Development of High Efficient and Low Toxic Oil Spill Dispersants based on Sorbitol Derivants Nonionic Surfactants and Glycolipid Biosurfactants

doi:10.4236/jep.2013.41B004

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Journal of Environmental Protec tion, 2013, 4, 16-22

doi:10.4236/jep.2013.41b004 Published Online January 2013 (http://www.SciRP.org/journal/jep)

Development of High Efficient and Low Toxic Oil Spill

Dispersants Based on Sorbitol Derivants Nonionic

Surfactants and Glycolipid Biosurfactants

Dandan Song1,2, Shengkang Liang1,2*, Qianqian Zhang1,2, Jiangtao Wang1,2, Limei Yan1,2

1Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Qingdao, China; 2College of Chemistry and

Chemical Engineering, Ocean University of China, Qingdao, China.

Email: *[email protected]

Received 2013

ABSTRACT

Dispersant s , usually blending with several surfactants and a solvent, are used to enhance oil spill dispersion as small

droplets in water column. Although there is growing acceptance of dispersants as a counter measure to marine oil spills

around the world, the two major issues with the dispersants are their toxicity to marine life and dispersion effectiveness

(DE) for crude-oil, especially for heavy oil. To develop more efficient and less toxic dispersants, two kinds of sorbitol

derivant nonionic surfactant (polysorbate 85 and sorbeth-40 tetraoleate), two kinds of glycolipid biosurfactants (rham-

nolipid and sophorolipid) and less toxic solvent ethylene glycol but yl ether were chosen in this study, and two di sper-

sant formulations were optimized by uniform design methods. Effects of dispersant-to-oil ratio, temperature, salinity

and pH on the performance of the two optimized dispersants were investigated. The two dispersants had hi gh d i spe r sio n

effectiveness (DE) for heavy crude oil, while both dispersants keep high DE at the dispersant-to-oil ratio below 1:25

and the temperature above 5˚C. In addition, the two dispersants also performed well in a wide range of salinity and pH

values. Finally, toxicity tests revealed that the two dispersan ts showed lo w toxicity to two kinds of fish (Danio rerio and

Microgobius gulosus).

Keywords: Oil Spill Dispersant; For mula Uniform Design; Ba ffled Flask Test; Ef fectiven e ss; Toxicit y

1. Introduction

Increasing exploitation, production, transportation, and

storage of oil around the world have caused more oil

spills in the oceans [1-3]. When an oil spill occurs, the

spills will most likely spread over a large area under gra-

vitational, surface tension, and viscous forces if the quick

response is not initiated and the environmental and eco-

nomic effects can be devastating, such as sensitive shore-

lines, local wildlife and plant life [4]. So, careful spill

response technologies are needed to minimize the dam-

age of oil spills at sea. Except for mechanical response

and in situ burning, the application of chemical disper-

sants is an efficient mean of reducing the environmental

and economic impact of spilled oil [5].

Dispersants are usually sprayed onto oil slicks to ac-

celerate the dispersion of oil from the sea surface into the

water column. A typical dispersant may consist of three

or mor e surfa ctant s and solve nt. S urfacta nts u suall y have

oil-soluble hydrocarbon chains and water-soluble groups,

are partially soluble in both oil and water, and the free

energy of the system is minimized when surfactants are

present at the oil-water interface [6]. Surfactant blends

show high dispersant effectiveness when compared with

individuals, means synergistic agonistic interactions be-

tween surfactants [7,8] Nevertheless, not all surfactant

compositions are suitable for consisting dispersants to

disperse spilled oil effectively, and many of the effective

ones have the drawbacks of being toxic and/or non-bio-

degradable [9]. During the 1970s and 80s, many coun-

tries resisted the use of dispersants. This was mainly be-

cause of high toxicity of dispersants, which contains

some aromatics compounds in the solvents. For example,

the Torrey Canyon spill, in which the use of toxic dis-

persant led to subsequent widespread environmental

damage and an impression that dispersant use only adds

to the problem. [10]. During the course of years, the

chemical compositions of dispersants have been chang-

ing from especially toxic ones to not so toxic products.

Man y o f mo d er n d isp e rsants a r e ve r y lo w i n toxi city – an

order of ma gnitude lo wer than man y common house hold

products, which could be related use of nonionic surfac-

tant and solvents such as the glycol ethers and water in-

*Corresponding author.

Development of High Efficient and Low Toxic Oil Spill Dispersants Based on Sorbitol Derivants Nonionic

Surfactants and Glycolipid Biosurfactants

stead of aromatic compounds [11,12]. In recently, lower-

toxicity and more effective dispersants have led to

broadly acceptance of dispersant use. Oil Spill Intelli-

gence once reported that 36 out of 149 countries rely on

dispersant use as their primary response option and

another 62 consider it a secondary option [13].

Many dispersants are effective in dispersing easily

dispersed oils. However, the effectiveness can become

highly disparate when testing more viscous products of

heavy oils or weathered crudes, especially in cold water,

and higher energy is required for breaking up the treated

higher vi scosity slick into small drople ts [ 14, 15].

Mixture design and response surface methods have

been used to optimize oil spill dispersants [7]. In this

study, uniform design method was used to optimize dis-

persants formulations. Dispersant effectiveness was

conducted according to an improved dispersant testing

protocol, named the Baffled Flask Test (BFT) [16]. Low

toxic and biodegradable sorbitol derivants nonionic sur-

factants and glycolipid biosurfactants were selected as

variables to compose oil spill dispersants formulations.

The aims of present work are to achieve optimized for-

mulations of dispersant by determining the effectiveness

in dispersing QHD32-6 crude oil and to investigate the

effects of some important factors, such as dispersant-to-

oil ratio (DOR), temperature, salinity and pH, on disper-

sion effec tiveness.

2. Materials and Methods

2.1. Material s

2.1.1. Crude oil and Seawater

The tested heavy crude oil was submitted by China Na-

tional Offshore Oil Corp. Its general physicochemical

properties are listed in Table 1. The seawater used was

conducted from East China Sea and its salinity, pH, and

temperature are 32 per thousand, 8.02, and 15˚C, respec-

tively.

2.1.2. Surfactants and Solvent

The sorbitol derivants nonionic surfactants were poly-

sorbate 85 (Tween 85) and sorbeth-40 tetraoleate (GO440)

purchased from Nikko Chemicals Co., Ltd. (Shanghai,

Table 1. Physicochemical properties of QHD32 -6 crude oil

used.

Properties Value

API gravity at 15˚C 15.7~16.5

Viscosity at 50˚C (mPa·s,) 408~634

Asphaltenes content (wt. %) 3.7

Paraffin cont ent (wt. %) 2.26~3.28

China), respectively. And the biosurfactants were sophoro-

lipid s (SLs) and rhamnolipids (RLs), produced by Can-

dida bombicola ATCC22214 grown on glucose/ rapeseed

oil culture medium and Pseudomonas aeruginosa O-2-2

grown on rapeseed oil culture medium in the laboratory,

respectively. The culture conditions, purification, and

characterization of the two glycolipid biosurfactant s ha ve

been reported in our early research [17,18]. The proper-

ties o f fo ur s u rfactants are shown in Ta ble 2 . T he s ol ve nt

of disperstant was ethylene glycol butyl ether (2-bu-

toxyethanol) and purchased from GuangCheng Chemical

Co., Ltd. (Tianjin, China)

2.2. Methods

2.2.1. Uniform Design Method

A uniform design (UD) seeks design points that are un-

iformly scattered on the domain, was proposed by Fang

[19] based on quasi-Monte Carlo method or number -

theoretic method. It has been popular since 1980. In UD

results, the levels of all variables or factors included in

chemical experiments are changed continuously, allows

the largest po ssible a mount of le vels for ea ch factor, that

is, the number of levels could be equal to the number of

experiment runs. In this study, dispersant formulation

contains 50 percent of surfactants, and the other half was

2-buto xyetha nol a s solve nt. F our sur facta nts use d as four

factors in UD, different compositions influenced the dis-

persion effectiveness. In order to get more information

from experiment, each factor took 24 levels, U24 (244)

was chosen for experimental design that was listed in

Table 3.

2.2.2. Dispersion Experiments

Baffled Flask Test (BFT) method was performed in de-

termining dispersant effectiveness [16,20]. A stock solu-

tion of dispersant-oil mixt ure i n dichlo romet hane ( DCM)

was prepared. Specific volume of stock standard solution

was added to 30 mL seawater in a 125 mL separatory

funnel and extracted with DCM. The final extract was

adjusted to 25 mL with DCM. For QHD32-6 crude oil,

Table 2. Propert ies of four surfactants used.

Surfactan tsa HLBb Min imal surf ace t ension

(mN/m) CMCc

(m g /L)

S Ls 12-13 30 30

R La 22-24 29 40

Tween 85 11.0 43 0.5

GO440 12.5 unkn own unknown

a. The abbreviations of SLs, RLa, Tween 85, and GO440 represent sophorol-

ipids, rhamnolipids, polysorbate 85, and sorbeth-40 tetraoleate surfactants,

respectively; b and c corres pond to hydrophilic-liphophilic-balance and Critical

Micelle Concentrations, respectively.

Development of High Efficient and Low Toxic Oil Spill Dispersants Based on Sorbitol Derivants Nonionic

Surfactants and Glycolipid Biosurfactants

Table 3. Factors and levels of surfactants in the oil dispersants

(Abbreviations as in Table 2, the solvent is 2-butoxye-

thanol).

No.

Disp ers ant formulati on composition HLB value of

dispersant

SLs (%)

R Ls (%)

Tween 85

(%) GO440

(%) Solvent

(%)

1 36.25 2.88 7.15 3.71 50.00 12.47

2 30.15 12.30 6.10 1.43 50.00 14.35

3 26.45 6.30 3.44 13.80 50.00 13.33

4 23.70 20.05 0.28 5.95 50.00 16.06

5 21.40 2.33 16.25 10. 05 50.00 12.25

6 19.40 13.05 10.35 7.15 50.00 14.46

7 17.65 1.68 5.55 25.15 50.00 12.48

8 16.05 17.10 1.08 15.75 50.00 15.55

9 14.60 13.25 21.85 0.28 50.00 14.21

10 13.30 9.15 13.60 13.95 50.00 13.70

11 12.05 26.05 7.45 4.45 50.00 17.11

12 10.85 10.05 2.10 27.00 50.00 14.24

13 9.75 9.45 27.25 3.515 50.00 13.37

14 8.75 3.58 16.70 21.00 50.00 12.60

15 7.75 19.85 9.40 13.00 50.00 15.91

16 6.80 0.83 3.28 39.10 5 0.00 12.49

17 5.85 5.30 33.10 5.75 50.00 12.46

18 5.00 23.60 19.85 1.57 50.00 16.34

19 4.16 12.20 11.45 22.20 50.00 14.43

20 3.35 34.15 4.60 7.90 50.00 18.82

21 2.56 1.57 40.60 5.30 50.00 11.56

22 1.80 17.25 23.10 7.85 50.00 15.07

23 1.07 3.68 13.60 31.65 50.00 12.78

24 0.35 24.50 6.05 19.05 50.00 16.96

the six calibration concentrations obtained were 0.0926,

0.2036, 0.3980, 0.7034, 0.9256 and 1.2958 g/L. Then the

absorbance of oil standard solutions relative to a DCM

blank was measured from 340 to 400 nm using a

UV-VIS Spectrophotometer (UV Prove 2.0, Shimadzu).

The area under the abso rbance vs. wavele ngth cur ve was

automatically integrated between 340 and 400 nm. For

dispersant effectiveness analysis, first, 100 mL seawater,

equilibrated at the desired temperature, was added to the

baffled flask, and then 90 mg oil was dispensed directly

onto the surface of seawater. Finally, the dispersant was

dispensed at the center of the oil slick in a flask. The

flask was placed on an orbital shaker and shaken for 10

min at a rotation speed of 150 rpm. After shaking, the

flask remained stationary for 10 min. Then, 30 mL sam-

ple was collected and processed according to oil standard

procedure. Control without adding dispersant and four

replicate flasks were performed for quality control.

2.2.3. Factors Affecting Dispersion

One factor experiments were conducted to investigate the

influence of every environmental factor on the dispersion

effectiveness of the optimal dispersants. The factors and

levels of each factor were as follows: temperature (0, 5,

10, 20, and 30˚C), salinity (0, 5, 10, 20, 30, 35, and 40

per thousand), pH (6, 7, 8, 8.5, 9, and 10), DOR (1:10,

1:15, 1:20, 1:25, and 1:30). The rotational speed was kept

constant at 150 rpm for all dispersion experiments.

2.2.4. Toxicity

The acute toxicity of two optimal dispersa nts to fish was

estimated by a short-term test on adult Danio rerio and

Microgobius gulosus. Danio rerio obtained from local

aquafarms and Microgobius gulosus caught from local

shallow mari ne tide p oo ls wer e r aised in o ur lab in ster ile

fresh water and seawater at 25˚C, respectively. No food

was provided before 24h and during the test. On the day

of experiment, test sol ution with concentration o f disper-

sants 600 mg/L and control solution were prepared and

aerated to restore the concentration of dissolved oxygen

to air saturation value. 10 fish were then placed in the 5 L

glass aquaria containing 3 L test and control solution.

Experiments were carried out in duplicate. The number

of dead was recorded after 1, 12, 24, and 48 h.

3. Results and Discussion

3.1. Formulation optimization

The UD method is employed in this study because its

principle is to replace the complete combination of expe-

rimental parameters by using relatively fewer experiment

trials uniformly distributed within the parameter space,

and emphasize the uniformity of space filling in experi-

mental domain. In experiment trails, four kinds of sur-

factants were arranged according to uniform design table

(Table 3 ). 24 baffled flask tests were performed, and two

optimized dispersant formulations (No.13 and No.16)

were identified. The DE of formulation No.13 and No.16

ranged from 30% to 60% from 5˚C to 30˚C at the DOR

of 1:25 and 150 rpm mixing speed. Under the same DOR

and mixing speed, Corexit 9500, which was developed

for dispersing heavy and weathered oils, its DE for heavy

Development of High Efficient and Low Toxic Oil Spill Dispersants Based on Sorbitol Derivants Nonionic

Surfactants and Glycolipid Biosurfactants

oil of IFO 180 and IFO 380 were both less than 30% at

5˚C and 16˚C [15]. Usually, hydrophilic- liphophil-

ic-balance (HLB) is important in determining oil disper-

sion effectiveness and oil spill dispersants should tradi-

tionally have (HLB) values in the range of 10-15 [21].

The HLB values of formulation No.13 and No.16 were

13.37 and 12.49, respectively, are in good agreement

with the value of oil spill dispersant proposed. In addi-

tion, the mixture of nonionic and ionic surfactants solu-

tions form mixed micelles exhibited better efficiency in

decreasing oil-water interfacial tensions and lower criti-

cal micelle concentration (CMC) than individual com-

ponents and facilitated the dispersion of the oil droplets;

Meanwhile, the mixed surfactants and solvents systems

also form a continuous film, which stabilizes the new

interface and prevents the coalescence of oil droplets

[22].

3.2. Factors Affecting Dispersion

The DE of oil spill dispersant depends on not only crude

oil nature, but also various environmental conditions,

such as mixing energy, temperature and salinity [23,24].

In order to optimize using conditions for the two dis-

persants developed in this study, the effects of DOR,

temperature, salinity, and pH of seawater were investi-

gated.

Figure 1(a) shows that the increase in DE as DOR in-

creased from 1:30 to 1:10 for both formulatio ns. The D E

of formulation No.13 was sensitive to changes in DOR

and decreased significantly when DOR was reduced to

1:25. However, DE of formulation No.16 decreased

slowly with decrease of DOR. These results indicated

that lower DOR (1:25 and 1:30) could be appropriate for

dispersing this style of oil effectively and dramatically

reduce the waste of dispersant. In addition, these two

formulations had almost equal high DE when DOR was

1:10. It means that high DOR is in favor of decreasing

oil-water interfacial tension and diluting the oil as small

droplets into the water column. Formulation No.16 exhi-

bits higher DE than that of for mu la tion No.13 a t the same

DOR (from 1:15 to 1:30) suggested that formulation

No.16 was superior to No.13 in this experiment condi-

tion.

Figure 1(b) shows temperature effects on the effec-

tiveness of two dispersant formulations in dispersing

QHD32-6 crude oil. It can be seen that DE of formula-

tion No.13 and No.16 were high (> 40%) at the low

temperature (5˚C). This result indicated that these two

dispersant formulations could be successfully applied as

oil dispersant in cold weather. For formulation No.13,

DE increased as temperature increased from 0 to 30˚C.

Srinivasan, et al. [15] found that DE of three dispersants

for IFO180 and IFO380 in the BFT were almost two

times more effective at 16˚C than at 5˚C when mixing

energy was sufficiently high. However, when the tem-

perature increased from 5˚C to 30˚C, DE declined for

formulation No.16. Here, it is interesting to note that

Figure 1. Effect s of dispers ant-to-oil ratio (DOR) (a), temperature (b, at 1:10 DOR), salinity (c, at 1:25 DOR), and pH (d, at

1:25 DOR) on dispersi on effective ness (DE ) of f ormulation No.13 and No.16. The levels of f ix ed factors in every studied factor

were consi stent with seawat er used in this study.

Development of High Efficient and Low Toxic Oil Spill Dispersants Based on Sorbitol Derivants Nonionic

Surfactants and Glycolipid Biosurfactants

high temperature exhibited negative effect on DE. The

conflicting trends i n DE with the increase in te mperature

have been observed by previous researchers [25,26]. It

can be concluded that temperature may play a double

role here: on the one hand, it decreases viscosity of the

crude oil and thereby causes an increase in dispersion; on

the other hand, it may change the physical properties of

dispersants, which results the decrease of DE. A further

study of the effect of temperature on DE should be per-

formed to obtain clearer understanding the influencing

mechanism.

Figure 1(c) shows that DE increased with a slowly in-

crease in salinity from 10 to 40 per thousand for formula-

tion No.13. As mentioned earlier, higher salinity in-

creased the effectiveness of dispersants by preventing

surfactant molecules from migrating into the water phase,

equivalent to a salting-out effect for the surfactant from

the saline solution [27]. This salting-out effect can pro-

mote association of surfactant molecules with oil at oil-

water interfaces, which is important for lowering oil-

water interfacial surface tensions in the oil-dispersant

mixture [38]. For formulation No.16, results exhibited

minimum DE when salinity was 40 per thousand, which

may be caused by salting-in effect. Overall, salinity was

not a significant factor affected DE of two formulations.

Combined with the results of no detectable toxic effect of

No.13 and No.16 on freshwater fish and saltwater fish, it

can be concluded that these two dispersants are suitable

for remediation of oil spill not only in fresh water but

also in seawater.

The result in Figure 1(d) shows that DE of two for-

mulations were both high when pH was in the range of

7-10, whic h cove red the values of most sea regions.

3.3. Dispersant Toxicity

Improvements in dispersant formulation mean that it is

not only the increase of dispersion capability but also the

decrease of toxicity. It was found that the landings of

crustaceans had no any apparent reduction after spraying

dispersant in Sea Empress incident off the coast of South

Wales [29]. The field trials indicated that the use of dis-

persant was unlikely to have acute toxic effects on the

marine environment. In this study, Danio rerio (fresh-

water fish) and Microgobius gulosus (saltwater fish)

were used to study the acute toxicity of formulation

No.13 and No.16. When these two types of fish were

exposed in dispersant solution, lethality rates for Danio

rerio at the end of 48 hours was 0 and for Microgobius

gulosus, lethality rates was also 0 at the end of 24 hours.

However, lethality rates for Danio rerio reached 60%

after exposed in one chemical dispersant (named GM)

solution at the end of 12 hours. The toxicity results indi-

cated that these two optimized dispersants formulations

had lower toxicity and were in accordance with Chinese

national standard [30], which requests lethality rates of

these two types of fish to be no more than 50 percent at

the end of 24 hours wit h the s ame exposure conc entrati on

of dispersant.

4. Conclusion

In this study, formula uniform design was successfully

used t o optimize d ispersant formulatio ns by arr anging t he

levels of four surfactants continuously. Two optimized

formulations were obtained in view of high dispersion

effectiveness and low toxicity. The effects of environ-

ment factors on dispersant effectiveness were as below:

First, the variation trend of DE increased with the in-

crease of DOR was found for both of two optimized

formulations. DE was still high when the formulation

No.16 was used at low DOR. Using the d ispersa nt at low

DOR can save the usage amount of dispersant and reduce

the possible damage of dispersant to environment. These

two dispersant formulations could be also effective in

response of oil spill in cold weather. Secondly, salting-

out effect of salinity lowered oil-water interfacial surface

tensions and promoted the dispersion o f oil, but t he effect

was not significant. Similarly, pH was not a significant

factor affected DE of two formulations, which were both

high when pH was in the range of 7-10. Finally, these

two optimized dispersants formulations had lower toxic-

ity towards two kinds of fish (Danio rerio and Microgo-

bius gulosus). Nonetheless, whether the two optimized

dispersant formulations could be introduced to apply in

oil spill response requires a further field test to better

understand the effects of oil properties, interaction of

environmental factors and d e tailed toxicity.

5. Acknowledge

The authors wish to acknowledge the insightful com-

ments and suggestions from the anonymous reviewers

and editors for improving the content of this manuscript.

The research was funded by China Offshore Environ-

mental Service Ltd. (2010098).

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