Huie Liu*, Hongjian Chen, Guanghui Huang, Yunfei Yu, Rujie Li, Shuang Chen

College of Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China

Keywords:

ABSTRACT

1. Introduction

In the process of petroleum exploration, transportation, processing and storage, a large amount of petroleum contaminated soil will inevitably be produced. If it is not effectively treated, not only a large amount of petroleum resources will be wasted, but also environmental damage will be caused by the penetration of petroleum from the oily soil to the aquifer, and hence human health will be threatened[1,2].The gradually stricter environmental laws and the increasing awareness of environmental protection[3]make more and more researchers pay attention to the harmless treatment and resource utilization of oily soil.

There are many kinds of methods for oil-bearing soil remediation, which can be divided into physical methods, chemical methods and biological methods [4]. Compared with traditional soil remediation technology, surfactant-aided washing [5–7] is attractive, among which, micro-emulsion washing technology has attracted attention as an oily soil remediation method [8–12].Micro-emulsion is a transparent or translucent,isotropic and thermodynamically stable system, composed of oil, water, surfactants and co-surfactants [9]. Compared with surfactant solution, microemulsion has stronger de-emulsification ability and lower interfacial tension,and can solubilize more organic matters[13].It can be divided into Winsor type I, type II and type III [14]. Among them,Winsor type I is the state of O/W micro-emulsion coexisting with excess oil phase, type II is W/O micro-emulsion coexisting with excess water phase and type III is a state where the microemulsion phase, excess water phase and excess oil phase are in equilibrium.

Surfactants are mainly divided into ionic and non-ionic types.Among them, non-ionic bio-surfactants are popular due to their low toxicity, biodegradability and stability in complex environments [15,16]. For example, sophorolipids (SLs) are degradable bio-surfactants, and their critical micelle concentration (CMC) is between 40–100 mg?L-1,which is much lower than that of general chemically synthesized surfactants [17,18]. In addition, SLs are produced by microbial fermentation, and the raw materials are readily available. They not only have the common properties of conventional surfactants, but also have the characteristics of low cytotoxicity, readily biodegradable, stable at extreme conditions and ecofriendly [18–20].

Minucelli et al.[21]used SLs produced from fat in the bioremediation of lubricating oil contaminated soil. It was found that the surfactant showed good emulsification effect on various hydrocarbons such as toluene,n-heptane and lubricating oil.Goswami et al.[22] explored the solubilization performance of various surfactant solutions on kerosene through kerosene extraction experiments,and found that SLs showed the best effect in extracting hydrocarbon compounds from contaminated soil. It was also found that SLs show promising effect in biodegradation of oil. For example,Kang et al. [23] found that the crude oil was effectively removed by the addition of SLs,resulting in 80% biodegradation in 8 weeks.When the SLs was used for bioremediation of lubricating-oil contaminated soil, Minucelli et al. [21] found that the addition of SLs could enhance significantly the CO2production in the first 4 days of incubation, indicating their fast effect.

It was found in the previous work of our group that Winsor type I micro-emulsion with sodium dodecyl benzene sulfonate(C18H29-NaO3S, SDBS) as surfactant could maintain its type unchanged when more oil was added into the system[8],with a free oil phase exist in equilibrium with O/W microe-mulsion phase, which will make the recovery of oil and the reuse of the micro-emulsion phase easy to carry out when used in oily soil treatment.However,SDBS is not an environment-friendly reagent.So,in this work,biological SLs,a biodegradable and green surfactant,was selected,and acidic SLs was used to prepare Winsor I type micro-emulsion for the treatment of an oil contaminated saline-alkaline sandy soil,taking advantage of its low interfacial tension, the entrainment effect of micelles in it,biodegradability of itself and promoting biodegradation of oil by it[20,22].Evaluation of corrosion performance of the micro-emulsion and plant growth in the reconditioned soil was carried out to provide basic data for the practical application of the micro-emulsion washing method in the treatment of oily soil.

2. Experimental

2.1. Materials and equipment

The crude oil sample was collected from an oil field in China(named GR oil field hereafter), with a density (20 °C) of 0.8511 g?cm-3and a viscosity (35 °C) of 99.52 mm2?s-1; The soil samples were also collected from GR oil field, dried in air and passed through a 2 mm sieve for use. The pH value is 8.56 and the total salt content is 1.48% for the soil, which is a typical saline-alkaline sandy soil.

A.R. grade sodium chloride (NaCl), anthrone, toluene, anhydrous dextrose and SDBS, and 98% (mass) sulfuric acid were all purchased from Sinopharm Chemical Reagent Co., Ltd. 0# diesel,industrial grade, was purchased from a gas station of Sinopec in Qingdao. Acidic SLs (active ingredient content >50%, pH = 2.33),obtained from Shandong Qilu Biotechnology Co., Ltd. Alfalfa and ryegrass are selected from Qianlvyuan Timber Farms.

Instruments include electronic balance (AL204/00, Mettler-Toledo International Trade(Shanghai)Co.,Ltd.),constant temperature water bath (HH series-4, Qingdao Juchuang Environmental Protection Equipment Co., Ltd.), pH meter (Lei Magnetic PHS-3C,Shanghai Shuli Instrument and Meter Co., Ltd.), intelligent digital display multifunctional oil–water bath (HH-WO, Gongyi Yuhua Instrument Co., Ltd.), electric heating blast drying oven (101A-1E,Shanghai Experimental Instrument Factory Co., Ltd.), UV–visible spectrophotometer (Precision Scientific Instrument (Shanghai)Co., Ltd.), biological microscope (CX31, Olympus Corporation) and precision electric mixer (JJ-1A, Changzhou Tianrui Instrument Co.,Ltd.).

2.2. Methods

2.2.1. Crude oil contaminated soil treatment through micro-emulsion washing

Contaminated soil with an oil content of 15% (mass fraction,based on dry soil)was prepared[8]using the crude oil and soil collected from GR oilfield. Winsor type I micro-emulsion was prepared with the biological acidic SLs surfactant, proper amount of diesel oil and brine, through simple mixing of these components.The two phases of the micro-emulsion were mixed together into a suspension and then used in the treatment of oily soil. For comparison, SLs aqueous solution (mixtures of acidic SLs and water) was also prepared and used in the washing of the oily soil.The prepared oily soil was put into a test tube with a stopper,and then a certain amount of washing agent,the micro-emulsion or the SLs aqueous solution was added into it,with the ratio between the contaminated soil and the water in the washing agent controlled at 1:1.25 (mass). The oily soil and the washing agent were mixed thoroughly and the test tube was then put into a 35°C water bath and stand for 24 h until the system was stable.The solid phase was then separated from the liquid phase, and washed twice with water. Then the washed soil was dried under 105 °C, and its oil content was determined by Soxhlet extraction method. To ensure the reliability of the experimental results, the average value of three parallel experiments was used as the final results in the experiments.

2.2.2. Determination of crude oil removal rate

The oil content of soil was determined by Soxhlet extraction method.Dried oily soil,with a mass of m1,g,was wrapped with filter paper,with the total mass of the package being m2,g.The package was then put into the Soxhlet extractor, with toluene as the solvent, refluxing until the toluene in the Soxhlet extractor is colorless. The package was dried, cooled and weighed. The mass was recorded as m3, g. Eq. (1) was used for calculating the drybased oil content of the oily soil.

The crude oil removal rate was calculated according to Eq. (2)[8], based on the supposition that no change existed for the total mass of soil particles before and after washing treatment.

where R is the crude oil removal rate,X1is the dry-based oil content of the initial oily soil and X2is that after washing treatment.

2.2.3. Characterization of crude oil and soil

The composition, viscosity, density and ash content were measured for the original crude oil and the recovered crude oil,according to the petroleum asphalt four-component determination method (NB/SH/T 0509-2010), viscosity measurement method(GB/T 10247-2008), density or relative density determination for crude oil and liquid or solid petroleum products-capillary stopped pyknometer method or double graduated capillary pyknometers method (GB/T 13377-2010) and petroleum products ash determination method (GB/T 508-1985), respectively.

The pH value, total salt content and organic matter content of the original soil and the soil after treatment were determined according to determination of soil pH (NY/T 1121.2-2006), gravimetric method for determination of total soil salt content (DB37/T 1303-2009) and determination of soil organic Matter (NY/T 1121.6-2006), respectively.

2.2.4. Corrosion evaluation of Micro-emulsion

The corrosion performance of the micro-emulsion on steel coupons was tested from the mass loss of corrosion coupons (NACE RP0775-2005), which is the most classic and commonly used corrosion detection technology in industry.The commonly used 316L stainless steel, 304 stainless steel, 45# steel and Q235 steel for chemical equipment were tested respectively. The experimental method is as follows.

The size of the stainless steel coupon was measured, with an accuracy of 0.02 mm, and its surface was wiped up. It was then put into the heated petroleum ether and absolute ethanol in turn,soaking in each for about 2 min. Thereafter the steel coupon was blow-dried to degrease and dehydrate. The blow-dried stainless steel coupon was then wrapped with filter paper and stored in a desiccator, standing for about 2 h and weighed with an accuracy of 0.0001 g. In the next step, an appropriate amount of microemulsion with optimized formula was put in a glass bottle and the stainless steel coupon was hung in it. It was required that the stainless steel coupon could not touch the inner wall of the glass bottle and the upper edge of the steel coupon was 1 cm lower than the liquid surface level. The image of the device is shown in Fig. 1. The glass bottle was then placed in a 35 °C water bath to conduct the test. The stainless steel coupon was taken out of the device after a certain period of time, immediately rinsed and soaked with water, petroleum ether and pickling solution in sequence.And then it was rinsed with tap water and absolute ethanol in turn to clean the residual acid solution on the surface.In the end, the steel coupon was blow-dried, wrapped with filter paper and placed in a desiccator.After 2 h,it was weighed again.The corrosion rate was then calculated using Eq. (3).

where rcorris the corrosion rate,mm?a-1;m is the initial mass of the steel coupon, g; m1is the mass of it after the test, g; S is the total area of it, cm2; t is the corrosion test time, h; and P is the density of the steel coupon, g?cm-3.

According to the basic regulations for design of steel chemical vessels (Hg/T 20580-2011), the corrosion degree can be divided into four grades, as shown in Table 1.

2.2.5. Germination and growth tests of plant seeds

The remediation effect of soil was evaluated through plant seeds germination and growth tests.The natural soil,contaminated soil and remediated soil from GR oilfield were tested. Pot experiments were carried out on ryegrass and alfalfa. 1.5 kg of the three kindsofabove-mentionedsoilwereputinto 160 mm × 120 mm × 60 mm flowerpots. Seeds with full grains and uniform sizes were selected to plant in the flowerpots. Three parallel samples of each plant in different soils were prepared and watered once a day to maintain moisture of the soil. A selfmade vinyl house was used as a mantle on the pots,so as to maintain the temperature of the system at ～25°C.The germination rate and growth of the seeds within 7 days were observed. The plant height, root length, stem and root mass, and the oil content in the soil 60 days later were measured. Seed germination rate was calculated according to Eq. (4).

Fig. 1. Image of the corrosion device.

Table 1 Specifications for the corrosion degree of petrochemical equipment

2.2.6. Determination of sophorolipids loss rate in washing operation

The SLs used in this experiment were produced by fermentation of glucose.Its molecular structure was similar to that of glucose.So the principle of anthrone chromogenic method was used to measure the glucose content in the micro-emulsion,and then the content of SLs was calculated according to their molecular weights.

Standard curve was prepared using glucose standard solutions and anthrone reagent. The anthrone reagent was prepared using fully mixed anthrone (0.5 g) and concentrated sulfuric acid(500 ml). 4 ml anthrone reagent and 1 ml standard glucose solution of different concentrations were put into stoppered test tubes,mixed well and cooled in an ice bath. The mixture was then put into a boiling water bath to react for 10 min and cooled in an ice bath. Absorbance at wavelength of 620 nm, OD620was measured using the UV–visible spectrophotometer. The standard curve of glucose was finally obtained by plotting OD620vs. glucose concentration(Cglucose),as shown in Fig.2.The fitted result for the plot can be expressed using Eq. (5), with R2= 0.9991.

To determine the effective content of SLs in micro-emulsions,the micro-emulsion was diluted with distilled water. 1 ml diluted liquid and 1 ml 95% absolute ethanol were fully mixed and then centrifugated under 5000 r?min-1for 10 min. 0.5 ml of the supernatant was put into a stoppered test tube afterwards and immersed in a boiling water bath for 10 min to evaporate the solvent in it.1 ml distilled water and 4 ml anthrone reagent were then added into the test tube, mixed thoroughly, and quickly cooled in an ice bath. Mixture of 1 ml distilled water and 4 ml anthrone reagent was used as a blank sample.OD620of the prepared sample was measured and mass content of SLs in the micro-emulsion can be calculated from Eq. (6).

Loss rate of sophorolipids was calculated according to Eq. (7).

where CSLis SLs concentration in the sample, mg?L-1; d is the dilution factor for determination of total SLs;CSL,0is the SLs concentration before washing treatment, mg?L-1; CSL,1is the SLs concentration after washing treatment, mg?L-1；s is the loss rate of SLs, % (mass).

3. Results and Discussion

3.1. Analysis of micro-emulsion washing effect on crude oil contaminated soil

3.1.1. Washing with sophorolipids aqueous solution

As described in Section 2.2.1, aqueous solution with different content of SLs was prepared and used as a kind of washing agent.The crude oil contaminated soil was aged more than 15 days in advance. The temperature was controlled at 35 °C in the washing tests. The change of crude oil removal rate with the mass fraction of SLs(based on the mass of water,the same below)in the solution was observed, as shown in Fig. 3. It can be seen that the removal rate of crude oil from soil increases first with the increase of SLs mass fraction, reaches the highest value of 87.96% at the SLs mass fraction of 12%and then tend to stabilize with the further increase of SLs mass fraction.Within a certain range,the increase of SL mass fraction will significantly reduce the surface tension of the system[24]. The increase of SLs concentration can provide increasing micelles for the system,which is conducive to the elution of crude oil in oily soil. The decrease of surface tension and increase of micelle number is considered the main reason for the increase of crude oil removal rate.

3.1.2. Washing with sophorolipids micro-emulsion

Specific contents of brine, diesel, and SLs were selected to ensure the spontaneous formation of Winsor type I microemulsions. The free oil phase mixed together with the microemulsion phase by strong agitation and then it was used in the oily soil washing. The operating temperature was also controlled at 35 °C. The results are shown in Fig. 4. Compared with the results in Fig.3,it can be found that the micro-emulsion systems all show significantly higher oil-removal rates than the SLs solution system.This is because micro-emulsions have very low interfacial tension and strong solubilization capabilities[8,9],and it can lower the viscosity of crude oil in oily soil [25], which will be conducive to the separation of oil and soil.It can be seen from Fig.4 that under different content of NaCl and diesel, when the mass fraction of SLs is less than 6%, the removal rate of crude oil increases with the increase of the mass fraction of SLs. And when the mass fraction of SLs is higher than 6%, the removal rate of crude oil tends to be stable.

Fig. 3. Effect of SLs aqueous solution on the oil-removal rate from oil-bearing soil.

Fig.4(a)shows that when the content of NaCl was controlled at 1%, the diesel content in the system shows some influence on the oil removal effect. The micro-emulsion with 13.6% diesel gave the highest removal rate of crude oil (95.6%). This is because the addition of diesel can not only improve the stability and reduce surface tension of the micro-emulsion system, but also helps to wet the oil droplets and reduce the adsorption force between the contaminant and the solid[13,26].SDBS was selected as the surfactant of micro-emulsions in the remediation of crude oil contaminated soil in our previous report [8]. Similarly, it was also found that the addition of diesel oil can weaken the adsorption of crude oil in the soil and increase its removal rate.However,excessive diesel oil cannot improve the oil removal effect further. This is because excessive diesel oil will lead to its significant adsorption onto the soil,causing the increase of oil content for the treated soil instead of decrease, thereby reducing the oil removal rate.

With the diesel content maintained constant at 13.6%,the content of NaCl in the washing agent also shows slight influence on the oil removal effect. The oil removal rate increases first with the increase of NaCl content, reaches the highest value of 95.6%at the NaCl content of 1% (mass) and then tend to stabilize (see Fig. 4(b)). Salting out effect of NaCl [27–29] leads to lower CMC and increasing adsorption of SLs at the surface,increasing its solubilization of crude oil in the soil. However,when the NaCl content increases to a certain level, it will squeeze out some water molecules from the hydrated hydrophilic shell of the micelle and consequently destabilize the micelle and reduce the cloud point,thereby reducing the crude oil removal effect.

SLs micro-emulsion with w(SLs) = 6%, w(NaCl) = 1%, w(diesel) = 13.36% was selected as the best formula (A), which gave a high oil removal rate of 95.6%. To clear whether the oil removal is mainly caused by the extraction effect of the diesel in the washing agent,a mixture of water and diesel(B),with w(diesel)=13.36%(without SLs and NaCl in comparison with the above microemulsion) was used in the contaminated soil washing treatment,which gave a very low oil removal rate, i.e., 66.54%. It was known that the washing result for aqueous solution of SLs with w(SLs) = 6% (C, see the data in Fig. 3) only gave an oil removal rate of 60.77%.The oil removal rates of A,B and C washing agents were all shown in Fig. 5 for comparison. The above comparison clearly shows that it is not simply the diesel or the SLs in the washing agent plays major role in the washing process. The Winsor type I micro-emulsion system as a whole showed significant effect on the oil removal.

Without specific explanation, the following experiments are all based on the selected SLs micro-emulsion formula,i.e.w(SLs)=6%,w(NaCl) = 1% and w(diesel) = 13.36%.

Consistent with the previous results for Winsor type I microemulsions with other kinds of surfactants of our group [8], the resulting eluate from the washing process still exhibits a Winsor type I state. As shown in Fig. 6, the top layer is the equilibrium oil phase, whose color is close to the original oil. The middle layer is the O/W micro-emulsion phase and the lower layer is the solid phase after oil removal.

It has been described that the micro-emulsion phase was mixed together with the equilibrium oil phase in the washing agent before washing operation. The state of the washing agent was observed using a biological microscope, the image was given in Fig. 7. It can be seen that micelles (those very small droplets, in nanoscale)and diesel oil droplets(those large droplets,in micrometer scale) coexist in the washing agent. Synergistic action was expected to exist between the micelle and diesel oil droplets during the washing process. The washing mechanism of the Winsor type I micro-emulsion was described in Fig.8.It is considered that the following actions take place when the washing agent contact the oil contaminated soil, i.e., the desorption of oil from the solid surface under the action of micelles, the swollen of micelles with solubilization of the desorbed oil and the dissolving of the oil into the diesel droplets when they contact with each other.Because the oil droplets in the mixture is in an unsteady-state, it tends to float up into the free oil phase,leading to the naturally separation of the oil phase and the micro-emulsion phase, just as shown in Fig. 6.

Fig. 4. Influences of SLs, NaCl and diesel content in micro-emulsion on oil-removal effect.

Fig. 5. Oil removal rates for different washing agents.

3.2. Comparison between original crude oil/ soil and the recovered ones

3.2.1. Physicochemical properties of the original crude oil and the recovered oil

As shown in Fig. 6, after the washing treatment, an oil phase naturally exists in equilibrium with the SL micro-emulsion phase,and hence the two liquid phases can be easily separated to obtain recovered crude oil. According to the methods described in Section 2.2.3,the basic physical and chemical properties of the original crude oil sample and the recovered crude oil were analyzed, as shown in Table 2.

Fig. 6. The states of oily soil and eluate after micro-emulsion washing.

Fig. 7. The microscopic image of the washing agent.

Fig. 8. The expected washing mechanism for Winsor type I micro-emulsion.

It can be seen from Table 2 that the original oil sample has a density of 0.8511 g?cm-3at 20 °C, which is a medium crude oil.The density of the recovered crude oil is 0.8825 g?cm-3, which is higher than that of the original one. It was observed that the ash content in the recovered oil sample (0.53%) is higher than that in the original one (0.15%), which should be an important reason for the higher density of the recovered oil sample. The higher ash content is attributed to the carrying of the fine particles from the soil by the washing agent during the washing process.Furthermore, compared with the original oil sample, the viscosity of the recovered oil sample decreased by an order of magnitude.By comparing the four-component analysis results of the two oil samples, it was found that the saturated content of the recovered oil sample increased, and the aromatic content, resin and asphaltene content decreased. The diesel components from the washing agent should be the main reason. Some diesel components should enter into the recovered crude oil, increasing the content of light components of the crude oil. At the same time, similar to the results of SDBS micro-emulsion washing [8], resin and asphaltene show difficulties in their desorption from the soil,which should be concerned in the future researches.All these led to the decrease of the viscosity of the recovered oil.

3.2.2. Physicochemical properties of natural soil and sophorolipids micro-emulsion remediated soil

According to the experimental method described in Section 2.2.3, the physicochemical properties of the original natural soil and SLs micro-emulsion remediated soil were analyzed. The results are shown in Table 3.

It can be seen from Table 3 that the pH value of the natural soil is 8.56 and the total salt content is 1.48% (mass). According to the local standard of Shanxi Province‘‘DB14/T 1415-2017”,it is classified into heavily saline-alkaline land and is not suitable for the normal growth of plants. Compared with the original natural soil, the pH value after remediation dropped by 0.88 unit,and the total salt content was reduced by 80.41%, which is mildly saline-alkaline. It is considered that the acidic SLs is one reason for reducing the pH value of the soil during the remediation. And during the washing process, the soluble salt in the soil will dissolve into the micro-emulsion, resulting in a decrease in the salt content of the soil.After micro-emulsion remediation, the organic matter content of the soil is 1.15 percentage point higher than that of the natural soil.It is expected that the remnant petroleum hydrocarbons and SLs in the soil are the main reasons.Both the mildly saline-alkaline property and the higher organic matter content of the remediated soil help to provide a better environment for plant growth than the original natural soil, which will be evaluated in Section 3.5.

Table 2 Data of physical and chemical properties of crude oil

3.3. Reuse of sophorolipids Micro-emulsion

3.3.1. Direct reuse of the recovered micro-emulsion phase

After the washing operation, the obtained liquid presents Winsor type I state, as shown in Fig. 6. It can be seen from the figure that the upper phase is the oil phase, the middle phase is the micro-emulsion phase, with the interface clearly observed. The upper oil phase can be easily separated and recovered.The remaining micro-emulsion phase was collected in a flask without any treatment,and a certain quality of oily soil was added into it again(the liquid/solid mass ratio was also controlled at 2:1).It was then maintained at 35 °C and stirred at 600 r?min-1for 20 min. The crude oil removal rate was measured after the washing operation.4 times of reuse of the recovered micro-emulsion was carried out.The results was shown in Fig.9,where the label‘‘Fresh”is the first time washing use the fresh micro-emulsion and the labels ‘‘Reuse 1”-‘‘Reuse 4” is the following 4 times of reusing of the recovered micro-emulsion phase.

It can be seen from the results in Fig.9 that as the times of reuse increases, the removal rate of crude oil shows a downward trend.After 5 times using of the washing agent,the removal rate of crude oil decreased to only 69.19%. This is mainly related to the loss of the ingredients in the micro-emulsion.For example,the initial diesel oil phase in the micro-emulsion system will enter into the crude oil during the washing and separation of the oil phase,which can be confirmed by the significantly lowered viscosity and increased saturation content for the recovered oil (see Table 2),and also part of the diesel oil and SLs will remain in the soil. All these loss of components from the micro-emulsion resulted in a decrease in the removal rate of crude oil.

3.3.2. Reuse of the recovered micro-emulsion phase through supplement of diesel oil

Considering the loss of diesel phase during the washing process,13.36% (mass) diesel oil based on the recovered micro-emulsion was supplemented, mixed and reused as new washing agent. The liquid–solid mass ratio was also controlled at 2:1 and stirred for 20 min under 35 °C and 600 r?min-1. this operation was repeated five times, and the results are shown in Fig. 10.

The oil removal rate is still higher than 92% in the 3rd reuse of the micro-emulsion phase, as shown in Fig. 10, and after the 4th reuse operation, the crude oil removal rate reduced to 87.83%.But when compare with the results in Fig. 9, it can be found that better effects can be reached when supplemented with diesel oil.

Using the method described in Section 2.2.6,the loss of SLs during washing was measured.It was found that the loss rate of SLs for one-time washing is 3.39%.This is mainly because some SLs in the micro-emulsion will adsorb onto the soil during washing,which isthe main reason for decrease of the crude oil removal rate during rewashing through diesel oil supplement.However,the loss of diesel shows more significant influence than SLs on the decrease of washing efficiency.

Table 3 Physicochemical properties of soil

Fig. 9. Washing results for direct reuse of the micro-emulsion phase.

3.4. Corrosion evaluation of sophorolipids Micro-emulsion

In view of the acidity of the SLs used,the method in Section 2.2.4 was used to evaluate the corrosion performance of the selected SLs micro-emulsion (w(SLs) = 6%, w(NaCl) = 1%, w(diesel) = 13.36%,pH = 3.67, which shows high washing performance, as shown in Section 3.1.2).Table 4 show the corrosion results of different types of steel coupons. It can be seen that in the SLs micro-emulsion system, the corrosion degree of 316L and 304 stainless steel is extremely low, and the corrosion rate is between 0.004 and 0.007 mm?a-1. According to the standard of ‘‘basic regulations for the design of steel chemical containers” (HG/T 20580-2011), they were very slightly corroded,which shows that 316L and 304 stainless steel have high corrosion resistance in the SL micro-emulsion system.The corrosion rates of SLs micro-emulsion on 45#steel and Q235 steel are both about 0.1 mm?a-1, being slight corrosion.Among the four kinds of steels, Q235 steel shows the most corrosive. So, when treating oily soil with SLs micro-emulsion, the equipment material is suggested to use the corrosion-resistant 316L or 304 stainless steels.

Fig. 10. Washing results for reuse of micro-emulsion through diesel oil supplement.

For comparison, the corrosion performance of another microemulsion system reported by our group [8], i.e., SDBS microemulsion was also evaluated (w(SDBS) = 10%, w(nbutnaol) = 4.8%, w(NaCl) = 0.8%, w(desel) = 13.6%). Table 5 gives the evaluation result.By comparing the data in Table 4 and Table 5,it can be found that the two kinds of micro-emulsion systems show similar trends on the four kinds of steels, i.e., 316L and 304 stainless steel showing lower corrosion rate and 45# and Q235 steels showing relatively higher corrosion rate.The highest corrosion rate for SDBS micro-emulsion on Q235 steel is only about 0.025 mm?a-1,much lower than SLs micro-emulsion system. This shows that the four types of steels all show good corrosion resistance to the SDBS micro-emulsion system.From the cost point of view,it is suggested to choose Q235 steel or 45# steel for the equipment construction materials when using SDBS micro-emulsion.And hence the equipment cost for SLs micro-emulsion system will be higher than SDBS system, but the biodegradability of it shows great attractive.

3.5. Evaluation on plant seed germination and growth

To investigate the remediation effect of the soil, seed germination and growth tests were carried out.Seed germination is a very important part of the plant life cycle,and it is susceptible to external environment,among which salt-alkali stress is one of the most serious factors [30,31]. The natural soil, crude oil contaminated soil,SDBS micro-emulsion[8]remediated soil(1#remediated soil)and SLs micro-emulsion remediated soil(2#remediated soil)were tested on the seed germination of ryegrass and alfalfa.

Table 6 gives the test results.It can be found that the germination rates of ryegrass and alfalfa seeds in the natural soil and contaminated soil were both 0%within 7 days.The germination rate of ryegrass in the SDBS micro-emulsion remediated soil(1#)showed better result, 25.71%, although the alfalfa still gave a 0% germination rate. However, the germination rates of these two kinds of plants in the SLs micro-emulsion remediated soil (2#) reached 97.14% and 76.66%, respectively. And the initiating of ryegrass seeds germination in SDBS micro-emulsion remediated soil is 4 days later than that in the SLs micro-emulsion remediated soil.

It was known from Section 3.2.2 that the natural soil from GR oilfield is a heavily saline-alkaline soil.A large number of salt ions accumulate on the surface of the soil, leading to a decrease in the air permeability of the soil and reduce the osmotic potential of the seed cells. It is difficult for seeds to absorb water, thereby inhibiting the normal germination and growth of plants. Studies have shown that the increase of salt content and pH will directly poison and inhibit the germination of seeds [31].

2# remediated soil shows better effect on the germination rate of ryegrass and alfalfa seeds than that of 1# remediated soil. The pH value of 1# soil was measured to be 8.34, which is 0.66 higher than that of 2# soil. The 1# soil is a moderate alkaline soil while the 2# soil is a mild alkaline soil. It is supposed that the pH of remediated soil plays important role in the seeds germination rate,and the lower pH of the 2# soil promoted the germination of plants. On the other hand, the SDBS is not easy to biodegraded,leading to secondary pollution to the soil. While the SLs is totally biodegradable and have no toxicity to plants,which can even nourish the plants.

After 20 days, the growth of ryegrass in 1# and 2# remediated soil was observed.The growth of ryegrass in 2#remediated soil is much better than that in 1#,see Fig.11(a)and(b),and the ryegrass in 1#remediated soil actually did not grow,instead,they all withered after 20 days (see Fig. 11(a)).

The growth state of ryegrass and alfalfa in 2# remediated soil was traced for 60 days (see Table 7). Both of the two plants showgood growth state and ryegrass shows better growing effect than that of alfalfa. SLs micro-emulsion improves the physical and chemical properties of the soil, such as lowering the pH value and the total salt content and increasing the content of organic matter, which provides a good living environment for plant growth. On the other hand, SLs can also effectively reduce the interfacial tension, enhance solubility and mobility of the remaining hydrocarbons in the soil, weaken the toxicity of the soil and make it suitable for plant growth [20].

Table 4 Corrosion evaluation results for SLs micro-emulsion on different types of steels

Table 5 Corrosion evaluation results for SDBS micro-emulsion on different types of steels

Table 6 7 days germination rate of ryegrass and alfalfa seeds in different soil (%)

Fig. 11. The growth state of ryegrass in different remediated soil after 20 days planting: (a) SDBS micro-emulsion remediated soil (1#); (b) SLs micro-emulsion remediated soil (2#).

3.6.Further remediation of the SLs micro-emulsion remediated soil by ryegrass

After treated with micro-emulsion, there are still some petroleum hydrocarbons remaining in the soil.When the content of petroleum hydrocarbons in the soil is low, phytoremediation can be used. It is already found that ryegrass grows vigorously in the soilafter SLs micro-emulsion remediation and is better than alfalfa.Therefore, the degradation of oil after ryegrass growing was observed. The experimental results are shown in Fig. 12.

Table 7 Plants growth state in the SLs micro-emulsion remediated soil after 60 days

Fig. 12. Degradation of residual oil in the SLs micro-emulsion remediated soil.

Comparing with the naturally degraded soil, the soil planting ryegrass shows significant decrease in oil content. After 60 days,the amount of residual oil dropped from 1.03% to about 0.8%. The degradation rate reached 22.33% for the soil planting ryegrass,while that of the naturally degraded soil was only 9.7%, which shows that ryegrass contributes obviously to the degradation of petroleum hydrocarbons in the soil. Similarly, Wei et al. [32] also found that ryegrass had a higher tolerances to oil pollution than alfalfa and gave a higher degradation effects than alfalfa and the non-planted one.So,ryegrass is suggested to use for phytoremediation after SLs micro-emulsion washing.

4. Conclusions

Winsor type I sophorolipids (SLs) micro-emulsion was used in 15% (mass) oil contaminated soil washing. The following conclusions can be drawn.

SLs micro-emulsion with w(SLs) = 6%, w(NaCl) = 1%, w(diesel) = 13.36% (Winsor type I) shows the best washing effect on the oil contaminated soil, with the oil removal rate reaching 95.6% and the final eluate was still in the state of Winsor type I.

The viscosity of recovered oil was lower than the original oil,attributing to the higher saturation content and the lower resin and asphaltene content. Both the total salt content and pH of the recovered soil were lower than the natural soil, because of the acidic property of the SLs and the dissolving out of the salt into the washing agent.

The recovered micro-emulsion phase shows good reuse performance through supplement of diesel oil,the oil removal rate being still higher than 92% in the 3rd reuse operation.

Germination rate of ryegrass in the SLs micro-emulsion remediated soil (2#) reached 97.14% and shows good growing state in a 60-day observation, and it shows good phytoremediation effect on the contaminated soil after SLs micro-emulsion washing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank the financial support of National Natural Science Foundation of China (22078366) and the supply of the sophorolipids by Shandong Qilu Biotechnology Co., Ltd.