing a single, fatty acyl ether surfactant were smaller than those
produced with other dispersants, and droplets formed by dispersants containing two fatty acyl
ester surfactants were larger. Figure 8 shows the interactions between dispersant HLB and (A)
surfactant chemistry or (B) dispersant composition. In general, the smallest droplets were
formed when dispersants with HLB of 8 or 10 containing fatty acyl ether surfactants were used.
sorbitan ester
surfactant chemistry
ether
Figure 7: Comparison of treatment effects on droplet-size distribution (DMV) due to the
surfactant chemistry-dispersant composition interaction. Error bars represent one
standard deviation of three independent replicate dispersion experiments. Single,
unequal, and equal refer to the fatty acyl chain from either a single surfactant or
two surfactants in unequal or equal composition levels.
20
-------
0)
E
o 2 --
n
0)
E
0)
0>
E
12
Figure 8: Comparison of treatment effects on droplet-size distribution
(DMV) due to the interactions between dispersant HLB and (A)
surfactant chemistry and (B) dispersant composition. Error bars
represent one standard deviation of three independent replicate
dispersion experiments. Single, unequal, and equal refer to the
fatty acyl chain from either a single surfactant or two surfactants
in unequal or equal composition levels.
The largest droplets, however, were produced by dispersants containing fatty acyl ester
surfactants with HLB of 8 or 10. Overall, the correlation between dispersant effectiveness and
droplet size is relatively weak, but the differences between the largest and smallest diameters of
mean volume were relatively small (i.e., 3.2 to 8.0 |j,m).
21
-------
The three-factor interactions for DMV and dispersion effectiveness are shown in Figure
9A and B, respectively. Once again, due to the very large number of pairwise comparisons that
are possible (351), the differences that are indicated are those identified using the LSD test. The
significant differences identified using Tukey's method for pairwise comparisons are shown in
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sorbitan ester ether sorbitan ester ether sorbitan ester ether
HLB = 8
HLB = 10
HLB = 12
Figure 9: Comparison of three-factor interactions for (A) droplet-size distribution (DMV)
and (B) dispersant effectiveness. Error bars represent one standard deviation of
three independent replicate dispersion experiments. The asterisk denotes a
significant difference (p < 0.05) as determined by the LST test.
22
-------
Tables 11 and 12. Tukey's method identifies many more significant differences than does the
LSD. For example, although the ANOVA determined that the three-factor interaction for
dispersion effectiveness was highly significant (p < 0.001; see Table 6), the LSD test was unable
to identify any differences between adjacent pairs of means when they were rank ordered,
whereas Tukey's method identified 104 significant differences when all possible pairwise
comparisons were evaluated (Table 12).
Table 8
Interaction
8-sorb
10-sorb
12 -sorb
8-ester
10-ester
12-ester
8-ether
10-ether
12-ether
: Probabi
two-fac
8-sorb
1.000
0.005
0.001
0.000
0.00 1
0.000
0.000
0.158
1.000
lities that
tor intera<
10-sorb
1.000
1.000
0.000
1.000
0.000
0.000
0.000
0.005
the mean
;tion betw
12-sorb
1.000
0.000
1.000
0.000
0.000
0.000
0.001
5 of any p;
een dispe
8-ester
1.000
0.000
0.380
0.000
0.000
0.000
lirofDM
rsant HLB
10-ester
1.000
0.000
0.000
0.000
0.001
Vs are sig
and surfc
12-ester
1.000
0.000
0.000
0.000
nificantly
ictant chei
8-ether
1.000
0.081
0.000
different
-nistry
10-ether
1.000
0.148
for the
12-ether
1.000
Table 9: Probabilities that the means of any pair of DMVs are significantly different for the
two-factor interaction between dispersant HLB and dispersant composition
Interaction
8-single
8-unequal
8-equal
10-single
10-
unequal
10-equal
12-single
12-
unequal
12-equal
8-single
1.000
0.000
0.000
1.000
0.008
0.000
0.000
0.000
0.000
8-
unequal
1.000
1.000
0.000
0.538
0.182
1.000
0.000
0.976
8-equal
1.000
0.000
0.238
0.450
0.983
0.000
1.000
10-
single
1.000
0.044
0.000
0.000
0.000
0.000
10-
unequal
1.000
0.001
0.837
0.000
0.072
10-equal
1.000
0.060
0.004
0.799
12-
single
1.000
0.000
0.822
12-
unequal
1.000
0.000
12-equal
1.000
23
-------
Table 10: Probabilities that the means of any pair of DMVs are significantly different for the
two-factor interaction between dispersant HLB and surfactant chemistry
Interaction
sorb-
single
sorb-uneq
sorb-equal
ester-
single
ester-uneq
ester-
equal
ether-
single
ether-uneq
ether-
equal
sorb-
single
1.000
0.999
0.673
0.988
0.000
0.000
0.000
0.308
0.000
sorb-
uneq
1.000
0.967
1.000
0.000
0.000
0.000
0.738
0.000
sorb-
equal
1.000
0.995
0.000
0.000
0.000
1.000
0.005
ester-
single
1.000
0.000
0.000
0.000
0.881
0.000
ester-
uneq
1.000
0.005
0.000
0.000
0.000
ester-
equal
1.000
0.000
0.000
0.000
ether-
single
1.000
0.000
0.000
ether-
uneq
1.000
0.029
ether-
equal
1.000
24
-------
Table 11: Probabilities that the means of any pair of DMVs are significantly different for the three-factor interaction between
disper sant HLB, surfactant chemistry, and dispersant composition
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estar
l.OOC
].30C
O.OOC
0.000
i :.oc
O.OOC
O.OOC
O.OL3
IX'C
J.'JUC
0.000
. ^ _;
a;:a:
1 OOC
000
OOC
OOC
ox
000
000
0 X
OOC
000
S'i
ether
L.OOO
L.OCO
0.443
O.L61
L.OOO
O.Q5?
0.2S"
0.000
L.OOO
8.1'
ethsr
1 OOC
0 501
02^6
1 OOC
0 09$
0-.::
o out
1 000
8,E
e±a[
l.OOi
1.000
0.50";
0.000
LCOO
O.IK
LO 5
2the:
L.OOO
0.212
0.000
L.XO
II [III
0.34S
10.U
ethfii
1 OOC
0 134
0 350
0 0 Ji
1 OOC
10 E.
ether
1.003
O.OCO
0.04S
0.452
L2i
sther
1 OOC
0.000
0.092
12'U
e:liar
I OOC
0.000
12 E
ather
• fifi"
_ .VL v
Note: All p-values shown as 0.000 signify < 0.001.
25
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Table 12: Probabilities that the means of any pair of dispersant effectivenesses are significantly different for the three-factor
interaction between dispersant HLB. surfactant chemistry, and dispersaut composition
draractM
S :: sorb
3 U sore
SE-Qrb
lO.Ssorb
10"J:Olb
lOEiBrb
L2.Si£Kb
irj'-ort-
U E iBit
3'S ;r.e:
S. fester
SE 'ester
1C 'i ester
lOU-astei
10 E era
12 'i e;tei
12U5ite:
12 H e;:er
£ S «ta
SUe-hei
SEe-Jier
lOieoer
10 I" eusr
3 OE. ether
.12 i ether
12 V e:;.ar
12-Eetber
S'i
iOtb
l.OOC
o.osc
0.003
0.1S7
0.003
0.453
0.003
0.130
0.1 £5
0.215
0.118
041"
0.405
0.972
0.1 ST
0.143
0.722
0.04S
0.75=
0.1 SI
o.ss?
0.116
0.6$ 5
0.1 S?
0.057
0.415
0.2M
Si-
sorb
l.OOC
01S9
:5:s
0 325
0.509
01S9
0.002
0 369
0599
0-42
0 V'i
0 J5C
0.0":
0 3
-------
6. Conclusions
In this research, dispersant effectiveness was measured using two methods: measurement
of the droplet-size distribution using an optical particle counter (OPC) and extraction of the
dispersed oil into dichloromethane followed by gravimetric measurement of the mass of
extracted oil. The gravimetric method produced systematically lower estimates of dispersion
effectiveness than the size-distribution method, probably due to evaporation of volatile oil
components during evaporation of the solvent. Therefore, treatment effects were evaluated
based on the size-distribution data.
All dispersants produced bimodal droplet-volume distributions, and the two major modes
had similar mean diameters (about 4 |im and 10 |j,m). This bimodal size distribution may be due
primarily to the characteristics of the baffled flask test, although experiments conducted in salt
water using the same dispersants tend to produce trimodal volume distributions (unpublished
data). In the latter case, an additional large volume mode (mean diameter of about 25 ^m) is
produced. Despite the similar characteristics of all of the dispersed-oil volume distributions,
dispersant formulations tended to change the distribution of oil between the two droplet-size
modes. The relative amount of oil in the large and small size modes was quantified using the
diameter of mean volume (DMV). Although the overall differences among DMV for dispersant
formulations was relatively small, the dispersant characteristics exerted highly significant effects
on DMV with all three main effects, all three two-factor interactions (i.e., HLB x dispersant
composition; HLB x surfactant chemistry; dispersant composition x surfactant chemistry), and
the three-factor interaction (i.e., HLB x dispersant composition x surfactant chemistry) being
significant at the 95% confidence level. The strong statistical significance (ANOVA results in
Tables 6 and 7) of these treatment effects and interactions appears to be due to the high
reproducibility of the droplet-size distributions that were produced by each dispersant
formulation. Dispersion effectiveness was much more variable than were the size distributions.
It is also of interest to note that divalent cations seem to exert a more important influence than
monovalent cations in terms of droplet size distribution effects from the use of dispersants (Belk,
etal., 1989)
Dispersants formulated with sorbitan ester surfactants and with HLB of between about 8
and 10 exhibited the best performance when the dispersion of a weathered Mars Blend crude oil
was tested in the synthetic lake freshwater. At least two of the dispersant formulations that were
tested were highly effective with >90% of the added oil transferred to the aqueous phase in the
baffled flask test. Dispersants formulated with fatty acyl ester or ether surfactants and higher
HLB were relatively ineffective. The least effective dispersants transferred less than 60% of the
added oil to the aqueous phase. It is unknown to what extent this observation applies to other
types of freshwater formulations.
This report showed that, at least with Mars Blend crude oil in simulated lake water,
dispersants can be designed to drive an oil slick into the freshwater column with the same
efficiency as in saltwater as long as the HLB is optimum. Clearly, many more oils would need to
be tested under different conditions (temperature, organic content, water composition, etc.) to
enable firm conclusions that oil can be dispersed in freshwater as a response tool. The ultimate
decision to use dispersants in treating freshwater petroleum oil spills is up to the federal on-scene
coordinator, the incident command team, the regional response teams, and EPA Headquarters,
since many other factors need to be considered before rendering a decision to disperse oil into
27
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the water column. It is beyond the scope of this report to consider such factors. Its purpose was
simply to determine if freshwater dispersion is possible and to determine whether effective
freshwater dispersants can be designed to produce stable oil droplet distributions in such an
environment.
7. References
Belk, J.L., D.J. Elliott and L.M. Flaherty (1989). The comparative effectiveness of dispersants in
fresh and low salinity waters. In: Proceedings, 1989 Oil Spill Conference, 333-336.
American Petroleum Institute. Washington, DC
Blondina, G.J., M.M. Singer, I. Lee, M.T. Ouano, M. Hodgins, R.S. Tjeerdema and M.L. Sowby
(1999). Influence of salinity on petroleum accommodation by dispersants. Spill Science
and Technology Bulletin 5(2): 127-134.
Bruheim, P., H. Bredholt and K. Eimhjellen (1999). Effects of surfactant mixtures, including
Corexit 9527, on bacterial oxidation of acetate and alkanes in crude oil. Applied and
Environmental Microbiology 65_(4): 1658-1661.
Bruheim, P. and K. Eimhjellen (2000). Effects of non-ionic surfactants on the uptake and
hydrolysis of fluoresceindiacetate by alkane-oxidizing bacteria. Canadian Journal of
Microbiology 46(4): 387-390.
Clayton, J.R., J.R. Payne and J.S. Farlow (1993). Oil Spill Dispersants: Mechanisms of Action
and Laboratory Tests. CRC Press, Inc. Boca Raton, FL
Clark, J. 2004. Dispersant basics: Mechanism, chemistry, and physics of dispersants in oil-spill
response. Presented to the National Research Council Committee on Oil-Spill Dispersants:
Efficacy and Effects, March 15, 2004.
Fingas, M., I. Bier, M. Bobra and S. Callaghan (1991). Studies on the physical and chemical
behavior of oil and dispersant mixtures. In: Proceedings, 1991 Oil Spill Conference.
American Petroleum Institute. Washinton, DC
Fingas, M.F., B. Kolokowski and EJ. Tennyson (1990). Study of oil spill dispersants
effectiveness and physical studies. Thirteenth Arctic and Marine Oilspill Program
Technical Seminar, Edmonton, Alberta, Canada, Environment Canada.
Georges-Ares, A. and J.R. Clark (2000). Aquatic toxicity of two Corexit dispersants.
Chemosphere 40: 897-906.
Lehtinen, C.M. and A.-M. Vesala (1984). Effectiveness of oil spill dispersants at low salinities
and low water temperatures. In: Oil Spill Chemical Dispersants: Research, Experience,
and Recommendations, 108-121. T. E. Allen. American Society for Testing and
Materials. Philadelphia, PA
Myers, D. (2006). Surfactant Science and Technology, Third Edition. John Wiley and Sons, Inc.
Hoboken, NJ
NRC (2005). Oil Spill Dispersants: Efficacy and Effects. National Academies Press.
Washington, DC
NRC, N.R.C. (1989). Using Oil Spill Dispersants on the Sea. National Academy Press.
Washington, DC
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Porter, M.R. (1991). Handbook of Surfactants. Blackie and Sons, Ltd. Glasgow, Scotland
Serial, G.A., A.D. Venosa, K.M. Koran, E. Holder and D.W. King (2004). Oil spill dispersant
effectiveness protocol. II: Performance of revised protocol. Journal of Environmental
Engineering (ASCE) 130(10): 1085-1093.
Stoffyn-Egli, P. and K. Lee (2002). Formation and characterization of oil-mineral aggregates.
Spill Science & Technology Bulletin 8(1): 31-44.
Van Hamme, J.D. and O.P. Ward (1999). Influence of chemical surfactants on the
biodegradation of crude oil by a mixed bacterial culture. Canadian Journal of
Microbiology 45: 130-137.
29
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