Elsevier

Analytica Chimica Acta

Volume 602, Issue 2, 29 October 2007, Pages 211-222
Analytica Chimica Acta

Extraction of polycyclic aromatic hydrocarbons and organochlorine pesticides from soils: A comparison between Soxhlet extraction, microwave-assisted extraction and accelerated solvent extraction techniques

https://doi.org/10.1016/j.aca.2007.09.023Get rights and content

Abstract

The methods of simultaneous extraction of polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides (OCPs) from soils using Soxhlet extraction, microwave-assisted extraction (MAE) and accelerated solvent extraction (ASE) were established, and the extraction efficiencies using the three methods were systemically compared from procedural blank, limits of detection and quantification, method recovery and reproducibility, method chromatogram and other factors. In addition, soils with different total organic carbon contents were used to test the extraction efficiencies of the three methods. The results showed that the values obtained in this study were comparable with the values reported by other studies. In some respects such as method recovery and reproducibility, there were no significant differences among the three methods for the extraction of PAHs and OCPs. In some respects such as procedural blank and limits of detection and quantification, there were significant differences among the three methods. Overall, ASE had the best extraction efficiency compared to MAE and Soxhlet extraction, and the extraction efficiencies of MAE and Soxhlet extraction were comparable to each other depending on the property such as TOC content of the studied soil. Considering other factors such as solvent consumption and extraction time, ASE and MAE are preferable to Soxhlet extraction.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental pollutants having both natural (forest fires and volcanic activity) and anthropogenic origin (incomplete combustion of fossil fuels, coke production, many industrial processes, and so on) [1]. Owing to the mutagenic and carcinogenic potential of some compounds of this group [2], the U.S. Environmental Protection Agency (EPA) lists 16 PAHs as priority pollutants. China suffers serious contamination of PAHs from combustion of fossil fuel and biomass. PAH contamination issues in China are of particular concern to local authorities and scientists [3], [4], [5].

Hexachlorocyclohexane isomers (HCHs) and dichlorodiphenyltrichloroethane plus its metabolites (DDTs) are two main sorts of organochlorine pesticides (OCPs) and are typically toxic and bioaccumulative persistent organic pollutants (POPs) [6]. They have been used extensively in China since 1950s, resulting in widespread environmental pollution [7]. In fact, in the environment soil acts as a sink for PAHs, DDTs and HCHs, and is therefore important in organic pollutants monitoring and research [8].

The analysis of PAHs, DDTs, HCHs in soils generally includes extraction, cleanup, column fractionation and gas chromatographic separation. In general, sample extraction procedures prior to analysis are often perceived as bottlenecks in analytical methods. Traditionally the extraction of PAHs, DDTs, HCHs from soil is carried out by means of Soxhlet extraction. Unfortunately, this technique is time-consuming and requires large volumes of organic solvents. In the last few years, new extraction techniques have been established in order to reduce the volume of extraction solvents and extraction times and improve the precision of analytes recoveries. Such techniques include microwave-assisted extraction (MAE), supercritical fluid extraction (SFE) and accelerated solvent extraction (ASE) [9].

MAE utilizes microwave energy, which is a non-ionizing radiation that causes molecular motion by migration of ions and rotation of dipoles but does not induce changes in molecular structure, to heat the solvent and the sample for extracting pollutants. The method of MAE was initiated by using a domestic microwave oven by Ganzler et al. [10]. MAE is gaining an important role in sample preparation techniques because it offers greatly reduced usage of organic solvents and extraction times, and it increases sample numbers through the use of multi-vessel systems that allow simultaneous extraction of multiple samples. In addition, wet samples can be extracted with MAE. ASE is performed at temperatures in the range of 40–200 °C to enhance the speed of elution and pressures in the range of 1000–2500 psi to keep the solvents in liquids state. This technique gained wide acceptance since it provided quantitative extraction with short extraction time. The technique is also simple to learn and apply in the laboratory. MAE and ASE as the US EPA standard methods 3546 and 3545 are in update of the EPA SW-846 method [11].

Various extraction parameters that have influence upon the extraction process will depend on a number of steps controlling the transport of analytes from the matrix to the bulk fluid. Generally these parameters include extraction solvent composition, solvent volume, extraction temperature, extraction time, system pressures and the matrix characteristics (including pH, water content and total organic matter content) [12], [13], [14], [15], [16]. Of all parameters, the amount of total organic carbon (TOC) and the hydrophobicity of soil organic matter were estimated to be more significant parameters in decreasing the environmental availability of POPs owing to the sequestration of these chemicals [17]. In a study by Lopez-Avila et al. [18], recoveries for spiked compounds in aged soil decreased for many of the contaminants due to effects of sequestration. In other studies [19], [20], the highest recoveries were obtained for the sand matrix that has low amount of organic matter, and the lowest recoveries were achieved for the sandy loam soil that has high content of organic matter.

Other workers have compared a range of techniques for the extraction of organic pollutants from environmental matrices. Dean et al. compared Soxhlet extraction with MAE for the extraction efficiencies of 16 PAHs from highly contaminated soils, and it was found that MAE gave higher extraction efficiencies than Soxhlet extraction did [21]. In another study Barriada-Pereira et al. compared ASE with MAE for the recoveries of organochlorine pesticides spiked into vegetable samples, and the results indicated that MAE had better extraction effect than ASE had by comparing the recoveries and limits of detection and chromatogram [22]. In a wider study Saim et al. compared Soxhelt extraction with ASE and MAE for effectiveness of extracting PAHs from contaminated soil samples, and the results showed that the highest recoveries of individual PAHs were obtained by Soxhlet extraction [23]. However previous studies about method comparison were very elementary and not systematic, and they did not consider the influence of the soil organic matter content on the extraction.

The objective of the present study was to systematically compare the extraction efficiency of three methods, i.e., Soxhlet extraction, MAE and ASE for extracting PAHs, DDTs and HCHs from soils by spiked recoveries experiments. Three sorts of soils with different organic matter contents were extracted for the comparison. The extraction conditions were chosen from the comprehensive and important reference list. They have not been optimized individually for the three methods.

Section snippets

Samples and chemicals

The recovery experiments were performed using spiked soil matrix samples (0–10 cm, 10.0 mgC g−1) near Weiming Lake in Peking University. These samples were relatively clean and uncontaminated.

The comparison experiments were carried out using three different sorts of soil samples from Hebei and Shanxi Provinces, China. TOC of the soil samples was determined using a TOC analyzer (Shimadzu 5000-A). The three sorts of soils have different TOC concentration ranging from low to high (0.02%, 0.55% and

Comparison of procedure blanks of the three methods

To check the contamination due to extraction procedure, a series of procedure blanks of the three extraction methods were performed. Table 1, Table 2 show the procedural blank values of 16 PAHs and OCPs obtained by using the three methods. These values were two orders of magnitude lower than the concentrations of most PAHs and OCPs of the actual samples, and thus they did not exhibit notable influence to the experimental results. Basheer et al. compared the extraction efficiencies of OCPs in

Conclusion

The methods of simultaneous extraction of PAHs, DDTs and HCHs from soils using Soxhlet extraction, MAE and ASE were established, and the extraction efficiencies using the three methods were systemically compared from procedural blank, limits of detection and quantification, method recovery and reproducibility, method chromatogram and other factors. The results showed that the values obtained in this study were comparable with the values reported by other studies. Overall, ASE had the best

Acknowledgements

The funding of this study was provided by National Science Foundation of China (40590390) and National Basic Research Program (2007CB407303).

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