m e d i ation Journal of Bioremediation & Biodegradation · Citation: Wang JG, Zhan XH, Liang JR, Zhou LX, Lin YS, et al. (2011) A Novel Method for the Determination of Total Hydrocarbon

ISSN: 2155-6199 JBRBD, an open access journalBioremediation: Environmental organic Geo-ChemistryJ Bioremed Biodegrad

Research Article Open Access

Wang, et al. J Bioremed Biodegrad 2011, S2 DOI: 10.4172/2155-6199.S2-001

Research Article Open Access

Keywords: Hydrocarbon mixture standard sample; Total hydrocar-bon content; Soil organic matter; Heavy metals; Thermal treatment

Introduction Contaminated soils, as a result of agricultural and industrial

activities, exist ubiquitously in the environment [1-4], which release a large amount of hydrocarbon contaminants with mutagenic, carcinogenic and toxic characteristics [5]. Government, industries, and the public have already recognized the potential dangers of these contaminants to human health and natural environment. In response to an increasing need to address environmental problems, many remediation technologies such as thermal treatment [6,7], washing [8,9], venting [10,11] and bioremediation [12,13] were developed to remediate these contaminated soils.

Although significant advances in understanding soils remediation have been achieved, there are still few reports on the remediation of hydrocarbon mixture-contaminated sites. Moreover, the determination of total hydrocarbon contents in these contaminated sites is a hard and time consuming work. The traditional methods for the determination of hydrocarbons mainly include gas chromatography (GC), high performance liquid chromatography (HPLC), the mass spectrum (MS) and their hyphenated techniques (GC-MS, HPLC-MS). However, these methods are always related to a time-consuming extraction process for target contaminants, large volumes of extraction solvents and high cost for instruments [14,15], etc. In some cases, a combination of expensive analysis instruments is needed for qualitative and quantitative analysis of unknown hydrocarbons. Therefore, it is necessary to develop simple, suitable, accurate, and sensitive methods to determine the extent of soil contamination by hydrocarbon mixture in place of the usual expensive analytical tools. The remediation efficiency of any technology can also be assessed rapidly by using the newly developed methods according to the removal of contaminants after remediation.

In China, there are many special sites for washing and recycling the used drums and containers located in Yangtze River Delta, where a wide variety of unknown organic reagents or contaminants are released into the soil around the site and some hazardous organic compounds may be migrated downward to reach the groundwater and the plant, resulting in severe site contamination and threatening human health. These contaminated sites are commonly close to farmland or channel and must be remediated as soon as possible due to their potential environmental risk [16]. Our previous investigations have demonstrated that in these contaminated sites, a variety of contaminants (C5-C40 hydrocarbons) exist and result in great difficult for qualitative and quantitative analysis of mixed hydrocarbon-contaminated sample in the process of remediation. These contaminants, originated from the dumping of oils or chemicals in recycled drums over the past three decades, may be mixtures of isomeric hydrocarbons. Besides, the universal GC-MS method [17] is not cost-effective and some time the combination of other analytical tools is needed as some of organic components with high boiling points may not be gasified in chromatographic column. The infrared method [18] has always a shortage in the determination of hydrocarbons with stronger polar property and

*Corresponding author: Li-xiang Zhou, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing, 210095, China, E-mail: [email protected]

Received September 16, 2011; Accepted November 08, 2011; Published November 14, 2011

Citation: Wang JG, Zhan XH, Liang JR, Zhou LX, Lin YS, et al. (2011) A Novel Method for the Determination of Total Hydrocarbon in the Hydrocarbon Mixture-Contaminated Soil. J Bioremed Biodegrad S2:001. doi:10.4172/2155-6199.S2-001

Copyright: © 2011 Wang JG, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

AbstractA simple, reliable and rapid analysis method, for the determination of total hydrocarbon content in a hydrocarbon

mixture-contaminated soil that derived from a drum washing factory, was described and validated for the remediation of the contaminated site. This method was based on an assumption that the possibly extracted total hydrocarbon by organic solvent in the hydrocarbon mixture-contaminated soil had a ultraviolet absorption peak and could be used as hydrocarbon mixture standard sample (MSS), by which total hydrocarbon content in the contaminated soil could be measured using ultraviolet spectrophotometry instead of traditional analysis methods (GC, HPLC, GC-MS and HPLC-MS). The results shown that after the available MSS was dissolved in dichloromethane, it stably exhibited ultraviolet absorption peak at 230 nm and the calibration curve for absorbance versus concentrations indicated a high correlation (r=0.9999, p<0.001). The recoveries of this method were in the range of 81.43-104.49% with total hydrocarbon content ranging from 150 to 9000 mg kg-1 when extracted with pure dichloromethane; and the soil organic matter (SOM) interference with this method was negligible due to the percentage of transported concentration of SOM in total hydrocarbon content of contaminated soil was less than 1%. In general, the proposed method had the advantages of convenience, simplicity and good repeatability, and could be used to rapidly determine the total hydrocarbon content in hydrocarbon mixture-contaminated soils in this study. However, heavy metals exhibited interference in this method and it was necessary to analyze the existence of heavy metals prior to the adoption of this method.

A Novel Method for the Determination of Total Hydrocarbon in the Hydrocarbon Mixture-Contaminated SoilJian-gang Wang1,2, Xin-hua Zhan1, Jian-ru Liang1, Li-xiang Zhou1*, Yu-suo Lin3 and Joanthan W.C. Wong4

1College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing, 210095, China 2State key laboratory of desert and oasis ecology, Xin Jiang institute of ecology and geography, chinese academy of science, Urumqi, 830011, China3Nanjing Institute of Environmental Sciences, Ministry of Environmental Protection of China, Nanjing, 210042, China4Department of Biology, Hong Kong Baptist University, Hong Kong, China

ISSN: 2155-6199

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small molecular weight. In addition, the gravimetric method [19,20] has been extensively applied for the quantitative analysis of petroleum hydrocarbon, however, the soil organic matter (SOM) interferences are always ignored during the test, and the method normally should not be utilized for quantitative analysis of contaminants in contaminated soils with low levels of petroleum hydrocarbons. With the existing analytical methods, it is very difficult to adopt a suitable method, in terms of these contaminated sites, to monitor the remediation process. Therefore, in this study, we choose one of these sites as a demonstration field and attempt to establish a suitable and economically viable method for the determination of the total hydrocarbon content rather than using the conventional analysis such as GC-MS and HPLC-MS.

The specific objectives of this proposed investigation are to (1) investigate the feasibility of extracting and separating the total hydrocarbon in the contaminated soil as the MSS; (2) identify the ultraviolet (UV) absorption peak of the extracted MSS dissolved in different organic solvents, by which the total hydrocarbon content in the contaminated soil can be quantitatively analyzed; (3) validate the accuracy of the method and possible interferences derived from SOM and heavy metals for their absorption intensity at the scanned maximum wavelength and: (4) use the proposed method to assess the reduction of total hydrocarbon content in contaminated soil before and after thermal treatment so that the accuracy and reliability of the method can be identified in practice.

Materials and MethodsSoil sample and properties

A hydrocarbon mixture-contaminated soil from a washing factory of recycled drums in Wujiang city of Jiangsu province, China, was used in the present study. The soil sample was air-dried and ground to pass through a 2 mm sieve to determine soil physicochemical properties. In addition, an uncontaminated soil sample was also collected from an uncontaminated area near the sites, and used as control to observe the recoveries of this method and possible effect of SOM on the determination of total hydrocarbon content by the proposed method. Two soil samples near the solvent-dumping point and adjacent to the contaminated site (with varying levels of contamination) were collected for identifying the reliability of the proposed analytical method before and after thermal treatment. Selected physiochemical properties of the soils were determined according to Standard Analysis Method for Soil and Plant [21], It was found that pH, total organic carbon (TC), total nitrogen (TN), and total phosphorus (TP) of the tested soil were 7.4, 2.9%, 0.1% and 0.1%, respectively, and the particle size distribution of the tested soil was 5.2% ( > 0.2 mm) for coarse sand, 26.0% (0.02–0.2 mm) for fine sand, 27.3% (0.002–0.02 mm) for silt and 41.1% (< 0.002 mm) for clay.

Novel method for determining total hydrocarbon content

The proposed method comprised of the preparation of MSS, verification of UV absorption peak of the MSS and preparation of calibration curve, recovery analysis and evaluation of possible interferences of SOM and heavy metals.

(1) Preparation of MSS: 100 g of the contaminated soil was initially weighed into 250 ml of clean glass flask with the addition of 200 ml of hexane (analytical grade, Nanjing chemical reagent Co. Ltd., China), placed in an ultrasonic bath for 1 h. Then, after the organic suspension was incubated overnight at room temperature, the supernatant was transferred to another evaporation flask. This separation process was repeated twice. Finally, the supernatant obtained was evaporated

and separated by rotary evaporators (KQ-250B, Kunshan ultrasonic instrument Co. Ltd., China). The residue in the flask was referred as MSS. It should be noted that almost 1 kg of the contaminated soil was used for extracting the final MSS obtained in this study.

(2) Verification of UV absorption peak and preparation of calibration curve: The UV absorption peak of the MSS after dissolved in organic solvents (12, 24 and 36 mg L-1 for hexane, 45 and 65 mg L-1 for dichloromethane) was identified by scanning wavelength from 190 to 280 nm using UV-visible spectrometer (UV-754, Shanghai). Further, the calibration curve was plotted by a series of standard concentrations of available MSS solutions versus the corresponding absorption values at the maximum absorption wavelength identified above, the prepared standard concentrations for MSS dissolved in hexane and dichloromethane were 0, 6, 12, 24 and 36 mg L-1, and 0, 12, 24, 36, 48 and 60 mg L-1, respectively.

(3) Recovery analysis and SOM interference: The uncontaminated soil was spiked with different concentrations of MSS solutions to obtain the contaminated soils with different standard concentrations (150, 300, 3000, 6000 and 9000 mg kg-1) of MSS, and the SOM interference was measured as the same method and was deducted. Therefore, the recovery was calculated by the following equation:

Recovery (%) = (A-B) 100 C× (1)

Where, A was the actual concentration determined (mg kg-1), B was the concentrations in the blank of the sample (mg kg-1), C was the standard concentration prepared (mg kg-1). The extraction of total hydrocarbon and SOM were performed by ultrasonic extraction [22]. Briefly, the 30 ml of glass tube, after the addition of 1g soil and 10 ml organic solvent (dichloromethane or hexane), was capped with PTFE cap and sonicated for 1 h in a ultrasonic bath. Subsequently, the tube was centrifuged at 3000 r v 22 μm PTFE organic membrane and diluted to the required volume for determination by UV spectrophotometry.

(4) Investigation of heavy metals interferences: The possible interferences of heavy metals were assessed by determining the total hydrocarbon content in indigenous contaminated soil spiked with different concentrations of heavy metals. Five soil samples with one of Cu2+, Cr3+, Zn2+, Pb2+, respectively, and their equivalent mixture were prepared in contaminated soil. The corresponding content of heavy metals were 160 mg kg-1 for Cu2+ and Cr3+, 200 mg kg-1 for Zn2+ and Pb2+, which were prepared in reference to the third standard value of the Soil Environmental Quality Standard, China [23]. Moreover, the soil without heavy metals was used as control. The interference degree resulted from heavy metals was evaluated by following equation:

Interference degree (%)= (H-CK) 100 CK × (2)

Where, H was the total hydrocarbon content (mg kg-1) after the addition of heavy metals, CK was the concentration of total hydrocarbon without heavy metals (mg kg-1). The total hydrocarbon content was determined one month after addition of heavy metals for aging.

Soil thermal treatment

Thermal treatment experiment was performed in a novel-designed apparatus, and capable of accurately manipulating the temperature and time to avoid the occurrence of calefactive process for soil. This new apparatus was different from the reported tube type resistance furnace [24] and fluidized bed reactor [25].

Figure 1 exhibits the employed apparatus for soil thermal


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experiments, which mainly consists of a thermostat and a furnace with heat preservation plate of 24 holes on the top. The 24 holes were designed to embed steel tubes (20 mm inner diameter and 70 mm length) with 30 g of contaminated soil at inner so that the contaminated soil could be thermally treated. Initially, the equipment was heated up to a desired temperature, and then these soil-containing steel tubes were rapidly inserted into the oven, and heating time was recorded immediately. When the treatment time was attained, all of the soils in the steel tubes were withdrawn, mixed together sufficiently, and air-cooled until room temperature, and finally stored in glass bottle for total hydrocarbon analysis. The thermal experiment was consisted of a 10 min treatment at each of five temperatures (25, 225, 325, 400 and 500ºC) according to the boiling points of hydrocarbons [25,26]. All the thermal treatment process was conducted in a powerful fume hood.

Data analysisAll the experiments were performed in triplicates, and the results

are presented as mean ± standard deviation (SD). Statistical analysis was performed with software Origin 7.5.

Results and DiscussionsIdentification of absorption peak of MSS and preparation of calibration curve

After the MSS extraction process, the red-brown MSS with aromatic

fragrance was obtained that demonstrated the presence of petroleum hydrocarbons and polycyclic aromatic hydrocarbons. Subsequently, it was necessary to identify the stable existence of UV absorption peak for the purpose of qualitative and quantitative analysis of MSS. The UV absorption peak of MSS after dissolved in common organic solvents (45 and 65 mg L-1 of concentration for hexane; 12, 24 and 36 mg L-1 for dichloromethane) was identified by the scanning wavelength from 190 to 280 nm (Figure 2a). It was demonstrated that the absorption peak was stable at 230 and 200 nm for the MSS solution dissolved in dichloromethane and hexane, respectively. Higher concentration responded to a stronger absorption value, which is almost consistent with the maximum absorption wavelength of petroleum hydrocarbons [27]. Further, the use of MSS for quantitative analysis depended, to a large extent, on the linear calibration curve between the MSS solution concentration and corresponding absorbance.

As shown in Figure 2b, the relationship between the concentrations of MSS solutions and corresponding UV absorbance conformed a linear correlation, as indicated by the high correlation coefficients for both the organic solvents, dichloromethane (r=0.9999, p<0.001) and hexane (r=0.9999, p<0.001). The above results clearly indicated that the UV spectrophotometry is a practical approach to quantitatively analyze the total hydrocarbon content in the soil matrix. However, the method accuracy was needed to be tested in the presence of SOM and heavy metals with acceptable levels of recovery.

Interferences and recoveries of the proposed method

SOM in the contaminated soil may affect quantitative determination of total hydrocarbon due to possible UV absorbance. However, it is very difficult to separate SOM from total hydrocarbon in the original contaminated soil. For evaluating the interferences of SOM, a set of uncontaminated soil, which has similar soil characteristics to the tested contaminated soil, was used for elucidating the SOM interference. As shown in Figure 3, SOM had a modest and a highest absorption band at 230 and 200 nm, when extracted by dichloromethane and hexane, respectively, and the corresponding UV absorbance of SOM without dilution was about 0.05. Accordingly, the hydrocarbon concentrations

Figure 1: The apparatus for soil thermal treatment.

180 200 220 240 260 2800.0

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a

Absorbance

b

dichloromethane hexane

Y=0.12483+36.1167XR=0.9999

Y=0.22015+72.36902XR=0.9999

Figure 2: The UV absorption band for different concentrations of mixture standard sample dissolved in dichloromethane and hexane (a), the calibration curves of mixture standard sample solution plotted (dissolved in dichloromethane and hexane) against absorbance.


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extracted in the bulk contaminated soils reached 38.98 and 28.22 mg kg-1. Similarly, the corresponding UV absorbance of total hydrocarbon, after diluted for 17 times by dichloromethane and hexane, was 0.48 and 0.31, respectively, and the extracted concentrations of total hydrocarbon in the contaminated soil were 5071 and 4504 mg kg-1. Namely, the percentage of SOM concentration in total hydrocarbon content in the bulk soil, for the proposed method, was less than 1%, which implied that the effect of the SOM in the tested soil on the measurement of the total hydrocarbon content was acceptable and could easily deducted. In addition, it was worthy to notice that for the original contaminated soil, the UV absorption peak of the total hydrocarbon extracted and dissolved with two organic solvents was identical with the MSS standard solutions (Figure 2a) after extraction as the proposed method.

For further understanding the accuracy of analysis method, the recoveries of the total hydrocarbon in the tested soil spiked with different concentrations of MSS standard solution to prepare the MSS-contaminated soils (150, 300, 3000, 6000 and 9000 mg kg-1) was evaluated using the proposed method. As listed in table 1, the recoveries

varied between 81.43-104.49% for dichloromethane and 74.98-117.76% for hexane with total hydrocarbon content ranging from 150 to 9000 mg kg-1 And there was not significant difference in the recoveries of total hydrocarbon between dichloromethane and hexane (p<0.01). However, the reproducibility for this method demonstrated that the dichloromethane solvent extraction appeared to be much better than the hexane solvent extraction due to a lower RSD ranging from 0.95 to 7.68% for the former, whereas the RSD for the latter varied from 2.38 to 8.58%. In general, the developed method exhibited a good repeatability and accuracy and had a shorten sample test time.

Despite the acceptable interference for SOM with the proposed method, the heavy metal interferences were also investigated to enhance the application. As shown in Figure 4, the heavy metals at 200 mg kg-1 of Zn2+, Pb2+ and 100 mg kg-1 of Cu2+, Cr3+ had an obvious interference with the absorbance of total hydrocarbon extracted with dichloromethane when compared with the control (contaminant level: 3472 mg kg-1, without heavy metals). The total hydrocarbon contents obtained after the addition of Cu2+, Zn2+, Pb2+, Cr3+ and heavy metal mixtures were 3143.74, 3609.19, 4468.01, 4118.56 and 5232 mg kg-1 with the interference degree of 1.71%, 3.93%, 28.65%, 18.59% and 50.65% respectively. Some studies have indicated that the organic matter is capable of complexing heavy metals due to the presence of functional group (e.g., COOH and phenolic-OH) [28,29], suggesting that the UV absorption intensity of total hydrocarbon may be interfered. Our study also demonstrated the interference of heavy metals for the determination of total hydrocarbon. Therefore, the proposed method has a limitation when the site was contaminated with higher levels of heavy metals. However, additional study is needed to test this method at lower concentrations of heavy metals.

Application of the proposed method in thermal remediation

As mentioned above, the proposed method was accurate and applicable for the determination of total hydrocarbon content in hydrocarbon mixture-contaminated sites. It was essential that the feasibility of the proposed method should be verified with soil remediation.

180 200 220 240 260 280 3000.0

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0.6 spiked soil extracted by dichloromethane spiked soil extracted by hexane blank soil extracted by dichloromethane blank soil extracted by hexane

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Figure 3: The absorption band of total hydrocarbon in the spiked soils and SOM in blank sample after extracted and dissolved by dichloromethane and hexane respectively.

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heavy metal mixture Cu2+ (160 mg kg-1)

Zn2+ (200 mg kg-1) Pb2+ (200 mg kg-1) Cr3+ (160 mg kg-1) CK

Figure.4. The absorption band of total hydrocarbon in the original contami-nated soil after the spiking with different heavy metal.

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Figure 5: The changes of total hydrocarbon content in contaminated soils during thermal treatment at different temperatures


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Using the proposed method, the total hydrocarbon content in both of the indigenous contaminated soils with high and low concentrations of pollutant were determined before and after thermal treatment at 25, 225, 325, 400 and 500 ºC. As shown in Figure 5, the contents of total contaminants in the two soils decreased drastically with the increasing temperatures from 25 to 500 ºC. The total hydrocarbon content, for the high concentration contaminated soil, had declined from the initial level of 6976.21 mg kg-1 to 835.10 mg kg-1 after the treatment at 500 ºC. As for the soil with low contaminants concentration, the total hydrocarbon content after the treatment at 500 ºC treatment decreased from 4754.18 mg kg-1 to 1151.26 mg kg-1, and the corresponding removal percentages were 88.03% and 75.79%, respectively. The results clearly indicate that the proposed method was feasible and efficient in determining the total hydrocarbon content accurately, suggesting that it can be used easily for thermal treatments.

ConclusionsIn this study, a novel analytical method for total hydrocarbon was

successfully established and applied to determine the contamination level of hydrocarbon mixture-contaminated soil before and after thermal treatment. The method was based on the extraction and separation of total hydrocarbon from the indigenous contaminated soil to obtain MSS for a quantitative analysis by UV spectrophotometry in the range of 190 to 280 nm. It was demonstrated that as for the contaminated site in this paper, the SOM interference on the determination of total hydrocarbon content was less than 1%, which was acceptable for this proposed method. The recoveries of the total hydrocarbon with concentration levels of 150-9000 mg kg-1, after extracted by dichloromethane, were in the range of 81.43-104.49%, and the RSD values varied from 0.95 to 7.68%, and the corresponding recoveries and RSD for the hexane extraction were 74.98-117.76% and 2.38 and 8.58%, indicating that the total hydrocarbon were extracted more efficiently with dichloromethane due to higher recoveries and lower RSD. In conclusion, the proposed method has advantages of convenience, simplicity and good repeatability, and can be used to determine the content of total hydrocarbon in hydrocarbon mixture-contaminated soils. However, the heavy metals may have some interference with the determination of total hydrocarbon when their concentrations were high, indicating the need to analyze the heavy metal levels prior to the adoption of this method.

Acknowledgements

The study was financially supported by the 863 program of China (2007AA061101, 2009AA063103) and Special Project for the National Soil Environmental Investigation of China. The authors would like to thank colleagues in the Institute of Solid Waste Treatment and Reuse in Nanjing Agricultural University of China for their kind help in experiments. In addition, the authors also thank Mr. Sheng-tian, Zhang, Mr. Meng Tian, Mr. Xin Zhao and Mr. Fu-xiang Yin of Institute of Environmental Sciences, Ministry of Environmental Protection, Nanjing, China for their support in soil sampling.

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m e d i ation Journal of Bioremediation & Biodegradation · Citation: Wang JG, Zhan XH, Liang JR, Zhou LX, Lin YS, et al. (2011) A Novel Method for the Determination of Total Hydrocarbon