Desorption investigation of crude oils in loess contaminated in Yumen mining area

Western loess area is one of the primary bases of petroleum and natural gas. New and high quality oil wells have been continuously exploited in Yumen and Qingyang of Ganshu province. In the area, during the processes of depositing, transplanting and handling, crude oils could leak solid toxic organic substances which could be likely to exert an everlasting impact on the soils of mining area and surrounding area, and exacerbate our weak natural environment. Oils resulting in soil contamination included remnant oil and soluble oils. Remnant oil was tough to move with water, and was also hard to be absorbed by crops. Therefore, oils effectively resulting in soil contamination in soil-water system were soluble oils. Some researches have indicated that the harm to aquatic organisms caused by oil substances dissolved in water could last for 10 years, which would have a serious impact on the life and health of local people. For example, there are cases that farmland irrigation water and even drinking water were greatly contaminated by crude oils leakage in Yumen and Qingyang oil fields. All these are tough contamination issues for oil producing and refining area. Consequently, desorption investigation of crude oils could not only provide theoretical basis for repairing loess contaminated in Yumen mining area, but also have its practical application values.

1 Materials and methods
1.1 Soil
Four sorts of soil were sampled from Yumen oil field of Ganshu. Sample 1 was collected far away from contamination source, surface layer 10cm; sample 2 was from 30m nearby discarded oil hole, surface layer 10cm; sample 3 was from 10m around oil well, surface layer 10cm; sample 4 was from 2m around oil well, surface layer 10cm. Due to Yumen oil field located in the Gobi desert area with rare rainfall where it was tough for the growth of plants and animals, few indigenous microorganisms exited in sample 1 and 2 and none in sample 3 and 4, so interference of microorganisms could be neglected in the present experiment. Soil samples dried by air were filtrated through 100 mesh sieves and stored for further investigation. Soil characteristics were assayed and results were listed in Table 1.

Table 1 Property of loess soil

1.2 Reagents and apparatus
Crude oils were sampled from Yumen oil field; petroleum ether(boiling point from 60-90 ℃, analytical pure(AR), No.1 factory of Tianjin chemical reagent, China); anhydrous calcium chloride(AR, Chemical factory of Tianjin Tanggu Dengzhong, China); anhydrous sodium carbonate(Chemical factory of Tianjin Tanggu Pengda, China); sodium dodecyl sulphate(SDS, Chemical pure(CR), No.6 factory of Tianjin chemical reagent, China); cetyltrimethylammonium bromide(HDTMAB, CR, Shanghai Chemical reagent company, Chinese Medical Group); polyvinylpyrrolidone(PVPL, AR, Tianjin Taida Fine Chemical Co. Ltd); PHS-4C acidimeter(Chengdu Fangzhou Science and Technology Development Company, China); 754 type UV-VIS Spectrophotometer(Shanghai Analytical Instrument General Factory); THZ-C desktop constant temperature oscillator(Jiangshu Taichang experimental equipment factory, China); SK3300H ultrasonic cleaner(Shanghai Kedao Ultrasonic Instrument Co. Ltd).
1.3 Working curve
Crude oil samples from Yumen oil field was prepared to 1.00mg/mL standard oil stock solutions by petroleum ether. Serial standard oil solutions were added to 50ml colorimetric tubes respectively, diluted by petroleum ether until attaining a constant volume of 50mL, and then the absorbency was measuredat 255nm. Working curve was plotted with absorbance A against oil concentration C_oil, and its linearity ranged at the scope of 0-0.16mg/mL. Its linear fitting equation and correlation coefficient was as follows:
    A=0.00639+8.21416C_oil, R=0.99987
1.4 Experiment and determination method
Serial different loess was added to 50ml iodine volumetric flasks, and 50mL0.01MCaCl₂ were transferred into 50ml volumetric flasks. The mixture solution was agitated at the THZ-C desktop constant temperature oscillator for 24h until completely achieved absorption equilibrium with plug tightened and then kept for 2 h. 20 ml accurate supernatant was ultrasonic extracted by 5ml petroleum ether twice and resultant solutions was diluted until attaining a constant volume of 10mL. Desorption capacity of crude oils was calculated after correcting measured by 754 type UV-VIS Spectrophotometer. With respect to the investigation of surfactant effects, 0.002% surfactant replaced 0.01MCaCl₂(Avoiding emulsification). With respect to the investigation of pH value effects, 5g soil sample 4 was added to serial iodine volumetric flasks respectively, and merged with 0.01MCaCl₂ of certain pH value adjusted by pHS-4C acidimeter. The other steps followed as previous described.

Table 2 Desorption constant of crude oil in Yumen diggings's contamination loess soils

Fig.1 Desorption isotherm of crude oil in soil 1

Fig.2. Desorption isotherm of crude oil in soil 2

Fig.3 Desorption isotherm of crude oil in soil 3

Fig.4 Desorption isotherm of crude oil in soil 4

2. Results and discussions
2.1 Isothermal desorption curve
Isothermal desorption curves(Figure 1, 2, 3 and 4) of crude oils in loess were in agreement with Freundich equation, which could be denoted as follows:
C_s=K_d·C_oilⁿ or lgC_s=nlgC_oil+lgK_d
    Where C_s was desorption capacity in equilibrium, mg/mL; C_oil was oil concentration in soil, mg/ml; K_d was adsorption constant; n was constant of exponential term. K_d represents the desorption capability of crude oils in contaminated loess, and desorption property of crude oils in soil-water system could be characterized by soil organic carbon constant K_oc(K_oc=K_d/organic carbon content). K_oc could be used as an index to evaluate the migration trend of crude oils in soil-water system.K_d, n, r and K_oc were calculated by regression analysis, and results were listed in Table 2. K_d and K_oc decreased gradually, which indicated that desorption capability decreased inversely with the increasing of organic substances. Crude oils are hydrophobic, due to hydrophobic bond and Van der Waals force, and were adsorbed in the soil organic substances. Organic substance level was higher, desorption capability of contaminations was lower. Elution rate of the four sorts of soil was 1.03%, 0.653%, 0.319% and 0.218%, respectively.


Fig.5 Temperature influence to desorption of crude oil in soil 4 Fig.6. pH influence to desorption of crude oil in soil 4

2.2 Effects of temperature, pH value and surfactant
2.2.1 Effects of temperature
Desorption of crude oils was affected by environmental temperature, as seen from Figure 5, desorption capacity of crude oils in loess increased with the increasing of temperature. Elution rate was 0.218%, 0.442% and 0.535% at the temperature of 25℃, 35℃ and 45℃, respectively. As temperature increased, the viscosity of crude oils decreased, fluidity increased and density difference between water and oil increased which was in favor of the desorption of crude oils from the surface of loess. Then crude oils floated in the water and were easy to separate. At low temperature, oil could be desorbed, but the desorbed oils accumulated into ropy semi-solid mass merged with loess, and thus were hard to be floating oil and separate out.
2.2.2 Effects of pH
pH value was an important factor affecting the desorption of chemical substances, which not only influenced charge distribution of soil colloid, but also affected configuration distribution of crude oil components. Soil sample 4 was undertaken to plot the desorption curve of crude oils along with the pH value. As seen from Figure 6, desorption capacity of crude oils in loess increased with the increasing of pH value. Elution rate of crude oils increased from 0.345% to 1.20% at pH values ranged from 1.85 to 10.55. When pH increased, inorganic base in solutions not only reacted with organic acid and acid components and produced surfactive substance; on the other hand, inorganic base interacted with soil surface, altered the surface structure and electrical properties of soils and thus enhanced desorption effect greatly.
2.2.3 Effects of surfactant
Light components in ground crude oils volatilized in the air as time went by, remnant crude oils were heavy components. Due to extreme low solubility, crude oils merged tightly with soil were tough to desorb. Surfactant was a sort of substance with both hydrophilic and hydrophobic groups in molecule, and could increase the water solubility of hydrophobic substances by forming micellars and ameliorate the efficacy of soil desorption. As seen from Figure 7 and 8, as far as soil sample 3 and 4 were concerned, compared to water desorption solutions, addition of anionic surfactant SDS enhanced the desorption capability of crude oils, while cationic surfactant HDTMAB and nonionic surfactant PVPL attenuated. Elution rate of SDS in soil sample 3 attained 1.07% which increased 5 times compared to water eluants; Elution rate of SDS in soil sample 4 attained 0.911 which increased 4.2 times compared to water eluants. Anionic surfactant could increase the negative potential of soil and crude oil surface, enhance the repulsive force between soil and crude oils, crude oils and crude oils as well, and thus could improve desorption capability of crude oils and prevent redeposition; cationic surfactant had weak desorption effect, due to the mutual attraction of cationic surfactant with positive charge and soil with negative charge, but addition of cationic surfactant could retain crude oils in the soil and increase recovery rate of crude oils; nonionic surfactant didn't ionize in water solutions with high stability, and had low capability of eluting crude oils.
    Taken together, with the increasing of pH value and temperature, in the presence of anionic surfactant, crude oils were easily desorbed. Rainfall could make soluble components transfer into farmland irrigation water and drinking water. In order to avoid the previous described issue, cationic surfactant should be used to retain crude oils in the surface layer of soil, and then reclaimed for further utilization.

Fig.7 Surfactant influence to desorption of crude oil in soil 3

Fig.8 Surfactant influence to desorption of crude oil in soil 4

3 CONCLUSIONS
3.1 Desorption curve of crude oils in loess contaminated by Yumen crude oils was nonlinear, and was in agreement with Freundich equation with low desorption capacity.
3.2 Desorption manner was affected by temperature, pH value and surfactant. With the increasing of temperature and pH value, desorption capacity increased; addition of anionic surfactant was in favor of the desorption of crude oils; addition of cationic surfactant was not in favor of the desorption of crude oils, but was beneficial to the retention of crude oils in the soil for further utilization.
3.3 Desorption investigation of crude oils could provide theoretical basis for repairing loess contaminated in Yumen mining area.

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玉门矿区污染黄土中原油的解吸研究
展惠英（甘肃联合大学化工学院 甘肃兰州 730070)
摘要 用振荡平衡法研究了玉门矿区原油污染黄土中原油的解吸行为，测得了解吸常数，绘出了解吸等温线，结果表明，Freundlich方程可较好地描述黄土中原油的解吸状况；黄土中原油的解吸行为受温度、溶液的pH值和表面活性剂的影响；温度和pH值越高，原油的解吸量越大；阴离子表面活性剂的加入有助于原油的解吸，在相同条件下使解吸率提高了5倍，阳离子和非离子表面活性剂的加入不利于原油解吸，但是阳离子表面活性剂的加入有利于原油在土壤中的截留以及回收再利用，本项研究为修复玉门原油污染黄土提供了理论依据。
关键词 原油 黄土 解吸