Chemosphere 88 (2012) 245–249

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Chemosphere

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Technical Note

Solvent extraction for heavy crude oil removal from contaminated soils

Xingang Li a,b, Yongliang Du a, Guozhong Wu a, Zhongyuan Li a, Hong Li a,b, Hong Sui a,b,⇑a School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, Chinab National Engineering Research Centre for Distillation Technology, Tianjin 300072, China

a r t i c l e i n f o

Article history:Received 14 December 2011Received in revised form 1 March 2012Accepted 2 March 2012Available online 5 April 2012

Keywords:Solvent extractionSoilOilnC7-asphaltenes

0045-6535/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.chemosphere.2012.03.021

⇑ Corresponding author. Address: No. 92 Weijin R300072, PR China. Tel.: +86 22 27404701; fax: +86 22

E-mail address: suihong@tju.edu.cn (H. Sui).

a b s t r a c t

A new strategy of heavy crude oil removal from contaminated soils was studied. The hexane–acetone sol-vent mixture was used to investigate the ability of solvent extraction technique for cleaning up soilsunder various extraction conditions. The mixtures of hexane and acetone (25 vol%) were demonstratedto be the most effective in removing petroleum hydrocarbons from contaminated soils and approx 90%of saturates, naphthene aromatics, polar aromatics, and 60% of nC7-asphaltenes were removed. Kineticexperiments demonstrated that the equilibrium was reached in 5 min and the majority of the oil pollu-tants were removed within 0.5 min. The effect of the ratio between solvent and soil on the extraction effi-ciency was also studied and results showed that the efficiency would increase following the highersolvent soil ratio. Then the multistage continuous extraction was considered to enhance the removal effi-ciency of oil pollutants. Three stages crosscurrent and countercurrent solvent extraction with the solventsoil ratio 6:1 removed 97% oil contaminants from soil. Clearly the results showed that the mixed-solventof hexane and acetone (25 vol%) with character of low-toxic, acceptable cost and high efficiency waspromising in solvent extraction to remove heavy oil fractions as well as petroleum hydrocarbons fromcontaminated soils.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Oil polluted soil usually originates from spills or leaks of storagetanks during oil exploitation and discharge operations. Crude oil isan extremely complex mixture of hydrocarbons. Usually smallamounts of heteroatom like nitrogen, oxygen, and sulfur, as wellas trace amounts of metals like vanadium and nickel are found(Hansen, 2007). Most oil components are hazardous to humanhealth and the soil biota (Villalobos et al., 2008). A number of tech-nologies have been developed for remediating soils contaminatedwith petroleum hydrocarbons (Urum et al., 2006; Avila-Chavezand Trejo, 2010).

Solvent extraction is a cleanup method that uses solvents to ex-tract or remove harmful chemicals from polluted materials. Suchmethod can be efficient in removing hydrophobic organic contam-inants from soils (USPEA, 2001). Solvent extraction has been ac-cepted as an alternative organic contaminants remediationmethod. Ethanol–water mixed solvents were used to remove pen-tachlorophenol from soils contaminated with wood treating wastes(Khodadoust et al., 1999b). The results obtained from 5:1 alkane–alcohol mixture showed that the majority of polychlorinateddibenzo-p-dioxins and polynuclear aromatic hydrocarbons were

ll rights reserved.

oad, Nankai District, Tianjin27404705.

removed from the soils (Nam et al., 2001). Solvent extraction hasalso been investigated for treating soils contaminated with chlori-nated compounds and hydrocarbons in the diesel range usingmixed solvents ethyl acetate–acetone–water (Silva et al., 2005;Murena and Gioia, 2009).

The contaminants mentioned above have a common feature thatthe molecular weight is less than 300 Da, and some of which can beidentified in the light fractions isolated from crude oil containingsaturated compounds and aromatic compounds using GC–MS(Wang et al., 2010). There are also many available soil remediationtechnologies and analytical methods concentrated solely on hydro-carbon contaminants with less than 40 carbon atoms (Khalladiet al., 2009; Xu et al., 2011). Most of the above studies focused onthe removal of light hydrocarbons contaminants from soils.

For heavy crude oil polluted soils, there is large amount of res-ins and asphaltenes in oil pollutants. Although they are low- orno-toxic, they have the capacity to adsorb onto soil surface, alter-ing the surface from water-wet to oil-wet, and also have the poten-tial risk to human health and environment (Dumitran et al., 2009).However, limited information was available for removing heavy oilfractions from soils.

Therefore, the aim of this study was to investigate the possibilityof applying solvent extraction using hexane–acetone mixed sol-vents to clean up soils contaminated with heavy crude oil. Basedon the most common separation procedure (ASTM D4124-01,2001), the oil pollutants were grouped into four major classes:

246 X. Li et al. / Chemosphere 88 (2012) 245–249

saturates (S), naphthene aromatics (NA), polar aromatics (PA), andnC7-asphaltenes (A) by polarity and solubility. The influence of var-ious extraction parameters (acetone concentration, extraction time,liquid solid ratio and extraction stages) on the removal of four oilfractions from soils was examined.

2. Materials and methods

2.1. Reagents

Analytical grade solvents were used in this study. Acetone, n-hexane, n-heptane, methanol, toluene, trichloroethylene andneutral aluminum oxide (0.074–0.15 mm) were purchased fromTianjin Jiangtian Technology, China. Aluminum oxide was chro-matographic grade and calcined at 413 �C for 16 h before using.

2.2. Soils

The crude oil contaminated soils and the crude oil sample wereobtained from Gudong Oil Production Plant at the Shengli Oil Fieldlocated in Shandong Province, China. The soils were air-dried,homogenized and sieved (40 mesh) to remove large vegetableroots and rocks (>0.42 mm), and stored in glass desiccators beforeusing.

2.3. Solvent extraction

To evaluate the capacity of the hexane–acetone solvent mixturefor removing the contaminants from the soil, the following proce-dure was adopted. Contaminated soil samples (10 g) were weighedinto a 100 mL conical flask. A given volume of solvent was added.The contact between the phases was promoted by magnetic agita-tion and the separation was done by centrifuging in tubes at5000 rpm for 5 min. Samples of soil and samples of the extractmixture were collected to analyze the residual oil content afterextraction and the removed oil fractions during extraction.

2.4. Continuous extraction

Crosscurrent and countercurrent solvent extraction experi-ments were conducted to investigate the effect of different extrac-tion styles and stages on oil removal. Crosscurrent solventextraction was performed at different solvent soil ratios (2:1, 3:1and 6:1). Countercurrent solvent extraction was performed at sol-vent soil ratio of 6:1. In each stage, the soil samples (10 g) and sol-vent were mixed for 20 min and separated by centrifuging in tubesat 5000 rpm for 5 min. Fig. 1 shows the schematic for crosscurrent

(a)

(b)

Fig. 1. Three-stage (a) crosscurrent and (b) co

and countercurrent solvent extraction in three stages. The detailsof these extraction experiments were reported by Khodadoustet al. (1999a).

2.5. Determination of oil content in soil

The concentration of oil in soils was determined gravimetricallyby ultrasonic extraction. Soil sample (5 g) was placed in 50 mL con-ical flask and sonicated with 25 mL toluene for 20 min, using anultrasonic instrument (KQ100KDB, Kunshan, China) at 20 �C and100 W. After sonication, the extract was separated from the sampleby centrifugation at 5000 rpm for 5 min. The above procedureswere repeated twice and the extracts were decanted off after eachextraction. Solvent was separated from the extract by distillationand drying.

2.6. Elution chromatography

The oil samples containing four oil fractions were first sepa-rated into n-heptane-insoluble A and the n-heptane-soluble partsusing 100 mL n-heptane. The calcined aluminum oxide (150 g)was added in a glass chromatographic column (510 cm � 25 cmi.d.). The column was preweted with 20 mL n-heptane, and the n-heptane-soluble parts (solvent was removed) were transferred intothe column using a minimum amount of n-heptane. Then the elu-ants were added at a drip rate of 2–3 mL min�1 and eluted as fol-lows: 65 mL n-heptane and 35 mL toluene for S, 100 mL tolueneand 100 mL 50% (v/v) methanol–toluene for NA, and 200 mL tri-chloroethylene for PA. The net mass of oil fractions was recordedafter removing all of the solvent from the eluates.

3. Results and discussion

3.1. Soil characterization

The soil samples were weathered more than 12 months beforeexperiment and the oil content was 98000 mg kg�1. The oil frac-tions of crude oil sample and soil pollutants are shown in Table1. The largest percentage mass loss was noticed for NA (7%) fol-lowed by S (2%), which was due to the evaporation of volatile com-pounds of light oil fractions (Urum et al., 2004). PA and A increasedby 4% and 6%, respectively. Because of PA and A with high molec-ular weight, high viscosity and non-volatilization, it was confirmedthat the heavy oil fractions in weathered soils were more concen-trated than those in the non-weathered contaminated soil. There-fore, it was vital to investigate the removal of heavy oil fractions insolvent extraction.

untercurrent solvent extraction process.

Table 1Distribution of oil fractions in crude oil and soil pollutants.

Oil fractions (% total oil)a Crude oil sample Soil pollutants

S 31 29NA 25 18PA 41 45A 2 8

a Weight content.

X. Li et al. / Chemosphere 88 (2012) 245–249 247

3.2. Solvent extraction

Extraction experiments were conducted for contaminated soilswith hexane–acetone solvent mixture, and the results obtainedare shown in Fig. 2. There are no obvious differences shown on S re-moval with increasing acetone content of the solvent mixture from0 to 0.75 volume fractions. Comparing with the pure acetone, all thesolvent mixtures were effective to extract saturates from soils withthe removal more than 90%. The removal of NA obviously increasedwith rising acetone content of the solvent up to 0.25 acetone vol-ume fractions in solvent mixture; thereafter, the removal of NAfrom soils decreased with the higher acetone content. PA removalalso presented an initial rising and then dropping tendency withthe increasing volume fractions of acetone in solvent mixture andthe highest removal was 88% at 0.375 acetone volume fractions.

Hexane is non-polar while acetone is polar. Therefore, the polar-ity of solvent mixture increases gradually while increasing the vol-ume fractions of acetone. Extraction solvent selection bases on‘‘like dissolves like’’ (Hansen, 2007). That means the solvent has gooddissolving efficiency when its polarity is similar to that of oil pollu-tants. So the polarity of solvent mixture is significant to the extrac-tion efficiency in removing oil contaminants. It can be concludedthat the polarity of solvent mixture with 0.25 and 0.375 acetone vol-ume fractions was similar to the polarity of NA and PA, respectively.

Solvent mixtures showed lower removal efficiency for A thanthe other three oil fractions. If the hydrocarbon liquid used todilute crude oil or bitumen is an n-alkane such as n-heptane or

0

20

40

60

80

100

Rem

oval

(%)

S

Rem

oval

(%)

Volume fraction ace0.0 0.2 0.4 0.6 0.8 1.0

0

20

40

60

80

100 PA

Fig. 2. Extraction oil from soils using hexane–acetone solvent mixture. Solvent soil ratideviation (n = 5).

n-pentane, the precipitate is called asphaltenes (Redelius, 2004).So A was not removed from soils when using pure hexane. Whenacetone volume fractions ranged from 0.25 to 0.5, about 60% of Awas removed from soils. Fig. 2 also indicated that solvent mixtureswith the acetone volume fractions ranging from 0.125 to 0.375were effective in extracting total petroleum hydrocarbons (TPH)from soils. The optimal acetone content was 25% where TPH re-moval reached 89%. Therefore, the 25% acetone solvent mixturewas used in the following experiments.

3.3. Extraction kinetics

The kinetics experiments showed that all the oil fractions re-moval rapidly reached equilibrium. Insignificant changes in the re-moval rates were observed after 5 min. Both oil pollutants andhexane–acetone solvent mixtures are organic compound, so oilcan rapidly dissolve in solvent. The removal of S, NA, PA and Areached 91%, 81%, 87% and 60% after 0.5 min, respectively. It alsoillustrated that oil fractions could be effectively removed usingthe 25% acetone solvent mixture.

3.4. Effect of solvent soil ratio

Fig. 3 shows the variation in four oil fractions removal with thesolvent soil ratio (v/w) increasing. The trends that were higher sol-vent soil ratio along with higher removal were similar for the dif-ferent oil fractions. The increase of solvent soil ratio will enhancethe interaction between the oil contaminants and extractionsolvent, as well as increase the concentrations gradient betweenliquid–solid phases. This suggested that solvent soil ratio playedsignificant role in extracting oil pollutants from soils. However,the large solvent soil ratio means that the large volume of solventis needed and the proper condition is important to save solventcost in extraction technology. Approx 90% of S, NA, PA and 60%of A were removed when the liquid phase was six times higherthan the solid one. Therefore, the optimal solvent soil ratio was 6:1.

tone in solvent mixture

NA

0.0 0.2 0.4 0.6 0.8 1.0

A

o: 6:1; extraction time: 20 min; temperature: 20 �C. Error bars represent standard

0

20

40

60

80

100

solvent soil ratio

Rem

oval

(%)

S

Rem

oval

(%)

solvent soil ratio

NA

2:1 4:1 6:1 8:1 10:1 12:10

20

40

60

80

100 PA

2:1 4:1 6:1 8:1 10:1 12:1

A

Fig. 3. Contaminants removed at different solvent soil ratio. Extraction time: 20 min; temperature: 20 �C. Error bars represent standard deviation (n = 5).

Cro 2:1 Cro 3:1 Cro 6:1 Coun0

4

860

70

80

90

100

(b)

Stage Stage Stage

Stage Stage Stage

Rem

oval

(%)

0

4

860

70

80

90

100

(a)

Rem

oval

(%)

Fig. 4. Crosscurrent (Cro) and countercurrent (Coun) extraction: (a) three-stageextraction; and (b) different stages extraction using 60 mL solvent mixtures.

248 X. Li et al. / Chemosphere 88 (2012) 245–249

3.5. Continuous extraction

Three-stage crosscurrent solvent extraction was performed atthe solvent soil ratio of 2:1, 3:1, 6:1 and countercurrent extractionwas performed at 6:1. The total volumes of solvent used were 60,90, 180 and 60 mL for each 10 g soils, respectively. Fig. 4a showedthat cumulative TPH removal of crosscurrent extraction after three

stages increased with solvent soil ratio rising. Crosscurrent extrac-tion with 6:1 solvent soil ratio had the highest efficiency with the97% TPH removal. Countercurrent extraction with the same solventsoil ratio also had the similar result. However, the solvent con-sumption (60 mL) was only one third of that used in crosscurrentextraction. Results also indicated that most contaminants were re-moved during the first and second stages in crosscurrent and coun-tercurrent solvent extraction.

Fig. 4b indicated that the oil removal increased with the increas-ing stages of crosscurrent extraction. Countercurrent extractionwas the most effective in removing oil pollutants on the conditionof the same volume solvent. In order to reduce solvent consuming,multistage extraction with small solvent soil ratio was necessary toachieve the high removal of oil. However, more stage extraction andcountercurrent process were always accompanied with the morecomplex operation in laboratory and industry. For the soils usedin this study, two stages crosscurrent solvent extraction with great-er than 95% removal at 3:1 solvent soil ratio was suitable in remov-ing oil contaminants.

4. Conclusions

The mixtures of hexane and acetone ranging from 0.125 to0.375 acetone fractions in mixed solvent were effective in extract-ing petroleum hydrocarbons from contaminated soils, especiallyfor PA and A which were more than one half in oil pollutants.The extraction kinetics rapidly reached equilibrium and the re-moval of oil fractions increased with rising solvent soil ratio. Coun-tercurrent extraction was more effective in removing oil from soils.Extraction temperature was not considered in this study due to thevolatility of hexane–acetone mixed solvent.

Acknowledgements

The authors gratefully acknowledge the support from the Na-tional Hi-Technology Research & Development Program of China(Grant No. 2009AA063102), Program for Changjiang Scholars andInnovative Research Team in University (Grant No. IRT0936) and

X. Li et al. / Chemosphere 88 (2012) 245–249 249

the Municipal Natural Science Foundation of Tianjin (Grant No.11JCYBJC05400).

References

ASTM D4124-01, 2001. Standard Test Methods for Separation of Asphalt into FourFractions. ASTM International, West Conshohocken.

Avila-Chavez, M.A., Trejo, A., 2010. Remediation of soils contaminated with totalpetroleum hydrocarbons and polycyclic aromatic hydrocarbons: extractionwith supercritical ethane. Ind. Eng. Chem. Res. 49, 3342–3348.

Dumitran, C., Onutu, I., Dinu, F., 2009. Extraction of hydrophobic organiccompounds from soils contaminated with crude oil. Rev. Chim. 60, 1224–1227.

Hansen, C.M., 2007. Hansen Solubility Parameters: A User’s Handbook, second ed.CRC Press, Boca Raton.

Khalladi, R., Benhabiles, O., Bentahar, F., Moulai-Mostefa, N., 2009. Surfactantremediation of diesel fuel polluted soil. J. Hazard. Mater. 164, 1179–1184.

Khodadoust, A.P., Suidan, M.T., Acheson, C.M., Brenner, R.C., 1999a. Remediation ofsoils contaminated with wood preserving wastes: crosscurrent andcountercurrent solvent washing. J. Hazard. Mater. 64, 167–179.

Khodadoust, A.P., Suidan, M.T., Acheson, C.M., Brenner, R.C., 1999b. Solventextraction of pentachlorophenol from contaminated soils using water–ethanolmixtures. Chemosphere 38, 2681–2693.

Murena, F., Gioia, F., 2009. Solvent extraction of chlorinated compounds from soilsand hydrodechlorination of the extract phase. J. Hazard. Mater. 162, 661–667.

Nam, P., Kapila, S., Liu, Q.H., Tumiatti, W., Porciani, A., Flanigan, V., 2001. Solventextraction and tandem dechlorination for decontamination of soil.Chemosphere 43, 485–491.

Redelius, P., 2004. Bitumen solubility model using Hansen solubility parameter.Energy Fuels 18, 1087–1092.

Silva, A., Delerue-Matos, C., Fiuza, A., 2005. Use of solvent extraction to remediatesoils contaminated with hydrocarbons. J. Hazard. Mater. 124, 224–229.

Urum, K., Pekdemir, T., Copur, M., 2004. Surfactants treatment of crude oilcontaminated soils. J. Colloid Interface Sci. 276, 456–464.

Urum, K., Grigson, S., Pekdemir, T., McMenamy, S., 2006. A comparison of theefficiency of different surfactants for removal of crude oil from contaminatedsoils. Chemosphere 62, 1403–1410.

USPEA, 2001. A Citizen’s Guide to Solvent Extraction. US Environmental ProtectionAgency, Washington, DC.

Villalobos, M., Avila-Forcada, A.P., Gutierrez-Ruiz, M.E., 2008. An improvedgravimetric method to determine total petroleum hydrocarbons incontaminated soils. Water Air Soil Pollut. 194, 151–161.

Wang, S.J., Guo, G.L., Yan, Z.G., Lu, G.L., Wang, Q.H., Li, F.S., 2010. The development ofa method for the qualitative and quantitative determination of petroleumhydrocarbon components using thin-layer chromatography with flameionization detection. J. Chromatogr. A 1217, 368–374.

Xu, J., Pancras, T., Grotenhuis, T., 2011. Chemical oxidation of cable insulating oilcontaminated soil. Chemosphere 84, 272–277.