ICES Journal of Marine Science, 63: 1418e1430 (2006)
doi:10.1016/j.icesjms.2006.04.019
Dispersant effectiveness on oil spills e impact of salinity
Subhashini Chandrasekar, George A. Sorial, and
James W. Weaver
Chandrasekar, S., Sorial, G. A., and Weaver, J. W. 2006. Dispersant effectiveness on oil
spills e impact of salinity. e ICES Journal of Marine Science, 63: 1418e1430.
When a dispersant is applied to an oil slick, its effectiveness in dispersing the spilled oil
depends on factors such as oil properties, wave-mixing energy, temperature, and salinity
of the water. Estuaries represent water with varying salinity, so in this study, three salinity
values in the range 10e34 psu were investigated, representing potential salinity concentrations found in typical estuaries. Three oils were chosen to represent light refined oil, light
crude oil, and medium crude oil. Each was tested at three weathering levels to represent
maximum, medium, and zero weathering. Two dispersants were chosen for evaluation. A
modified trypsinizing flask termed a baffled flask was used to conduct the experimental
runs. A full factorial experiment was conducted for each oil. The interactions between
the effects of salinity and three environmental factors, temperature, oil weathering, and mixing energy, on dispersion effectiveness were investigated. Each experiment was replicated
four times in order to evaluate the accuracy of the test. Statistical analyses of the experimental data were performed for each of the three oils independently for each dispersant
treatment (two dispersants and oil controls). A linear regression model representing the
main factors (salinity, temperature, oil weathering, flask speed) and second-order interactions among the factors was fitted to the experimental data. Salinity played an important
role in determining the significance of temperature and mixing energy on dispersant effectiveness for almost all the oiledispersant combinations. The impact of salinity at different
weathering was only significant for light crude oil with dispersant A.
Ó 2006 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
Keywords: baffled-flask test, dispersant, dispersant effectiveness, environmental factors, oil
remediation, oil spill.
Received 29 September 2005; accepted 3 April 2006.
S. Chandrasekar and G. A. Sorial: Department of Civil and Environmental Engineering,
University of Cincinnati, Cincinnati, OH 45221-0071, USA. J. W. Weaver: Ecosystems
Research Division, National Exposure Research Laboratory, US EPA, Athens, GA
30605, USA. Correspondence to G. A. Sorial: tel: þ1 513 5562987; fax: þ1 513
5562599; e-mail: [email protected].
Introduction
In the event of unintentional releases of oil into coastal waters, oil from slicks can have deleterious impacts on biota in
exposed ecosystems. Effects will depend in large part on the
ultimate location of the oil as well as on its chemical composition at the time of interaction with the biota (NRC,
1985). Oil slicks usually spread rapidly to a large area because of the action of gravitational and viscous forces, so
quick response has to be initiated (Hoult, 1972). Four
cleanup strategies that frequently receive consideration
include (i) mechanical cleanup or recovery, (ii) burning,
(iii) bioremediation, and (iv) treatment with chemical
dispersants (NRC, 1989).
Chemical dispersants are made of surfactants that are usually sprayed onto oil slicks to remove oil from the surface and
1054-3139/$32.00
disperse it into the water column, significantly reducing the
impact on shorelines and habitats (Lessard and Demarco,
2000). The essential components in dispersant formulations
are surfactants, which contain both oil-compatible (lipophilic)
and water-compatible (hydrophilic) groups. Following successful application of a chemical dispersant formulation to
an oil slick on water, the surfactant molecules reside at
oilewater interfaces and reduce the interfacial surface
tension. In the presence of mixing energy (provided by
wave or wind action), this might result in dispersion of the
oil as small droplets into the underlying water column.
Such dispersion leads to dilution of the oil in the water and
increased oilewater interfacial surface area, which might
eventually favour microbial degradation of the oil.
Studies have shown that the salinity of receiving waters
can impact dispersion of oil by chemical dispersants (Clayton
Ó 2006 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
Dispersant effectiveness on oil spills e impact of salinity
et al., 1993). Specifically, the intent of dispersant formulations for marine use is to provide maximum dispersion at normal seawater salinity. Mackay et al. (1984) note that higher
salinity increases the effectiveness of dispersants by deterring
migration of surfactant molecules into the water phase,
equivalent to a salting-out effect for the surfactant from the
saline medium. Such a situation will tend to promote association of surfactant molecules with oil at oilewater interfaces,
which is important for lowering oilewater interfacial surface
tensions in the oiledispersant mixture. In general, increasing
salinity will decrease the solubility of dispersants in water,
resulting in more surfactant being available to interact and
mix with the oil. Experimental studies have demonstrated
a general increase in dispersant effectiveness with increasing
salinity. Clayton et al. (1993) and Byford et al. (1983) performed tests to determine the effect of salinity on dispersant
effectiveness under low temperatures and high-energy conditions, using the Labofina-rotating flask test method. Those
tests were conducted with seven dispersants and two types
of crude oil. Test conditions were intended to simulate those
commonly found in the Arctic environment. Results indicated an overall increase in dispersion with increasing salinity in five of the dispersants tested. Clayton et al. (1993) and
Fingas (1991) studied the role of salinity on the effectiveness
of three dispersants on three types of crude oil, using the
swirling flask test method. These tests also showed an increase in dispersion with an increase in salinity from 0 to
45 psu. However, there could have existed maximum dispersion effectiveness or no effect when the salinity was investigated (Blondina et al., 1999; Moles et al., 2002). It depends
on the dispersanteoil combination and the mixing energy.
A number of other factors, such as mixing energy, oil
weathering, and temperature also influence dispersant effectiveness. Emulsions of oil droplets in water begin to form
when sea energy is sufficient (Fingas, 1991; Fingas et al.,
1995). After addition of the dispersants, mixing energy is
further required to disperse the oil droplets formed. Clayton
et al. (1993) reported that applications of dispersants reduce
the interfacial tension between oil and water, resulting in
the formation of oil droplets. Experimental studies performed by a number of scientists have indicated that the
size of the oil droplets is inversely related to the amount
of mixing energy input into test vessels. For example,
Clayton et al. (1993) and Fingas et al. (1993) conducted experiments which indicated that the mixing energy reduces
the size of the oil droplets. However, the available database
for droplet size distribution is very limited. The chemical
composition and physical properties of crude oil also determine the behaviour of the oil, and the way its properties
will change when the oil is spilled at sea (Kristiansen
et al., 1997). Weathering increases the viscosity of the oil
through evaporation of the lighter components. Oil viscosity is perceived as a major factor affecting the dispersibility
of oil (Canevari et al., 2001). As the oil weathers and the
viscosity increases, the effectiveness of the chemical dispersant declines (Daling, 1989). Lower water temperature
1419
increases the viscosity of both the oil and the dispersant.
A higher water temperature usually increases the solubility
of dispersants in water, and also affects the spilled oil temperature. Hence, an increase in temperature reduces oil viscosity and leads to improvement of dispersion. Mackay and
Szeto (1981), Byford et al. (1983), Lentinen and Vesala
(1984), and Fingas (1991) conducted studies which indicated an increase in dispersion efficiency with increasing
temperature. However, there have been conflicting results
in the trend of dispersant effectiveness with either increasing or decreasing water temperature. For example, the results of studies performed by Byford et al. (1983) differed
from those conducted by Fingas (1991).
To assess the impact of dispersant usage on oil spills,
US EPA is currently developing and evaluating models
(Weaver, 2004). Because of the complexity of chemical
and physical interactions between spilled oils, dispersants,
and the sea, an empirical approach to the interaction between the dispersant and the oil slick may provide a useful
or practical approach for including dispersants in these
models. The overall objective of this research was therefore
to create a set of empirical data on three oils and two dispersants, by studying the variation in the effectiveness of
the dispersants caused by changes in the salinity of seawater, temperature, oil type, oil weathering, dispersant
type, and rotation speed. Recently, the US EPA (Sorial
et al., 2004a, b) developed an improved dispersant testing
protocol, called the baffled-flask test (BFT), which was
the basis of the experiments conducted in the present study.
The specific objectives were (i) to conduct a factorial experimental setup with four replicates to determine which
of the factors salinity, temperature, oil type, oil weathering,
dispersant type, and rotation speed of the BFT are related to
the effectiveness of a dispersant used in oil spill response,
and (ii) to determine empirical relationships between the
amount of oil dispersed and the variables studied.
Material and methods
Modified 150-ml glass baffled trypsinizing flasks with
screw caps at the top and Teflon stopcocks near the bottom
were used in all experiments (Chandrasekar et al., 2005).
An orbital shaker (Lab-Line Instruments Inc, Melrose
Park, IL) with a variable speed control unit (40e400 rpm)
and an orbital diameter of 0.75 inches (2 cm) was used to
provide turbulence to solutions in test flasks. The shaker
has a control speed dial to provide a reading of rpm within
the instrument. The accuracy is within 10%, as determined by the manufacturer. A Brinkmann Eppendorf
repeater plus pipettor (Fisher Scientific, Pittsburgh, PA), capable of dispensing 4 ml of dispersant and 100 ml of oil with
an accuracy of 0.3% and a precision of 0.25%, was used
with 100-ml and 5-ml syringe tip attachments. Glassware
consisting of graduated cylinders, 125-ml separatory funnels with Teflon stopcocks, pipettes, 50-ml crimp-style
1420
S. Chandrasekar et al.
amber glass vials, and 50, 100, and 1000-ml gas-tight syringes were also used. A UV mini-1240 UVeVIS Spectrophotometer (UVeVIS spec) (Shimadzu Scientific Instruments,
Inc, Wood Dale, IL) was used in all experiments to measure
the dispersed oil concentration after extraction.
The synthetic sea salt ‘‘Instant Ocean’’ (Aquarium Systems, Mentor, OH) was used for all experiments at concentrations (salinity) of 10, 20, and 34 psu (the salinity of
ocean water ranges between 32 and 37 psu). The synthetic
seawater was prepared by adding sufficient water to 10, 20,
or 34 g of salt to make 1 l of solution. Three types of oil
samples provided by US EPA, South Louisiana Crude Oil
(SLC), Prudhoe Bay Crude Oil (PBC), and Number 2
Fuel Oil (2FO), were used in the study. Of the three oils
studied, SLC and PBC are light and medium weight
EPA/API standard reference crude oils, respectively, and
2FO is light refined oil. Two dispersants that Venosa
et al. (2002) found to be particularly effective dispersants
were used in this study, here referred to as dispersants A
and B. The solvent dichloromethane (DCM, pesticide
quality) was used to extract all sample water and oile
standard water samples.
Weathering of oils
The three oils, SLC, PBC, and 2FO, were used in the study
at three levels of weathering. They were weathered by bubbling nitrogen gas at a low flow rate at a room temperature
of 22 1(C to achieve volume losses of 0%, 10% (after 3
days), and 20% (after 7 days) for SLC and PBC, and 0%,
3.8% (after 3 days), and 7.6% (after 7 days) for 2FO.
oil, 80 ml of the dispersant, and 18 ml of DCM was measured with a 1-ml gas-tight syringe, and the concentration
was then determined. These stock solutions were used to
prepare standard solutions as mentioned above.
Dispersant effectiveness procedure
A volume of 120 ml of synthetic seawater equilibrated at the
desired temperature was added to the test flask, followed by
the sequential addition of oil, and finally the dispersant.
Then, 100 ml of oil was dispensed directly onto the surface
of the synthetic seawater with an Eppendorf repeater pipettor with a 5-ml syringe tip attachment. The dispersant was
then dispensed onto the centre of the oil slick with a 100ml syringe tip attachment set to dispense 4 ml, giving a dispersant-to-oil ratio (DOR) of 1:25. The flask was placed
on an orbital shaker and mixed for 10 min at the desired rotation speed, at the end of which it was removed from the
shaker and allowed to remain stationary for another
10 min. At the end of the settling period, the first 2 ml of
sample was drained from the stopcock and discarded, then
30 ml of sample was collected in a 50 ml measuring cylinder. The 30 ml sample was then transferred to a 125 ml separatory funnel, and extracted three times with fresh 5 ml
DCM. The extract was then adjusted to a final volume of
20 ml and transferred to a 50-ml crimp-style glass vial
with a Teflon/aluminium seal. These vials were stored at
4 2(C until analysis (a maximum of 5 days). The oil standard procedure and test procedures were conducted according to the procedures given by Sorial et al. (2004a, b).
Sample analysis
Oil standard procedure
Standard solutions of oil for calibrating the UVeVIS spec
were prepared with the specific reference oils and dispersant used for a particular set of experimental test runs.
For control treatments with no dispersant, i.e. oil control
experiments, only oil was used to make the standard solution. Initially, oil alone stock standard was prepared. The
density of 2 ml of the specific reference oil with 18 ml
DCM added was measured with a 1-ml gas-tight syringe,
and the concentration of the oil solution was determined.
Specific volumes of 20, 50, 100, 150, 200, or 300 ml of
SLCeDCM stock, or 11, 20, 50, 100, 125, or 150 ml of
PBCeDCM stock, or 150, 200, 400, 600, 800, or 1000 ml
of 2FOeDCM stock were added to 30 ml of synthetic seawater in a separatory funnel, and extracted three times with
DCM. The final DCM volume for the combined extracts
was adjusted to 20 ml with DCM. The extracts were transferred to a 50-ml crimp-style glass vial with a Teflon/
aluminium seal, wrapped with parafilm, mixed by inverting
many times, and stored in a refrigerator at 4 2(C until
time of analysis (a maximum of 5 days). For treatments
with oil plus dispersant, oil plus dispersant stock standard
was first prepared. The density of 2 ml specific reference
The experimental sample extracts and the standard solutions were removed from the refrigerator and allowed to
equilibrate at laboratory temperature. First, a blank solution
(DCM) was introduced to zero the spectrophotometer.
Then, the standard solutions were analysed in order of increasing concentration, and the absorbance values were
noted at wavelengths of 340, 370, and 400 nm to generate
a calibration curve (see below). After this, the experimental
samples were analysed.
Calculation procedure
The area under the absorbance vs. wavelength curve for the
standards and experimental samples between wavelengths
340 and 400 nm was calculated using the trapezoidal rule,
according to the following equation:
Area ¼
ðAbs340 þ Abs370 Þ30 ðAbs370 þ Abs400 Þ30
þ
;
2
2
ð1Þ
where Abs340, Abs370, and Abs400 are the absorbances measured at wavelengths of 340, 370, and 400 nm, respectively.
The dispersant performance (i.e. the percentage of oil
dispersed, or the effectiveness), based on the ratio of oil
Dispersant effectiveness on oil spills e impact of salinity
dispersed in the test system to the total oil added to the system, was determined from
Effectiveness ð%Þ ¼ Total oil dispersed 100=roil Voil ;
ð2Þ
1
where roil is the density of the test oil (g l ), Voil is the volume of oil added to the test flask (100 ml, i.e. 104 l), the
total oil dispersed is the mass of oil 120 ml/30 ml, the
mass of oil (g) is the concentration of oil VDCM, where
VDCM is the final volume of the DCM-extract of water sample (0.020 l), and the concentration of oil (g l1) is the area
determined by Equation (1) divided by the slope of the
calibration curve.
Factorial experimental design
The main aim was to determine the environmental factors
that are related to the effectiveness of a dispersant used in
oil remediation. The response variable for the experiment
was the percentage effectiveness of the dispersant. The factors and levels of each of the factors are the following:
salinity (10, 20, and 34 psu), weathering (0, 10, and 20%
for SLC and PBC; 0, 3.8, and 7.6% for 2FO), dispersant (A
or B), temperature (5(C, 22(C, and 35(C), and flask speed
(150, 200, and 250 rpm). A complete factorial experiment
was conducted with these levels for each factor. The total
number of experimental samples prepared for each oil was
648 (3 salinities 3 weathering levels 3 temperatures 3
flask speeds 4 replicates 2 dispersants). The factorial
experiment was also performed for each of the three oils separately, i.e. with no dispersant added. The total number of oil
control experimental samples prepared for each oil was 324 (3
salinities 3 weathering levels 3 temperatures 3 flask
speeds 4 replicates).
Results and discussion
Statistical analysis was performed separately on each of the
nine oiledispersant combinations, i.e. three oils, with
1421
dispersants (A or B) and the oils alone. The results were
analysed using analysis of variance, with a ¼ 0.05. The
highest order interaction in all cases was assumed to be
non-significant, and its degrees of freedom were used for
error determination. A significant interaction means that
the effect of one input parameter varies at differing levels
of another input parameter. The t-test from the REG (regression) procedure was used to test the level of significance for
each factor studied. The REG procedure is a general-purpose
procedure that performs linear regression analysis (SAS
Institute, 2000). The condition for significance as determined by statistical analysis was that the probability of
a run being greater than the corresponding Student’s t-test
value should be <0.0001 (Sorial et al., 2004b). Using this
procedure, significant factors were determined for each
oiledispersant combination (see Table 1). More explanation
is provided in Empirical relationships.
A four-replicate study was also conducted for all experiments to determine the precision of the experimental
results for the range of variables studied. The precision
objectives were determined using the relative standard
deviation (RSD) for percentage effectiveness, based on
four-replicate flasks. The acceptance criterion was based
upon RSD < 15% (Venosa et al., 2002). The RSD was calculated as standard deviation 100/average effectiveness.
The effect of salinity at different
mixing energies
The mixing energy was provided in the form of revolutions
per minute (rpm). Figure 1 shows the results for percentage
effectiveness of dispersant A for the three oils in their unweathered condition (0% weathering) and at room temperature (22 1(C). Clearly, the percentage effectiveness for
a given oil at 0% weathering and given salinity increased
as the speed of the orbital shaker increased from 150 to
250 rpm. This trend in dispersant effectiveness with increase in flask speed was true for oil þ dispersant B experiments as well as for oil control experiments (results not
shown).
Table 1. Significant factors for various oiledispersant combinations.
Oil
Oil control experiments
SLC
Temperature, speed,
temperature by speed
interaction
Salinity, temperature by
speed interaction, temperature
by salinity interaction, speed
by salinity interaction
Temperature by salinity
interaction
PBC
2FO
Oil þ dispersant A
experiments
Oil þ dispersant B
experiments
Weathering, speed
Temperature, speed, speed
by salinity interaction
Temperature, speed
Temperature, temperature
by weathering interaction,
temperature by speed
interaction
Temperature, speed
Temperature, speed,
temperature by speed
interaction
1422
S. Chandrasekar et al.
For PBC, Figure 1 shows that the impact of salinity is
more pronounced at the intermediate speed (200 rpm)
than at the other speeds. This behaviour was further confirmed for the other temperatures and weathering conditions
studied, by calculating the RSD values of dispersant effectiveness at differing salinity. The RSD values were 4.01,
15.1, and 2.6 at 150, 200, and 250 rpm flask speed, respectively. Comparing the RSD value of 15.1 with the other
values obtained could imply a significant impact of salinity
at 200 rpm. However, the RSD was very close to 15%,
and hence no significance role of salinity could be
confirmed.
In the case of 2FO, the RSD values for dispersant effectiveness at different salinity for the three temperatures and weathering conditions were 8.4, 5.9, and 1.4 at 150, 200, and
250 rpm flask speed, respectively. As the RSD values
are <15%, it is concluded that salinity played no significant
role.
The effect of salinity at different temperatures
Figure 1. Flask speed vs. percentage effectiveness of dispersant A
at 22 1(C.
Figure 1 also shows that the dispersant effectiveness increased with an increase in salinity from 10 to 34 psu for
SLC at a given flask speed. For SLC, the RSD values for
dispersant effectiveness among the three salinities at the
three temperatures and three weathering levels studied
were 7.6, 6.7, and 5.6 at 150, 200, and 250 rpm flask speed,
respectively. Therefore, in the case of SLC, the impact of
salinity on dispersant effectiveness is nearly the same at
the three levels of mixing, and this behaviour is also evident
in Figure 1.
Figure 2 shows the results for percentage effectiveness of
dispersant A for the three oils at their maximum weathering
level (20% for SLC and PBC, 7.8% for 2FO) and at maximum flask speed (250 rpm). In the case of SLC at 10 and
34 psu, the percentage effectiveness increased with increase
in temperature from 5 1(C to 22 1(C, but decreased at
35 1(C. For example, the dispersant effectiveness values
at 5 1(C, 22 1(C, and 35 1(C, were 73.3, 81.5, and
77.7%, respectively, at 10 psu, and 84.1, 92.8, and 90.7%,
respectively, at 34 psu. However, the results obtained at
20 psu show that the percentage effectiveness increased
with a rise in temperature. For example, the dispersant effectiveness values at 5 1(C, 22 1(C, and 35 1(C
were 77.1, 80.9, and 84.7%, respectively. Figure 2a also
shows that dispersant effectiveness increased with an increase in salinity from 10 to 34 psu at 5 1(C and
35 1(C. Also, for SLC, the RSD values for dispersant effectiveness at different salinity for the weathering and speed
conditions studied were 6.9, 7.9, and 7.7 at 5 1(C,
22 1(C, and 35 1(C, respectively. As the RSD values
are <15%, we conclude that there was no impact of salinity
on dispersant effectiveness. This conclusion is also evident
from Table 1.
For PBC, for all three salinities, the percentage effectiveness first increased with a rise in temperature from 5 1(C
to 22 1(C, then decreased at 35 1(C. This means that
PBC, a medium crude oil, resists dispersion even at high
temperature and a high salinity of 34 psu. This behaviour
may be due to weathering of the oil during the test at
35(C. For PBC, the RSD values for dispersant effectiveness
at the different salinities were 5.1, 2.3, and 5.8 at 5 1(C,
22 1(C, and 35 1(C, respectively. For PBC, the impact of salinity at the three flask speeds and weathering
conditions was less pronounced than for SLC, a finding
also justified in Table 1.
Dispersant effectiveness on oil spills e impact of salinity
1423
However, the RSD values are less than or close to 15%,
and Table 1 indicates clearly that salinity was not a significant
factor.
The effect of salinity at different weathering
The effect of salinity at the three weathering conditions studied is presented in Figures 3 and 4, in order to observe the
behaviour at the lowest and highest temperatures studied.
Figure 3 shows the results for percentage effectiveness of
Figure 2. Temperature vs. percentage effectiveness of dispersant A
at 250 rpm.
In the case of 2FO at 10 and 34 psu, the dispersant percentage effectiveness increased with a rise in temperature from
5 1(C to 22 1(C, but it decreased at 35 1(C. However, the results obtained at 20 psu show that the percentage
effectiveness increased with a rise in temperature. For 2FO,
the RSD values for dispersant effectiveness at different salinity were 5.6, 15.3, and 10.9 at 5 1(C, 22 1(C, and
35 1(C, respectively. Hence, the significance of salinity
on dispersant effectiveness is more pronounced at the higher
temperatures of 22 1(C and 35 1(C than at 5 1(C.
Figure 3. Weathering vs. percentage effectiveness of dispersant A
at 5 1(C.
1424
S. Chandrasekar et al.
conclude that dispersant effectiveness increased with increase in salinity from 10 to 34 psu for all three oils at given
weathering. For SLC, the RSD values for dispersant effectiveness at different salinity were 3.6, 7.6, and 6.7 at 0%,
10%, and 20% weathering, respectively. This implies that salinity has a greater impact on dispersant effectiveness at the
higher weathering levels of 10% and 20%. However, the
RSD values are <15%. For PBC, the RSD values for dispersant effectiveness among different salinities were 6.1, 7.0, and
6.5 at 0%, 10%, and 20% weathering, respectively, showing
a similar behaviour to that of SLC. For 2FO, the RSD values
for dispersant effectiveness at different salinity were 7.7, 5.0,
and 6.0 at 0%, 3.8%, and 7.6% weathering, respectively. In
the case of 2FO, the impact of salinity on dispersant effectiveness was more pronounced at 0% weathering than at the other
weathering conditions, but again the RSD values are <15%.
Figure 4 shows the results for percentage effectiveness of
dispersant A on all three oils at an intermediate flask speed
of 200 rpm and a high temperature (35 1(C). Again, as
the degree of weathering of the oil increased, the dispersant
effectiveness decreased. For SLC, the RSD values for dispersant effectiveness at different salinity were 6.8, 14.8,
and 11.4 at 0%, 10%, and 20% weathering, respectively.
This implies that salinity has a greater impact on dispersant
effectiveness at weathering levels of 10% and 20% than it
does at 0%. For PBC, the RSD values for dispersant effectiveness at different salinity were 11.5, 11.6, and 10.8 at
0%, 10%, and 20% weathering, respectively. For 2FO,
the RSD values for dispersant effectiveness at different salinity were 17.7, 16.6, and 13.9 at 0%, 3.8%, and 7.6%
weathering, respectively. In the case of 2FO, Figure 4
shows that with increase in salinity from 10 to 20 psu, the
dispersant percentage effectiveness increased, but that it decreased at 34 psu at all three levels of weathering, indicating a negative impact of salinity.
General discussion
Figure 4. Weathering vs. percentage effectiveness of dispersant A
at 35 1(C.
dispersant A on all three oils, at a flask speed of 200 rpm, and
at a temperature of 5 1(C. In general, for any oil at a given
salinity, as the degree of weathering of the oil increased, the
dispersant effectiveness decreased. For example, for SLC at
200 rpm flask speed and 10 psu, the values of dispersant effectiveness at 0%, 10%, and 20% weathering were 72%,
61.5%, and 60.9%, respectively. This is true for oil þ dispersant B experiments as well as oil control experiments conducted (results not shown). From Figure 3, we also
Table 2 shows the effect of salinity at different mixing
energies. The percentage effectiveness values for each
oiledispersant combination and oil control experiment are
shown. The range in values of effectiveness is shown for
all temperatures and weathering levels studied. The corresponding RSD values were calculated. Overall, for SLC,
the RSD range among dispersant effectiveness values was
9e46%, 18e51%, and 26e110%, for experiments with dispersants A, B, and oil controls, respectively. Similarly for
PBC, the RSD range was 20e53%, 17e54%, and
24e100%, for experiments with dispersants A, B, and oil
controls, respectively. For 2FO, the ranges of RSD among
dispersant effectiveness values were 17e64%, 24e54%,
and 15e52%, for experiments with dispersants A, B, and
oil controls, respectively. If an RSD value > 15% is considered to be significant, then salinity plays an important role
in determining the significance of flask speed on dispersant
effectiveness for all three oils. This is also evident from
Dispersant effectiveness on oil spills e impact of salinity
1425
Table 2. Effect of salinity at different mixing energies.
Range of average percentage effectiveness
10 psu
Oiledispersant
SLCeA
SLCeB
SLC)
PBCeA
PBCeB
PBC)
2FOeA
2FOeB
)
2FO
20 psu
34 psu
150 rpm
200 rpm
250 rpm
150 rpm
200 rpm
250 rpm
150 rpm
200 rpm
250 rpm
24e63
42e63
1e7
17e47
20e58
1e2
17e56
20e55
1e2
53e84
62e75
1e11
48e85
42e66
1e8
37e89
40e75
2e5
70e91
74e89
2e13
57e96
69e86
3e11
65e95
66e93
3e7
44e65
44e61
2e7
17e58
22e64
1e2
16e48
23e46
1e4
62e88
64e78
1e12
50e69
43e67
3e6
38e79
42e74
2e6
77e91
75e90
2e13
60e94
73e87
3e11
70e94
70e95
3e7
34e81
26e51
1e5
20e60
26e63
2e9
18e61
24e56
2e7
69e91
70e77
2e10
55e86
46e69
2e10
41e87
47e78
1e7
84e98
77e92
4e11
64e97
80e87
3e12
73e98
72e98
2e10
)Oil control experiments.
Table 1, which lists the significant factors for each of the
oiledispersant combinations. Speed was also a significant
factor for SLC and PBC oil control experiments.
The effect of salinity at different temperatures is reflected
by the results listed in Table 3, which lists the percentage
effectiveness values for each oiledispersant combination
and oil control experiment. The range in effectiveness
values is shown for all flask speeds and weathering levels
studied. The corresponding RSD values for these results
were calculated. In the case of SLC, the RSD among dispersant effectiveness values varied between 3% and 23%, 3%
and 30%, and 44% and 124%, for experiments with dispersants A, B, and oil controls, respectively. Similarly for
PBC, the RSD ranged between 10% and 54%, 1% and
44%, and 3% and 80%, for the same experiments, respectively. For 2FO, the RSD among dispersant effectiveness
values ranged between 8% and 65%, 14% and 45%, and
3% and 82%, for the same experiments, respectively.
Again, if an RSD value > 15% is considered to be significant, then salinity plays an important role in determining
the significance of temperature on dispersant effectiveness,
for all three oils except for SLC with dispersant A, which
gave comparatively less RSD than the other experiments.
This is also evident from Table 1, which shows that
temperature is indeed a significant factor for all oile
dispersant combinations except SLC with dispersant A.
Temperature was also a significant factor for SLC oil
control experiments.
The effect of salinity at different degrees of weathering
can be seen in Table 4, which lists the percentage effectiveness values for each oiledispersant combination and oil
control experiment. The range in effectiveness values is
shown for all flask speeds and temperatures studied. The
corresponding RSD values for these results were calculated.
Table 3. Effect of salinity at different temperatures.
Range of average percentage effectiveness
10 psu
Oiledispersant
SLCeA
SLCeB
SLC)
PBCeA
PBCeB
PBC)
2FOeA
2FOeB
)
2FO
20 psu
34 psu
5(C
22(C
35(C
5(C
22(C
35(C
5(C
22(C
35(C
41e83
42e79
1e3
17e61
20e71
1e3
17e74
20e67
1e3
24e86
50e87
5e13
37e96
47e81
1e11
41e95
31e91
1e8
52e90
54e89
6e13
30e85
49e86
1e12
42e76
34e93
2e6
44e85
44e81
1e4
17e66
22e78
1e6
16e77
23e71
1e4
44e91
52e88
2e7
36e94
47e84
2e5
45e94
32e93
2e7
57e91
55e90
7e14
48e86
53e87
2e11
45e94
44e95
4e8
50e89
46e83
1e5
20e69
26e84
2e7
18e79
24e73
1e5
34e97
26e90
1e9
40e95
47e87
2e6
48e98
33e98
1e7
73e98
41e92
4e12
31e75
56e89
2e11
32e89
41e98
6e11
)Oil control experiments.
1426
S. Chandrasekar et al.
Table 4. Effect of salinity at different weatherings.
Range of average percentage effectiveness
10 psu
Oiledispersant
SLCeA
SLCeB
SLC)
PBCeA
PBCeB
PBC)
2FOeA
2FOeB
2FO)
20 psu
34 psu
0%
10%
20%
0%
10%
20%
0%
10%
20%
49e90
45e89
1e13
18e91
21e86
1e11
24e79
44e87
2e12
17e96
21e80
1e9
30e81
42e83
1e13
17e92
20e82
1e10
51e91
49e90
1e13
19e94
27e87
1e11
44e85
47e89
1e13
18e91
24e83
1e10
44e84
44e85
1e13
17e88
22e82
2e9
54e98
28e92
1e11
21e95
30e89
1e12
34e97
27e91
1e9
33e97
28e85
1e11
39e92
26e88
1e11
20e90
26e83
1e10
0%
3.8%
7.6%
0%
3.8%
7.6%
0%
3.8%
7.6%
18e92
20e93
1e6
18e95
21e91
1e5
17e75
22e91
1e7
20e94
23e95
1e7
18e90
23e93
1e7
16e86
23e93
1e7
21e95
24e98
2e10
19e97
24e97
1e10
18e98
23e96
1e10
)Oil control experiments.
Overall, for dispersant A experiments, the RSD among dispersant effectiveness values varied between 3% and 36%
for SLC, between 1% and 12% for PBC, and between 1%
and 14% for 2FO. For dispersant B experiments, the RSD
among dispersant effectiveness values varied between 1%
and 6% for SLC, between 1% and 9% for PBC, and between 1% and 9% for 2FO. For oil control experiments,
the RSD among dispersant effectiveness values varied between 1% and 15% for SLC, between 1% and 11% for
PBC, and between 1% and 14% for 2FO. Based on these
RSD values, we conclude that the impact of salinity on
weathering was significant for SLC with dispersant A experiments only (also evident from Table 1).
Empirical relationships
A linear regression model was fitted to the experimental
data for each of the oiledispersant combinations, utilizing
the REG procedure (SAS Institute, 2000). All factor terms
and their interactions were included in the model, regardless of their significance. This is needed to define all interactions and quadratic relationships. The model takes the
form
yi ¼b0 þ bv xvðiÞ þ bt xtðiÞ þ bs xsðiÞ þ bl xlðiÞ þ bvt xvðiÞ xtðiÞ
þ bvs xvðiÞ xsðiÞ þ bts xtðiÞ xsðiÞ þ btl xtðiÞ xlðiÞ þ bsl xsðiÞ xlðiÞ
þ blv xlðiÞ xvðiÞ þ b2s x2sðiÞ þ b2t x2tðiÞ þ b2v x2vðiÞ þ b2l x2lðiÞ ;
ð3Þ
for i ¼ 1, 2, 3, the treatment levels. Here yi is the effectiveness value at the corresponding levels of the factors (x), b0
is the intercept, bv the oil weathering effect, bt the temperature effect, bs the speed effect, bl the salinity effect, bvt the
effect of the weathering by temperature interaction, bvs the
effect of the weathering by speed interaction, bts the effect
of temperature by speed interaction, btl the effect of temperature by salinity interaction, bsl the effect of speed by salinity interaction, blv the effect of salinity by weathering
interaction, b2s the effect of the second-order interaction of
speed, b2t the effect of the second-order interaction of temperature, b2v the effect of second-order interaction of weathering, and b2l is the effect of the second-order interaction
of salinity. The equation contains all the main effects
and second-order interactions for all factors. The various
b parameters for the various oiledispersant combinations
are given in Table 5 together with r2-values that indicate
the linearity of the model. Except for the oil control
experiments, all r2-values were >90%. The significant
terms in Equation (3) are due to the factors listed in
Table 1. This is obtained by determining the t-value
(parameter value/s.e.), then determining the probability
from the following equation:
s
p ¼ X tn1;1a pffiffiffi;
n
ð4Þ
where p is the probability, X the mean of the samples, s the
standard deviation, and tn1,1a is the tabulated t-test value
at a ¼ 0.05 (Montgomery, 1991). A parameter is significant
if the probability of a run being greater than the corresponding calculated t-value is <0.0001.
Figure 5 shows a comparison of measured and estimated
values of dispersant effectiveness on SLC. Each of the plots
shows the data cluster along the 1:1 line, indicating a close
match between estimated and measured values. Figure 5b,
for SLC with dispersant A, and Figure 6c, for SLC with dispersant B, especially show a good match between measured
and estimated values owing to the high values of r2. Similarly, Figure 6 shows a comparison of measured and estimated values of dispersant effectiveness on PBC. In the
Table 5. Coefficients of regression equation (Equation (3)).
Prudhoe Bay Crude Oil
Number 2 Fuel Oil
Factor)
No dispersant
Dispersant A
Dispersant B
No dispersant
Dispersant A
Dispersant B
No dispersant
Dispersant A
Dispersant B
b0
bv
bt
bs
bl
b2v
b2t
b2s
b2l
bvt
bvs
blv
bts
btl
bsl
r2
20.05044
0.10360
0.30690
0.19971
0.21127
0.00193
0.00369
0.0004423
0.00410
0.00103
0.00031019
0.00059386
0.00100
0.00308
0.00005366
0.8958
115.96645
2.03166
0.07442
1.5252
0.08709
0.06792
0.01891
0.00283
0.00827
0.00274
0.00038116
0.01226
0.00234
0.00927
0.0009232
0.9246
66.77451
0.05252
0.77095
1.00915
0.35897
0.00830
0.00775
0.00194
0.02081
0.00173
0.0009320
0.00913
0.0000826
0.00936
0.00654
0.9409
6.23968
0.03741
0.10179
0.00239
0.31155
0.00234
0.00438
0.0001129
0.0001460
0.00180
0.0003129
0.0002707
0.00157
0.00444
0.00143
0.8729
180.85008
0.76673
4.16418
1.52901
0.35435
0.01261
0.07657
0.00252
0.00516
0.00293
0.00183
0.01077
0.00186
0.00197
0.0009026
0.9137
11.00662
0.12229
3.7982
0.10247
0.09902
0.00198
0.03229
0.00151
0.00123
0.01012
0.00019923
0.00123
0.00814
0.00226
0.00184
0.9803
0.64197
0.04481
0.02456
0.01378
0.12144
0.00996
0.0007854
0.00009731
0.00216
0.00027196
0.00005556
0.00649
0.00036340
0.00383
0.00008335
0.8421
109.75982
1.91720
5.74749
0.72230
0.14205
0.14260
0.09288
0.000426
0.00809
0.00968
0.01146
0.05541
0.00596
0.00477
0.00286
0.9209
101.66665
0.32559
2.98224
0.75067
0.84530
0.16016
0.05608
0.000704
0.01018
0.00576
0.00353
0.000317
0.00176
0.01265
0.0004338
0.9588
Dispersant effectiveness on oil spills e impact of salinity
South Louisiana Crude Oil
)v ¼ weathering; t ¼ temperature; s ¼ speed; and l ¼ salinity.
1427
1428
S. Chandrasekar et al.
Figure 5. Comparison of measured and estimated values (Equation
(3)) of dispersant effectiveness on South Louisiana Crude Oil.
Figure 6. Comparison of measured and estimated values (Equation
(3)) of dispersant effectiveness on Prudhoe Bay Crude Oil.
case of PBC, there is tight clustering along the 1:1 line for
PBC with dispersants A and B, the r2-values being 91.37%
and 98.03%, respectively.
Figure 7 is a comparison of measured and estimated
values of dispersant effectiveness on 2FO. Figure 7b and
c, for dispersants A and B, especially shows a tight cluster
along the 1:1 line for 2FO, for which the r2-values were as
high as 92.09% and 95.88%, respectively.
Summary and conclusions
A full factorial experiment with four replicates was conducted to determine the impact of salinity on three environmental factors, the mixing energy, the temperature, and the
oil weathering. All experiments were analysed using an
analysis of variance with a ¼ 0.05. The REG procedure
was used to perform linear regression analysis.
Dispersant effectiveness on oil spills e impact of salinity
1429
each oil, depending on the oil properties. The impact
of salinity on dispersant effectiveness is more pronounced at higher temperature than at lower temperature, i.e. the significance of salinity on dispersant
effectiveness increased with increase in temperature
for all three oils. In general, salinity plays an important role in determining the significance of temperature on dispersant effectiveness for all oiledispersant
combinations, except SLC with dispersant A.
(iii) Dispersion efficiency decreased with increase in the
level of weathering for only one oiledispersant combination. The impact of weathering is only significant
for SLC with dispersant A.
(iv) In general, dispersion efficiency increased with increase
in salinity for most oiledispersant combinations.
(v) This research work has successfully created a set of
empirical data on three oils and two dispersants that
could serve as an input to the oil-spill simulation
models being developed by EPA. The empirical correlation for the collected experimental data predicted
with good accuracy the effectiveness of the dispersant.
The results of this research are expected to be incorporated into EPA’s model of oil spills (Weaver, 2004).
Acknowledgements
This research was supported by the U.S. Environmental
Protection Agency (US EPA) under Contract no. 68-C00-159. Although this work was reviewed by EPA and approved for publication, it may not reflect official agency
policy. The comments of the reviewers were much
appreciated.
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