Determination of polycyclic aromatic hydrocarbons in oil
contaminated soil
by
V. Tomos, K. Anifantaki, N. Pasadakis, N. Varotsis
PVT and Core Analysis Laboratory
Department of Mineral Resources Engineering
Technical University of Crete
Chania 73100, GREECE
Abstract
The detailed determination of the composition of multicomponent hydrocarbon mixtures is a quite complex analytical procedure, which becomes even
more tedious when Polycyclic Aromatic Hydrocarbons (PAH) are involved. The
absence of standardized procedures for the identification and quantitation of the
latter is widely recognized.
The objective of this work was to estimate the pollution level in the soil
recovered from a Greek Refinery. Different soil pretreatment, drying and extraction procedures were applied. Thermal and chemical drying, sonication, mechanical shaking, liquid–liquid extraction and solid phase extraction were utilised,
among others, for the preconcentration of PAH from soil samples.
For the detection of the pollutants, Gas Chromatography–Mass Spectrometry (GC-MS) was used. An improved method for the rapid estimation of the
total oil content in the soil sample was developed using an Infrared (IR) detection
technique. Experimental results derived from the above methods are presented.
The analytical results which were obtained, indicated that the quantitative
determination of the PAH depends strongly on the analytical procedure employed.
This procedure should combine information derived from different analytical
measurements in order to provide reliable characterization of the samples
Introduction
Petroleum-derived compounds constitute an important potential source of environmental pollution. The estimation of oil pollution in the soil is a difficult
task due to the complexity of the composition of the petroleum fractions and to
the different changes that the hydrocarbon components undergo, as a result of
weathering. The identification and the accurate quantitative measurement of the
pollutants is required for locating the pollution source, for estimating toxicity, biodegradability and for determining the appropriate remediation strategy.
Despite the considerable effort payed towards the enhancement of the
analysis of the hydrocarbon components in the environment, the absence of a generally accepted analytical protocol is widely recognized 1. Furthermore, the com-
1
plexity of the matrix of the petroleum fractions, complicates analytical work and
often leads to contradictory results when the samples are analysed from different
laboratories2. The approach for analysing petroleum-derived components in the
environment is twinfold. Either the total hydrocarbon content is globally measured or specific methods for the identification and quantitation of individual components or component groups are applied3.
IR and FT-IR absorption, ultraviolet (UV) and UV-fluoresence sprectrometry methods are used for the global quantification of total hydrocarbons 4. They
have been utilised as rapid screening tools for the determination of the hydrocarbon content in soil and water. The above methods are considered as semiquantitative ones as they fail to identify the pollutants. The response of the IR detector can be directly related to the mass of the pollutants present in the soil samples. Furthermore, the selection of the standards for the calibration of the IR detector does not influence the accuracy of the measurements to the same extent as
in the UV and UV-fluoresence methods. On the other hand, IR spectrometry is not
very accurate at low concentration levels, it can be used only in the adsorption region of the C-H bands (2850-3000 cm-1) and the accuracy of the quantitative results that it provides, varies with the composition of the sample5.
For the identification of the individual hydrocarbon components found in
trace levels in the environment, various analytical protocols have been adapted6.
They include several sample pretreatment steps for the isolation of the components of interest and for their further analysis by GC or GC-MS7. The applicability
of these methods on heavily polluted samples, such as the ones involved in this
study, is rather limited8. One of the main difficulties that are encountered, is the
rather incomplete separation of the PAH components from the complex matrix
that can be achieved by the commonly used clean-up procedures9.
The objective of this study was the determination of the oil content and of
the PAH concentration in soil samples. Different analytical procedures were applied for sample pretreatment and analysis and their performances in the analysis
of heavily contaminated soil samples were compared.
Experimental Procedures and Results
Samples
Three surficial samples (0-20cm depth), referred as SI, were recovered
from an area in the yard of a Greek Refinery where contaminated soil from different places exposed to weathering for several months was disposed. The recovered
samples were stored in dark glass bottles at 4oC. Visual inspection indicated poorly graded soil particles and heavy contamination by dark oil compounds.
Solvents, Chemicals
All solvents used in this study were of analytical grade from Merck. A
standard mixture of the 16 EPA priority PAHs, 2000 μg/ml each from Supelco
2
was used for the GC-MS calibration and n-dodecane (99 %) from Aldrich as internal standard. Aluminium oxide (90 active neutral), 70-230 mesh from Merck
and Silica gel, 100-200 mesh from Aldrich, were also used.
Sample pretreatment
Initially, all samples were thoroughly homogenized by a sieve shaker on a
2mm metal sieve. The water content (10-14% w.) was determined on portions of
homogenized samples after 24h drying at 1050C. Different drying procedures, including thermal and chemical drying, were applied. Thermal drying was performed at 400C for five days and at 1050C for 24h. Chemical drying was performed by mixing the samples with activated (6500C for 12 h) anhydrous sodium
sulfate (Na2SO4) at a 1:1 (w/w) basis. Both techniques gave similar results of the
oil content.
Extraction techniques
Ultrasonic extraction: Samples S1, S2, S3 (2,5g) were subjected to ultrasonic extraction according to the analytical scheme described by Berset et al 1 .
Briefly, the samples were put into glass conic bottles with 25ml of a solvent mixture (hexane/acetone 85/15 v/v) and were left in the ultrasonic bath (300W) for 30
min. The extracts were filtered twice through Whatman paper No 42 (2.5 μm pore
size) and stored in the refrigerator in dark amber glass flasks, closed with teflon
screw caps for subsequent analysis. The total Hydrocarbon Content (THC) of the
samples determined gravimetrically was found to be 13, 10 and 6 %w on dry
weight (dw) basis respectively.
Soxhlet extraction: The soil sample S1 was subsequently extracted using
the Soxhlet method10. 20g of the sample were extracted over-night with 160 ml of
a 10:5:1 hexane/acetone/toluene mixture. After cooling, the solvent was shaken
for 30 seconds with 300 ml of distilled water. The lower aqueous layer was discarded and the extract was washed twice with distilled water (300ml). The organic
phase was dried using anhydrous sodium sulfate (10g) and the solvent was evaporated under nitrogen. The oil content was found to be 14 %w. In order to investigate the distribution of the oil components between soil particles of different size,
the S1 sample was subdivided into five basic fractions according to the ASTM D422 soil classification system. The grain size analysis gave cobbles, gravel,
coarse, medium and fine sand. No silt or clay yield was observed. These fractions
are referred as S1F1-5. The Soxhlet extraction procedure, as described above, was
subsequently applied to these samples. The first two soil fractions were found to
be free of oil components. The determined oil content on dry basis are presented
in Table 1.
Clean-up procedures
The concentrates from ultrasonic extraction (samples S1, S2, S3) were separated
into component groups following an open column chromatography method11.
Firstly, the oil extracts were de-asphalted with 35ml n-heptane. The de-asphalted
3
oil was loaded onto a chromatography column (500mm x 8.5mm), wet packed
with 10g of a 2:1 w/w mixture of activated (at 2400C for 12h) silica gel and alumina. The saturates were eluted with 35ml n-pentane and the aromatics with 40ml
of a 1:1 (v/v) mixture of n-pentane/dichloromethane. The polar compounds were
eluted with 40ml methanol and the asphaltenes with 40ml chloroform. All fractions were gravimetrically measured after solvent evaporation and the obtained
results are presented in Table 2.
Table 1. Soil fractions characteristics and oil content
Soil Fraction
Grain size (mm) Soil Fraction yield (% w)
S1F1
>19.00
4.9
S1F2
>4.75
25.0
S1F3
>2.00
18.0
S1F4
>0.425
36.1
S1F5
>0.075
16.0
Oil Content (% dw)
----10.2
14.3
14.7
The extracts obtained from the samples S1F3-5 using the Soxhlet extraction
procedure were also separated into component groups (saturates, aromatics and
polars) using a simplified open-column chromatography procedure. Briefly, the
extracts were loaded onto a glass column (340 mm x 6mm), wet packed with 2g
silica gel on the bottom and 1g alumina. Aliphatic hydrocarbons were eluted with
hexane (10ml) and the aromatics using benzene (25ml). The quantitative results
are presented in Table 2.
Table 2. Concentration of the component groups in the extracts
Sample
Saturates
Aromatics
Resins
Asphaltenes
%w
S1
34.0
34.7
17.3
14.0
S2
40.3
35.4
13.9
10.4
S3
31.3
40.5
15.0
13.2
S1F3
22.0
41.0
37.0
S1F4
23.7
38.3
38.0
S1F5
30.3
36.0
33.7
Determination of Total Hydrocarbon Content in soil by IR spectroscopy
The THC in soil samples was measured using a Foxboro Miran 1ACVF
analyzer12. The soil samples S1, S2, S3, S1F3, S1F4 and S1F5 (2g) were extracted
with CCl4 (20 ml) in an ultrasonic bath for 2h. The extracts were injected for
analysis after filtration (Whatman paper No 42 and Acrodisc PTFE 0.45μm). The
calibration of the analyzer was performed successively using (i) a mixture of n-
4
Concentration (ppm)
tetradecane, cyclohexane and toluene (38:38:24 %v/v), (ii) a heavy gas oil fraction produced in the refinery and (iii) the oil extract obtained from ultrasonic extraction of the sample S2. The calibration curves are presented in Figure 1. The
maximum relative error measured using these curves, for concentrations up to
5g/l, were found to be 5%. Undoubtedly, the proper selection of the calibration
standard is essential for obtaining reliable quantitative results using the IR method.
Sample
S1
S2
S3
S1F3
S1F4
S1F5
Gas Oil
S2 sample
RH mix
THC (% dw)
13.8
12.8
7.0
14.5
14.1
9.8
Absorbance
Table 3. Total Hydrocarbon Content in
soil samples (THC) by IR analysis
Figure 1. Calibration curves obtained
for the IR analyzer
GC-MS analysis
The determination of selected polyaromatic components was performed by
GC-MS analysis. The oil extracts S1F3, S1F4, S1F5 were analyzed using GC-MS Fisson MD-800 system. The GC was equipped with a Chrompack WCOT column CP-Sil
5 CB/MS column (30m, 0.32mm ID) at He flow rate of 1.1 ml/min. Due to the wide
range of the molecular weights of the components of interest (128 for naphthalene to
276 for benzo(a)pyrene) the on-column injection technique was used to avoid discrimination. The oven temperature was programmed from 50oC to 65oC at 15oC/min,
from 65oC to 150oC at 8oC/min and from 150oC to 300oC at 3oC/min. Samples were
injected in hexane solution (100 ppm) with n-dodecane as internal standard. The ion
source was operated at 70eV. The temperature of the transfer line and of the source
was set at 300oC and 250oC respectively. The MS was functioning in total ion monitoring mode. A sample chromatogram of a mixture containing the 16 EPA priority
polyaromatic hydrocarbons and the aromatic fraction of sample S1F3 are presented in
Figure 2. As it is shown in the chromatogram, it is rather impossible to separate and
quantify the individual constituents in the complex hydrocarbon mixture. The shape
of the obtained chromatogram, which is characteristic for heavy petroleum mixtures,
is referred as the Unresolved Complex Mixture (UCM)3. Although using extracted ion
chromatograms, specific components of interest can be identified. In this work, the
5
presence of the 16 EPA priority PAH was investigated using selected ions as is presented in Table 4. The quantitation was performed using response factors for each individual PAH with four point calibration curves. The results are presented in Table 4.
Table 4. PAH determination by GC-MS
Name
Monitored Ions
M/z
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Naphthalene
Acenaphthylene
Acenaphthene
Fluorene
Phenanthrene
Anthracene
Fluoranthene
Pyrene
Benzo(a)anthracene
Chrysene
Benzo(b)fluoranthene
Benzo(k)fluoranthene
Benzo(a)pyrene
Indeno(1,2,3-cd)pyrene
Dibenz(a,h)anthracene
Benzo(ghi)perylene
128
152
154
166
178
178
202
202
228
228
252
252
252
276
278
276
129
151
153
165
179
176
101
200
229
226
253
253
253
138
139
138
Concentration
ppm
S1F3 S1F4 S1F5
127
153
152 3
167 2
176 33 25
179 13 10
203 53 44 39
203 118 113 124
226
229 117 127 143
125 87 90 74
125
125 70 96 92
227
279
277 64 55 71
Intensity
C12
`
5
15
25
35
45
Retention time (min)
55
65
Figure 2. Total ion chromatograms of the 16 PAH mixture and of the S1F3 aromatics
Discussion
Although the extraction by Soxhlet has been reported as a more effective method than
the ultrasonic one due to its more intensive conditions, our results, derived from duplicate analyses, indicate similar recoveries between the two methods due perhaps to
the high oil content of the samples. The selection of the appropriate separation scheme
for the clean-up procedures is very important as it strongly influences the composition
of the component groups.
The analytical results presented in Table 2, show that the measured concentrations of saturates and aromatics in sample S1 depend on the selected solvents and
chromatographic conditions. In the case of heavy petroleum fractions, components
6
belonging to the same functional group may be found in more than one chromatographic fraction. Therefore, the analysis of all the derived fractions from the clean-up
procedures is necessary for the complete compositional characterization.The oil content determined on soil fractions of different grain size indicates that, the fraction with
the finer particles exhibits lower concentration of oil components. This can be possibly explained by the limited exposure of this fraction to the pollutant. The GC-MS
results show that the concentration of the individual PAHs in the extracts from soil
fractions is independent from their particle size. The determined concentration level
of PAHs lies within the reported ranges of heavily polluted samples9.
Acknowledgments
We would like to thank the Applied Geology Laboratory of the Technical University
of Crete for assisting with the characterisation of the soil samples and the Greek General Secretariat of Research and Technology for partially financing this work.
References
1.
Berset J.D., Ejem M., Holzer R. and Lischer P. Comparison of different drying, extraction and
detection techniques for the determination of priority polycyclic aromatic hydrocarbons in background
contaminated soil samples. Analytica Chimica Acta 383, 263,1999.
2.
Zemanek G.M., Pollard J.T.S., Kenefick L.S. and Hrudey S.E. Multi-phase partioning and cosolvent effects for polynuclear aromatic hydrocarbons (PAH) in authentic petroleum- and creosotecontaminated soils. Envir. Pol. 98, No2, 239, 1997.
3.
Albaiges J. and Grimalt J. A Quality Assurance Study for the Analysis of H yd ro c arb o n s in
Sediments. Intern. J. Environ. Anal. Chem. 31, 281, 1987.
4.
Morel G., Samhan O., Literathy P., Al-Hashash H., Moulin L., Saeed T., Al-Matrouk K., Martin-Bouyer M., Paturel L., Jarosz J., Vial M., Combet E., Fachinger C. and Suptil J. Evaluation of
chromatographic and spectroscopic methods for the analysis of petroleum-derived compounds in the
environment. Fresenius J. Anal. Chem. 339, 699, 1991.
5.
Romero T.M. and Ferrer N. Determination of oil and grease by solid phase extraction and
infrared spectroscopy. Anal. Chim. Acta 395, 77, 1999.
6.
Pollard J.T.S and Hrudey S.E. Hydrocarbons Wastes at Petroleum and Creosote-Contaminated
Sites: Rapid Characterixation of Component Classes by Thin-Layer Chromatography with Flame Ionization Detection. Environ. Sci. Technol. 26, 2528, 1992 .
7.
Optimized Polycyclic Aromatic Hydrocarbon (PAH) Separations Using the HP 6890 and 5890
Series GC. Hewlett Packard, Application Note 228, 1998.
8.
Saim N., Dean J.R., Abdullah M. P. and Zuriati Z. An Experimental Design Approach for the
determination of Polycyclic Aromatic Hydrocarbons from Highly Contaminated Soil Using Accelerated Solvent Extraction. Anal. Chem. 70, 420, 1998.
9.
Domeno C. and Nerin C. Determination of polyaromatic hydrocarbons and some related compounds in industrial waste oils by GPC-HPLC-UV. Analyst 124, 67, 1999.
10.
Smith J. M., Lethbridge G. and Burns G.R. Fate of phenanthrene, pyrene and benzo[a]pyrene
during biodegradation of crude oil added to two soils. Elsevier FEMS Microbiology Letters 173, 445,
1999.
11.
Haeseler F., Blanchet D., Druelle V. and Vandecasteele J.P. Analytical characterization of
contaminated soil from former manufactured gas plant Envir. Science & Technol. 33,No6, 825, 1999.
12.
American Petroleum Institute Publication No. 4449. Health and Environmental Sciences Department.
7