Stability Research on the Effect of Oil Spill Dispersant I-Separation Characteristics of Oil Spill Dispersant

With the continuous research and development of the low-toxic and high-efficiency oil spill dispersants (OSDs), there are an increasing number of evaluation methods of the effect of the OSDs. Currently, the majority of the OSDs effect evaluation is related with the testing of the emulsification in the laboratory, while the stability researches on the OSDs effect are still few. In this paper, the separation characteristics of the oil spill dispersant (OSD) and the oil were investigated, and the stability of the effect of the OSD was also studied. Firstly, the mixture of the oil and the OSD which have been poured into the seawater was thoroughly stirred, left to stand and observed. Later, the greatest separation degree with the oil and the final stability of the OSD was obtained through the analysis. Then, the stability of the combination between the oil and the OSD was studied under the conditions of the no wave, intermittent wave and continuous wave. The study shows the OSD will gradually move away from the oil, which is influenced by the time and duration of the wave action. This study provides experimental basis and methods for the research on the stability mechanism of the OSD effect, as well as the theoretical basis for the comprehensive evaluation and the rational use of the OSD.


INTRODUCTION
Large-scale oil spills happened frequently, although the international agreements on oil pollution with strict preventive strategies [1].The oil spill dispersants (OSDs) play an important role to deal with the frequent oil spills in the marine environment, which can effectively reduce the interfacial tension between the oil and the seawater, greatly speed up the oil dispersing into the seawater from the surface, rapidly clean up the oil spills in the sea, and significantly reduce the impact on shorelines and habitats [2].In order to better develop and select the appropriate OSDs, some standardized effect tests in the laboratory are often used to evaluate different kinds of the OSDs effect [3].
Currently, the majority of the OSDs effect evaluation in the laboratory is concerned with the testing of the OSDs emulsification while the researches on the emulsion stability are few.The OSDs effect is evaluated in the laboratory scale, which mainly means the OSDs effect is evaluated in the controlled environment.The mixing approach of the oil and the OSDs used in the laboratory is different with the testing method differs slightly.The reason why emulsification decreases in the experiment is generally considered to be the emulsion oil floating.Therefore, the OSDs have been mistakenly assumed to be of continuously good effect because of the emulsification tested is not based on the timeliness and dynamics in a long time under the wave action.Many research results show that the OSDs effect value in the laboratory is higher than that in the field.It is mainly because the surfactant is separated from the spilled oils during the initial usage period or the mixing period after the spraying, and the hydrophilic part of the surfactant directly merges with the seawater, that is, there is an exchange of surfactants between the oil droplet and the surrounding seawater [4,5] and the removal of surfactants was equivalent to a proportional loss of the entire dispersant [6].There is a study showing the rate of dispersant loss and decline in dispersion potential was related to slick thickness or the quantity of the test oil used [7], beyond that the behavior of the oil is also determined by the chemical composition and physical properties of crude oil, and the way its properties will change when the oil is spilled at sea [8].For that reason, the accurate effect of the OSDs is determined by the actual number of the OSDs in the oils instead of the total number of the OSDs that have been used.Therefore, determining the actual number of the OSDs in the oil spills under different conditions is a new focus of the OSDs effect, which overcomes the former shortcomings that only the OSDs emulsifying efficiency is taken into consideration.This approach is also an important means to determine whether the OSDs suffer physical failure directly.In addition, whether the OSD is constantly evaluated in a longterm with effectiveness will cause the concerns on the consequence of using OSDs, namely, whether the dispersive oil will be floating in the remote place after a period of time.This will cause a latently great threat to the marine, which may appear abruptly at any time.Besides, it is difficult to ascertain where the responsibility lies for the oil spills.
Taking into account the internal and external factors described above, representative crude oil and the oil spill dispersant (OSD) were selected as the experimental subjects in this paper, that is, Bohai crude oil which is suitable to be disposed with the OSDs and the common OSD.The standing experiment and the dynamic experiment were investigated to study the stability of the OSD and the characteristics of the wave impacting on the stability.Firstly, the OSD and the oil were thoroughly mixed, poured into the seawater, then stirred, and left standing.Its stability was observed and analyzed; beyond that, whether the time and duration of the wave action impacted the stability of the OSD effect or not was studied, based on which the stability of the OSD in combination with the oil is also discussed.In this study, the analysis methods focused on the relative content of the OSD in the oil slick and the concentration of the dispersive oil in the seawater.

Materials
The experimental oil was from a platform in Bohai and is medium viscosity.The OSD was the FuKen-OSD from Qingdao Huahai Environmental Protection Industry Co., Ltd.The salinity of the artificial seawater was about 34 ‰.

The Experiments about the Stability
The mixture of the oil and the OSD was prepared by the volume ratio about 0.1, 0.2, 0.4 and 0.6 according to the use range of the OSD.The mixture described above was respectively poured into the seawater (V oil : V seawater = 1:80).After being sufficiently stirred, the mixture was left standing for 72h; The floating oil formed after standing was moved to the clean seawater (V oil : V seawater = 1:200), followed by sufficiently stir, and let stand for 48h.Repeat this process three times.The ratio of the oil and seawater were 1:500 and 1:1000 respectively.The proportion of seawater was increased to simulate the process that the oil diffuses into the seawater.

The Experiments about the Waves Impacting the Stability
The mixture of the oil and the OSD was poured into the seawater tank, and three conditions were provided, which were respectively the no wave, the intermittent wave (the vibration frequency was 33±1 times/min; the wave had been on for 2 hours and been off for 1 hour), and the continuous wave (the vibration frequency was identical to the intermittent wave).Each experiment group had been maintained for 12 hours.During the experiment, the oil slick was sampled for gas chromatography analysis every three hours, which were aimed at measuring the relative content of the OSD in the oil slick.All the experiments were finished in the laboratory of 15±1 o C

Test Methods
The relative content of the OSD in the oil slick was measured by the gas chromatography (Shimadzu Co., Ltd.GC-2010 gas chromatography).The pretreatment of the oil slick: The chromatography column was successively filled by cotton, 5.5g silica gel with column chromatography reagent grade, and 1.2g anhydrous sodium sulfate placed on the top.Each sample accurately weighed was fully dissolved by 15mL configured mixed solution of dichloromethane and nhexane.Rinse the filled column by 10mL hexane.When anhydrous sodium sulfate just exposed, immediately pour the dissolved oil sample prepared into the column and collect the chromatography liquid for testing.
Testing conditions of the GC The temperature of the inlet and the detector were respectively 280 o C and 300 o C, and the carrier gas was high purity N 2 .Temperature program: The initial temperature was 60 o C at first, then increased to 100 o C by 20 o C/min, and kept for 2 min; then increased to 280 o C by 8 o C/min, and kept for 50 min.

The Stability Results of the Oil in Combination with the OSD
The standing experimental investigation was to study the OSD's stability, which was completed by intermittently increasing the amount of seawater and alternately stirredstanding operation.In other words, the floating mixture in the surface of the seawater was poured into the seawater, then stirred, and left standing, and the sampling analysis was made.The floating mixture in last experiment was transferred to the cleaning seawater, and the above operation was constantly repeated in the different amount of the seawater.Stirring adequately, intermittently increasing seawater and letting it stand for a long time were to know the maximum dispersion (the degree of the dispersion and the amount of the dispersion), the greatest separation degree with the oil and the final stability of the OSD.The relative content of the OSD was measured by the gas chromatography.The test result is shown in the Fig. (1).As can be seen from the Fig. (1), with the increase of the operation times, the trend line is slightly rising when the ratio of the OSD and the oil is 0.1(the ratio of the OSD used is below the use range), and the relative content of the OSD in the oil slick is linearly declining when the ratio of the OSD and the oil are 0.2, 0.4 and 0.6, which indicates that the OSD will be away from the oil slick.The degree of the separation varies with the initial ratio of the OSD and the oil.The larger the initial ratio is, the greater the loss of the OSD will be.For example, when the relative content of the OSD is 0.6 initially, about 7.5% floating oil slick will be lost when mixed with the clean seawater each time.
In the case of approximating OSD effect, the OSD usage principle "as little as possible" should be followed in order to avoid the secondary pollution to the marine environment.As is shown in Fig. (1), in this experiment when the ratio of the OSD and the oil is 0.4, the best amount of the OSD is obtained, so the following experiment is controlled by this ratio.

The Experimental Results of the Wave Impacting on the Stability
The relative content of the OSD in the oil slick is measured under the condition of the no wave, intermittent wave and continuous wave, which mainly study the effect of the wave's continuity and shows different relative contents of the OSD of the oil slick contrasted among these three cases.The results are shown in Fig. (2).As can be seen from Fig. (2), the relative content of the OSD is significantly decreased under the condition of the no wave; when the wave exits, the continuous wave is better than the intermittent wave at keeping the OSD in the oil slick; the OSD has decreased under the different wave action, but with the wave time increasing, the relative content of the OSD decreases, which may be relative to the increase of the entrained seawater.
From what has been discussed above, the wave plays a key role in the combination stability of the oil and the OSD.The wave is not only beneficial to the mixing of the OSD and the oil, which fully interact with each other, but also can help the OSD remain for a long retention time in the oil.

CONCLUSION
The stability of the OSD effect was investigated by the standing experiment, and the dynamic characteristics of the OSD being away from the oil were also studied under the different wave actions.The conclusions are listed below.
The changes of the OSD in the oil slick were analyzed by the gas chromatography, which manifested the OSD will move away from the oil slick whatever the conditions are.
Waves play a key role in the combination stability of the OSD and the oil, which mainly reflects the duration and the action time.The time of the wave is longer.The OSD is harder to separate from the oil slick.
The study preliminarily shows the stability of the OSD effect is influenced by the wave complexly and repeatedly, which is possibly related with the emulsified oil Entrained with water.

Fig. ( 1 ).
Fig. (1).The change of the relative content of the OSD in the oil slick with the time in the standing experiment.

Fig. ( 2 ).
Fig. (2).The change of the relative content of the OSD in the oil slick under the condition of the no wave, intermittent wave and continuous wave.